Thermalization Of Hot Carriers Research Articles

Evidence of abnormal hot carrier thermalization at van Hove singularity of twisted bilayer graphene

Interlayer twist evokes revolutionary changes to the optical and electronic properties of twisted bilayer graphene (TBG) for electronics, photonics and optoelectronics. Although the ground state responses in TBG have been vastly and clearly studied, the dynamic process of its photoexcited carrier states mainly remains elusive. Here, we unveil the photoexcited hot carrier dynamics in TBG by time-resolved ultrafast photoluminescence (PL) autocorrelation spectroscopy. We demonstrate the unconventional ultrafast PL emission between the van Hove singularities (VHSs) with a ∼4 times prolonged relaxation lifetime. This intriguing photoexcited carrier behavior is ascribed to the abnormal hot carrier thermalization brought by bottleneck effects at VHSs and interlayer charge distribution process. Our study on hot carrier dynamics in TBG offers new insights into the excited states and correlated physics of graphene twistronics systems.

Impact of Magnetism on (e-ph) self-energy and thermalization of hot carriers in 2D half-functionalized GeC-hybrid

The present work delves into the analysis of carrier scattering rates, emphasizing the effective strategy of chemical functionalization for introducing magnetism into two-dimensional, initially non-magnetic materials. Specifically, our investigation focuses on the distinct alterations observed in the electron linewidth within two half-fluorinated configurations, namely GeC-F and F-GeC. Leveraging density functional and many-body perturbation theories, our findings highlight a substantial increase in the linewidth upon inducing ferromagnetism and antiferromagnetism in the GeC hybrid. Additionally, we identify the significant contribution of different phonon modes to the linewidth. The outcomes reveal that the out-of-plane acoustic mode ZA exerts global control over the valence and conduction bands. Moreover, the relaxation time of electrons following sunlight illumination demonstrates that electron thermalization occurs within 1.5fs to 130fs for ferromagnetic GeC-F and within 2.32fs to 42fs for antiferromagnetic F-GeC. This study provides a detailed and accurate description of electron behavior at sub-picosecond timescales, a realm that is experimentally challenging to attain.

Strong Dimensional and Structural Dependencies of Hot Carrier Effects in InGaAs Nanowires: Implications for Photovoltaic Solar Cells.

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.

The Shockley–Queisser Efficiency Limit of Solar Thermophotovoltaic (STPV) Cells Using Different Photovoltaic Cells and a Radiation Shield Considering the Étendue of Solar Radiation

A theoretical model is developed to determine the Shockley–Queisser efficiency limit of solar thermophotovoltaic (STPV) cells with single- or double-junction photovoltaic (PV) cells and a simple radiation shield considering the divergence nature of concentrated solar radiation. A combination of adaptive parametric sweep and graphic-based methods is developed to solve the highly nonlinear correlations of energy and carrier transports in the theoretical model to find the optimized operating conditions of STPVs with high stability. The theoretical model predicts that the Shockley–Queisser efficiency limit of STPV under 1000× solar concentration and a simple radiation shield is ~50.1% with InGaAsSb PV cells, ~49.1% with GaSb PV cells, and ~53.2% with InGaAsSb/GaSb double-junction PV cells. The operating temperatures are ~1719.5 K, ~1794.1 K, and 1640.0 K, respectively. An observation from the modeling is that the energy loss due to the thermalization of hot carriers in the STPV with spectrally selected emitters is ~40% less than that in single-junction solar cells. Also determined from the modeling is that ~20% of the collected solar energy is still lost through thermal radiation, even with a simple radiation shield to block the radiative heat loss to the surroundings. Following this understanding, a further improvement in the Shockley–Queisser efficiency of STPVs can be achieved by adopting advanced designs of radiation shields that can separate the absorber of the STPVs far away from the aperture of the radiation shield without using a large-area absorber.

Multiple k-Point Nonadiabatic Molecular Dynamics for Ultrafast Excitations in Periodic Systems: The Example of Photoexcited Silicon.

With the rapid development of ultrafast experimental techniques for the research of carrier dynamics in solid-state systems, a microscopic understanding of the related phenomena, particularly a first-principle calculation, is highly desirable. Nonadiabatic molecular dynamics (NAMD) offers a real-time direct simulation of the carrier transfer or carrier thermalization. However, when applied to a periodic supercell, there is no cross-k-point transitions during the NAMD simulation. This often leads to a significant underestimation of the transition rate with the single-k-point band structure in a supercell. In this work, based on the surface hopping scheme used for NAMD, we propose a practical method to enable the cross-k transitions for a periodic system. We demonstrate our formalism by showing that the hot electron thermalization process by the multi-k-point NAMD in a small silicon supercell is equivalent to such simulation in a large supercell with a single Γ point. The simulated hot carrier thermalization process of the bulk silicon is compared with the recent ultrafast experiments, which shows excellent agreements. We have also demonstrated our method for the hot carrier coolings in the amorphous silicons and the GaAlAs_{2} solid solutions with the various cation distributions.

Hot Carrier Thermalization and Josephson Inductance Thermometry in a Graphene-Based Microwave Circuit

Due to its exceptional electronic and thermal properties, graphene is a key material for bolometry, calorimetry, and photon detection. However, despite graphene's relatively simple electronic structure, the physical processes responsible for the heat transport from the electrons to the lattice are experimentally still elusive. Here, we measure the thermal response of low-disorder graphene encapsulated in hexagonal boron nitride by integrating it within a multiterminal superconducting microwave resonator. The device geometry allows us to simultaneously apply Joule heat power to the graphene flake while performing calibrated readout of the electron temperature. We probe the thermalization rates of both electrons and holes with high precision and observe a thermalization scaling exponent not consistent with cooling through the graphene bulk and argue that instead it can be attributed to processes at the graphene-aluminum interface. Our technique provides new insights into the thermalization pathways essential for the next-generation graphene thermal detectors.

Auger scattering dynamic of photo-excited hot carriers in nano-graphite film

Charge carrier scattering channels in graphite bridging its valence and conduction band offer an efficient Auger recombination dynamic to promote low energy charge carriers to higher energy states. It is of importance to answer the question whether a large number of charge carriers can be promoted to higher energy states to enhance the quantum efficiency of photodetectors. Here, we present an experimental demonstration of an effective Auger recombination process in the photo-excited nano-graphite film. The time-resolved hot carrier thermalization was analyzed based on the energy dissipation via the Auger scattering channels. We split the Auger recombination occurrence centered at 0.40 eV energy state into scattering and recombination parts, for characterizing the scattering rate in the conduction band and the recombination rate toward the valence band. The scattering time with respect to the energy state was extracted as 8 ps · eV−1, while the recombination time with respect to the energy state was extracted as 24 ps · eV−1. Our study indicates a 300 fs delay between the hot carrier recombination and generation, leading to a 105 ps−1 · cm−3 Auger scattering efficiency. The observed duration for the Auger recombination to generate hot carriers is prolonged for 1 ps, due to the hot carriers energy relaxation bottleneck with optical-phonons in the nano-graphite. The presented analytic expression gives valuable insights into the Auger recombination dynamic to estimate its most efficient energy regime for mid-infrared photodetection.

(Digital Presentation) Energy Resolved Dynamics of Mid-Gap and Surface States in ZnTe Photocathodes

Transient x-ray spectroscopy has become a powerful tool for photoelectrochemistry. X-rays can measure the coupled electronic and structural dynamics that underlie excited state polaron formation, molecular reaction dynamics, and hot carrier thermalization pathways. Element-specific responses can be used to measure electron or hole transport between each component of a photoelectrode (i.e. light absorber, oxide protection layer, catalyst). In this talk, we discuss how improvements in the signal to noise ratio of our transient XUV spectrometer now allow for the measurement of mid-gap and surface state filling dynamics. The energy of electrons and holes as a function of time is measured for a ZnTe photocathode. The thermalization, mid-gap state filling, and recombination are measured in terms of the energies of electrons and holes. An ab-initio approach using the Bethe-Salpeter equation with time-dependent electron and phonon populations is used to predict and confirm the transient XUV measurements. The few nanometer surface sensitivity of grazing angle, transient XUV measurements make it favorable for measuring interfaces in photoelectrochemistry.

Impact of excitation energy on hot carrier properties in InGaAs multi‐quantum well structure

AbstractHot carrier solar cells aim to overcome the theoretical limit of single‐junction photovoltaic devices by suppressing the thermalization of hot carriers and extracting them through energy selective contacts. Designing efficient hot carrier absorbers requires further investigation on hot carrier properties in materials. Although the thermalization of hot carriers is responsible for a large portion of energy loss in solar cells, it is still one of the least understood phenomena in semiconductors. Here, the impact of excitation energy on the properties of photo‐generated hot carriers in an InGaAs multi‐quantum well (MQW) structure at various lattice temperatures and excitation powers is studied. Photoluminescence (PL) emission of the sample is detected by a hyperspectral luminescence imager, which creates spectrally and spatially resolved PL maps. The thermodynamic properties of hot carriers, such as temperature and quasi‐Fermi‐level splitting, are carefully determined via applying full PL spectrum fitting, which solves the Fermi–Dirac integral and considers the band‐filling effect in the nanostructured material. In addition, the impact of thermalized power density and carrier scattering with longitudinal optical phonons on the spectral linewidth broadening under two excitation energies is studied.

Enhanced Hot Carrier Up‐Conversion in Graphene By Quantum Dot Coating

AbstractGraphene with unique linear band structure and enhanced electron–electron interaction exhibits peculiar hot‐carrier generation. The thermalized hot carriers in graphene have potential to achieve high energy which is well above energy of the incident photons. Together with its outstanding optical and electrical properties, the exotic thermalization of hot carriers makes graphene a promising material for applications in optical up‐conversion devices and ultrafast photonics. However, the excitation efficiency of hot carriers is limited by its weak absorption and atomically thin light–matter interaction length. Here, the efficient hot‐carrier amplification is demonstrated in graphene by adjacent quantum dots (QD). With pump–probe experiments, this hot carrier amplification is found to stem from the charge collection of the two‐photon excited hot carriers in QD. Taking advantages of strong absorption of QD, the hot‐carrier excitation efficiency of graphene can be efficiently improved under near‐infrared sub‐bandgap excitation of QD. The transferred hot carriers are with energy higher than those originally excited in graphene, leading to raised hot‐carrier temperature and enhanced hot‐carrier up‐conversion. This research demonstrates QD's immense potential to realize high efficiency hot‐carrier injection in graphene‐based devices, which will promote their ability for hot‐carrier‐driven devices, optical up‐conversion devices and high‐frequency optoelectronics.

(Invited) Prediction of Excited State Dynamics for Element-Specific Transient X-Ray Spectroscopy

Transient x-ray spectroscopy is evolving into a critical tool in photoelectrochemistry. X-rays can measure the coupled electronic and structural dynamics that underlie excited state polaron formation, molecular reaction dynamics, and hot carrier thermalization pathways. Element-specific responses can be used to measure electron or hole transport between each component of a photoelectrode (i.e. light absorber, oxide protection layer, catalyst). In this talk, we discuss progress towards the ab-initio prediction and interpretation of excited state x-ray spectra on a tens of femtoseconds to nanosecond timescale. The static and dynamic x-ray spectra are unique for each element or bonding scheme. It is typically unknown whether an element of choice will provide relevant dynamical information before performing the technically demanding transient x-ray experiment. We implement a Bethe-Salpeter equation approach with time-dependent electron and phonon populations from solving the Boltzmann transport equation. The calculation accurately includes core-hole excitonic effects in a range of test materials without needing to assume a model of the excited state processes (i.e. renormalization, lattice expansion, state-filling). The eventual goal is a distributable goal that can pre-screen transient x-ray experiments and interpret the measured excited state spectra.

Hot carrier relaxation and inhibited thermalization in superlattice heterostructures: The potential for phonon management

One of the main loss mechanisms in photovoltaic solar cells is the thermalization of photogenerated hot carriers via phonon-mediated relaxation. By inhibiting these relaxation mechanisms and reducing thermalization losses, it may be possible to improve the power conversion efficiency of solar cells beyond the single gap limit. Here, type-II InAs/AlAsSb multi-quantum well (MQW) structures are investigated to study the impact of the phononic properties of the AlAsSb barrier material in hot carrier thermalization. Experimental and theoretical results show that by increasing the barrier thickness (increasing the relative contribution of AlAsSb content in the superlattices), the relaxation of hot carriers is reduced as observed in power-dependent photoluminescence and thermalization analysis. This is attributed to an increase in the phononic bandgap of the MQW with increasing AlAsSb composition reducing the efficiency of the dominant Klemens mechanism as the phononic properties shift toward a more AlSb-like behavior.

Identification of surface and volume hot-carrier thermalization mechanisms in ultrathin GaAs layers

Hot-carrier solar cells offer the opportunity to harvest more energy than the limit set by the Shockley–Queisser model by reducing the losses due to the thermalization of photo-generated carriers. Previous reports have shown lower thermalization rates in thinner absorbers, but the origin of this phenomenon is not precisely understood. In this work, we investigate a series of ultrathin GaAs absorber layers sandwiched between AlGaAs barriers and transferred on host substrates with a gold back mirror. We perform power-dependent photoluminescence characterizations at different laser wavelengths from which we determine the carrier temperature in four absorber thicknesses between 20 and 200 nm. We observe a linear relationship between the absorbed power and the carrier temperature increase. By relating the absorbed and thermalized power, we extract a thermalization coefficient for all samples. It shows an affine dependence with the thickness, leading to the identification of distinct volume and surface contributions to thermalization. We confirm that volume thermalization is linked to LO phonon decay. We discuss the origin of the interface-related thermalization, showing that the effect of LO phonon transport is negligible. Overall, this work sheds new light on thermalization processes in ultrathin semiconductor layers and introduces a method to compare the performance of hot-carrier absorbers.

Room-Temperature Hot-Polaron Photovoltaics in the Charge-Ordered State of a Layered Perovskite Oxide Heterojunction

Harvesting of solar energy by hot carriers from optically induced intraband transitions offers new perspectives for photovoltaic energy conversion. Clearly, mechanisms slowing down hot-carrier thermalization constitute a fundamental core of such pathways of third-generation photovoltaics. The intriguing concept of hot polarons stabilized by long-range phonon correlations in charge-ordered strongly correlated three-dimensional metal-oxide perovskite films has emerged and been demonstrated for ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{Mn}\mathrm{O}}_{3}$ at low temperature. In this work, a tailored approach to extending such processes to room temperature is presented. It consists of a specially designed epitaxial growth of two-dimensional Ruddlesden-Popper ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{1.5}{\mathrm{Mn}\mathrm{O}}_{4}$ films on $\mathrm{Nb}$:${\mathrm{Sr}\mathrm{Ti}\mathrm{O}}_{3}$ with a charge-ordering transition at ${T}_{\mathrm{CO}}$ \ensuremath{\sim} 320 K. This opens the route to a different phonon-bottleneck strategy of slowing down carrier relaxation by strong coupling of electrons to cooperative lattice modes.

Investigation of the spatial distribution of hot carriers in quantum-well structures via hyperspectral luminescence imaging

Observation of robust hot carrier effects in quantum-well structures has prompted hopes to increase the efficiency of solar cells beyond the Shockley–Queisser limit (33% for single junction solar cells at AM1.5G). One of the main studies in hot carrier effects is the determination of carrier temperature, which provides information on the thermalization mechanisms of hot carriers in semiconductor materials. Here, we investigate the spatial distribution of photo-generated hot carriers in a InGaAs multi-quantum-well structure via hyperspectral luminescence imaging. We discuss proper methods of extracting the temperature of carriers from a photoluminescence spectrum. Robust hot carrier effects are observed at the center of the laser spot at various lattice temperatures. In addition, it is seen that the local carrier temperature scales linearly with the local laser intensity as long as the illumination exceeds a threshold power; the carrier temperature at regions with local intensities below the threshold drops to the lattice temperature, i.e., experiences no hot carrier effects. Moreover, at large distances from the concentrated light, where the level of illumination is negligible, evidence of carrier radiative recombination is observed, which is attributed to carrier diffusion in the planar structure. The results of this study can be applied to investigate the influence of carrier diffusion and thermoelectric effects on the thermalization of hot carriers.

Ultra-low thermal conductivity and super-slow hot-carrier thermalization induced by a huge phononic gap in multifunctional nanoscale boron pnictides

Abstract Recent synthesis of thin films of boron monophosphide (BP) motivates us to report herewith the lattice thermal conductivity calculated in nanoscale boron pnictides. The lattice thermal conductivity in BX (X = P, As, Sb) monolayers are calculated to be 51.3, 20.7 and 5.4 Wm−1K−1 respectively at 300 K. The ultra-low lattice thermal conductivity in BSb, which is comparable to that in SnSe, is attributed to low elastic and bond stiffness, low Debye temperature, small group velocity and large mode Gruneisen parameter near zone center. Phonon group velocities in BSb are comparable to that in SnSe and Bi2Te3. Up to 50% reduction in lattice thermal conductivity is noticed at 300 K upon shortening the phonon mean free path, which is realizable, e.g., in 1D nanoribbons, or via defects, vacancies, nanoengineering, etc. A comparative study based on different models to compute the lattice thermal conductivity will provide useful insights into the experimentally measured ones. The widest ever phononic gap (436 & 438 cm−1) is found in BAs and BSb monolayers, where the transverse and longitudinal optical phonons meet the criterion for the prevention of Klemens decay and hence, they can be gainfully exploited in hot-carrier solar cells. BX (X = P, As, Sb) monolayers show a direct bandgap and piezoelectricity. Nanoscale boron pnictides are promising materials in futuristic thermoelectrics, bolometers, third-generation solar cells and nanopiezotronics.

(Invited) Polaron Mediated Cooling Dynamics in Perovskite Materials: Effect of Temperature

During last decade huge number of research investigations are going on different kind of halide perovskite materials due to their exciting optoelectronic properties which includes ability to provide very efficient solar devices. Charge carrier cooling process is one of the key process for design and development of any higher efficient optoelectronic devices. Ultrafast hot carrier cooling is the key loss channel which limits achievable solar conversion efficiency. Delaying the carrier cooling high efficient hot-carrier solar cell can be realized. Thus, delayed hot carrier cooling in the cell absorber layer can make hot carrier extraction a relatively easier 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. In our recent investigation, we have demonstrated dramatic dip in the cooling rate in CsPbBr3 NC after growing Cs4PbBr6 shell over it, in contrast to the bare CsPbBr3 core NCs. By employing ultrafast 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. Role of phonon in the cooling process has also been investigated by carrying out the ultrafast transient studies in the temperature range of 5K-300K.

Photoemission signatures of nonequilibrium carrier dynamics from first principles

Time- and angle-resolved photoemission spectroscopy (tr-ARPES) constitutes a powerful tool to inspect the dynamics and thermalization of hot carriers. The identification of the processes that drive the dynamics, however, is challenging even for the simplest systems owing to the coexistence of several relaxation mechanisms. Here, we devise a Green's function formalism for predicting the tr-ARPES spectral function and establish the origin of carrier thermalization entirely from first principles. The predictive power of this approach is demonstrated by an excellent agreement with experiments for graphene over time scales ranging from a few tens of femtoseconds up to several picoseconds. Our work provides compelling evidence of a non-equilibrium dynamics dominated by the establishment of a hot-phonon regime.

Electron-phonon coupling and hot electron thermalization in titanium nitride

We have studied the thermalization of hot carriers in both pristine and defective titanium nitride (TiN) using a two-temperature model. All parameters of this model, including the electron-phonon coupling parameter, were obtained from first-principles density-functional theory calculations. The virtual-crystal approximation was used to describe defective systems. We find that thermalization of hot carriers occurs on much faster timescales than in gold as a consequence of the significantly stronger electron-phonon coupling in TiN. Specifically, the largest thermalization times, on the order of 200 femtoseconds, are found in TiN with nitrogen vacancies for electron temperatures around 4000 K.

Highly efficient hot electron harvesting from graphene before electron-hole thermalization.

Although the unique hot carrier characteristics in graphene suggest a new paradigm for hot carrier-based energy harvesting, the reported efficiencies with conventional photothermoelectric and photothermionic emission pathways are quite low because of inevitable hot carrier thermalization and cooling loss. Here, we proposed and demonstrated the possibility of efficiently extracting hot electrons from graphene after carrier intraband scattering but before electron-hole interband thermalization, a new regime that has never been reached before. Using various layered semiconductors as model electron-accepting components, we generally observe ultrafast injection of energetic hot electrons from graphene over a very broad photon energy range (visible to mid-infrared). The injection quantum yield reaches as high as ~50%, depending on excitation energy but remarkably, not on fluence, in notable contrast with conventional pathways with nonlinear behavior. Hot electron harvesting in this regime prevails over energy and carrier loss and closely resembles the concept of hot carrier solar cell.