Thermalization Of Hot Carriers Research Articles

Polaron-Mediated Slow Carrier Cooling in a Type-1 3D/0D CsPbBr3@Cs4PbBr6 Core-Shell Perovskite System.

Rapid hot carrier cooling is the key loss channel overriding all possible energy loss pathways that limit 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 eventually triggered interest in 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 a growing Cs4PbBr6 shell over CsPbBr3 core nanocrystals (NCs), in contrast to the bare CsPbBr3 core NCs. Using transient absorption spectroscopy, we investigated the disparity in the hot carrier thermalization pathways in the CsPbBr3 and CsPbBr3@Cs4PbBr6 core-shell NCs under the same laser fluence, which can be validated as a corollary of polaron formation in the later NCs.

Strong Hot Carrier Effects in Single Nanowire Heterostructures.

We use transient Rayleigh scattering to study the thermalization of hot photoexcited carriers in single GaAs0.7Sb0.3/InP nanowire heterostructures. By comparing the energy loss rate in single core-only GaAs0.7Sb0.3 nanowires which do not show substantial hot carrier effects with the core-shell nanowires, we show that the presence of an InP shell substantially suppresses the longitudinal optical phonon emission rate at low temperatures which then leads to strong hot carrier effects.

Asymmetric hot-carrier thermalization and broadband photoresponse in graphene-2D semiconductor lateral heterojunctions.

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.

Temperature and excitation dependence of stimulated emission and spontaneous emission in InGaN epilayer**Project supported by the National Key Research Program of China (Grant No. 2016YFB0501604) and the National Natural Science Foundation of China (Grant Nos. 10874127 and 61227902).

Temperature and excitation dependent photoluminescence (PL) of InGaN epilayer grown on c-plane GaN/sapphire template by molecular beam epitaxy (MBE) has been systematically investigated. The emission spectra of the sample consisted of strong multiple peaks associated with one stimulated emission (SE) located at 430 nm and two spontaneous emissions (SPE) centered at about 450 nm and 480 nm, indicating the co-existence of shallow and deep localized states. The peak energy of SE exhibiting weak s-shaped variation with increasing temperature revealed the localization effect of excitons. Moreover, an abnormal increase of the SPE intensity with increasing temperature was also observed, which indicated that the carrier transfer between the shallow and deeper localized states exists. Temperature dependent time-resolved PL (TRPL) demonstrated the carrier transfer processes among the localized states. In addition, a slow thermalization of hot carriers was observed in InGaN film by using TRPL and transient differential reflectivity, which is attributed to the phonon bottleneck effect induced by indium aggregation.

Quantifying Wavelength-Dependent Plasmonic Hot Carrier Energy Distributions at Metal/Semiconductor Interfaces.

Hot carriers generated from the nonradiative decay of localized surface plasmons are capable of driving charge-transfer reactions at the surfaces of metal nanostructures. Photocatalytic devices utilizing plasmonic hot carriers are often based on metal nanoparticle/semiconductor heterostructures owing to their efficient electron-hole separation ability. The rapid thermalization of hot carriers generated at the metal nanoparticles yields a distribution of carrier energies that determines the capability of the photocatalytic device to drive redox reactions. Here, we quantify the thermalized hot carrier energy distribution generated at Au/TiO2 nanostructures using wavelength-dependent scanning electrochemical microscopy and a series of molecular probes with different redox potentials. We determine the quantum efficiencies and oxidizing power of the hot carriers from wavelength-dependent reaction rates and photocurrent across the metal/semiconductor interface. The wavelength-dependent reaction efficiency tracks the surface plasmon resonance spectrum of the Au nanoparticles, showing that the reaction is facilitated by plasmon excitation, while the responses from molecules with different redox potentials shed light on the energy distribution of the hot holes generated at metal nanoparticle/semiconductor heterostructures. The results provide important insight into the energies of the plasmon-generated hot carriers and quantum efficiencies of plasmonic photocatalytic devices.

Efficient carrier multiplication in CsPbI3 perovskite nanocrystals

The all-inorganic perovskite nanocrystals are currently in the research spotlight owing to their physical stability and superior optical properties—these features make them interesting for optoelectronic and photovoltaic applications. Here, we report on the observation of highly efficient carrier multiplication in colloidal CsPbI3 nanocrystals prepared by a hot-injection method. The carrier multiplication process counteracts thermalization of hot carriers and as such provides the potential to increase the conversion efficiency of solar cells. We demonstrate that carrier multiplication commences at the threshold excitation energy near the energy conservation limit of twice the band gap, and has step-like characteristics with an extremely high quantum yield of up to 98%. Using ultrahigh temporal resolution, we show that carrier multiplication induces a longer build-up of the free carrier concentration, thus providing important insights into the physical mechanism responsible for this phenomenon. The evidence is obtained using three independent experimental approaches, and is conclusive.

Hot phonon and carrier relaxation in Si(100) determined by transient extreme ultraviolet spectroscopy.

The thermalization of hot carriers and phonons gives direct insight into the scattering processes that mediate electrical and thermal transport. Obtaining the scattering rates for both hot carriers and phonons currently requires multiple measurements with incommensurate timescales. Here, transient extreme-ultraviolet (XUV) spectroscopy on the silicon 2p core level at 100 eV is used to measure hot carrier and phonon thermalization in Si(100) from tens of femtoseconds to 200 ps, following photoexcitation of the indirect transition to the Δ valley at 800 nm. The ground state XUV spectrum is first theoretically predicted using a combination of a single plasmon pole model and the Bethe-Salpeter equation with density functional theory. The excited state spectrum is predicted by incorporating the electronic effects of photo-induced state-filling, broadening, and band-gap renormalization into the ground state XUV spectrum. A time-dependent lattice deformation and expansion is also required to describe the excited state spectrum. The kinetics of these structural components match the kinetics of phonons excited from the electron-phonon and phonon-phonon scattering processes following photoexcitation. Separating the contributions of electronic and structural effects on the transient XUV spectra allows the carrier population, the population of phonons involved in inter- and intra-valley electron-phonon scattering, and the population of phonons involved in phonon-phonon scattering to be quantified as a function of delay time.

Enhanced hot electron lifetimes in quantum wells with inhibited phonon coupling

Hot electrons established by the absorption of high-energy photons typically thermalize on a picosecond time scale in a semiconductor, dissipating energy via various phonon-mediated relaxation pathways. Here it is shown that a strong hot carrier distribution can be produced using a type-II quantum well structure. In such systems it is shown that the dominant hot carrier thermalization process is limited by the radiative recombination lifetime of electrons with reduced wavefunction overlap with holes. It is proposed that the subsequent reabsorption of acoustic and optical phonons is facilitated by a mismatch in phonon dispersions at the InAs-AlAsSb interface and serves to further stabilize hot electrons in this system. This lengthens the time scale for thermalization to nanoseconds and results in a hot electron distribution with a temperature of 490 K for a quantum well structure under steady-state illumination at room temperature.

Ultrafast transient absorption measurements of photocarrier dynamics in monolayer and bulk ReSe2.

Recently, transition metal dichalcogenides have been extensively studied as new functional materials for electronic and optoelectronic applications. Most initial efforts have been focused on several members of this material family with 2H lattice structure. ReSe2 as a less-study transition metal dichalcogenide has gained significant momentum due to its 1T lattice structure and in-plane anisotropic electronic and optical properties. Extensive efforts have shown its promising future as a novel material for optoelectronics. However, little was known about the photocarrier dynamics in this material. Here we report the first transient absorption measurement of photocarrier dynamics in ReSe2 bulk and monolayer sample in reflection geometry. We observed ultrafast thermalization and relaxation of hot carriers in bulk ReSe2, and obtained a photocarrier lifetime on the order of 80 ps and decreases slightly with increasing the carrier density. Pronounced anisotropic response of differential reflection was observed. We also studied monolayer ReSe2 samples obtained by chemical vapor deposition and deduced a photocarrier lifetime on the order of 10 ps. These results provide fundamental information for using this material in various optoelectronic devices.

Ultrafast Spectral Dynamics of CsPb(BrxCl1–x)3 Mixed-Halide Nanocrystals

In this work we investigated the spectral dynamics of cesium lead mixed-halide, CsPb(BrxCl1–x)3 perovskite nanocrystals probed with complementary spectral techniques: time-resolved photoluminescence and transient absorption spectroscopy. Mixed-halide perovskite nanocrystals were synthesized via a hot-injection method followed by anion exchange reactions. Our results show that increased Cl content in perovskite nanocrystals (a) diminished the photoluminescence quantum yield and gave rise to rapid radiative recombination of carriers; (b) resulted in rapid thermalization of hot carriers and low carrier temperatures, which suggests weaker hot-phonon bottleneck and Burstein–Moss effects; (c) decreased the bandgap renormalization energy, which suggests high exciton binding energy and poor charge extraction in Cl substituted perovskite nanocrystals; and (d) increased the number of carriers undergoing Auger losses, where Auger processes dominate over trap-assisted recombination. These findings provide a generaliz...

Hot electron-hole plasma dynamics and amplified spontaneous emission in ZnTe nanowires.

Key to optimizing and tailoring the optoelectronic properties of semiconductor nanostructures for practical applications is a clear understanding of their carrier interactions and recombination dynamics. Herein, the electron-hole (e-h) plasma dynamics and the electron-phonon coupling interactions in zincblende ZnTe nanowires (NWs) were systematically investigated by time-resolved photoluminescence (TRPL) spectroscopy over a wide range of lattice temperatures (4-300 K) and pump densities. Following intense, non-resonant femtosecond (fs) laser pulse excitation, the excited carriers thermalize to quasi-equilibrium distribution through carrier-carrier and carrier-phonon scattering within a few picoseconds. The peak temperature of the hot electron gas (Te0) is much higher than the lattice temperature and increases sub-linearly with the pump fluence. The hot electron gas thermalizes in two characteristic carrier density-dependent regimes - i.e., within 35 ps under high carrier densities (e-h plasma) while persisting to 360 ps under low carrier densities (exciton). Temperature-dependent studies of the ZnTe NWs revealed that the acoustic phonons play a significant role in the cooling of the hot e-h plasma in these NWs and the emission band broadening arises from the interplay of the contributions from crystal imperfections, LA and LO phonon scattering and most importantly, from the hot carrier thermalization. For demonstration, e-h plasma-amplified spontaneous emission in ZnTe NWs at room temperature by one- and two-photon excitation was realized. The results provide new insights into carrier interactions and recombination dynamics of ZnTe NWs and highlight their potential for high-efficiency e-h plasma light emitters, sensors and in plasma photochemotherapy.

Ultrafast carrier dynamics in bimetallic nanostructure-enhanced methylammonium lead bromide perovskites.

In this work, we examine the impact of hybrid bimetallic Au/Ag core/shell nanostructures on the carrier dynamics of methylammonium lead tribromide (MAPbBr3) mesoporous perovskite solar cells (PSCs). Plasmon-enhanced PSCs incorporated with Au/Ag nanostructures demonstrated improved light harvesting and increased power conversion efficiency by 26% relative to reference devices. Two complementary spectral techniques, transient absorption spectroscopy (TAS) and time-resolved photoluminescence (trPL), were employed to gain a mechanistic understanding of plasmonic enhancement processes. TAS revealed a decrease in the photobleach formation time, which suggests that the nanostructures improve hot carrier thermalization to an equilibrium distribution, relieving hot phonon bottleneck in MAPbBr3 perovskites. TAS also showed a decrease in carrier decay lifetimes, indicating that nanostructures enhance photoinduced carrier generation and promote efficient electron injection into TiO2 prior to bulk recombination. Furthermore, nanostructure-incorporated perovskite films demonstrated quenching in steady-state PL and decreases in trPL carrier lifetimes, providing further evidence of improved carrier injection in plasmon-enhanced mesoporous PSCs.

Quantification of hot carrier thermalization in PbS colloidal quantum dots by power and temperature dependent photoluminescence spectroscopy

PbS QDs are studied as attractive candidates to be applied as hot carrier solar cell absorbers.

Effect of Halide Treatments on PbSe Quantum Dot Thin Films: Stability, Hot Carrier Lifetime, and Application to Photovoltaics

Air stability and efficient multiple exciton generation (MEG) are crucial for the application of PbSe quantum dot (QD) thin film in next generation photovoltaics. Recently it was reported that PbS QD thin films with solid-state halide atomic ligands exhibited superior performance in terms of stability and efficiency. There is great interest in applying these halide ligands to PbSe QD thin films to improve the stability, while their additional effects on the hot carrier dynamics and hence MEG efficiency remain unknown. Here, we demonstrate that proper halide treatments can modify both the stability and hot carrier thermalization of a PbSe QD thin film. This confirms that using proper halide ligands in the solid-state ligand exchange step for film fabrication can significantly improve the stability. The film subjected to an iodide treatment exhibited the best air stability, and additionally its hot carrier thermalization time can be three times longer than that with a chloride treatment. We suggest that str...

Tracking Primary Thermalization Events in Graphene with Photoemission at Extreme Time Scales.

Direct and inverse Auger scattering are amongst the primary processes that mediate the thermalization of hot carriers in semiconductors. These two processes involve the annihilation or generation of an electron-hole pair by exchanging energy with a third carrier, which is either accelerated or decelerated. Inverse Auger scattering is generally suppressed, as the decelerated carriers must have excess energies higher than the band gap itself. In graphene, which is gapless, inverse Auger scattering is, instead, predicted to be dominant at the earliest time delays. Here, <8 fs extreme-ultraviolet pulses are used to detect this imbalance, tracking both the number of excited electrons and their kinetic energy with time-and angle-resolved photoemission spectroscopy. Over a time window of approximately 25fs after absorption of the pump pulse, we observe an increase in conduction band carrier density and a simultaneous decrease of the average carrier kinetic energy, revealing that relaxation is in fact dominated by inverse Auger scattering. Measurements of carrier scattering at extreme time scales by photoemission will serve as a guide to ultrafast control of electronic properties in solids for petahertz electronics.

Discrete Electronic Bands in Semiconductors and Insulators: Potential High-Light-Yield Scintillators

Bulk semiconductors and insulators typically have continuous valence and conduction bands. In this paper, we show that valence and conduction bands of a multinary semiconductor or insulator can be split to narrow discrete bands separated by large energy gaps. This unique electronic structure is demonstrated by first-principles calculations in several quaternary elpasolite compounds, i.e., Cs2NaInBr6, Cs2NaBiCl6, and Tl2NaBiCl6. The narrow discrete band structure in these quaternary elpasolites is due to the large electronegativity difference among cations and the large nearest-neighbor distances in cation sublattices. We further use Cs2NaInBr6 as an example to show that the narrow bands can stabilize self-trapped and dopant-bound excitons (in which both the electron and the hole are strongly localized in static positions on adjacent sites) and promote strong exciton emission at room temperature. The discrete band structure should further suppress thermalization of hot carriers and may lead to enhanced impact ionization, which is usually considered inefficient in bulk semiconductors and insulators. Finally, these characteristics can enable efficient room-temperature light emission in low-gap scintillators and may overcome the light-yield bottleneck in current scintillator research.

Carrier Thermalization Dynamics in Single Zincblende and Wurtzite InP Nanowires

Using transient Rayleigh scattering (TRS) measurements, we obtain photoexcited carrier thermalization dynamics for both zincblende (ZB) and wurtzite (WZ) InP single nanowires (NW) with picosecond resolution. A phenomenological fitting model based on direct band-to-band transition theory is developed to extract the electron-hole-plasma density and temperature as a function of time from TRS measurements of single nanowires, which have complex valence band structures. We find that the thermalization dynamics of hot carriers depends strongly on material (GaAs NW vs InP NW) and less strongly on crystal structure (ZB vs WZ). The thermalization dynamics of ZB and WZ InP NWs are similar. But a comparison of the thermalization dynamics in ZB and WZ InP NWs with ZB GaAs NWs reveals more than an order of magnitude slower relaxation for the InP NWs. We interpret these results as reflecting their distinctive phonon band structures that lead to different hot phonon effects. Knowledge of hot carrier thermalization dynamics is an essential component for effective incorporation of nanowire materials into electronic devices.

The impact of hot charge carrier mobility on photocurrent losses in polymer-based solar cells.

A typical signature of charge extraction in disordered organic systems is dispersive transport, which implies a distribution of charge carrier mobilities that negatively impact on device performance. Dispersive transport has been commonly understood to originate from a time-dependent mobility of hot charge carriers that reduces as excess energy is lost during relaxation in the density of states. In contrast, we show via photon energy, electric field and film thickness independence of carrier mobilities that the dispersive photocurrent in organic solar cells originates not from the loss of excess energy during hot carrier thermalization, but rather from the loss of carrier density to trap states during transport. Our results emphasize that further efforts should be directed to minimizing the density of trap states, rather than controlling energetic relaxation of hot carriers within the density of states.

Ab initio study of hot carriers in the first picosecond after sunlight absorption in silicon.

Hot carrier thermalization is a major source of efficiency loss in solar cells. Because of the subpicosecond time scale and complex physics involved, a microscopic characterization of hot carriers is challenging even for the simplest materials. We develop and apply an abinitio approach based on density functional theory and many-body perturbation theory to investigate hot carriers in semiconductors. Our calculations include electron-electron and electron-phonon interactions, and require no experimental input other than the structure of the material. We apply our approach to study the relaxation time and mean free path of hot carriers in Si, and map the band and k dependence of these quantities. We demonstrate that a hot carrier distribution characteristic of Si under solar illumination thermalizes within 350fs, in excellent agreement with pump-probe experiments. Our work sheds light on the subpicosecond time scale after sunlight absorption in Si, and constitutes a first step towards abinitio quantification of hot carrier dynamics in materials.

Enhanced carrier multiplication in engineered quasi-type-II quantum dots.

One process limiting the performance of solar cells is rapid cooling (thermalization) of hot carriers generated by higher-energy solar photons. In principle, the thermalization losses can be reduced by converting the kinetic energy of energetic carriers into additional electron-hole pairs via carrier multiplication (CM). While being inefficient in bulk semiconductors this process is enhanced in quantum dots, although not sufficiently high to considerably boost the power output of practical devices. Here we demonstrate that thick-shell PbSe/CdSe nanostructures can show almost a fourfold increase in the CM yield over conventional PbSe quantum dots, accompanied by a considerable reduction of the CM threshold. These structures enhance a valence-band CM channel due to effective capture of energetic holes into long-lived shell-localized states. The attainment of the regime of slowed cooling responsible for CM enhancement is indicated by the development of shell-related emission in the visible observed simultaneously with infrared emission from the core.