Unbalanced Hole and Electron Diffusion in Lead Bromide Perovskites

Solution-Processed Micro-Nanostructured Electron Transport Layer via Bubble-Assisted Assembly for Efficient Perovskite Photovoltaics

Organic-inorganic halide perovskite solar cells (PSCs) have attracted significant attention in photovoltaic research, owing to their superior optoelectronic properties and cost-effective manufacturing techniques. However, the unbalanced charge carrier diffusion length in perovskite materials leads to the recombination of photogenerated electrons and holes. The inefficient charge carrier collecting process severely affects the power conversion efficiency (PCE) of the PSCs. Herein, a solution-processed SnO2 array electron transport layer with precisely tunable micro-nanostructures is fabricated via a bubble-template-assisted approach, serving as both electron transport layers and scaffolds for the perovskite layer. Due to the optimized electron transporting pathway and enlarged perovskite grain size, the PSCs achieve a PCE of 25.35% (25.07% certificated PCE).

Colloidal Quasi-2D Methylammonium Lead Bromide Perovskite Nanostructures with Tunable Shape and High Chemical Stability

Control over the lateral dimensions of colloidal nanostructures is a complex task which requires a deep understanding of the formation mechanism and reactivity in the corresponding systems. As a result, it provides a well-founded insight to the physical and chemical properties of these materials. In this work, the preparation of quasi-2D methylammonium lead bromide nanostripes and discuss the influence of some specific parameters on the morphology and stability of this material is demonstrated. The variation in the amount of the main ligand dodecylamine gives a large range of structures beginning with 3D brick-like particles at low concentrations, nanostripes at elevated and ultimately nanosheets at large concentrations. The amount of the co-ligand trioctylphosphine can alter the width of the nanostripe shape to a certain degree. The thickness can be adjusted by the amount of the second precursor methylammonium bromide. Additionally, insights are given for the suggested formation mechanism of these anisotropic structures as well as for stability against moisture at ambient conditions in comparison with differently synthesized nanosheet samples.

Direct detection of minimum ionizing charged particles in a perovskite single crystal detector with single particle sensitivity

We report the detection of high energy electrons of some hundreds of MeV, crossing a methylammonium lead bromide single crystal device with sensitivity down to a single electron. In the device, the released energy is close to the energy released by minimum-ionizing particles. This is the first demonstration of a perovskite-based device that can be used for tracking and counting minimum-ionizing charged particles. The device reaches single particle sensitivity with a low bias voltage of 5 V. It also shows a good linearity of the response as a function of the number of electrons in a dynamic range of approximately 104.

Heterointerface engineering of layered double hydroxide/MAPbBr3 heterostructures enabling tunable synapse behaviors in a two-terminal optoelectronic device

Solution-processable semiconductor heterostructures enable scalable fabrication of high performance electronic and optoelectronic devices with tunable functions via heterointerface control. In particular, artificial optical synapses require interface manipulation for nonlinear signal processing. However, the limited combinations of materials for heterostructure construction have restricted the tunability of synaptic behaviors with simple device configurations. Herein, MAPbBr3 nanocrystals were hybridized with MgAl layered double hydroxide (LDH) nanoplates through a room temperature self-assembly process. The formation of such heterostructures, which exhibited an epitaxial relationship, enabled effective hole transfer from MAPbBr3 to LDH, and greatly reduced the defect states in MAPbBr3. Importantly, the ion-conductive nature of LDH and its ability to form a charged surface layer even under low humidity conditions allowed it to attract and trap holes from MAPbBr3. This imparted tunable synaptic behaviors and short-term plasticity (STP) to long-term plasticity (LTP) transition to a two-terminal device based on the LDH-MAPbBr3 heterostructures. The further neuromorphic computing simulation under varying humidity conditions showcased their potential in learning and recognition tasks under ambient conditions. Our work presents a new type of epitaxial heterostructure comprising metal halide perovskites and layered ion-conductive materials, and provides a new way of realizing charge-trapping induced synaptic behaviors.

Optical Sensing Capability Evaluation for Methylammonium Based Perovskites for Explosive

Here, we have synthesized methylammonium based two metal halide perovskites (MHP) such as MAPbBr3, and MAPbI3 using methylammonium bromide, methylammonium iodide, lead bromide, respective at room temperature under certain experimental conditions. All synthesized MHPs have been confirmed through X-ray diffraction technique (XRD), scanning electron microscope (SEM), Fourier transform infra-red (FTIR) and photoluminescence (PL) analysis. Afterward, comparative evaluation on optical sensing capability has been made for both MHPs using PL in different solvents. Importantly, we find out that MAPbBr3 exhibit an excellent optical feature over MAPbI3 in hexane only. Afterward, MAPbBr3 has also been explored to know the sensing capability for nitrobenzene sensing. Our model study confirms that MAPbBr3 is an excellent sensing material with R square (0.87), selectivity (16.9%) and Stern Volmer constant (Ksv=10- 2 × 0.464) for nitrobenzene in hexane.

Solution-Grown MAPbBr3 Single Crystals for Self-Powered Detection of X-rays with High Energies above One Megaelectron Volt

Perovskite single crystals are actively studied as X-ray detection materials with enhanced sensitivity. Moreover, the feasibility of using perovskites for self-powered devices such as photodetectors, UV detectors, and X-ray detectors can significantly expand their application range. In this work, the charge carrier transport and photocurrent properties of MAPbBr3 single crystals (MSCs) are improved by the mechanochemical surface treatment using glycerin combined with an additional electrode design that forms an ohmic contact. The sensitivity of MSC-based detectors and pulse shape generated by X-rays are enhanced at various bias voltages. The synthesized MSC detectors generate direction-dependent photocurrents, which indicate the presence of a polarization-induced internal electric field. In addition, photocurrent signals are produced by X-rays with energies greater than 1 MeV under a zero-bias voltage. This work demonstrates a high application potential of perovskites as self-powered detectors for X-rays with energies exceeding 1 MeV.

Numerical Analysis of Stable (FAPbI3)0.85(MAPbBr3)0.15-Based Perovskite Solar Cell with TiO2/ZnO Double Electron Layer

Although perovskite solar cells have achieved excellent photoelectric conversion efficiencies, there are still some shortcomings, such as defects inside and at the interface as well as energy level dislocation, which may lead to non-radiative recombination and reduce stability. Therefore, in this study, a double electron transport layer (ETL) structure of FTO/TiO₂/ZnO/(FAPbI₃)_0.85(MAPbBr₃)_0.15/Spiro-OMeTAD is investigated and compared with single ETL structures of FTO/TiO₂/(FAPbI₃)_0.85(MAPbBr₃)_0.15/Spiro-OMeTAD and FTO/ZnO/(FAPbI₃)_0.85(MAPbBr₃)_0.15/Spiro-OMeTAD using the SCAPS-1D simulation software, with special attention paid to the defect density in the perovskite active layer, defect density at the interface between the ETL and the perovskite active layer, and temperature. Simulation results reveal that the proposed double ETL structure could effectively reduce the energy level dislocation and inhibit the non-radiative recombination. The increases in the defect density in the perovskite active layer, the defect density at the interface between the ETL and the perovskite active layer, and the temperature all facilitate carrier recombination. Compared with the single ETL structure, the double ETL structure has a higher tolerance for defect density and temperature. The simulation outcomes also confirm the possibility of preparing a stable perovskite solar cell.

Mixed 3D-2D Perovskite Flexible Films for the Direct Detection of 5 MeV Protons

This study reports on a novel, flexible, proton beam detector based on mixed 3D-2D perovskite films deposited by solution onto thin plastic foils. The 3D-2D mixture allows to obtain micrometer-thick and highly uniform films that constitute the detector's active layer. The devices demonstrate excellent flexibility with stable electric transport properties down to a bending radius of 3.1 mm. The detector is characterized under a 5 MeV proton beam with fluxes in the range [4.5 × 105 - 1.4 × 109 ] H+ cm-2 s-1 , exhibiting a stable response to repetitive irradiation cycles with sensitivity up to (290 ± 40) nC Gy-1 mm-3 and a limit of detection down to (72±2) µGy s-1 . The detector radiation tolerance is also assessed up to a total of 1.7 × 1012 protons impinging on the beam spot area, with a maximum variation of the detector's response of 14%.

Thermochemical Study of CH3NH3Pb(Cl1-xBrx)3 Solid Solutions

Hybrid organic-inorganic perovskite halides, and, in particular, their mixed halide solid solutions, belong to a broad class of materials which appear promising for a wide range of potential applications in various optoelectronic devices. However, these materials are notorious for their stability issues, including their sensitivity to atmospheric oxygen and moisture as well as phase separation under illumination. The thermodynamic properties, such as enthalpy, entropy, and Gibbs free energy of mixing, of perovskite halide solid solutions are strongly required to shed some light on their stability. Herein, we report the results of an experimental thermochemical study of the CH₃NH₃Pb(Cl_1-xBr_x)₃ mixed halides by solution calorimetry. Combining these results with molecular dynamics simulation revealed the complex and irregular shape of the compositional dependence of the mixing enthalpy to be the result of a complex interplay between the local lattice strain, hydrogen bonds, and energetics of these solid solutions.

Extreme-Ultraviolet Excited Scintillation of Methylammonium Lead Bromide Perovskites

Inorganic-Organic lead halide materials have been recognized as potential high-energy X-ray detectors because of their high quantum efficiencies and radiation hardness. Surprisingly little is known about whether the same is true for extreme-ultraviolet (XUV) radiation, despite applications in nuclear fusion research and astrophysics. We used a table-top high-harmonic generation setup in the XUV range between 20 and 45 eV to photoexcite methylammonium lead bromide (MAPbBr₃) and measure its scintillation properties. The strong absorbance combined with multiple carriers being excited per photon yield a very high carrier density at the surface, triggering photobleaching reactions that rapidly reduce the emission intensity. Concurrent to and in spite of this photobleaching, a recovery of the emission intensity as a function of dose was observed. X-ray photoelectron spectroscopy and X-ray diffraction measurements of XUV-exposed and unexposed areas show that this recovery is caused by XUV-induced oxidation of MAPbBr₃, which removes trap states that normally quench emission, thus counteracting the rapid photobleaching caused by the extremely high carrier densities. Furthermore, it was found that preoxidizing the sample with ozone was able to prolong and improve this intensity recovery, highlighting the impact of surface passivation on the scintillation properties of perovskite materials in the XUV range.

Highly Mobile Excitons in Single Crystal Methylammonium Lead Tribromide Perovskite Microribbons

Excitons are often given negative connotation in solar energy harvesting in part due to their presumed short diffusion lengths. We investigate exciton transport in single-crystal methylammonium lead tribromide (MAPbBr₃) microribbons via spectrally, spatially, and temporally resolved photocurrent and photoluminescence measurements. Distinct peaks in the photocurrent spectra unambiguously confirm exciton formation and allow for accurate extraction of the low temperature exciton binding energy (39 meV). Photocurrent decays within a few μm at room temperature, while a gate-tunable long-range photocurrent component appears at lower temperatures (about 100 μm below 140 K). Carrier lifetimes of 1.2 μs or shorter exclude the possibility of the long decay length arising from slow trapped-carrier hopping. Free carrier diffusion is also an unlikely source of the highly nonlocal photocurrent, due to their small fraction at low temperatures. We attribute the long-distance transport to high-mobility excitons, which may open up new opportunities for novel exciton-based photovoltaic applications.

Charge transfer excitons in unfunctionalized graphite-wrapped MAPbBr3 nanocrystal composites with different morphologies

Charge transfer from perovskite nanocrystals to graphite sheets.

Approaching the intrinsic exciton physics limit in two-dimensional semiconductor diodes

Two-dimensional (2D) semiconductors have attracted intense interest for their unique photophysical properties, including large exciton binding energies and strong gate tunability, which arise from their reduced dimensionality^1-5. Despite considerable efforts, a disconnect persists between the fundamental photophysics in pristine 2D semiconductors and the practical device performances, which are often plagued by many extrinsic factors, including chemical disorder at the semiconductor-contact interface. Here, by using van der Waals contacts with minimal interfacial disorder, we suppress contact-induced Shockley-Read-Hall recombination and realize nearly intrinsic photophysics-dictated device performance in 2D semiconductor diodes. Using an electrostatic field in a split-gate geometry to independently modulate electron and hole doping in tungsten diselenide diodes, we discover an unusual peak in the short-circuit photocurrent at low charge densities. Time-resolved photoluminescence reveals a substantial decrease of the exciton lifetime from around 800 picoseconds in the charge-neutral regime to around 50 picoseconds at high doping densities owing to increased exciton-charge Auger recombination. Taken together, we show that an exciton-diffusion-limited model well explains the charge-density-dependent short-circuit photocurrent, a result further confirmed by scanning photocurrent microscopy. We thus demonstrate the fundamental role of exciton diffusion and two-body exciton-charge Auger recombination in 2D devices and highlight that the intrinsic photophysics of 2D semiconductors can be used to create more efficient optoelectronic devices.

Superior photo-carrier diffusion dynamics in organic-inorganic hybrid perovskites revealed by spatiotemporal conductivity imaging

The outstanding performance of organic-inorganic metal trihalide solar cells benefits from the exceptional photo-physical properties of both electrons and holes in the material. Here, we directly probe the free-carrier dynamics in Cs-doped FAPbI₃ thin films by spatiotemporal photoconductivity imaging. Using charge transport layers to selectively quench one type of carriers, we show that the two relaxation times on the order of 1 μs and 10 μs correspond to the lifetimes of electrons and holes in FACsPbI₃, respectively. Strikingly, the diffusion mapping indicates that the difference in electron/hole lifetimes is largely compensated by their disparate mobility. Consequently, the long diffusion lengths (3~5 μm) of both carriers are comparable to each other, a feature closely related to the unique charge trapping and de-trapping processes in hybrid trihalide perovskites. Our results unveil the origin of superior diffusion dynamics in this material, crucially important for solar-cell applications.

s-p Mixing in Stereochemically Active Lone Pairs Drives the Formation of 1D Chains of Lead Bromide Square Pyramids

In lead(II) halide compounds including virtually all lead halide perovskites, the Pb²⁺ 6s lone pair results in distorted octahedra, in accordance with the pseudo-Jahn-Teller effect, rather than generating hemihedral coordination polyhedra. Here, in contrast, we report the characterization of an organic-inorganic hybrid material consisting of one-dimensional edge-sharing chains of Pb-Br square pyramids, separated by [Mn(DMF)₆]²⁺ (DMF = dimethylformamide) octahedra. Molecular orbital analysis and density-functional theory calculations indicate that square pyramidal coordination about Pb²⁺ results from the occupancy of the empty ligand site by a Pb²⁺ lone pair that has both s and p orbital character rather than the exclusively 6s lone pair. These results demonstrate that a Pb²⁺ lone pair can be exploited to behave like a ligand in lead halide compounds, greatly expanding the realm of possible lead halide materials to include extended solids with nonoctahedral coordination environments.

Prospects of Z-Scheme Photocatalytic Systems Based on Metal Halide Perovskites

Considering the attractive optoelectronic properties of metal halide perovskites (MHPs), their introduction to the field of photocatalysis was only a matter of time. Thus far, MHPs have been explored for the photocatalytic generation of hydrogen, carbon dioxide reduction, organic synthesis, and pollutant degradation applications. Of growing research interest and possible applied significance are the currently emerging developments of MHP-based Z-scheme heterostructures, which can potentially enable efficient photocatalysis of highly energy-demanding redox processes. In this Perspective, we discuss the advantages and limitations of MHPs compared to traditional semiconductor materials for applications as photocatalysts and describe emerging examples in the construction of MHP-based Z-scheme systems. We discuss the principles and material properties that are required for the development of such Z-scheme heterostructure photocatalysts and consider the ongoing challenges and opportunities in this emerging field.

Solution-Grown Formamidinium Hybrid Perovskite (FAPbBr3) Single Crystals for α-Particle and γ-Ray Detection at Room Temperature

Compared with the widely reported MAPbBr₃ single crystals, formamidinium-based (FA-based) hybrid perovskites FAPbBr₃ (FPB) with superior chemical and structure stability are expected to be more efficient and perform as more reliable radiation detectors at room temperature. Here, we employ an improved inverse temperature crystallization method to grow FPB bulk single crystals, where issues associated with the retrograde solubility behavior are resolved. A crystal growth phase diagram has been proposed, and accordingly, growth parameters are optimized to avoid the formation of NH₄Pb₂Br₅ secondary phase. The resulting FPB crystals exhibit a high resistivity of 2.8 × 10⁹ Ω·cm and high electron and hole mobility-lifetime products (μτ) of 8.0 × 10^-4 and 1.1 × 10^-3 cm²·V^-1, respectively. Simultaneously, the electron and hole mobilities (μ) are evaluated to be 22.2 and 66.1 cm²·V^-1·s^-1, respectively, based on the time-of-flight technique. Furthermore, a Au/FPB SC/Au detector is constructed that demonstrates a resolvable gamma peak from 59.5 keV ²⁴¹Am γ-rays at room temperature for the first time. An energy resolution of 40.1% is obtained at 30 V by collecting the hole signals. These results demonstrate the great potential of FAPbBr₃ as a hybrid material for γ-ray spectroscopy and imaging.

Crystallization of CsPbBr3 single crystals in water for X-ray detection

Metal halide perovskites have fascinated the research community over the past decade, and demonstrated unprecedented success in optoelectronics. In particular, perovskite single crystals have emerged as promising candidates for ionization radiation detection, due to the excellent opto-electronic properties. However, most of the reported crystals are grown in organic solvents and require high temperature. In this work, we develop a low-temperature crystallization strategy to grow CsPbBr₃ perovskite single crystals in water. Then, we carefully investigate the structure and optoelectronic properties of the crystals obtained, and compare them with CsPbBr₃ crystals grown in dimethyl sulfoxide. Interestingly, the water grown crystals exhibit a distinct crystal habit, superior charge transport properties and better stability in air. We also fabricate X-ray detectors based on the CsPbBr₃ crystals, and systematically characterize their device performance. The crystals grown in water demonstrate great potential for X-ray imaging with enhanced performance metrics.

Perovskite single crystals are commonly grown in organic solvents, which require relatively high temperature condition. Here, the authors develop a low-temperature crystallisation strategy to grow CsPbBr₃ single crystals in water with improved charge transport properties and stability.

Directly visualizing carrier transport and recombination at individual defects within 2D semiconductors

Two-dimensional semiconductors (2DSCs) are promising materials for a wide range of optoelectronic applications. While the fabrication of 2DSCs with thicknesses down to the monolayer limit has been demonstrated through a variety of routes, a robust understanding of carrier transport within these materials is needed to guide the rational design of improved practical devices. In particular, the influence of different types of structural defects on transport is critical, but difficult to interrogate experimentally. Here, a new approach to visualizing carrier transport within 2DSCs, Carrier Generation-Tip Collection Scanning Electrochemical Cell Microscopy (CG-TC SECCM), is described which is capable of providing information at the single-defect level. In this approach, carriers are locally generated within a material using a focused light source and detected as they drive photoelectrochemical reactions at a spatially-offset electrolyte interface created through contact with a pipet-based probe, allowing carrier transport across well-defined, µm-scale paths within a material to be directly interrogated. The efficacy of this approach is demonstrated through studies of minority carrier transport within mechanically-exfoliated n-type WSe2 nanosheets. CG-TC SECCM imaging experiments carried out within pristine basal planes revealed highly anisotropic hole transport, with in-plane and out-of-plane hole diffusion lengths of 2.8 µm and 5.8 nm, respectively. Experiments were also carried out to probe recombination across individual step edge defects within n-WSe2 which suggest a significant surface charge (∼5 mC m-2) exists at these defects, significantly influencing carrier transport. Together, these studies demonstrate a powerful new approach to visualizing carrier transport and recombination within 2DSCs, down to the single-defect level.

High efficiency and stable photoluminescence of CH3NH3PbBr3@CsPbBr3 perovskite quantum dots

CH₃NH₃PbBr₃@CsPbBr₃ quantum dots were prepared by epitaxially growing a CsPbBr₃ shell on the surface of CH₃NH₃PbBr₃ due to their similar crystal structures. The inorganic CsPbBr₃ shell provides enhanced stability for the CH₃NH₃PbBr₃ core. Compared with that of CH₃NH₃PbBr₃, the photoluminescence of CH₃NH₃PbBr₃@CsPbBr₃ quantum dots is not only strong, but also stable for months, in addition to having a high quantum yield.

Understanding the Instability of the Halide Perovskite CsPbI3 through Temperature-Dependent Structural Analysis

Despite the tremendous interest in halide perovskite solar cells, the structural reasons that cause the all-inorganic perovskite CsPbI₃ to be unstable at room temperature remain mysterious, especially since many tolerance-factor-based approaches predict CsPbI₃ should be stable as a perovskite. Here single-crystal X-ray diffraction and X-ray pair distribution function (PDF) measurements characterize bulk perovskite CsPbI₃ from 100 to 295 K to elucidate its thermodynamic instability. While Cs occupies a single site from 100 to 150 K, it splits between two sites from 175 to 295 K with the second site having a lower effective coordination number, which, along with other structural parameters, suggests that Cs rattles in its coordination polyhedron. PDF measurements reveal that on the length scale of the unit cell, the PbI octahedra concurrently become greatly distorted, with one of the IPbI angles approaching 82° compared to the ideal 90°. The rattling of Cs, low number of CsI contacts, and high degree of octahedral distortion cause the instability of perovskite-phase CsPbI_3. These results reveal the limitations of tolerance factors in predicting perovskite stability and provide detailed structural information that suggests methods to engineer stable CsPbI₃ -based solar cells.

Perovskite Multiple Quantum Wells on Layered Materials toward Narrow-Band Green Emission for Backlight Display Applications

Quasi-two-dimensional perovskite multiple quantum wells (MQWs) have garnered attention on account of their good stability in comparison with three-dimensional analogs for light-emitting diode (LED) applications, but they exhibit a declined photoluminescence quantum yield (PLQY) for the backlight display technology. Herein, we report an in situ process by introducing layered materials as substrates to prepare the perovskite MQW powder and utilize hexagonal boron nitride (h-BN) as an example to investigate the luminescence characteristics and mechanism of h-BN@(PMA)₂MA_n-1Pb_nBr_3n+1 (h-BN@PMPB). Remarkably, the deposition strategy enables an enhanced PLQY from 2.6 to 32.4%. A synergetic effect of low reabsorption, low defect concentration, and high absorptivity leads to high-efficiency cascade energy transfer and also an improvement in stability. Moreover, the h-BN@PMPB composite powder shows a narrow-band green emission band peaking at 532 nm with a full width at half maximum of 32 nm, and the white LED backlight reaches a wide color gamut 106.1% of the National Television Standards Committee (NTSC). This work reveals a general synthesis method for preparing perovskite powder and paves the way to achieve new solid-state luminescent materials with controlled size and morphology for backlight display applications.

Highly Efficient CsPbBr3 Planar Perovskite Solar Cells via Additive Engineering with NH4SCN

Improving stability is a major aspect for commercial application of perovskite solar cells (PSCs). The all-inorganic CsPbBr₃ perovskite material has been proven to have excellent stability. However, the CsPbBr₃ film has a small range of light absorption and serious charge recombination at the interface or inside the device, so the power conversion efficiency is still lower than that of the organic-inorganic hybrid one. Here, we successfully fabricate high-quality CsPbBr₃ films via additive engineering with NH₄SCN. By incorporating NH₄⁺ and pseudo-halide ion SCN^- into the precursor solution, a smooth and dense CsPbBr₃ film with good crystallinity and low trap state density can be obtained. At the same time, the results of a series of photoluminescence and electrochemical analyses including electrical impedance spectroscopy, space-charge limited current method, Mott-Schottky data, and so on reveal that the NH₄SCN additive can greatly reduce the trap state density of the CsPbBr₃ film and also effectively inhibit interface recombination and promote charge transport in the CsPbBr₃ planar PSC. Finally, the CsPbBr₃ planar PSC prepared with a molar ratio of 1.5% NH₄SCN achieves a champion efficiency of 8.47%, higher than that of the pure one (7.12%).

Micron-Size Two-Dimensional Methylammonium Lead Halide Perovskites

Hybrid lead halide perovskites with 2D stacking structures have recently emerged as promising materials for optoelectronic applications. We report a method for growing 2D nanosheets of hybrid lead halide perovskites (I, Br and Cl), with tunable lateral sizes ranging from 0.05 to 8 μm and a structure consisting of n stacked monolayers separated by long alkylamines, tunable from bulk down to n = 1. The key to obtaining such a wide range of perovskite properties hinged on utilizing the respective lead halide nanosheets as precursors in a hot-injection synthesis that afforded careful control over all process parameters. The layered, quantum-confined ( n ≤ 4) nanosheets were comprised of major and minor fractions with differing n. Energy funneling from low to high n (high to low energy) regions within a single sheet, mediated by the length of the ligands between stacks, produced photoluminescent quantum yields as high as 49%. These large, tunable 2D nanosheets could serve as convenient platforms for future high-efficiency optoelectronic devices.

Photoinduced photoluminescence enhancement in self-assembled clusters of formamidinium lead bromide perovskite nanocrystals

Nanocrystals of formamidinium lead bromide perovskite (FAPbBr3) self-assemble into clusters in powder and film samples and provide a prolonged photoluminescence lifetime, which is attributed to the diffusion of charge carriers through interparticle states formed among nanocrystals. Interestingly, the photoluminescence lifetime decreases and the emission intensity increases for the clusters, which is with the increase in the intensity of excitation light. By doping the nanocrystal clusters with C60, we successfully harvested the photogenerated charge carriers. Nonetheless, at high intensities of excitation, the rate of radiative recombination becomes comparable to that of the electron transfer to C60. Thus, the optimum rate of electron transfer to C60 is accomplished by minimally exciting the self-assembled nanocrystals.

Photodriven Dipole Reordering: Key to Carrier Separation in Metalorganic Halide Perovskites

Photodriven dipole reordering of the intercalated organic molecules in halide perovskites has been suggested to be a critical degree of freedom, potentially affecting physical properties, device performance, and stability of hybrid perovskite-based optoelectronic devices. However, thus far a direct atomically resolved dipole mapping under device operation condition, that is, illumination, is lacking. Here, we map simultaneously the molecule dipole orientation pattern and the electrostatic potential with atomic resolution using photoexcited cross-sectional scanning tunneling microscopy and spectroscopy. Our experimental observations demonstrate that a photodriven molecule dipole reordering, initiated by a photoexcited separation of electron-hole pairs in spatially displaced orbitals, leads to a fundamental reshaping of the potential landscape in halide perovskites, creating separate one-dimensional transport channels for holes and electrons. We anticipate that analogous light-induced polarization order transitions occur in bulk and are at the origin of the extraordinary efficiencies of organometal halide perovskite-based solar cells as well as could reconcile apparently contradictory materials' properties.

Unravelling the Effects of A-Site Cations on Nonradiative Electron-Hole Recombination in Lead Bromide Perovskites: Time-Domain ab Initio Analysis

Lead bromide perovskites APbBr₃ (A = Cs, MA, FA) hold great promise in optoelectronics and photovoltaics. Because the band gaps of the three materials are similar, and also because the A-site cation does not contribute to band edges, one would expect a minor influence of A-site cation on the excited-state lifetime of the perovskites. Experiments defy that expectation. By performing ab initio nonadiabatic (NA) molecular dynamics combined with time-domain density functional simulations, we demonstrate that the nonradiative electron-hole recombination times are in the order FAPbBr₃ > MAPbBr₃ > CsPbBr₃, which are determined by the NA electron-phonon coupling because decoherence times are similar. The simulations show that the larger A-site cation and the smaller NA coupling because larger A-site cation suppresses the Pb-Br cages' motion. The electron-hole recombination is slow, ranging from subnanosecond to nanoseconds, because the NA coupling is small, less than 3 meV, and because decoherence time is slow, less than 7 fs. Both the trend of recombination and the time scales show excellent agreement with experiments. The time-domain atomistic simulations rationalize the experimental observations and advance our understanding of the cations' influence on perovskite excited-state lifetimes.

Metal Halide Perovskites: Synthesis, Ion Migration, and Application in Field-Effect Transistors

The past several years have witnessed tremendous developments of metal halide perovskite (MHP)-based optoelectronics. Particularly, the intensive research of MHP-based light-emitting diodes, photodetectors, and solar cells could probably reform the optoelectronic semiconductor industry. In comparison, in spite of the large intrinsic charge carrier mobility of MHPs, the development of MHP-based field-effect transistors (MHP-FETs) is relatively slow, which is essentially due to the gate-field screening effect induced by the ion migration and accumulation in MHP-FETs. This work mainly aims to summarize the recent important work on MHP-FETs and propose solutions in terms of the development bottleneck of perovskite-based transistors, in an attempt to boost the research of MHP transistors further. First, the advantages and potential applications of MHP-FETs are briefly introduced, which is followed by a detailed description of the MHP crystalline structure and various material fabrication techniques. Afterward, MHP-FETs are discussed, including transistors based on hybrid organic-inorganic perovskites, all-inorganic perovskites, and lead-free perovskites.

Influence of Eu-substitution on luminescent CH3NH3PbBr3 quantum dots

We report a series of Eu-substituted methylammonium lead tribromide quantum dots (MAPb1-xEuxBr3 QDs). The crystallinity of these QDs increases with increasing Eu content, while there is only a small change in the lattice constant, and the morphology of the MAPb1-xEuxBr3 QDs is unaffected by the Eu content. This demonstrates that Eu is a suitable element for substituting Pb while retaining the original crystal structure. We observe blue photoluminescence (PL) consistent with Eu2+, which, at high Eu content (x = 0.3), contributes luminescence intensity equal to the green PL of unsubstituted MAPbBr3 QDs. As Eu is a less toxic substitute for Pb, that also provides blue luminescence, MAPb1-xEuxBr3 QDs may prove to be a valuable optoelectronic material.

What Remains Unexplained about the Properties of Halide Perovskites?

The notion that halide perovskite crystals (ABX₃ , where X is a halide) exhibit unique structural and optoelectronic behavior deserves serious scrutiny. After decades of steady and half a decade of intense research, the question which attributes of these materials are unusual, is discussed, with an emphasis on the identification of the most important remaining issues. The goal is to stimulate discussion rather than to merely present a community consensus.

Halide Composition Controls Electron-Hole Recombination in Cesium-Lead Halide Perovskite Quantum Dots: A Time Domain Ab Initio Study

We demonstrate that halide content strongly affects nonradiative electron-hole recombination in all-inorganic perovskite quantum dots (QDs). Using time domain density functional theory and nonadiabatic molecular dynamics, we show that replacing half of the bromines with iodines in a CsPbBr₃ QD extends the charge carrier lifetime by a factor of 5, while complete replacement extends the lifetime by a factor of 8. Doping with iodines decreases the nonadiabatic charge-phonon coupling because iodines are heavier and slower than bromines and because the overlap between the electron and hole wave functions is reduced. In general, the nonradiative electron-hole recombination proceeds slowly, on a nanosecond time scale, due to small sub-1 meV nonadiabatic coupling and short sub-10 fs coherence times. The obtained recombination times and their dependence on the halogen content show excellent agreement with experiments. Our study suggests that the power conversion efficiencies of solar cells can be controlled by changing the halide composition in all-inorganic perovskite QDs.

Single-crystalline perovskite wafers with a Cr blocking layer for broad and stable light detection in a harsh environment

Herein, ultrathin (∼35 μm) CH3NH3PbI3 (MAPbI3) single-crystalline wafers have been successfully prepared by using an appropriate geometry-regulated dynamic-flow reaction system. The measurement results proved that the obtained wafers have high crystallinity, and showed broad light absorption from ultraviolet to near infrared (850 nm) which can be attributed to the indirect bandgap. Straight after, such an MAPbI3 wafer was used to fabricate high-quality photodetectors (PDs). On account of its faster carrier transport and significantly reduced defect density, the device exhibits a high photoresponse (R) of 5 A/W and short on/off response (0.039 s/0.017 s). Interestingly, by introducing a Cr interlayer between the MAPbI3 wafer and the Au electrode to avoid the migration of Au, the PD shows nearly no degradation when it works at 200 °C. Furthermore, the device performance shows very little degradation over the course of 60 days of storage under ambient conditions owing to its lack of grain boundaries. We believe the strategy reported here is very promising for achieving broad photodetection in a harsh environment.

Electrons, Excitons, and Phonons in Two-Dimensional Hybrid Perovskites: Connecting Structural, Optical, and Electronic Properties

Two-dimensional (2D) hybrid perovskites are stoichiometric compounds consisting of alternating inorganic metal-halide sheets and organoammonium cationic layers. This materials class is widely tailorable in composition, structure, and dimensionality and is providing an intriguing playground for the solid-state chemistry and physics communities to uncover structure-property relationships. In this Perspective, we describe semiconducting 2D perovskites containing lead and tin halide inorganic frameworks. In these 2D perovskites, charges are typically confined to the inorganic framework because of strong quantum and dielectric confinement effects, and exciton binding energies are many times greater than kT at room temperature. We describe the role of the heavy atoms in the inorganic framework; the geometry and chemistry of organic cations; and the "softness" of the organic-inorganic lattice on the electronic structure and dynamics of electrons, excitons, and phonons that govern the physical properties of these materials.

All-Ambient Processed Binary CsPbBr3-CsPb2Br5 Perovskites with Synergistic Enhancement for High-Efficiency Cs-Pb-Br-Based Solar Cells

All-inorganic CsPbBr₃ perovskite solar cells display outstanding stability toward moisture, light soaking, and thermal stressing, demonstrating great potential in tandem solar cells and toward commercialization. Unfortunately, it is still challenging to prepare high-performance CsPbBr₃ films at moderate temperatures. Herein, a uniform, compact CsPbBr₃ film was fabricated using its quantum dot (QD)-based ink precursor. The film was then treated using thiocyanate ethyl acetate (EA) solution in all-ambient conditions to produce a superior CsPbBr₃-CsPb₂Br₅ composite film with a larger grain size and minimal defects. The achievement was attributed to the surface dissolution and recrystallization of the existing SCN^- and EA. More specifically, the SCN^- ions were first absorbed on the Pb atoms, leading to the dissolution and stripping of Cs⁺ and Br^- ions from the CsPbBr₃ QDs. On the other hand, the EA solution enhances the diffusion dynamics of surface atoms and the surfactant species. It is found that a small amount of CsPb₂Br₅ in the composite film gives the best surface passivation, while the Br-rich surface decreases Br vacancies (V_Br) for a prolonged carrier lifetime. As a result, the fabricated device gives a higher solar cell efficiency of 6.81% with an outstanding long-term stability.

Structural effects on optoelectronic properties of halide perovskites

This review gives a perspective on different synthetic methodologies for the preparation of halide perovskites and highlights the structural effects on their optoelectronic properties.

Impact of Interfacial Layers in Perovskite Solar Cells

Perovskite solar cells (PCSs) are composed of organic-inorganic lead halide perovskite as the light harvester. Since the first report on a long-term-durable, 9.7 % efficient, solid-state perovskite solar cell, organic-inorganic halide perovskites have received considerable attention because of their excellent optoelectronic properties. As a result, a power conversion efficiency (PCE) exceeding 22 % was certified. Controlling the grain size, grain boundary, morphology, and defects of the perovskite layer is important for achieving high efficiency. In addition, interfacial engineering is equally or more important to further improve the PCE through better charge collection and a reduction in charge recombination. In this Review, the type of interfacial layers and their impact on photovoltaic performance are investigated for both the normal and the inverted cell architectures. Four different interfaces of fluorine-doped tin oxide (FTO)/electron-transport layer (ETL), ETL/perovskite, perovskite/hole-transport layer (HTL), and HTL/metal are classified, and their roles are investigated. The effects of interfacial engineering with organic or inorganic materials on photovoltaic performance are described in detail. Grain-boundary engineering is also included because it is related to interfacial engineering and the grain boundary in the perovskite layer plays an important role in charge conduction, recombination, and chargecarrier life time.

Phonon Speed, Not Scattering, Differentiates Thermal Transport in Lead Halide Perovskites

Thermal management plays a critical role in the design of solid state materials for energy conversion. Lead halide perovskites have emerged as promising candidates for photovoltaic, thermoelectric, and optoelectronic applications, but their thermal properties are still poorly understood. Here, we report on the thermal conductivity, elastic modulus, and sound speed of a series of lead halide perovskites MAPbX₃ (X = Cl, Br, I), CsPbBr₃, and FAPbBr₃ (MA = methylammonium, FA = formamidinium). Using frequency domain thermoreflectance, we find that the room temperature thermal conductivities of single crystal lead halide perovskites range from 0.34 to 0.73 W/m·K and scale with sound speed. These results indicate that regardless of composition, thermal transport arises from acoustic phonons having similar mean free path distributions. A modified Callaway model with Born von Karmen-based acoustic phonon dispersion predicts that at least ∼70% of thermal conductivity results from phonons having mean free paths shorter than 100 nm, regardless of whether resonant scattering is invoked. Hence, nanostructures or crystal grains with dimensions smaller than 100 nm will appreciably reduce thermal transport. These results are important design considerations to optimize future lead halide perovskite-based photovoltaic, optoelectronic, and thermoelectric devices.