High-Performance Planar-Type Photodetector on (100) Facet of MAPbI3 Single Crystal

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.

Synthesis, Structure, and Optoelectronic Properties of a Hybrid Organic-Inorganic Perovskite with a Monoethanolammonium Cation MAxMEA1-xPbI3

Hybrid organic-inorganic perovskites have emerged as promising materials for next-generation optoelectronic devices owing to their tunable properties and low-cost fabrication. We report the synthesis of 3D hybrid perovskites with monoethanolammonium cations. Specifically, we investigated the optoelectronic properties and morphological characteristics of polycrystalline films of hybrid perovskites MAxMEA1-xPbI3, which contain methylammonium (MA) and monoethanolammonium (MEA) cations. MAxMEA1-xPbI3 crystallizes in a tetragonal perovskite structure. The substitution of methylammonium cations with monoethanolammonium ions led to an increase in the lattice parameters and the bandgap energy. Energy level diagrams of the synthesized samples were also constructed. The bandgap of MA0.5MEA0.5PbI3 makes it a promising material for use in tandem solar cells. These polycrystalline films, namely MA0.5MEA0.5PbI3 and MA0.25MEA0.75PbI3 were fabricated using a one-step spin-coating method without an antisolvent. These films exhibit a uniform surface morphology under the specified deposition parameters. Within the scope of this study, no evidence of dendritic structures or pinhole-type defects were observed. All synthesized samples demonstrated photocurrent generation under visible light illumination. Moreover, using monoethanolammonium cations reduced the hysteresis of the I-V characteristics, indicating improved device stability.

A high-performance X-ray detector based on large-size perovskite MAPbI3 single crystals grown by environmentally friendly solvents and advanced systems

In this paper, we report a novel hermetically sealed and precisely temperature-controlled growth system that grows MAPbI3 single crystals up to a size of 60 × 48 × 22 mm3. The grown crystals have lower defects and higher quality. It is expected to facilitate the commercialization of perovskite single crystals for optoelectronic applications.

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.

Single Crystal Ruddlesden-Popper and Dion-Jacobson Metal Halide Perovskites for Visible Light Photodetectors: Present Status and Future Perspectives

2D metal halide perovskites (MHPs), mainly the studied Ruddlesden-Popper (RP) and Dion-Jacobson (DJ) phases, have gained enormous popularity as optoelectronic materials owing to their self-assembled multiple quantum well structures, tunable semiconducting properties, and improved structural stability compared to their bulk 3D counterparts. The performance of polycrystalline thin film devices is limited due to the formation of defects and trap states. However, as studied so far, single crystal-based devices can provide a better platform to improve device performance and investigate their fundamental properties more reliably. This Review provides the first comprehensive report on the emerging field of RP and DJ perovskite single crystals and their use in visible light photodetectors of varied device configurations. This Review structurally summarizes the 2D MHP single crystal growth methods and the parameters that control the crystal growth process. In addition, the characterization techniques used to investigate their crystal properties are discussed. The review further provides detailed insights into the working mechanisms as well as the operational performance of 2D MHP single crystal photodetector devices. In the end, to outline the present status and future directions, this Review provides a forward-looking perspective concerning the technical challenges and bottlenecks associated with the developing field of RP and DJ perovskite single crystals. Therefore, this timely review will provide a detailed overview of the fast-growing field of 2D MHP single crystal-based photodetectors as well as ignite new concepts for a wide range of applications including solar cells, photocatalysts, solar H2 production, neuromorphic bioelectronics, memory devices, etc.

Crystal Structure and Spectroscopic Characterization of a New Hybrid Compound, (C12H17N2)2[CdBr4], for Energy Storage Applications

Organic-inorganic hybrid materials have recently found a vast variety of applications in the fields of energy storage and microelectronics due to their outstanding electric and dielectric characteristics, including high dielectric constant, low conductivity, and low dielectric loss. However, despite the promising properties of these materials, there remains a need to explore novel compounds with improved performance for practical applications. In this research paper, the focus is on addressing this scientific challenge by synthesizing and characterizing the new-centrosymmetric (C12H17N2)2[CdBr4] crystal. This compound offers potential advancements in energy storage technologies and microelectronics due to its unique structural and electronic properties. The chemical mentioned above crystallizes in the monoclinic system, and its protonated amine (C12H17N2)+ and isolated anion [CdBr4]2- are bound by C-H···π and N-H···Br hydrogen bonds to form its zero-dimensional structure. Through optical absorption analysis, the semiconductor nature of the material is verified, showcasing a band gap of around 2.9 eV. Furthermore, an in-depth examination of Nyquist plots reveals the material's electrical characteristics' sensitivity to frequency and temperature variations. By applying Jonscher's power law to analyze ac conductivity plots, it is observed that the variation in the exponent "s" accurately characterizes the conduction mechanism, aligning with CBH models. The compound exhibits low dielectric loss values and a high permittivity value (ε ∼ 105), making it a promising candidate for energy storage applications. By managing the scientific challenge of improving material performance for energy storage and microelectronics, this research contributes to advancing the field and opens avenues for further exploration and application of organic-inorganic hybrid materials.

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.

Metal-Free Perovskites for X-Ray Detection

Metal-free perovskites are a promising class of materials for X-ray detection due to their unique structural, optical, and electrical properties. Here, we first delve into the stoichiometry and geometric argument of metal-free perovskites. Followed, the alternative A/B/X ions and hydrogen-bonding are clearly introduced to further optimize the materials' stability and properties. Finally, we provide a comprehensive overview of their potential applications for flexible X-ray images and prospects for metal-free perovskite development. In conclusion, metal-free perovskite is a promising material for X-ray detection. Its stoichiometric and geometric parameters, ion, and hydrogen bond selection, and application prospects are worthy of further study.

Tricolor narrowband planar perovskite photodetectors based on FP microcavity structure

This paper presents a novel tunable narrowband photodetector based on Ag-MgF2-Ag (metal-dielectric-metal: MDM) Fabry-Perot (FP) microcavity structure. The tunability is achieved through precise adjustment of the thickness of the metal and intermediate dielectric layers of the FP microcavity, taking into account the response spectral range of planar perovskite. After optimizing the parameters mentioned above, the prototype devices were prepared by combining the perovskite layer and MDM layer. The center wavelength of the planar detector can be tuned from 430 nm to 680 nm within the detection band of 400-800 nm, with a narrow FWHM about 30 nm and a relatively high response of 0.05 A/W @ 5 V bias voltage for 500 nm. Meanwhile the rise and fall times of the detector are 375 ms and 550 ms, respectively. The experimental results are corroborated by the theory. Our design is highly beneficial to such applications as hyperspectral photography and color-related active optical devices, which paves the way to design this kind of triple structure.

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.

Constructing high-quality 1D nano/microwire hybrid structure for high-performance photodetectors based on CdSe nanobelt/perovskite microwire

All-inorganic perovskite CsPbBr3 is considered as a promising photoelectric material due to its high environmental stability and excellent photoelectric properties. Constructing low-dimension hybrid structures by combining CsPbBr3 with semiconductor materials have recently attracted particular attention because they may bring new functionalities or generate synergistic effects in optoelectronic devices. Herein, the high-quality 1D CdSe nanobelt (NB)/CsPbBr3 microwire (MW) photodetectors are designed first time, which exhibit excellent performance as integrating I on/I off ratio of 5.02 × 104, responsivity of 1.63 × 103 A/W, external quantum efficiency of 3.8 × 105% and detectivity up to 5.33 × 1012 Jones. These properties are all improved at least one order of magnitude compared to those of single CsPbBr3 photodetectors. Moreover, the response range is broadened from the 300-570 nm (the single CsPbBr3 device) to 300-740 nm (the hybrid photodetector). Then, the first-principles calculations are carried out to reveal the physical mechanism from the atomic scale. The remarkably improved optoelectronic properties are attributed to the high crystalline quality as well as unique band alignment of hybrid structure that facilitate the effective separation and transport of photogenerated carriers. These works indicate that 1D CdSe/CsPbBr3 hybrid devices have promising applications in building high-performance and broader spectral response photodetectors and other optoelectronic devices.

Near-infrared polarization-sensitive photodetection via interfacial symmetry engineering of an Si/MAPbI3 heterostructural single crystal

Methylammonium lead iodide (MAPbI₃) single crystals (SCs) have drawn particular attention in the optoelectronics field, due to their outstanding photoelectric performance. However, the structures of those MAPbI₃ SCs are isotropic, which limits the further application of the materials for polarization-sensitive photodetection. Here, we propose a strategy of symmetry modulation by heterogeneously integrating large-sized MAPbI₃ SCs with silicon (Si) wafers and we give the first demonstration of self-powered near-infrared (NIR) polarization-sensitive photodetection using MAPbI₃ SCs. Created via a delicate solution method, the MAPbI₃/Si heterostructures show a high crystalline quality and a solid interfacial connection. More importantly, the built-in electric field resulting from the band bending at the MAPbI₃/Si heterostructure interface generates polar symmetry, which enables directional transport of photogenerated carriers, making the MAPbI₃/Si heterostructures highly polarization-sensitive. Consequently, in the self-powered mode, NIR photodetectors of MAPbI₃/Si heterostructures exhibit large polarization ratios of 3.3 at 785 nm and 2.8 at 940 nm. Moreover, a high detectivity of 7.35 × 10¹² Jones of the present devices is also achieved. Our work gives the first demonstration of self-powered polarization-sensitive photodetection of MAPbI₃ SCs and provides a strategy to design polarization-sensitive materials beyond the conventional limitations induced by isotropic structures.

High-performance layer-structured Si/Ga2O3/CH3NH3PbI3 heterojunction photodetector based on a Ga2O3 buffer interlayer

Organic-inorganic metal halide perovskite-based photodetectors (PDs) have attracted great attention because they exhibit extraordinary optoelectronic performances due to advantages such as a low trap-state density and large absorption coefficient. As a buffer layer, G a ₂ O ₃ can block electron hole recombination, passivate an Si surface, reduce trap density, and improve the ability of electron tunneling. Here, we demonstrate a trilayer hybrid structure (S i/G a ₂ O ₃/C H ₃ N H ₃ P b I ₃) composed of an n-type silicon wafer, G a ₂ O ₃ interlayer, and C H ₃ N H ₃ P b I ₃ thin film. The effect of different G a ₂ O ₃ layer thicknesses on the characteristics of a PD was studied, which shows that the responsivity first increases and then decreases with an increase in the G a ₂ O ₃ film thickness; the optimized G a ₂ O ₃ thickness is 300 nm. Additionally, the optimal responsivity, detectivity, and the rise and decay times are 7.2m A W ^-1, 7.448×10¹⁰ Jones, and 39 and 1.7 ms, respectively. This device has a better performance because G a ₂ O ₃ and perovskite have a matched energy level. We believe our work could provide a new way to fabricate high-performance optoelectronic devices.

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.

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.

Ultraviolet-to-infrared broadband photodetector and imaging application based on a perovskite single crystal

The organic-inorganic hybrid perovskite CH₃NH₃PbBr₃(MAPbBr₃) has been well developed in the X-ray to visible light band due to its superior optoelectronic properties, but this material is rarely studied in the infrared band. In this paper, a UV-NIR broadband optical detector based on MAPbBr₃ single crystal is studied, and the response range can reach the near-infrared region. In the visible light band, the optical response of the device is mainly caused by the photoelectric effect; in the near-infrared band, the optical response of the device is mainly caused by the thermal effect. The carrier response of MAPbBr₃ material under different wavelengths of light was investigated using a non-contact measurement method (optical pump terahertz (THz) probe spectroscopy). This paper also builds a set of photoelectric sensor array components, and successfully realizes the conversion of optical image signals to electrical image signals in the visible light band and infrared band. The experimental results show that MAPbBr₃ crystals provide a new possibility for UV-NIR broadband photodetectors.

Unraveling the Effect of Halogen Ion Substitution on the Noise of Perovskite Single-Crystal Photodetectors

Halide mixing in perovskites has been an efficient way to engineer the bandgap, stability, and charge carrier mobility of lead halide perovskites, while its effect on the noise of lead halide perovskite photodetectors is still unknown. We present the preparation of mixed halide methylammonium lead perovskite single crystals by the solution temperature-decreasing method. Planar-structured photodetectors were constructed on the basis of these mixed halide perovskite single crystals. The effect of halogen ion substitution on the noise of devices was investigated by analyzing their dark current spectra. It is shown that the single-crystal photodetectors with higher levels of chloride suffer from larger noise thus have lower detectivity. Density functional theory calculations have also been proposed to reveal the effect of halogens on band structure. These results provide a comprehensive understanding of mixed halide perovskites and may help in the design and preparation of higher-performance devices.

γ-ray Radiation Hardness of CsPbBr3 Single Crystals and Single-Carrier Devices

The superior environmental stability of all-inorganic metal halide perovskites compared to their organic-inorganic counterparts makes them more promising in practical applications. Here, the stability of an archetypical all-inorganic CsPbBr₃ single crystal and its single-carrier devices under ⁶⁰Co γ-ray irradiation was investigated. The CsPbBr₃ single crystal itself shows ostensible hardness as its structural and optical properties present imperceptible changes even with a total ionizing dose of 800 krad. Unexpectedly, the single crystal-based single-carrier devices exhibit apparent dose-dependent hardness. The performance of the hole-only device suffers from more deterioration than the electron-only device under high irradiation doses (>400 krad). Our results reveal that such a discrepancy originates from the different influences of γ-ray irradiation-induced defects on the transport behaviors of holes and electrons in CsPbBr₃ single-crystal devices. These findings offer a new understanding of the interaction mechanism between γ-photons and all-inorganic metal halide perovskite-based devices.

Physics of defects in metal halide perovskites

Metal halide perovskites are widely used in optoelectronic devices, including solar cells, photodetectors, and light-emitting diodes. Defects in this class of low-temperature solution-processed semiconductors play significant roles in the optoelectronic properties and performance of devices based on these semiconductors. Investigating the defect properties provides not only insight into the origin of the outstanding performance of perovskite optoelectronic devices but also guidance for further improvement of performance. Defects in perovskites have been intensely studied. Here, we review the progress in defect-related physics and techniques for perovskites. We survey the theoretical and computational results of the origin and properties of defects in perovskites. The underlying mechanisms, functions, advantages, and limitations of trap state characterization techniques are discussed. We introduce the effect of defects on the performance of perovskite optoelectronic devices, followed by a discussion of the mechanism of defect treatment. Finally, we summarize and present key challenges and opportunities of defects and their role in the further development of perovskite optoelectronic devices.

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.

A high-responsivity CsPbBr3 nanowire photodetector induced by CdS@CdxZn1-xS gradient-alloyed quantum dots

Benefitting from excellent thermal and moisture stability, inorganic halide perovskite materials have established themselves quickly as promising candidates for fabricating photoelectric devices. However, due to their high trap state density and rapid carrier recombination rate, the photoelectric conversion efficiencies of current inorganic halide perovskite materials are still lower than expected. Here, after systematic research on the optoelectronic properties of CsPbBr₃ nanowires (NWs) decorated with binary CdS quantum dots (QDs), CdS@ZnS core/shell QDs, and gradient-alloyed CdS@Cd_xZn_1-xS QDs, respectively, we proposed a facile method to improve the quantum efficiency of perovskite-based photodetectors with low cost, in which the aforementioned QDs are firstly integrated with CsPbBr₃ NWs, which act as a photosensitive layer. Notably, the responsivity of the CsPbBr₃ NW photodetector decorated with CdS@Cd_xZn_1-xS QDs was enhanced about 10-fold compared to that of pristine CsPbBr₃ NW devices. This value is far superior to those for hybrids composed of binary CdS QDs and CdS@ZnS core/shell QDs. The high responsivity enhancement phenomena are interpreted based on the unique funnel-shaped energy level of CdS@Cd_xZn_1-xS QDs, which is favorable for light-harvesting and photocarrier separation. This work indicates that our unique QD/NW hybrid nanostructure is a desirable building block for fabricating high-performance photodetectors.

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.

Ternary Hybrid Perovskite Solid Solution Single Crystals: Growth, Composition Determination and Phase Stability in Highly Moist Atmosphere

Ternary hybrid perovskite solid solutions have shown superior optoelectronic properties and better stability than their ABX₃ simple perovskite counterparts under ambient conditions. However, crystal growth and identification of the accurate composition of these complex crystalline compounds remain challenging, and their stability under extreme conditions such as in highly moist atmosphere is unknown. Herein, large-size (up to 2 cm) single crystals of ternary perovskite 0.80FAPbI₃  ⋅ x'FAPbBr₃  ⋅ y'CsPbI₃ (x'+y'=0.20) are grown. An elemental analysis method based on wavelength dispersive X-ray fluorescence is proposed to determine their accurate compositions. Among these single crystals, the composition with y'=0.12 shows the best moisture stability at 90 % relative humidity for 15 days. Other components with richer or poorer Cs⁺ ions undergo different phase segregation behaviours. The performance and stability of photodetectors based on these single crystals are tested. This work offers a deeper insight into phase stability of ternary hybrid perovskite solid solution crystals in highly moist atmosphere.

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.

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.

Charge-Transfer Effect and Enhanced Photoresponsivity of WS2- and MoSe2-Based Field Effect Transistors with π-Conjugated Polyelectrolyte

The characteristics of field effect transistors (FETs) fabricated using two-dimensional (2D) transition-metal dichalcogenides (TMDCs) can be modulated by surface treatment of the active layers. In this study, an ionic π-conjugated polyelectrolyte, poly(9,9-bis(4'-sulfonatobutyl)fluorene-alt-1,4-phenylene) potassium (FPS-K), was used for the surface treatment of MoSe₂ and WS₂ FETs. The photoluminescence (PL) intensities of monolayer (1L)-MoSe₂ and 1L-WS₂ clearly decreased, and the PL peaks were red-shifted after FPS-K treatment, suggesting a charge-transfer effect. In addition, the n-channel current of both the MoSe₂ and WS₂ FETs increased and the threshold voltage (V_th) shifted negatively after FPS-K treatment owing to the charge-transfer effect. The photoresponsivity of the MoSe₂ FET under light irradiation (λ_ex = 455 nm) increased considerably, from 5300 A W^-1 to approximately 10 000 A W^-1, after FPS-K treatment, and similar behavior was observed in the WS₂ FET. The results can be explained in terms of the increase in electron concentration due to photogating. The external quantum efficiency and photodetectivity of both FETs were also enhanced by the charge-transfer effect resulting from surface treatment with FPS-K containing mobile cations (K⁺) and fixed anions (SO₃^-), as well as by the photogating effect. The variation in charge-carrier density due to the photogating and charge-transfer effects is estimated to be approximately 2 × 10¹² cm^-2. The results suggest that π-conjugated polyelectrolytes such as FPS-K can be a promising candidate for the passivation of TMDC-based FETs and obtaining enhanced photoresponsivity.

Carrier Diffusion and Recombination Anisotropy in the MAPbI3 Single Crystal

MAPbI₃, one of the archetypical metal halide perovskites, is an exciting semiconductor for a variety of optoelectronic applications. The photoexcited charge-carrier diffusion and recombination are important metrics in optoelectronic devices. Defects in grain interiors and boundaries of MAPbI₃ films cause significant nonradiative recombination energy losses. Besides defect impact, carrier diffusion and recombination anisotropy introduced by structural and electronic discrepancies related to the crystal orientation are vital topics. Here, large-sized MAPbI₃ single crystals (SCs) were grown, with the (110), (112), (100), and (001) crystal planes simultaneously exposed through the adjusting ratios of PbI₂ to methylammonium iodide (MAI). Such MAPbI₃ SCs exhibit a weak n-type semiconductor character, and the Fermi levels of these planes were slightly different, causing a homophylic p-n junction at crystal ledges. Utilizing MAPbI₃ SCs, the photoexcited carrier diffusion and recombination within the crystal planes and around the crystal ledges were investigated through time-resolved fluorescence microscope. It is revealed that both the (110) and (001) planes were facilitated to be exposed with more MAI in the growth solutions, and the photoluminescence (PL) of these planes manifesting a red-shift, longer carrier lifetime, and diffusion length compared with the (100) and (112) planes. A longer carrier diffusion length promoted photorecycling. However, excessive MAI-assisted grown MAPbI₃ SCs could increase the radiative recombination. In addition, it revealed that the carrier excited within the (001) and (112) planes was inclined to diffuse toward each other and was favorable to be extracted out of the grain boundaries or crystal ledges.

Solid-State Neutron Detection Based on Methylammonium Lead Bromide Perovskite Single Crystals

Perovskite-based semiconductors, such as methylammonium and cesium lead halides (MPbX₃: M = CH₃NH₃⁺ or Cs⁺; X = I^-, Br^-, or Cl^-), have attracted immense attention for several applications, including radiation detection, due to their excellent electronic and optical properties.^1,2,3,4,5,6 In addition, the combination of perovskites with other materials enables unique device structures. For example, robust and reliable diodes result when combined with metal oxide semiconductors. This device can be used for detection of nonionizing and ionizing radiation. In this paper, we report a unique perovskite single-crystal-based neutron detector using a heterojunction diode based on single-crystal MAPbBr₃ and gallium oxide (Ga₂O₃) thin film. The MAPbBr₃/Ga₂O₃ diodes demonstrate a leakage current of ∼7 × 10^-10 A/mm², an on/off ratio of ∼10², an ideality factor of 1.41, and minimal hysteresis that enables alpha particle, gamma-ray, and neutron detection at a bias as low as (-5 V). Gamma discrimination is further improved by 85% by optimizing the thickness of the perovskite single crystal. The MAPbBr₃/Ga₂O₃ diodes also demonstrate a neutron detection efficiency of ∼3.92% when combined with a ¹⁰B neutron conversion layer.

Recent Advances in Perovskite Photodetectors for Image Sensing

In recent years, metal halide perovskites have been widely investigated to fabricate photodetectors for image sensing due to the excellent photoelectric performance, tunable bandgap, and low-cost solution preparation process. In this review, a comprehensive overview of the recent advances in perovskite photodetectors for image sensing is provided. First, the key performance parameters and the basic device types of photodetectors are briefly introduced. Then, the recent developments of image sensors on the basis of different dimensional perovskite materials, including 0D, 1D, 2D, and 3D perovskite materials, are highlighted. Besides the device structures and photoelectric properties of perovskite image sensors, the preparation methods of perovskite photodetector arrays are also analyzed. Subsequently, the single-pixel imaging of perovskite photodetectors and the strategies to fabricate narrowband perovskite photodetectors for color discrimination are discussed. Finally, the potential challenges and possible solutions for the future development of perovskite image sensors are presented.

Limits to Electrical Mobility in Lead-Halide Perovskite Semiconductors

Semiconducting polycrystalline thin films are cheap to produce and can be deposited on flexible substrates, yet high-performance electronic devices usually utilize single-crystal semiconductors, owing to their superior charge-carrier mobilities and longer diffusion lengths. Here we show that the electrical performance of polycrystalline films of metal-halide perovskites (MHPs) approaches that of single crystals at room temperature. Combining temperature-dependent terahertz conductivity measurements and ab initio calculations we uncover a complete picture of the origins of charge-carrier scattering in single crystals and polycrystalline films of CH₃NH₃PbI₃. We show that Fröhlich scattering of charge carriers with multiple phonon modes is the dominant mechanism limiting mobility, with grain-boundary scattering further reducing mobility in polycrystalline films. We reconcile the large discrepancy in charge-carrier diffusion lengths between single crystals and films by considering photon reabsorption. Thus, polycrystalline films of MHPs offer great promise for devices beyond solar cells, including light-emitting diodes and modulators.

Unravelling a Zigzag Pathway for Hot Carrier Collection with Graphene Electrode

The capture of photoexcited deep-band hot carriers, excited by photons with energies far above the bandgap, is of significant importance for photovoltaic and photoelectronic applications because it is directly related to the quantum efficiency of photon-to-electron conversion. By employing time-resolved photoluminescence and state-of-the-art time-domain density functional theory, we reveal that photoexcited hot carriers in organic-inorganic hybrid perovskites prefer a zigzag interfacial charge-transfer pathway, i.e., the hot carriers transfer back and forth between CH₃NH₃PbI₃ and graphene electrode, before they reach a charge-separated state. Driven by quantum coherence and interlayer vibrational modes, this pathway at the semiconductor-graphene interface takes about 400 fs, much faster than the relaxation process within CH₃NH₃PbI₃ (several picoseconds). Our work provides new insight into the fundamental understanding and precise manipulation of hot carrier dynamics at the complex interfaces, paving the way for highly efficient photovoltaic and photoelectric device optimization.

Photocurrent in Metal-Halide Perovskite/Organic Semiconductor Heterostructures: Impact of Microstructure on Charge Generation Efficiency

Hybrid organic-inorganic metal-halide perovskites have emerged as versatile materials for enabling low-cost, mechanically flexible optoelectronic applications. The progress has been commendable; however, technological breakthroughs have outgrown the basic understanding of processes occurring in bulk and at device interfaces. Here, we investigated the photocurrent at perovskite/organic semiconductor interfaces in relation to the microstructure of electronically active layers. We found that the photocurrent response is significantly enhanced in the bilayer structure as a result of a more efficient dissociation of the photogenerated excitons and trions in the perovskite layer. The increase in the grain size within the organic semiconductor layer results in reduced trapping and further enhances the photocurrent by extending the photocarriers' lifetime. The photodetector responsivity and detectivity have improved by 1 order of magnitude in the optimized samples, reaching values of 6.1 ± 1.1 A W^-1, and 1.5 × 10¹¹ ± 4.7 × 10¹⁰ Jones, respectively, and the current-voltage hysteresis has been eliminated. Our results highlight the importance of fine-tuning film microstructure in reducing the loss processes in thin-film optoelectronics based on metal-halide semiconductors and provide a powerful interfacial design method to consistently achieve high-performance photodetectors.

Growth of two-dimensional formamidine lead halide perovskite single-crystalline sheets and their optoelectronic properties

Formamidine-based hybrid perovskite is an excellent optoelectronic material; however, its intrinsic non-layered crystalline structure makes it hard to isolate the corresponding 2D counterparts. In this work, a unique liquid-epitaxy technique was introduced to grow micro-sized two-dimensional FAPbX3 perovskite sheets. Such ultrathin sheets exhibited excellent photo-induced carrier properties with high crystalline quality, as well as provided new opportunities for next-generation optoelectronic devices.

Low-Dimensional Metal Halide Perovskite Photodetectors

Metal halide perovskites (MHPs) have been a hot research topic due to their facile synthesis, excellent optical and optoelectronic properties, and record-breaking efficiency of corresponding optoelectronic devices. Nowadays, the development of miniaturized high-performance photodetectors (PDs) has been fueling the demand for novel photoactive materials, among which low-dimensional MHPs have attracted burgeoning research interest. In this report, the synthesis, properties, photodetection performance, and stability of low-dimensional MHPs, including 0D, 1D, 2D layered and nonlayered nanostructures, as well as their heterostructures are reviewed. Recent advances in the synthesis approaches of low-dimensional MHPs are summarized and the key concepts for understanding the optical and optoelectronic properties related to the PD applications of low-dimensional MHPs are introduced. More importantly, recent progress in novel PDs based on low-dimensional MHPs is presented, and strategies for improving the performance and stability of perovskite PDs are highlighted. By discussing recent advances, strategies, and existing challenges, this progress report provides perspectives on low-dimensional MHP-based PDs in the future.

Room-temperature synthesis, growth mechanisms and opto-electronic properties of organic–inorganic halide perovskite CH3NH3PbX3 (X = I, Br, and Cl) single crystals

Growth of MAPbX₃ (X = I, Br, and Cl) single crystals by room temperature crystallization (RTC) method, and the crystallization pathway illustrated by the solubility curve of MAPbCl₃ in DMSO, compared with inverse temperature crystallization (ITC) method.

Light illumination and temperature-induced current–voltage hysteresis in single-crystal perovskite photodiodes

Recently, current–voltage (IV) hysteresis, which is more frequently observed in thin film perovskite solar cells, has been intensively studied due to the destruction of data accuracy in device measurement.

Recent Advances and Challenges in Halide Perovskite Crystals in Optoelectronic Devices from Solar Cells to Other Applications

Organic-inorganic hybrid perovskite materials have attracted tremendous attention as a key material in various optoelectronic devices. Distinctive optoelectronic properties, such as a tunable energy band position, long carrier diffusion lengths, and high charge carrier mobility, have allowed rapid progress in various perovskite-based optoelectronic devices (solar cells, photodetectors, light emitting diodes (LEDs), and lasers). Interestingly, the developments of each field are based on different characteristics of perovskite materials which are suitable for their own applications. In this review, we provide the fundamental properties of perovskite materials and categorize the usages in various optoelectronic applications. In addition, the prerequisite factors for those applications are suggested to understand the recent progress of perovskite-based optoelectronic devices and the challenges that need to be solved for commercialization.

Long and Ultrastable All-Inorganic Single-Crystal CsPbBr3Microwires: One-Step Solution In-Plane Self-Assembly at Low Temperature and Application for High-Performance Photodetectors

As ideal building blocks for optoelectronic devices, one-dimensional (1D) single-crystal perovskite microwires (MWs) have received widespread attention due to their unique physical and chemical properties. Herein, a one-step solution in-plane self-assembly method is proposed to directly grow millimeter-long CsPbBr₃ MWs with superior crystal quality at atmospheric environment. This method effectively avoids the use of toxic antisolvents. Furthermore, a MW-based photodetector is successfully fabricated, showing high photoresponsivity (20 A/W) and fast response (less than 0.3 ms). The stability of the photodetector is also confirmed by aging MW in air for 60 days, which shows a negligible change of photocurrent from 1.29 to 1.25 nA (-3 V) under the same experimental conditions. This work provides a low-cost and fast synthesis method for the preparation of single-crystal perovskite MWs and demonstrates their potential application for high-performance and stable photoelectronic device.

Bication-Mediated Quasi-2D Halide Perovskites for High-Performance Flexible Photodetectors: From Ruddlesden-Popper Type to Dion-Jacobson Type

Quasi-2D halide perovskites, especially the Ruddlesden-Popper perovskites (RPPs), have attracted great attention because of their promising properties for optoelectronics; however, there are still serious drawbacks, such as inefficient charge transport, poor stability, and unsatisfactory mechanical flexibility, restricting further utilization in advanced technologies. Herein, high-quality quasi-2D halide perovskite thin films are successfully synthesized with the introduction of the unique bication ethylenediammonium (EDA) via a one-step spin-coating method. This bication EDA, with short alkyl chain length, can not only substitute the typically bulky and weakly van der Waals-interacted organic bilayer spacer cations forming the novel Dion-Jacobson phase to enhance the mechanical flexibility of the quasi-2D perovskite (e.g., EDA(MA)_n-1Pb_nI_3n+1; MA = CH₃NH₃⁺) but also serve as a normal cation to achieve the more intact films (e.g., (iBA)₂(MA)_3-2x(EDA)_xPb₄I₁₃). When fabricated into photodetectors, these optimized EDA-based perovskites deliver an excellent responsivity of 125 mA/W and a fast response time down to 380 μs under 532 nm irradiation. More importantly, the device with the Dion-Jacobson phase perovskite can be bent down to a radius of 2 mm and processed with 10,000 cycles of the bending test without any noticeable performance degradation because of its superior structure to RPPs. Besides, these films do not exhibit any material deterioration after ambient storage for 30 days. All these performance parameters are already comparable or even better than those of the state-of-the-art RPPs recently reported. This work provides valuable design guidelines of the quasi-2D perovskites to obtain high-performance flexible photodetectors for next-generation optoelectronics.

Halide Perovskite Single Crystals: Optoelectronic Applications and Strategical Approaches

Halide perovskite is one of the most promising semiconducting materials in a variety of fields such as solar cells, photodetectors, and light-emitting diodes. Lead halide perovskite single crystals featuring long diffusion length, high carrier mobility, large light absorption coefficient and low defect density, have been attracting increasing attention. Fundamental study of the intrinsic nature keeps revealing the superior optoelectrical properties of perovskite single crystals over their polycrystalline thin film counterparts, but to date, the device performance lags behind. The best power conversion efficiency (PCE) of single crystal-based solar cells is 21.9%, falling behind that of polycrystalline thin film solar cells (25.2%). The oversized thickness, defective surfaces, and difficulties in depositing functional layers, hinder the application of halide perovskite single crystals in optoelectronic devices. Efforts have been made to synthesize large-area single crystalline thin films directly on conductive substrates and apply defect engineering approaches to improve the surface properties. This review starts from a comprehensive introduction of the optoelectrical properties of perovskite single crystals. Then, the synthesis methods for high-quality bulk crystals and single-crystalline thin films are introduced and compared, followed by a systematic review of their optoelectronic applications including solar cells, photodetectors, and X-ray detectors. The challenges and strategical approaches for high-performance applications are summarized at the end with a brief outlook on future work.

Broad-Band Photodetectors Based on Copper Indium Diselenide Quantum Dots in a Methylammonium Lead Iodide Perovskite Matrix

Low-temperature solution-processed methylammonium lead iodide (MAPbI₃) crystalline films have shown outstanding performance in optoelectronic devices. However, their high dark current and high noise equivalent power prevent their application in broad-band photodetectors. Here, we applied a facile solution-based antisolvent strategy to fabricate a hybrid structure of CuInSe₂ quantum dots (CISe QDs) embedded into a MAPbI₃ matrix, which not only enhances the photodetector responsivity, showing a large on/off ratio of 10⁴ at 2 V bias compared with the bare perovskite films, but also significantly (for over 7 days) improves the device stability, with hydrophobic ligands on the CuInSe₂ QDs acting as a barrier against the uptake of environmental moisture. MAPbI₃/CISe QD-based lateral photodetectors exhibit high responsivities of >0.5 A/W and 10.4 mA/W in the visible and near-infrared regions, respectively, partly because of the formation of a type II interface between the respective semiconductors but most significantly because of the efficient trap-state passivation of the perovskite grain surfaces, and the reduction in the twinning-induced trap density, which stems from both CISe QDs and their organic ligands. A large specific detectivity of 2.2 × 10¹² Jones at 525 nm illumination (1 μW/cm²), a fast fall time of 236 μs, and an extremely low noise equivalent power of 45 fW/Hz^1/2 have been achieved.

Growth of centimeter-scale perovskite single-crystalline thin film via surface engineering

Modern electronic and photonic devices rely on single-crystalline thin film semiconductors for high performance and reproducibility. The emerging halide perovskites have extraordinary electronic and photonic properties and can be synthesized via low cost solution-based methods. They have been used in a variety of devices with performance approaching or over the devices based on conventional materials. However, their solution based growth method is intrinsically challenge to grow large scale single-crystalline thin film due to the random nucleation and isotropous growth of the crystal. Here, we report the growth of centimeter-scale perovskite single-crystalline thin films by controlling the nucleation density and growth rate of the crystal under a spatially confined growth condition. The hydrophobic treatment on substrates inhibits nucleation and accelerates the growth of single-crystalline thin film, providing enough space for initial nucleus growing up quickly without touching each other. Single-crystalline perovskite thin-film with an aspect ratio of 1000 (1 cm in side length, 10 μm in thickness) has been successfully grown. The low trap density and the high mobility of the as-grown thin film show a high crystallinity. The photodetector based on the perovskite thin film has achieved a gain ~ 104, benefitting from the short transit time of the carries due to the high mobility and thin thickness of the active layer. Our work opens up a new route to grow large scale perovskite single-crystalline thin films, providing a platform to develop high- performance devices.

Enhanced stability in CH3NH3PbI3 hybrid perovskite from mechano-chemical synthesis: structural, microstructural and optoelectronic characterization

Among the hybrid organic-inorganic perovskites MAPbX₃ (MA: methyl-ammonium CH₃-NH₃⁺, X = halogen), the triiodide specimen (MAPbI₃) is still the material of choice for solar energy applications. Although it is able to absorb light above its 1.6 eV bandgap, its poor stability in humid air atmosphere has been a major drawback for its use in solar cells. However, we discovered that this perovskite can be prepared by ball milling in a straightforward way, yielding specimens with a superior stability. This fact allowed us to take atomic-resolution STEM images for the first time, with sufficient quality to unveil microscopic aspects of this material. We demonstrated full Iodine content, which might be related to the enhanced stability, in a more compact PbI₆ framework with reduced unit-cell volume. A structural investigation from neutron powder diffraction (NPD) data of an undeuterated specimen was essential to determine the configuration of the organic MA unit in the 100-298 K temperature range. A phase transition is identified, from the tetragonal structure observed at RT (space group I4/mcm) to an orthorhombic (space group Pnma) phase where the methyl-ammonium organic units are fully localized. Our NPD data reveal that the MA changes are gradual and start before reaching the phase transition. Optoelectronic measurements yield a photocurrent peak at an illumination wavelength of 820 nm, which is redshifted by 30 nm with respect to previously reported measurements on MAPbI₃ perovskites synthesized by crystallization from organic solvents.

Reversible Decomposition of Single-Crystal Methylammonium Lead Iodide Perovskite Nanorods

Perovskite solar cells offer remarkable performance, but further advances will require deeper understanding and control of the materials and processing. Here, we fabricate the first single crystal nanorods of intermediate phase (MAI-PbI₂-DMSO), allowing us to directly observe the phase evolution while annealing in situ in a high-vacuum transmission electron microscope, which lets up separate thermal effects from other environmental conditions such as oxygen and moisture. We attain the first full determination of the crystal structures and orientations of the intermediate phase, evolving perovskite, precipitating PbI₂, and e-beam induced PbI₂ during phase conversion and decomposition. Surprisingly, the perovskite decomposition to PbI₂ is reversible upon cooling, critical for long-term device endurance due to the formation of MAI-rich MAPbI₃ and PbI₂ upon heating. Quantitative measurements with a thermodynamic model suggest the decomposition is entropically driven. The single crystal MAPbI₃ nanorods obtained via thermal cycling exhibit excellent mobility and trap density, with full reversibility up to 100 °C (above the maximum temperature for solar cell operation) under high vacuum, offering unique potential for high-performance flexible solar cells.