General Space-Confined On-Substrate Fabrication of Thickness-Adjustable Hybrid Perovskite Single-Crystalline Thin Films

Perovskite Solar Cells and Light Emitting Diodes: Materials Chemistry, Device Physics and Relationship

Solution-processed perovskite solar cells (PSCs) and perovskite light emitting diodes (PeLEDs) represent promising next-generation optoelectronic technologies. This Review summarizes recent advancements in the application of metal halide perovskite materials for PSC and PeLED devices to address the efficiency, stability and scalability issues. Emphasis is placed on material chemistry strategies used to control and engineer the composition, deposition process, interface and micro-nanostructure in solution-processed perovskite films, leading to high-quality crystalline thin films for optimal device performance. Furthermore, we retrospectively compare the device physics of PSCs and PeLEDs, their working principles and their energy loss mechanisms, examining the similarities and differences between the two types of devices. The reciprocity relationship suggests that a great PSC should also be a great PeLED, motivating the search for interconverting photoelectric bifunctional devices with maximum radiative recombination and negligible non-radiative recombination. Specific requirements of PSCs and PeLEDs in terms of bandgap, thickness, band alignment and charge transport to achieve this target are discussed in detail. Further challenges and issues are also illustrated, together with prospects for future development. Understanding these fundamentals, embracing recent breakthroughs and exploring future prospects pave the way toward the rational design and development of high-performance PSC and PeLED devices.

Advances in Single-Crystal Films: Synergistic Insights from Perovskites and Organic Molecules for High-Performance Optoelectronics

Semiconductor single-crystal thin films are crucial for the advancement of high-performance optoelectronic devices. Despite significant progress in fabricating perovskite and organic single-crystal films, interdisciplinary insights between these domains remain unexplored. This review aims to bridge this gap by summarizing recent advances in fabrication strategies for perovskite and organic molecular single-crystal films. Five preparation methods-solution-phase epitaxy, solid-phase epitaxy, meniscus-induced crystallization, antisolvent-induced crystallization, and space-confined growth-are analyzed with a focus on their principles, functional properties, and distinct advantages. By comparing these approaches across material systems, this review identifies transferable insights that can drive the development of large-scale, high-quality single-crystal films. Furthermore, the optoelectronic applications of these films are explored, including solar cells, photodetectors, light-emitting devices, and transistors, while addressing challenges such as scalability, defect control, and integration. This work highlights the importance of cross-disciplinary innovation and provides an effective pathway for integrating perovskite and organic molecular processing to advance the next generation of single-crystal film technologies.

Mechanistic insights and optimization strategies for perovskite single-crystal thin film growth

Perovskite materials, with their tunable band gaps, high optical absorption, and excellent carrier mobility, are key candidates for lasers, LEDs, photodetectors, and solar cells. Polycrystalline thin films dominate current applications but suffer from efficiency and stability losses largely due to grain boundaries. Perovskite single-crystal thin films (SCTFs) offer optimized carrier diffusion and reduced recombination losses, though challenges in achieving high-quality SCTFs remain. Fabrication techniques and device applications of SCTFs have been widely explored, yet the crystallization mechanisms that critically influence film quality and device performance offer significant opportunities for further investigation. This review aims to provide a comprehensive analysis of SCTF nucleation, growth dynamics, and structural optimization, highlighting the role of external factors like substrate properties and solution chemistry. By advancing the understanding of these mechanisms, we hope to guide efficient SCTF fabrication and inspire innovations in high-performance, stable perovskite-based optoelectronics.

In-liquid Superspreading Space-confined Epitaxy on Superamphiphilic Surfaces for Pt(II) Complex Crystalline Film Growth

Solution-based method is regarded as a promising approach to fabricate large-area, high-quality crystalline films, owing to its low-cost manufacturing and facile features. However, traditional solution-based methods still suffer from random simultaneous nucleation and uncontrollable crystal growth which result in polycrystalline films and coffee-ring effect. Herein, it is proposed that an in-liquid superspreading space-confined epitaxy approach on a superamphiphilic surface to fabricate crystalline films with controllable initial nucleation and crystal morphology. With delicate control of the liquid environment, concentration, and superspreading space-confined solvent film thickness, planar crystalline films with high crystallinity and smooth morphology are obtained. A controllable dewetting crystallization mechanism is proposed, indicating that the diffusion coefficient, regulated by liquid environment, can control the dewetting process during crystallization. With the balance of solvent diffusion and solute precipitation in crystallization, the ordered in-plane and out-of-plane molecular stacking is achieved. And the as-prepared planar Pt(II) complex crystalline film exhibits multi-signal sensing ability, which can be further used to fabricate the reaffirmed sensing detector for precise gas sensing in complex and unstable conditions. This study demonstrates a facile approach for crystalline film fabrication with controllable nucleation and morphology in a liquid environment, which holds promising applications in the construction of oxygen or water-sensitive organic/inorganic devices.

Graphite-Based Localized Heating Technique for Growing Large Area Methylammonium Lead Bromide Single Crystalline Perovskite Wafers and Their Charge Transfer Characteristics

Development of a reproducible technique to grow large area single crystalline perovskite wafers is an open research gap in the field of single crystalline perovskite solar cells. A graphite-based localized heating technique for growing large area methylammonium lead bromide (CH3NH3PbBr3; MAPBr) single crystalline thin film (SCTF) on different buffer layers, such as glass/indium doped tin oxide (ITO), glass/ITO/poly(triaryl amine) (PTAA), and glancing angle deposition (GLAD) coated glass/ITO/TiO2 substrates is reported, and their charge transport properties are discussed. It is observed that the localized heating technique can confine the supersaturation of the precursor mainly to the center of the substrate, leading to a restricted number of nucleations within a specific area on the substrate. Here, such 2-3 seed crystals obtained initially are allowed to grow to a larger size of up to 65 mm2. The X-ray diffraction (XRD) analysis indicated that the large area SCTF is an actual single crystal and not a heterogeneous group of small crystals merged together with a crystallinity index (CI) of 92.60 ± 0.11% which was comparable to that of the bulk single crystal (97.74 ± 0.47%). The atomic force microscopy (AFM) image depicted a smooth SCTF surface (R a = 4.37 ± 0.01 nm), and the wave-like pattern is attributed to the substrate morphology, implying that the topography of the substrate plays a crucial role in obtaining a planar SCTF. The XRD, UV-visible, photoluminescence (PL), Raman, and FTIR spectra analyses revealed that the large area SCTF is phase pure and free of residual impurities. The charge injection characteristics of the SCTFs grown on different buffer layers were investigated using PL emission (PLE) and PL decay analyses. The decrease in the PLE intensity for the SCTFs grown on PTAA and TiO2 substrates implied exciton quenching behavior, indicating the injection of the photogenerated charge carriers into the charge transfer layers (CTLs). The decrease of the fast decay component from τ1 = 4.77 ± 0.18 ns for glass to τ1 = 3.32 ± 0.07 ns for TiO2 and τ1 = 3.15 ± 0.33 ns for PTAA is ascribed to the interfacial recombination of the charges accumulated at the CTL/perovskite interface. These results propose that the localized heating technique can be employed for growing large area single crystalline perovskite wafers for optoelectronic and photovoltaic device applications.

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.

Preparation Techniques for Perovskite Single Crystal Films: From Nucleation to Growth

Thickness-controllable perovskite single crystal films exhibit tremendous potential for various optoelectronic applications due to their capacity to leverage the relationship between diffusion length and absorption depth. However, the fabrication processes have suffered from difficulties in large-area production and poor quality with abundant surface defects. While post-treatments, such as passivation and polishing, can provide partial improvement in surface quality, the fundamental solution lies in the direct growth of high-quality single crystal films. In this work, we firstly summarize the basic principles of nucleation and growth phenomenon of crystalline materials. Advanced growth methods of perovskite single crystal films, including solution-based, vapor phase epitaxial growth, and top-down method, are discussed, highlighting their respective advantages and limitations. Finally, we also present future directions and the challenges that lie ahead in perovskite single crystal films.

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.

Interfacial Engineering for Controlled Crystal Mosaicity in Single-Crystalline Perovskite Films

Organic-inorganic hybrid perovskites have attracted significant attention for optoelectronic applications due to their efficient photoconversion properties. However, grain boundaries and irregular crystal orientations in polycrystalline films remain issues. This study presents a method for producing crystalline-orientation-controlled perovskite single-crystal films using retarded solvent evaporation. It is shown that single-crystal films, grown via inverse temperature crystallization within a confined space, exhibit enhanced optoelectronic property. Using interfacial polymer layer, this method produces high-quality perovskite single-crystalline films with varying crystal orientations. Density functional theory calculations confirm favorable adsorption energies for (110) surfaces with methylammonium iodide and PbI2 terminations on poly(3-hexylthiophene), and stronger adsorption for (224) surfaces with I and methylammonium terminations on polystyrene, influenced by repulsive forces between the thiophene group and the perovskite surface. The correlation between charge transport characteristics and perovskite single-crystalline properties highlights potential advancements in perovskite optoelectronics, improving device performance and reliability.

Recent Progress of Thin Crystal Engineering for Perovskite Solar Cells

Metal halide perovskite single crystals hold promise for photovoltaics with high efficiency and stability due to their superior optoelectronic properties and weak bulk ion migration. The past several years have witnessed rapid development of single-crystal perovskite solar cells (PSCs) with efficiency rocketed from 6.5 % to 24.3 %, however, which still lags behind their polycrystalline counterparts. Moreover, the poor device stability under light illumination is contrary to the high ion migration barrier of perovskite single crystals. The key limiting factors should be the low crystalline quality and high surface defect density of solution-grown thin single crystals. Under this circumstance, a review paper summarizing the recent progress and challenges will be instructive for future development of this emerging field. In this manuscript, the crystal engineering used to enhance carrier transport and suppress carrier recombination in vertical single-crystal PSCs will be summarized initially, including crystal growth, component control, surface and interface modification. Subsequently, the application of perovskite single crystals in lateral single-crystal PSCs will be discussed and compared with the conventionally vertical structure. Finally, the challenges and proposed strategies for the development of single-crystal PSCs are provided.

Space-Confined Growth for Thickness-Controlled Cs3Bi2I9 Perovskite Single Crystal Wafers for X-Ray Detectors

The Cs3Bi2I9 single crystal, as an all-inorganic non-lead perovskite, offers advantages such as stability and environmental friendliness. Its superior photoelectric properties, attributed to the absence of grain boundary influence, make it an outstanding X-ray detection material compared to polycrystals. In addition to material properties, X-ray detector performance is affected by the thickness of the absorption layer. Addressing this, a space-confined method is proposed. The temperature field is determined through finite element simulation, effectively guiding the design of the space-confined method. Through this innovative method, a series of thickness-controlled perovskite single crystal wafers (PSCWs) are successfully prepared. Corresponding X-ray detectors are then prepared, and the impact of single crystal thickness on device performance is investigated. With an increase in single crystal thickness, a rise followed by a decline in device sensitivity is observed, reaching an optimal value at 0.7 mm thickness at 40V mm-1 with a device performance of 11313.6µC Gy-1 cm-2. This space-confined method enables the direct growth of high-quality perovskite single crystals with specified thickness, eliminating the need for slicing or etching.

Recent Advances in Structure Design and Application of Metal Halide Perovskite-Based Gas Sensor

Metal halide perovskites (MHPs) are emerging gas-sensing materials and have attracted considerable attention in gas sensors due to their unique bandgap structure and tunable optoelectronic properties. The past decade has witnessed significant developments in the gas-sensing field; however, their intrinsic structural instability and ambiguous gas-sensing mechanisms hamper their practical applications. Herein, we summarize the recent advances in MHP-based gas sensors. The physicochemical properties of MHPs are discussed at first. The structure design, including dimension design and engineering design, is overviewed as well as their fabrication methods, and we put forward our insights into the gas-sensing mechanism of MHPs. It is believed that enhanced understanding of gas-sensing mechanisms of MHPs are helpful for their application as gas-sensing materials, and structure design can enhance their stability, sensing sensitivity, and selectivity to target gases as gas sensors. Subsequently, the latest developments in MHP-based gas sensors are summarized according to their different application scenarios. Finally, we conclude with the current status and challenges in this field and propose future perspectives.

Unified Crystal Phase Control with MACl for Inducing Single-Crystal-Like Perovskite Thin Films in High-Pressure Fusion Toward High Efficiency Perovskite Solar Cell Modules

Perovskite solar cells, recognized for their high photovoltaic conversion efficiency (PCE), cost-effectiveness, and simple fabrication, face challenges in PCE improvement due to structural defects in polycrystalline films. This study introduces a novel fabrication method for perovskite films using methylammonium chloride (MACl) to align grain orientation uniformly, followed by a high-pressure process to merge these grains into a texture resembling single-crystal perovskite. Employing advanced visual fluorescence microscopy, charge dynamics in these films are analyzed, uncovering the significant impact of grain boundaries on photo-generated charge transport within perovskite crystals. A key discovery is that optimal charge transport efficiency and speed occur in grain centers when the grain size exceeds 10 µm, challenging the traditional view that efficiency peaks when grain size surpasses film thickness to form a monolayer. Additionally, the presence of large-sized grains enhances ion activation energy, reducing ion migration under light and improving resistance to photo-induced degradation. In application, a perovskite solar cell module with large grains achieve a PCE of 22.45%, maintaining performance with no significant degradation under continuous white LED light at 100 mA cm-2 for over 1000 h. This study offers a new approach to perovskite film fabrication and insights into optimizing perovskite solar cell modules.

van der Waals Integration of Large-Area Monocrystalline 3D Perovskite Thin Films on Arbitrary Semiconductor Substrates for Heterojunctions

Perovskite monocrystalline films are regarded as desirable candidates for the integration of high-performance optoelectronics due to their unique photophysical properties. However, the heterogeneous integration of a perovskite monocrystalline film with other semiconductors is fundamentally limited by the lattice mismatch, which hinders direct epitaxy. Herein, the van der Waals (vdW) integration strategy for 3D perovskites is developed, where perovskite monocrystalline films are epitaxially grown on the mother substrate, followed by its peeling off and transferring to arbitrary semiconductors, forming monocrystalline heterojunctions. The as-achieved CsPbBr3-Nb-doped SrTiO3 (Nb:STO) vdW p-n heterojunction exhibited comparable performance to their directly epitaxial counterpart, demonstrating the feasibility of vdW integration for 3D perovskites. Furthermore, the vdW integration could be extended to silicon substrates, rendering the CsPbBr3-n-Si and CsPbCl3-p-Si p-n heterojunction with apparent rectification behaviors and photoresponse. The vdW integration significantly enriches the selections of semiconductors hybridizing with perovskites and provides opportunities for monocrystalline perovskite optoelectronics with complex configurations and multiple functionalities.

Gas-Liquid Interface Route to Hybrid Copper Bromine Perovskite Single-Crystal Membrane with Dielectric Transitions and Ferromagnetic Exchanges

Single-crystal membranes (SCMs) show great promise in the fields of sensors, light-emitting diodes, and photodetection. However, the growth of a large-area single-crystal membranes is challenging. We report a new organic-inorganic SCMs [HCMA]2CuBr4 (HCMA = cyclohexanemethylamine) crystallized at the gas-liquid interface. It also has low-temperature ferromagnetic order, high-temperature dielectric anomalies, and narrow band gap indirect semiconductor properties. Specifically, the reversible phase transition of the compound occurs at 350/341 K on cooling/heating and exhibits dielectric anomalies and stable switching performance near the phase transition temperature. The ferromagnetic exchange interaction in the inorganic octahedra and the organic layer enables ferromagnetic ordering at low-temperature 10 K. Finally, the single crystal exhibits an indirect semiconducting property with a narrow band gap of 0.99 eV. Such rich multichannel physical properties make it a potential application in photodetection, information storage and sensors.

A Polarization-Sensitive Photodetector with Patterned CH3NH3PbCl3 Film

Perovskite photodetectors with polarization-sensitive properties have gained significant attention due to their potential applications in fields such as imaging and remote sensing. Most perovskite photodetectors concentrate on iodine (I) or bromine (Br)-based materials, primarily due to their straightforward fabrication techniques. The utilization of chloride (Cl)-based perovskites with wider bandgaps, such as CH3NH3PbCl3, is relatively limited. In this work, polarized perovskite photodetectors are prepared by a patterned spatially confined method with polarization sensitivity and excellent optoelectronic properties. The patterned perovskite photodetectors (PP-PDs) not only exhibit outstanding photoelectric conversion performance but also demonstrate polarization sensitivity. PP-PDs showcase remarkable performance, including on/off ratios of 3.4 × 104, an extremely low dark current of 1.56 × 10-11 A, and a rapid response time of microseconds. The responsivity and detectivity of PP-PDs reach 10.6 A W-1 and 3 × 1012 Jones, respectively, positioning them as among the highest-performing MAPbCl3-based photodetectors reported to date. Furthermore, polarization layered imaging sensing is achieved using stepwise scanning of the device. This work provides innovative ideas for realizing high-performance polarized perovskite photodetectors.

Can thick metal-halide perovskite single crystals have narrower optical bandgaps with near-infrared absorption?

Metal-halide perovskite (MHP) single crystals are emerging as potential competitors to their polycrystalline thin-film counterparts. These materials have shown the specific feature of extended absorbance towards the near-infrared (NIR) region, which promises further extension of their applications in the field of photovoltaics and photodetectors. This notable expansion of absorbance has been explained by the narrower effective optical bandgap of MHP single crystals promoted by their large thickness over several micrometres to millimetres. Herein, the attributes of the material's thickness and the measurement technique used to estimate these characteristics are discussed to elucidate the actual origins of the extended absorbance of MHP single crystals. Contrary to the general belief of the narrower bandgap of the MHP single crystals, we demonstrate that the extended NIR absorption in the MHP single crystals mainly originates from the combination of unique below-bandgap absorption of MHPs, the thickness of single crystals, and the technical limitation of the spectrophotometer, with the key attributes of (i) significantly large thickness of the MHP single crystals by suppressing the transmitted light and (ii) the detector's limited dynamic range. Combining the theoretical and experimental characterizations, we clarify the significant role of the large thickness together with the limited sensitivity of the detector in promoting the well-known red shift of the absorption onset of the MHP single crystals. The observations evidently show that in some special circumstances, the acquired absorption spectrum cannot reliably represent the optical bandgap of MHP materials. This highlights some misinterpretations in the estimation of the narrower optical bandgap of the MHP single crystals from conventional optical methods, while the optical bandgap is an inherent property independent of the thickness. The proposed broad applications of the MHP single crystals are dictated by their fascinating properties, and therefore, a deep insight into these features should be considered besides device applications, because much of their property-function relationships are still ambiguous and a subject of debate.

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.

Vapor Phase Growth of Air-Stable Hybrid Perovskite FAPbBr3 Single-Crystalline Nanosheets

Organic-inorganic hybrid perovskites have attracted tremendous attention owing to their fascinating optoelectronic properties. However, their poor air stability seriously hinders practical applications, which becomes more serious with thickness down to the nanoscale. Here we report a one-step vapor phase growth of HC(NH2)2PbBr3 (FAPbBr3) single-crystalline nanosheets of tunable size up to 50 μm and thickness down to 20 nm. The FAPbBr3 nanosheets demonstrate high stability for over months of exposure to air with no degradation in surface roughness and photoluminescence efficiency. Besides, the FAPbBr3 photodetectors exhibit superior overall performance as compared to previous devices based on nonlayered perovskite nanosheets, such as an ultralow dark current of 24 pA, an ultrahigh responsivity of 1033 A/W, an external quantum efficiency over 3000%, a rapid response time around 25 ms, and a high on/off ratio of 104. This work provides a strategy to tackle the challenges of hybrid perovskites toward integrated optoelectronics with requirements of nanoscale thickness, high stability, and excellent performance.

Diffusion of Solute Atoms in an Evaporated Liquid Droplet

Controlling the evaporation of a solvent has made it possible to grow crystals, nanoparticles, and microparticles from liquid droplets. At the heart of this process is the evaporation-induced diffusion of solute atoms, causing the liquid solution of the solute atoms to be in a supersaturated state. In this work, we analyze the mass transport in a spherical liquid droplet, which experiences the loss or evaporation of the solvents across the droplet surface. Using a pseudo-steady-state method, two approximate solutions are derived for the moving boundary problem: one is a linear function of the square of radial variable with a constraint to the loss rate of the solvent, and the other is an exponential function of the square of radial variable without any constraint to the loss rate of the solvent. The numerical results obtained from both approximate solutions are in accord with the numerical results from the finite element method, validating the approximate solutions. The results reveal that a small evaporation/loss rate of the solvent is needed to maintain a relatively uniform distribution of solute atoms in a liquid droplet during the solvent evaporation/loss.

General Spatial Confinement Recrystallization Method for Rapid Preparation of Thickness-Controllable and Uniform Organic Semiconductor Single Crystals

Organic semiconductor single crystals (OSSCs) are ideal materials for studying the intrinsic properties of organic semiconductors (OSCs) and constructing high-performance organic field-effect transistors (OFETs). However, there is no general method to rapidly prepare thickness-controllable and uniform single crystals for various OSCs. Here, inspired by the recrystallization (a spontaneous morphological instability phenomenon) of polycrystalline films, a spatial confinement recrystallization (SCR) method is developed to rapidly (even at several second timescales) grow thickness-controllable and uniform OSSCs in a well-controlled way by applying longitudinal pressure to tailor the growth direction of grains in OSCs polycrystalline films. The relationship between growth parameters including the growth time, temperature, longitudinal pressure, and thickness is comprehensively investigated. Remarkably, this method is applicable for various OSCs including insoluble and soluble small molecules and polymers, and can realize the high-quality crystal array growth. The corresponding 50 dinaphtho[2,3-b:2″,3″-f]thieno[3,2-b]thiophene (DNTT) single crystals coplanar OFETs prepared by the same batch have the mobility of 4.1 ± 0.4 cm2 V-1 s-1 , showing excellent uniformity. The overall performance of the method is superior to the reported methods in term of growth rate, generality, thickness controllability, and uniformity, indicating its broad application prospects in organic electronic and optoelectronic devices.

Perovskite single-crystal thin films: preparation, surface engineering, and application

Perovskite single-crystal thin films (SCTFs) have emerged as a significant research hotspot in the field of optoelectronic devices owing to their low defect state density, long carrier diffusion length, and high environmental stability. However, the large-area and high-throughput preparation of perovskite SCTFs is limited by significant challenges in terms of reducing surface defects and manufacturing high-performance devices. This review focuses on the advances in the development of perovskite SCTFs with a large area, controlled thickness, and high quality. First, we provide an in-depth analysis of the mechanism and key factors that affect the nucleation and crystallization process and then classify the methods of preparing perovskite SCTFs. Second, the research progress on surface engineering for perovskite SCTFs is introduced. Third, we summarize the applications of perovskite SCTFs in photovoltaics, photodetectors, light-emitting devices, artificial synapse and field-effect transistor. Finally, the development opportunities and challenges in commercializing perovskite SCTFs are discussed.

Anisotropic Growth of Centimeter-Size CsCu2 I3 Single Crystals with Ultra-Low Trap Density for Aspect-Ratio-Dependent Photodetectors

Low-dimensional ternary copper iodide metal halide with strong quantum confinement effects has made great progress in optoelectronic fields. However, efficient regulation of anisotropic growth of metal halides single crystal still remains a great challenge. Herein, 2 cm size CsCu₂ I₃ single crystals with tunable aspect ratio and the trap states (n_trap ) as low as 5.38 × 10⁹  cm^-3 are fabricated by optimized anti-solvent vapor-assisted method, in which the growth cycle is shortened by half. Evidenced by real-time observation and the LaMer growth model, the rapid and anisotropic growth mechanism is ascribed to preferential 1D growth, promoted by high concentration and fast vapor rate. Furthermore, the aspect-ratio-dependent optoelectronic performance is observed, the on-off ratio for 2 cm CsCu₂ I₃ single crystal are enhanced 350 times compared with those of short and thick single crystal, which shows ultrahigh on-off ratio of 1570, D* of 1.34 × 10¹²  Jones, R_λ of 276.94 mA W^-1 , t _rise /t _decay of 0.37 and 1.08 ms, and EQE of 95.53%, which are clearly at very high level among lead-free perovskite-based photodetectors. This study not only provides a new strategy for overcoming anisotropic growth limitations of low-dimensional metal halides, but also paves a way for high-performance optoelectronic applications.

Excessive Iodine Enabled Ultrathin Inorganic Perovskite Growth at the Liquid-Air Interface

The liquid-air interface offers a platform for the in-plane growth of free-standing materials. However, it is rarely used for inorganic perovskites and ultrathin non-layered perovskites. Herein the liquid-air interfacial synthesis of inorganic perovskite nanosheets (Cs₃ Bi₂ I₉ , Cs₃ Sb₂ I₉ ) is achieved simply by drop-casting the precursor solution with only the addition of iodine. The products are inaccessible without iodine addition. The thickness and lateral size of these nanosheets can be adjusted through the iodine concentration. The high volatility of the iodine spontaneously drives precursors that normally stay in the liquid to the liquid-air interface. The iodine also repairs in situ iodine vacancies during perovskite growth, giving enhanced optical and optoelectronic properties. The liquid-air interfacial growth of ultrathin perovskites provides multi-degree-of-freedom for constructing perovskite-based heterostructures and devices at atomic scale.

Synthetic approaches for perovskite thin films and single-crystals

Halide perovskites are compelling candidates for the next generation of photovoltaic technologies owing to an unprecedented increase in power conversion efficiency and their low cost, facile fabrication and outstanding semiconductor properties.

New Approaches to Produce Large-Area Single Crystal Thin Films

Wafer-scale growth of single crystal thin films of metals, semiconductors, and insulators is crucial for manufacturing high-performance electronic and optical devices, but still challenging from both scientific and industrial perspectives. Recently, unconventional advanced synthetic approaches have been attempted and have made remarkable progress in diversifying the species of producible single crystal thin films. This review introduces several new synthetic approaches to produce large-area single crystal thin films of various materials according to the concepts and principles.

Zero-Dimensional Cs3BiX6 (X = Br, Cl) Single Crystal Films with Second Harmonic Generation

The development of atomically thin single crystal films is necessary to potential applications in the 2D semiconductor field, and it is significant to explore new physical properties in low-dimensional semiconductors. Since, zero-dimensional (0D) materials without natural layering are connected by strong chemical bonds, it is challengeable to break symmetry and grow 0D Cs₃BiX₆ (X = Br, Cl) single crystal thin films. Here, we report the successful growth of 0D Cs₃BiX₆ (X = Br, Cl) single crystal films using a solvent evaporation crystallization strategy. Their phases and structures are both well evaluated to confirm 0D Cs₃BiX₆ (X = Br, Cl) single crystal films. Remarkably, the chemical potential dependent morphology evolution phenomenon is observed. It gives rise to morphology changes of Cs₃BiBr₆ films from rhombus to hexagon as BiBr₃ concentration increased. Additionally, the robust second harmonic generation signal is detected in the Cs₃BiBr₆ single crystal film, demonstrating the broken symmetry originated from decreased dimension or shape change.

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.

Rationalizing the Effect of Polymer-Controlled Growth of Perovskite Single Crystals on Optoelectronic Properties

To improve and modulate the optoelectronic properties of single-crystal (SC) metal halide perovskites (MHPs), significant progress has been achieved. Polymer-assisted techniques are a great approach to control the growth rate of SCs effectively. However, the resultant optoelectrical properties induced by polymers are ambiguous and need to be taken into the consideration. In this study, we have synthesized methylammonium lead triiodide (MAPbI3) SCs using polyethylene glycol (PEG) and polystyrene (PS) polymers where PEG contains oxygen functionalities and PS does not. We studied the electrical properties of these SCs under dark and illumination conditions. It was observed that PEG-assisted SCs showed few defects with lower photocurrent as compared to the PS-assisted ones because of defect-mediated conductivity. The results are further verified by transient current response, responsivity, and capacitance-frequency measurements. The present study sheds light on the polymer selection for the growth of MHP SCs and their optoelectronic properties.

Single-crystal organometallic perovskite optical fibers

Semiconductors in their optical-fiber forms are desirable. Single-crystal organometallic halide perovskites have attractive optoelectronic properties and therefore are suitable fiber-optic platforms. However, single-crystal organometallic perovskite optical fibers have not been reported before due to the challenge of one-directional single-crystal growth in solution. Here, we report a solution-processed approach to continuously grow single-crystal organometallic perovskite optical fibers with controllable diameters and lengths. For single-crystal MAPbBr₃ (MA = CH₃NH₃⁺) perovskite optical fiber made using our method, it demonstrates low transmission losses (<0.7 dB/cm), mechanical flexibilities (a bending radius down to 3.5 mm), and mechanical deformation-tunable photoluminescence in organometallic perovskites. Moreover, the light confinement provided by our organometallic perovskite optical fibers leads to three-photon absorption (3PA), in contrast with 2PA in bulk single crystals under the same experimental conditions. The single-crystal organometallic perovskite optical fibers have the potential in future optoelectronic applications.

Improving Photoelectric Conversion with Broadband Perovskite Metasurface

The miniaturization and integration of optoelectronic devices require progressive size reduction of active layers, resulting in less optical absorption and lower quantum efficiency. In this work, we demonstrate that introducing a metasurface made of hybrid organic-inorganic perovskite (HOIP) can significantly enhance broadband absorption and improve photon-to-electron conversion, which roots from exciting Mie resonances together with suppressing optical transmission. On the basis of the HOIP metasurface, a broadband photodetector has been fabricated where photocurrent boosts more than 10 times in the frequency ranging from ultraviolet to visible. The device response time is less than 5.1 μs at wavelengths 380, 532, and 710 nm, and the relevant 3 dB bandwidth is over 0.26 MHz. Moreover, this photodetector has been applied as a signal receiver for transmitting 2D color images in broadband optical communication. These results accentuate the practical applications of HOIP metasurfaces in novel optoelectronic devices for broadband optical communication.

Halide perovskite single crystals: growth, characterization, and stability for optoelectronic applications

Recently, metal halide perovskite materials have received significant attention as promising candidates for optoelectronic applications with tremendous achievements, owing to their outstanding optoelectronic properties and facile solution-processed fabrication. However, the existence of a large number of grain boundaries in perovskite polycrystalline thin films causes ion migration, surface defects, and instability, which are detrimental to device applications. Compared with their polycrystalline counterparts, perovskite single crystals have been explored to realize stable and excellent properties such as a long diffusion length and low trap density. The development of growth techniques and physicochemical characterizations led to the widespread implementation of perovskite single-crystal structures in optoelectronic applications. In this review, recent progress in the growth techniques of perovskite single crystals, including advanced crystallization methods, is summarized. Additionally, their optoelectronic characterizations are elucidated along with a detailed analysis of their optical properties, carrier transport mechanisms, defect densities, surface morphologies, and stability issues. Furthermore, the promising applications of perovskite single crystals in solar cells, photodetectors, light-emitting diodes, lasers, and flexible devices are discussed. The development of suitable growth and characterization techniques contributes to the fundamental investigation of these materials and aids in the construction of highly efficient optoelectronic devices based on halide perovskite single crystals.

Large-Area Uniaxial-Oriented Growth of Free-Standing Thin Films at the Liquid-Air Interface with Millimeter-Sized Grains

Manipulating materials at the atomic scale and assembling them into macroscopic structures with controlled dimensionalities and single-crystal quality are grand scientific challenges. Here, we report a general solvent evaporation method to synthesize large-area uniaxial-oriented growth of free-standing thin films at the liquid-air interface. Crystals nucleate at the solution surface and rotate into the same orientation under electrostatic interaction and then merge as large crystals and grow laterally into a large-area uniform thin film with millimeter-sized grains. The lateral dimension is confined only by the size of containers. The film thickness can be tuned by adjusting solvent evaporation rate (R) and solute diffusivity (D), and a characteristic length, L * ∼ D R , was derived to estimate the film thickness. Molecular dynamic (MD) simulations reveal a concentration spike at the liquid-air interface during fast solvent evaporation, leading to the lateral growth of thin films. The large-area uniaxial oriented films are demonstrated on both inorganic metal halides and hybrid metal halide perovskites. The solvent evaporation approach and the determination of key parameters enabling film thickness prediction are beneficial to the high throughput and scalable production of single crystal-quality thin film materials under controlled evaporation conditions.

Heterogeneous Nucleation of an Embryo in the Shape of Square Prism: Effect of Surface Roughness

The solution-based synthesis in a confined space between two parallel plates has been demonstrated to be a potential approach to grow single-crystal perovskite films of large sizes, such as CH₃NH₃PbX₃ (X = Br and I) single-crystal films. In this work, we study the effects of surface roughness on the separation between two parallel, rough plates, and heterogeneous nucleation of an embryo in the shape of square prism for the Wenzel contact between the embryo and the rough surface. Analytical relations are derived for the separation of two parallel, rough plates under mechanical loading and the critical dimensions of a square embryo on a rough surface. The analytical relation reveals that one can control the thickness of perovskite films grown between two parallel plates by changing the surface tension of the precursor solution and mechanical loading. The critical dimensions of a square embryo and the corresponding formation energy are dependent on interface energies and the root mean square of surface roughness. There exists a critical root mean square of surface roughness, above which it is very difficult to form an embryo in the shape of square prism. The results illustrate the important roles of the interface energies and surface roughness of substrates in the growth of single-crystal films, including perovskites and ionic crystals, and the need to include the anisotropic characteristics of surface/interface energies in the nucleation analysis of crystalline materials.

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.

High-Luminance Microsized CH3NH3PbBr3 Single-Crystal-Based Light-Emitting Diodes via a Facile Liquid-Insulator Bridging Route

Micro-/nanosized organic-inorganic hybrid perovskite single crystals (SCs) with appropriate thickness and high crystallinity are promising candidates for high-performance electroluminescent (EL) devices. However, their small lateral size poses a great challenge for efficient device construction and performance optimization, causing perovskite SC-based light-emitting diodes (PSC-LEDs) to demonstrate poor EL performance. Here, we develop a facile liquid-insulator bridging (LIB) strategy to fabricate high-luminance PSC-LEDs based on single-crystalline CH₃NH₃PbBr₃ microflakes. By introducing a blade-coated poly(methyl methacrylate) (PMMA) insulating layer to effectively overcome the problems of leakage current and possible short circuits between electrodes, we achieve the reliable fabrication of PSC-LEDs. The LIB method also allows us to systematically boost the device performance through crystal growth regulation and device architecture optimization. Consequently, we realize the best CH₃NH₃PbBr₃ microflake-based PSC-LED with an ultrahigh luminance of 136100 cd m^-2 and a half-lifetime of 88.2 min at an initial luminance of ∼1100 cd m^-2, which is among the highest for organic-inorganic hybrid perovskite LEDs reported to date. Moreover, we observe the strong polarized edge emission of the microflake-based PSC-LEDs with a high degree of polarization up to 0.69. Our work offers a viable approach for the development of high-performance perovskite SC-based EL devices.

Pattern-Selective Molecular Epitaxial Growth of Single-Crystalline Perovskite Arrays toward Ultrasensitive and Ultrafast Photodetector

The emergence of organic-inorganic perovskite has provided great flexibility for creating optoelectronic devices with unprecedented performance or unique functionality. However, the perovskite films explored so far have been difficult to be patterned to arrays owing to their poor solvent and moisture stability, which usually lead to serious structural damage of perovskites. The successful preparation of perovskite microarrays with uniform shape and size is more challenging. Here we report a straightforward approach to realize single-crystalline perovskite arrays through a relatively simple pattern-selective molecular epitaxial growth. This approach is applied to create diverse shaped perovskite arrays, such as hexagon, triangle, circle, square, and rectangle. A vertically aligned perovskite photodetector displays both an ultrasensitive and ultrafast photoresponse arising from the reduction in carrier diffusion paths and the high optical absorption. This work demonstrates a general approach to creating perovskite arrays with uniform shape, size, and morphology and provides a rich platform for producing high-performance photodetectors and photovoltage devices.

Perovskite-Type 2D Materials for High-Performance Photodetectors

Photodetectors are light sensors in widespread use in image sensing, optical communication, and consumer electronics. In current smart optoelectronic technology, conventional semiconductors have encountered a bottleneck caused by inflexibility and opacity. With the ever-increasing demands for versatile optoelectronic applications, perovskite-type 2D materials demonstrate great potential for advanced photodetectors inspired by molecularly thin 2D materials. Through the reduction of thickness to thin or molecularly thin levels, single-crystalline 2D perovskites can exhibit superior optoelectronic performance characteristics, such as tunable absorption property by chemical design, enhanced carrier separation by remarkable photosensing capability, and improved carrier extraction by versatile band engineering. More importantly, perovskite-type 2D materials exhibit great potential for large-scale monolithic integration to achieve all-in-one sensing-memory-computing optoelectronic devices. In this Perspective, recent progress in 2D perovskite-based photodetectors is presented in detail. The focus is on growth strategies for reducing thickness, thickness-dependent optical and electrical properties, device engineering, heterojunction fabrication, and device performance. Finally, the current challenges and future prospects in this field are presented.

Competitive Displacement Triggering DBP Photoelectrochemical Aptasensor via Cetyltrimethylammonium Bromide Bridging Aptamer and Perovskite

Here, a label-free perovskite-based photoelectrochemical (PEC) aptasensor was rationally designed for the displacement assay of dibutyl phthalate (DBP), a well-known endocrine disruptor, with the aid of cetyltrimethylammonium bromide (CTAB). In this method, CTAB significantly enhanced the PEC response and humidity resistance of the CH₃NH₃PbI₃ perovskite by forming a protecting layer and passivating the X- and A-sites vacancies of CH₃NH₃PbI₃. In addition, CTAB facilitated the immobilization of an aptamer through van der Waals and hydrophobicity forces, as well as the electrostatic interactions between the phosphate group of the aptamer and the cationic group of CTAB. When exposed to DBP in the affinity solution, the DBP aptamer was released from the electrode because the affinity between DBP and its aptamer competes with the interaction of the aptamer and CTAB. The displacement of the aptamer from the perovskite surface relieves the block effect and thus enhances the photoelectric signal of perovskite. By virtue of the good photoelectrochemical characters of CH₃NH₃PbI₃ and the specific recognition ability of aptamer, the linear range of the PEC sensor was 1.0 × 10^-13 to 1.0 × 10^-8 M and the detection and quantification limits were down to 2.5 × 10^-14 and 8.2 × 10^-14 M (S/N = 3), respectively. This work offers a novel strategy for designing aptasensors for the detection of various targets and exhibits the marvelous potential of organic-inorganic perovskite in the field of PEC analysis.

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.

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.

Toward Microlasers with Artificial Structure Based on Single-Crystal Ultrathin Perovskite Films

A perovskite microlaser is potentially valuable for integrated photonics due to its excellent properties. The artificial microlasers were mostly made on polycrystalline films. Though a perovskite single crystal has significantly improved properties in comparison with its polycrystalline counterpart, an artificial microlaser based on single-crystal perovskite has been much less explored due to the difficulty in producing an ultrathin-single-crystal (UTSC) film. Here we show a device processing based on a perovskite UTSC film, confirming the high performance of the UTSC device with a quality factor of 1250. The single-crystal device shows 4.5 times the quality factor and 8 times the radiation intensity in comparison with its polycrystalline counterpart. The experiment first proved that hybrid perovskite microlasers with a subwavelength fine structure can be processed by focused ion beams (FIB). In addition, a wavelength-tunable distributed feedback (DFB) laser is demonstrated, with a tuning range of ∼4.6 nm. The research provides an easily applicable approach for perovskite photonic devices with excellent performance.

Biphasic Liquid-Liquid Interface Limit Architecture of High-Quality Perovskite Single-Crystal Sheets for UV Photodetection

Thin organic-inorganic MAPbX₃ perovskite single-crystal sheets have become the hotspot of smart photodetectors because of their low number of trap states, high carrier mobility, long diffusion length, and effective light-receiving area. However, MAPbX₃ single crystals are so fragile that single-crystal perovskite sheets with a thickness of ≤100 μm are hard to obtain by cutting. Thin single-crystal MAPbX₃ sheets were synthesized by the biphasic liquid-liquid interface limit method with dimethicone/DMSO biphasic films and could be obtained with an adjustable thickness of 1-50 μm and improved crystal quality of the perovskite sheets. The thin MAPbX₃ single-crystal sheet-based photodetector exhibits a superior responsivity of 0.88 A W^-1, an external quantum efficiency of 276.8%, and an ultrahigh detectivity of 2.26 × 10¹¹ Jones under 395 nm irradiation at 3 V. These values are more than 500% as high as those of the bulk-crystal-based photodetector. In particular, the sheet-based photodetector could retain long-time stability after 4200 on-off cycles.

Supersaturation-Controlled Growth of Monolithically Integrated Lead-Free Halide Perovskite Single-Crystalline Thin Film for High-Sensitivity Photodetectors

Monolithical integration of the promising optoelectronic material with mature and inexpensive silicon circuitry contributes to simplifying device geometry, enhancing performance, and expanding new functionalities. Herein, a lead-free halide perovskite Cs₃ Bi₂ I₉ single-crystalline thin film (SCTF), with thickness ranging from 900 nm to 4.1 µm and aspect ratio up to 1666, is directly integrated on various substrates including Si wafer, through a facile and low-temperature solution-processing method. The growth kinetics of the lead-free halide perovskite SCTF are elucidated by in situ observation, and the solution supersaturation is controlled to reduce the inverse-temperature crystallization nucleation density and elongate the evaporation growth. The excellent lattice match and band alignment between Si(111) and Cs₃ Bi₂ I₉ (001) facets promote photogenerated charge dissociation and extraction, resulting in boosting the photoelectric sensitivity by 10-200 times compared with photodetectors based on other substrates. More importantly, this silicon-compatible perovskite SCTF photodetector exhibits a high switching ratio of 3000 and a fast response of 1.5 µs, which are higher than most reported state-of-the-art lead-free halide perovskite photodetectors. This work not only gives an in-depth understanding of the perovskite precursor solution chemistry, but also demonstrates the great potential of monolithical integration of lead-free halide perovskite SCTF with a silicon wafer for high-performance photodetectors.

Halide perovskites scintillators: unique promise and current limitations

The widespread use of X- and gamma-rays in a range of sectors including healthcare, security and industrial screening is underpinned by the efficient detection of the ionising radiation. Such detector applications are dominated by indirect detectors in which a scintillating material is combined with a photodetector. Halide perovskites have recently emerged as an interesting class of semiconductors, showing enormous promise in optoelectronic applications including solar cells, light-emitting diodes and photodetectors. Here, we discuss how the same superior semiconducting properties that have catalysed their rapid development in these optoelectronic devices, including high photon attenuation and fast and efficient emission properties, also make them promising scintillator materials. By outlining the key mechanisms of their operation as scintillators, we show why reports of remarkable performance have already emerged, and describe how further learning from other optoelectronic devices will propel forward their applications as scintillators. Finally, we outline where these materials can make the greatest impact in detector applications by maximally exploiting their unique properties, leading to dramatic improvements in existing detection systems or introducing entirely new functionality.

Recent Progress of Chiral Perovskites: Materials, Synthesis, and Properties

Chiral materials with intrinsic inversion-symmetric structures possess many unique physicochemical features, including circular dichroism, circularly polarized photoluminescence, nonlinear optics, ferroelectricity, and spintronics. Halide perovskites have attracted considerable attention owing to their excellent optical and electrical properties, which are particularly suitable for realizing high power-conversion efficiency in solar cells. Recent studies have shown that chirality can be transferred from chiral organic ligands into halide perovskites and the resultant chiral perovskites combine the advantages of both chiral materials and halide perovskites; this provides an ideal platform to design next-generation optoelectronic and spintronic devices. In this progress report, the most recent advances are summarized in various chemical structures of chiral perovskites, their synthesis strategies, chirality generation mechanisms, and physical properties. Furthermore, the potential chiral-halide-perovskite-based applications are presented and the challenges and prospects of chiral perovskites are discussed. This report outlines the diverse construction strategies of and proposes research directions for chiral halide perovskites; thus, it provides insights into the design of novel chiral perovskites and facilitates investigation of the optoelectronic applications that employ chirality.

Metasurface-assisted broadband optical absorption in ultrathin perovskite films

Ultrathin hybrid organic-inorganic perovskite (HOIP) films have significant potential for use in integrated high-performance photoelectric devices. However, the relatively low optical absorption capabilities of thinner films, particularly in the long-wavelength region, pose a significant challenge to the further improvement of photoelectrical conversion in ultrathin HOIP films. To address this problem, we propose a combining of ultrathin HOIP film with plasmonic metasurface to enhance the absorption of the film effectively. The metasurface excites localized surface plasmon resonances and deflects the reflected light within the HOIP film, resulting in an obvious enhancement of film absorption. Finite-difference time-domain simulation results reveal that the far-field intensities, deflection angles, and electric field distributions can be effectively varied by using metasurfaces with different arrangements. Examination of the reflection and absorption spectra reveals that embedding a specifically designed metasurface into the HOIP film produces an obvious enhancement in broadband optical absorption compared with pure HOIP films. We further demonstrate that this broadband absorption promotion mechanism can be effective at a wide range of HOIP film thicknesses. Comparison of the absorption spectra at various incidence angles of ultrathin HOIP films with and without underlying metasurfaces indicates that the addition of a metasurface can effectively promote absorption under wide-angle incident light illumination. Moreover, by extending the metasurface structure to a two-dimensional case, absorption enhancements insensitive to the incident polarization states have also been demonstrated. This proposed metasurface-assisted absorption enhancement method could be applied in designing novel high-performance thin-film solar cells and photodetectors.

Ligand assisted growth of perovskite single crystals with low defect density

A low defect density in metal halide perovskite single crystals is critical to achieve high performance optoelectronic devices. Here we show the reduction of defect density in perovskite single crystals grown by a ligand-assisted solution process with 3‐(decyldimethylammonio)‐propane‐sulfonate inner salt (DPSI) as an additive. DPSI ligands anchoring with lead ions on perovskite crystal surfaces not only suppress nucleation in solution, but also regulate the addition of proper ions to the growing surface, which greatly enhances the crystal quality. The grown CH₃NH₃PbI₃ crystals show better crystallinity and a 23-fold smaller trap density of 7 × 10¹⁰ cm⁻³ than the optimized control crystals. The enhanced material properties result in significantly suppressed ion migration and superior X-ray detection sensitivity of CH₃NH₃PbI₃ detectors of (2.6 ± 0.4) × 10⁶ µC Gy⁻¹air cm⁻² for 60 kVp X-ray and the lowest detectable dose rate reaches (5.0 ± 0.7) nGy s⁻¹, which enables reduced radiation dose to patients in medical X-ray diagnostics.

The performance of a metal halide perovskite single crystal is governed by the defect density. Here, the authors report a high quality single crystal perovskite grown by a ligand-assisted solution process with DPSI achieving 23-fold smaller trap density than that without DPSI.

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.

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.