Halide perovskites exhibit unique slow hot-carrier cooling properties capable of unlocking disruptive perovskite photon–electron conversion technologies (e.g., high-efficiency hot-carrier photovoltaics, photo-catalysis, and photodetectors). Presently, the origins and mechanisms of this retardation remain highly contentious (e.g., large polarons, hot-phonon bottleneck, acoustical–optical phonon upconversion etc.). Here, we investigate the fluence-dependent hot-carrier dynamics in methylammonium lead triiodide using transient absorption spectroscopy, and correlate with theoretical modeling and first-principles calculations. At moderate carrier concentrations (around 1018 cm−3), carrier cooling is mediated by polar Fröhlich electron–phonon interactions through zone-center delayed longitudinal optical phonon emissions (i.e., with phonon lifetime τ LO around 0.6 ± 0.1 ps) induced by the hot-phonon bottleneck. The hot-phonon effect arises from the suppression of the Klemens relaxation pathway essential for longitudinal optical phonon decay. At high carrier concentrations (around 1019 cm−3), Auger heating further reduces the cooling rates. Our study unravels the intricate interplay between the hot-phonon bottleneck and Auger heating effects on carrier cooling, which will resolve the existing controversy.

The slow bimolecular recombination that drives three-dimensional lead-halide perovskites' outstanding photovoltaic performance is conversely a fundamental limitation for electroluminescence. Under electroluminescence working conditions with typical charge densities lower than 1015 cm−3, defect-states trapping in three-dimensional perovskites competes effectively with the bimolecular radiative recombination. Herein, we overcome this limitation using van-der-Waals-coupled Ruddlesden-Popper perovskite multi-quantum-wells. Injected charge carriers are rapidly localized from adjacent thin few layer (n≤4) multi-quantum-wells to the thick (n≥5) multi-quantum-wells with extremely high efficiency (over 85%) through quantum coupling. Light emission originates from excitonic recombination in the thick multi-quantum-wells at much higher decay rate and efficiency than bimolecular recombination in three-dimensional perovskites. These multi-quantum-wells retain the simple solution processability and high charge carrier mobility of two-dimensional lead-halide perovskites. Importantly, these Ruddlesden-Popper perovskites offer new functionalities unavailable in single phase constituents, permitting the transcendence of the slow bimolecular recombination bottleneck in lead-halide perovskites for efficient electroluminescence.

Organic–inorganic halide perovskite single crystals possess many outstanding properties conducive for photovoltaic and optoelectronic applications. However, a clear photophysics picture is still elusive, particularly, their surface and bulk photophysics are inexorably convoluted by the spectral absorbance, defects, coexisting photoexcited species, etc. In this work, an all‐optical study is presented that clearly distinguishes the surface kinetics from those of the bulk in the representative methylammonium‐lead bromide (MAPbBr3) and ‐lead iodide (MAPbI3) single crystals. It is found that the bulk recombination lifetime of the MAPbBr3 single crystal is shortened significantly by approximately one to two orders (i.e., from ≈34 to ≈1 ns) at the surface with a surface recombination velocity of around 6.7 × 103 cm s−1. The surface trap density is estimated to be around 6.0 × 1017 cm−3, which is two orders larger than that of the bulk (5.8 × 1015 cm−3). Correspondingly, the diffusion length of the surface excited species is ≈130–160 nm, which is considerably reduced compared to the bulk value of ≈2.6–4.3 μm. Furthermore, the surface region has a wider bandgap that possibly arises from the strong lattice deformation. The findings provide new insights into the intrinsic photophysics essential for single crystal perovskite photovoltaics and optoelectronic devices.

Developing environmentally friendly perovskites has become important in solving the toxicity issue of lead-based perovskite solar cells. Here, the first double perovskite (Cs AgBiBr ) solar cells using the planar structure are demonstrated. The prepared Cs AgBiBr films are composed of high-crystal-quality grains with diameters equal to the film thickness, thus minimizing the grain boundary length and the carrier recombination. These high-quality double perovskite films show long electron-hole diffusion lengths greater than 100 nm, enabling the fabrication of planar structure double perovskite solar cells. The resulting solar cells based on planar TiO exhibit an average power conversion efficiency over 1%. This work represents an important step forward toward the realization of environmentally friendly solar cells and also has important implications for the applications of double perovskites in other optoelectronic devices.

Halide perovskites possess enormous potential for various optoelectronic applications. Presently, a clear understanding of the interplay between the lattice and electronic effects is still elusive. Specifically, the weakly absorbing tail states and dual emission from perovskites are not satisfactorily described by existing theories based on the Urbach tail and reabsorption effect. Herein, through temperature-dependent and time-resolved spectroscopy on metal halide perovskite single crystals with organic or inorganic A-site cations, we confirm the existence of indirect tail states below the direct transition edge to arise from a dynamical Rashba splitting effect, caused by the PbBr6 octahedral thermal polar distortions at elevated temperatures. This dynamic effect is distinct from the static Rashba splitting effect, caused by non-spherical A-site cations or surface induced lattice distortions. Our findings shed fresh perspectives on the electronic-lattice relations paramount for the design and optimization of emergent perovskites, revealing broad implications for light harvesting/photo-detection and light emission/lasing applications.

Low-temperature solution-processed CH3 NH3 PbI3 interfaced with TiO2 has recently been demonstrated as a highly successful type-II light harvesting heterojunction with ≈20% efficiency. Therefore, an efficient ultrafast photoexcited electron transfer from CH3 NH3 PbI3 to TiO2 is expected. However, by probing the photoexcited charge carrier dynamics in CH3 NH3 PbI3 /quartz, CH3 NH3 PbI3 /TiO2 (compact), and CH3 NH3 PbI3 /PCBM in a comparative study, an electron transfer potential barrier between CH3 NH3 PbI3 and the compact TiO2 (prepared with the spray pyrolysis method) formed by surface states is uncovered. Consequently, the CH3 NH3 PbI3 photoluminescence intensity and lifetime is enhanced when interfaced to compact TiO2 . The electron accumulation within CH3 NH3 PbI3 needed to overcome this interfacial potential barrier results in the undesirable large current-voltage hysteresis observed for CH3 NH3 PbI3 /TiO2 planar heterojunctions. The findings in this study indicate that careful surface engineering to reduce this potential barrier is key to pushing perovskite solar cell efficiencies toward the theoretical limit.

cell (i.e., mesoscopic, meso-superstructured and planar heterojunctions), the power conversion effi ciencies (PCEs) of these cells have improved tremendously from 3.8% to 20% within a few years. [1][2][3][4][5][6][7] The high effi ciencies obtained with the hybrid perovskites are attributed to the high absorbance and long-range balanced charge transport lengths within the hybrid perovskites. [ 8,9 ] While most studies focus on improving the device performance, equal emphasis should also be given to the fundamental device physics. Among the several open questions on perovskite solar cells, the most challenging issue to date is the hysteresis effect (or dynamic lag) in current -voltage ( I-V ) measurements. [ 10,11 ] It was found that the PCEs measured is highly dependent on scan rate, scan direction, scan history, and light exposure. This could lead to the inaccurate reporting of PCEs, which would undermine the credibility and progress of this nascent photovoltaic technology. Consensus on the origin(s) of the hysteresis has proven elusive. Proposed origins include slow trapping and detrapping of charges due to subgap traps of solution-processed perovskites; changes to the ferroelectric structure and ion migration, etc. [10][11][12][13] Detailed investigations are therefore urgently needed to unravel their complicated mechanisms and elucidate their physical origins. Such fi ndings would be highly essential for establishing clear design rules needed for further performance improvements in halide organic-inorganic perovskite solar cells.The electrical properties and optical properties of an optoelectronic material are intimately coupled; both are the macroscopic refl ection of the intrinsic electronic physics. Studying both the electrical and optical behavior in a photovoltaic device is an ideal approach to uncover the physics shared by the two. Till now, very few reports have concurrently studied the optical and electrical phenomena that occur in perovskite solar cells with hysteresis. Herein, through versatile combined electrical and optical measurements, we uncover that the hysteresis effect in CH 3 NH 3 PbI 3 (MAPbI 3 ):TiO 2 -based perovskite solar cells is dominated by distinct slow processes persisting from hundreds of milliseconds to tens of seconds. These processes originate from the dynamic rearrangement of the perovskite structure that is mediated by applied electric fi elds and accumulated

Sn-based perovskites are promising Pb-free photovoltaic materials with an ideal 1.3 eV bandgap.However, to date, Sn-based thin film perovskite solar cells have yielded relatively low power conversion efficiencies (PCEs). This is traced to their poor photophysical properties (i.e., short diffusion lengths (<30 nm) and two orders of magnitude higher defect densities) than Pb-based systems. Herein we reveal that melt-synthesized cesium tin iodide (CsSnI 3 ) ingots containing high quality large single crystal (SC) grains transcend these fundamental limitations. Through detailed optical spectroscopy, we uncover their inherently superior properties, with bulk carrier lifetimes reaching 6.6 ns, doping concentrations of around 4.5 × 10 17 cm -3 , and minority-carrier diffusion lengths approaching 1 µm, as compared to their polycrystalline counterparts: ~54 ps, >9.2 × 10 18 cm -3 , and 16 nm, respectively. CsSnI 3 SCs also exhibit very low surface recombination velocity of <2 × 10 3 cm.s -1 similar to Pb-based perovskites. Importantly, these key parameters are comparable to highperformance p-type photovoltaic materials (e.g., InP crystals). Our findings predict a PCE of ~23% for optimized CsSnI 3 SCs solar cells, highlighting their great potential.