Compositionally Graded Absorber for Efficient and Stable Near-Infrared-Transparent Perovskite Solar Cells

Compositional grading has been widely exploited in highly efficient Cu(In,Ga)Se2, CdTe, GaAs, quantum dot solar cells, and this strategy has the potential to improve the performance of emerging perovskite solar cells. However, realizing and maintaining compositionally graded perovskite absorber from solution processing is challenging. Moreover, the operational stability of graded perovskite solar cells under long-term heat/light soaking has not been demonstrated. In this study, a facile partial ion-exchange approach is reported to achieve compositionally graded perovskite absorber layers. Incorporating compositional grading improves charge collection and suppresses interface recombination, enabling to fabricate near-infrared-transparent perovskite solar cells with power conversion efficiency of 16.8% in substrate configuration, and demonstrate 22.7% tandem efficiency with 3.3% absolute gain when mechanically stacked on a Cu(In,Ga)Se2 bottom cell. Non-encapsulated graded perovskite device retains over 93% of its initial efficiency after 1000 h operation at maximum power point at 60 °C under equivalent 1 sun illumination. The results open an avenue in exploring partial ion-exchange to design graded perovskite solar cells with improved efficiency and stability.

Electric-Field-Induced Dynamic Electronic Junctions in Hybrid Organic-Inorganic Perovskites for Optoelectronic Applications

Organic-inorganic metal halide perovskites have attracted great attention as optoelectronic materials because of their low cost, relative insensitivity to defects, and solution-processible properties. However, some of their properties, such as thermal instability, toxicity, and current-voltage hysteresis still remain elusive. Ion migration, which has been proven to be a thermal-activated process, is regarded as one of the major origins of the hysteresis and thus detrimental to the long-term stability of the optoelectronic devices. Nevertheless, by using the external electric field to pole the perovskite, ion migration would be possible to be utilized to create dynamic electronic junctions. In this paper, electric-field-induced dynamic electronic junctions have been manipulated for photodetection and energy harvesting through the ion migration under external electric field. Ion-migration-induced p-n or n-p junction has been successfully created via tuning the polarity of the external applied voltage, which is used for photodetection with a relatively fast response. By freezing out of the nonuniformly distributed ions after migration at low temperature, we demonstrate that the ion-migration-induced dynamic junctions can function as an energy harvesting device with an external quantum efficiency of 20%.

Precursor non-stoichiometry to enable improved CH3NH3PbBr3 nanocrystal LED performance

High photoluminescence quantum yields and narrow emission wavelengths, combined with low temperature solution processing, make CH₃NH₃PbBr₃ nanocrystals (NCs) favorable candidates for light-emitting applications. Herein, we describe the synthesis of CH₃NH₃PbBr₃ NC inks by a convenient room-temperature ligand assisted reprecipitation protocol. We further investigate the effect of modulation of the CH₃NH₃Br : PbBr₂ ratio during NC synthesis on the optical properties, crystallinity, particle size distribution and film formation of the NC ink. Subsequently, we fabricate LEDs using these NCs as the emissive layer and the highest efficiency (1.75% external quantum efficiency) and brightness (>2700 cd m^-2) is achieved for the 1.15 : 1 precursor ratio. It is inferred that the NC surface properties and film coverage are more crucial than the photoluminescence intensity to achieve high device efficiency. Moreover, by separating the NC synthesis and thin film formation processes, we can exert more control during device fabrication, which makes it very promising for scale-up applications.

Synthetic Control over Quantum Well Width Distribution and Carrier Migration in Low-Dimensional Perovskite Photovoltaics

Metal halide perovskites have achieved photovoltaic efficiencies exceeding 22%, but their widespread use is hindered by their instability in the presence of water and oxygen. To bolster stability, researchers have developed low-dimensional perovskites wherein bulky organic ligands terminate the perovskite lattice, forming quantum wells (QWs) that are protected by the organic layers. In thin films, the width of these QWs exhibits a distribution that results in a spread of bandgaps in the material arising due to varying degrees of quantum confinement across the population. Means to achieve refined control over this QW width distribution, and to examine and understand its influence on photovoltaic performance, are therefore of intense interest. Here we show that moving to the ligand allylammonium enables a narrower distribution of QW widths, creating a flattened energy landscape that leads to ×1.4 and ×1.9 longer diffusion lengths for electrons and holes, respectively. We attribute this to reduced ultrafast shallow hole trapping that originates from the most strongly confined QWs. We observe an increased PCE of 14.4% for allylammonium-based perovskite QW photovoltaics, compared to 11-12% PCEs obtained for analogous devices using phenethylammonium and butylammonium ligands. We then optimize the devices using mixed-cation strategies, achieving 16.5% PCE for allylammonium devices. The devices retain 90% of their initial PCEs after >650 h when stored under ambient atmospheric conditions.

High-Performance Photodetectors Based on Solution-Processed Epitaxial Grown Hybrid Halide Perovskites

Hybrid organic-inorganic halide perovskites (HOIPs) have recently attracted tremendous attention because of their excellent semiconducting and optoelectronic properties, which exist despite their morphology and crystallinity being far inferior to those of more mature semiconductors, such as silicon and III-V compound semiconductors. Heteroepitaxy can provide a route to achieving high-performance HOIP devices when high crystalline quality and smooth morphology are required, but work on heteroepitaxial HOIPs has not previously been reported. Here, we demonstrate epitaxial growth of methylammonium lead iodide (MAPbI₃) on single crystal KCl substrates with smooth morphology and the highest carrier recombination lifetime (∼213 ns) yet reported for nonsingle crystalline MAPbI₃. Experimental Raman spectra agree well with theoretical calculations, presenting in particular a sharp peak at 290 cm^-1 for the torsional mode of the organic cations, a marker of orientational order and typically lacking in previous reports. Photodetectors were fabricated showing excellent performance, confirming the high quality of the epitaxial MAPbI₃ thin films. This work provides a new strategy to enhance the performance of all HOIPs-based devices.

High-Quality CH3NH3PbI3 Films Obtained via a Pressure-Assisted Space-Confined Solvent-Engineering Strategy for Ultrasensitive Photodetectors

High-quality organic-inorganic hybrid perovskite films are crucial for excellent performance of photoelectric devices. Herein, we demonstrate a pressure-assisted space-confined solvent-engineering strategy to grow highly oriented, pinhole-free thin films of CH₃NH₃PbI₃ with large-scale crystalline grains, high smoothness, and crystalline fusion on grain boundaries. These single-crystalline grains vertically span the entire film thickness. Such a film feature dramatically reduces recombination loss and then improves the transport property of charge carriers in the films. Consequently, the photodetector devices, based on the high-quality CH₃NH₃PbI₃ films, exhibit high photocurrent (105 μA under 671 nm laser with a power density of 20.6 mW/cm² at 10 V), good stability, and, especially, an ultrahigh on/off ratio (I_light/I_dark > 2.2 × 10⁴ under an incident light of 20.6 mW/cm²). These excellent performances indicate that the high-quality films will be potential candidates in other CH₃NH₃PbI₃-based photoelectric devices.

Nonreciprocal current from electron interactions in noncentrosymmetric crystals: roles of time reversal symmetry and dissipation

In noncentrosymmetric crystals with broken inversion symmetry [Formula: see text], the I-V (I: current, V: voltage) characteristic is generally expected to depend on the direction of I, which is known as nonreciprocal response and, for example, found in p-n junction. However, it is a highly nontrivial issue in translationally invariant systems since the time-reversal symmetry T plays an essential role, where the two states at crystal momenta k and -k are connected in the band structure. Therefore, it has been considered that the external magnetic field (B) or the magnetic order which breaks the T-symmetry is necessary to realize the nonreciprocal I-V characteristics, i.e., magnetochiral anisotropy. Here we theoretically show that the electron correlation in T-broken multi-band systems can induce nonreciprocal I-V characteristics without T-breaking. An analog of Onsager's relation shows that nonreciprocal current response without T -breaking generally requires two effects: dissipation and interactions. By using nonequilibrium Green's functions, we derive general formula of the nonreciprocal response for two-band systems with onsite interaction. The formula is applied to Rice-Mele model, a representative 1D model with inversion breaking, and some candidate materials are discussed. This finding offers a coherent understanding of the origin of nonreciprocal I-V characteristics, and will pave a way to design it.

Connecting the solution chemistry of PbI2 and MAI: a cyclodextrin-based supramolecular approach to the formation of hybrid halide perovskites

Cyclodextrin macrocycles are able to modify and control the solvation equilibria of hybrid perovskite components in solution by establishing supramolecular interactions.

The evolution from solvated precursors to hybrid halide perovskite films dictates most of the photophysical and optoelectronic properties of the final polycrystalline material. Specifically, the complex equilibria and the importantly different solubilities of lead iodide (PbI₂) and methylammonium iodide (MAI) induce inhomogeneous crystal growth, often leading to a defect dense film showing non-optimal optoelectronic properties and intrinsic instability. Here, we explore a supramolecular approach based on the use of cyclodextrins (CDs) to modify the underlying solution chemistry. The peculiar phenomenon demonstrated is a tunable complexation between different CDs and MA⁺ cations concurrent to an out of cage PbI₂ intercalation, representing the first report of a connection between the solvation equilibria of the two perovskite precursors. The optimal conditions in terms of CD cavity size and polarity translate to a neat enhancement of PbI₂ solubility in the reaction media, leading to an equilibration of the availability of the precursors in solution. The macroscopic result of this is an improved nucleation process, leading to a perovskite material with higher crystallinity, better optical properties and improved moisture resistance. Remarkably, the use of CDs presents a great potential for a wide range of device-related applications, as well as for the development of tailored composite materials.

Amine treatment induced perovskite nanowire network in perovskite solar cells: efficient surface passivation and carrier transport

In the fabrication of high efficiency organic-inorganic metal halide perovskite solar cells (PSCs), an additional interface modifier is usually applied for enhancing the interface passivation and carrier transport. In this paper, we develop an innovative method with in-situ growth of one-dimensional perovskite nanowire (1D PNW) network triggered by Lewis amine over the perovskite films. To our knowledge, this is the first time to fabricate PSCs with shape-controlled perovskite surface morphology, which improved power conversion efficiency (PCE) from 14.32% to 16.66% with negligible hysteresis. The amine molecule can passivate the trap states on the polycrystalline perovskite surface to reduce trap-state density. Meanwhile, as a fast channel, the 1D PNWs would promote carrier transport from the bulk perovskite film to the electron transport layer. The PSCs with 1D PNW modification not only exhibit excellent photovoltaic performances, but also show good stability with only 4% PCE loss within 30 days in the ambient air without encapsulation. Our results strongly suggest that in-situ grown 1D PNW network provides a feasible and effective strategy for nanostructured optoelectronic devices such as PSCs to achieve superior performances.

Pure zero-dimensional Cs4PbBr6 single crystal rhombohedral microdisks with high luminescence and stability

Zero-dimensional (0D) perovskite Cs₄PbBr₆ has been speculated to be an efficient solid-state emitter, exhibiting strong luminescense on achieving quantum confinement. Although several groups have reported strong green luminescence from Cs₄PbBr₆ powders and nanocrystals, doubts that the origin of luminescence comes from Cs₄PbBr₆ itself or CsPbBr₃ impurities have been a point of controversy in recent investigations. Herein, we developed a facile one-step solution self-assembly method to synthesize pure zero-dimensional rhombohedral Cs₄PbBr₆ micro-disks (MDs) with a high PLQY of 52% ± 5% and photoluminescence full-width at half maximum (FWHM) of 16.8 nm. The obtained rhombohedral MDs were high quality single-crystalline as demonstrated by XRD and SAED patterns. We demonstrated that Cs₄PbBr₆ MDs and CsPbBr₃ MDs were phase-separated from each other and the strong green emission comes from Cs₄PbBr₆. Power and temperature dependence spectra evidenced that the observed strong green luminescence of pure Cs₄PbBr₆ MDs originated from direct exciton recombination in the isolated octahedra with a large binding energy of 303.9 meV. Significantly, isolated PbBr₆^4- octahedra separated by a Cs⁺ ion insert in the crystal lattice is beneficial to maintaining the structural stability, depicting superior thermal and anion exchange stability. Our study provides an efficient approach to obtain high quality single-crystalline Cs₄PbBr₆ MDs with highly efficient luminescence and stability for further optoelectronic applications.

Near-Band-Edge Optical Responses of CH_{3}NH_{3}PbCl_{3} Single Crystals: Photon Recycling of Excitonic Luminescence

The determination of the band gap and exciton energies of lead halide perovskites is very important from the viewpoint of fundamental physics and photonic device applications. By using photoluminescence excitation (PLE) spectra, we reveal the optical properties of CH_{3}NH_{3}PbCl_{3} single crystals in the near-band-edge energy regime. The one-photon PLE spectrum exhibits the 1s exciton peak at 3.11 eV. On the contrary, the two-photon PLE spectrum exhibits no peak structure. This indicates photon recycling of excitonic luminescence. By analyzing the spatial distribution of the excitons and photon recycling, we obtain 3.15 eV for the band gap energy and 41 meV for the exciton binding energy.

Formation and Diffusion of Metal Impurities in Perovskite Solar Cell Material CH3NH3PbI3: Implications on Solar Cell Degradation and Choice of Electrode

Solar cells based on methylammonium lead triiodide (MAPbI3) have shown remarkable progress in recent years and have demonstrated efficiencies greater than 20%. However, the long-term stability of MAPbI3-based solar cells has yet to be achieved. Besides the well-known chemical and thermal instabilities, significant native ion migration in lead halide perovskites leads to current-voltage hysteresis and photoinduced phase segregation. Recently, it is further revealed that, despite having excellent chemical stability, the Au electrode can cause serious solar cell degradation due to Au diffusion into MAPbI3. In addition to Au, many other metals have been used as electrodes in MAPbI3 solar cells. However, how the external metal impurities introduced by electrodes affect the long-term stability of MAPbI3 solar cells has rarely been studied. A comprehensive study of formation energetics and diffusion dynamics of a number of noble and transition metal impurities (Au, Ag, Cu, Cr, Mo, W, Co, Ni, Pd) in MAPbI3 based on first-principles calculations is reported herein. The results uncover important general trends of impurity formation and diffusion in MAPbI3 and provide useful guidance for identifying the optimal metal electrodes that do not introduce electrically active impurity defects in MAPbI3 while having low resistivities and suitable work functions for carrier extraction.

Perovskite Quantum Dots with Near Unity Solution and Neat-Film Photoluminescent Quantum Yield by Novel Spray Synthesis

In this study, a novel perovskite quantum dot (QD) spray-synthesis method is developed by combining traditional perovskite QD synthesis with the technique of spray pyrolysis. By utilizing this new technique, the synthesis of cubic-shaped perovskite QDs with a homogeneous size of 14 nm is demonstrated, which shows an unprecedented stable absolute photoluminescence quantum yield ≈100% in the solution and even in the solid-state neat film. The highly emissive thin films are integrated with light emission devices (LEDs) and organic light emission displays (OLEDs). The color conversion type QD-LED (ccQD-LED) hybrid devices exhibit an extremely saturated green emission, excellent external quantum efficiency of 28.1%, power efficiency of 121 lm W^-1 , and extraordinary forward-direction luminescence of 8 500 000 cd m^-2 . The conceptual ccQD-OLED hybrid display also successfully demonstrates high-definition still images and moving pictures with a 119% National Television System Committee 1931 color gamut and 123% Digital Cinema Initiatives-P3 color gamut. These very-stable, ultra-bright perovskite QDs have the properties necessary for a variety of useful applications in optoelectronics.

Spinel Co3O4 nanomaterials for efficient and stable large area carbon-based printed perovskite solar cells

Carbon based perovskite solar cells (PSCs) are fabricated through easily scalable screen printing techniques, using abundant and cheap carbon to replace the hole transport material (HTM) and the gold electrode further reduces costs, and carbon acts as a moisture repellent that helps in maintaining the stability of the underlying perovskite active layer. An inorganic interlayer of spinel cobaltite oxides (Co₃O₄) can greatly enhance the carbon based PSC performance by suppressing charge recombination and extracting holes efficiently. The main focus of this research work is to investigate the effectiveness of Co₃O₄ spinel oxide as the hole transporting interlayer for carbon based perovskite solar cells (PSCs). In these types of PSCs, the power conversion efficiency (PCE) is restricted by the charge carrier transport and recombination processes at the carbon-perovskite interface. The spinel Co₃O₄ nanoparticles are synthesized using the chemical precipitation method, and characterized by X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and UV-Vis spectroscopy. A screen printed thin layer of p-type inorganic spinel Co₃O₄ in carbon PSCs provides a better-energy level matching, superior efficiency, and stability. Compared to standard carbon PSCs (PCE of 11.25%) an improved PCE of 13.27% with long-term stability, up to 2500 hours under ambient conditions, is achieved. Finally, the fabrication of a monolithic perovskite module is demonstrated, having an active area of 70 cm² and showing a power conversion efficiency of >11% with virtually no hysteresis. This indicates that Co₃O₄ is a promising interlayer for efficient and stable large area carbon PSCs.

Lead-Free Hybrid Perovskite Absorbers for Viable Application: Can We Eat the Cake and Have It too?

Many years since the booming of research on perovskite solar cells (PSCs), the hybrid perovskite materials developed for photovoltaic application form three main categories since 2009: (i) high-performance unstable lead-containing perovskites, (ii) low-performance lead-free perovskites, and (iii) moderate performance and stable lead-containing perovskites. The search for alternative materials to replace lead leads to the second group of perovskite materials. To date, a number of these compounds have been synthesized and applied in photovoltaic devices. Here, lead-free hybrid light absorbers used in PV devices are focused and their recent developments in related solar cell applications are reviewed comprehensively. In the first part, group 14 metals (Sn and Ge)-based perovskites are introduced with more emphasis on the optimization of Sn-based PSCs. Then concerns on halide hybrids of group 15 metals (Bi and Sb) are raised, which are mainly perovskite derivatives. At the same time, transition metal Cu-based perovskites are also referred. In the end, an outlook is given on the design strategy of lead-free halide hybrid absorbers for photovoltaic applications. It is believed that this timely review can represent our unique view of the field and shed some light on the direction of development of such promising materials.

Alkoxybenzothiadiazole-Based Fullerene and Nonfullerene Polymer Solar Cells with High Shunt Resistance for Indoor Photovoltaic Applications

We synthesized three semicrystalline polymers (PTTBT_BO, PDTBT_BO, and P2FDTBT_BO) by modulating the intra- and intermolecular noncovalent Coulombic interactions and investigated their photovoltaic characteristics under various light intensities. Low series (R_s) and high shunt (R_sh) resistances are essential prerequisites for good device properties under standard illumination (100 mW cm^-2). Considering these factors, among three polymers, PDTBT_BO polymer solar cells (PSCs) exhibited the most desirable characteristics, with peak power conversion efficiencies (PCE) of 7.52 and 9.60% by being blended with PC₇₁BM under standard and dim light (2.5 mW cm^-2), respectively. P2FDTBT_BO PSCs exhibited a low PCE of 3.69% under standard light due to significant charge recombination with high R_s (9.42 Ω cm²). However, the PCE was remarkably improved by 2.3 times (8.33% PCE) under dim light, showing negligible decrease in open-circuit voltage and remarkable increase in fill factor, which is due to an exceptionally high R_sh of over 1000 kΩ cm². R_s is less significant under dim light because the generated current is too small to cause noticeable R_s-induced voltage losses. Instead, high R_sh becomes more important to avoid leakage currents. This work provides important tips to further optimize PSCs for indoor applications with low-power electronic devices such as Internet of things sensors.

Colloids of Naked CH3NH3PbBr3 Perovskite Nanoparticles: Synthesis, Stability, and Thin Solid Film Deposition

A novel preparation of lead halide, CH₃NH₃PbBr₃, perovskite nanoparticle solid films from colloidal "naked" nanoparticles, that is, dispersible nanoparticles without any surfactant, is reported. The colloids are obtained by simply adding potassium ions, whose counterions are both more lipophilic and less coordinating than bromide ions, to the perovskite precursor solutions (CH₃NH₃Br/PbBr₂ in dimethylformamide) following the reprecipitation strategy. The naked nanoparticles exhibit a low tendency to aggregate in solution, and they effectively self-assembled on a substrate by centrifugation of the colloid, leading to homogeneous nanoparticle solid films with arbitrary thickness. These results are expected to spur further the interest in lead halide perovskites due to the new opportunities offered by these films.

An optical dynamic study of MAPbBr3 single crystals passivated with MAPbCl3/I3-MAPbBr3 heterojunctions

Recently, perovskite based solar cells have attracted lots of research interest, some of which is in the passivation of perovskite surfaces, particularly the heterojunction based surface passivation. In this study, the optical dynamics of MAPbBr₃ single crystals with and without heterojunction passivation were studied systematically by means of a time-resolved spectroscopic technique for the first time. The emission lifetime of MAPbBr₃ single crystals under two-photon (1064 nm) excitation is a few orders of magnitude longer than that measured under one-photon (355 nm or 532 nm) excitation. Interestingly, with surface passivation, the lifetime measured at 355 nm excitations could be tuned significantly, whereas the lifetime change under 1064 nm excitations was considerably less. Our results give a direct evidence of surface quench by comparing the lifetimes before and after surface passivation. Furthermore, the results demonstrate that proper MAPbCl₃-MAPbBr₃ heterojunctions can dramatically reduce the recombination channels in the surface region, which can be potentially useful for perovskite based solar cells, light emitting diodes (LED), and sensitive detectors.

Low temperature excitonic spectroscopy and dynamics as a probe of quality in hybrid perovskite thin films

We have developed a framework for using temperature dependent static and dynamic photoluminescence (PL) of hybrid organic-inorganic perovskites (PVSKs) to characterize lattice defects in thin films, based on the presence of nanodomains at low temperature. Our high-stability PVSK films are fabricated using a novel continuous liquid interface propagation technique, and in the tetragonal phase (T > 120 K), they exhibit bi-exponential recombination from free charge carriers with an average PL lifetime of ∼200 ns. Below 120 K, the emergence of the orthorhombic phase is accompanied by a reduction in lifetimes by an order of magnitude, which we establish to be the result of a crossover from free carrier to exciton-dominated radiative recombination. Analysis of the PL as a function of excitation power at different temperatures provides direct evidence that the exciton binding energy is different in the two phases, and using these results, we present a theoretical approach to estimate this variable binding energy. Our findings explain this anomalous low temperature behavior for the first time, attributing it to an inherent fundamental property of the hybrid PVSKs that can be used as an effective probe of thin film quality.

Highly Enhanced Third-Harmonic Generation in 2D Perovskites at Excitonic Resonances

Two-dimensional hybrid organic-inorganic Ruddlesden-Popper perovskites (RPPs) have attracted considerable attention due to their rich photonic and optoelectronic properties. The natural multi-quantum-well structure of 2D RPPs has been predicted to exhibit a large third-order nonlinearity. However, nonlinear optical studies on 2D RPPs have previously been conducted only on bulk polycrystalline samples, in which only weak third-harmonic generation (THG) has been observed. Here, we perform parametric nonlinear optical characterization of 2D perovskite nanosheets mechanically exfoliated from four different lead halide RPP single crystals, from which we observe ultrastrong THG with a maximum effective third-order susceptibility (χ⁽³⁾) of 1.12 × 10^-17 m² V^-2. A maximum conversion efficiency of 0.006% is attained, which is more than 5 orders of magnitude higher than previously reported values for 2D materials. The THG emission is resonantly enhanced at the excitonic band gap energy of the 2D RPP crystals and can be tuned from violet to red by selecting the RPP homologue with the requisite resonance. Due to signal depletion effects and phase-matching conditions, the strongest nonlinear response is achieved for thicknesses less than 100 nm.

Long-lived hot-carrier light emission and large blue shift in formamidinium tin triiodide perovskites

A long-lived hot carrier population is critical in order to develop working hot carrier photovoltaic devices with efficiencies exceeding the Shockley-Queisser limit. Here, we report photoluminescence from hot-carriers with unexpectedly long lifetime (a few ns) in formamidinium tin triiodide. An unusual large blue shift of the time-integrated photoluminescence with increasing excitation power (150 meV at 24 K and 75 meV at 293 K) is displayed. On the basis of the analysis of energy-resolved and time-resolved photoluminescence, we posit that these phenomena are associated with slow hot carrier relaxation and state-filling of band edge states. These observations are both important for our understanding of lead-free hybrid perovskites and for an eventual future development of efficient lead-free perovskite photovoltaics.

Polarization-Dependent Optoelectronic Performances in Hybrid Halide Perovskite MAPbX3 (X = Br, Cl) Single-Crystal Photodetectors

Hybrid organic-inorganic lead halide perovskites (HOIPs) have received significant attention because of their impressive performances in the fields of solar cells and photoelectric detection. In the past five years, great efforts have been made to improve the crystallinity, reduce grain boundaries, and enhance the stabilities of perovskite films. Compared with films, HOIP single crystals possess fewer grain boundaries and stronger optoelectronic properties and can be applied in optoelectronic devices. As the most popular HOIP member, single crystals of MAPbX₃ (X = Br, Cl) are deemed as important candidates for ultraviolet-visible photodetectors, in which the crystal structure anisotropy largely affects the detection performance. In this study, high-quality cubic single crystals of MAPbBr₃ and MAPbCl₃ were successfully grown from solutions. Taking advantages of their smooth (100) facets, planar metal-semiconductor-metal photodetectors were fabricated using Au interdigitated electrodes. The optoelectronic performances under nonpolarized and linearly polarized lights were explored. The optoelectronic performances were dependent on linearly polarized lights. Interestingly, both responsivity and external quantum efficiency were greatly enhanced under the excitation with linearly polarized lights. Moreover, the polarization-related optical absorptions and the electron densities within the (100) plane could be used to interpret different optoelectronic performances of single crystals of MAPbX₃ (X = Br, Cl) under various linearly polarized lights.

Ionic-Electronic Ambipolar Transport in Metal Halide Perovskites: Can Electronic Conductivity Limit Ionic Diffusion?

Ambipolar transport describes the nonequilibrium, coupled motion of positively and negatively charged particles to ensure that internal electric fields remain small. It is commonly invoked in the semiconductor community where the motion of excess electrons and holes drift and diffuse together. However, the concept of ambipolar transport is not limited to semiconductor physics. Materials scientists working on ion conducting ceramics understand ambipolar transport dictates the coupled diffusion of ions and the rate is limited by the ion with the lowest diffusion coefficient. In this Perspective, we review a third application of ambipolar transport relevant to mixed ionic-electronic conducting materials for which the motion of ions is expected to be coupled to electronic carriers. In this unique situation, the ambipolar diffusion model has been successful at explaining the photoenhanced diffusion of metal ions in chalcogenide glasses and other properties of materials. Recent examples of photoenhanced phenomena in metal halide perovskites are discussed and indicate that mixed ionic-electronic ambipolar transport is similarly important for a deep understanding of these emerging materials.

Design Growth of MAPbI3 Single Crystal with (220) Facets Exposed and Its Superior Optoelectronic Properties

MAPbI₃ is deemed as the most prominent member in hybrid perovskites family because of its extremely optoelectronic properties. However, some issues and puzzles are still in expectation of their answers, such as stabilities, hysteresis, ferroelectricity, and so on. To bridge the distinctions between MAPbI₃ single crystal and thin films, large-size single crystals are demanded. On the contrary, crystal structure anisotropy-dependent optoelectronic properties is an inevitable topic. A series of large-size MAPbI₃ single crystals with (220) facets exposed were successfully grown, using high concentration solutions and large-size seed crystals to match growth rates of (100) and (220) facets. The optoelectronic properties of photocurrents, responsivity, EQE, and detectivity clearly showed significant anisotropy of optoelectronic properties in MAPbI₃ single crystal. According to ion migration theory, the anisotropy of optoelectronic properties was interpreted. We hope this result will be helpful to guide oriented growth MAPbI₃ thin films.

Surface potential mapping and n-type conductivity in organic–inorganic lead iodide crystals

We demonstrated the electronic structure of CH₃NH₃PbI₃ and its variation on the surface via surface potential and work function distribution.

Characterization of the influences of morphology on the intrinsic properties of perovskite films by temperature-dependent and time-resolved spectroscopies

The influences of morphology on the intrinsic properties of perovskite films are quantitatively characterized by temperature-dependent and time-resolved spectroscopies.

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.

Precursor purity effects on solution-based growth of MAPbBr3 single crystals towards efficient radiation sensing

In low purity crystals, (bottom) smaller more disordered crystallite sizes lead to increased charge trapping, compared to high purity (top).

Controllable growth of two-dimensional perovskite microstructures

Controllable synthesis of 2D (C₄H₉NH₃)₂(CH₃NH₃)_n−1Pb_nI_3n+1 microstructures with various shapes and sizes for functional optoelectronics.

The mixing effect of organic cations on the structural, electronic and optical properties of FAxMA1−xPbI3perovskites

The stability and optical performance of MAPbI3 perovskites can be effectively improved by doping FA cations.

Cations substitution tuning phase stability in hybrid perovskite single crystals by strain relaxation

Methylammonium (MA) and formamidinium (FA) are two typical A site cations in lead halide perovskites. Instability of synthesised crystals will degrade the properties of the photoelectrical device constructed by such perovskites. MAPbI₃ and FAPbI₃ in cubic crystal structure have been demonstrated to be the most stable at room temperature. Herein we synthesised MA(EA)PbI₃ and FA(MA)PbI₃ single crystals using an inverse-temperature crystallization strategy by partially substituting the methylammonium (MA) with ethylammonium (EA) and the formamidinium (FA) with methylammonium (MA) respectively. The XRD results show that both crystal structures are cubic, which means organic incorporation can stabilize the crystal structure of lead halide perovskites. The lattice distortion decrease and strain relaxation in single crystals were considered to be the reason leading to higher stability. The single crystals of MA(EA)PbI₃ and FA(MA)PbI₃ with low trap state density exhibit excellent light-absorbing properties, indicating their potential applications in photoelectric devices.

Cations size induced phase tuning in hybrid perovskite single crystals: interplay of lattice distortion and strain relaxation.

Recent Progress in Single-Crystalline Perovskite Research Including Crystal Preparation, Property Evaluation, and Applications

Organic-inorganic lead halide perovskites are promising optoelectronic materials resulting from their significant light absorption properties and unique long carrier dynamics, such as a long carrier lifetime, carrier diffusion length, and high carrier mobility. These advantageous properties have allowed for the utilization of lead halide perovskite materials in solar cells, LEDs, photodetectors, lasers, etc. To further explore their potential, intrinsic properties should be thoroughly investigated. Single crystals with few defects are the best candidates to disclose a variety of interesting and important properties of these materials, ultimately, showing the increased importance of single-crystalline perovskite research. In this review, recent progress on the crystallization, investigation, and primary device applications of single-crystalline perovskites are summarized and analyzed. Further improvements in device design and preparation are also discussed.

The Electrical and Optical Properties of Organometal Halide Perovskites Relevant to Optoelectronic Performance

Organometal halide perovskites are under intense study for use in optoelectronics. Methylammonium and formamidinium lead iodide show impressive performance as photovoltaic materials; a premise that has spurred investigations into light-emitting devices and photodetectors. Herein, the optical and electrical material properties of organometal halide perovskites are reviewed. An overview is given on how the material composition and morphology are tied to these properties, and how these properties ultimately affect device performance. Material attributes and techniques used to estimate them are analyzed for different perovskite materials, with a particular focus on the bandgap, mobility, diffusion length, carrier lifetime, and trap-state density.

High-Pressure-Induced Comminution and Recrystallization of CH3 NH3 PbBr3 Nanocrystals as Large Thin Nanoplates

High pressure (HP) can drive the direct sintering of nanoparticle assemblies for Ag/Au, CdSe/PbS nanocrystals (NCs). Instead of direct sintering for the conventional nanocrystals, this study experimentally observes for the first time high-pressure-induced comminution and recrystallization of organic-inorganic hybrid perovskite nanocrystals into highly luminescent nanoplates with a shorter carrier lifetime. Such novel pressure response is attributed to the unique structural nature of hybrid perovskites under high pressure: during the drastic cubic-orthorhombic structural transformation at ≈2 GPa, (301) the crystal plane fully occupied by organic molecules possesses a higher surface energy, triggering the comminution of nanocrystals into nanoslices along such crystal plane. Beyond bulk perovskites, in which pressure-induced modifications on crystal structures and functional properties will disappear after pressure release, the pressure-formed variants, i.e., large (≈100 nm) and thin (<10 nm) perovskite nanoplates, are retained and these exhibit simultaneous photoluminescence emission enhancing (a 15-fold enhancement in the photoluminescence) and carrier lifetime shortening (from ≈18.3 ± 0.8 to ≈7.6 ± 0.5 ns) after releasing of pressure from 11 GPa. This pressure-induced comminution of hybrid perovskite NCs and a subsequent amorphization-recrystallization treatment offer the possibilities of engineering the advanced hybrid perovskites with specific properties.

White perovskite based lighting devices

Hybrid organic–inorganic and all-inorganic metal halide perovskites have been one of the most intensively studied materials during the last few years.

Formamidinium Lead Bromide (FAPbBr3) Perovskite Microcrystals for Sensitive and Fast Photodetectors

Because of the good thermal stability and superior carrier transport characteristics of formamidinium lead trihalide perovskite HC(NH₂)₂PbX₃ (FAPbX₃), it has been considered to be a better optoelectronic material than conventional CH₃NH₃PbX₃ (MAPbX₃). Herein, we fabricated a FAPbBr₃ microcrystal-based photodetector that exhibited a good responsivity of 4000 A W^-1 and external quantum efficiency up to 10⁶% under one-photon excitation, corresponding to the detectivity greater than 10¹⁴ Jones. The responsivity is two orders of magnitude higher than that of previously reported formamidinium perovskite photodetectors. Furthermore, the FAPbBr₃ photodetector's responsivity to two-photon absorption with an 800-nm excitation source can reach 0.07 A W^-1, which is four orders of magnitude higher than that of its MAPbBr₃ counterparts. The response time of this photodetector is less than 1 ms. This study provides solid evidence that FAPbBr₃ can be an excellent candidate for highly sensitive and fast photodetectors.

Defects in metal triiodide perovskite materials towards high-performance solar cells: origin, impact, characterization, and engineering

The progress of defect science in metal triiodide perovskite is critically reviewed, including the origin, impacts, characterization, and engineering.

Organic semiconductor crystals

A comprehensive overview of organic semiconductor crystals is provided, including the physicochemical features, the control of crystallization and the device physics.

Colloidal synthesis of lead-free all-inorganic cesium bismuth bromide perovskite nanoplatelets

Lead halide two-dimensional (2D) nanoplatelets (NPLs) have attracted intense interest due to their unique optoelectronic properties.

Recent Advances in Synthesis and Properties of Hybrid Halide Perovskites for Photovoltaics

The progress made by the scientific community in emerging photovoltaic technologies over the past two decades has been outstanding. Numerous methods have been developed for the preparation of hybrid organic-inorganic perovskite solar cells. The power conversion efficiency has been up to 14% by a one-step vacuum deposition technique. A serious concern is the toxicity of the materials. In this review, several methods aimed at resolving these problems to some extent have been compiled, including eco-friendly synthesis. Further efficiency enhancements are expected following optimization, and a better fundamental understanding of the internal electron charge transfer, electron-hole diffusion to the corresponding layers, flexibility, and stability-dependent bandgaps is reported. This paper explores the green synthesis of organic-inorganic perovskites for industrialization. Concerning the above facts, a simple low-cost model called "dispersed photovoltaic cells" is presented.

The effect of SrI2 substitution on perovskite film formation and its photovoltaic properties via two different deposition methods

A 10 mol% Sr-substituted mesoscopic perovskite solar cell fabricated via a two-step spin-coating method exhibited the highest power conversion efficiency of 15.52%.

Formic acid: an accelerator and quality promoter for nonseeded growth of CH3NH3PbI3 single crystals

With the aid of formic acid, CH₃NH₃PbI₃ single crystals of 9 mm length were directly harvested within 3 days via a nonseeded solution temperature-lowering (STL) method.

Photodetectors Based on Organic-Inorganic Hybrid Lead Halide Perovskites

Recent years have witnessed skyrocketing research achievements in organic-inorganic hybrid lead halide perovskites (OIHPs) in the photovoltaic field. In addition to photovoltaics, more and more studies have focused on OIHPs-based photodetectors in the past two years, due to the remarkable optoelectronic properties of OIHPs. This article summarizes the latest progress in this research field. To begin with, the factors influencing the performance of photodetectors are discussed, including both internal and external factors. In particular, the channel width and the incident power intensities should be taken into account to precisely and objectively evaluate and compare the output performance of different photodetectors. Next, photodetectors fabricated on single-component perovskites in terms of different micromorphologies are discussed, namely, 3D thin-film and single crystalline, 2D nanoplates, 1D nanowires, and 0D nanocrystals, respectively. Then, bilayer structured perovskite-based photodetectors incorporating inorganic and organic semiconductors are discussed to improve the optoelectronic performance of their pristine counterparts. Additionally, flexible OIHPs-based photodetectors are highlighted. Finally, a brief conclusion and outlook is given on the progress and challenges in the field of perovskites-based photodetectors.