Perovskite CH3NH3PbI3(Cl) Single Crystals: Rapid Solution Growth, Unparalleled Crystalline Quality, and Low Trap Density toward 108 cm–3

Enhanced Efficiency of Mixed-Halide Perovskite Solar Cells Through Optimization of the Layer Thicknesses, Defect Density, and Metal Contact Work Function

Mixed-halide perovskites enable the creation of high-performance and low-cost solar cells. Chloride incorporation enhances film morphology, carrier diffusion length, and stability, improving device performance. Nevertheless, optimizing film thickness, defect density, and metal contact work function remains insufficiently explored, despite its potential to enhance power conversion efficiency. In this study, a numerical simulation was performed using SCAPS-1D (version 3.3.10) to identify the optimal parameters for the FTO/TiO2/CH3NH3Pb3-xClx/Spiro-OMeTAD/Au configuration. The best performance parameters that have been published in the literature based on experimental results are as follows: VOC = 1.077 V, JSC = 21.45 mA/cm2, FF = 77.57%, and PCE = 17.97%. In contrast, the performance parameters obtained from numerical simulations for the same structure are VOC = 1.28 V, JSC = 21.63 mA/cm2, FF = 78%, and PCE = 21.53%. In our numerical analysis, we achieved efficiencies that were comparable to those reported in experimental studies, and after optimization, superior performance parameters were attained, including VOC = 1.179 V, JSC = 27.26 mA/cm2, FF = 81.03%, and PCE = 26.07%. These results indicate that optimized parameters can be integrated into the design and fabrication of mixed-halide perovskite solar cells to enhance performance.

MAPbX3 Perovskite Single Crystals for Advanced Optoelectronic Applications: Progress, Challenges, and Perspective

Perovskite single crystals have garnered significant attention due to their impressive properties in optoelectronic applications, including long carrier diffusion lengths, low trap-state densities, and enhanced stability. Methylamino lead halide perovskite (MAPbX3, where X is a halogen such as Cl, Br, or I) is a notable example of a metal halide perovskite with desirable properties and ideal cubic perovskites with a tolerance factor between 0.9 and 1.0. MAPbX3 has adjustable bandgap, high thermal and chemical stability, and excellent light absorption capacity. Here the unique characteristics of MAPbX3, including molecular structure, optical absorption properties, and carrier transport of MAPbX3 single crystals are summarized. Universal growth technologies for MAPbX3 single crystals, including inverse temperature crystallization, anti-solvent evaporation crystallization, solvent evaporation method, and single-crystalline thin film, including epitaxial method and space limiting method, are briefly introduced. Additionally, a comprehensive review of MAPbX3 single crystals in various optoelectronic device applications, including photodetectors, X-ray detectors, light-emitting diodes, lasers, and solar cells is mainly discussed. Finally, the current challenges and future prospects of the large-scale preparation and growth of MAPbX3 single crystals are put forward. With the continuous progress of photoelectric technology, more innovative photoelectric applications in the future are expected to bring more convenience and progress.

Nuclear Quantum Effects Enhance Structural Stability but Accelerate Charge Carrier Recombination in MHyPbBr3 Perovskite

Hybrid organic-inorganic perovskites exhibit significant nuclear quantum effects (NQEs) due to their light hydrogen atoms. By performing ring polymer molecular dynamics, ab initio molecular dynamics, and nonadiabatic molecular dynamics simulations on the MHyPbBr3 (MHy+ = CH3NH2NH2+) perovskites, we demonstrate that NQEs stabilize the lattice by suppressing atomic motions and accelerate nonradiative charge recombination. This stabilization arises from the synergistic effects of the Pb-N coordination bonds and N-H···Br hydrogen bonds, which enhance organic-inorganic interactions. As a result, Pb-Br octahedra, particularly [Pb(1)Br6]4- octahedra supporting electron and hole, are well-preserved, promoting electronic wavefunction delocalization and increasing electron-hole overlap. These effects enhance nonadiabatic coupling by overcoming the reduced atomic motions. Overall, this and the prolonged decoherence time accelerate the nonradiative electron-hole recombination due to NQEs. Our study highlights the unique influence of NQEs on the geometrical stability and charge carrier dynamics in MHyPbBr3, offering fundamental insights for future material design.

Structural Modifications due to Bi-Doping in MAPbBr3 Single Crystals and Their Impact on Electronic Transport and Stability

Doping strategy in lead halide perovskites is essential to enhance its optoelectrical properties and expand the potential applications. In this work, the mechanisms, for how dopants affect the overall structural, optical, electrical, and chemical properties and stability of lead halide perovskite materials, are investigated. This is done by specifically considering various bismuth (Bi) doping concentrations in MAPbBr3 single crystals grown using the inverse temperature crystallization method. The resultant doped single crystals exhibit a saturation point when Bi concentration exceeds 0.063% which is considered an optimum doping point. The highest thermal stability is also achieved at this doping concentration among the doped single crystals. This study clearly identifies how Bi doping affects the properties of MAPbBr3 and extends to consider stability, which has not been fully considered for MA-based perovskites previously. This will provide a clear understanding of evaluating doped perovskite materials for enhanced material properties, device performance, and stability.

Single-Crystal Perovskite for Solar Cell Applications

The advent of organic-inorganic hybrid metal halide perovskites has revolutionized photovoltaics, with polycrystalline thin films reaching over 26% efficiency and single-crystal perovskite solar cells (IC-PSCs) demonstrating ≈24%. However, research on single-crystal perovskites remains limited, leaving a crucial gap in optimizing solar energy conversion. Unlike polycrystalline films, which suffer from high defect densities and instability, single-crystal perovskites offer minimal defects, extended carrier lifetimes, and longer diffusion lengths, making them ideal for high-performance optoelectronics and essential for understanding perovskite material behavior. This review explores the advancements and potential of IC-PSCs, focusing on their superior efficiency, stability, and role in overcoming the limitations of polycrystalline counterparts. It covers device architecture, material composition, preparation methodologies, and recent breakthroughs, emphasizing the importance of further research to propel IC-PSCs toward commercial viability and future dominance in photovoltaic technology.

Characterization of Cl-doped two-dimensional (PEA)2PbBr4 perovskite single crystals for fast neutron and gamma ray detection

In this paper, a high-quality Cl-doped two-dimensional halide perovskite (PEA)2Pb(Br0.95Cl0.05)4 crystal was prepared using a seed-induced volatile solvent method. On optimizing the Cl- doping concentration, we found that 5% Cl-doping results in (PEA)2PbBr4 with the highest optical and photon yield. Based on the Cl-doped (PEA)2PbBr4 single crystal, the response characterization of the (PEA)2Pb(Br0.95Cl0.05)4 crystal in the mixed field of neutrons and gamma rays (n/γ) has been verified. Using the time-of-flight method and the linear relationship between integral charge and neutron yield, it was proved that (PEA)2Pb(Br0.95Cl0.05)4 crystal can be used for n/γ screening. The time difference between the fast neutron released by a single nuclear reaction and the γ photon arriving at the detector was 130 ns, and the arrival time of the γ photon is earlier than that of the fast neutron. This work has a broad application prospect in the study of nuclear reaction kinetics, the monitoring of the neutron yield of fusion devices and the total energy released by nuclear reactions.

Spectrally Tunable Lead-Free Perovskite Rb2ZrCl6:Te for Information Encryption and X-ray Imaging

A series of lead-free Rb2ZrCl6:xTe4+ (x = 0%, 0.1%, 0.5%, 1.0%, 2.0%, 3.0%, 5.0%, 10.0%) perovskite materials were synthesized through a hydrothermal method in this work. The substitution of Te4+ for Zr in Rb2ZrCl6 was investigated to examine the effect of Te4+ doping on the spectral properties of Rb2ZrCl6 and its potential applications. The incorporation of Te4+ induced yellow emission of triplet self-trapped emission (STE). Different luminescence wavelengths were regulated by Te4+ concentration and excitation wavelength, and under a low concentration of Te4+ doping (x ≤ 0.1%), different types of host STE emission and Te4+ triplet state emission could be achieved through various excitation energies. These luminescent properties made it suitable for applications in information encryption. When Te4+ was doped at high concentrations (x ≥ 1%), yellow triplet state emission of Te4+ predominated, resulting in intense yellow emission, which stemmed from strong exciton binding energy and intense electron-phonon coupling. In addition, a Rb2ZrCl6:2%Te4+@RTV scintillating film was fabricated and a spatial resolution of 3.7 lp/mm was achieved, demonstrating the potential applications of Rb2ZrCl6:xTe4+ in nondestructive detection and bioimaging.

Crystallization Kinetics of Hybrid Perovskite Solar Cells

Metal halide perovskites (MHPs) are considered ideal photovoltaic materials due to their variable crystal material composition and excellent photoelectric properties. However, this variability in composition leads to complex crystallization processes in the manufacturing of Metal halide perovskite (MHP) thin films, resulting in reduced crystallinity and subsequent performance loss in the final device. Thus, understanding and controlling the crystallization dynamics of perovskite materials are essential for improving the stability and performance of PSCs (Perovskite Solar Cells). To investigate the impact of crystallization characteristics on the properties of MHP films and identify corresponding modulation strategies, we primarily discuss the relevant aspects of MHP crystallization kinetics, systematically summarize theoretical methods, and outline modulation techniques for MHP crystallization, including solution engineering, additive engineering, and component engineering, which helps highlight the prospects and current challenges in perovskite crystallization kinetics.

Dimensionality Engineering of Organic-Inorganic Halide Perovskites for Next-Generation X-Ray Detector

The next-generation X-ray detectors require novel semiconductors with low material/fabrication cost, excellent X-ray response characteristics, and robust operational stability. The family of organic-inorganic hybrid perovskites (OIHPs) materials comprises a range of crystal configuration (i.e., films, wafers, and single crystals) with tunable chemical composition, structures, and electronic properties, which can perfectly meet the multiple-stringent requirements of high-energy radiation detection, making them emerging as the cutting-edge candidate for next-generation X-ray detectors. From the perspective of molecular dimensionality, the physicochemical and optoelectronic characteristics of OIHPs exhibit dimensionality-dependent behavior, and thus the structural dimensionality is recognized as the key factor that determines the device performance of OIHPs-based X-ray detectors. Nevertheless, the correlation between dimensionality of OIHPs and performance of their X-ray detectors is still short of theoretical guidance, which become a bottleneck that impedes the development of efficient X-ray detectors. In the review, the advanced studies on the dimensionality engineering of OIHPs are critically assessed in X-ray detection application, discussing the current understanding on the "dimensionality-property" relationship of OIHPs and the state-of-the-art progresses on the dimensionality-engineered OIHPs-based X-ray detector, and highlight the open challenges and future outlook of this field.

Universal growth of perovskite thin monocrystals from high solute flux for sensitive self-driven X-ray detection

Metal-halide perovskite thin monocrystals featuring efficient carrier collection and transport capabilities are well suited for radiation detectors, yet their growth in a generic, well-controlled manner remains challenging. Here, we reveal that mass transfer is one major limiting factor during solution growth of perovskite thin monocrystals. A general approach is developed to overcome synthetic limitation by using a high solute flux system, in which mass diffusion coefficient is improved from 1.7×10-10 to 5.4×10-10 m2 s-1 by suppressing monomer aggregation. The generality of this approach is validated by the synthesis of 29 types of perovskite thin monocrystals at 40-90 °C with the growth velocity up to 27.2 μm min-1. The as-grown perovskite monocrystals deliver a high X-ray sensitivity of 1.74×105 µC Gy-1 cm-2 without applied bias. The findings regarding limited mass transfer and high-flux crystallization are crucial towards advancing the preparation and application of perovskite thin monocrystals.

Perovskite Single Crystals with Self-Cleaning Surface for Efficient Photovoltaics

Metal halide perovskite single crystals are promising for diverse optoelectronic applications. As a universal issue of solution-grown perovskite single crystals, surface contamination causes adverse effect on material properties and device performance. Herein, learning from the self-cleaning effect of lotus leaf, we address the surface contamination issue by introducing an amphiphilic long-chain organic amine into the perovskite crystal growth solution. Self-assembly of CTAC provides a hydrophobic crystal surface, inducing spontaneous removal of residual growth solution, which results in clean surface and better optoelectronic properties of perovskite single crystals. An impressive efficiency of 23.4 % is obtained, setting a new record for FAx MA1-x PbI3 single-crystal perovskite solar cells (PSCs). Moreover, our strategy also applies to perovskite single crystals with different morphology and composition, which may contribute to improvement of other single-crystal perovskite optoelectronic devices.

Growth methods' effect on the physical characteristics of CsPbBr3 single crystal

This study offers an extensive exploration into approaches for cultivating CsPbBr3 SCs using inverse temperature crystallization (ITC), with a specific focus on seed-induced (method (1)) and nucleation-mediated (method (2)) growth techniques. Our findings reveal that leveraging seed-assisted growth at lower temperatures yields noteworthy enhancements in the material's optical and electrical behaviors, outperforming the outcomes achieved through nucleation-driven growth. Concretely, through the employment of the space charge limited current (SCLC) technique, an evident contrast emerges in the trap-populated threshold voltage between the seed-facilitated crystal (SC1) (measuring 0.309 V) and its nucleation-facilitated counterpart (SC2) (measuring 1.513 V), consequently giving rise to discernable dissimilarities in trap density assessments. Evidence from temperature-dependent analysis of space charge limited currents substantiates these findings, revealing trap density values of 8.81 × 109 cm-3 for SC1, juxtaposed with 2.08 × 1010 cm-3 for SC2. Additionally, the SC1 displays a notably diminished trap energy level. Furthermore, the investigation underscores the affirmative influence of method (1) at lower temperatures on the optical and crystalline characteristics of the substance. This effect is evidenced by enhanced photoluminescence (PL) reactions and reduced lattice strain (Ls), as determined through X-ray diffraction (XRD) techniques. Moreover, the research establishes the substantial impact of this enhanced crystallization technique on the photodetector (PD) attributes of the crystal. This effect induces elevated levels of detectivity and responsivity for method (1).

Embedded Host/Guest Alloy Aggregations Enable High-Performance Ternary Organic Photovoltaics

The ternary strategy has been intensively studied to improve the power conversion efficiencies of organic photovoltaics. Thereinto, the location of the guest component plays a critical role, but few reports have been devoted to this concern. Hereon, the distribution of LA1 as a guest acceptor in a variety of ternary scenarios is reported and the dominating driving forces of managing the guest distribution and operating modes are outlined. Governed by the appropriate relationship of compatibility, crystallinity, and surface energies between host and guest acceptors, as well as interfacial interactions between donor and dual acceptors, most of the LA1 molecules permeate into the internal of host acceptor phases, forming embedded host/guest alloy-like aggregations. The characteristic distributions greatly optimize the morphologies, maximize energy transfer, and enhance exciton/charge behaviors. Particularly, PM6:IT-4F:LA1 ternary cells afford high efficiency of 15.27% with impressive fill factors (FF) over 81%. The popularization studies further verify the superiority of the LA1-involved alloy structures, and with the Y6-family acceptor as the host component, an outstanding efficiency of 19.17% is received. The results highlight the importance of guest distribution in ternary systems and shed light on the governing factors of distributing the guests in ternary cells.

A New Metal-Free Molecular Perovskite-Related Single Crystal with Quantum Wire Structure for High-Performance X-Ray Detection

The family of metal-free molecular perovskites, an emerging novel class of eco-friendly semiconductor, welcomes a new member with a unique 1D hexagonal perovskite structure. Lowering dimensionality at molecular level is a facile strategy for crystal structure conversion, optoelectronic property regulation, and device performance optimization. Herein, the study reports the design, synthesis, packing structure, and photophysical properties of the 1D metal-free molecular perovskite-related single crystal, rac-3APD-NH4 I3 (rac-3APD= racemic-3-Aminopiperidinium), that features a quantum wire structure formed by infinite chains of face-sharing NH4 I6 octahedra, enabling strong quantum confinement with strongly self-trapped excited (STE) states to give efficient warm orange emission with a photoluminescence quantum yield (PLQY) as high as ≈41.6%. The study accordingly unveils its photoexcited carrier dynamics: rac-3APD-NH4 I3 relaxes to STE state with a short lifetime of 10 ps but decays to ground state by emitting photons with a relatively longer lifetime of 560 ps. Additionally, strong quantum confinement effect is conducive to charge transport along the octahedral channels that enables the co-planar single-crystal X-ray detectors to achieve a sensitivity as high as 1556 µC Gyair -1 cm-2 . This work demonstrates the first case of photoluminescence mechanism and photophysical dynamics of 1D metal-free perovskite-related semiconductor, as well as the promise for high-performance X-ray detector.

Surface Engineering of Methylammonium Lead Bromide Perovskite Crystals for Enhanced X-ray Detection

The surface quality of lead halide perovskite crystals can extremely influence their optoelectronic properties and device performance. Here, we report a surface engineering crystallization technique in which we in situ grow a polycrystalline methylammonium lead tribromide (MAPbBr3) film on top of bulk mm-sized single crystals. Such MAPbBr3 crystals with a MAPbBr3 passivating film display intense green emission under UV light. X-ray photoelectron spectroscopy demonstrates that these crystals with emissive surfaces are compositionally different from typical MAPbBr3 crystals that show no emission under UV light. Time-resolved photoluminescence and electrical measurements indicate that the MAPbBr3 film/MAPbBr3 crystals possess less surface defects compared to the bare MAPbBr3 crystals. Therefore, X-ray detectors fabricated using the surface-engineered MAPbBr3 crystals provide an almost 5 times improved sensitivity to X-rays and a more stable baseline drift with respect to the typical MAPbBr3 crystals.

Carriers, Quasi-particles, and Collective Excitations in Halide Perovskites

Halide perovskites (HPs) are potential game-changing materials for a broad spectrum of optoelectronic applications ranging from photovoltaics, light-emitting devices, lasers to radiation detectors, ferroelectrics, thermoelectrics, etc. Underpinning this spectacular expansion is their fascinating photophysics involving a complex interplay of carrier, lattice, and quasi-particle interactions spanning several temporal orders that give rise to their remarkable optical and electronic properties. Herein, we critically examine and distill their dynamical behavior, collective interactions, and underlying mechanisms in conjunction with the experimental approaches. This review aims to provide a unified photophysical picture fundamental to understanding the outstanding light-harvesting and light-emitting properties of HPs. The hotbed of carrier and quasi-particle interactions uncovered in HPs underscores the critical role of ultrafast spectroscopy and fundamental photophysics studies in advancing perovskite optoelectronics.

Compositional gradient engineering and applications in halide perovskites

Organic-inorganic halide perovskites (HPs) have attracted respectable interests as active layers in solar cells, light-emitting diodes, photodetectors, etc. Besides the promising optoelectronic properties and solution-processed preparation, the soft lattice in HPs leads to flexible and versatile compositions and structures, providing an effective platform to regulate the bandgaps and optoelectronic properties. However, conventional solution-processed HPs are homogeneous in composition. Therefore, it often requires the cooperation of multiple devices in order to achieve multi-band detection or emission, which increases the complexity of the detection/emission system. In light of this, the construction of a multi-component compositional gradient in a single active layer has promising prospects. In this review, we summarize the gradient engineering methods for different forms of HPs. The advantages and limitations of these methods are compared. Moreover, the entropy-driven ion diffusion favors compositional homogeneity, thus the stability issue of the gradient is also discussed for long-term applications. Furthermore, applications based on these compositional gradient HPs will also be presented, where the gradient bandgap introduced therein can facilitate carrier extraction, and the multi-components on one device facilitate functional integration. It is expected that this review can provide guidance for the further development of gradient HPs and their applications.

Phenyl-Terminated Coupling Interface Enabled Highly Efficient and Stable Multiwavelength Perovskite Single Crystal/Silicon Integrated Photodetector

The use of amino-terminated siloxanes as coupling interface for perovskite single crystals (PSCs)/silicon integrated devices has been demonstrated to be an effective method toward CMOS compatible optoelectronics; however, it suffers from the coupling stability against the hydrophilicity of the exposed terminal amino groups. In this work, a phenyl-terminated interfacial molecule, anilino-methyl-triethoxysilane (AMTES), is proposed to achieve the effectively galvanic coupling between PSCs and silicon, which can not only improve the device environmental reliability but also lower the surface energy of the silicon substrate so as to facilitate the epitaxial growth of PSCs. Benefiting from the interfacial coupling of AMTES, the obtained MAPbI₃ SC/silicon integrated device possesses highly efficient multiwavelength photodetection properties across the X-ray and NIR range, which exhibits a specific detectivity D* of 3.84 × 10¹³ cm Hz^1/2 W^-1 in the visible-NIR region and an X-ray sensitivity of 1.18 × 10⁴ μC Gy_air^-1 cm^-2 with the lowest detection limit of 49.6 nGy_air s^-1. The ultra wide -3 dB bandwidth of 67,300 Hz and the linear dynamic range (LDR) of 112 dB also prove its impressive dynamic response capabilities. Moreover, the AMTES modified integrated device almost maintains 96% of the initial photodetection performance even after keeping in the atmosphere environment for 28 days. This work opens a new avenue for interfacial engineering toward the development of on-chip PSC integrated silicon optoelectronic devices.

Detector-grade perovskite single-crystal wafers via stress-free gel-confined solution growth targeting high-resolution ionizing radiation detection

Solution-processed organic‒inorganic halide perovskite (OIHP) single crystals (SCs) have demonstrated great potential in ionizing radiation detection due to their outstanding charge transport properties and low-cost preparation. However, the energy resolution (ER) and stability of OIHP detectors still lag far behind those of melt-grown inorganic perovskite and commercial CdZnTe counterparts due to the absence of detector-grade high-quality OIHP SCs. Here, we reveal that the crystallinity and uniformity of OIHP SCs are drastically improved by relieving interfacial stress with a facial gel-confined solution growth strategy, thus enabling the direct preparation of large-area detector-grade SC wafers up to 4 cm with drastically suppressed electronic and ionic defects. The resultant radiation detectors show both a small dark current below 1 nA and excellent baseline stability of 4.0 × 10^-8nA cm^-1 s^-1 V^-1, which are rarely realized in OIHP detectors. Consequently, a record high ER of 4.9% at 59.5 keV is achieved under a standard ²⁴¹Am gamma-ray source with an ultralow operating bias of 5 V, representing the best gamma-ray spectroscopy performance among all solution-processed semiconductor radiation detectors ever reported.

Intrinsic Formamidinium Tin Iodide Nanocrystals by Suppressing the Sn(IV) Impurities

The long search for nontoxic alternatives to lead halide perovskites (LHPs) has shown that some compelling properties of LHPs, such as low effective masses of carriers, can only be attained in their closest Sn(II) and Ge(II) analogues, despite their tendency toward oxidation. Judicious choice of chemistry allowed formamidinium tin iodide (FASnI₃) to reach a power conversion efficiency of 14.81% in photovoltaic devices. This progress motivated us to develop a synthesis of colloidal FASnI₃ NCs with a concentration of Sn(IV) reduced to an insignificant level and to probe their intrinsic structural and optical properties. Intrinsic FASnI₃ NCs exhibit unusually low absorption coefficients of 4 × 10³ cm^-1 at the first excitonic transition, a 190 meV increase of the band gap as compared to the bulk material, and a lack of excitonic resonances. These features are attributed to a highly disordered lattice, distinct from the bulk FASnI₃ as supported by structural characterizations and first-principles calculations.

Over 19% Efficiency Organic Solar Cells by Regulating Multidimensional Intermolecular Interactions

Research on organic solar cells (OSCs) has progressed through material innovation and device engineering. However, well-known and ubiquitous intermolecular interactions, and particularly their synergistic effects, have received little attention. Herein, the complicated relationship between photovoltaic conversion and multidimensional intermolecular interactions in the active layers is investigated. These interactions are dually regulated by side-chain isomerization and end-cap engineering of the acceptors. The phenylalkyl featured acceptors (LA-series) exhibit stronger crystallinity with preferential face-on interactions relative to the alkylphenyl attached isomers (ITIC-series). In addition, the PM6 and LA-series acceptors exhibit moderate donor/acceptor interactions compared to those of the strongly interacting PM6/ITIC-series pairs, which helps to enhance phase separation and charge transport. Consequently, the output efficiencies of all LA series acceptors are over 14%. Moreover, LA-series acceptors show appropriate compatibility, host/guest interactions, and crystallinity relationships with BTP-eC9, thereby leading to uniform and well-organized "alloy-like" mixed phases. In particular, the highly crystalline LA23 further optimizes multiple interactions and ternary microstructures, which results in a high efficiency of 19.12%. Thus, these results highlight the importance of multidimensional intermolecular interactions in the photovoltaic performance of OSCs.

Automated Graph Neural Networks Accelerate the Screening of Optoelectronic Properties of Metal-Organic Frameworks

The numerous organic and inorganic components of metal-organic framework (MOF) materials provide intriguing optoelectronic properties. Accurately predicting the electronic structural properties of MOFs has become the main focus. This work establishes two graph neural network models, crystal graph convolutional neural networks and a materials graph network, for predicting the band gaps of more than 10 000 MOF structures and promotes to improve the prediction accuracy through automatic hyperparameter tuning algorithms. Subsequently, for exploring machine learning-assisted screening of MOFs for the broader electronic properties, the screened copper-based MOFs are compared with lead-based MAPbI₃ solar cells with respect to the band gaps, densities of states, and charge density distributions, and the results have demonstrated that the overlap of the wave functions between the initial and final states of MOFs is weakened, which is conducive to the improvement of photoelectric performance. The chlorine doping strategy further enhances the advantage. The tuning of the machine learning model and hyperparameters and the doping strategy of halogen elements furnish empirical rules for the design of MOFs with excellent optoelectronic properties.

Chemical approaches for electronic doping in photovoltaic materials beyond crystalline silicon

Electronic doping is applied to tailor the electrical and optoelectronic properties of semiconductors, which have been widely adopted in information and clean energy technologies, like integrated circuit fabrication and PVs. Though this concept has prevailed in conventional PVs, it has achieved limited success in the new-generation PV materials, particularly in halide perovskites, owing to their soft lattice nature and self-compensation by intrinsic defects. In this review, we summarize the evolution of the theoretical understanding and strategies of electronic doping from Si-based photovoltaics to thin-film technologies, e.g., GaAs, CdTe and Cu(In,Ga)Se₂, and also cover the emerging PVs including halide perovskites and organic solar cells. We focus on the chemical approaches to electronic doping, emphasizing various chemical interactions/bonding throughout materials synthesis/modification to device fabrication/operation. Furthermore, we propose new classifications and models of electronic doping based on the physical and chemical properties of dopants, in the context of solid-state chemistry, which inspires further development of optoelectronics based on perovskites and other hybrid materials. Finally, we outline the effects of electronic doping in semiconducting materials and highlight the challenges that need to be overcome for reliable and controllable doping.

Halide Perovskite Crystallization Processes and Methods in Nanocrystals, Single Crystals, and Thin Films

Halide perovskite semiconductors with extraordinary optoelectronic properties have been fascinatedly studied. Halide perovskite nanocrystals, single crystals, and thin films have been prepared for various fields, such as light emission, light detection, and light harvesting. High-performance devices rely on high crystal quality determined by the nucleation and crystal growth process. Here, the fundamental understanding of the crystallization process driven by supersaturation of the solution is discussed and the methods for halide perovskite crystals are summarized. Supersaturation determines the proportion and the average Gibbs free energy changes for surface and volume molecular units involved in the spontaneous aggregation, which could be stable in the solution and induce homogeneous nucleation only when the solution exceeds a required minimum critical concentration (C_min ). Crystal growth and heterogeneous nucleation are thermodynamically easier than homogeneous nucleation due to the existent surfaces. Nanocrystals are mainly prepared via the nucleation-dominated process by rapidly increasing the concentration over C_min , single crystals are mainly prepared via the growth-dominated process by keeping the concentration between solubility and C_min , while thin films are mainly prepared by compromising the nucleation and growth processes to ensure compactness and grain sizes. Typical strategies for preparing these three forms of halide perovskites are also reviewed.

Organic-Inorganic Lead Halide Perovskite Single Crystal: From Synthesis to Applications

Organic-inorganic lead halide perovskite is widely used in the photoelectric field due to its excellent photoelectric characteristics. Among them, perovskite single crystals have attracted much attention due to its lower trap density and better carrier transport capacity than their corresponding polycrystalline materials. Owing to these characteristics, perovskite single crystals have been widely used in solar cells, photodetectors, light-emitting diode (LED), and so on, which have greater potential than polycrystals in a series of optoelectronic applications. However, the fabrication of single-crystal devices is limited by size, thickness, and interface problems, which makes the development of single-crystal devices inferior to polycrystalline devices, which also limits their future development. Here, several representative optoelectronic applications of perovskite single crystals are introduced, and some existing problems and challenges are discussed. Finally, we outlook the growth mechanism of single crystals and further the prospects of perovskite single crystals in the further field of microelectronics.

Asymmetrical Electrical Performance across Different Planes of Solution-Grown MAPbBr3 Crystals of mm Dimensions

Throughout a few years, carrier transport studies across HaP single crystals have gained enormous importance for current generation photovoltaic and photodetector research with their superior optoelectronic properties compared to commercially available polycrystalline materials. Utilizing the room-temperature solution-grown method, we synthesized MAPbBr₃ crystals and examined their electrical transport properties. Although the X-ray diffraction reveals the cubical nature of the crystals, we have observed anisotropy in the electrical transport behavior and variation in dielectric constant across the three opposite faces of the crystals of mm dimensions. The face with a higher dielectric constant depicts improved parameters from electrical characteristics such as lower trap densities and higher mobility values. We further explore the origin of its anisotropic nature by performing X-ray diffraction on three opposite faces of crystals. Our studies define the specific faces of cuboid-shaped MAPbBr₃ crystals for efficient electrical contact in the fabrication of optoelectronic devices.

Structural Disorder in Higher-Temperature Phases Increases Charge Carrier Lifetimes in Metal Halide Perovskites

Solar cells and optoelectronic devices are exposed to heat that degrades performance. Therefore, elucidating temperature-dependent charge carrier dynamics is essential for device optimization. Charge carrier lifetimes decrease with temperature in conventional semiconductors. The opposite, anomalous trend is observed in some experiments performed with MAPbI₃ (MA = CH₃NH₃⁺) and other metal halide perovskites. Using ab initio quantum dynamics simulation, we establish the atomic mechanisms responsible for nonradiative electron-hole recombination in orthorhombic-, tetragonal-, and cubic MAPbI₃. We demonstrate that structural disorder arising from the phase transitions is as important as the disorder due to heating in the same phase. The carrier lifetimes grow both with increasing temperature in the same phase and upon transition to the higher-temperature phases. The increased lifetime is rationalized by structural disorder that induces partial charge localization, decreases nonadiabatic coupling, and shortens quantum coherence. Inelastic and elastic electron-vibrational interactions exhibit opposite dependence on temperature and phase. The partial disorder and localization arise from thermal motions of both the inorganic lattice and the organic cations and depend significantly on the phase. The structural deformations induced by thermal fluctuations and phase transitions are on the same order as deformations induced by defects, and hence, thermal disorder plays a very important role. Since charge localization increases carrier lifetimes but inhibits transport, an optimal regime maximizing carrier diffusion can be designed, depending on phase, temperature, material morphology, and device architecture. The atomistic mechanisms responsible for the enhanced carrier lifetimes at elevated temperatures provide guidelines for the design of improved solar energy and optoelectronic materials.

Efficient Eco-Friendly Flexible X-ray Detectors Based on Molecular Perovskite

Next-generation wearable electronics requires mechanical robustness. In addition to the previously reported eco-friendliness, low cost, and light weight of molecular perovskites, flexibility is also a desired merit for their practical use. Here we design a flexible X-ray detector based on a novel molecular perovskite, DABCO-CsBr₃ (DABCO = N-N'-diazabicyclo[2.2.2]octonium), which is the missing link between metal-free molecular perovskites A(NH₄)X₃ (A = divalent organic ammoniums) and conventional metal halide based ABX₃ (B = divalent metal cations) perovskites. DABCO-CsBr₃ inherits its band nature from A(NH₄)X₃, while it exhibits a stronger stopping power. DABCO-CsBr₃ shows potential for high-performance ionizing radiation detectors due to low dark current, low ion migration, and an efficient mobility-lifetime (μτ) product. Finally, a molecular-perovskite-based flexible X-ray detector is demonstrated on the basis of the DABCO-CsBr₃/poly(vinylidene fluoride) composite, with a sensitivity of 106.7 μC Gy_air^-1 cm^-2. This work enriches the molecular perovskite family and highlights the promise of molecular perovskites for the next-generation eco-friendly and wearable optoelectronic devices.

Electrode Spacing as a Determinant of the Output Performance of Planar-Type Photodetectors Based on Methylammonium Lead Bromide Perovskite Single Crystals

Methylammonium lead bromide is a very perspective hybrid semiconductor material, suitable for high-sensitive, filter-free photodetection of electromagnetic radiation. Herein, we studied the effect of electrode spacing on the output performance and stability of planar-type photodetectors based on high-quality MAPbBr₃ single crystals. Such crystals, as large as 4.5×4.5×1.2 mm³ were synthesized via the inverse temperature crystallization method and were further used for the fabrication of planar Au/MAPbBr₃/Au photodetectors with variable electrode spacing (in the range between 125 and 25 μm). We report that the electrode spacing has a profound impact on photocurrent densities and key detector parameters (responsivity R, external quantum efficiency EQE, and specific detectivity D*). In the studied fivefold electrode spacing, the photocurrent density increased over 4 times, with decreasing active area of the devices. This effect is attributed to intrinsic photocurrent amplification. Based on the transient photocurrent measurements and calculated key parameters, we determined the device sample with the best output performance. The champion sample with an electrode spacing of 50 μm exhibited great detection ability, especially for a low light intensity of 200 nWcm^-2, for which we calculated the R of 19.55 A W^-1, EQE of 4253%, and D* of 3.42 × 10¹² Jones (cm Hz^1/2 W^-1). Moreover, the functional stability of this device showed a minimal reduction of photodetection ability after 2000 cycles, which makes it very promising for the next generation of optoelectronic devices.

In Situ Growth Mechanism for High-Quality Hybrid Perovskite Single-Crystal Thin Films with High Area to Thickness Ratio: Looking for the Sweet Spot

The development of in situ growth methods for the fabrication of high-quality perovskite single-crystal thin films (SCTFs) directly on hole-transport layers (HTLs) to boost the performance of optoelectronic devices is critically important. However, the fabrication of large-area high-quality SCTFs with thin thickness still remains a significant challenge due to the elusive growth mechanism of this process. In this work, the influence of three key factors on in situ growth of high-quality large-size MAPbBr₃ SCTFs on HTLs is investigated. An optimal "sweet spot" is determined: low interface energy between the precursor solution and substrate, a slow heating rate, and a moderate precursor solution concentration. As a result, the as-obtained perovskite SCTFs with a thickness of 540 nm achieve a record area to thickness ratio of 1.94 × 10⁴  mm, a record X-ray diffraction peak full width at half maximum of 0.017°, and an ultralong carrier lifetime of 1552 ns. These characteristics enable the as-obtained perovskite SCTFs to exhibit a record carrier mobility of 141 cm² V^-1 s^-1 and good long-term structural stability over 360 days.

Searching for High-Quality Halide Perovskite Single Crystals toward X-ray Detection

Metal halide perovskite materials, which combine outstanding physical properties, large absorption coefficient, tailored composition, and low-cost solution-processing, have aroused wide attention for use in various optoelectronic devices. Recently, perovskite single crystals have been rapidly outpacing traditional semiconductor materials in the field of radiation detection. As a prerequisite, achieving high-quality single crystals under controllable solution-phase growth must be tackled to fulfill their full potential as a new paradigm in this stagnated field. This Perspective summarizes the advances in X-ray detectors based on lead halide perovskite single crystals, presenting a comprehensive picture of the relationship among composition engineering, synthesis, and device properties. Additionally, we share our thoughts on several outstanding challenges of perovskite single crystals as promising X-ray detectors and propose possible approaches to the unresolved issues. We anticipate that this Perspective can open up new opportunities to improve their optoelectronic properties, which confers fascinating photonics applications with above and beyond state-of-the-art performance.

Effect of Carrier Gas Flow Rate on the Morphology and Luminescence Properties of CsPbBr3 Microcrystals

All-inorganic halide perovskites, especially lead perovskite microcrystals, have attracted more and more attention because of their excellent photoelectric properties and chemical stability. Herein, high quality CsPbBr3 microcrystals with three different stable morphologies, namely microplate, frustum of a square pyramid and pyramid, were synthesized by the chemical vapor deposition (CVD) method through altering the flow rate of a carrier gas and were comparatively studied in structure and optical property. The photoluminescence (PL) results showed that the CsPbBr3 microplate has the best luminescence property. The structural characterization results by scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray rocking curves (XRC) and Raman revealed that the flow rate of the carrier gas could manipulate the morphology evolution of CsPbBr3 microcrystals and further impact their luminescence properties.

Suppressing Oxygen-Induced Deterioration of Metal Halide Perovskites by Alkaline Earth Metal Doping: A Quantum Dynamics Study

Exposure to oxygen undermines stability and charge transport in metal halide perovskites, because molecular oxygen, as well as photogenerated superoxide and peroxide, erodes the perovskite lattice and creates charge traps. We demonstrate that alkaline earth metals passivate the oxygen species in CH₃NH₃PbI₃ by breaking the O-O bond and forming new bonds with the oxygen atoms, shifting the trap states of the antibonding O-O orbitals from inside the bandgap into the bands. In addition to eliminating the oxidizing species and the charge traps, doping with the alkaline earth metals slightly increases the bandgap and partially localizes the electron and hole wavefunctions, weakening the electron-hole and charge-phonon interactions and making the charge carrier lifetimes longer than even those in pristine CH₃NH₃PbI₃. Relative to CH₃NH₃PbI₃ exposed to oxygen and light, the charge carrier lifetime of the passivated CH₃NH₃PbI₃ increases by 2-3 orders of magnitude. The ab initio quantum dynamics simulations demonstrate that alkaline earth metals passivate efficiently not only intrinsic perovskite defects, but also the foreign species, providing a viable strategy to suppress perovskite degradation.

Highly oriented MAPbI3 crystals for efficient hole-conductor-free printable mesoscopic perovskite solar cells

Highly crystalline perovskite films with large grains and few grain boundaries are conducive for efficient and stable perovskite solar cells. Current methods for preparing perovskite films are mostly based on a fast crystallization process, with rapid nucleation and insufficient growth. In this study, MAPbI3 perovskite with inhibited nucleation and promoted growth in the TiO2/ZrO2/carbon triple mesoscopic scaffold was crystallized by modulating the precursor and the crystallization process. N-methylformamide showed high solubility for both methylammonium iodide and PbI2 and hampered the formation of large colloids in the MAPbI3 precursor solution. Furthermore, methylammonium chloride was added to reduce large colloids, which are a possible source of nucleation sites. During the crystallization of MAPbI3, the solvent was removed at a slow controlled speed, to avoid rapid nucleation and provide sufficient time for crystal growth. As a result, highly oriented MAPbI3 crystals with suppressed non-radiative recombination and promoted charge transport were obtained in the triple mesoscopic layer with disordered pores. The corresponding hole-conductor-free, printable mesoscopic perovskite solar cells exhibited a highest power conversion efficiency of 18.82%. The device also exhibited promising long-term operational stability of 1000 h under continuous illumination at maximum power point at 55 ± 5 °C and damp-heat stability of 1340 h aging at 85 °C as well as 85% relative humidity.

Improving the Conductivity of the PEDOT:PSS Layers in Photovoltaic Cells Based on Organometallic Halide Perovskites

Among conductive polymers, PEDOT films find the widest application in electronics. For photovoltaic applications, studies of their optical properties, stability, and electrical conductivity are of greatest interest. However, the PEDOT:PSS transport layers, when used in photovoltaic cells, have a high electrical resistance, which prevents solar cells from increasing their efficiency. One of the promising ways to improve their electrical properties is the use of composite materials based on them, in which the conductivity can be increased by introducing various additives. In this work, conductive polymer films PEDOT:PSS (poly (3,4-ethylenedioxythiophene):polystyrene sulfonate acid) doped with a number of amines (Pentylamine, Octylamine, Diethylamine, Aniline with carbon nanotubes) were obtained and studied. It is shown that, depending on the concentration of dopants, the electrical conductivity of PEDOT:PSS films can be significantly improved. In this case, the light transmission of the films practically does not change. The process of improving the conductivity by treating the surface of the finished film with amines, followed by heat treatment, was studied. It is assumed that the improvement in conductivity is the result of the self-assembly of monolayers of organic molecules on the surface of the PEDOT:PSS film leading to its p-doping due to intermolecular interaction.

Further Advancement of Perovskite Single Crystals

Halide perovskite (HP) single crystals (SCs) are garnering extensive attention as active materials to substitute polycrystalline counterparts in solar cells, photodiodes, and photodetectors, etc. Nevertheless, the large thickness and defect-rich surface results in severe carrier recombination and becomes the major bottleneck for augmented performance. In this perspective, we are looking forward to explaining in detail why the SCs hardly unleash their engrossing potential and introduce two parallel paths for further advancement. First is the modification of thick SCs by reducing the prepared thickness or surface passivation. Second is the large thickness that is conducive to the sufficient absorption of high-energy rays with strong penetrating ability and is beneficial to the thermoelectric effect due to the ultralow thermal conductivity of HPs. These applications provide a roundabout strategy to exploit freestanding SCs with a large thickness. Herein, direct modification and application of thick SCs are systematically introduced, expecting to give rise to the prosperity of HP SCs.

Counterdiffusion-in-gel growth of high optical and crystal quality MAPbX3 (MA = CH3NH3+, X = I−, Br−) lead-halide perovskite single crystals

Halide perovskites are promising semiconductor materials for optoelectronics.

Monocrystalline Methylammonium Lead Halide Perovskite Materials for Photovoltaics

Lead halide perovskite solar cells have been gaining more and more interest. In only a decade, huge research efforts from interdisciplinary communities enabled enormous scientific advances that rapidly led to energy conversion efficiency near that of record silicon solar cells, at an unprecedented pace. However, while for most materials the best solar cells were achieved with single crystals (SC), for perovskite the best cells have been so far achieved with polycrystalline (PC) thin films, despite the optoelectronic properties of perovskite SC are undoubtedly superior. Here, by taking as example monocrystalline methylammonium lead halide, the authors elaborate the literature from material synthesis and characterization to device fabrication and testing, to provide with plausible explanations for the relatively low efficiency, despite the superior optoelectronics performance. In particular, the authors focus on how solar cell performance is affected by anisotropy, crystal orientation, surface termination, interfaces, and device architecture. It is argued that, to unleash the full potential of monocrystalline perovskite, a holistic approach is needed in the design of next-generation device architecture. This would unquestionably lead to power conversion efficiency higher than those of PC perovskites and silicon solar cells, with tremendous impact on the swift deployment of renewable energy on a large scale.

Three-Dimensional CA-LBM Numerical Model and Experimental Verification of Cs2AgBiBr6 Perovskite Single Crystals Grown by Solution Method

A three-dimensional cellular automata-lattice Boltzmann (CA-LBM) coupling model is established to simulate the facet growth process and the controlled cooling growth process of Cs2AgBiBr6 perovskite single crystals. In this model, the LBM method is used to calculate the real-time solute field, the CA method is used to simulate the crystal growth process driven by supersaturation of solute, and the geometric parameter g related to the adjacent grid is introduced to reduce the influence of grid anisotropy. The verification of the model is achieved by comparing the simulation results with the experimental results. The comparison results show that a smaller cooling rate is helpful for the growth of large-size single crystals, which verifies the rationality and correctness of the model.

Highly Luminescent Metal-Free Perovskite Single Crystal for Biocompatible X-Ray Detector to Attain Highest Sensitivity

Solution-processed metal-based halide perovskites have taken a dominant position for perovskite optoelectronics including light emission and X-ray detection; however, the toxicity of the included heavy metals severely restricts their applications for wearable, lightweight, and transient optoelectronic devices. Here, the authors describe investigations of large (4 × 6 × 2 mm³ ) 3D metal-free perovskite MDABCO-NH₄ I₃ (MDBACO = methyl-N'-diazabicyclo[2.2.2]octonium) single crystal and its charge recombination and extraction behavior for light emission and X-ray detection. Unlike conventional 3D metal-based perovskites, this lightweight and biocompatible perovskite large crystal is processed from aqueous solution at room temperature, and can achieve both an extremely long carrier lifetime up to ≈1.03 µs and the formation of self-trapped excited states for luminescence. These features contribute to a photoluminescence quantum yield (PLQY) as high as ≈53% at room temperature and an X-ray sensitivity up to 1997 ± 80 μC Gy cm^-2 at 50 V bias (highest among all metal-free detectors). The ability to tune the perovskite band gap by modulating the structure under high pressure is also demonstrated, which opens up applications for the crystal as colored emitters. These attributes make it a molecular alternative to metal-based perovskites for biocompatible and transient optoelectronics.

Introducing Intermolecular Cation-π Interactions for Water-Stable Low Dimensional Hybrid Lead Halide Perovskites

Optoelectronically active hybrid lead halide perovskites dissociate in water. To prevent this dissociation, here, we introduce long-range intermolecular cation-π interactions between A-site cations of hybrid perovskites. An aromatic diamine like 4,4'-trimethylenedipyridine, if protonated, can show a long-range cation-π stacking, and therefore, serves as our A-site cation. Consequently, 4,4'-trimethylenedipyridinium lead bromide [(4,4'-TMDP)Pb₂ Br₆ ], a one-dimensional hybrid perovskite, remains completely stable after continuous water treatment for six months. Mechanistic insights about the cation-π interactions are obtained by single-crystal X-ray diffraction and nuclear magnetic resonance spectroscopy. The concept of long-range cation-π interaction is further extended to another A-site cation 4,4'-ethylenedipyridinium ion (4,4'-EDP), forming water-stable (4,4'-EDP)Pb₂ Br₆ perovskite. These water-stable perovskites are then used to fabricate white light-emitting diode and for light up-conversion through tunable third-harmonic generation. Note that the achieved water stability is the intrinsic stability of perovskite composition, unlike the prior approach of encapsulating the unstable perovskite material (or device) by water-resistant materials. The introduced cation-π interactions can be a breakthrough strategy in designing many more compositions of water-stable low-dimensional hybrid perovskites.

II-VI Organic-Inorganic Hybrid Nanostructures with Greatly Enhanced Optoelectronic Properties, Perfectly Ordered Structures, and Shelf Stability of Over 15 Years

Organic-inorganic hybrids may offer material properties not available from their inorganic components. However, they are typically less stable and disordered. Long-term stability study of the hybrid materials, over the anticipated lifespan of a real-world electronic device, is practically nonexistent. Disordering, prevalent in most nanostructure assemblies, is a prominent adversary to quantum coherence. A family of perfectly ordered II-VI-based hybrid nanostructures has been shown to possess many unusual properties and potential applications. Here, using a prototype structure β-ZnTe(en)_0.5-a hybrid superlattice-and applying an array of optical, structural, surface, thermal, and electrical characterization techniques, in conjunction with density-functional theory calculations, we have performed a comprehensive and correlative study of the crystalline quality, structural degradation, electronic, optical, and transport properties on samples from over 15 years old to the recently synthesized. The findings show that not only do they exhibit an exceptionally high level of crystallinity in both macroscopic and microscopic scale, comparable to high-quality binary semiconductors; and greatly enhanced material properties, compared to those of the inorganic constituents; but also, some of them over 15 years old remain as good in structure and property as freshly made ones. This study reveals (1) what level of structural perfectness is achievable in a complex organic-inorganic hybrid structure or a man-made superlattice, suggesting a nontraditional strategy to make periodically stacked heterostructures with abrupt interfaces; and (2) how the stability of a hybrid material is affected differently by its intrinsic attributes, primarily formation energy, and extrinsic factors, such as surface and defects. By correlating the rarely found long-term stability with the calculated relatively large formation energy of β-ZnTe(en)_0.5 and contrasting with the case of hybrid perovskite, this work illustrates that formation energy can serve as an effective screening parameter for the long-term stability potential of hybrid materials. The results of the prototype II-VI hybrid structures will, on one hand, inspire directions for future exploration of the hybrid materials, and, on the other hand, provide metrics for assessing the structural perfectness and long-term stability of the hybrid materials.

Elimination of Charge Recombination Centers in Metal Halide Perovskites by Strain

Metal halide perovskites exhibit enhanced photoluminescence and long-lived carriers in experiments under strain. Using ab initio nonadiabatic molecular dynamics, we demonstrate that compressive and tensile strain can eliminate charge recombination centers created by defect states, by shifting traps from bandgap into bands. A compressive strain enhances coupling of Pb-s and I-p orbitals, pushes the valence band (VB) up in energy, and moves the trap state due to iodine interstitial (I_i) into the VB. The strain distorts the system and breaks the I-dimer responsible for the I_i trap. A tensile strain increases Pb-Pb distance, weakens overlap of Pb-p orbitals, and pushes the conduction band (CB) down in energy. The trap state created by replacement of iodine with methylammonium (MA_I) is moved into the CB. Application of strain to the defective systems not only eliminates midgap traps but also creates moderate disorder that reduces overlap of electron and hole wave functions, activates phonon modes accelerating coherence loss within the electronic subsystem, and extends carrier lifetimes even beyond those in pristine MAPbI₃. Our investigation rationalizes the high performance of perovskites solar cells under strain and reveals how strain passivates I_i and MA_I defects in MAPbI₃, providing a new nonchemical strategy for defect control and engineering.

Revealing Charge Carrier Mobility and Defect Densities in Metal Halide Perovskites via Space-Charge-Limited Current Measurements

Space-charge-limited current (SCLC) measurements have been widely used to study the charge carrier mobility and trap density in semiconductors. However, their applicability to metal halide perovskites is not straightforward, due to the mixed ionic and electronic nature of these materials. Here, we discuss the pitfalls of SCLC for perovskite semiconductors, and especially the effect of mobile ions. We show, using drift-diffusion (DD) simulations, that the ions strongly affect the measurement and that the usual analysis and interpretation of SCLC need to be refined. We highlight that the trap density and mobility cannot be directly quantified using classical methods. We discuss the advantages of pulsed SCLC for obtaining reliable data with minimal influence of the ionic motion. We then show that fitting the pulsed SCLC with DD modeling is a reliable method for extracting mobility, trap, and ion densities simultaneously. As a proof of concept, we obtain a trap density of 1.3 × 10¹³ cm^-3, an ion density of 1.1 × 10¹³ cm^-3, and a mobility of 13 cm² V^-1 s^-1 for a MAPbBr₃ single crystal.

Zero-Dimensional Lead-Free FA3Bi2I9 Single Crystals for High-Performance X-ray Detection

Direct X-ray detectors based on metal halide perovskites and their derivatives exhibit high sensitivity and low limit of detection (LoD). Compared with three-dimensional (3D) hybrid lead halide perovskites, low-dimensional A₃Bi₂I₉ perovskite derivatives (A = Cs, Rb, NH₄, CH₃NH₃(MA)) present better stability, greater environmental friendliness, and comparable X-ray detection performance. Here, we report FA₃Bi₂I₉ (FA= CH(NH₂)₂) single crystals (SCs) as a new member of the A₃Bi₂I₉ series for X-ray detection, which were prepared by the nucleation-controlled secondary solution constant temperature evaporation (SSCE) method. Centimeter-sized FA₃Bi₂I₉ SCs show a full width at half-maximum (fwhm) of 0.0096°, which is superior to that of recently reported Cs₃Bi₂I₉ (0.058°) and MA₃Bi₂I₉ SCs (0.024°) obtained by inverse temperature crystallization (ITC). The as-grown FA₃Bi₂I₉ SC shows a large resistivity of 7.8 × 10¹⁰ Ω cm and a high ion migration activation energy (E_a) of 0.56 eV, which can guarantee a low noise level and good operational stability under a large external bias. The FA₃Bi₂I₉ SC detector exhibits a LoD of 0.2 μGy_air s^-1, a sensitivity of 598.1 μC Gy_air ^-1 cm ^-2, and an X-ray detection efficiency of 33.5%, which are much better than those of the commercialized amorphous selenium detector. Results presented here will provide a new lead-free perovskite-type material to achieve green, sensitive, and stable X-ray detectors.