Hot Carrier Extraction Research Articles

(Invited) Energy Level Alignment at Monolayer MoS2/Electrolyte Interfaces

The fundamental problem that limits the solar energy conversion efficiency of semiconductors such as CdTe and Si is that all excess solar photon energy above the band gap is lost as heat. Avoiding thermalization energy losses is of paramount significance for solar energy conversion because hot-carrier-based systems theoretically achieve 66% efficiency, which breaks the detailed balance limit of 33%.Of all the candidate materials, 2D semiconductors such as monolayer (ML) MoS2 have unique physical and photophysical properties that could make hot-carrier energy conversion possible. The knowledge gap in the field is that the electronic states of 2D materials move with carrier density, due to either light absorption or an applied electrochemical potential. The energy level movements are significant because the real fundamental driving force for charge transfer (ΔG 0´) is unclear for a given reaction and applied potential. In principle, quantifying ΔG 0´ under working conditions opens up the possibility to tune the hot carrier extraction rate relative to the cooling rate. Our research team has employed photocurrent spectroscopy, steady-state absorption spectroscopy, and in situ femtosecond transient absorption spectroscopy as a function of applied potential to characterize underlying steps in a ML MoS2 photoelectrochemical cell. The rich data set informs us on the timescales for hot-carrier generation/cooling and exciton formation/recombination, as well as the magnitudes of changes in exciton energy levels, exciton binding energies, and the electronic band gap. These findings open the possibility of tuning the hot-carrier extraction rate relative to the cooling rate to ultimately utilize hot-carriers for solar energy conversion applications.

Hot carrier extraction from plasmonic-photonic superimposed heterostructures.

Plasmonic nanostructures have been exploited in photochemical and photocatalytic processes owing to their surface plasmon resonance characteristics. This unique property generates photoinduced potentials and currents capable of driving chemical reactions. However, these processes are hampered by low photon conversion and utilization efficiencies, which are issues that need to be addressed. In this study, we integrate plasmonic photochemistry and simple tunable heterostructure characteristics of a dielectric photonic crystal for the effective control of electromagnetic energy below the diffraction limit of light. The nanostructure comprises high-density Ag nanoparticles on nanocavity arrays of SrTiO3 and TiO2, where two oxides constitute a chemical heterojunction. Such a nanostructure is designed to form intense electric fields and a vectorial electron flow channel of Ag → SrTiO3 → TiO2. When the plasmonic absorption of Ag nanoparticles matched the photonic stopband, we observed an apparent quantum yield of 3.1 × 10-4 e- per absorbed photon. The contributions of light confinement and charge separation to the enhanced photocurrent were evaluated.

Construction of Plasmonic Metal@Semiconductor Core–Shell Photocatalysts: From Epitaxial to Nonepitaxial Strategies

In recent decade, plasmonic metal@semiconductor core–shell nanostructures have sparked tremendous research interest toward development of advanced photocatalysts. This is because such materials facilitate to harness the localized surface plasmon resonant absorption of plasmonic metal nanoparticles tunable across the entire ultraviolet–visible–near infrared (UV–vis–NIR) region, for improving the photocatalytic efficiency of surrounding semiconductors via multiple energy and charge carrier flow channels. Herein, the recent synthetic progresses achieved in wet chemical preparation of plasmonic metal@semiconductor core–shell photocatalysts are outlined. Tentatively, the reported synthetic approaches are classified into three major groups including the epitaxial seeded growth, the kinetically overdriven nonepitaxial seeded growth, and the cation exchange‐facilitated (CEf) nonepitaxial growth dictated by chemical thermodynamics. Special focuses are laid on elucidating the fundamental correlation of the synthetic mechanism with the crystallographic and physical properties of the resultant nanostructures that strongly dominate the synergistic coupling efficiency between plasmonic metal and semiconductor. Of these, the promise of CEf nonepitaxy in curbing the formation of interfacial defect states and promoting extraction of plasmonic hot carriers from the large lattice‐mismatched hybrid photocatalysts is detailedly elaborated. A comprehensive understanding in this scope is of significance for rationally architecting the plasmonic metal@semiconductor core–shell nanostructures toward disclosing fascinating horizons in photocatalytic solar‐to‐fuel conversion.

Optical-Beam-Induced Current in InAs/InP Nanowires for Hot-Carrier Photovoltaics.

Using the excess energy of charge carriers excited above the band edge (hot carriers) could pave the way for optoelectronic devices, such as photovoltaics exceeding the Shockley–Queisser limit or ultrafast photodetectors. Semiconducting nanowires show promise as a platform for hot-carrier extraction. Proof of principle photovoltaic devices have already been realized based on InAs nanowires, using epitaxially defined InP segments as energy filters that selectively transmit hot electrons. However, it is not yet fully understood how charge-carrier separation, relaxation, and recombination depend on device design and on the location of optical excitation. Here, we introduce the use of an optical-beam-induced current (OBIC) characterization method, employing a laser beam focused close to the diffraction limit and a high precision piezo stage, to study the optoelectric performance of the nanowire device as a function of the position of excitation. The photocurrent response agrees well with modeling based on hot-electron extraction across the InP segment via diffusion. We demonstrate that the device is capable of producing power and estimate the spatial region within which significant hot-electron extraction can take place to be on the order of 300 nm away from the barrier. When comparing to other experiments on similar nanowires, we find good qualitative agreement, confirming the interpretation of the device function, while the extracted diffusion length of hot electrons varies. Careful control of the excitation and device parameters will be important to reach the potentially high device performance theoretically available in these systems.

Ultrastrong Coupling Leads to Slowed Cooling of Hot Excitons in Few-Layer Transition-Metal Dichalcogenides

The excited-state dynamics of few-layer molybdenum disulfide (FL-MoS2) are studied in conditions of strong light–matter coupling to plasmon polaritons. Hot carrier extraction in these systems has been proposed due to the observation of slow cooling of the high-energy C exciton relative to the bandgap A exciton. Here, we show that in conditions of ultrastrong coupling to plasmon polaritons, the lifetimes of the two slowest C exciton decay processes are extended by factors of 1.5 and 5.8 in FL-MoS2. We hypothesize that strong coupling delocalization suppresses multiple decay mechanisms throughout the visible spectrum in MoS2 such as defect scattering, intervalley scattering of band-nested excitations to the Κ-valley band edges, and exciton–exciton annihilation. We also find that decay from the upper to the lower hybrid mode is not ultrafast as seen in organic systems but in fact introduces an additional delay of ∼20 ps. Our observations show that strong coupling can be used to extend the lifetimes of hot excitons in FL-MoS2 and potentially in other two-dimensional transition-metal dichalcogenides, with potential for above-bandgap photophysics and photochemistry applications such as hot carrier extraction, which could lead to more efficient solar energy conversion.

Hot-carrier photocatalysts for artificial photosynthesis.

We applied hot-carrier extraction to particulate photocatalysts for artificial photosynthetic reactions including water splitting for H2 production and CO2 reduction to CO and HCOOH, and elucidated promising features of hot-carrier photocatalysts (HC-PCs). We designed a specific structure of the HC-PC; a semiconductor core in which thermalization of photo-generated carriers is significantly suppressed is surrounded by a shell whose bandgap is wider than that of the core. Among the photo-generated hot carriers in the core, only carriers whose energies are larger than the shell bandgap are extracted passing through the shell to the active sites on the shell surface. Thus, the shell functions as an energy-selective contact. We calculated the upper bounds of the rates of the carrier supply from the core to the active sites using a newly constructed detailed-balance model including partial thermalization and nonradiative recombination of the carriers. It has been revealed that the HC-PCs can yield higher carrier-supply rates and thus potentially higher solar-to-chemical energy conversion efficiencies for H2 and CO production than those of conventional photocatalysts with the assistance of intraband transition and Auger recombination/impact ionization. It should be noted, however, that one of the necessary conditions for efficient hot-carrier extraction is sufficiently large carrier density in the core, which, in turn, requires concentrated solar illumination by several hundreds. This would raise rate-limiting problems of activities of the chemical reactions induced by the photo-generated carriers and material-transfer properties.

Recent Progress and Future Opportunities for Hot Carrier Photodetectors: From Ultraviolet to Infrared Bands

AbstractThe hot carriers generated from the nonradiative decay of surface plasmons in metallic nanostructures can inject into the conduction band of a semiconductor, allowing for the sub‐bandgap photodetection under room temperature. By the controllable interfacial barrier height between the plasmonic and semiconductor/insulator materials, the hot carrier photodetectors working from ultraviolet to infrared bands are extensively demonstrated with significant progress. In this review, hot carrier dynamics are briefly discussed from generation, transport, and emission perspectives. The state‐of‐the‐art progress of hot carrier photodetectors with various configurations, material constitutions, and plasmonic nanostructures are surveyed. To further promote hot carrier extraction efficiency toward the practical applications, the thermodynamic loss analysis, and the potential strategies from the optical, electrical, and material perspectives are addressed. The performances of the developed hot carrier photodetectors are also summarized, particularly addressing the novel functionalities, challenges, and future opportunities.

Subpicosecond Hot Hole Transfer in a Graphene Quantum Dot Composite with High Efficiency.

Extraction of hot carriers is of prime importance because of its potential to overcome the energy loss that limits the efficiency of an optoelectronic device. Employing a femtosecond upconversion setup, herein we report a few picoseconds carrier cooling time of colloidal graphene quantum dots (GQDs) is at least an order of magnitude slower compared to that in its bulk form. A slower carrier cooling time of GQDs compared to that of the other semiconductor quantum dots and their bulk materials is indeed a coveted property of GQDs that would allow one easy harvesting of high energy species employing a suitable molecular system as shown in this study. A subpicosecond hot hole transfer time scale has been achieved in a GQD-molecular system composite with high transfer efficiency. Our finding suggests a dramatic enhancement of the efficiency of GQD based optoelectronic devices can possibly be a reality.

Hot carrier exploitation strategies and model for efficient solar cell applications

Hot carriers are electrons or holes that are created in semiconductors upon the absorption of photons with energies greater than the fundamental bandgap. The excess energy of the hot carrier cools to the lattice temperature via carrier–phonon scattering and wasted as heat in [the] picoseconds timescale. The hot-carrier cooling represents a severe loss in the solar cells that have significantly limits their power conversion efficiencies. Hot carrier solar cells aim to mitigate this optical limitation by effective utilization of carriers at elevated energies. However, exploitation of hot carrier energy is extremely challenging as hot carriers rapidly lose their excess energy in phonon emission and therefore requires a substantial delay of carrier cooling in absorber material. In this paper a simple model was formulated to study the kinetic energies and hence the energy levels of the photo excited carriers in the quantum dots (QDs) whereas Schaller model was used to investigate the threshold energies of considered QDs. Results strongly indicate low threshold photon energies within the energy conservation limit for PbSe, PbTe, PbS, InAs, and InAs QDs. These materials seem to be good candidates for efficient carrier multiplication. It is found also that PbSe, PbTe, PbS, InAs, ZnS and InAs QDs exhibit promising potential for possible hot carrier absorber due to their widely spaced energy levels predicted to offer a large phononic gap between the optical and acoustic branches in the phonon dispersion. This in principle enhances phonon bottleneck effect that dramatically slows down hot carrier cooling leading to retention of hot carriers long enough to enable their exploitation. Two novel strategies were employed for the conversion of hot carriers into usable energies. The first approach involves the extraction of the energetic hot carriers while they are ‘hot’ to create higher photo voltage while the second approach uses the hot carrier to produce more carriers through impact ionization to create higher photo current. These mechanisms theoretically give rise to high overall conversion efficiencies of hot carrier energy well above Shockley and Queisser limit of conventional solar cells.

Dynamics of hot carriers in plasmonic heterostructures

Abstract Understanding and optimising the mechanisms of generation and extraction of hot carriers in plasmonic heterostructures is important for applications in new types of photodetectors, photochemistry and photocatalysis, as well as nonlinear optics. Here, we show using transient dynamic measurements that the relaxation of the excited hot-carriers in Au/Pt hetero-nanostructures is accelerated through the transfer pathway from Au, where they are generated, to Pt nanoparticles, which act as a hot-electron sink. The influence of the environment on the dynamics was also demonstrated. The time-resolved photoluminescence measurements confirm the modified hot-electron dynamics, revealing quenching of the photoluminescence signal from Au nanoparticles in the presence of Pt and an increased photoluminescence lifetime. These observations are signatures of the improved extraction efficiency of hot-carriers by the Au/Pt heterostructures. The results give insight into the time-dependent behaviour of excited compound nanoscale systems and provide a way of controlling the relaxation mechanisms involved, with important consequences for engineering nonlinear optical response and hot-carrier-assisted photochemistry.

Cascade Electron Transfer Induces Slow Hot Carrier Relaxation in CsPbBr3 Asymmetric Quantum Wells

We report an engineering approach not only to delay hot carrier equilibrium but also to slow the cooling rate of CsPbBr3-based multiple quantum wells (MQWs), as evident from femtosecond transient absorption measurements and density functional theory calculations. Three energetically cascaded CsPbBr3 perovskite layers (stacked with thicknesses of 3, 7, and 20 nm for asymmetric MQWs and 20, 20, and 20 nm for symmetric MQWs) are separated by a 5 nm organic barrier of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline. Time-resolved data demonstrate that the sequential hot-electron transfer between CsPbBr3 layers mediates the delayed hot carrier equilibrium in the asymmetric MQWs. Interestingly, the delayed hot carrier equilibrium is followed by a much slower relaxation in asymmetric MQWs (40 ps) than symmetric ones (3.2 ps), which could be attributed to the decoupling of a hot electron–hole originating from hot electron transfer. Our findings provide a promising approach for efficient hot carrier extraction in solar cells that exceed the Shockley–Queisser limit.

Tribovoltaic Device Based on the W/WO3 Schottky Junction Operating through Hot Carrier Extraction

Tribovoltaic Device Based on the W/WO₃ Schottky Junction Operating through Hot Carrier Extraction

(Invited) Advanced Concepts for Ultra-High Conversion Efficiency of Solar Photons into Photovoltaics and Solar Fuels Based on Semiconductor Nanostructures and Molecular Singlet Fissionajn

In order to utilize solar power for the production of solar electricity and solar fuels on a global scale, it is necessary to develop solar photon conversion systems that have an appropriate combination of very high efficiency (delivered watts/m2) and low capital cost ($/m2). One potential promising approach to attain high conversion efficiencies well above the Shockley-Queisser limit of 33% is to utilize the unique properties of semiconductor nanostructures (viz., quantum dots/rods/films) and singlet fission chromophores to control the relaxation dynamics of photogenerated carriers and photochemical excited states, respectively. The former process creates enhanced photocurrent through efficient photogenerated electron-hole pair multiplication (electron-hole pairs exist as excitons in size-quantized nanostructures), or enhanced photopotential through hot carrier transport, transfer, and extraction processes. The latter process of singlet fission generates two triplets from an excited singlet state. These fundamental dynamics in various bulk and nanoscale semiconductors and in molecular chromophores have been studied using transient absorption, photoluminescence, photocurrent and THz spectroscopy with fs to ns time resolution. The predictions that the generation of more than one electron-hole pair or exciton per absorbed photon would be an efficient process in nanostructures, and via singlet fission, have been confirmed in different classes of materials, molecules, and their architectures in photon conversion devices. Selected aspects of this work will be summarized and recent advances will be discussed, including the very remarkable and beneficial theoretical effects of combining MEG or singlet fission with solar concentration and controlling the thicknesses of the photoactive layers in a two-photon photoelectrosynthesis cell structure for solar fuels.

Hot-carrier optoelectronic devices based on semiconductor nanowires

In optoelectronic devices such as solar cells and photodetectors, a portion of electron-hole pairs is generated as so-called hot carriers with an excess kinetic energy that is typically lost as heat. The long-standing aim to harvest this excess energy to enhance device performance has proven to be very challenging, largely due to the extremely short-lived nature of hot carriers. Efforts thus focus on increasing the hot carrier relaxation time and on tailoring heterostructures that allow for hot-carrier extraction on short time and length scales. Recently, semiconductor nanowires have emerged as a promising system to achieve these aims, because they offer unique opportunities for heterostructure engineering as well as for potentially modified phononic properties that can lead to increased relaxation times. In this review we assess the current state of theory and experiments relating to hot-carrier dynamics in nanowires, with a focus on hot-carrier photovoltaics. To provide a foundation, we begin with a brief overview of the fundamental processes involved in hot-carrier relaxation and how these can be tailored and characterized in nanowires. We then analyze the advantages offered by nanowires as a system for hot-carrier devices and review the status of proof-of-principle experiments related to hot-carrier photovoltaics. To help interpret existing experiments on photocurrent extraction in nanowires we provide modeling based on non-equilibrium Green's functions. Finally, we identify open research questions that need to be answered in order to fully evaluate the potential nanowires offer toward achieving more efficient, hot-carrier based, optoelectronic devices.

Efficiency enhancement in a single bandgap silicon solar cell considering hot-carrier extraction using selective energy contacts.

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.

Rational design of bimetallic photocatalysts based on plasmonically-derived hot carriers.

Hot carriers generated by plasmonic excitations have recently opened up new avenues in photocatalysis. The transfer of these energetic carriers to adjacent molecules can promote chemical transformations that are important for hydrogen generation by water splitting, CO2 reduction and degradation of organic pollutants. Here, we have developed and optimised a plasmonic hot-carrier catalytic system based on silica nanoparticles decorated with plasmonic gold nanoparticles as a source of hot carriers, equipped with platinum nanoclusters as co-catalyst for the enhancement of hot-carrier extraction. The latter plays a triple role by providing: a surface favourable for molecular adsorption; hot-electron generation near the nanoclusters due to field enhancement effects and electron momentum relaxation facilitating the electron transfer across the metal surface, exactly where molecules are adsorbed. The combination of plasmonic and catalytic metals in nano-heterostructured devices provides a new platform for photocatalytic processes and is of significant interest for future solar-based clean technologies.

Promoting Hot Carrier Extraction in Zn-Ag-In-Te Nanocrystals By Irradiation of High-Energy Light

Colloidal semiconductor nanocrystals (NCs), so-called quantum dots, offer attractive advantages such as low cost, large-scale solution processing, and tunable physicochemical properties depending on their size. Due to these advantages, it is expected to be used for highly efficient and low-cost energy conversion systems such as solar cells and photocatalysts. Since the relaxation rates of carriers in highly excited states are remarkably reduced by the discrete energy levels of semiconductor NCs, unique photoresponses of semiconductor NCs are induced by the absorption of photons with energy higher than the energy gap (Eg) of NCs. It has been reported that by extending the relaxation time of hot carriers, it is possible to transfer excess kinetic energy before releasing it as heat. Thus, next-generation solar cells of outstandingly high efficiency beyond the Shockley-Queisser limit can be developed if photogenerated carriers are harvested from a semiconductor NC without energy loss of incident photons. I-III-VI2 semiconductor-based NCs, such as CuInS2 or AgInS2 and their solid solution with ZnS, have been intensively studied as a visible-light-driven photocatalyst because of their excellent optical properties. Recently, we prepared ZnTe-AgInTe2 solid solution ((AgIn)xZn2(1-x)Te2: ZAITe) NCs with a rod shape, the Eg of which was tunable in near-IR region from 1.20 to 1.60 eV with an increase in Zn content.1 These NCs are expected to act as near-IR photosensitizers and enable the use of NC-generated hot carriers. However, their photoelectrochemical properties and hot carrier behavior have not been investigated to date. In this study, we clarified the behavior of hot carrier in ZAITe NCs by measurering the photoelectrochemical properties of ZAITe NC-immobilized photoelectrodes.2 Furthermore, we prepared ZAITe NC-Methylviologen (MV2+) complexes to evaluate the dynamics of hot electron transfer from ZAITe NCs to MV2+ under the excitation of lights with various photon energies3. ZAITe NCs were synthesized by a thermal reaction of corresponding metal acetates and a Te precursor in 1-dodecanethiol at 180°C for 180 min. TEM observations revealed that rod-shaped NCs with width of ca. 4 nm and length of ca. 11 nm were formed regardless of chemical composition of particles, that is, the x value. We revealed the electronic energy structures of ZAITe NCs with the compositions of x= 1.0, 0.75, and 0.5, the Eg values of which were 1.20, 1.27, and 1.46 eV, respectively. The band-edge potentials of conduction band (ECB) and valence band (EVB) were negatively and positively shifted with an increase in the Eg of NCs, the degree being more remarkable for the change in ECB. The photoelectrochemical behavior of ZAITe NCs immobilized on ITO electrode with thickness of less than one monoparticle, resembling a p-type semiconductor photoresponse, was remarkably different from that of thicker ZAITe NC multilayer films: The onset of cathodic photocurrent was shifted more positively than that of EVB of ZAITe NCs and the photocurrent generation efficiency was significantly enlarged, when photons of energy higher than ca. 2.5 eV were irradiated. The dependence of photocurrent-potential curves on the energy of irradiaited photons for thin ZAITe NC films were investivated. The magnitude of the cathodic photocurrent was increased with a negative shift of the applied pototential, regardless of the energy of excitation photons. Irradiation of photons with lower energy than 2.43 eV gave the onset potential at around 0 V vs. Ag/AgCl, being comparable to the EVB of ZAITe NCs with x = 0.75, -0.05 V vs. Ag/AgCl. In contrast, the increases of photon energy to 2.95 and 3.40 eV gave more positive onset potentials of 0.1 and 0.3 V vs. Ag/AgCl, respectively. we conclude that excitation with photons of > ca. 2.5 eV enabled direct injection of hot holes from ZAITe NCs into ITO electrodes, resulting in enhancement of the photocurrent generetion efficiency of ZAITe NCs. By using transient absorption spectroscopy, the migration of hot carriers in NCs can be directly observed, enabling better clarification of the photochemical properties of ZAITe NCs. Therefore, we also investivated the hot electron transfer from ZAITe NCs to MV2+ by masureing transient absorption spectra of the complexes. The details of this result will be discussed in the presentation. Reference Kameyama, et al., J. Mater. Chem. C, 2018, 6, 2034.Kameyama, et al., ChemNanoMat, 2019, 5, 1028.Kameyama, et al., ACS Adv., 2020, 10, 16361.

Interfacial Modulation with Aluminum Oxide for Efficient Plasmon‐Induced Water Oxidation

AbstractPlasmon‐induced photocatalysts hold great promise for solar energy conversion owing to their strong light‐harvesting ability and tunable optical properties. However, the complex process of interfacial extraction of hot carriers and the roles of metal/semiconductor interfaces in plasmonic photocatalysts are still not clearly understood. Herein, the manipulation of the interface between a plasmon metal (Au) and a semiconductor (rutile TiO2) by introducing an interfacial metal oxide (Al2O3) is reported. The resulting Au/Al2O3/TiO2 exhibits remarkable enhancement in photocatalytic water oxidation activity compared with Au/TiO2, giving an apparent quantum efficiency exceeding 1.3% at 520 nm for photocatalytic water oxidation. Such an interfacial modulation approach significantly prolongs the lifetime of hot carriers in the Au/TiO2 system, which conclusively improves the utilization of hot carriers for plasmon‐induced water oxidation reaction upon irradiation. This work emphasizes the essential role of the interfacial structure in plasmonic devices and provides an alternative method for designing efficient plasmonic photocatalysts for solar energy conversion.

Slow carrier cooling in methylammonium lead bromide quantum dots: Evidence of hot phonon bottleneck

Photoinduced charge carrier dynamics studies in lead halide perovskites helps in explaining key processes like carrier trapping, diffusion and multiexciton dynamics etc. which are of importance to understand the fate of excitons. In the present study, we investigate the dynamics in methylammonium lead bromide perovskite nanocrystals under low and high pump intensity to study hot carrier cooling mechanisms. The single exciton studies conducted under low pump fluences (<N> = 0.3) revealed slow carrier decay with a significant fraction of >1 ns decay times as expected due to long radiative carrier recombination times. This also indicated that fast carrier trapping does not occur as evidenced by absence of any fast decay components. The hot carrier (HC) cooling mechanisms were verified by studying carrier density dependence of carrier temperature decays as extracted from the higher energy part of exciton bleach. These studies revealed a very fast cooling of ∼150 fs under low carrier density 1.5 × 1017/ cm3. However additional experiments under higher carrier densities of ∼1018-1019/cm3 showed a very slow carrier temperature decay with decay time ∼100 ps for the highest intensity investigated. The analysis of hot carrier lifetimes revealed Auger heating and acoustic phonon up-conversion as reason for hot carrier relaxations at higher intensities. Additionally, we carried out experiments under hole quencher, we found lower initial carrier temperatures and faster decay indicating highly efficient hole transfer to the quencher in a time scale faster than cooling of carrier at higher intensities. These results hint at possibility of molecular design for enabling efficient hot carrier extraction from these Quantum Dots (QDs).

Strong band filling induced significant excited state absorption in MAPbI3 under high pump power

Perovskite has been considered as promising material for solar cell application. Understanding the carrier dynamics in perovskite material plays a vital role for further improvement of the power conversion efficiency of solar cells. In this article, carrier dynamics of MAPbI3 under different pump powers have been investigated by ultrafast pump-probe transient absorption spectroscopy. Complete processes of carrier cooling and recombination under different pump powers have been characterized and compared carefully. It is found that a significant carrier band filling happened during cooling process under high pump power, which results in a strong excited state absorption at the photon energy of 2.353 eV. High carrier density induced strong Auger effect appears not only in the band edge but also the higher conduction band. And subsequently, a large part of carriers keeps hot for more than 5 ns indicating that MAPbI3 is meaningful for hot carrier extraction in solar cells. A critical carrier concentration of 9.32 × 1017 cm−3 has been determined at which Auger effect starts to work. It could provide some guidance for perovskite solar cell application.