热载流子

Lead‐free double‐perovskites offer high photo‐thermal stability, enhanced conductivity, and suitable bandgap‐tuning, making them excellent light absorbers for photovoltaic applications. However, abundant surface trap states in this promising family of materials drastically reduce the photovoltaic efficiencies, posing a significant bottleneck for their commercial applications to be used as active materials in devices. The use of capping ligands during synthesis has long been a widely‐acknowledged strategy for minimizing surface defect states. However, insights into the molecular‐level mechanism, particularly whether these ligands are more effective in sweeping out electron or hole trap states, remain elusive. To address this, oleylamine is employed as a surface capping ligand when synthesizing Cs4CuSb2Cl12 (CCSC) nanocrystals. Based on structural and spectral characterization, femtosecond transient absorption spectroscopy, and electronic structure calculations, our in‐depth study demonstrates that oleylamine modifies structural, electronic, and optical properties of CCSC nanocrystals by exclusively passivating electron trap states, which significantly impacts hot carrier relaxation dynamics, and underscores the significance of defect passivation strategies in managing hot carrier cooling in CCSC nanocrystals, thereby offering a framework for developing high‐performance and durable perovskite optoelectronic devices. The concept of selective surface passivation paves the way to explore similar phenomena within broader classes of materials.

Germanium arsenide (GeAs) is a layered semiconductor with remarkably anisotropic thermoelectric and optical properties and a promising candidate for multifunctional devices based on in-plane polarization dependent response. Understanding the underlying mechanism of such devices requires knowledge of GeAs electronic band structure and of the hot carrier dynamics in its conduction band, whose details are still unclear. In this work, we investigate the properties of occupied and photoexcited states of GeAs, by combining scanning tunneling spectroscopy, angle-resolved photoemission spectroscopy (ARPES), and time-resolved ARPES. We find that GeAs is an ∼0.8 eV indirect gap semiconductor, for which the conduction band minimum (CBM) is located at the Γ¯ point while the valence band maximum is out of Γ¯. A Stark broadening of the valence band is observed immediately after photoexcitation, which can be attributed to the effects of the electrical field at the surface induced by inhomogeneous screening. Moreover, the hot electron relaxation time of 1.56 ps is down to the CBM, which is dominated by electron–phonon coupling. Besides their relevance for our understanding of GeAs, these findings present general interest for the design of high performance thermoelectric and optoelectronic devices based on 2D semiconductors.

Significance Solar cell materials absorb a large fraction of sunlight, but most of the solar energy is not converted to electricity. Instead, most of the absorbed solar energy is lost as heat. Here, we show that ultrathin 2D materials can extract the excess solar energy before it is lost as heat.

The broadband photothermoelectric effect has been studied on a graphene-2D semiconductor lateral heterojunction. The massless Dirac electron transport in graphene has led to a variety of unique light-matter interaction phenomena, which promise many novel optoelectronic applications. Most of the effects are only accessible by breaking the spatial symmetry, through introducing edges, p-n junctions, or heterogeneous interfaces. The recent development of direct synthesis of lateral heterostructures offers new opportunities to achieve the desired asymmetry. As a proof of concept, we study the photothermoelectric effect in an asymmetric lateral heterojunction between the Dirac semimetallic monolayer graphene and the parabolic semiconducting monolayer MoS2. Very different hot-carrier cooling mechanisms on the graphene and the MoS2 sides allow us to resolve the asymmetric thermalization pathways of photoinduced hot carriers spatially with electrostatic gate tunability. We also demonstrate the potential of graphene-2D semiconductor lateral heterojunctions as broadband infrared photodetectors. The proposed structure shows an extreme in-plane asymmetry and provides a new platform to study light-matter interactions in low-dimensional systems.

No abstract available

The changes of off-current, threshold voltage, and on-current due to Off-Sate Stress on PMOS devices are successfully simulated by introducing an energy resonant form of the reaction cross-section instead of the Keldysh-like reaction cross-section in the energy-driven hot carrier degradation model, for changing the drain stress bias conditions. The degradation mechanism is analyzed, and the primary cause for the degradation is identified as Single Vibrational Excitation (SVE) by the secondary generated electron carriers at the drain side. It is demonstrated that additional hole traps at $\text{Si}-\text{Oxide}$ interface can lead to a counter-intuitive decrease of the linear mode on-current, unlike saturation-mode on-current, and a turn-around effect at very long time and high drain bias stress.

Experimental observations of vibronic coherences in electronically excited colloidal semiconductor nanocrystals offer a window into the ultrafast dynamics of hot carrier cooling. In previous work, we showed that, in amine-passivated quantum dots (QDs), these coherences arise during relaxation through a cascade of conical intersections between electronically excited states. Here, we demonstrate the generality of this framework by application to QDs with surface-bound carboxylate ligands. A model involving a similar cascade of conical intersections accurately reproduces the frequencies of vibronic coherences observed with broadband multidimensional spectroscopy. The impact of ligands on the relaxation dynamics is attributed to two distinct mechanisms involving either electronic or vibrational coupling between the core and ligands. Compared to the amine-passivated QDs studied previously, the electronic coupling mechanism is less prominent in carboxylate-passivated QDs. Furthermore, comparison of acetate and formate ligands reveals that truncating the ligand alkyl chains alters the relaxation behavior predicted by the model.

No abstract available

In order to develop an efficient metal-free solar energy harvester, we herein performed the electronic structure calculation, followed by the hot carrier relaxation dynamics of two dimensional (2D) aza-covalent organic framework by time domain density functional calculations in conjunction with non-adiabatic molecular dynamics (NAMD) simulation. The electronic structure calculation shows that the aza-covalent organic framework (COF) is a direct bandgap semiconductor with acute charge separation and effective optical absorption in the UV-visible region. Our study of non-adiabatic molecular dynamics simulation predicts the sufficiently prolonged electron-hole recombination process (6.8 nanoseconds) and the comparatively faster electron (22.48 ps) and hole relaxation (0.51 ps) dynamics in this two-dimensional aza-COF. According to our theoretical analysis, strong electron-phonon coupling is responsible for the rapid charge relaxation, whereas the electron-hole recombination process is slowed down by relatively weak electron-phonon coupling, relatively lower non-adiabatic coupling, and quick decoherence time. We do hope that our results of NAMD simulation on exciton relaxation dynamics will be helpful for designing photovoltaic devices based on this two dimensional aza-COF.

The integration of 55-nm radio frequency (RF) and mixed-signal technology on the low power (LP) platform has enabled highly integrated system on chip (SoC) solutions. The RF CMOS technology, known for its high integration and cost-effectiveness, is widely used in RF front-end modules like voltage-controlled oscillators (VCO), power amplifiers (P A), low noise amplifiers (LNA), and switches. Hot carrier injection (HCI) poses a reliability challenge in analog/RF circuit design on the 55nm LP platform. This paper presents an assessment of HCI reliability in two groups of thick gate oxide NIO FETs with different 10 LDD implant conditions. By optimizing the 10 LDD implant, the HCI lifetime is shown to be 3.5 times higher with a lighter arsenic dose (LDD-) compared to a higher dose (LDD+). Low frequency noise is another critical parameter for RF and mixed-signal applications, particularly in the Internet of Things (IoT) field. The low frequency noise upconverted to phase noise of VCO can degrade the performance of RF front-end chips. In this paper, hot carrier degradation and low frequency noise characteristics are investigated for NIO FETs. The coupling results between HCI and low frequency noise are also investigated, contributing to the advancement of RF and mixed-signal platforms.

In this work, an ultralow specific ON-resistance (<inline-formula> <tex-math notation="LaTeX">${R}_{ \mathrm{\scriptscriptstyle ON},\text {sp}}$ </tex-math></inline-formula>) silicon-on-insulator lateral double diffusion metal-oxide-semiconductor (SOI-LDMOS) applied in automotive circuits has been fabricated based on the <inline-formula> <tex-math notation="LaTeX">$0.18~\mu \text{m}$ </tex-math></inline-formula> process technology with 16.5 <inline-formula> <tex-math notation="LaTeX">$\text{m}\Omega \cdot $ </tex-math></inline-formula>mm2 which leads about 30% than that of reported studies. However, the poor hot carrier reliability of the SOI-LDMOS cannot fulfill the automotive circuits. To solve it, a new device has been proposed with linearly doped technology based on the discussions on the inner mechanisms of the <inline-formula> <tex-math notation="LaTeX">${R}_{ \mathrm{\scriptscriptstyle ON}}$ </tex-math></inline-formula> degradation. Thanks to the linear doping concentration in lateral and vertical directions near the damage points (poly-gate edge and bird’s beak), the impact ionization and vertical electric field have been weakened evidently. As a result, the <inline-formula> <tex-math notation="LaTeX">${R}_{ \mathrm{\scriptscriptstyle ON}}$ </tex-math></inline-formula> degradation of the proposed device (Device A) has been improved effectively and decreased from 11% to lower 1% when stressing 10000 s under the hot carrier stress. Meanwhile, the static electrical parameters of the Device A are still in an acceptable changes with OFF-state breakdown voltage (BVOFF) about 55 V and <inline-formula> <tex-math notation="LaTeX">${R}_{ \mathrm{\scriptscriptstyle ON},\text {sp}}$ </tex-math></inline-formula> about 17.5 <inline-formula> <tex-math notation="LaTeX">$\text{m}\Omega \cdot $ </tex-math></inline-formula>mm2.

In 25V High-Voltage NMOS manufactured by 28nm High-K Metal Gate (HKMG) process, the first peak of Isub appears at Vg=6.7V. Due to the hot carrier injection (HCI, stress condition @Vg=6.7V, Vd=27.5V), the linear current Idlin (Vd=0.05V) firstly increases and then decreases with time when the gate voltage Vg is low (<4V), and Idlin monotonously decreases when Vg is high (>4V), which is different from previous research. Under a certain stress time, the degradation of drain current (Vg=25V) is the largest when Vd=4.5V. Two competing mechanisms were proposed to explain the maximum degradation at Vd=4.5V. The mechanisms above were confirmed utilizing TCAD simulation and were integrated in manufacturing process. The obtained results confirmed the presume and helped to study and improve the HCI reliability.

In semiconductor‐based microlasers, the generated excited carriers can affect both the optical absorption coefficient and refractive index, influencing the lasing behavior and device performance. In this study, the lasing behavior of individual MAPbBr3 microrods pumped by femtosecond laser pulses is investigated using an ultrafast optical Kerr gating technique. By analyzing the spectral and temporal evolution of the lasing behavior, it is found that the band‐filling (BF) effect can cause a significant change in the refractive index of the material, which results in an unfavorable red‐shift and broadening of the resonant modes, deteriorating the laser linewidth and quality factor. The hot carrier cooling process can provide a buffer for alteration of the energy level occupation state, resulting in a small transient refractive index change and slight red‐shift. These results offer insights into the lasing behavior driven by photogenerated carrier dynamics and provide an optimization strategy for semiconductor‐based microlasers.

A single photon avalanche diode (SPAD) cell using N-channel extended-drain metal oxide semiconductor (N-EDMOS) is tested for its hot-carrier damage (HCD) resistance. The stressing gate-voltage (VGS) dependence is compared to hot-hole (HH) injection, positive bias temperature (PBT) instability and off-mode (VGS = 0). The goal was to check an accurate device lifetime extraction using accelerated DC to AC stressing by applying the quasi-static (QS) lifetime technique. N-EDMOS device is devoted to 3D bonding with CMOS imagers obtained by an optimized process with an effective gate-length Leff = 0.25 µm and a SiO2 gate-oxide thickness Tox = 5 nm. The operating frequency is 10 MHz at maximum supply voltage VDDmax = 5.5 V. TCAD simulations are used to determine the real voltage and timing configurations for the device in a mixed structure of the SPAD cell. AC device lifetime is obtained using worst-case DC accelerating degradation, which is transferred by QS technique to the AC waveforms applied to N-EDMOS device. This allows us to accurately obtain the AC device lifetime as a function of the delay and load for a fixed pulse shape. It shows the predominance of the high energy hot-carriers involved in the first substrate current peak during transients.

Spin-conserving transport of carriers is an essential requirement for the practical semiconductor-based spintronic devices. Kinetics of optical phonons and Dyakonov-Perel (DP) depolarization of spins in drift transport in semiconductor gallium arsenide (GaAs) is theoretically investigated. We consider electrons in n-type bulk GaAs subjected to a strong electric field, where the electron distribution is assumed to be drifted Maxwellian. The momentum drift of this distribution results in the enhanced drift velocity, and electrons with the corresponding energy emit optical hot phonons in the drifting process. The hot phonons are incorporated via the longitudinal polar optical phonon (POP) mechanism in the momentum relaxation. It is found that a finite phonon lifetime can reduce the momentum relaxation rate, which results in a delay in the runaway to higher fields, where the effect increases with the electron density. The electron spin is found to relax with the DP relaxation frequencies, and the DP spin lifetimes are found to decrease with increasing the drift field. However, a high field completely depolarizes the electron spin due to an increase of the DP spin precession frequency of the hot electrons in the POP scattering process. It is also found that the DP spin precession frequency decreases with decreasing electron temperature or increasing electron density in the moderate range. However, the findings resulting from this investigation demonstrate the hot carrier effect in the spin transport in semiconductors. The results are discussed in comparison with those obtained in earlier experimental and theoretical studies with different approaches.

Hot carrier photovoltaics have offered a promising solution to overcome the efficiency bottleneck known as Shockley‐Queisser limit. However, the development of hot carrier photovoltaics lags far behind the conventional ones, one of the long‐standing challenges is how to efficiently extract hot carriers. Here, a hot‐hole extraction efficiency is demonstrated up to 91% in CsPbI3/WSe2 van der Waals heterostructures at carrier densities comparable to the continuous solar excitation. In the CsPbI3/WSe2 heterostructure, the epitaxial CsPbI3 perovskite with reduced disorders functions as a hot carrier chromophore with an extended relaxation time, monolayer WSe2 with a unique band nesting structure serves as energy selective contact, the resonant high‐lying energy‐level matching at the ultraclean hetero‐interface with large orbit overlap provides energetically aligned transport channels for efficiently delivering hot holes. The observations also reveal that the resonant high‐lying state accessibility at the hetero‐interface plays a key role in the efficient extraction of hot carriers. The findings provide a design principle toward tailoring out‐of‐equilibrium carrier dynamics for designing hot‐carrier‐driven optoelectronics.

Hot carrier solar cells (HCSCs), harvesting the excess energy of hot carriers generated by above-band gap photoexcitation, are crucial for pushing the solar cell efficiency beyond the Shockley-Queisser limit, which is challenging to realize mainly due to fast hot-carrier cooling. By performing transient reflectance spectroscopy in a MoSe2/hBN/WS2 junction, we demonstrate the gate-tunable harvest of hot electrons from MoSe2 to WS2. By spectrally distinguishing hot-electron extraction from lattice temperature increase, we find that electrostatically doped electrons in MoSe2 can boost hot-electron extraction density (nET) by a factor up to several tens. Such enhancement arises from the interaction between hot excitons and doped electrons, which converts the excess energy of hot excitons to excitations of the Fermi sea and hence generates hot electrons. Moreover, nET can be further enhanced by reducing the conduction band offset with an external electric field. Our results provide in-depth insights into the design of HCSCs with electrostatic strategies.

Significance Semiconductor heterojunctions are crucial for optoelectronic devices. Despite the remarkable performance achieved, a complete understanding of the intricate interplay of the junction electrical potentials and charge transport phenomena across the heterojunction interface is missing. In particular, the “hot” photocarriers immediately after optical excitation play a crucial role in photovoltaic, photocatalytic, and photosensing devices, but their interaction with the heterojunction remains not understood. In this work, we apply scanning ultrafast electron microscopy to provide a holistic view of photoexcited charge dynamics in a Si/Ge heterojunction. We find that the built-in potential and the band offsets drastically modify the diffusion process of hot photocarriers across the heterojunction due to charge trapping, with significant implications for hot-carrier-based applications.

We investigate the ultrafast carrier and phonon dynamics in the ferrimagnetic semiconductor Mn$_3$Si$_2$Te$_6$ using time-resolved optical pump-probe spectroscopy. Our results reveal that the electron-phonon thermalization process with a subpicosecond timescale is prolonged by the hot-phonon bottleneck effect. We identify the subsequent relaxation processes associated with two non-radiative recombination mechanisms, i.e., phonon-assisted electron-hole recombination and defect-related Shockley-Read-Hall recombination. Temperature-dependent measurements indicate that all three relaxation components show large variation around 175 and 78 K, which is related to the initiation of spin fluctuation and ferrimagnetic order in Mn$_3$Si$_2$Te$_6$. In addition, two pronounced coherent optical phonons are observed, in which the phonon with a frequency of 3.7 THz is attributed to the $A_{1g}$ mode of Te precipitates. Applying the strain pulse propagation model to the coherent acoustic phonons yields a penetration depth of 506 nm and a sound speed of 2.42 km/s in Mn$_3$Si$_2$Te$_6$. Our results develop understanding of the nonequilibrium properties of the ferrimagnetic semiconductor Mn$_3$Si$_2$Te$_6$, and also shed light on its potential applications in optoelectronic and spintronic devices.

Traditional silicon solar cells can only absorb the solar spectrum at wavelengths below 1.1 μm. Here we proposed a breakthrough in harvesting solar energy below Si bandgap through conversion of hot carriers generated in the metal into a current using an energy barrier at the metal–semiconductor junction. Under appropriate conditions, the photo-excited hot carriers can quickly pass through the energy barrier and lead to photocurrent, maximizing the use of excitation energy and reducing waste heat consumption. Compared with conventional silicon solar cells, hot-carrier photovoltaic conversion Schottky device has better absorption and conversion efficiency for an infrared regime above 1.1 μm, expands the absorption wavelength range of silicon-based solar cells, makes more effective use of the entire solar spectrum, and further improves the photovoltaic performance of metal–silicon interface components by controlling the evaporation rate, deposition thickness, and annealing temperature of the metal layer. Finally, the conversion efficiency 3.316% is achieved under the infrared regime with a wavelength of more than 1100 nm and an irradiance of 13.85 mW/cm^2.

Many silicon-on-insulator (SOI) metal–oxide–semiconductor field-effect transistors (MOSFETs) are used in deep space detection systems because they have higher radiation resistance than bulk silicon devices. However, SOI devices have to face the double challenge of radiation and conventional reliability problems, such as hot carrier stress, at the same time. Thus, we wondered whether there is any interaction between reliability degradation and irradiation damage. In this paper, the effect of hot-carrier injection (HCI) on γ-ray-irradiated partially depleted (PD) SOI n-MOSFETs with a T-shaped gate structure is investigated. A strange phenomenon that accelerated the annealing effect on irradiation devices caused by HCI in 5 s was observed. That is, HCI has fast recovery ability on the irradiated narrow-channel n-MOSFETs. We explain the physical mechanism of this recovery effect qualitatively. Moreover, we designed a comparable experiment to evaluate the effect on the wide-channel devices. These results show that the narrow-channel devices are more sensitive to irradiation and HCI effects than wide-channel devices.

High-electron-mobility transistor (HEMT) based on Gallium Nitride (GaN) material is often designed for high-voltage operating conditions because of the high electric critical field of GaN material. However, such devices are often prone to the hot carrier stress (HCS) effect under high drain voltage and on-state conditions. Therefore, the HCS effect is an important consideration for reliable GaN HEMT devices. The Al2O3/Si3N4 bilayer gate dielectric AlGaN/GaN metal–insulator–semiconductor (MIS) HEMT has many advantages such as low gate leakage current and interface defects. However, the degradation phenomena observed in this device under HCS is very different from those of Si3N4 MIS HEMTs discussed in several reported studies. In this work, the HCS degradation results of Si3N4 MIS HEMTs and Al2O3/Si3N4 bilayer MIS HEMTs are both investigated and compared. The HCS degradations in Al2O3/Si3N4 bilayer MIS HEMTs are also examined and illustrated in depth. Finally, different stress voltage conditions of HCS are applied and the C-V measurements are carried out in order to confirm the degradation behaviors of MIS HEMT device.

No abstract available

Lead halide perovskites (LHPs) have excellent semiconductor properties. They have been used in many applications such as solar cells. Recently, the hot carrier dynamics in this type of material have received much attention as they are useful for enhancing the performance of optoelectrical devices fabricated from it. Here, we study the ultrafast hot carrier dynamics of a single CsPbBr3 microplate using femtosecond Kerr-gated wide-field fluorescence spectroscopy. The transient photoluminescence spectra have been measured under a variety of excitation fluences. The temporal evolution of bandgap renormalization and the competition between hot carrier cooling and the recovery of the renormalized bandgap are clearly revealed.

Conventional models of p-MOSFET hot carrier degradation (HCD) are based on interface trap generation and Arrhenius type temperature scaling. This study investigates p-MOSFET HCD across a wide temperature range (77–370 K) to ensure the reliability of peripheral CMOS in quantum-classical hybrid computing systems. Our key findings reveal a significant deviation from conventional models. Stress causes a positive threshold voltage shift (ΔVT > 0) at all temperatures because impact ionization generates electrons that are trapped in the gate oxide; the subthreshold slope changes little, indicating minimal interface trap creation. Crucially, at extremely low temperatures, suppressed phonon scattering the mean free path and allows carriers to retain higher kinetic energy before collision, favoring ‘lucky electron’ capture, leading to non-Arrhenius |ΔVT| scaling. A distinct anomaly at 77 K is the observation that the post-stress substrate current ΔIsub does not decrease after stress. Device lifetime estimated with an energy driven model significantly increases as temperature decreases. These results clarify the HCD mechanism at extremely low temperatures and offer physical insight that can support reliable CMOS operation from 77 to 370 K.

Junction profiles of hot electron stressed high-voltage N-channel field effect transistor (NFET) devices were measured by scanning capacitance microscopy. Deactivation of phosphorous was observed on the drain side. To directly establish a link between phosphorus deactivation and hydrogen, junction profiles were measured on an unstressed NFET (N-type metal-oxide-semiconductor) device with and without H2 plasma treatment and with subsequent 400 °C annealing in helium. Phosphorus deactivation was observed in the device after H2 plasma treatment, while subsequent 400 °C annealing led to dissociation of the P–H (or H–Si–P) bond and recovery of the device junctions.

No abstract available

Controlling the high-power laser transmittance is built on the diverse manipulation of multiple nonlinear absorption (NLA) processes in the nonlinear optical (NLO) materials. According to standard saturable absorption (SA) and reverse saturable absorption (RSA) model adapted for traditional semiconductor materials, the coexistence of SA and RSA will result in SA induced transparency at low laser intensity, yet switch to RSA with pump fluence increasing. Here, we observed, in contrast, an unusual RSA to SA conversion in quasi-two-dimensional (2D) perovskite film with a low threshold around 2.6 GW cm−2. With ultrafast transient absorption (TA) spectra measurement, such abnormal NLA is attributed to the competition between excitonic absorption enhancement and non-thermalized carrier induced bleaching. TA singularity from non-thermalized “Fermi Sea” is observed in quasi-2D perovskite film, indicating an ultrafast carrier thermalization within 100 fs. Moreover, the comparative study between the 2D and 3D perovskites uncovers the crucial role of hot-carrier effect to tune the NLA response. The ultrafast carrier cooling of quasi-2D perovskite is pointed out as an important factor to realize such abnormal NLA conversion process. These results provide fresh insights into the NLA mechanisms in low-dimensional perovskites, which may pave a promising way to diversify the NLO material applications. Controlling the high-power laser transmittance is built on the diverse manipulation of multiple nonlinear absorption processes in the nonlinear optical materials. Here, the authors demonstrate the crucial role of hot-carrier effect to tune the nonlinear absorption response in quasi-2D perovskite films.

No abstract available

Dynamic semiconductor diode generators (DDGs) offer a potential portable and miniaturized energy source, with the advantages of high current density, low internal impedance, and independence of the rectification circuit. However, the output voltage of DDGs is generally as low as 0.1–1 V, owing to energy loss during carrier transport and inefficient carrier collection, which requires further optimization and a deeper understanding of semiconductor physical properties. Therefore, this study proposes a vertical graphene/silicon DDG to regulate the performance by realizing hot carrier transport and collection. With instant contact and separation of the graphene and silicon, hot carriers are generated by the rebounding process of built‐in electric fields in dynamic graphene/silicon diodes, which can be collected within the ultralong hot electron lifetime of graphene. In particular, monolayer graphene/silicon DDG outputs a high voltage of 6.1 V as result of ultrafast carrier transport between the monolayer graphene and silicon. Furthermore, a high current of 235.6 nA is generated due to the carrier multiplication in graphene. A voltage of 17.5 V is achieved under series connection, indicating the potential to supply electronic systems through integration design. The graphene/silicon DDG has applications as an in situ energy source for harvesting mechanical energy from the environment.

A thorough understanding of the photocarrier relaxation dynamics in semiconductor quantum dots (QDs) is essential to optimize their device performance. However, resolving hot carrier kinetics under high excitation conditions with multiple excitons per dot is challenging because it convolutes several ultrafast processes, including Auger recombination, carrier-phonon scattering, and phonon thermalization. Here, we report a systematic study of the lattice dynamics induced by intense photoexcitation in PbSe QDs. By probing the dynamics from the lattice perspective using ultrafast electron diffraction together with modeling the correlated processes collectively, we can differentiate their roles in photocarrier relaxation. The results reveal that the observed lattice heating time scale is longer than that of carrier intraband relaxation obtained previously using transient optical spectroscopy. Moreover, we find that Auger recombination efficiently annihilates excitons and speeds up lattice heating. This work can be readily extended to other semiconductor QDs systems with varying dot sizes.

Extracting hot carriers prior to thermalization is a long-standing challenge for surpassing the Shockley-Queisser limit in photovoltaic and optoelectronic devices. Antimony selenide (Sb2Se3), featuring quasi-one-dimensional ribbon-like crystal motifs, has recently emerged as a promising platform for hot-carrier utilization. However, directly resolving the associated ultrafast extraction current remains elusive. Here, by employing polarization-phase-resolved THz emission spectroscopy, we visualized the directed transient hot-electron extraction current at the Sb2Se3/SnO2 interface and identify an ∼1.2 eV pump photon energy threshold by a Fowler-type photoemission model, consistent with the direct band gap of the Sb2Se3. These results position THz emission spectroscopy as a powerful, noncontact metrology for mapping ultrafast and anisotropic hot-carrier dynamics and provide design principles for directional hot-carrier management in Sb2Se3-based energy-conversion devices.

Abstract: The hot carrier energy loss rate in a two-dimensioal electron gas in SiGe/Si quantum well has been theoretically studied and carrier concentration ranging from 1.0x1012 to 5.0x1014 m-2. The energy loss rate in this highly non-parabolic system is dominated by acoustic deformation potential scattering, whereas the acoustic piezoelectric scattering is negligible. We also studied variation of energy loss rate with thickness of various quantum wells

The electron emission properties of a single-crystal nitrogen-doped diamond(001) photocathode inserted in a 10kV DC photoelectron gun are determined using a tunable (235-410nm) ultraviolet laser radiation source for photoemission from both the back nitrogen-doped substrate face and the front homo-epitaxially grown and undoped diamond crystal face. The measured spectral trends of the mean transverse energy (MTE) and quantum efficiency (QE) of the emitted electrons are both anomalous and non-monotonic, but are shown to be consistent with (i) the known physics of electron photoexcitation from the nitrogen substitution states into the conduction bands of diamond, (ii) the energy position and dispersion characteristics of the conduction bands of diamond in the (001) emission direction, (iii) the effective electron affinity of the crystal faces, (iv) the strong electron-(optical)phonon coupling in diamond, and (v) the associated hot electron transport dynamics under energy equipartition with the optical phonons. Notably, the observed hot electron emission is shown to be restricted parallel to the photocathode surface by the low transverse effective masses of the emitting band states - a transverse momentum filtering effect.

A ‘toy model’—aimed at capturing the essential physics—is presented that jointly describes spin-polarized hot electron transport and spin pumping driven by local heating. These two processes both contribute to spin-current generation in laser-excited magnetic heterostructures. The model is used to compare the two contributions directly. The spin-polarized hot electron current is modeled as one generation of hot electrons with a spin-dependent excitation and relaxation scheme. Upon decay, the excess energy of the hot electrons is transferred to a thermalized electron bath. The elevated electron temperature leads to an increased rate of electron-magnon scattering processes and yields a local accumulation of spin. This process is dubbed as spin pumping by local heating. The built-up spin accumulation is effectively driven out of the ferromagnetic system by (interfacial) electron transport. Within our model, the injected spin current is dominated by the contribution resulting from spin pumping, while the hot electron spin current remains relatively small. We derive that this observation is related to the ratio between the Fermi temperature and Curie temperature, and we show what other fundamental parameters play a role.

Understanding the energy transport properties of hot energy carriers is of great importance for a diverse range of topics from nanoelectronics and photochemistry to the discovery of quantum materials. While much progress has been made in the study of hot carrier dynamics using ultrafast far-field time-resolved spectroscopies, it remains a great challenge to understand hot carrier transport and interaction dynamics at the nanoscale. Existing theoretical models yield only qualitative predictions that are difficult to validate against experiments. Here we present a theoretical framework that extends the study of near-field thermal radiation into the ultrafast time domain, enabling sensitive local probing and quantitative study of nanoscale hot electron and phonon transport effects that have been challenging to quantify. The proposed technique of near-field hot carrier nanoscopy directly links the features of different nonequilibrium effects to near-field thermal absorption and scattering by a scanning nanotip. Our model predicts ultrafast thermal radiation in response to photoexcitation, as well as elucidates the nanoscopic radiation properties of a number of hot carrier dissipation pathways, including nonlinear electron supercollision, second sound, and nonlocal phonon transport. This work is expected to guide experiments to identify the fundamental constraints unlocking thermal wave (second sound) propagation and address the roles of competing hydrodynamic and ballistic phonon effects at the nanoscale.

Static heterojunction-based electronic devices have been widely applied because carrier dynamic processes between semiconductors can be designed through band gap engineering. Herein, we demonstrate a tunable direct-current generator based on the dynamic heterojunction, whose mechanism is based on breaking the symmetry of drift and diffusion currents and rebounding hot carrier transport in dynamic heterojunctions. Furthermore, the output voltage can be delicately adjusted and enhanced with the interface energy level engineering of inserting dielectric layers. Under the ultrahigh interface electric field, hot electrons will still transfer across the interface through the tunneling and hopping effect. In particular, the intrinsic anisotropy of black phosphorus arising from the lattice structure produces extraordinary electronic, transport, and mechanical properties exploited in our dynamic heterojunction generator. Herein, the voltage of 6.1 V, current density of 124.0 A/m2, power density of 201.0 W/m2, and energy-conversion efficiency of 31.4% have been achieved based on the dynamic black phosphorus/AlN/Si heterojunction, which can be used to directly and synchronously light up light-emitting diodes. This direct-current generator has the potential to convert ubiquitous mechanical energy into electric energy and is a promising candidate for novel portable and miniaturized power sources in the in situ energy acquisition field.

Heterostructures comprising silicon, molybdenum disulfide (MoS2) and graphene are investigated with respect to the vertical current conduction mechanism. The measured current-voltage (I-V) characteristics exhibit temperature dependent asymmetric current, indicating thermally activated charge carrier transport. The data is compared and fitted to a current transport model that confirms thermionic emission as the responsible transport mechanism across the devices. Theoretical calculations in combination with the experimental data suggest that the heterojunction barrier from Si to MoS2 is linearly temperature dependent for T = 200 to 300 K with a positive temperature coefficient. The temperature dependence may be attributed to a change in band gap difference between Si and MoS2, strain at the Si/MoS2 interface or different electron effective masses in Si and MoS2, leading to a possible entropy change stemming from variation in density of states as electrons move from Si to MoS2. The low barrier formed between Si and MoS2 and the resultant thermionic emission demonstrated here makes the present devices potential candidates as the emitter diode of graphene-base hot electron transistors for future high-speed electronics.

We demonstrate the ultrafast conversion of statically passive dielectrics (e.g., amorphous TiO2) to transient second-order nonlinear media upon the sub-picosecond transfer of hot electrons, enabling active control of second-order optical processes.

The transport of hot, relativistic electrons produced by the interaction of an intense petawatt laser pulse with a solid has garnered interest due to its potential application in the development of innovative x-ray sources and ion-acceleration schemes. We report on spatially and temporally resolved measurements of megagauss magnetic fields at the rear of a 50-μm thick plastic target, irradiated by a multi-picosecond petawatt laser pulse at an incident intensity of ~1020 W/cm2. The pump-probe polarimetric measurements with micron-scale spatial resolution reveal the dynamics of the magnetic fields generated by the hot electron distribution at the target rear. An annular magnetic field profile was observed ~5 ps after the interaction, indicating a relatively smooth hot electron distribution at the rear-side of the plastic target. This is contrary to previous time-integrated measurements, which infer that such targets will produce highly structured hot electron transport. We measured large-scale filamentation of the hot electron distribution at the target rear only at later time-scales of ~10 ps, resulting in a commensurate large-scale filamentation of the magnetic field profile. Three-dimensional hybrid simulations corroborate our experimental observations and demonstrate a beam-like hot electron transport at initial time-scales that may be attributed to the local resistivity profile at the target rear.

Since the invention of transistors, the flow of electrons has become controllable in solid-state electronics. The flow of energy, however, remains elusive, and energy is readily dissipated to lattice via electron-phonon interactions. Hence, minimizing the energy dissipation has long been sought by eliminating phonon-emission process. Here, we report a different scenario for facilitating energy transmission at room temperature that electrons exert diffusive but quasiadiabatic transport, free from substantial energy loss. Direct nanothermometric mapping of electrons and lattice in current-carrying GaAs/AlGaAs devices exhibit remarkable discrepancies, indicating unexpected thermal isolation between the two subsystems. This surprising effect arises from the overpopulated hot longitudinal-optical (LO) phonons generated through frequent emission by hot electrons, which induce equally frequent LO-phonon reabsorption (“hot-phonon bottleneck”) cancelling the net energy loss. Our work sheds light on energy manipulation in nanoelectronics and power-electronics and provides important hints to energy-harvesting in optoelectronics (such as hot-carrier solar-cells). Minimizing the energy dissipation is usually sought by eliminating phonon-emission process. Here, the authors find a different approach for facilitating energy transmission at room temperature that electrons exert diffusive but quasiadiabatic transport, free from substantial energy loss.

A voltage-tunable negative electron affinity (NEA) semiconductor photocathode offers one key advantage over current materials such as cesiated NEA photocathodes: stability under ambient conditions. A semiconductor/insulator/graphene heterostructure can inject electrons into the conduction band of the insulator, where an electric field "heats" them up so that the effective emission barrier seen by the "hot" electrons is negative, enabling a voltage-tunable NEA surface. Here, we have experimentally demonstrated a peak emission current density of 2.253 × 10-3 A/cm2 and a peak external quantum efficiency (EQE) of ∼1.53% from a p-Si/amorphous-Al2O3/graphene-based hot electron laser-assisted cathode (HELAC). We have developed a full-band Monte Carlo Boltzmann Transport Equation (MCBTE) solver to study the hot electron transport behavior in three different crystalline insulators: SiO2, Al2O3, and MgO. Through MCBTE and semiconductor device simulations, we have predicted a peak emission current density of ∼103 A/cm2, far above our experimental value, indicating that the optimal performance of state-of-the-art HELACs has not yet been realized. This theoretical framework provides an understanding of the key performance limitations of the device and can be used to guide the optimal design (e.g., through the selection of new materials) of voltage-tunable NEA semiconductor photocathodes.

With advances in device miniaturization, understanding and manipulating nanoscale hot electron dynamics in semiconductors is recognized as an essential factor for improving performance and energy efficiency in optoelectronics and logic devices in the post‐Moore era. This work demonstrates an effective strategy to modulate hot electron dynamics through nonequilibrium phonon excitations, utilizing first‐principles‐based mode‐resolved electron‐phonon coupled Boltzmann transport equation calculations. Two different phonon‐mediated pathways for perturbing hot electron relaxation dynamics in doped semiconductors are illustrated, i.e., high‐frequency optical phonons (e.g., longitudinal optical phonons in GaN) and low‐frequency acoustic phonons, both of which exhibit strong coupling with electrons. While exciting high‐frequency optical phonons to significant nonequilibrium states can quickly reheat and elevate electron temperatures, their rapid energy decay to other phonons fails to continuously slow down the subsequent hot electron relaxation. In contrast, the weak coupling of low‐frequency acoustic phonons with other phonons facilitates the excitation of long‐lived phonon nonequilibrium, which effectively prolongs the hot electron relaxation process from a few to tens of picoseconds for GaN, AlN, and Si. These findings reveal a general mechanism to modulate hot electron dynamics in device semiconductors, offering promising approaches to enhance the energy efficiency of advanced nanoscale devices.

The InSb/CdTe heterojunction material, characterized by low effective mass and high electron mobility, exhibits interfacial energy band bending, leading to the Rashba spin–orbit coupling effect and nonreciprocal transport, which make it suitable for the fabrication of spintronic devices with broad applications in logic and storage fields. However, the complex heterojunction interfaces of InSb/CdTe, composed of group III–V and group II–VI semiconductors, are prone to interdiffusion. Therefore, characterization and study of the interfacial properties of InSb/CdTe heterojunctions are crucial for the growth improvement of the InSb/CdTe material system as well as its application in the field of spintronics. In this study, a novel scanning probe , called a scanning noise microscope, is applied to visualize hot electron scattering in InSb/CdTe nano-devices. The results demonstrate that the near-field signal originates from the Coulomb scattering of charged ions on electrons at the interface of the embedded layer heterojunction. This real-space, nondestructive characterization of the heterojunction interface properties offers a new tool for enhancing the performance of heterojunctions.

Abstract The purpose of this work is to incorporate numerically, in a discontinuous Galerkin (DG) solver of a Boltzmann–Poisson model for hot electron transport, an electronic conduction band whose values are obtained by the spherical averaging of the full band structure given by a local empirical pseudopotential method (EPM) around a local minimum of the conduction band for silicon, as a midpoint between a radial band model and an anisotropic full band, in order to provide a more accurate physical description of the electron group velocity and conduction energy band structure in a semiconductor. This gives a better quantitative description of the transport and collision phenomena that fundamentally define the behavior of the Boltzmann–Poisson model for electron transport used in this work. The numerical values of the derivatives of this conduction energy band, needed for the description of the electron group velocity, are obtained by means of a cubic spline interpolation. The EPM-Boltzmann–Poisson transport with this spherically averaged EPM calculated energy surface is numerically simulated and compared to the output of traditional analytic band models such as the parabolic and Kane bands, numerically implemented too, for the case of 1D n + − n − n + silicon diodes with 400 and 50 nm channels. Quantitative differences are observed in the kinetic moments related to the conduction energy band used, such as mean velocity, average energy, and electric current (momentum), as well as the I V -curves.

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Recently, planar and broadband hot-electron photodetectors (HE PDs) were established but exhibited degraded performances due to the adoptions of the single-junction configurations and the utilizations of absorbable films with thicknesses larger than the electronic mean free path. In this work, we present a five-layer design for planar HE PDs assisted by triple junctions in which an ultrathin Pt layer dominates the broadband and displays strong optical absorption (>0.9 from 900 nm to 1700 nm). Optical studies reveal that the optical admittance matching between optical admittances of designed device and air at all interested wavelengths is responsible for broadband light-trapping that induces prominent energy depositions in Pt layers. Electrical investigations show that, benefitting from suppressed hot-electron transport losses and increased hot-electron harvesting junctions, the predicted responsivity of the designed HE PD is up to 8.51 mA/W at 900 nm. Moreover, the high average absorption (responsivity) of 0.96 (3.66 mA/W) is substantially sustained over a broad incidence angle regardless of the polarizations of incident light. The comparison studies between five-layer and three-layer devices emphasize the superiority of five-layer design in strong optical absorption in Pt layers and efficient hot-electron extraction.

Highly efficient hot electron transport represents one of the most important properties required for applications in photovoltaic devices. Whereas the fabrication of efficient hot electron capture and lost-cost devices remains a technological challenge, regulating the energy level of acceptor-donor system through the incorporation of foreign ions using the solution-processed technique is one of the most promising strategies to overcome this obstacle. Here we present a versatile acceptor-donor system by incorporating MoO3:Eu nanophosphors, which reduces both the ‘excess’ energy offset between the conduction band of acceptor and the lowest unoccupied molecular orbital of donor and that between the valence band and highest occupied molecular orbital. Strikingly, the hot electron transfer time has been shortened. This work demonstrates that suitable energy level alignment can be tuned to gain the higher hot electron/hole transport efficiency in a simple approach without the need for complicated architectures. This work builds up the foundation of engineering building blocks for third-generation solar cells.

Elucidation of hot carrier transport and cooling mechanisms at the micro‐/nanoscale is critical for optoelectronics, thermal management, and photocatalysis. Spatiotemporal evolution of hot electrons is usually convoluted with their ultrafast dynamics. Herein, an ultrafast microscopy is employed to directly track the spatiotemporal distribution of photoexcited hot electrons, providing a transformative approach to unravel the competitive relationship of transport and cooling. In the temporal evolution profiles of hot electron distribution, an anomalous contracting stage showing obvious thickness and fluence dependency is observed, with a characteristic end time indicating the completion of electron–phonon (e‐ph) thermalization. Hot electron transport plays a prominent role in the competition with e‐ph coupling, while interfacial heat dissipation dominates nonequilibrium state evolution with thickness below ballistic length. This work significantly enriches the tool kit of ultrafast techniques and provides guidance for rational design and optimization of micro‐/nanodevices.

Hot electron photodetection based on metallic nanostructures is attracting significant attention due to its potential to overcome the limitation of the traditional semiconductor bandgap. To enable efficient hot electron photodetection for practical applications, it is necessary to achieve broadband and perfect light absorption within extremely thin plasmonic nanostructures using cost‐effective fabrication techniques. In this study, an ultrahigh optical absorption (up to 97.3% in average across the spectral range of 1200−2400 nm) is demonstrated in the ultrathin plasmonic nanoneedle arrays (NNs) with thickness of 10 nm, based on an all‐wet metal‐assisted chemical etching process. The efficient hot electron generation, transport, and injection at the nanoscale apex of the nanoneedles facilitate the photodetector to achieve a record low noise equivalent power (NEP) of 4.4 × 10−12 W Hz−0.5 at the wavelength of 1300 nm. The hot‐electron generation and injection process are elucidated through a transport model based on a Monte Carlo approach, which quantitatively matches the experimental data. The photodetector is further integrated into a light imaging system, as a demonstration of the exceptional imaging capabilities at the near‐IR regime. The study presents a lithography‐free, scalable, and cost‐effective approach to enhance hot electron photodetection, with promising prospects for future imaging systems.

The problem of hot electron transport and energy loss at high electric fields in insulators is of considerable interest in the context of dielectric breakdown and hot carrier induced degradation. Hot electron transport is discussed in terms of electron-phonon scattering in polymeric dielectrics. Monte Carlo (MC) simulation provides the basis for study of hot electron transport in thin polyethylene (PE) films. Electron trajectories and spatial evolution of the electron energy distribution are presented. Possible molecular degradation mechanisms are discussed.

Oxide heterointerfaces are ideal for investigating strong correlation effects to electron transport, relevant for oxide-electronics. Using hot-electrons, we probe electron transport perpendicular to the La0.7Sr0.3MnO3 (LSMO)- Nb-doped SrTiO3 (Nb:STO) interface and find the characteristic hot-electron attenuation length in LSMO to be 1.48 ± 0.10 unit cells (u.c.) at −1.9 V, increasing to 2.02 ± 0.16 u.c. at −1.3 V at room temperature. Theoretical analysis of this energy dispersion reveals the dominance of electron-electron and polaron scattering. Direct visualization of the local electron transport shows different transmission at the terraces and at the step-edges.

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We experimentally show that the ballistic length of hot electrons in laser-heated gold films can exceed ∼150 nm, which is ∼50% greater than the previously reported value of 100 nm inferred from pump–probe experiments. We also find that the mean free path of electrons at the peak temperature following interband excitation can reach upward of ∼45 nm, which is higher than the average value of 30 nm predicted from our parameter-free density functional perturbation theory. Our first-principles calculations of electron–phonon coupling reveal that the increase in the mean free path due to interband excitation is a consequence of drastically reduced electron–phonon coupling from lattice stiffening, thus providing the microscopic understanding of our experimental findings.

Ab-initio calculations of charge transport properties in materials without adjustable parameters have provided microscopic insights into electron-phonon interactions which govern charge transport properties. Other transport properties such as the diffusion coefficient provide additional microscopic information and are readily accessible experimentally, but few ab-initio calculations of these properties have been performed. Here, we report first-principles calculations of the hot electron diffusion coefficient in Si and its dependence on electric field over temperatures from 77 -- 300 K. While qualitative agreement in trends such as anisotropy at high electric fields is obtained, the quantitative agreement that is routinely achieved for low-field mobility is lacking. We examine whether the discrepancy can be attributed to an inaccurate description of f-type intervalley scattering by computing the microwave-frequency noise spectrum and piezoresistivity. These calculations indicate that any error in the strength of f-type scattering is insufficient to explain the diffusion coefficient discrepancies. Our findings suggest that the measured diffusion coefficient is influenced by factors such as space charge effects which are not included in ab-initio calculations, impacting the interpretation of this property in terms of charge transport processes.

Through this work, a unique substrate temperature dependent evolution of hot electron distribution is reported in GaN HEMTs on C-doped GaN buffer, and its reliability consequences are discussed. With rise in substrate temperature, significant rise in hot electron concentration, its energy, and interaction with buffer traps is observed at the drain edge, in contrast to an expected reduction in hot electron population. A mechanism based on carrier de-trapping and transport to drain is proposed and experimentally validated.

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We propose a Monte Carlo framework including (de)trapping to describe the non-equilibrium operation of charge trap flash memories. We thereby base the empirical carrier distribution that was proposed in recent studies on physical parameters. After an outline of the simulation procedure, we show how carrier trapping and detrapping is included. Finally, we illustrate the simulators' capabilities via simulations of the programming and retention operations, highlighting the insight into the carrier dynamics that this approach enables.

It is commonly accepted that electron-electron scattering (EES) alters the high-energy tail of the energy distribution function [1] [2], and thus plays an important role in the physically-based modeling of hot carrier degradation [3]. One can distinguish between selfconsistent models which assume the actual or an approximate non-equilibrium distribution for the partner electrons, and non-selfconsistent models which assume an equilibrium distribution for the partner electrons. The latter approach is suitable to describe the interaction of channel hot electrons with a reservoir of cold electrons in the drain region. This case is studied in the present work. We briefly discuss the details about the derivation of the single-particle scattering rate and the implementation in a Monte Carlo simulator for both parabolic bands and full-band structures.

Photoluminescence blinking is a common phenomenon that occurs across various low‐dimensional materials, like 0D quantum dots or 1D nanowires. Two blinking types in 0D and 1D systems have been observed and extensively studied, revealing the mechanisms of non‐equilibrium photocarrier kinetics, thereby enhancing emission stability and optimizing emitter performance. However, the origin of blinking in 2D materials is still less understood compared to those in quantum dots and single molecules and only the A‐type blinking has been reported. Here, a B‐type photoluminescence blinking is identified at the WS2/Si heterointerface through the statistics of fluorescence lifetime‐intensity distribution. Temperature‐dependent photoluminescence and transient absorption spectra show that the blinking arises from the dynamic competition between two hot carrier relaxation pathways: one leading to A exciton emission and the other to localized exciton recombination. Moreover, Förster resonance energy transfer modulates the localized exciton density at the heterointerface and sustains the blinking phenomenon, which is distinct from other B‐type blinking. This B‐type blinking broadens the understanding of photocarrier dynamics in 2D/3D systems, which will benefit the development of optoelectronic devices based on 2D materials.

In recent years, phonon–electron carrier dragging has emerged as an innovative approach for modulating energy transfer in low-dimensional systems. In this letter, we explore the fundamental mechanisms of electron–phonon coupling and the role of thermal lag behavior in ultrafast heat transport. We present a theoretical investigation of non-equilibrium (n-eq) thermal dynamics in graphene under femtosecond laser excitation, emphasizing the role of phonon-branch-resolved electron–phonon coupling. This framework provides new insight into ultrafast energy transfer processes at femtosecond timescales and illustrates key deviations from the predictions of the classical two-temperature model, particularly in spatially localized heat transport. Our results show that a 190 fs laser pulse induces a strong n-eq state, followed by momentum redistribution among the excited carriers. This is then followed by effective cooling of the carrier distribution on a 450 fs timescale through phonon emission.

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Hot carrier distributions generated by the absorption of high-energy photons typically etherealize rapidly via various phonon-mediated relaxation processes. In this paper, it is shown that type-II quantum wells (QWs) exhibit stable non-equilibrium carrier distributions, which can be manipulated and stabilized independent of the photonic dispersion of the constituent materials. Moreover, it will be shown that the reduced overlap between electron and hole wave functions in type-II QWs, which inhibit radioactive recombination, play a critical role in hot carrier thermalization in these systems. Current-voltage characteristics from analogous MQW p-i-n structures show enhanced hot carrier photocurrent extraction with increasing optical excitation power. Specifically, upon increased photo-excitation the elevated thermal energy in the carrier distribution results in the tunneling of hot electrons from the QW and increased photo carrier collection.

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This paper presents a study on the performance of an axially graded Ge-Si<inf>(1-x)</inf>Ge<inf>x</inf>-Si p-n nanowire solar cell using Monte Carlo simulation. The nanowire structure consists of a germanium (Ge) cathode, silicon (Si) anode, and a variable Si<inf>(1-x)</inf>Ge<inf>x</inf> middle region. The Silvaco code incorporates quantum mechanical effects such as Fermi statistics for carrier distribution, SRH recombination for trap-assisted processes and non-equilibrium transport using the Bi-Conjugate Gradient method. The study focuses on key photovoltaic parameters, including short-circuit current density (Jsc), open-circuit voltage (Voc), maximum power (Pm), and fill factor (FF), to assess the solar cell's efficiency. Through extensive simulations with different x compositions in the middle region, it is found that the Ge-Ge-Si<inf>0.5</inf>Ge<inf>0.5</inf>-Si structure delivers the best results, offering enhanced photovoltaic performance. We also conducted a comparison of the three different solar cell structures: (a) Ge-Si(0.5)Ge<inf>0.5</inf>-Si, (b) axially graded Ge-Si<inf>0.1</inf>Ge<inf>0.9</inf>-Si<inf>0.5</inf>Ge<inf>0.5</inf>-Si<inf>0.9</inf>Ge<inf>0.l</inf>-Si, and (c) core-to-shell graded Ge-Si<inf>(1-x)</inf>Ge<inf>x</inf>-Si. These findings provide insights into the optimal design of graded nanowire solar cells for improved efficiency.

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THz quantum cascade lasers (QCLs) have been limited to operation below 200 K [1], despite large efforts towards improving active region design and minimizing device losses [2]. The former has typically been done using simplified models with fast calculation times, but which often do not predict the correct lasing frequency, current density, or carrier distribution of the THz active region [3]. Until recently [4], advanced modelling schemes has exclusively been used for analyzing specific devices due to their computational complexity. However, with the growing availability of large computer clusters, it has become feasible to perform optimization on a larger scale also with more complex models, such as the non-equilibrium Green's function (NEGF) method.

We explore an instantaneous decoherence correction (IDC) approach for the decoherence and energy relaxation in the quantum-classical dynamics of charge transport in organic semiconducting crystals. These effects, originating from environmental fluctuations, are essential ingredients of the carrier dynamics. The IDC is carried out by measurement-like operations in the adiabatic representation. While decoherence is inherent in the IDC, energy relaxation is taken into account by considering the detailed balance through the introduction of energy-dependent reweighing factors, which could be either Boltzmann (IDC-BM) or Miller-Abrahams (IDC-MA) type. For a non-diagonal electron-phonon coupling model, it is shown that IDC tends to enhance diffusion while energy relaxation weakens this enhancement. As expected, both the IDC-BM and IDC-MA achieve a near-equilibrium distribution at finite temperatures in the diffusion process, while in the Ehrenfest dynamics the electronic system tends to infinite temperature limit. The resulting energy relaxation times with the two kinds of factors lie in different regimes and exhibit different dependences on temperature, decoherence time, and electron-phonon coupling strength, due to different dominant relaxation processes.

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Controlling doping is key to optimizing graphene for high-speed electronic and optoelectronic devices. However, its impact on non-equilibrium carrier lifetimes remains debated. Here, we systematically tune the doping level of quasi-freestanding epitaxial graphene on SiC(0001) via potassium deposition and probe its ultrafast carrier dynamics directly in the band structure using time- and angle-resolved photoemission spectroscopy. We find that increased doping lowers both the peak electronic temperature and the cooling rate of Dirac carriers, which we attribute to higher electronic heat capacity and reduced phonon emission phase space. Comparing quasi-freestanding graphene with graphene on a carbon buffer layer reveals faster relaxation in the latter, likely due to additional electronic states or phonon modes being available for heat dissipation. These findings offer new insights for optimizing graphene in electronic and photonic technologies.

We analyze the time-resolved photoluminescence (PL) from the bulklike continuum of excited states of 450 nm emitting InGaN/GaN device structures that are based on wide quantum wells with 25 nm thickness. In its spectrally integrated form, the PL behaves as if it were dominated by radiative recombination, including an almost exponential decay reflecting the radiative lifetime. If the PL is resolved spectrally, a very broad spectrum of time constants is found, from ps to ns. At low temperatures, we detected PL decay as fast as (16 ± 2) ps. This PL is assigned to hot carrier PL within the bulklike quasi-continuum of states of these promising structures. Theoretical modeling supports this interpretation of our experimental results.

Vertical graphene (VG), a complex thin-film material with hierarchical microstructures, has been successfully implemented in various applications. In this work, we have measured the ultrafast dynamics of VG, which is grown vertically on SiO2 wafers, by optical-pump terahertz-probe (OPTP) spectroscopy. We have found the relaxation of non-equilibrium charge carriers with two decay times. Furthermore, we also measure the photoconductivity of VG in the different pump-probe delays, which indicates distinct Drude-Smith characteristics. This work investigates the light-matter interactions between VG and terahertz(THz) waves and paves the way for the development of high-speed THz optoelectronic devices.

Slow relaxation of highly excited (hot) charge carriers can be used to increase efficiencies of solar cells and related devices as it allows hot carriers to be extracted and utilized before they relax and lose energy. Using a combination of real-time density functional theory and nonadiabatic molecular dynamics, we demonstrate that nonradiative relaxation of excited holes in an Au film slows down 30-fold as holes relax across the energy range -2 to -1.5 eV below the Fermi level. This effect arises due to sharp decreases in density of states (DOS) and reduced hole-phonon coupling in this energy range. Furthermore, to improve adhesion, a thin film of transition metal, such as Ti, is often inserted between the noble metal layer and its underlying substrate; we demonstrate that this adhesion layer completely eliminates the hot-hole bottleneck because it significantly, 7-fold per atom, increases the DOS in the critical energy region between -1.5 eV and the Fermi level, and because Ti atoms are 4-times lighter than Au atoms, high frequency phonons are introduced and increase the charge-phonon coupling. The detailed ab initio analysis of the charge-phonon scattering emphasizes the nonequilibrium nature of the relaxation processes and provides important insights into the energy flow in metal films. The study suggests that energy losses to heat can be greatly reduced by judicious selection of adhesion layers that do not involve light atoms and have relatively low DOS in the relevant energy range. Inversely, narrow Ti adhesion layers assist heat dissipation needed in electronics applications.

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It is still controversial whether the phonon bottleneck effect, traditionally considered to be negligible in strongly confined quantum dots (QDs) due to the efficient electron-hole energy exchange in Auger processes, could become more significant in n-doped QDs with fully occupied valence bands. In this study, we employ a generalized Holstein-Peierls Hamiltonian to describe the electron-vibration couplings in QDs and study the hot electron relaxation dynamics by large-scale nonadiabatic dynamics simulations based on the proposed effective Hamiltonian method. The high-energy electron undergoes rapid relaxation, driven by the high electronic density of states (DOS) and strong effective electron-vibration coupling in the energy space. In contrast, the time propagation of medium- and low-energy electrons shows an evident phonon bottleneck effect, characterized by the large energy separations that necessitate multiphonon processes, resulting in the relaxation time on the nanosecond scale. The electron relaxation rate is found to exhibit a power-law dependence on the electronic DOS, with an exponent of approximately 1.62. We also identify the key role of the quantum decoherence effect, which suppresses unphysical overcoherent electron propagation and enables exponential population decay to the conduction band minimum. The electron relaxation time calculated by the real-time simulations further validates the theoretical prediction by the obtained power-law formula. These theoretical findings elucidate the intricate interplay of electronic transitions, electron-vibration couplings, and quantum decoherence in QDs, providing insights for further optimization of QD-based optoelectronic devices.

The performance of semiconductor-based optoelectronic devices is dominated by hot-carrier/exciton (HC/HE) relaxation [1]. Past thermodynamic calculations have revealed that the power conversion efficiency of single-junction solar cells under 1 sun illumination can reach around 66% if the electron/hole excess energy is fully harvested [2]. Such efficient HC/HE extraction necessitates retarded intra-band relaxation. Nevertheless, the rapid HC/HE cooling constitutes a major loss channel for solar cell efficiency, and how to decelerate the HC/HE cooling still remains one of the cruxes for high-efficiency photovoltaic devices.

Understanding and manipulating hot electron dynamics in semiconductors may enable disruptive energy conversion schemes. Hot electrons in bulk semiconductors usually relax via electron-phonon scattering on a sub-picosecond timescale. Quantum-confined semiconductors such as quantum dots offer a unique platform to prolong hot electron lifetime through their size-tunable electronic structures. Here, we study hot electron relaxation in electron-doped ( n -doped) colloidal CdSe quantum dots. For lightly-doped dots we observe a slow 1P e hot electron relaxation (~10 picosecond) resulting from a Pauli spin blockade of the preoccupying 1S e electron. For heavily-doped dots, a large number of electrons residing in the surface states introduce picosecond Auger recombination which annihilates the valance band hole, allowing us to observe 300-picosecond-long hot electrons as a manifestation of a phonon bottleneck effect. This brings the hot electron energy loss rate to a level of sub-meV per picosecond from a usual level of 1 eV per picosecond. These results offer exciting opportunities of hot electron harvesting by exploiting carrier-carrier, carrier-phonon and spin-spin interactions in doped quantum dots. Hot electrons in bulk semiconductors usually relax via electron-phonon scattering on a sub-picosecond timescale. Here, the authors observe hot electron lifetime as long as 320 picoseconds by performing a photochemical reduction reaction on colloidal quantum dots.

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Two-dimensional (2D) heterostructures composed of transition-metal dichalcogenide atomic layers are the new frontier for novel optoelectronic and photovoltaic device applications. Some key properties that make these materials appealing, yet are not well understood, are ultrafast hole/electron dynamics, interlayer energy transfer and the formation of interlayer hot excitons. Here, we study photoexcited electron/hole dynamics in a representative heterostructure, the MoS2/WSe2 interface, which exhibits type II band alignment. Employing time-dependent density functional theory in the time domain, we observe ultrafast charge dynamics with lifetimes of tens to hundreds of femtoseconds. Most importantly, we report the discovery of an interfacial pathway in 2D heterostructures for the relaxation of photoexcited hot electrons through interlayer hopping, which is significantly faster than intralayer relaxation. This finding is of particular importance for understanding many experimentally observed photoinduced processes, including charge and energy transfer at an ultrafast time scale (<1 ps).

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Colloidal quantum dots (QDs) have attracted interest as materials for opto-electronic applications, wherein their efficient energy use requires the understanding of carrier relaxation. In QDs capped by bifunctional thiols, used to attach the QDs to a surface, the relaxation is complicated by carrier traps. Using 2D spectroscopy at 77 K, we follow excitations in thiol-capped CdSe QDs with state specificity and high time resolution. We unambiguously identify the lowest state as an optically allowed hole trap, and identify an electron trap with excited-state absorption. The presence of traps changes the initial dynamics entirely by offering a different relaxation channel. 2D electronic spectroscopy enables us to pinpoint correlations between states and to easily separate relaxation from different starting states. We observe the direct rapid trapping of 1S3/2, 2S3/2, and 1S1/2 holes, and several competing electron relaxation processes from the 1Pe state.

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Transient extreme ultraviolet (XUV) spectroscopy is becoming a valuable tool for characterizing solar energy materials because it can separate photoexcited electron and hole dynamics with element specificity. Here, we use surface-sensitive femtosecond XUV reflection spectroscopy to separately measure photoexcited electron, hole, and band gap dynamics of ZnTe, a promising photocathode for CO2 reduction. We develop an ab initio theoretical framework based on density functional theory and the Bethe-Salpeter equation to robustly assign the complex transient XUV spectra to the material's electronic states. Applying this framework, we identify the relaxation pathways and quantify their time scales in photoexcited ZnTe, including subpicosecond hot electron and hole thermalization, surface carrier diffusion, ultrafast band gap renormalization, and evidence of acoustic phonon oscillations.

Nonradiative processes govern efficiencies of semiconductor nanocrystal (NC)-based devices. A central process is hot exciton cooling, or the nonradiative relaxation of a highly excited electron/hole pair to form a band-edge exciton. Due to quantum confinement effects, the timescale and mechanism of cooling are not well understood. A mismatch between electronic energy gaps and phonon frequencies has led to the hypothesis of a phonon bottleneck and extremely slow cooling, while enhanced electron-hole interactions have suggested ultrafast cooling. Experimental measurements of the cooling timescale range six orders of magnitude. Here, we develop an atomistic approach to describe phonon-mediated exciton dynamics and simulate cooling in NCs of experimentally relevant sizes. We find that cooling occurs on ~30 fs timescales in CdSe NCs, in agreement with the most recent measurements, and that the phonon bottleneck is circumvented through a cascade of multiphonon-mediated relaxation events. Furthermore, we identify NC handles for tuning the cooling timescale.

Hot carriers (HCs) in lead halide perovskites are prone to rapidly relax at the band edge and waste plentiful photon energy, severely limiting their conversion efficiency as HC photovoltaic devices. Here, the HC cooling dynamics of MAPbI3 perovskite with common vacancy point defects (e.g., MAv+ and Iv-) and an interstitial point defect (e.g., Ii-) is elucidated, and the underlying physics is explicated using ab initio nonadiabatic molecular dynamics. Contrary to vacancy point defects, the interstitial point defect reduces the band degeneracy, decreases the HC -phonon interaction, weakens the nonadiabatic coupling, and ultimately slows down hot electron cooling by a factor of 1.5-2. Furthermore, the band-by-band relaxation pathway and direct relaxation pathway are uncovered for hot electron cooling and hot hole cooling, respectively, explaining why hot electrons can store more energy than hot holes during the cooling process. Besides, oxygen molecules interacting with Ii- sharply accelerate the hot electron cooling, making it even faster than that of the pristine system and revealing the detrimental effect of oxygen on HC cooling. This work provides significant insights into the defect-dependent HC cooling dynamics and suggests a new strategy to design high-efficiency HC photovoltaic devices.

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Understanding and controlling carrier dynamics in two-dimensional (2D) van der Waals heterostructures through strain are crucial for their flexible applications. Here, femtosecond transient absorption spectroscopy is employed to elucidate the interlayer electron transfer and relaxation dynamics under external tensile strains in a WSe2/MoS2 heterostructure. The results show that a modest ∼1% tensile strain can significantly alter the lifetimes of electron transfer and nonradiative electron-hole recombination by >30%. Ab initio non-adiabatic molecular dynamics simulations suggest that tensile strain weakens the electron-phonon coupling, thereby suppressing the transfer and recombination dynamics. Theoretical predictions indicate that strain-induced energy difference increases along the electron transfer path could contribute to the prolongation of the transfer lifetime. A subpicosecond decay process, related to hot-electron cooling, remains almost unaffected by strain. This study demonstrates the potential of tuning interlayer carrier dynamics through external strains, offering insights into flexible optoelectronic device design with 2D materials.

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Impurity doping of low-dimensional semiconductors is an interesting route towards achieving control over carrier dynamics and energetics, e.g., to improve hot carrier extraction, or to obtain strongly Stokes shifted luminescence. Such studies remain, however, underexplored for the emerging family of III-V colloidal quantum dots (QDs). Here, we show through a detailed global analysis of multiresonant pump-probe spectroscopy that electron cooling in copper-doped InP quantum dot (QDs) proceeds on subpicosecond time scales. Conversely, hole localization on Cu dopants is remarkably slow (1.8 ps), yet still leads to very efficient subgap emission. Due to this slow hole localization, common Auger assisted pathways in electron cooling cannot be blocked by Cu doping III-V systems, in contrast with the case of II-VI QDs. Finally, we argue that the structural relaxation around the Cu dopants, estimated to impart a reorganization energy of 220 meV, most likely proceeds simultaneously with the localization itself leading to efficient luminescence.

Thermal transport at nanoscale metal-semiconductor interfaces via electron-phonon coupling is crucial for applications of modern microelectronic, electro-optic and thermoelectric devices. To enhance the device performance, the heat flow can be regulated by modifying the interfacial atomic interactions. We use ab initio time-dependent density functional theory combined with non-adiabatic molecular dynamics to study how the hot electron and hole relaxation rates change on incorporating a thin Ti adhesion layer at the Au/WSe2 interface. The excited charge carrier relaxation is much faster in Au/Ti/WSe2 due to the enhanced electron-phonon coupling, rationalized by the following reasons: (1) Ti atoms are lighter than Au, W and Se atoms and move faster. (2) Ti has a significant contribution to the electronic properties in the relevant energy range. (3) Ti interacts strongly with WSe2 and promotes its bond-scissoring which causes Fermi-level pinning, making WSe2 contribute to electronic properties around the Fermi level. The changes in the relaxation rates are more pronounced for excited electrons compared to holes because both relative and absolute Ti contributions to the electronic properties are larger above than below the Fermi level. The results provide guidance for improving the design of novel and robust materials by optimizing the heat dissipation at metal-semiconductor interfaces.

Interfacing perovskites with two-dimensional materials such as metal-organic frameworks (MOFs) for improved stability and electron or hole extraction has emerged as a promising path forward for the generation of highly efficient and stable solar cells. In this work, we examine the structural properties and excitation dynamics of two MOF-perovskite systems: UMCM309-a@MAPbI3 and ZrL3@MAPbI3. We find that precise band alignment and electronegativity of the MOF-linkers are necessary to facilitate the capture of excited charge carriers. Furthermore, we demonstrate that intraband relaxation of hot electrons to the MOF subsystem results in optically disallowed transitions across the band gap, suppressing radiative recombination. Furthermore, we elucidate the key mechanisms associated with improved structural stability afforded to the perovskites by the two-dimensional MOFs, highlighting the necessity of broad surface coverage and strong MOF-perovskite interaction.

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The optical hot carrier solar cell (OHCSC) has potential to achieve an efficiency above 60%. Compared to its electrical counterpart, the OHCSC has several advantages, such as the ability to separately optimise the absorber for long hot carrier lifetime, the optical contact for a narrow window with high transmission and the collecting solar cell for high conversion efficiency. Hafnium oxynitride has shown hot carrier lifetimes of several nanoseconds, whilst these are enhanced when the HfON is sandwiched between high acoustic impedance materials in an AIM structure. Long hot carrier lifetimes are essential in a HCSC and also in an OHCSC in order to maintain the energy of carriers above the band gap. Also critical in an OHCSC is that the absorber material has a high radiative efficiency such that this energy is recycled into emitted photons without any energy loss. HfON is a luminescent material with reasonable radiative effiociency due to its bandgap of about 1.8eV. This work presents the design of an OHCSC with a luminescent HfON absorber, coupled to a distributed Bragg reflector as a narrow band pass filter at 1.8-2eV and a high efficiency GaN or perovskite collector cell of appropriate band gap. The collector cell will operate at high power output because the light filtered by the band pass filter is close to monochromatic, the filter will have high transmission in the window and reflect energies either side of the window with high efficiency, the absorber will have a long hot carrier lifetime. Results will be presented that indicate the principle of high efficiencies with such an OHCSC.

Abstract. Hot-carrier solar cells (HCSCs) offer the potential to enhance the energy-conversion efficiency of photovoltaic devices up to 86%. However, most HCSC models to date assume that electrons and holes have the same temperature, whereas many reports in III to V materials indicate that electrons can be much hotter than their counterparts. We present a detailed balance HCSC model that includes different temperatures for electrons and holes. We focus on the impact of the temperature imbalance on the voltage of such an HCSC and its power-conversion efficiency. Surprisingly, a temperature imbalance at a fixed effective temperature leads to a slight power-conversion efficiency increase, up to 1 to 2 percentage points, primarily due to an increase in fill factor and possibly open-circuit voltage. Yet, we show that the knowledge of the effective temperature alone is sufficient to design a satisfying HCSC.

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Efficient photo-excited hot carrier extraction and long lifetimes are achieved in MoS2/van der Waals Au contact, minimizing recombination. Ultrafast hole extraction (310 fs) and suppressed exciton formation highlight optimized interfaces, advancing electrode design for optoelectronic devices.

Hot carrier solar cells could achieve efficiencies exceeding the Shockley–Queisser limit by collecting hot carriers before they cool down. With hot-phonon bottle neck effect, hot carrier collection may be favorable...

The hot carrier solar cell (HCSC) is a novel concept to utilize the maximum energy from the sun in a photovoltaic device. It requires a hot carrier absorber (HCA) material which can absorb a broad range of solar energy and maintain ‘hot’ carriers for a long enough time to allow extraction of the carriers whilst they are still hot, thus minimizing the thermalization loss. It also requires energy and charge carrier selective contacts incorporated in the device. Recent literature presents many potential HCA materials with their pros and cons. In this paper, we explore the structural and optical properties of atomic layer deposited HfOxNy thin film, as a candidate HCSC material.

Photovoltaic devices directly convert incident solar energy to electricity. Above bandgap incident photons excite carriers (electron and holes) deep into their respective bands. These excited carriers are called hot carriers and they undergo several physical loss processes to lose their excess energy to the lattice and thermalize to the band edge. In conventional photovoltaics, these carriers are collected from the band-edge before they recombine. The Hot Carrier Solar Cell (HCSC) is a promising third generation photovoltaics concept for energy loss reduction in solar cells by inhibiting the thermalization losses in the absorber. It has a limiting efficiency of 85%, well beyond the Shockley–Queisser limit of 33% which is the upper threshold for conventional photovoltaic cells e.g., Si solar cells. The primary mission of the hot carrier absorber engineering is to achieve a cooling rate of the carriers from 100s of picoseconds to nanoseconds, comparable to radiative recombination. One way to achieve a slower hot carrier cooling rate in the absorber is to acquire a phonon bottleneck effect. This would impede phonon assisted hot carrier relaxation. We have investigated the use of transition metal hydrides as HCSC absorber materials because of several potential advantages. The large atomic mass difference between the transition metal and hydrogen atoms leads to a large phononic band gap and a separation of the optical and acoustic phonon branches. This large phononic gap can inhibit the Klemens mechanism whereby longitudinal optical phonon decays into two acoustic phonons – the main route by which optical phonons decay - hence slowing down the hot carrier thermalization. In this work, we report on the fabrication and preliminary results of non-stoichiometric titanium hydride, vanadium hydride and zirconium hydride thin films for their quality and useability as HCSC absorbers. The thin films were prepared using electron beam evaporation and characterized using XRD, UV-VIS/IR, Ellipsometry and Raman spectroscopy. The results show that the amount of hydrogen in TiHx is such that x=1, for VHx x= 0.81 and for ZrHx x=1. From UV/VIS/IR, the reflection from all samples is dominated by a Pd layer used to prevent oxidation. The suitability of these materials as hot carrier absorbers is yet to be fully assessed, but the data presented here are important initial parameters in making that determination.

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In order to achieve a hot carrier solar cell, one must: (a) prevent significant energy loss to phonons; and (b) extract primarily the hot carriers. We have recently suggested a new approach to these problems by using metastable upper energy levels to store the photogenerated carriers. A proof of concept cell based upon an InGaAs/InAlAs structure has yielded encouraging results that suggest that valley storage is occurring and yielding a $V_{OC} > E_{G}$. Current output, however, is restricted apparently due to a barrier between the L valleys in the two materials. New materials may provide the solution to this problem and lead to a new HCSC with greatly enhanced efficiency.

The most vital key to realize hot carrier solar cell is reducing carrier relaxation time to nanoseconds by phonon bottleneck effect often observed in nanostructure. However, the mechanisms underlying this are still not well understood. In this paper, we systematically investigated the mechanisms of phonon interfacial mismatch and carrier quantum confinement over phonon bottleneck effect in InN/InxGa(1-x)N multiple quantum wells (MQWs). Highly promising hot carrier lifetimes due to enhanced phonon bottleneck effect were observed in these MQWs, where the longest hot carrier lifetime is 3.2±0.12 ns. It was found the quantum confinement of carriers could play more important role in the reduction of carrier cooling rate, while the optical phonon confinement is more likely to dominate the initial carrier temperature. This study clarifies two of the most important mechanisms of phonon bottleneck effect and directs a promising application of III-V MQWs on the absorber of hot carrier solar cell.

In hot carrier solar cells (HCSC), photo generated carriers are collected before they relax their excess of kinetic energy. Their increased temperature translates into a voltage boost, opening perspectives for high power conversion efficiency. These systems have been investigated assuming that electrons and holes share the same temperature; yet several processes tend to imbalance the two distributions and disturb the operation of the device. In this presentation, we report the first direct observation of a 2-temperatures (2T) hot carrier system. To do so, we perform a detailed analysis of the band filling signature on the photoluminescence signal of an InGaAsP QW sample. We then calculate analytically the influence of Te≠Th on the voltage of the cell, and adapt a detailed balance model to evaluate the operation of a 2T HCSC. We show that the temperature imbalance firstly impacts the fill factor of the device. This analysis also allows us to conclude that the key parameter to capture the performance of a HCSC is not Te or Th, but an effective temperature which is readily accessible through photoluminescence. This result illustrates the robustness of HCSC, which can theoretically maintain high efficiency of broad range of parameters.

The hot carrier solar is a novel photovoltaic concept for exceeding the conventional single band gap limit by reducing thermalization losses. Whilst there are reports of having observed hot carrier based solar power conversion, the performances reported lag well behind the promised limits. In this report we examine the operation of a hot carrier solar cell at its most fundamental level. The extraction process and its impacts on limiting efficiency are examined for the case of a zero band gap absorber with thermionic barriers as the energy selective contacts, with it shown that a heat flow from the absorber to the contact layer is present at open circuit voltage. The operation of this type of hot carrier solar cell is examined for forward bias in the dark, showing marked differences with expected current voltage characteristics for conventional solar cells The impact of carrier cooling, with heat transferred from the carriers to the lattice is investigated for both the dark and illuminated cases.

Determination of Seebeck coefficient is a practical technique to investigate the direct conversion of hot carrier energy to electric voltage. However, this study is challenging, especially in nanostructured materials using traditional measurements via heaters and electric contacts. Here, we investigate photo-induced Seebeck effects of InGaAs multi-quantum-well structure via a contact-less measurement (photoluminescence spectroscopy). We have determined thermodynamic properties of hot carriers via fitting the emitted photoluminescence spectra with the generalized Planck's law. We have observed a linear dependence between the gradient of carrier temperature and the quasi-Fermi level splitting of photo-generated hot carriers at various lattice temperatures, which is associated with thermoelectric effects in the system.

Carrier thermalization in a superlattice solar cell made of polar semiconductors is studied theoretically by considering a minimal model where electron-phonon scattering is the principal channel of carrier energy loss. Importantly, the effect of an intrinsic quantum mechanical property; the phonon coherence, on carrier thermalization is investigated, within semiclassical picture in terms of phonon wave packet. It turns out that coherent longitudinal optical (LO) phonons weaken the effective electron-phonon coupling, thus supposedly lowering the carrier energy loss rate in solar cell. The resulting thermalization power is indeed significantly reduced by the coherent phonons, resulting in enhanced hot carrier effect, particularly for thin enough well layer where carrier confinement is also strong. A recent experiment on superlattice solar cell prototype is shown to manifest the coherent phonons-driven phenomenon. Our results demonstrate the practical implications of the fundamental quantum coherence property of phonons in semiconductors for improving superlattice solar cell performance, via hot carrier effect.

Hot carrier solar cell is proposed where charge carriers are cooled adiabatically in the charge transport layers adjoining the absorber. The device resembles an ideal thermoelectric converter where thermopower and therefore also carrier entropy are maintained constant during cooling from the temperature attained in the absorber to the temperature at contacts.

Hot carrier solar cells are a concept of photovoltaic devices, which offers the opportunity to harvest solar energy beyond the Shockley-Queisser limit. Unlike conventional photovoltaic devices, hot carrier solar cells convert excess kinetic energy into useful electrical power rather than losing it through thermalisation mechanisms. To extract the carriers while they are still “hot”, efficient energy-selective contacts must be developed. In previous studies, the presence of the hot carrier population in a p-i-n solar cell based on a single InGaAsP quantum well on InP substrate at room temperature has been demonstrated by means of complementary optical and electrical measurements, leading to an operating condition for this device beyond the limit for classical device operation. This result allows to design a new generation of devices to increase the hot carrier conversion contribution. In this work, we study InGaAs/AlInAs type II heterojunction as a selective contact for a future hot carrier solar cell device epitaxially grown on (001) oriented InP substrate. Two p-i-n solar cells have been grown by molecular beam epitaxy on InP. The absorber is a 50 nm-thick InGaAs layer surrounded by AlInAs barriers, all lattice-matched to InP. Two architectures are compared, the first with two symmetrical AlInAs barriers and the second with a single InGaAs quantum well in the center of the n-side barrier to allow electron tunneling across the barrier. Electrical characteristics under laser illumination with two different wavelengths have been measured to investigate the effect of the selective contact compared to the barrier. This preliminary study of InGaAs/AlInAs-based selective contacts show that such III–V combination is adapted for a future hot carrier solar cell in the InP technology.

Hot carrier cells offer the potential for very high efficiencies if slowed carrier cooling can be demonstrated effectively. Various mechanisms of phonon interaction have been identified that can lead to such slowed cooling with classes of materials exhibiting long hot carrier lifetimes. The exact reasons for these are not completely clear, but a study and comparison of the materials and mechanisms will give greater insight into constructed robust hot carrier absorber materials.

Exploring the harnessing of excess electron energy through the suppression of thermalization represents a promising way for achieving single-junction devices that surpass the Shockley-Queisser limit. This strategy necessitates slow cooling processes to allow the extraction of carriers before a substantial energy loss occurs. Currently, these mechanisms are extensively investigated optically using time-resolved characterization techniques, particularly transient absorption spectroscopy, with a focus on perovskite semiconductors. The application of this spectroscopic method aims to derive a cooling time, serving as a metric to assess the material's potential as a hot-carrier absorber. However, the interpretation of this parameter raises questions, given its variability with injection conditions and its determination in a transient regime, distinct from the steady-state conditions characteristic of solar cell operation. To address this uncertainty, we scrutinize the appropriateness of the cooling time as a metric for quantifying reductions in thermalization losses and whether it accurately reflects an effective carrier temperature in a steady-state scenario. Our investigation involves a comparative analysis of time-resolved and steady-state experiments conducted on thin films of gallium arsenide. Gallium arsenide is selected due to its robustness, stability, and extensive prior research, making it an ideal reference material. The transient absorption spectroscopy employs a high temporal resolution of 250 fs, and the extracted characteristics are systematically contrasted with steady-state properties obtained through photoluminescence experiments. We delve into the impact of various parameters such as injection level and excitation energy. To bolster our experimental findings, we undertake a modeling approach grounded in phonon rate equations. This integrated experimental and modeling investigation aims to provide an in-depth understanding of the relationship between transient and steady-state characteristics, shedding light on the properties to use as a metric for thermalization losses in hot-carrier absorbers.

One of the most important and challenging loss factors of photovoltaics is the heat production of energetic carriers excited by high energy incident photons. The present work shows that if carriers are extracted at their high energies before cooling down due to scattering, the conversion efficiency can be noticeably enhanced. To increase the efficiency of a single-band gap solar cell in this work, selective energy contacts are introduced to a p-i-n structure to extract hot carriers. A selective energy contact solar cell is made up of many collecting contacts with particular energy differences from the conduction band of the cell. In other words, each contact could extract carriers with a special range of energies. The concept of selective energy contact solar cells is to collect high energy carriers, i.e. electrons in this case, within a range of energies onto external electrodes before they cool down. The comparison between conventional solar cells and selective energy contact solar cells shows a significant enhancement in electron collection and efficiency. Based on simulation results, it is observed that the efficiency of the selective energy contact solar cell has been enhanced substantially exceeding almost twice as much as a conventional solar cell's and reaching a significant 34% efficiency.

Hot carrier cells offer the potential for very high efficiencies if slowed carrier cooling can be demonstrated effectively. Various mechanisms of phonon interaction have been identified that can lead to such slowed cooling with classes of materials exhibiting long hot carrier lifetimes. The exact reasons for these are not completely clear, but a study and comparison of the materials and mechanisms will give greater insight into constructed robust hot carrier absorber materials.

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The hot carrier solar cell (HCSC) concept has been proposed to overcome the Shockley Queisser limit of a single p–n junction solar cell by harvesting carriers before they have lost their surplus energy. A promising family of materials for these purposes is metal halide perovskites (MHP). MHPs have experimentally shown very long cooling times, the key requirement of a HCSC. By using ensemble Monte Carlo simulations, light is shed on why cooling times are found to be extended. This article concentrates on the role of thermalization in the cooling process. The role of carrier–phonon and carrier–carrier interactions in thermalization and cooling is specified, while showing how these processes depend on material parameters, such as the dielectric constant and effective mass. It is quantified how thermalization acts as a cooling mechanism via the cold background effect. The importance of a low degree of background doping is to achieve the observed extended cooling times. Herein, it is mapped out how perovskites should be tuned, their material parameters, carrier concentration, and purity, in order to realize a HCSC. It contributes to the debate on the cooling times in MHPs and the suitability of tin perovskites for HCSCs.

The extraction of charge carriers from a hot carrier solar cell using energy selective contacts, and the impact on limiting efficiency is analyzed. It is shown that previous analyses imply zero power output at all operating points and, as a consequence, the limiting conversion efficiencies are overestimates. The relationship between the limiting efficiency of a hot carrier solar cell and more general thermodynamic models is also discussed.

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In conventional solar cells, photogenerated carriers lose part of their energy before they can be extracted to make electricity. The aim of hot-carrier solar cells is to extract the carriers before this energy loss, thereby turning more energy into electrical power. This requires extracting the carriers in a nonequilibrium (nonthermal) energy distribution. Here, we investigate the performance of hot-carrier solar cells for such nonequilibrium distributions. We propose a quantum transport model in which each energy-loss process (carrier thermalization, relaxation, and recombination) is simulated by a B\"uttiker probe. We study charge and heat transport to analyze the hot-carrier solar cell's power output and efficiency, introducing partial efficiencies for different loss processes and the carrier extraction. We show that producing electrical power from a nonequilibrium distribution has the potential to improve the output power and efficiency. Furthermore, in the limit where the distribution is thermal, we prove that a boxcar-shaped transmission for the carrier extraction maximizes the efficiency at any given output power.

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III–V nanowire structures are among the promising material systems with applications in hot carrier solar cells. These nanostructures can meet the requirements for such photovoltaic devices, i.e., the suppression of thermalization loss, an efficient hot carrier transport, and enhanced photoabsorption thanks to their unique one-dimensional (1D) geometry and density-of-states. Here, we investigate the effects of spatial confinement of photogenerated hot carriers in InGaAs-InAlAs core–shell nanowires, which presents an ideal class of hot carrier solar cell materials due to its suitable electronic properties. Using steady-state photoluminescence spectroscopy, our study reveals that by increasing the degree of spatial confinement and Auger recombination, the effects of hot carriers increase, which is in good agreement with theoretical modeling. However, for thin nanowires, the temperature of hot carriers decreases as the effects of crystal disorder increase. This observation is confirmed by probing the extent of the disorder-induced Urbach tail and linked to the presence of a higher density of stacking defects in the limit of thin nanowires. These findings expand our knowledge of hot carrier thermalization in nanowires, which can be applied for designing efficient 1D hot carrier absorbers for advanced-concept photovoltaic solar cells.

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The presence of hot carriers is presented in the operational properties of an (FA,Cs)Pb(I, Br, Cl)3 solar cell at ambient temperatures and under practical solar concentration. Albeit, in a device architecture that is not suitably designed as a functional hot carrier solar cell. At 100 K, clear evidence of hot carriers is observed in both the high energy tail of the photoluminescence spectra and from the appearance of a nonequilibrium photocurrent at higher fluence in light J–V measurements. At room temperature, however, the presence of hot carriers in the emission at elevated laser fluence is shown to compete with a gradual red shift in the PL peak energy as photoinduced halide segregation begins to occur at higher lattice temperature. The effects of thermionic emission of hot carriers and the presence of a nonequilibrium carrier distribution are also shown to be distinct from simple lattice heating. This results in large unsaturated photocurrents at high powers as the Fermi distribution exceeds that of the heterointerface controlling carrier transport and rectification.

Hot carrier solar cells with theoretical efficiency upon to 66% in one sun condition by utilizing thermalization energy have been intensively studied in recent years. However, practical device of this cell has not yet been realized. Because it is difficult to utilize highly energetic carriers before thermalization. One approach to overcome this is effectively reducing thermalization rate via phonon bottleneck effect. A slowed carrier intervalley scattering will also enhance this effect via Frohlich interaction. Herein, the relationship between intervalley scattering and phonon bottleneck effect in Alx Ga(1-x) As/AlAs heterojunctions was studied. It shows that the carrier thermalization rate was significantly reduced once the carriers could access the satellite valleys. Moreover, the carrier lifetime extends with increasing GaAs molar fraction when intervalley scattering occurs. Because GaAs could effectively store the carriers in the satellite valleys due to much smaller Frohlich constant, which enhances the phonon bottleneck effect and reduces carrier cooling rate.

The hot carrier multi-junction solar cell (HCMJSC) is one of the promising advanced conceptual solar cells with theoretical efficiency greater than 65%, consisting of a thin top junction with a wide bandgap and a thicker junction at the bottom with a medium bandgap for absorption of high and low energy photons. The wide bandgap CdSe/CdS low-dimensional systems (e.g. quantum dots, QDs and nanoplatelets, NPLs) widely used in optoelectrical devices are expected to be an appropriate candidate for the top junction. However, the mechanisms underlying the carrier relaxation rate reduction (or phonon bottleneck effect, PBE) for HCMJSC in these material systems are not well understood so far. In this work, the carrier relaxation mechanisms in CdSe/CdS core/shell QDs and NPLs are quantitatively analyzed by calculating the thermalization coefficient (Qth) through steady state photoluminescence (SSPL) and picosecond-time resolved photoluminescence (ps-TRPL). A significantly extended carrier relaxation time of more than 20 ns was observed in the TRPL of QDs. This could be attributed to both the Auger reheating (AR) at the initial fast decay stage and acoustic phonon folding at the slow decay stage. For SSPL, the Qth value of QDs is much lower due to a 1 order of magnitude higher AR rate. A strong coupling may exist between AR and Qth with a high probability of PBE, where a lower Qth gives a higher AR rate. The AR may dominate carrier thermalization if the PBE level is high. Meanwhile, other mechanisms like acoustic phonon folding will also affect the carrier relaxation if the PBE is at a much lower level.

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The hot carrier cell (HCSCs) is a promising advanced photovoltaic concept. It aims to minimize major losses in p-n junction solar cells and is predicted to have energy conversion efficiency over 65% at one sun. Energy selective contacts (ESCs) is a critical component of the HCSC. In this paper, potential of ESCs based on quantum dot and quantum well double barrier resonant tunneling structures is presented. Results of I-V measurements these show negative differential resistance (NDR) characteristics both at low temperature and room temperature, thus indicating that these structures can be suitable for ESC application.

Here, a new approach to extract hot carriers and circumvent the thermalization losses in single energy gap solar cells is introduced. Specifically, it is proposed to harness the properties of metastable satellite valleys in the conduction band via enhanced inter-valley carrier scattering, which inhibits and subsequently facilitates hot carrier extraction. Two established III-V systems are proposed to demonstrate the potential of this protocol and provide a route to the long-sought hot carrier solar cell under standard AM 1.5G conditions

Hot electron transistors (HETs) represent an exciting new device for integration into semiconductor technology, holding the promise of high‐frequency electronics beyond the limits of SiGe bipolar hetero transistors. With the exploration of 2D materials such as graphene and new device architectures, hot electron transistors have the potential to revolutionize the landscape of modern electronics. This study highlights a novel hot electron transistor structure with a record output current density of 800 A cm−2 and a high current gain α, fabricated using a scalable fabrication approach. The hot electron transistor structure comprises 2D hexagonal boron nitride and graphene layers wet transferred to a germanium substrate. The combination of these materials results in exceptional performance, particularly in terms of the highly saturated output current density. The scalable fabrication scheme used to produce the hot electron transistor opens up opportunities for large‐scale manufacturing. This breakthrough in hot electron transistor technology holds promise for advanced electronic applications, offering high current capabilities in a practical and manufacturable device.

Since quantum computers have been gradually introduced in countries around the world, the development of the many related quantum components that can operate independently of temperature has become more important for enabling mature products with low power dissipation and high efficiency. As an alternative to studying cryo-CMOSs (complementary metal-oxide-semiconductors) to achieve this goal, quantum tunneling devices based on 2D materials can be examined instead. In this work, a vertical graphene-based quantum tunneling transistor has been used as a frequency modulator. The transistor can operate via different quantum tunneling mechanisms and generates, by applying the appropriate bias, voltage-resistance curves characteristic of variable nonlinear resistance for both base and emitter voltages. We experimentally demonstrate frequency modulation from input signals over the range of 100 kHz to 10 MHz, enabling a tunable frequency doubler or tripler in just a single transistor. This frequency multiplication with a tunneling mechanism makes the graphene-based tunneling device a promising option for frequency electronics in the emerging field of quantum technologies.

This work presents an analysis of the high-frequency behavior of $Ga_{2}O_{3}$ hot-electron transistors (HETs). We propose a small-signal model for $Ga_{2}O_{3} HET$ for the first time. The developed equivalent circuit can be easily integrated into the SPICE program providing a fast simulation result compared to the numerical simulation. The proposed equivalent circuit includes capacitances, current source, RC-delay circuit, and resistances to capture the intrinsic behavior of the transistor. The proposed circuit model is implemented in Advanced Design System (ADS), and the S-parameters are simulated over a wide frequency range. The results show that a positive power gain is realized as the collector current density increases. The cut-off frequency is also found to increase with the current density. The analysis provides valuable insights into the performance of $Ga_{2}O_{3}$ HET s and their potential for high-frequency applications.

The integration of graphene and other two-dimensional (2D) materials with existing silicon semiconductor technology is highly desirable. This is due to the diverse advantages and potential applications brought about by the consequent miniaturization of the resulting electronic devices. Nevertheless, such devices that can operate at very high frequencies for high-speed applications are eminently preferred. In this work, we demonstrate a vertical graphene base hot-electron transistor that performs in the radio frequency regime. Our device exhibits a relatively high current density (∼200 A/cm2), high common base current gain (α* ∼ 99.2%), and moderate common emitter current gain (β* ∼ 2.7) at room temperature with an intrinsic current gain cutoff frequency of around 65 GHz. Furthermore, cutoff frequency can be tuned from 54 to 65 GHz by varying the collector-base bias. We anticipate that this proposed transistor design, built by the integrated 2D material and silicon semiconductor technology, can be a potential candidate to realize extra fast radio frequency tunneling hot-carrier electronics.

Hot electron transistors (HETs) containing two-dimensional (2D) materials promise great potential in high-frequency analog and digital applications. Here, we experimentally demonstrate all-2D van der Waals (vdW) HETs formed by graphene, hBN, and WSe2 on both rigid and flexible substrates, in which the polarity of carriers could be tuned by changing bias conditions. We proposed a theoretical model to distinguish hot hole and hot electron components in the ambipolar vdW HET. Importantly, both hot hole and hot electron modes are achieved with pronounced saturation behavior as well as record-high collection efficiency approaching theoretical limit (99.9%) at room temperature. The vdW HET show maximum output current density of 400 A/cm2. The observed ambipolar hot carrier transport with high collection efficiency is promising for high-speed nanoelectronics and 2D hot electron spectroscopy.

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In this Letter, a three-terminal hot electron transistor is developed to explore the energy alignment between metal and amorphous indium–gallium–zinc oxide (a-IGZO). Utilizing detailed analysis of the hot electron current as a function of the applied bias voltage, the intrinsic Schottky barrier height is precisely determined at 1.2 eV between metal contact (Au) and a-IGZO. Systematic reduction of the sputtering oxygen flow rate during a-IGZO film deposition from 6 to 4.5 sccm and subsequently to 3 sccm resulted in a progressive decrease in the Schottky barrier height to 0.94 and 0.85 eV, respectively. This could be attributed to the increased concentration of oxygen vacancies in a-IGZO, which induces changes in bandwidth as confirmed by further ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy characterizations. Compared to the methods of traditional thermionic emission theory and Fowler–Nordheim tunneling model to extract Schottky barrier height, this study provides a more feasible approach for evaluating the barriers at the metal–semiconductor interface.

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We experimentally demonstrate DC functionality of graphene-based hot electron transistors, which we call graphene base transistors (GBT). The fabrication scheme is potentially compatible with silicon technology and can be carried out at the wafer scale with standard silicon technology. The state of the GBTs can be switched by a potential applied to the transistor base, which is made of graphene. Transfer characteristics of the GBTs show ON/OFF current ratios exceeding 10(4).

Intrinsic limits to device performance arise from fundamental material properties that define the best achievable operation, independent of engineering constraints. In GaN high-electron-mobility transistors (HEMTs), hot longitudinal optical (LO) phonons can act as an intrinsic performance bottleneck by reducing electron saturation velocity, output current, and transconductance—metrics that are important for device operation. While bulk GaN studies report LO phonon lifetimes of ∼1 ps, leading to strong nonequilibrium phonon populations, ungated heterostructures show much shorter lifetimes of only tens of femtoseconds. Because direct measurement in HEMTs is challenging, the true impact of hot phonons remains uncertain. Full-band transport simulations of a fabricated GaN HEMT presented here reveal that LO phonon lifetimes must be ≲40 fs to reproduce measured I–V characteristics, consistent with ultrafast decay observed in GaN heterostructures. We show that even this ultrafast LO-phonon decay is insufficient to fully suppress hot-phonon effects: the residual nonequilibrium LO population continues to limit the current density at high bias. Moreover, when the LO-phonon lifetime exceeds a few tens of femtoseconds, a pronounced hot-phonon bottleneck emerges, leading to a substantial current-density suppression that is inconsistent with experimental observations.

We have investigated the electrical properties and reliability of AlGaN/GaN high electron mobility transistors (HEMT) under high-temperature RF overdrive stress. The experimental results show that the drain current and transconductance of the device decrease at 25 °C and 55 °C but do not change significantly at 85 °C before and after the stress. The decline rate of the saturation drain current, the degradation amplitude of transconductance, and the drift amplitude of threshold voltage decrease with the increase in temperature. The results of pulse I–V and low-frequency noise tests show that the current collapse is inhibited, and the trap density is reduced at higher temperatures. The Electroluminescence (EL) test shows that the luminescence characteristics of the device after RF overdrive stress are more scattered and weaker. We believe that the degradation at lower temperatures is mainly due to the influence of the hot electron effect (HEE), while the change in electrical properties at higher temperatures is due to the weakening of HEE and the improvement of the Schottky interface.

To overcome the problem of minority carrier storage time in bipolar transistors, a hot electron transistor (HET) has been proposed. This device has the advantage of high working speed and some complex logic functions can be completed by using one component. Here, we demonstrate a mixed-dimensional HET composed of GaN/AlN microwires, graphene (Gr), and Si. The electrons between GaN/AlN are injected into graphene by an F-N tunneling mechanism to achieve high speed hot electrons, then cross graphene by ballistic transport, and are collected in a nearly lossless manner through a low-barrier Si. Therefore, the device shows a record DC gain of 16.2, a collection efficiency close to the limit of 99.9% based on the graphene hot electron transistor (GHET), an emitter current density of about 68.7 A/cm2, and a high on/off current ratio reaching ∼107. Meanwhile, the current saturation range is wide, beyond those of most GHETs. It has potential applications as a power amplifier.

A multi-grooves barrier-etched structure between barrier layer and passivation layer is proposed in this paper to suppress the hot electron effect at the gate edge on the drain side in the p-GaN gate AlGaN/GaN high-electron-mobility transistor. In the TCAD simulations, the groove structure induces extra electric field concentration region and AlGaN/SiN interface area, which can lower the high electric field peak and electron temperature in the channel at the gate edge, leading to the alleviated capture and release of hot electrons. The static I–V characteristic and dynamic switching performance and breakdown characteristic show that the multi-grooves barrier-etched structure improves the current collapse and switching time and breakdown voltage. Our work exhibits the great potential of multi-grooves barrier-etched structure on the stability and reliability of the AlGaN/GaN HEMT.

— Previously we demonstrated high-current, high-gain III-nitride hot electron transistors (HETs), utilizing collimated electron injection and an undoped base region. Here, we compare the behavior of these devices from 300 to 77 and 4.2 K to elucidate the role of hot electron scattering. Under cryogenic operation and Gummel biasing, we obtain a maximum current gain of 3.5 at a collector current density of 1.35 MA/cm 2 , limited by the onset of intervalley electron transfer, and common-emitter current gain b > 20. Our results point to the promise of nitride HETs for realizing the long-proposed coherent transistor.

An infrared hot-electron transistor (IHET) 5 × 8 array with a common base configuration that allows two-terminal readout integration was investigated and fabricated for the first time. The IHET structure provides a maximum factor of six in improvement in the photocurrent to dark current ratio compared to the basic quantum well infrared photodetector (QWIP), and hence it improved the array S/N ratio by the same factor. The study also showed for the first time that there is no electrical cross-talk among individual detectors, even though they share the same emitter and base contacts. Thus, the IHET structure is compatible with existing electronic readout circuits for photoconductors in producing sensitive focal plane arrays.

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Hot Carrier Injection (HCI) behavior of 3.3V NMOS with 3D-NAND hydrogen-rich process has been studied, and an anomalous HCI degradation is observed. To explain this anomalous degradation, the interface trap evaluation has been studied by TCAD and a novel model for current (IDS and IDL) degradation is proposed for the first time.

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In this work, we present a 2-Valley energy band model of electron transport that delivers more accurate solutions compared with the Farahmand model but with improved convergence and a faster solution time for very high electric fields. This was achieved by implementing the Fermi-Dirac integral distribution as a substitution for the Boltzmann exponential, electron carrier temperature due to heat generation and conduction in the semiconductor lattice, and additional electron concentration modeling for a second conduction energy band minima. The model was primarily tuned by varying the electron temperature relaxation time constant. It was tested using a GaN-based High Electron Mobility Transistor using the Finite-Element Quasi Fermi method.

A review and perspective is presented of the classical, semi-classical and fully quantum routes to the simulation of electro-thermal phenomena in ultra-scaled silicon nanowire field-effect transistors. It is shown that the physics of ultra-scaled devices requires at least a coupled electron quantum transport semi-classical heat equation model outlined here. The importance of the local density of states (LDOS) is discussed from classical to fully quantum versions. It is shown that the minimal quantum approach requires self-consistency with the Poisson equation and that the electronic LDOS must be determined within at least the self-consistent Born approximation. To bring in this description and to provide the energy resolved local carrier distributions it is necessary to adopt the non-equilibrium Green function (NEGF) formalism, briefly surveyed here. The NEGF approach describes quantum coherent and dissipative transport, Pauli exclusion and non-equilibrium conditions inside the device. There are two extremes of NEGF used in the community. The most fundamental is based on coupled equations for the Green functions electrons and phonons that are computed at the atomically resolved level within the nanowire channel and into the surrounding device structure using a tight binding Hamiltonian. It has the advantage of treating both the non-equilibrium heat flow within the electron and phonon systems even when the phonon energy distributions are not described by a temperature model. The disadvantage is the grand challenge level of computational complexity. The second approach, that we focus on here, is more useful for fast multiple simulations of devices important for TCAD (Technology Computer Aided Design). It retains the fundamental quantum transport model for the electrons but subsumes the description of the energy distribution of the local phonon sub-system statistics into a semi-classical Fourier heat equation that is sourced by the local heat dissipation from the electron system. It is shown that this self-consistent approach retains the salient features of the full-scale approach. For focus, we outline our electro-thermal simulations for a typical narrow Si nanowire gate all-around field-effect transistor. The self-consistent Born approximation is used to describe electron-phonon scattering as the source of heat dissipation to the lattice. We calculated the effect of the device self-heating on the current voltage characteristics. Our fast and simpler methodology closely reproduces the results of a more fundamental compute-intensive calculations in which the phonon system is treated on the same footing as the electron system. We computed the local power dissipation and “local lattice temperature” profiles. We compared the self-heating using hot electron heating and the Joule heating, i.e., assuming the electron system was in local equilibrium with the potential. Our simulations show that at low bias the source region of the device has a tendency to cool down for the case of the hot electron heating but not for the case of Joule heating. Our methodology opens the possibility of studying thermoelectricity at nano-scales in an accurate and computationally efficient way. At nano-scales, coherence and hot electrons play a major role. It was found that the overall behaviour of the electron system is dominated by the local density of states and the scattering rate. Electrons leaving the simulated drain region were found to be far from equilibrium.

Random telegraph noise (RTN) after hot carrier injection (HCI) is studied with respect to RTN measurement conditions and HCI stress conditions. Both the number and amplitude of observed RTN increase after HCI. RTN after HCI shows that RTN amplitude is roughly proportional to the threshold voltage shift by HCI. As injected carrier by HCI increases, the degradation in gate oxide occurs introducing the trap sites which generate larger RTN regardless of the trap position and inversion carrier distribution. Meanwhile, RTN amplitude shows a weak correlation with drain current degradation by HCI. Large drain current degradation with high stress drain voltage is caused not only by threshold voltage shift but also by the mobility degradation which has a small impact on RTN amplitude.

The strong Coulombic interactions in miniaturized structures can lead to efficient carrier multiplication, which is essential for many-body physics and design of efficient photonic devices beyond thermodynamic conversion limits. However, carrier multiplication has rarely been realized in layered semiconducting materials despite strong electronic interactions. Here, we report the experimental observation of unusual carrier multiplication in a multilayer black phosphorus device. Electric field-dependent Hall measurements confirm a substantial increase of carrier density in multilayer black phosphorus channel, which is attributed to the impact ionization by energetic carriers. This mechanism relies on the generation of self-heating induced charge carriers under the large electric field due to competition between electron–electron and electron–phonon interactions in the direct and narrow band gap (0.3 eV) of the multilayer black phosphorus. These findings point the way toward utilization of carrier multiplication to enhance the performance of electronics and optoelectronics devices based on two-dimensional materials. Carrier multiplication processes based on new electron-hole pair generation is instrumental to realizing ultrafast and efficient optoelectronic devices. Here, the authors demonstrate multilayered black phosphorous-based transistors that show enhanced performance due to carrier multiplication.

Abstract. Hot carrier solar cells require an absorber layer, which inhibits the thermalization of photogenerated carriers. Although such absorbers have been experimentally demonstrated, the specific mechanics of carrier thermalization are not fully understood. AlAs0.16Sb0.84/InAs in particular is an interesting material system due to its type-II band alignment and the possibility of a phonon bottleneck. We calculate steady-state carrier distributions under laser excitation in AlAs0.16Sb0.84/InAs multi-quantum well structures. Carrier temperatures are extracted from these distributions and compared with experimental results. Due to the large depth of the wells, a more sophisticated Schrödinger solver that accounted for non-parabolic effects was implemented. Carrier recombination was modeled via an ABC model that used parameters obtained from experimental data. We achieved insight into how the non-equilibrium phonon interacts with the hot carriers and acts to inhibit thermalization as well as how Pauli blocking can affect this interaction. The picture is more complicated than previously described, with different ranges of the phonon wavenumber q interacting preferentially with different ranges of carrier energies. In addition, the effect of the electron–hole interaction on the carrier temperature over varying barrier widths was investigated. To match the experiment, larger longitudinal optical phonon lifetimes had to be used compared with bulk values used previously.

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DCR drift (ΔDCR) modeling in Single-Photon Avalanche Diodes (SPADs) is proposed based on hot-carrier degradation (HCD) mechanism. The bond dissociation rate constant is modeled at various stress temperatures and voltages by the carrier energy distribution coupled with the current density considering carriers reaching the mean threshold Si-H bond dissociation energy. The carrier distribution energy was achieved by a Full-Band Monte-Carlo simulation accounting for the band structure and the scattering mechanisms. Hot electrons contributes mostly to the degradation of the top SPAD interface. The carrier density is then extracted from dark- and photo-generated currents together with multiplication current by means of experiment and modeling. ΔDCR is then computed by integrating the carrier generation rate from these stress-induced defects together with the position-dependent breakdown probability. This physic-based compact model allows to predict ΔDCR along stress time under a whole set of characterization and stress conditions.

The existence of leakage current pathways leading to the appearance of impact ionization and the potential device breakdown in planar Gunn GaN diodes is analyzed by means of a combined Monte Carlo (MC)-deep learning approach. Front-view (lateral) MC simulations of the devices show the appearance of a high-field hotspot at the anode corner of the etched region, just at the boundaries between the dielectric, the GaN-doped layer, and the buffer. Thus, if the isolation created by the etched trenches is not complete, a relevant hot carrier population within the buffer is observed at sufficiently high applied voltages, provoking the appearance of a very significant number of impact ionizations and the consequent avalanche process before the onset of Gunn oscillations. A neural network trained from MC simulations allows predicting with extremely good precision the breakdown voltage of the diodes depending on the doping of the GaN active layer, the permittivity of the isolating dielectric, and the lattice temperature. Low doping, high temperature, and high permittivity provide larger operational voltages, which implies a tradeoff with the conditions required to achieve terahertz (THz) Gunn oscillations at low voltages.

Charged traps are involved in relevant transistor degradation mechanisms, such as random telegraph noise (RTN), bias temperature instability (BTI), and hot carrier degradation (HCD). This work studies the impact of a charged oxide trap on the current of an n-type nanowire tri-gate silicon-on-insulator (SOI) MOSFET. The position of the charged trap is varied between the gate dielectric and the buried oxide to determine its effect on the current variation. To perform this analysis, a quantum-corrected Monte Carlo (MC) device simulator calibrated with experimental data is used. The results show that a charged trap reduces the number of electrons in the channel, degrading the transistor on-current. The maximum current variation is observed when the charged trap is placed in the buried oxide and closer to the middle of the channel. For this case, the variation is 10.16% with respect to the nominal current. It is observed that current variation is dominated by electron-number fluctuations rather than mobility fluctuations.

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Amorphous selenium lacks the structural long-range order present in crystalline solids. However, the stark similarity in the short-range order that exists across its allotropic forms, augmented with a shift to non-activated extended-state transport at high electric fields beyond the onset of impact ionization, allowed us to perform this theoretical study, which describes the high-field extended-state hole transport processes in amorphous selenium by modeling the band-transport lattice theory of its crystalline counterpart trigonal selenium. An in-house bulk Monte Carlo algorithm is employed to solve the semiclassical Boltzmann transport equation, providing microscopic insight to carrier trajectories and relaxation dynamics of these non-equilibrium “hot” holes in extended states. The extended-state hole–phonon interaction and the lack of long-range order in the amorphous phase is modeled as individual scattering processes, namely acoustic, polar and non-polar optical phonons, disorder and dipole scattering, and impact ionization gain, which is modeled using a power law Keldysh fit. We have used a non-parabolic approximation to the density functional theory calculated valence band density of states. To validate our transport model, we calculate and compare our time of flight mobility, impact ionization gain, ensemble energy and velocity, and high field hole energy distributions with experimental findings. We reached the conclusion that hot holes drift around in the direction perpendicular to the applied electric field and are subject to frequent acceleration/deceleration caused by the presence of high phonon, disorder, and impurity scattering. This leads to a certain determinism in the otherwise stochastic impact ionization phenomenon, as usually seen in elemental crystalline solids.

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The high field phenomena of inter-valley transfer and avalanching breakdown have long been exploited in devices based on conventional semiconductors. In this Article, we demonstrate the manifestation of these effects in atomically-thin WS2 field-effect transistors. The negative differential conductance exhibits all of the features familiar from discussions of this phenomenon in bulk semiconductors, including hysteresis in the transistor characteristics and increased noise that is indicative of travelling high-field domains. It is also found to be sensitive to thermal annealing, a result that we attribute to the influence of strain on the energy separation of the different valleys involved in hot-electron transfer. This idea is supported by the results of ensemble Monte Carlo simulations, which highlight the sensitivity of the negative differential conductance to the equilibrium populations of the different valleys. At high drain currents (>10 μA/μm) avalanching breakdown is also observed, and is attributed to trap-assisted inverse Auger scattering. This mechanism is not normally relevant in conventional semiconductors, but is possible in WS2 due to the narrow width of its energy bands. The various results presented here suggest that WS2 exhibits strong potential for use in hot-electron devices, including compact high-frequency sources and photonic detectors.

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The hot phonon bottleneck (HPB) effect has been proposed as one of the main phenomena behind the slow cooling in metal halide perovskites. Even though consensus has been reached regarding its existence, open questions remain concerning the HPB’s specific applicability and potential regarding hot carrier solar cell (HCSC) applications. We present a full investigation using ensemble Monte Carlo simulations of the HPB effect in metal halide perovskites (MHP). After describing the HPB effect in detail, we quantify how the HPB effect can extend carrier cooling times by orders of magnitude. We show how the HPB effect depends on carrier concentration, longitudinal optical (LO) phonon lifetime, and LO phonon frequency and connect these findings to how MHPs should be tuned concretely. Using ensemble Monte Carlo simulations, we can accurately model the interplay between carrier–phonon and carrier–carrier interactions up to high carrier density, yielding precise predictions regarding the HPB effect. This study provides important insights into the governing dynamics behind the HPB effect and shows how cooling times can be extended far beyond the phonon lifetime. Furthermore, it contributes to the discussion on cooling times in MHPs and their suitability for HCSC applications.

Under continuous-wave laser excitation in a lattice-matched In_0.53Ga_0.47As/In_0.8Ga_0.2As_0.44P_0.56 multi-quantum-well (MQW) structure, the carrier temperature extracted from photoluminescence rises faster for 405 nm compared with 980 nm excitation, as the injected carrier density increases. Ensemble Monte Carlo simulation of the carrier dynamics in the MQW system shows that this carrier temperature rise is dominated by nonequilibrium LO phonon effects, with the Pauli exclusion having a significant effect at high carrier densities. Further, we find a significant fraction of carriers reside in the satellite L-valleys for 405 nm excitation due to strong intervalley transfer, leading to a cooler steady-state electron temperature in the central valley compared with the case when intervalley transfer is excluded from the model. Good agreement between experiment and simulation has been shown, and detailed analysis has been presented. This study expands our knowledge of the dynamics of the hot carrier population in semiconductors, which can be applied to further limit energy loss in solar cells.

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Fast hot-carrier cooling process in the solar-absorbers fundamentally limits the photon-energy conversion efficiencies. It is highly desirable to develop the solar absorber with long-lived hot-carriers at sun-illumination level, which can be used to develop the hot-carrier solar cells with enhanced efficiency. Herein, we reveal that zinc-doped (0.34%) halide perovskites have the slower hot-carrier cooling compared with the pristine sample through the transient absorption spectroscopy measurements and theoretical calculations. The hot-carrier energy loss rate at the low photoexcitation level of 10 17 cm -3 is found to be ~3 times smaller than that of un-doped perovskites for 500-K hot carriers, and up to ten times when the hot-carrier temperature approaching the lattice temperature. The incorporation of zinc-dopant into perovskites can reduce the nonadiabatic couplings between conduction bands, which retards the photogenerated hot-carriers relaxation process. Our findings present a practical strategy to slow down the hot-carrier cooling in perovskites at low carrier densities, which are valuable for the further development of practical perovskite hot-carrier photovoltaics .

Hybrid organic-inorganic perovskites are promising for optoelectronic applications, yet the impact of intrinsic defects on hot carrier dynamics remains poorly understood. Here, we investigate hot carrier dynamics in methylammonium lead...

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The rapid relaxation of hot carriers leads to energy loss in the form of heat and consequently restricts the theoretical efficiency of single‐junction solar cells; However, this issue has not received much attention in tin‐lead perovskites solar cells. Herein, tin(II) oxalate (SnC2O4) is introduced into tin‐lead perovskite precursor solution to regulate hot‐carrier cooling dynamics. The addition of SnC2O4 increases the length of carrier diffusion, extends the lifetime of carriers, and simultaneously slows down the cooling rate of carriers. Furthermore, SnC2O4 can bond with uncoordinated Sn2+ and Pb2+ ions to regulate the crystallization of perovskite and enable large grains. The strongly reducing properties of the C2O42− can inhibit the oxidation of Sn2+ to Sn4+ and minimize the formation of Sn vacancies in the resulting perovskite films. Additionally, as a substitute for tin(II) fluoride, the introduction of SnC2O4 avoids the carrier transport issues caused by the aggregation of F– ions at the interface. As a result, the SnC2O4‐treated Sn‐Pb cells show a champion efficiency of 23.36%, as well as 27.56% for the all‐perovskite tandem solar cells. Moreover, the SnC2O4‐treated devices show excellent long‐term stability. This finding is expected to pave the way toward stable and highly efficient all‐perovskite tandem solar cells.

The next-generation hot-carrier solar cells, which can overcome the Shockley–Queisser limit by harvesting excess energy from hot carriers, are receiving increasing attention. Lead halide perovskite (LHP) materials are considered as promising candidates due to their exceptional photovoltaic properties, good stability and low cost. The cooling rate of hot carriers is a key parameter influencing the performance of hot-carrier solar cells. In this work, we successfully detected hot carrier dynamics in operando LHP devices using the two-pulse photovoltage correlation technique. To enhance the signal-to-noise ratio, we applied the delay-time modulation method instead of the traditional power modulation. This advancement allowed us to detect the intraband hot carrier cooling time for the organic LHP CH3NH3PbBr3, which is as short as 0.21 ps. In comparison, the inorganic Cs-based LHP CsPbBr3 exhibited a longer cooling time of around 0.59 ps due to different phonon contributions. These results provide us new insights into the optimal design of hot-carrier solar cells and highlight the potential of LHP materials in advancing solar cell technology.

Hot carrier (HC) cooling represents a dominant nonradiative loss pathway that ultimately constrains the efficiency of perovskite solar cells (PSCs). Despite the ubiquity of intrinsic vacancy defects in halide perovskites, their mechanistic influence on HC relaxation dynamics has remained elusive and is often overlooked, largely because the ultrafast time scales, intricate defect-phonon interactions, and subtle band-edge perturbations make these effects difficult to isolate and quantify. Here, first-principles calculations coupled with nonadiabatic molecular dynamics (NAMD) are employed to systematically assess how intrinsic vacancy defects affect HC cooling behavior in FAPbI3. We demonstrate that while vacancy defects significantly modulate the bandgap of FAPbI3, the carrier relaxation rate does not scale directly with the gap magnitude. Notably, iodine and formamidinium vacancies selectively hinder the cooling of hot electrons and hot holes, respectively. This defect-mediated suppression stems from two synergistic mechanisms: weakened electron-phonon (e-ph) coupling and a transition of carrier relaxation pathways from fast, direct relaxation to slower, stepwise processes. These effects synergistically prolong the HC lifetimes and mitigate energy dissipation during the cooling process. Our findings establish a defect-type-specific framework for tuning HC dynamics and highlight defect engineering as a powerful strategy to enhance hot carrier utilization in next-generation high-efficiency photovoltaic devices.

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Quantum-confined GaN/InGaN-based heterostructures are a natural choice for light-emitting devices due to their enhanced luminescence and superior efficiency. The reduced density of states and improved quantum confinement lead to improved radiative efficiency. However, under high excitation, quantum-confined structures exhibit band tail filling at elevated energies, giving rise to several effects, including altered carrier capture dynamics, extended radiative lifetimes, hot-carrier accumulation, and phonon bottlenecks. These effects may reduce the efficacy of the radiative process in the active region. Here, we show that the persistence of hot carriers—characterized by their elevated effective temperature and cooling dynamics—serves as a good metric for evaluating the overall efficacy of quantum-confined structures. To demonstrate this, we employ GaN/InGaN nanodisks as the host material and present a methodology for extracting hot-carrier temperature and cooling behavior using power-dependent photoluminescence and time-correlated single-photon counting measurements. While the radiative efficiency is measured around the peak emission wavelength, the short-wavelength tail reveals clear signatures of high-energy carrier occupation in both ground and excited states.

Carrier multiplication (CM) is an effective mechanism that makes it possible to use hot-carriers (HC) tobypassthe Shockley-Queisser limit for solar-cell efficiency. In this paper, we present a detailed study of both CM and HC cooling dynamics in quantum-confined CsPbI3 perovskite nanocrystals (NCs), using femtosecond transient absorption-spectroscopy. Surprisingly, our results show that barrierless CM, with an efficiency exceeding 90%, can be achieved in strongly-confinedNCs for a time scale << 200 fs.No CM, however,is observed in weakly-confinedNCs. HC cooling dynamics suggests the absence of an intrinsic phonon bottleneck in strongly-confined NCs. Furthermore, the bi-exciton Auger rate increased 4-fold in strongly-confined NCs compared to weakly-confined NCs.These results suggest thatthe efficient CM likely originates from enhanced Coulomb-coupling and relaxed momentum conservation.

Precise control of the hot carrier (HC) relaxation dynamics in halide perovskites is crucial for optimizing the performance of optoelectronic applications requiring efficient energy harvesting and charge transport. Herein, we investigate the effects of Bi, Ag, and Ag/Bi co-doping on HC relaxation in CsPbBr3 nanocrystals (NCs) using femtosecond transient absorption spectroscopy (TAS) and assess their impact on self-powered photodetectors (PDs). Temperature-dependent photoluminescence (PL) and HC energy loss rate analysis reveal an almost twofold reduction in the coupling strength and a threefold enhancement in the longitudinal optical (LO) phonon lifetimes after doping/co-doping. The extended LO phonon lifetime and suppressed coupling strength after Ag and Ag/Bi co-doping enhance the hot phonon bottleneck effect and prolong the HC relaxation process. These findings and improved self-powered PD performance provide a practical strategy for tailoring the HC dynamics in perovskite NCs, paving the way for designing high-efficiency photodetectors and solar cells.

Herein, we investigated the carrier-phonon relaxation process in a two-dimensional (2D) BA2PbBr4 perovskite and its heterostructure with MoS2. Energy transfer was observed in the van der Waals heterostructure of 2D perovskite and monolayer MoS2, leading to enhancement in the photoluminescence intensity of MoS2. Femtosecond pump-probe spectroscopy was used to study the carrier and lattice dynamics of pristine 2D materials and their heterostructure. A generalized two-temperature model was introduced to include competing effects of electron cooling in the rate equation of electron and lattice relaxation dynamics. The hot phonon bottleneck effect is more enhanced in the BA2PbBr4/MoS2 heterostructure than in pristine BA2PbBr4, resulting in a longer electron relaxation time. By developing a heterostructure platform with 2D BA2PbBr4 and MoS2 hybrid materials, this work provides a unique opportunity to understand and tailor carrier dynamics, interfacial coupling, and long-lived hot electrons, ultimately enhancing the efficiency of optoelectronic devices.

: Hot carrier solar cells (HCSC) have attracted extensive attention due to the efficient utilization of high-energy photons. Two-dimensional (2D) perovskites is one of the materials suitable as HCSC due to the hot phonon bottleneck effect as well as the quantum well structure. Providing methods to regulate the hot carrier cooling rate of 2D perovskites is crucial for further technological development. In this study, we systematically investigate the role of organic molecules in regulating hot carrier relaxation in 2D n = 1 perovskites through time-resolved spectroscopic measurements. The results of transient absorption and time-resolved photoluminescence reveal that hot carrier relaxation in 2D perovskites takes place on sub-picosecond time scales and can be effectively modulated by component engineering of organic molecules. These insightful results contribute to deep understanding of the hot carrier relaxation process of 2D perovskites and provide valuable information for the future development of higher performance perovskite solar cells.

Rapid hot-carrier/exciton cooling constitutes a major loss channel for photovoltaic efficiency. How to decelerate the hot-carrier/exciton relaxation remains a crux for achieving high-performance photovoltaic devices. Here, we demonstrate slow hot-exciton cooling that can be extended to hundreds of picoseconds in colloidal HgTe quantum dots (QDs). The energy loss rate is 1 order of magnitude smaller than bulk inorganic semiconductors, mediated by phonon bottleneck and interband biexciton Auger recombination (BAR) effects, which are both augmented at reduced QD sizes. The two effects are competitive with the emergence of multiple exciton generation. Intriguingly, BAR dominates even under low excitation fluences with a decrease in interparticle distance. Both experimental evidence and numerical evidence reveal that such efficient BAR derives from the tunneling-mediated interparticle excitonic coupling induced by wave function overlap between neighboring HgTe QDs in films. Thus, our study unveils the potential for realizing efficient hot-carrier/exciton solar cells based on HgTe QDs. Fundamentally, we reveal that the delocalized nature of quantum-confined wave function intensifies BAR. The interparticle excitonic coupling may cast light on the development of next-generation photoelectronic materials, which can retain the size-tunable confinement of colloidal semiconductor QDs while simultaneously maintaining high mobilities and conductivities typical for bulk semiconductor materials.

As typical representatives of group III chalcogenides, InSe, α-In2Se3, and β'-In2Se3 have drawn considerable interest in the domain of photoelectrochemistry. However, the microscopic mechanisms of carrier dynamics in these systems remain largely unexplored. In this work, we first reveal that hot electrons in the three systems have different cooling rate stages and long-lived hot electrons, through the utilization of density functional theory calculations and nonadiabatic molecular dynamics simulations. Furthermore, the ferroelectric polarization of α-In2Se3 weakens the nonadiabatic coupling of the nonradioactive recombination, successfully competing with the narrow bandgap and slow dephasing process, and achieving both high optical absorption efficiency and long carrier lifetime. In addition, we demonstrate that the ferroelectric polarization of α-In2Se3 not only enables the formation of the double type-II band alignment in the InSe/α-In2Se3/InSe heterostructure, with the top and bottom InSe sublayers acting as acceptors and donors, respectively, but also eliminates the hindrance of the built-in electric field at the interface, facilitating an ultrafast interlayer carrier transfer in the heterojunction. This work establishes an atomic mechanism of carrier dynamics in InSe, α-In2Se3, and β'-In2Se3 and the regulatory role of the ferroelectric polarization on the charge carrier dynamics, providing a guideline for the design of photoelectronic materials.

Perovskite solar cells (PSCs) have been propelled into the limelight over the past decade due to the rapid‐growing power conversion efficiency (PCE). However, the internal defects and the interfacial energy level mismatch are detrimental to the device performance and stability. In this study, it is demonstrated that a small amount of indium (In3+) ions in mixed cation and halide perovskites can effectively passivate the defects, improve the energy‐level alignment, and reduce the exciton binding energy. Additionally, it is confirmed that In3+ ions can significantly elevate the initial carrier temperature, slow down the hot‐carrier cooling rate, and reduce the heat loss before carrier extraction. The device with 1.5% of incorporated In3+ achieves a PCE of 22.4% with a negligible hysteresis, which is significantly higher than that of undoped PSCs (20.3%). In addition, the unencapsulated PSCs achieve long‐term stability, which retain 85% of the original PCE after 3,000 h of aging in dry air. The obtained results demonstrate and promote the development of practical, highly efficient, and stable hot‐carrier‐enhanced PSCs.

In photon-conversion processes, rapid cooling of photo-induced hot carriers is a dominant energy loss channel. We herein report a 3-fold reduced hot carrier cooling rate in CsPbBr 3 nanocrystals capped with a cross-linked polysiloxane shell in comparison to single alkyl-chain oleylamine ligands. Relaxation of hot charge carriers depends on the carrier-phonon coupling (CPC) process as an important channel to dissipate energies in nanostructured perovskite materials. The CPC strengths in the two samples were measured through cryogenic photoluminescence spectroscopic measurements. The effect of organic ligands on the CPC in CsPbBr 3 nanocrystals is elucidated based on a damped oscillation model. This supplements the conventional polaron-based CPC model, by involving a damping effect on the CPC from the resistance of the ligands against nanocrystal lattice vibrations. The model also accounts for the observed linear temperature-dependence of the CPC strength. Our work enables predictions about the effect of the ligands on the performance of perovskite nanocrystals in future applications.

Rapid hot carrier cooling is the key loss channel overriding all the possible energy loss pathways that limit the achievable solar conversion efficiency. Thus, delayed hot carrier cooling in the cell absorber layer can make hot carrier extraction a less cumbersome task, assisting in the realization of hot carrier solar cells. There have been plentitude of reports concerning the slow carrier cooling in perovskite materials, which has eventually triggered interest in the radical understanding of the native photophysics driving the device design. Here in this finding, a further dramatic dip in the cooling rate has been discerned upon growing Cs4PbBr6 shell over CsPbBr3 core NCs in contrast to the bare CsPbBr3 core NCs. Using Transient Absorption spectroscopy, we have investigated the disparity in the hot carrier thermalization pathways in the CsPbBr3 and CsPbBr3@Cs4PbBr6 core-shell NCs under same laser fluence, which can be validated as a corollary of polaron formation in the later NCs.

Optimization of the optoelectronic performance of lead halide perovskite (LHP) nanocrystals calls for understanding and manipulation of their hot carrier relaxation processes. In this work, the hot carrier relaxation in a nanocube (NC) and a nanoplate (NPL) of CsPbBr3 is studied using non-adiabatic molecular dynamics based on first-principles calculations. Strong electron-hole asymmetry in the relaxation processes is observed. Regardless of the nanocrystal shape, the hot hole cooling rate is much faster than that of hot electrons. Moreover, while the hot-hole relaxation is insensitive to the excitation energy, faster relaxation of hot electrons is observed with a lower excitation energy. The origin of the asymmetry is associated with the orbital characters and density of states at the band edges. The hot-hole relaxation is strongly affected by the shape of the nanocrystal. It is faster in the NPL than in the NC. This is attributed to the larger atomic displacements in the NPL due to its higher surface/volume ratio. These results provide theoretical insights for fundamental understanding of excited-state dynamics in LHPs and may help the development of hot-carrier optoelectronic devices.

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Carrier cooling is of widespread interest in the field of semiconductor science. It is linked to carrier-carrier and carrier-phonon coupling, and has profound implications for the photovoltaic performance of materials. Recent transient optical studies have shown that a high carrier density in lead-halide perovskites (LHPs) can reduce the cooling rate through a "phonon bottleneck". However, the role of carrier-carrier interactions, and the material properties that control cooling in LHPs, are still disputed. To address these factors, we utilize ultrafast "pump-push-probe" spectroscopy on LHP nanocrystal (NC) films. We find that the addition of cold carriers to LHP NCs increases the cooling rate, competing with the phonon bottleneck. By comparing different NCs and bulk samples, we deduce that the cooling behavior is intrinsic to the LHP composition, and independent of the NC size or surface. This can be contrasted with other colloidal nanomaterials, where confinement and trapping considerably influence the cooling dynamics.

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Understanding cooling of hot charge carriers in semiconductor quantum dots (QDs) is of fundamental interest and useful to enhance the performance of QDs in photovoltaics. We study electron and hole cooling dynamics in PbSe QDs up to high energies where carrier multiplication occurs. We characterize distinct cooling steps of hot electrons and holes and build up a broadband cooling spectrum for both charge carriers. Cooling of electrons is slower than of holes. At energies near the band gap we find cooling times between successive electronic energy levels in the order of 0.5 ps. We argue that here the large spacing between successive electronic energy levels requires cooling to occur by energy transfer to vibrational modes of ligand molecules or phonon modes associated with the QD surface. At high excess energy the energy loss rate of electrons is 1–5 eV/ps and exceeds 8 eV/ps for holes. Here charge carrier cooling can be understood in terms of emission of LO phonons with a higher density-of-states in the valence band than the conduction band. The complete mapping of the broadband cooling spectrum for both charge carriers in PbSe QDs is a big step toward understanding and controlling the cooling of hot charge carriers in colloidal QDs.

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Semiconductor nanocrystals (NCs) with size-tuned energy gaps present unique and desirable properties for optoelectronic applications. Recent synthetic advancements offer routes to spheroidal CsPbBr3 perovskite NCs in the strong quantum confinement regime with narrow size dispersion. Using tunable femtosecond laser pulses, we examine intraband carrier relaxation using transient absorption spectroscopy and show that, across the transition from weak to strong confinement, hot carrier lifetime increases compared to larger bulk-like particles. However, further increases of confinement subsequently lead to a reduction of the hot carrier lifetime and increase of the non-radiative Auger recombination rate. Finally, we show that hot carrier lifetimes increase as a function of excess energy above the band gap less sensitively under high confinement in comparison to the bulk. Understanding such unique trends is important for maximizing hot carrier lifetimes for use in next-generation hot carrier devices as well as evaluating the transition from weak to strong confinement.

Prolonging hot carrier cooling, a crucial factor in optoelectronic applications, including hot carrier photovoltaics, presents a significant challenge. High-energy band-nesting excitons within parallel bands offer a promising and underexplored avenue for addressing this issue. Here, we exploit an exceptional D exciton cooling prolongation of 2 to 3 orders of magnitude compared to sub-picosecond in typical transition metal dichalcogenides (TMDs) owing to the complex Coulomb environment and the sequential and mismatch-valley relaxation. Simultaneously, the intervalley scattering upconversion of band-edge excitons with the slow D exciton formation in the metastable Γ valley/hill also reduces the cooling rate. We successfully extract D and C excitons as hot carriers through integrating with various thicknesses of TiOx, achieving the highest efficiency of 98% and 85% at a Ti thickness of 2 nm. Our findings highlight the potential of band-nesting excitons for extending hot carrier cooling time, paving the way for advancements in hot carrier-based optoelectronic devices.

While SnF2 is reported as an effective additive for improving the efficiency of lead-free tin-based perovskite solar cells, the mechanism is still unclear and requires further studies. Upon incorporating SnF2 into MASnI3, SnF2 reduces the intrinsic carrier density from 1018 to 1012 cm–3 and produces a longer carrier diffusion length as confirmed by the Hall measurements. The femtosecond transient absorption spectroscopy shows that SnF2 doping enhances the hot-phonon bottleneck effect of MASnI3. The slow cooling process of hot carriers may help to reduce non-radiative recombination, increase the fluorescence lifetime, and, therefore, improve the utilization rate of carriers. Finally, lead-free low bandgap perovskite MASnI3 is utilized as a light absorbing layer in solar cells, achieving high optical current and high voltage in tin-based perovskite solar cells. The final power conversion efficiency is 10.2%, while the power conversion efficiency for the control unit is 6.69%.

This Letter reports the facile harvesting of hot carriers (HCs) in a composite of 12-faceted dodecahedron CsPbBr3 nanocrystal (NC) and a scavenger molecule. We recorded ∼3.3 × 1011 s-1 HC cooling rate in NC when excited with ∼1.4 times the band gap energy (Eg), increasing to >3 × 1012 s-1 in the presence of scavengers at high concentration due to the HC extractions. Since the observed intrinsic charge transfer rate (∼1.7 × 1012 s-1) in our NC-scavenger complex is about an order of magnitude higher than the HC cooling rate (∼3.3 × 1011 s-1), carriers are harvested before their cooling. Further, a fluorescence correlation spectroscopy study reveals NC tends to form a quasi-stable complex with a scavenger molecule, ensuring charge transfer completed (τct ≈ 0.6 ps) much before the complex breaks apart (>600 μs). The overall results of our study highlight the promise shown by 12-faceted NCs and their implications in modern applications, including hot carrier solar cells.

No abstract available

The hot-phonon bottleneck effect in lead-halide perovskites (APbX3) prolongs the cooling period of hot charge carriers, an effect that could be used in the next-generation photovoltaics devices. Using ultrafast optical characterization and first-principle calculations, four kinds of lead-halide perovskites (A=FA+/MA+/Cs+, X=I−/Br−) are compared in this study to reveal the carrier-phonon dynamics within. Here we show a stronger phonon bottleneck effect in hybrid perovskites than in their inorganic counterparts. Compared with the caesium-based system, a 10 times slower carrier-phonon relaxation rate is observed in FAPbI3. The up-conversion of low-energy phonons is proposed to be responsible for the bottleneck effect. The presence of organic cations introduces overlapping phonon branches and facilitates the up-transition of low-energy modes. The blocking of phonon propagation associated with an ultralow thermal conductivity of the material also increases the overall up-conversion efficiency. This result also suggests a new and general method for achieving long-lived hot carriers in materials. Slow cooling of hot charge carriers in lead halide perovskite could be used in photovoltaics devices. Here, Yanget al. study hot carrier dynamics by transient absorption spectroscopy. They relate the phonon bottleneck to the up-conversion of low-energy phonons, facilitated by the presence of organic cations.

As a promising high mobility p-type wide bandgap semiconductor, copper iodide has received increasing attention in recent years. However, the defect physics/evolution are still controversial, and particularly the ultrafast carrier and exciton dynamics in copper iodide has rarely been investigated. Here, we study these fundamental properties for copper iodide thin films by a synergistic approach employing a combination of analytical techniques. Steady-state photoluminescence spectra reveal that the emission at ~420 nm arises from the recombination of electrons with neutral copper vacancies. The photogenerated carrier density dependent ultrafast physical processes are elucidated with using the femtosecond transient absorption spectroscopy. Both the effects of hot-phonon bottleneck and the Auger heating significantly slow down the cooling rate of hot-carriers in the case of high excitation density. The effect of defects on the carrier recombination and the two-photon induced ultrafast carrier dynamics are also investigated. These findings are crucial to the optoelectronic applications of copper iodide. Deep understanding of defect physics, excitonic properties and the ultrafast carrier dynamics in the high mobility p-type transparent CuI is vital for its optoelectronic applications. Here, Liu et al. employ a synergistic approach to unveil these fundamental properties.

Carrier multiplication (CM) is the process in which multiple electron–hole pairs are created upon absorption of a single photon in a semiconductor. CM by an initially hot charge carrier occurs in competition with cooling by phonon emission, with the respective rates determining the CM efficiency. Up until now, CM rates have only been calculated theoretically. We show for the first time how to extract a distinct CM rate constant from experimental data of the relaxation time of hot charge carriers and the yield of CM. We illustrate this method for PbSe quantum dots. Additionally, we provide a simplified method using an estimated energy loss rate to estimate the CM rate constant just above the onset of CM, when detailed experimental data of the relaxation time is missing.

Manipulation of intrinsic carrier relaxation is crucial for designing efficient lead halide perovskite nanocrystal (NC) based optoelectronic devices. The influence of heterovalent Bi3+ doping on the ultrafast carrier dynamics and hot carrier (HC) cooling relaxation of CsPbBr3 NCs has been studied using femtosecond transient absorption spectroscopy and first-principles calculations. The initial HC temperature and LO phonon decay time point to a faster HC relaxation rate in the Bi3+-doped CsPbBr3 NCs. The first-principles calculations disclose the acceleration of carrier relaxation in Bi3+-doped CsPbBr3 NCs due to the appearance of localized bands (antitrap states) within the conduction band. The higher Born effective charges (Z*) and higher soft energetic optical phonon density of states cause higher electron-phonon scattering rates in the Bi-doped CsPbBr3 system, which is responsible for the faster HC cooling rate in doped systems.