Slow cooling and highly efficient extraction of hot carriers in colloidal perovskite nanocrystals

Optoelectronic properties and ultrafast carrier dynamics of copper iodide thin films

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.

Biexciton dynamics in halide perovskite nanocrystals

Lead halide perovskite nanocrystals are attracting considerable interest as next-generation optoelectronic materials. Optical responses of nanocrystals are determined by excitons and exciton complexes such as trions and biexcitons. Understanding of their dynamics is indispensable for the optimal design of optoelectronic devices and the development of new functional properties. Here, we summarize the recent advances on the exciton and biexciton photophysics in lead halide perovskite nanocrystals revealed by femtosecond time-resolved spectroscopy and single-dot spectroscopy. We discuss the impact of the biexciton dynamics on controlling and improving the optical gain.

Control of Hot Carrier Cooling in Lead Halide Perovskites by Point Defects

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., MA_v⁺ and I_v^-) and an interstitial point defect (e.g., I_i^-) 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 I_i^- 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.

Strain propagation in layered two-dimensional halide perovskites

Impulsive light excitation presents a powerful tool for investigating the interdependent structural and electronic responses in layered two-dimensional (2D) halide perovskites. However, detailed understanding of the nonlinear lattice dynamics in these soft hybrid materials remains limited. Here, we explicate the intrinsic strain propagation mechanisms in 2D perovskite single crystals using transient reflection spectroscopy. Ultrafast photoexcitation leads to the generation of strain pulses via thermoelastic (TE) stress and deformation potential (DP) interaction whence their detection proceed via Brillouin scattering. Using a two-temperature model together with strain wave propagation, we discern the TE and DP contributions in strain generation. Hot carrier cooling plays a dominant role in effecting the weak modulation amplitude. Out-of-plane lattice stiffness is reduced by the weak van der Waals bond between organic layers, resulting in a slow strain propagation velocity. Our findings inject fresh insights into the basic strain properties of layered perovskites critical for manipulating their functional properties for new applications.

Efficient Exciton Dislocation and Ultrafast Charge Extraction in CsPbI3 Perovskite Quantum Dots by Using Fullerene Derivative as Semiconductor Ligand

CsPbI₃ quantum dots (QDs) are of great interest in new-generation photovoltaics (PVs) due to their excellent optoelectronic properties. The long and insulative ligands protect their phase stability and enable superior photoluminescence quantum yield, however, limiting charge transportation and extraction in PV devices. In this work, we use a fullerene derivative with the carboxylic anchor group ([SAM]C60) as the semiconductor ligand and build the type II heterojunction system of CsPbI₃ QDs and [SAM]C60 molecules. We find their combination enables obvious exciton dislocation and highly efficient photogenerated charge extraction. After the introduction of [SAM]C₆₀, the exciton-binding energy of CsPbI₃ decreases from 30 meV to 7 meV and the fluorescence emission mechanism also exhibits obvious changes. Transient absorption spectroscopy visualizes a ~5 ps electron extraction rate in this system. The findings gained here may guide the development of perovskite QD devices.

Highly Orientated Perovskite Quantum Dot Solids for Efficient Solar Cells

Perovskite quantum dots (PQDs) have emerged as competitive optoelectronic materials for photovoltaic applications due to their ideal bandgap energy, high defect tolerance, and solution processability. However, the highly dynamic surface and imperfect cubic structure of PQDs generally result in unfavorable charge-carrier transport within the PQD solids and serious nonradiative recombination. Herein, a highly orientated PQD solid is demonstrated using precursor engineering accompanied by a chemical stripping treatment (CST). A combination of systematic experimental studies and theoretical calculations is conducted to fundamentally understand the resurfacing of PQDs using the CST approach. The results reveal that the highly ordered PQDs can result in a high orientation of PQD solids, significantly promoting charge-carrier transport within the PQD solids. Meanwhile, the ideal cubic-structured PQD with an iodine-rich surface dramatically decreases surface trap states, thereby substantially diminishing trap-assisted nonradiative recombination. Consequently, the inorganic PQD solar cell delivers a power conversion efficiency of up to 16.25%. This work provides a feasible avenue to construct highly orientated PQD solids with improved photophysical properties for high-performance optoelectronic devices.

Chemically Engineered Avenues: Opportunities for Attaining Desired Carrier Cooling in Perovskites

Hot carrier extraction-based devices are presently being persuaded as the most revolutionary means of surpassing the theoretical thermodynamic conversion efficiency limit (∼67 % for a model hot carrier solar cell). However, for practical realisation, there stand various hurdles that need to be surmounted, a major among all being the rapid hot carrier cooling rate. Though, the perovskite family has already demonstrated itself to exhibit slower cooling in contrast to the prototypical semiconductors. Decelerating this entire process of cooling further can prove to be a crucial stride in this regard. Quite contrarily, for the optoelectronic applications the situation is entirely conflicting where quick rate of cooling is a chief prerequisite. In the recent times, there have been various key developments that have targeted altering this cooling rate by various chemically engineered strategies. This review highlights such blueprints that can be utilized towards the advantageous alteration of the carrier cooling in accordance with the device requirements.

Modulating the Carrier Relaxation Dynamics in Heterovalently (Bi3+) Doped CsPbBr3 Nanocrystals

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.

Phonon-Mediated Slow Hot Carrier Dynamics in Lead-Free Cs3Bi2I9 Perovskite Single Crystal

In this report, we study the hot carrier cooling mechanism of the Cs3Bi2I9 single crystal by using femtosecond transient reflectance (fs-TR) spectroscopy. We find an unusual slow hot carrier cooling associated with longitudinal optical (LO) and coherent longitudinal acoustic phonons (CLAPs) emission during the deexcitation of the hot carriers. We posit the interplay between the hot-carriers and the LO and CLA phonons in subpicosecond to subnanosecond time scales, respectively, by analyzing the TR kinetics upon perturbation with excess energy. Furthermore, we measured the CLAPs propagation velocity in Cs3Bi2I9, the crystal, ranging from 1820 to 2000 ms-1. The elastic constants and frequency of Brillouin oscillations were estimated as 20.08 GPa and 14.66 GHz, respectively. Our discovery delivers new physical insights into how the hot carriers in Cs3Bi2I9 single crystal are coupled with a crystal lattice that controls the hot carrier dynamics.

Electronic Coupling of Highly Ordered Perovskite Nanocrystals in Supercrystals

Assembled perovskite nanocrystals (NCs), known as supercrystals (SCs), can have many exotic optical and electronic properties different from the individual NCs due to energy transfer and electronic coupling in the dense superstructures. We investigate the optical properties and ultrafast carrier dynamics of highly ordered SCs and the dispersed NCs by absorption, photoluminescence (PL), and femtosecond transient absorption (TA) spectroscopy to determine the influence of the assembly on the excitonic properties. Next to a red shift of absorption and PL peak with respect to the individual NCs, we identify signatures of the collective band-like states in the SCs. A smaller Stokes shift, decreased biexciton binding energy, and increased carrier cooling rates support the formation of delocalized states as a result of the coupling between the individual NC states. These results open perspectives for assembled perovskite NCs for application in optoelectronic devices, with design opportunities exceeding the level of NCs and bulk materials.

Optimizing the quasi-equilibrium state of hot carriers in all-inorganic lead halide perovskite nanocrystals through Mn doping: fundamental dynamics and device perspectives

Hot carrier (HC) cooling accounts for the significant energy loss in lead halide perovskite (LHP) solar cells. Here, we study HC relaxation dynamics in Mn-doped LHP CsPbI3 nanocrystals (NCs), combining transient absorption spectroscopy and density functional theory (DFT) calculations. We demonstrate that Mn2+ doping (1) enlarges the longitudinal optical (LO)-acoustic phonon bandgap, (2) enhances the electron-LO phonon coupling strength, and (3) adds HC relaxation pathways via Mn orbitals within the bands. The spectroscopic study shows that the HC cooling process is decelerated after doping under band-edge excitation due to the dominant phonon bandgap enlargement. When the excitation photon energy is larger than the optical bandgap and the Mn2+ transition gap, the doping accelerates the cooling rate owing to the dominant effect of enhanced carrier-phonon coupling and relaxation pathways. We demonstrate that such a phenomenon is optimal for the application of hot carrier solar cells. The enhanced electron-LO phonon coupling and accelerated cooling of high-temperature hot carriers efficiently establish a high-temperature thermal quasi-equilibrium where the excessive energy of the hot carriers is transferred to heat the cold carriers. On the other hand, the enlarged phononic band-gap prevents further cooling of such a quasi-equilibrium, which facilitates the energy conversion process. Our results manifest a straightforward methodology to optimize the HC dynamics for hot carrier solar cells by element doping.

Exciton Cooling in 2D Perovskite Nanoplatelets: Rationalized Carrier-Induced Stark and Phonon Bottleneck Effects

Using femtosecond transient absorption (fs-TA), we investigate the hot exciton relaxation dynamics in strongly confined lead iodide perovskite nanoplatelets (NPLs). The large quantum and dielectric confinement leads to discrete excitonic transitions and strong Stark features in the TA spectra. This prevents the use of conventional relaxation analysis methods extracting the carrier temperature or measuring the buildup of the band-edge bleaching. Instead, we show that the TA spectral line shape near the band-edge reflects the state of the system, which can be used to probe the exciton cooling dynamics. The ultrafast hot exciton relaxation in one- to three- monolayer-thick NPLs confirms the absence of intrinsic phonon bottleneck. However, excitation fluence-dependent measurements reveal a hot phonon bottleneck effect, which is found to be independent of the nature of the internal cations but strongly affected by the ligands and/or sample surface state. Together, these results suggest a role of the surface ligands in the cooling process.

Improved One- and Multiple-Photon Excited Photoluminescence from Cd2+-Doped CsPbBr3 Perovskite NCs

Metal halide perovskite nanocrystals (NCs) attract much attention for light-emitting applications due to their exceptional optical properties. More recently, perovskite NCs have begun to be considered a promising material for nonlinear optical applications. Numerous strategies have recently been developed to improve the properties of metal halide perovskite NCs. Among them, B-site doping is one of the most promising ways to enhance their brightness and stability. However, there is a lack of study of the influence of B-site doping on the nonlinear optical properties of inorganic perovskite NCs. Here, we demonstrate that Cd²⁺ doping simultaneously improves both the linear (higher photoluminescence quantum yield, larger exciton binding energy, reduced trap states density, and faster radiative recombination) and nonlinear (higher two- and three-photon absorption cross-sections) optical properties of CsPbBr₃ NCs. Cd²⁺ doping results in a two-photon absorption cross-section, reaching 2.6 × 10⁶ Goeppert-Mayer (GM), which is among the highest reported for CsPbBr₃ NCs.

Carrier-Specific Hot Phonon Bottleneck in CH3NH3PbI3 Revealed by Femtosecond XUV Absorption

Femtosecond carrier cooling in the organohalide perovskite semiconductor CH₃NH₃PbI₃ is measured using extreme ultraviolet (XUV) and optical transient absorption spectroscopy. XUV absorption between 44 and 58 eV measures transitions from the I 4d core to the valence and conduction bands and gives distinct signals for hole and electron dynamics. The core-to-valence-band signal directly maps the photoexcited hole distribution and provides a quantitative measurement of the hole temperature. The combination of XUV and optical probes reveals that upon excitation at 400 nm, the initial hole distribution is 3.5 times hotter than the electron distribution. At an initial carrier density of 1.4 × 10²⁰ cm^-3 both carriers are subject to a hot phonon bottleneck, but at 4.2 × 10¹⁹ cm^-3 the holes cool to less than 1000 K within 400 fs. This result places significant constraints on the use of organohalide perovskites in hot-carrier photovoltaics.

Control of the Surface Disorder by Ion-Exchange to Achieve High Open-Circuit Voltage in HC(NH2 )2 PbI3 Perovskite Solar Cell

The ionic nature of organic trihalide perovskite leads to structural irregularity and energy disorder at the perovskite surface, which seriously affects the photovoltaic performance of perovskite solar cells. Here, the origin of the perovskite surface disorder is analyzed, and a facial ion-exchange strategy is designed to regulate the surface chemical environment. By the reconstruction of terminal irregular Pb-I bonds and random cations, the repaired surface is characteristic of the reduced band tail states, consequent to the suppression of the uplift of quasi-Fermi level splitting and photocarrier scattering. The optimized device gets a high open-circuit voltage and operational stability. These findings fully elaborate the underlying mechanism concerning perovskite surface problem, giving guidance on tailoring the energy disorder.

Interface Engineering for Perovskite Solar Cells Based on 2D-Materials: A Physics Point of View

The last decade has witnessed the advance of metal halide perovskites as a promising low-cost and efficient class of light harvesters used in solar cells (SCs). Remarkably, the efficiency of lab-scale perovskite solar cells (PSCs) reached a power conversion efficiency of 25.5% in just ~10 years of research, rivalling the current record of 26.1% for Si-based PVs. To further boost the performances of PSCs, the use of 2D materials (such as graphene, transition metal dichalcogenides and transition metal carbides, nitrides and carbonitrides) has been proposed, thanks to their remarkable optoelectronic properties (that can be tuned with proper chemical composition engineering) and chemical stability. In particular, 2D materials have been demonstrated as promising candidates for (i) accelerating hot carrier transfer across the interfaces between the perovskite and the charge extraction layers; (ii) improving the crystallization of the perovskite layers (when used as additives in the precursor solution); (iii) favoring electronic bands alignment through tuning of the work function. In this mini-review, we discuss the physical mechanisms underlying the increased efficiency of 2D material-based PSCs, focusing on the three aforementioned effects.

Slowing Hot-Electron Relaxation in Mix-Phase Nanowires for Hot-Carrier Photovoltaics

Hot carrier harvest could save 30% energy loss in solar cells. So far, however, it is still unreachable as the photoexcited hot carriers are short-lived, ∼1 ps, determined by a rapid relaxation process, thus invalidating any reprocessing efforts. Here, we propose and demonstrate a feasible route to reserve hot electrons for efficient collection. It is accomplished by an intentional mix of cubic zinc-blend and hexagonal wurtzite phases in III-V semiconductor nanowires. Additional energy levels are then generated above the conduction band minimum, capturing and storing hot electrons before they cool down to the band edges. We also show the superiority of core/shell nanowire (radial heterostructure) in extracting hot electrons. The strategy disclosed here may offer a unique opportunity to modulate hot carriers for efficient solar energy harvest.

There is plenty of room at the top: generation of hot charge carriers and their applications in perovskite and other semiconductor-based optoelectronic devices

Hot charge carriers (HC) are photoexcited electrons and holes that exist in nonequilibrium high-energy states of photoactive materials. Prolonged cooling time and rapid extraction are the current challenges for the development of future innovative HC-based optoelectronic devices, such as HC solar cells (HCSCs), hot energy transistors (HETs), HC photocatalytic reactors, and lasing devices. Based on a thorough analysis of the basic mechanisms of HC generation, thermalization, and cooling dynamics, this review outlines the various possible strategies to delay the HC cooling as well as to speed up their extraction. Various materials with slow cooling behavior, including perovskites and other semiconductors, are thoroughly presented. In addition, the opportunities for the generation of plasmon-induced HC through surface plasmon resonance and their technological applications in hybrid nanostructures are discussed in detail. By judiciously designing the plasmonic nanostructures, the light coupling into the photoactive layer and its optical absorption can be greatly enhanced as well as the successful conversion of incident photons to HC with tunable energies can also be realized. Finally, the future outlook of HC in optoelectronics is highlighted which will provide great insight to the research community.

Repression of Interlayer Recombination by Graphene Generates a Sensitive Nanostructured 2D vdW Heterostructure Based Photodetector

Great success in 2D van der Waals (vdW) heterostructures based photodetectors is obtained owing to the unique electronic and optoelectronic properties of 2D materials. Performance of photodetectors based 2D vdW heterojunctions at atomic scale is more sensitive to the nanointerface of the heterojunction than conventional bulk heterojunction. Here, a nanoengineered heterostructure for the first-time demonstration of a nanointerface using an inserted graphene layer between black phosphorus (BP) and InSe which inhibits interlayer recombination and greatly improves photodetection performances is presented. In addition, a transition of the transport characteristics of the device is induced by graphene, from diffusion motion of minority carriers to drift motion of majority carriers. These two reasons together with an internal photoemission effect make the BP/G/InSe-based photodetector have ultrahigh specific detectivity at room temperature. The results demonstrate that high-performance vdW heterostructure photodetectors can be achieved through simple structural manipulation of the heterojunction interface on nanoscale.

State of the Art and Prospects for Halide Perovskite Nanocrystals

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Manipulation of hot carrier cooling dynamics in two-dimensional Dion-Jacobson hybrid perovskites via Rashba band splitting

Hot-carrier cooling processes of perovskite materials are typically described by a single parabolic band model that includes the effects of carrier-phonon scattering, hot phonon bottleneck, and Auger heating. However, little is known (if anything) about the cooling processes in which the spin-degenerate parabolic band splits into two spin-polarized bands, i.e., the Rashba band splitting effect. Here, we investigated the hot-carrier cooling processes for two slightly different compositions of two-dimensional Dion–Jacobson hybrid perovskites, namely, (3AMP)PbI₄ and (4AMP)PbI₄ (3AMP = 3-(aminomethyl)piperidinium; 4AMP = 4-(aminomethyl)piperidinium), using a combination of ultrafast transient absorption spectroscopy and first-principles calculations. In (4AMP)PbI₄, upon Rashba band splitting, the spin-dependent scattering of hot electrons is responsible for accelerating hot-carrier cooling at longer delays. Importantly, the hot-carrier cooling of (4AMP)PbI₄ can be extended by manipulating the spin state of the hot carriers. Our findings suggest a new approach for prolonging hot-carrier cooling in hybrid perovskites, which is conducive to further improving the performance of hot-carrier-based optoelectronic and spintronic devices.

Hybrid perovskite is a promising class of material for optoelectronic applications due to the slow hot-carrier cooling, yet the process is not well-understood in material with Rashba band splitting. Here, the authors reveal spin-flipping and spin-dependent scattering of hot electrons are responsible for accelerating the cooling at longer delays.

How to apply metal halide perovskites to photocatalysis: challenges and development

Semiconductor photocatalysts are widely used in environmental remediation and energy conversion processes that affect social development. These processes involve, for example, hydrogen production from water splitting, carbon dioxide reduction, pollutant degradation, and the conversion of raw organic chemical materials into high-value-added chemicals. Metal halide perovskites (MHPs) have become a new class of promising cheap and easy to manufacture candidate materials for use in photocatalytic semiconductors due to their advantages of high extinction coefficients, optimal band gaps, high photoluminescence quantum yields, and long electron-hole diffusion lengths. However, their unstable ion-bonded crystal structures (very low theoretical decomposition energy barriers) limit their widespread application. In this review, we introduce the physical properties of MHP materials suitable for photocatalysis, and MHP-based photocatalytic particle suspension systems, photoelectrode thin film systems, and photovoltaic-photo(electro)chemical systems. Then, numerous studies realizing efficient and stable photocatalytic water splitting, carbon dioxide reduction, organic conversion, and other reactions involving MHP materials were highlighted. In addition, we conducted rigorous analysis of the potential problems that could hinder progress in this new scientific research field, such as Pb element toxicity and material instability. Finally, we outline the potential opportunities and directions for photocatalysis research based on MHPs.

Accelerated Hot-Carrier Cooling in MAPbI3 Perovskite by Pressure-Induced Lattice Compression

Hot-carrier cooling (HCC) in metal halide perovskites above the Mott transition is significantly slower than in conventional semiconductors. This effect is commonly attributed to a hot-phonon bottleneck, but the influence of the lattice properties on the HCC behavior is poorly understood. Using pressure-dependent transient absorption spectroscopy, we find that at an excitation density below the Mott transition, pressure does not affect the HCC. On the contrary, above the Mott transition, HCC in methylammonium lead iodide is around 2-3 times faster at 0.3 GPa than at ambient pressure. Our electron-phonon coupling calculations reveal ∼2-fold stronger electron-phonon coupling for the inorganic cage mode at 0.3 GPa. However, our experiments reveal that pressure promotes faster HCC only above the Mott transition. Altogether, these findings suggest a change in the nature of excited carriers above the Mott transition threshold, providing insights into the electronic behavior of devices operating at such high charge-carrier densities.

Slow Hot-Carrier Cooling Enabled by Uniformly Arranging Different-n-Value Nanoplates in Quasi-2D Perovskites through Long-Range Orbit-Orbit Interaction toward Enhancing Photovoltaic Actions

This paper reports a slow hot-carriers cooling process enabled by uniformly arranging different-n-value nanoplates in quasi-2D perovskites (PEA₂MA_n-1Pb_nI_3n+1). Here, the energetic carriers are slowly relaxing from 1.743 to 1.688 eV within the time window of 91.7 ps, shown in transient absorption (TA) dynamics. It verifies an unusual long-order electron-lattice coupling to slow down the relaxation of hot carriers, generating long-lived energetic carriers in quasi-2D perovskites. Furthermore, the long-range orbit-orbit interaction is established through orbital polarizations between excited carriers, identified by polarization-dependent photocurrent. This indicates that the long-range orbit-orbit interaction provides the necessary condition to enable the unusual long-order electron-lattice coupling, causing a slow hot-carrier cooling effect through the energy-momentum dispersion curvature within the crystalline lattice. Interestingly, with the slow hot-carrier cooling and long-range orbit-orbit interaction, the quasi-2D perovskite demonstrates superior carrier transport shown by high fill-factor values (>80%) with reduced photovoltaic loss in solar cells at all different carrier densities.

Hydrostatic pressure effect of photocarrier dynamics in GaAs probed by time-resolved terahertz spectroscopy

Pressure effects on photocarrier dynamics such as interband relaxations and intraband cooling in GaAs have been investigated using in situ time-resolved terahertz spectroscopy with a diamond anvil cell. The interband photocarrier lifetime significantly decreases by nearly two orders of magnitude as the external hydrostatic pressure is increased up to 10 GPa. Considerable pressure tuning for the intervalley scattering processes has also been observed, and the time constants under different pressures are extracted based on the three-state rate model. This work provides new perspectives on tailoring nonequilibrium carrier dynamics in semiconductors using hydrostatic pressure and may serve as the impetus for the development of high-pressure terahertz spectroscopy.

Effect of Zinc-Doping on the Reduction of the Hot-Carrier Cooling Rate in Halide Perovskites

The fast hot-carrier cooling process in the solar-absorbers fundamentally limits their photon-conversion efficiencies. It is highly desirable to develop a solar absorber with long-lived hot-carriers at sun-illumination intensity, 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 10¹⁷  cm^-3 is found to be ≈3 times smaller than that of un-doped perovskites for T=500 K hot carriers, and up to ten times when the hot-carrier temperature approaches 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 processes. Our findings present a practical strategy to slow down the hot-carrier cooling in perovskites at low carrier densities, which would be invaluable for the further development of practical hot-carrier photovoltaics based on perovskites.

Achieving Optical Gain of the CsPbBr3 Perovskite Quantum Dots and Influence of the Variable Stripe Length Method

High-quality inorganic cesium lead halide perovskite quantum dot (CsPbBr₃ PQD) thin films were successfully deposited directly from a powdered source and used as an active laser medium following the examination of their distinctive surface and structural properties. To determine the suitability of the CsPbBr₃ PQDs as an active laser medium, amplified spontaneous emission (ASE) and optical gain properties were investigated under picosecond pulse excitation using the variable stripe length (VSL) method. The thin film of CsPbBr₃ PQDs has exhibited a sufficient value of the optical absorption coefficient of ∼0.86 × 10⁵ cm^-1 near the band edge and a direct band gap energy E _g ∼2.38 eV. The samples showed enhanced emission, and ASE was successfully recorded at a low threshold. The light emitted from the edge was observed near 2.40 and 2.33 eV for the stimulated emission (SE) and ASE regimes, respectively. The nonradiative decay contributes excitons dominant over biexcitons in the sample edge emission above the ASE threshold, making it practical for CsPbBr₃ PQDs to be used as optical gain media without undergoing repeated SE processes above the threshold over long periods. A high value of the optical gain coefficient was recorded at 346 cm^-1.

Achieving Optical Gain of the CsPbBr3 Perovskite Quantum Dots and Influence of the Variable Stripe Length Method

High-quality inorganic cesium lead halide perovskite quantum dot (CsPbBr₃ PQD) thin films were successfully deposited directly from a powdered source and used as an active laser medium following the examination of their distinctive surface and structural properties. To determine the suitability of the CsPbBr₃ PQDs as an active laser medium, amplified spontaneous emission (ASE) and optical gain properties were investigated under picosecond pulse excitation using the variable stripe length (VSL) method. The thin film of CsPbBr₃ PQDs has exhibited a sufficient value of the optical absorption coefficient of ∼0.86 × 10⁵ cm^-1 near the band edge and a direct band gap energy E _g ∼2.38 eV. The samples showed enhanced emission, and ASE was successfully recorded at a low threshold. The light emitted from the edge was observed near 2.40 and 2.33 eV for the stimulated emission (SE) and ASE regimes, respectively. The nonradiative decay contributes excitons dominant over biexcitons in the sample edge emission above the ASE threshold, making it practical for CsPbBr₃ PQDs to be used as optical gain media without undergoing repeated SE processes above the threshold over long periods. A high value of the optical gain coefficient was recorded at 346 cm^-1.

Effects of Crystal Morphology on the Hot-Carrier Dynamics in Mixed-Cation Hybrid Lead Halide Perovskites

Ultrafast pump-probe spectroscopies have proved to be an important tool for the investigation of charge carriers dynamics in perovskite materials providing crucial information on the dynamics of the excited carriers, and fundamental in the development of new devices with tailored photovoltaic properties. Fast transient absorbance spectroscopy on mixed-cation hybrid lead halide perovskite samples was used to investigate how the dimensions and the morphology of the perovskite crystals embedded in the capping (large crystals) and mesoporous (small crystals) layers affect the hot-carrier dynamics in the first hundreds of femtoseconds as a function of the excitation energy. The comparative study between samples with perovskite deposited on substrates with and without the mesoporous layer has shown how the small crystals preserve the temperature of the carriers for a longer period after the excitation than the large crystals. This study showed how the high sensitivity of the time-resolved spectroscopies in discriminating the transient response due to the different morphology of the crystals embedded in the layers of the same sample can be applied in the general characterization of materials to be used in solar cell devices and large area modules, providing further and valuable information for the optimization and enhancement of stability and efficiency in the power conversion of new perovskite-based devices.

Spin blockade and phonon bottleneck for hot electron relaxation observed in n-doped colloidal quantum dots

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.

Size-Dependent Hot Carrier Dynamics in Perovskite Nanocrystals Revealed by Two-Dimensional Electronic Spectroscopy

The lifetimes of hot carriers have been predicted to be prolonged in small nanocrystals with an inter-level spacing larger than phonon energy. Nevertheless, whether such a phonon bottleneck is present in perovskite semiconductor nanocrystals remains highly controversial. Here we report compelling evidence of a phonon bottleneck in CsPbI₃ nanocrystals with marked size-dependent relaxation of hot carriers by using broadband two-dimensional electronic spectroscopy (2DES). By combining high resolutions in both the time (<10 fs) and excitation energy domains, 2DES allows the clear disentanglement of the thermalization and cooling processes. The lifetime is over doubled for hot carriers when the average edge length of the nanocrystals decreases from 8.2 nm down to 4.6 nm. The confirmation of the phonon bottleneck effect suggests the feasibility of controlling hot carrier dynamics in perovskite semiconductors with nanocrystal size for potential applications of hot carrier devices.

Deciphering the excited-state dynamics and multicarrier interactions in perovskite core-shell type hetero-nanocrystals

Deciphering and modulating the carrier dynamics of perovskite nanocrystals (Pe-NCs) is crucial for their optoelectronic applications, which remains elusive to date. Herein, we, for the first time, explore the ultrafast dynamics of perovskite core-shell type NCs using CsPbBr₃@ZnS as a model system. According to the transient spectroscopic characterization, a physical picture of the ultrafast dynamics in core-shell Pe-NCs is built. Specifically, we directly observed the "hot" hole transfer from CsPbBr₃ to ZnS and confirmed the formation of charge-transfer state in CsPbBr₃@ZnS NCs. Such ultrafast (<100 fs) hole rearrangement speeds up the carrier cooling and breaks the hot phonon bottleneck effect in Pe-NCs. Moreover, thanks to the charge separation in CsPbBr₃@ZnS NCs, the Auger recombination is largely suppressed and the Auger lifetime is increased nearly 5-fold compared to that of "pure" CsPbBr₃ NCs, which endows CsPbBr₃@ZnS NCs with unique optical gain properties. These results are informative for halide perovskite-based applications, such as photocatalysis, hot-carrier photovoltaics and lasers.

Origins of the long-range exciton diffusion in perovskite nanocrystal films: photon recycling vs exciton hopping

The outstanding optoelectronic performance of lead halide perovskites lies in their exceptional carrier diffusion properties. As the perovskite material dimensionality is reduced to exploit the quantum confinement effects, the disruption to the perovskite lattice, often with insulating organic ligands, raises new questions on the charge diffusion properties. Herein, we report direct imaging of >1 μm exciton diffusion lengths in CH₃NH₃PbBr₃ perovskite nanocrystal (PNC) films. Surprisingly, the resulting exciton mobilities in these PNC films can reach 10 ± 2 cm² V^-1 s^-1, which is counterintuitively several times higher than the carrier mobility in 3D perovskite films. We show that this ultralong exciton diffusion originates from both efficient inter-NC exciton hopping (via Förster energy transfer) and the photon recycling process with a smaller yet significant contribution. Importantly, our study not only sheds new light on the highly debated origins of the excellent exciton diffusion in PNC films but also highlights the potential of PNCs for optoelectronic applications.

Phonon, thermal, and thermo-optical properties of halide perovskites

Metal halide perovskites are semiconductors with many fascinating characteristics and their widespread use in optoelectronic devices has been expected. High-quality thin films and single crystals can be fabricated by simple chemical solution processes and their fundamental electrical, optical, and thermal properties can be changed significantly by compositional substitution, in particular halogen ions. In this perspective, we provide an overview of phonon and thermal properties of metal halide perovskites, which play a decisive role in determining device performance. After a brief introduction to fundamental material properties, longitudinal-optical phonons and unusual thermal properties of metal halide perovskites are discussed. Remarkably, they possess very low thermal conductivities and very large thermal expansion coefficients despite their crystalline nature. In line with these discussions, we present optical properties governed by the strong electron-phonon interactions and the unusual thermal properties. By showing their unique thermo-optic responses and novel application examples, we highlight some aspects of the unusual thermal properties.

Multiphonon Raman Scattering and Strong Electron-Phonon Coupling in 2D Ternary Cu2MoS4 Nanoflakes

The interaction between photogenerated carriers with lattice vibrations plays a fundamental role in the nonradiative recombination and charge-transfer processes occurring in photocatalysis and photovoltaics. Here, we employ Raman spectroscopy to investigate the electron-phonon interaction in ternary layered Cu₂MoS₄ nanoflakes. Multiphonon Raman scattering with up to fourth-order longitudinal optical (LO) overtones is observed under above-band gap excitation, indicating a strong electron-phonon coupling (EPC) that could be described by the cascade model. The Huang-Rhys factor was derived to characterize the strength of EPC and was found to be increasing with decreasing temperature. First-principles calculations of lattice dynamics and electron-phonon matrix elements suggest that the strong EPC in Cu₂MoS₄ is dominated by Fröhlich coupling between electron and the electric fields, which is induced by the localized phonon mode originating from a flat phonon branch. Our findings facilitate the understanding of electron-phonon interaction in 2D ternary Cu₂MoS₄ and pave the way for developing and optimizing optoelectronic devices.

Protecting hot carriers by tuning hybrid perovskite structures with alkali cations

Successful implementation of hot carrier solar cells requires preserving high carrier temperature as carriers migrate through the active layer. Here, we demonstrated that addition of alkali cations in hybrid organic-inorganic lead halide perovskites led to substantially elevated carrier temperature, reduced threshold for phonon bottleneck, and enhanced hot carrier transport. The synergetic effects from the Rb, Cs, and K cations result in ~900 K increase in the effective carrier temperature at a carrier density around 10¹⁸ cm^-3 with an excitation 1.45 eV above the bandgap. In the doped thin films, the protected hot carriers migrate 100 s of nanometers longer than the undoped sample as imaged by ultrafast microscopy. We attributed these improvements to the relaxation of lattice strain and passivation of halide vacancies by alkali cations based on x-ray structural characterizations and first principles calculations.

One-step vapour phase growth of two-dimensional formamidinium-based perovskite and its hot carrier dynamics

Formamidinum lead iodide perovskite is one of the most promising materials for application in solar cells due to its narrow band gap and higher thermal stability. In this work, we demonstrate the facile synthesis of square-shaped formamidinium lead iodide single crystals on indium tin oxide (ITO) substrates using a one-step vapour phase deposition method. Formamidinium lead iodide-based two-dimensional layered perovskite crystals were successfully synthesized by controlling the deposition conditions. These crystals exhibited a blue-shifted photoluminescence (PL) compared to the conventional formaminium lead iodide perovskite crystals. Power law fittings of the excitation power dependent PL spectra revealed that Auger heating becomes dominant at high excitation densities. In addition, we observed an asymmetric broadening of the PL peak tail at the high energy side, indicating light emission from hot carriers even under steady-state illumination conditions. Phonon-bottleneck effect and Auger heating were considered as the main mechanisms for retardation of hot carrier cooling. Further analysis of the high energy tails using Maxwell-Boltzmann fitting revealed hot-carrier temperatures as high as 690 K. Our findings provide an important aspect of the synthetic approach of perovskites for their potential application in hot carrier solar cells.

Charge Carrier Relaxation in Colloidal FAPbI3 Nanostructures Using Global Analysis

We study the hot charge carrier relaxation process in weakly confined hybrid lead iodide perovskite colloidal nanostructures, FAPbI₃ (FA = formaminidium), using femtosecond transient absorption (TA). We compare the conventional analysis method based on the extraction of the carrier temperature (T_c) by fitting the high-energy tail of the band-edge bleach with a global analysis method modeling the continuous evolution of the spectral lineshape in time using a simple sequential kinetic model. This practical approach results in a more accurate way to determine the charge carrier relaxation dynamics. At high excitation fluence (density of charge carriers above 10¹⁸ cm⁻³), the cooling time increases up to almost 1 ps in thick nanoplates (NPs) and cubic nanocrystals (NCs), indicating the hot phonon bottleneck effect. Furthermore, Auger heating resulting from the multi-charge carrier recombination process slows down the relaxation even further to tens and hundreds of picoseconds. These two processes could only be well disentangled by analyzing simultaneously the spectral lineshape and amplitude evolution.

Exciton and Carrier Dynamics in Two-Dimensional Perovskites

Two-dimensional Ruddlesden-Popper hybrid lead halide perovskites have become a major topic in perovskite optoelectronics. Here, we aim to unravel the ultrafast dynamics governing the evolution of charge carriers and excitons in these materials. Using a combination of ultrabroadband time-resolved THz (TRTS) and fluorescence upconversion spectroscopies, we find that sequential carrier cooling and exciton formation best explain the observed dynamics, while exciton-exciton interactions play an important role in the form of Auger heating and biexciton formation. We show that the presence of a longer-lived population of carriers is due to the latter processes and not to a Mott transition. Therefore, excitons still dominate at laser excitation densities. We use kinetic modeling to compare the phenethylammonium and butylammonium organic cations while investigating the stability of the resulting films. In addition, we demonstrate the capability of using ultrabroadband TRTS to study excitons in large binding energy semiconductors through spectral analysis at room temperature.

Direct Visualization and Determination of the Multiple Exciton Generation Rate

Multiple exciton generation (MEG) takes place in competition to other hot carrier cooling processes. While the determination of carrier cooling rates is well established, direct information on MEG dynamics has been lacking. Here, we present a methodology to obtain the MEG rate directly in the initial ultrafast transient absorption dynamics. This method is most effective to systems with slow carrier cooling rates. Perovskite quantum dots exhibit this property and are used to illustrate this approach. They show a delayed carrier concentration buildup following an excitation pulse above the MEG threshold energy, which is accompanied by a faster carrier relaxation, providing a direct evidence of the MEG process. Numerical modeling within a simple framework of two competing cooling mechanisms allows us to extract the MEG rate and carrier energy cooling rates for this material. The presented methodology could provide new insights in carrier generation physics and valuable information for MEG investigations.

Efficient hot-electron extraction in two-dimensional semiconductor heterostructures by ultrafast resonant transfer

Energy loss from hot-carrier cooling sets the thermodynamic limit for the photon-to-power conversion efficiency in optoelectronic applications. Efficient hot-electron extraction before cooling could reduce the energy loss and leads to efficient next generation devices, which, unfortunately, is challenging to achieve in conventional semiconductors. In this work, we explore hot-electron transfer in two-dimensional (2D) layered semiconductor heterostructures, which have shown great potential for exploring new physics and optoelectronic applications. Using broadband micro-area ultrafast spectroscopy, we firmly established a type I band alignment in the WS₂-MoTe₂ heterostructure and ultrafast (∼60 fs) hot-electron transfer from photoexcited MoTe₂ to WS₂. The hot-electron transfer efficiency increases with excitation energy or excess energy as a result of a more favorable continuous competition between resonant electron transfer and cooling, reaching 90% for hot electrons with 0.3 eV excess energy. This study reveals exciting opportunities of designing extremely thin absorber and hot-carrier devices using 2D semiconductors and also sheds important light on the photoinduced interfacial process including charge transfer and generation in 2D heterostructures and optoelectronic devices.

Kinetic modelling of intraband carrier relaxation in bulk and nanocrystalline lead-halide perovskites

The relaxation of high-energy "hot" carriers in semiconductors is known to involve the redistribution of energy between hot and cold carriers, as well as the transfer of energy from hot carriers to phonons. Over the past few years, these two processes have been identified in lead-halide perovskites (LHPs) using ultrafast pump-probe experiments, but their interplay is not fully understood. Here we present a practical and intuitive kinetic model that accounts for the effects of both hot and cold carriers on carrier relaxation in LHPs. We apply this model to describe the dynamics of hot carriers in bulk and nanocrystalline CsPbBr₃ as observed by multi-pulse "pump-push-probe" spectroscopy. The model captures the slowing of the relaxation dynamics in the materials as the number of hot carriers increases, which has previously been explained by a "hot-phonon bottleneck" mechanism. The model also correctly predicts an acceleration of the relaxation kinetics as the number of cold carriers in the samples is increased. Using a series of natural approximations, we reduce our model to a simple form containing terms for the carrier-carrier and carrier-phonon interactions. The model can be instrumental for evaluating the details of carrier relaxation and carrier-phonon couplings in LHPs and other soft optoelectronic materials.