Role of Microstructure in the Electron-Hole Interaction of Hybrid Lead-Halide Perovskites

Solar cells based on hybrid inorganic-organic halide perovskites have demonstrated high power conversion efficiencies in a range of architectures. The existence and stability of bound electron-hole pairs in these materials, and their role in the exceptional performance of optoelectronic devices, remains a controversial issue. Here we demonstrate, through a combination of optical spectroscopy and multiscale modeling as a function of the degree of polycrystallinity and temperature, that the electron-hole interaction is sensitive to the microstructure of the material. The long-range order is disrupted by polycrystalline disorder and the variations in electrostatic potential found for smaller crystals suppress exciton formation, while larger crystals of the same composition demonstrate an unambiguous excitonic state. We conclude that fabrication procedures and morphology strongly influence perovskite behaviour, with both free carrier and excitonic regimes possible, with strong implications for optoelectronic devices.

Ultrafast Interfacial Electron and Hole Transfer from CsPbBr3 Perovskite Quantum Dots

Recently reported colloidal lead halide perovskite quantum dots (QDs) with tunable photoluminescence (PL) wavelengths covering the whole visible spectrum and exceptionally high PL quantum yields (QYs, 50-90%) constitute a new family of functional materials with potential applications in light-harvesting and -emitting devices. By transient absorption spectroscopy, we show that the high PL QYs (∼79%) can be attributed to negligible electron or hole trapping pathways in CsPbBr3 QDs: ∼94% of lowest excitonic states decayed with a single-exponential time constant of 4.5 ± 0.2 ns. Furthermore, excitons in CsPbBr3 QDs can be efficiently dissociated in the presence of electron or hole acceptors. The half-lives of electron transfer (ET) to benzoquinone and subsequent charge recombination are 65 ± 5 ps and 2.6 ± 0.4 ns, respectively. The half-lives for hole transfer (HT) to phenothiazine and the subsequent charge recombination are 49 ± 6 ps and 1.0 ± 0.2 ns, respectively. The lack of electron and hole traps and fast interfacial ET and HT rates are key properties that may enable the development of efficient lead halide perovskite QDs-based light-harvesting and -emitting devices.

Wafer-scale growth of large arrays of perovskite microplate crystals for functional electronics and optoelectronics

Methylammonium lead iodide perovskite has attracted intensive interest for its diverse optoelectronic applications. However, most studies to date have been limited to bulk thin films that are difficult to implement for integrated device arrays because of their incompatibility with typical lithography processes. We report the first patterned growth of regular arrays of perovskite microplate crystals for functional electronics and optoelectronics. We show that large arrays of lead iodide microplates can be grown from an aqueous solution through a seeded growth process and can be further intercalated with methylammonium iodide to produce perovskite crystals. Structural and optical characterizations demonstrate that the resulting materials display excellent crystalline quality and optical properties. We further show that perovskite crystals can be selectively grown on prepatterned electrode arrays to create independently addressable photodetector arrays and functional field effect transistors. The ability to grow perovskite microplates and to precisely place them at specific locations offers a new material platform for the fundamental investigation of the electronic and optical properties of perovskite materials and opens a pathway for integrated electronic and optoelectronic systems.

CH3NH3PbCl3 Single Crystals: Inverse Temperature Crystallization and Visible-Blind UV-Photodetector

Single crystals of hybrid perovskites have shown remarkably improved physical properties compared to their polycrystalline film counterparts, underscoring their importance in the further development of advanced semiconductor devices. Here we present a new method of growing sizable CH3NH3PbCl3 single crystals based on the retrograde solubility behavior of hybrid perovskites. We show, for the first time, the energy band structure, charge recombination, and transport properties of CH3NH3PbCl3 single crystals. These crystals exhibit trap-state density, charge carrier concentration, mobility, and diffusion length comparable with the best quality crystals of methylammonium lead iodide or bromide perovskites reported so far. The high quality of the crystal along with its suitable optical band gap enabled us to build an efficient visible-blind UV-photodetector, demonstrating its potential in optoelectronic applications.

Two-Photon Absorption in Organometallic Bromide Perovskites

Organometallic trihalide perovskites are solution-processed semiconductors that have made great strides in third-generation thin film light-harvesting and light-emitting optoelectronic devices. Recently, it has been demonstrated that large, high-purity single crystals of these perovskites can be synthesized from the solution phase. These crystals' large dimensions, clean bandgap, and solid-state order have provided us with a suitable medium to observe and quantify two-photon absorption in perovskites. When CH3NH3PbBr3 single crystals are pumped with intense 800 nm light, we observe band-to-band photoluminescence at 572 nm, indicative of two-photon absorption. We report the nonlinear absorption coefficient of CH3NH3PbBr3 perovskites to be 8.6 cm GW(-1) at 800 nm, comparable to epitaxial single-crystal semiconductors of similar bandgap. We have leveraged this nonlinear process to electrically autocorrelate a 100 fs pulsed laser using a two-photon perovskite photodetector. This work demonstrates the viability of organometallic trihalide perovskites as a convenient and low-cost nonlinear absorber for applications in ultrafast photonics.

Mixed Iodide-Bromide Methylammonium Lead Perovskite-based Diodes for Light Emission and Photovoltaics

Vacuum deposition techniques are used to prepare mixed iodide-bromide methylammonium lead perovskite diodes via an intermediate double layer of the pure iodide and bromide perovskites. The diodes lead to bright electroluminescence, whose emission spectra maxima shift from the infrared toward the visible with increasing bromide content. When illuminated with AM1.5 simulated sunlight the devices function as efficient solar cells with power conversion efficiencies as high as 12.9%.

Single-Layer Light-Emitting Diodes Using Organometal Halide Perovskite/Poly(ethylene oxide) Composite Thin Films

Organometal halide perovskite and poly(ethylene oxide) composite thin films are studied. Single-layer light-emitting diodes using the composite thin film sandwiched between indium tin oxide and indium-gallium eutectic alloy exhibit a low turn-on voltage and high brightness because of the ionic conductivity of the composite film and the formation of a p-i-n homojunction.

Two-Inch-Sized Perovskite CH3 NH3 PbX3 (X = Cl, Br, I) Crystals: Growth and Characterization

Two-inch-sized perovskite crystals, CH3 NH3 PbX3 (X=I, Br, Cl), with high crystalline quality are prepared by a solution-grown strategy. The availability of large perovskite crystals is expected to transform its broad applications in photovoltaics, optoelectronics, lasers, photodetectors, LEDs, etc., just as crystalline silicon has done in revolutionizing the modern electronics and photovoltaic industries.

Energetics and dynamics in organic-inorganic halide perovskite photovoltaics and light emitters

The rapid transcendence of organic-inorganic metal halide perovskite solar cells to above the 20% efficiency mark has captivated the broad photovoltaic community. As the efficiency race continues unabated, it is essential that fundamental studies keep pace with these developments. Further gains in device efficiencies are expected to be increasingly arduous and harder to come by. The key to driving the perovskite solar cell efficiencies towards their Shockley-Queisser limit is through a clear understanding of the interfacial energetics and dynamics between perovskites and other functional materials in nanostructured- and heterojunction-type devices. In this review, we focus on the current progress in basic characterization studies to elucidate the interfacial energetics (energy-level alignment and band bending) and dynamical processes (from the ultrafast to the ultraslow) in organic-inorganic metal halide perovskite photovoltaics and light emitters. Major findings from these studies will be distilled. Open questions and scientific challenges will also be highlighted.

Solvent-Mediated Crystallization of CH3NH3SnI3 Films for Heterojunction Depleted Perovskite Solar Cells

Organo-lead halide perovskite solar cells have gained enormous significance and have now achieved power conversion efficiencies of ∼20%. However, the potential toxicity of lead in these systems raises environmental concerns for widespread deployment. Here we investigate solvent effects on the crystallization of the lead-free methylammonium tin triiodide (CH3NH3SnI3) perovskite films in a solution growth process. Highly uniform, pinhole-free perovskite films are obtained from a dimethyl sulfoxide (DMSO) solution via a transitional SnI2·3DMSO intermediate phase. This high-quality perovskite film enables the realization of heterojunction depleted solar cells based on mesoporous TiO2 layer but in the absence of any hole-transporting material with an unprecedented photocurrent up to 21 mA cm(-2). Charge extraction and transient photovoltage decay measurements reveal high carrier densities in the CH3NH3SnI3 perovskite device which are one order of magnitude larger than CH3NH3PbI3-based devices but with comparable recombination lifetimes in both devices. The relatively high background dark carrier density of the Sn-based perovskite is responsible for the lower photovoltaic efficiency in comparison to the Pb-based analogues. These results provide important progress toward achieving improved perovskite morphology control in realizing solution-processed highly efficient lead-free perovskite solar cells.

Low-threshold amplified spontaneous emission and lasing from colloidal nanocrystals of caesium lead halide perovskites

Metal halide semiconductors with perovskite crystal structures have recently emerged as highly promising optoelectronic materials. Despite the recent surge of reports on microcrystalline, thin-film and bulk single-crystalline metal halides, very little is known about the photophysics of metal halides in the form of uniform, size-tunable nanocrystals. Here we report low-threshold amplified spontaneous emission and lasing from ∼10 nm monodisperse colloidal nanocrystals of caesium lead halide perovskites CsPbX₃ (X=Cl, Br or I, or mixed Cl/Br and Br/I systems). We find that room-temperature optical amplification can be obtained in the entire visible spectral range (440–700 nm) with low pump thresholds down to 5±1 μJ cm⁻² and high values of modal net gain of at least 450±30 cm⁻¹. Two kinds of lasing modes are successfully observed: whispering-gallery-mode lasing using silica microspheres as high-finesse resonators, conformally coated with CsPbX₃ nanocrystals and random lasing in films of CsPbX₃ nanocrystals.

Lead halide perovskite colloidal nanocrystals have promising optoelectronic properties, such as high photoluminescence quantum yields and narrow emission linewidths. Here, the authors report low-threshold amplified spontaneous emission and two kinds of lasing in nanostructured caesium lead halide perovskites.

Growth and Anion Exchange Conversion of CH3NH3PbX3 Nanorod Arrays for Light-Emitting Diodes

The nanowire and nanorod morphology offers great advantages for application in a range of optoelectronic devices, but these high-quality nanorod arrays are typically based on high temperature growth techniques. Here, we demonstrate the successful room temperature growth of a hybrid perovskite (CH3NH3PbBr3) nanorod array, and we also introduce a new low temperature anion exchange technique to convert the CH3NH3PbBr3 nanorod array into a CH3NH3PbI3 nanorod array while preserving morphology. We demonstrate the application of both these hybrid perovskite nanorod arrays for LEDs. This work highlights the potential utility of postsynthetic interconversion of hybrid perovskites for nanostructured optoelectronic devices such as LEDs, which enables new strategies for the application of hybrid perovskites.

Low surface recombination velocity in solution-grown CH3NH3PbBr3 perovskite single crystal

Organic-inorganic hybrid perovskites are attracting intense research effort due to their impressive performance in solar cells. While the carrier transport parameters such as mobility and bulk carrier lifetime shows sufficient characteristics, the surface recombination, which can have major impact on the solar cell performance, has not been studied. Here we measure surface recombination dynamics in CH₃NH₃PbBr₃ perovskite single crystals using broadband transient reflectance spectroscopy. The surface recombination velocity is found to be 3.4±0.1 × 10³ cm s⁻¹, ∼2–3 orders of magnitude lower than that in many important unpassivated semiconductors employed in solar cells. Our result suggests that the planar grain size for the perovskite thin films should be larger than ∼30 μm to avoid the influence of surface recombination on the effective carrier lifetime.

Surface recombination velocity can have a major impact on solar cell performance. Here, Yang et al. measure surface recombination dynamics in perovskite single crystals using broadband transient reflectance spectroscopy. Grain size is crucial to avoid the effects of surface recombination on carrier lifetime.

Controlling CH3NH3PbI(3-x)Cl(x) Film Morphology with Two-Step Annealing Method for Efficient Hybrid Perovskite Solar Cells

The methylammonium lead halide perovskite solar cells have become very attractive because they can be prepared with low-cost solution-processable technology and their power conversion efficiency have been increasing from 3.9% to 20% in recent years. However, the high performance of perovskite photovoltaic devices are dependent on the complicated process to prepare compact perovskite films with large grain size. Herein, a new method is developed to achieve excellent CH3NH3PbI3-xClx film with fine morphology and crystallization based on one step deposition and two-step annealing process. This method include the spin coating deposition of the perovskite films with the precursor solution of PbI2, PbCl2, and CH3NH3I at the molar ratio 1:1:4 in dimethylformamide (DMF) and the post two-step annealing (TSA). The first annealing is achieved by solvent-induced process in DMF to promote migration and interdiffusion of the solvent-assisted precursor ions and molecules and realize large size grain growth. The second annealing is conducted by thermal-induced process to further improve morphology and crystallization of films. The compact perovskite films are successfully prepared with grain size up to 1.1 μm according to SEM observation. The PL decay lifetime, and the optic energy gap for the film with two-step annealing are 460 ns and 1.575 eV, respectively, while they are 307 and 327 ns and 1.577 and 1.582 eV for the films annealed in one-step thermal and one-step solvent process. On the basis of the TSA process, the photovoltaic devices exhibit the best efficiency of 14% under AM 1.5G irradiation (100 mW·cm(-2)).

Mobile Charge-Induced Fluorescence Intermittency in Methylammonium Lead Bromide Perovskite

Organic-inorganic halide perovskite has emerged as a very promising material for solar cells due to its excellent photovoltaic enabling properties resulting in rapid increase in device efficiency over the last 3 years. Extensive knowledge and in-depth physical understanding in the excited state carrier dynamics are urgently required. Here we investigate the fluorescence intermittency (also known as blinking) in vapor-assisted fabricated CH3NH3PbBr3 perovskite. The evident fluorescence blinking is observed in a dense CH3NH3PbBr3 perovskite film that is composed of nanoparticles in close contact with each other. In the case of an isolated nanoparticle no fluorescence blinking is observed. The "ON" probability of fluorescence is dependent on the excitation intensity and exhibits a similar power rule to semiconductor quantum dots at higher excitation intensity. As the vapor-assisted fabricated CH3NH3PbBr3 perovskite film is a cluster of nanoparticles forming a dense film, it facilitates mobile charge migration between the nanoparticles and charge accumulation at the surface or at the boundary of the nanoparticles. This leads to enhanced Auger-like nonradiative recombination contributing to the fluorescence intermittency observed. This finding provides unique insight into the charge accumulation and migration and thus is of crucial importance for device design and improvement.

Heterogeneous Charge Carrier Dynamics in Organic-Inorganic Hybrid Materials: Nanoscale Lateral and Depth-Dependent Variation of Recombination Rates in Methylammonium Lead Halide Perovskite Thin Films

We reveal substantial luminescence yield heterogeneity among individual subdiffraction grains of high-performing methylammonium lead halide perovskite films by using high-resolution cathodoluminescence microscopy. Using considerably lower accelerating voltages than is conventional in scanning electron microscopy, we image the electron beam-induced luminescence of the films and statistically characterize the depth-dependent role of defects that promote nonradiative recombination losses. The highest variability in the luminescence intensity is observed at the exposed grain surfaces, which we attribute to surface defects. By probing deeper into the film, it appears that bulk defects are more homogeneously distributed. By identifying the origin and variability of a surface-specific loss mechanism that deleteriously impacts device efficiency, we suggest that producing films homogeneously composed of the highest-luminescence grains found in this study could result in a dramatic improvement of overall device efficiency. We also show that although cathodoluminescence microscopy is generally used only to image inorganic materials it can be a powerful tool to investigate radiative and nonradiative charge carrier recombination on the nanoscale in organic-inorganic hybrid materials.

High-quality bulk hybrid perovskite single crystals within minutes by inverse temperature crystallization

Single crystals of methylammonium lead trihalide perovskites (MAPbX₃; MA=CH₃NH₃⁺, X=Br⁻ or I⁻) have shown remarkably low trap density and charge transport properties; however, growth of such high-quality semiconductors is a time-consuming process. Here we present a rapid crystal growth process to obtain MAPbX₃ single crystals, an order of magnitude faster than previous reports. The process is based on our observation of the substantial decrease of MAPbX₃ solubility, in certain solvents, at elevated temperatures. The crystals can be both size- and shape-controlled by manipulating the different crystallization parameters. Despite the rapidity of the method, the grown crystals exhibit transport properties and trap densities comparable to the highest quality MAPbX₃ reported to date. The phenomenon of inverse or retrograde solubility and its correlated inverse temperature crystallization strategy present a major step forward for advancing the field on perovskite crystallization.

Hybrid perovskites are a promising class of materials for photovoltaic applications. Here, addressing the need for high-quality hybrid perovskite materials, the authors achieve the rapid growth of hybrid perovskite single crystals of high quality by inverse temperature crystallization.

How Important Is the Organic Part of Lead Halide Perovskite Photovoltaic Cells? Efficient CsPbBr3 Cells

Hybrid organic-inorganic lead halide perovskite photovoltaic cells have already surpassed 20% conversion efficiency in the few years that they have been seriously studied. However, many fundamental questions still remain unanswered as to why they are so good. One of these is "Is the organic cation really necessary to obtain high quality cells?" In this study, we show that an all-inorganic version of the lead bromide perovskite material works equally well as the organic one, in particular generating the high open circuit voltages that are an important feature of these cells.

Large-Size CH3NH3PbBr3 Single Crystal: Growth and In Situ Characterization of the Photophysics Properties

We reported a facile single-solution fabrication method to grow large-scale CH3NH3PbBr3 hybrid perovskite single crystal at room temperature. The obtained single crystal in this experiment was 14 × 14 mm. The sample's in situ photophysics properties under dark and illumination, including the surface morphology, work function, surface current distribution, microcosmic I-V curves, as well as the polarization behavior, were in situ characterized by integrated utilization of a scanning probe microscopy, respectively. Piezoresponse force microscopy (PFM) phase angles indicated the existence of "polarization" in CH3NH3PbBr3 lattice. Interestingly, the "polarization effect" was enhanced by the plus light source. Moreover, a surface potential shift as large as 200 mV was observed under the condition of the illumination on and off. This research is proposed to provide an opportunity to take a fresh look at the architectural design and photovoltaic performance origin of the hybrid perovskite solar cells.

Detection of X-ray photons by solution-processed organic-inorganic perovskites

The evolution of real-time medical diagnostic tools such as angiography and computer tomography from radiography based on photographic plates was enabled by the development of integrated solid-state X-ray photon detectors, based on conventional solid-state semiconductors. Recently, for optoelectronic devices operating in the visible and near infrared spectral regions, solution-processed organic and inorganic semiconductors have also attracted immense attention. Here we demonstrate a possibility to use such inexpensive semiconductors for sensitive detection of X-ray photons by direct photon-to-current conversion. In particular, methylammonium lead iodide perovskite (CH₃NH₃PbI₃) offers a compelling combination of fast photoresponse and a high absorption cross-section for X-rays, owing to the heavy Pb and I atoms. Solution processed photodiodes as well as photoconductors are presented, exhibiting high values of X-ray sensitivity (up to 25 µC mGy_air^-1 cm^-3) and responsivity (1.9×10⁴ carriers/photon), which are commensurate with those obtained by the current solid-state technology.

Facile route to freestanding CH3NH3PbI3 crystals using inverse solubility

CH₃NH₃PbI₃ was found to exhibit inverse solubility at high temperatures in γ-butyrolactone. Making use of this unusual, so far unreported phenomenon, we present a facile method for the growth of freestanding crystals of CH₃NH₃PbI₃ from solution without addition of any capping agents or seed particles. Large, strongly faceted crystals could be grown within minutes. This finding may aid in understanding the crystallization process of CH₃NH₃PbI₃ from solution that may lead to improved morphological control of film deposition for a range of device architectures. Our process offers a facile and rapid route to freestanding crystals for use in a broad range of characterization techniques.

Lead iodide perovskite light-emitting field-effect transistor

Despite the widespread use of solution-processable hybrid organic–inorganic perovskites in photovoltaic and light-emitting applications, determination of their intrinsic charge transport parameters has been elusive due to the variability of film preparation and history-dependent device performance. Here we show that screening effects associated to ionic transport can be effectively eliminated by lowering the operating temperature of methylammonium lead iodide perovskite (CH₃NH₃PbI₃) field-effect transistors. Field-effect carrier mobility is found to increase by almost two orders of magnitude below 200 K, consistent with phonon scattering-limited transport. Under balanced ambipolar carrier injection, gate-dependent electroluminescence is also observed from the transistor channel, with spectra revealing the tetragonal to orthorhombic phase transition. This demonstration of CH₃NH₃PbI₃ light-emitting field-effect transistors provides intrinsic transport parameters to guide materials and solar cell optimization, and will drive the development of new electro-optic device concepts, such as gated light-emitting diodes and lasers operating at room temperature.

Hybrid organic–inorganic perovskites have shown great potential for use in optoelectronic applications. Here, the authors create solution-processed lead iodide perovskite light-emitting field-effect transistors and demonstrate both ambipolar behaviour and gate-assisted electroluminescence.

Spatial and temporal imaging of long-range charge transport in perovskite thin films by ultrafast microscopy

Charge carrier diffusion coefficient and length are important physical parameters for semiconducting materials. Long-range carrier diffusion in perovskite thin films has led to remarkable solar cell efficiencies; however, spatial and temporal mechanisms of charge transport remain unclear. Here we present a direct measurement of carrier transport in space and in time by mapping carrier density with simultaneous ultrafast time resolution and ∼50-nm spatial precision in perovskite thin films using transient absorption microscopy. These results directly visualize long-range carrier transport of ∼220 nm in 2 ns for solution-processed polycrystalline CH3NH3PbI3 thin films. Variations of the carrier diffusion coefficient at the μm length scale have been observed with values ranging between 0.05 and 0.08 cm(2) s(-1). The spatially and temporally resolved measurements reported here underscore the importance of the local morphology and establish an important first step towards discerning the underlying transport properties of perovskite materials.

Identification of Trap States in Perovskite Solar Cells

Thermally stimulated current (TSC) measurements are used to characterize electronic trap states in methylammonium lead iodide perovsite solar cells. Several TSC peaks were observed over the temperature range from 20 K to room temperature. To elucidate the origins of these peaks, devices with various organic charge transport layers and devices without transport layers were tested. Two peaks appear at very low temperatures, indicating shallow trap states that are mainly attributed to the PCBM/C60 electron transport bilayer. However, two additional peaks appear at higher temperatures, that is, they are deeper in energy, and are assigned to the perovskite layer. At around T = 163 K, a sharp peak, also present in the dark TSC measurements, is assigned to the orthorhombic-tetragonal phase transition in the perovskite. However, a peak at around T = 191 K is assigned to trap states with activation energies of around 500 meV but with a rather low concentration of 1 × 10(21) m(-3).

Perovskite solar cells prepared by flash evaporation

A simple vacuum deposition method for the preparation of high quality hybrid organic-inorganic methylammonium lead iodide perovskite thin films is reported. When sandwiched in between organic charge transporting layers, such films lead to solar cells with a power conversion efficiency of 12.2%.

Multifaceted Excited State of CH3NH3PbI3. Charge Separation, Recombination, and Trapping

A need to understand the excited-state behavior of organic-inorganic hybrid perovskites, such as CH3NH3PbI3, has arisen due to the rapid development of perovskite solar cells. The photoinduced processes leading to the efficient charge separation observed in these materials remain somewhat elusive. This Perspective presents an overview of the initial attempts to characterize the excited-state and charge recombination dynamics in the prototypical material CH3NH3PbI3. While much has been accomplished in designing high-efficiency solar cells, the multifaceted nature of the CH3NH3PbI3 excited state offers ample challenges for the photovoltaic community to better comprehend. Building on this foundation may enable us to tackle the stability concerns that have shadowed the rise of perovskite solar cells. Furthermore, a better understanding of the excited-state properties can provide insight into the specific properties that have thrust this material to the forefront of photovoltaic research.

The dynamics of methylammonium ions in hybrid organic-inorganic perovskite solar cells

Methylammonium lead iodide perovskite can make high-efficiency solar cells, which also show an unexplained photocurrent hysteresis dependent on the device-poling history. Here we report quasielastic neutron scattering measurements showing that dipolar CH₃NH₃⁺ ions reorientate between the faces, corners or edges of the pseudo-cubic lattice cages in CH₃NH₃PbI₃ crystals with a room temperature residence time of ∼14 ps. Free rotation, π-flips and ionic diffusion are ruled out within a 1–200-ps time window. Monte Carlo simulations of interacting CH₃NH₃⁺ dipoles realigning within a 3D lattice suggest that the scattering measurements may be explained by the stabilization of CH₃NH₃⁺ in either antiferroelectric or ferroelectric domains. Collective realignment of CH₃NH₃⁺ to screen a device's built-in potential could reduce photovoltaic performance. However, we estimate the timescale for a domain wall to traverse a typical device to be ∼0.1–1 ms, faster than most observed hysteresis.

Hysteresis often exists in the characterization of methylammonium lead halide-based solar cells, but is not well understood. Here, the authors use quasielastic neutron scattering to study the dynamics of dipolar organic cations and shed light on the hysteresis behaviour.

Metal-halide perovskites for photovoltaic and light-emitting devices

Metal-halide perovskites are crystalline materials originally developed out of scientific curiosity. Unexpectedly, solar cells incorporating these perovskites are rapidly emerging as serious contenders to rival the leading photovoltaic technologies. Power conversion efficiencies have jumped from 3% to over 20% in just four years of academic research. Here, we review the rapid progress in perovskite solar cells, as well as their promising use in light-emitting devices. In particular, we describe the broad tunability and fabrication methods of these materials, the current understanding of the operation of state-of-the-art solar cells and we highlight the properties that have delivered light-emitting diodes and lasers. We discuss key thermal and operational stability challenges facing perovskites, and give an outlook of future research avenues that might bring perovskite technology to commercialization.

Interfacial Charge Transfer Anisotropy in Polycrystalline Lead Iodide Perovskite Films

Solar cells based on organic-inorganic lead iodide perovskite (CH3NH3PbI3) exhibit remarkably high power conversion efficiency (PCE). One of the key issues in solution-processed films is that often the polycrystalline domain orientation is not well-defined, which makes it difficult to predict energy alignment and charge transfer efficiency. Here we combine ab initio calculations and photoelectron spectroscopy to unravel the electronic structure and charge redistribution at the interface between different surfaces of CH3NH3PbI3 and typical organic hole acceptor Spiro-OMeTAD and electron acceptor PCBM. We find that both hole and electron interfacial transfer depend strongly on the CH3NH3PbI3 surface orientation: while the (001) and (110) surfaces tend to favor hole injection to Spiro-OMeTAD, the (100) surface facilitates electron transfer to PCBM due to surface delocalized charges and hole/electron accumulation at the CH3NH3PbI3/organic interfaces. Molecular dynamic simulations indicate that this is due to strong orbital interactions under thermal fluctuations at room temperature, suggesting the possibility to further improve charge separation and extraction in perovskite-based solar cells by controlling perovskite film crystallization and surface orientation.

Morphology control of the perovskite films for efficient solar cells

In the past two years, the power conversion efficiency (PCE) of organic–inorganic hybrid perovskite solar cells has significantly increased up to 20.1%.

Antisolvent diffusion-induced growth, equilibrium behaviours in aqueous solution and optical properties of CH3NH3PbI3 single crystals for photovoltaic applications

Antisolvent diffusion-induced growth, equilibrium behaviours in aqueous solution and optical properties of CH₃NH₃PbI₃ single crystals.

A large grain size perovskite thin film with a dense structure for planar heterojunction solar cells via spray deposition under ambient conditions

A facile spray deposition process was developed to prepare high-quality perovskite films with full surface coverage and large grain size.

The chemical origin of the p-type and n-type doping effects in the hybrid methylammonium–lead iodide (MAPbI3) perovskite solar cells

A field-induced and photoinduced self-doping chemistry of the MAPbI₃ perovskite films affecting their photovoltaic performance and stability is proposed.

Modulation of photovoltage in mesoscopic perovskite solar cell by controlled interfacial electron injection

The controlled electron injection by surface modification of mesoporous TiO₂in the mesoscopic perovskite solar cell plays important role in determining open-circuit voltage and performance of perovskite solar cell.

High-performance perovskite solar cells fabricated by vapor deposition with optimized PbI2 precursor films

High-performance CH₃NH₃PbI₃ solar cells were achieved by hybrid physical–chemical vapor deposition (HPCVD) method with optimized spin coating process for PbI₂ precursor films.

Thickness of the hole transport layer in perovskite solar cells: performance versus reproducibility

We systematically studied the effect of the thickness of a spiro-MeOTAD hole transport layer on photovoltaic properties and reproducibility. We find that an interplay exists between the photovoltaic performance and reproducibility.

Rotational dynamics of organic cations in the CH3NH3PbI3perovskite

Rotational dynamics of organic cations in the CH₃NH₃PbI₃perovskite are revealed by elastic and quasi-elastic neutron scattering and group theoretical analysis.

Cubic structure of the mixed halide perovskite CH3NH3PbI3−xClxvia thermal annealing

A methodology has been developed to obtain a cubic structure of the mixed halide perovskite CH₃NH₃PbI_3−xCl_x that involves thermal annealing of a vacuum-deposited perovskite layer.

Solid state transformation of the crystalline monohydrate (CH3NH3)PbI3(H2O) to the (CH3NH3)PbI3 perovskite

Crystals of (CH₃NH₃·H₂O)PbI₃ spontaneously transform to the black (CH₃NH₃)PbI₃ perovskite as a porous microcrystalline solid. The dihydrate (CH₃NH₃)₄PbI₆·2H₂O) requires more forcing conditions to produce (CH₃NH₃)PbI₃.

Retrograde solubility of formamidinium and methylammonium lead halide perovskites enabling rapid single crystal growth

Based on the right choice of solvent(s), the retrograde solubility of hybrid perovskites can be achieved enabling rapid inverse temperature crystallization.