Air-Stable Lead-Free Perovskite Thin Film Based on CsBi3I10 and Its Application in Resistive Switching Devices

Reliable and Robust Two-Dimensional Perovskite Memristors for Flexible-Resistive Random-Access Memory Array

Two-dimensional (2D) halide perovskites have become a promising class of memristive materials due to their low power consumption, compositional versatility, and microstructural anisotropy in electronics. However, implementing high-performance resistive random-access memory requires a higher reliability and moisture resistance. To address these issues, component studies and attempts to improve the phase stability have been reported but have not been able to achieve sufficient reliability. Here, highly textured thin films grown perpendicular to the substrate in Ruddlesden-Popper 2D perovskites exhibited highly stable and reliable binary memory performance. We further built a flexible crossbar array to verify data storage capability, achieving a high device yield, robust endurance, long retention, reliability to operate under bending conditions, and moisture stability over a year. These device performances are attributed to preformed vertically oriented nanocrystals that allow the conductive filaments to operate reliably. Our finding provides the material design strategy that can be extended to the development of semiconductor materials for next-generation memory devices.

Light-Adapted Optoelectronic-Memristive Device for the Artificial Visual System

With the development of artificial intelligence systems, it is necessary to develop optoelectronic devices with photoresponse and storage capacity to simulate human visual perception systems. The key to an artificial visual perception system is to integrate components with both sensing and storage capabilities of illumination information. Although module integration components have made useful progress, they still face challenges such as multispectral response and high energy consumption. Here, we developed a light-adapted optoelectronic-memristive device integrated by an organic photodetector and ferroelectric-based memristor to simulate human visual perception. ITO/P3HT:PC71BM/Au as the light sensor unit shows a high on/off ratio (Iph/Id) reaching ∼5 × 104 at 0 V. The memristor unit, consisting of ITO/CBI@P(VDF-TrFE)/Cu, has a RON/ROFF ratio window of ∼106 under 0.05 V read voltage and ultralow power consumption of ∼1 pW. Moreover, the artificial visual perception unit shows stable light-adapted memory windows under different wavelengths of irradiation light (400, 500, and 600 nm; they meet the spectral range of human visual recognition) and can clearly identify the target image ("T" shape) because of the apparent contrast, which results from the high ROFF/RON ratio values. These results provide a potential design strategy for the development of intelligent artificial vision systems.

Self-Assembly of Delta-Formamidinium Lead Iodide Nanoparticles to Nanorods: Study of Memristor Properties and Resistive Switching Mechanism

In the quest for advanced memristor technologies, this study introduces the synthesis of delta-formamidinium lead iodide (δ-FAPbI3) nanoparticles (NPs) and their self-assembly into nanorods (NRs). The formation of these NRs is facilitated by iodide vacancies, promoting the fusion of individual NPs at higher concentrations. Notably, these NRs exhibit robust stability under ambient conditions, a distinctive advantage attributed to the presence of capping ligands and a crystal lattice structured around face-sharing octahedra. When employed as the active layer in resistive random-access memory devices, these NRs demonstrate exceptional bipolar switching properties. A remarkable on/off ratio (105) is achieved, surpassing the performances of previously reported low-dimensional perovskite derivatives and α-FAPbI3 NP-based devices. This enhanced performance is attributed to the low off-state current owing to the reduced number of halide vacancies, intrinsic low dimensionality, and the parallel alignment of NRs on the FTO substrate. This study not only provides significant insights into the development of superior materials for memristor applications but also opens new avenues for exploring low-dimensional perovskite derivatives in advanced electronic devices.

Piezo-Acoustic Resistive Switching Behaviors in High-Performance Organic-Inorganic Hybrid Perovskite Memristors

Memristors are regarded as promising candidates for breaking the problems including high off-chip memory access delays and the hash rate cost of frequent data moving induced by algorithms for data-intensive applications of existing computational systems. Recently, organic-inorganic halide perovskites (OIHPs) have been recognized as exceptionally favorable materials for memristors due to ease of preparation, excellent electrical conductivity, and structural flexibility. However, research on OIHP-based memristors focuses on modulating resistive switching (RS) performance through electric fields, resulting in difficulties in moving away from complex external circuits and wire connections. Here, a multilayer memristor has been constructed with eutectic gallium and indium (EGaIn)/ MAPbI3 /poly(3,4-ethylenedioxythiophene): poly(4-styrenesulphonate) (PEDOT: PSS)/indium tin oxide (ITO) structure, which exhibits reproducible and reliable bipolar RS with low SET/RESET voltages, stable endurance, ultrahigh average ON/OFF ratio, and excellent retention. Importantly, based on ion migration activated by sound-driven piezoelectric effects, the device exhibits a stable acoustic response with an average ON/OFF ratio greater than 103 , thus realizing non-contact, multi-signal, and far-field control in RS modulation. This study provides a single-structure multifunctional memristor as an integrated architecture for sensing, data storage, and computing.

Quasi-2D Lead-Tin Perovskite Memory Devices Fabricated by Blade Coating

Two terminal passive devices are regarded as one of the promising candidates to solve the processor-memory bottleneck in the Von Neumann computing architectures. Many different materials are used to fabricate memory devices, which have the potential to act as synapses in future neuromorphic electronics. Metal halide perovskites are attractive for memory devices as they display high density of defects with a low migration barrier. However, to become promising for a future neuromorphic technology, attention should be paid on non-toxic materials and scalable deposition processes. Herein, it is reported for the first time the successful fabrication of resistive memory devices using quasi-2D tin-lead perovskite of composition (BA)2 MA4 (Pb0.5 Sn0.5 )5 I16 by blade coating. The devices show typical memory characteristics with excellent endurance (2000 cycles), retention (105  s), and storage stability (3 months). Importantly, the memory devices successfully emulate synaptic behaviors such as spike-timing-dependent plasticity, paired-pulse facilitation, short-term potentiation, and long-term potentiation. A mix of slow (ionic) transport and fast (electronic) transport (charge trapping and de-trapping) is proven to be responsible for the observed resistive switching behavior.

A Dual-Functional Perovskite-Based Photodetector and Memristor for Visual Memory

The imitation of human visual memory demands the multifunctional integration of light sensors similar to the eyes, and image memory, similar to the brain. Although humans have already implemented electronic devices with visual memory functions, these devices require a combination of various components and logical circuits. However, the combination of visual perception and high-performance information storage capabilities into a single device to achieve visual memory remains challenging. In this study, inspired by the function of human visual memory, a dual-functional perovskite-based photodetector (PD) and memristor are designed to realize visual perception and memory capacities. As a PD, it realizes an ultrahigh self-powered responsivity of 276 mA W-1 , a high detectivity of 4.7 × 1011  Jones (530 nm; light intensities, 2.34 mW cm-2 ), and a high rectification ratio of ≈100 (±2 V). As a memristor, an ultrahigh on/off ratio (≈105 ), an ultralow power consumption of 3 × 10-11  W, a low setting voltage (0.15 V), and a long retention time (>7000 s) are realized. Moreover, the dual-functional device has the capacity to perceive and remember light paths and store data with good cyclic stability. This device exhibits perceptual and cyclic erasable memory functions, which provides new opportunities for mimicking human visual memory in future multifunctional applications.

Novel Semiconductive Ternary Hybrid Heterostructures for Artificial Optoelectronic Synapses

Synaptic devices that mimic biological synapses are considered as promising candidates for brain-inspired devices, offering the functionalities in neuromorphic computing. However, modulation of emerging optoelectronic synaptic devices has rarely been reported. Herein, a semiconductive ternary hybrid heterostructure is prepared with a D-D'-A configuration by introducing polyoxometalate (POM) as an additional electroactive donor (D') into a metalloviologen-based D-A framework. The obtained material features an unprecedented porous 8-connected bcu-net that accommodates nanoscale [α-SiW12 O40 ]4- counterions, displaying uncommon optoelectronic responses. Besides, the fabricated synaptic device based on this material can achieve dual-modulation of synaptic plasticity due to the synergetic effect of electron reservoir POM and photoinduced electron transfer. And it can successfully simulate learning and memory processes similar to those in biological systems. The result provides a facile and effective strategy to customize multi-modality artificial synapses in the field of crystal engineering, which opens a new direction for developing high-performance neuromorphic devices.

Advances in the Application of Perovskite Materials

Nowadays, the soar of photovoltaic performance of perovskite solar cells has set off a fever in the study of metal halide perovskite materials. The excellent optoelectronic properties and defect tolerance feature allow metal halide perovskite to be employed in a wide variety of applications. This article provides a holistic review over the current progress and future prospects of metal halide perovskite materials in representative promising applications, including traditional optoelectronic devices (solar cells, light-emitting diodes, photodetectors, lasers), and cutting-edge technologies in terms of neuromorphic devices (artificial synapses and memristors) and pressure-induced emission. This review highlights the fundamentals, the current progress and the remaining challenges for each application, aiming to provide a comprehensive overview of the development status and a navigation of future research for metal halide perovskite materials and devices.

Multilevel resistive switching in stable all-inorganic n-i-p double perovskite memristor

Memristors are promising information storage devices for commercial applications because of their long endurance and low power consumption. Particularly, perovskite memristors have revealed excellent resistive switching (RS) properties owing to the fast ion migration and solution fabrication process. Here, an n-i-p type double perovskite memristor with "ITO/SnO₂/Cs₂AgBiBr₆/NiO_x/Ag" architecture was developed and demonstrated to reveal three resistance states because of the p-n junction electric field coupled with ion migration. The devices exhibited reliable filamentary with an on/off ratio exceeding 50. The RS characteristics remained unchanged after 1000 s read and 300 switching cycles. The synaptic functions were examined through long-term depression and potentiation measurements. Significantly, the device still worked after one year to reveal long-term stability because of the all-inorganic layers. This work indicates a novel idea for designing a multistate memristor by utilizing the p-n junction unidirectional conductivity during the forward and reverse scanning.

The Role of Polymers in Halide Perovskite Resistive Switching Devices

Currently, halide perovskites (HPs) are gaining traction in multiple applications, such as photovoltaics and resistive switching (RS) devices. In RS devices, the high electrical conductivity, tunable bandgap, good stability, and low-cost synthesis and processing make HPs promising as active layers. Additionally, the use of polymers in improving the RS properties of lead (Pb) and Pb-free HP devices was described in several recent reports. Thus, this review explored the in-depth role of polymers in optimizing HP RS devices. In this review, the effect of polymers on the ON/OFF ratio, retention, and endurance properties was successfully investigated. The polymers were discovered to be commonly utilized as passivation layers, charge transfer enhancement, and composite materials. Hence, further HP RS improvement integrated with polymers revealed promising approaches to delivering efficient memory devices. Based on the review, detailed insights into the significance of polymers in producing high-performance RS device technology were effectively understood.

Enhanced Resistive Switching Performance through Air-Stable Cu2AgSbI6 Thin Films for Flexible and Multilevel Storage Application

Herein, the lead-free halide perovskite films with different Cu-to-Ag ratios (Cu_3-xAg_xSbI₆, x = 0, 1, 2, or 3) have been prepared by a spin-coating method at low temperature. The enhanced resistive switching (RS) performance of more uniform SET/RESET voltages and the endurance up to at least 1600 cycles are found in the RS memory with a device structure of Ag/PMMA/Cu₂AgSbI₆/ITO. The device performance is not degraded under different bending angles and after 10³ bending cycles, which is beneficial for flexible memory applications. The appropriately increased activation energy of the perovskites with the partial substitution of Ag atoms, which would lead to a more robust filament formed, is proposed to explain the enhanced RS mechanism. Importantly, the effective size and number of filaments measured by conductive AFM are introduced to confirm the multilevel storage effect of Cu₂AgSbI₆. The multilevel storage characteristics with four resistance levels are demonstrated by various compliance currents. Moreover, the Cu₂AgSbI₆ memory devices still exhibit enhanced RS properties and multilevel storage after 75 days of exposure to ambient conditions. Our study provides a strategy for improving the stability and high-density storage applications of halide perovskite RS memory devices.

Recent Advances in Halide Perovskite Resistive Switching Memory Devices: A Transformation from Lead-Based to Lead-Free Perovskites

Due to their remarkable electrical and light absorption characteristics, hybrid organic-inorganic perovskites have recently gained popularity in several applications such as optoelectronics, lasers, and light-emitting diodes. Through this, there has recently been an increase in the use of halide perovskites (HPs) in resistive switching (RS) devices. However, lead-based (Pb-based) perovskites are notorious for being unstable and harmful to the environment. As a result, lead-free (Pb-free) perovskite alternatives are being investigated in achieving the long-term and sustainable use of RS devices. This work describes the characteristics of Pb-based and Pb-free perovskite RS devices. It also presents the recent advancements of HP RS devices, including the selection strategies of perovskite structures. In terms of resistive qualities, the directions of both HPs appear to be identical. Following that, the possible impact of switching from Pb-based to Pb-free HPs is examined to determine the requirement in RS devices. Finally, this work discusses the opportunities and challenges of HP RS devices in creating a stable, efficient, and sustainable memory storage technology.

Low-Dimensional Metal-Halide Perovskites as High-Performance Materials for Memory Applications

Metal-halide perovskites have drawn profuse attention during the past decade, owing to their excellent electrical and optical properties, facile synthesis, efficient energy conversion, and so on. Meanwhile, the development of information storage technologies and digital communications has fueled the demand for novel semiconductor materials. Low-dimensional perovskites have offered a new force to propel the developments of the memory field due to the excellent physical and electrical properties associated with the reduced dimensionality. In this review, the mechanisms, properties, as well as stability and performance of low-dimensional perovskite memories, involving both molecular-level perovskites and structure-level nanostructures, are comprehensively reviewed. The property-performance correlation is discussed in-depth, aiming to present effective strategies for designing memory devices based on this new class of high-performance materials. Finally, the existing challenges and future opportunities are presented.

Ambient Stable All Inorganic CsCu2I3 Artificial Synapses for Neurocomputing

In resistive switching memories or artificial synaptic devices, halide perovskites have attracted attention for their unusual features such as rapid ion migration, adjustable composition, and facile synthesis. Herein, the environmentally friendly and highly air stable CsCu₂I₃ perovskite films are used as the active layer in the Au/CsCu₂I₃/ITO/glass artificial synapses. The device shows variable synaptic plasticities such as long-term and short-term synaptic plasticity, paired-pulse facilitation, and spike-timing-dependent plasticity by combining potentiation and depression along the formation of conductive filaments. The performances of the devices are maintained for 160 days under ambient conditions. Additionally, the accuracy evaluation of the CsCu₂I₃-based artificial synapses performs exceptionally well with the MNIST and Fashion MNIST data sets, demonstrating high learning accuracy in deep neural networks. Using the novel B-site engineered halide perovskite material with extreme air stability, this study paves the way for artificial synaptic devices for next-generation in-memory hardware.

Nanostructured perovskites for nonvolatile memory devices

Perovskite materials have driven tremendous advances in constructing electronic devices owing to their low cost, facile synthesis, outstanding electric and optoelectronic properties, flexible dimensionality engineering, and so on. Particularly, emerging nonvolatile memory devices (eNVMs) based on perovskites give birth to numerous traditional paradigm terminators in the fields of storage and computation. Despite significant exploration efforts being devoted to perovskite-based high-density storage and neuromorphic electronic devices, research studies on materials' dimensionality that has dominant effects on perovskite electronics' performances are paid little attention; therefore, a review from the point of view of structural morphologies of perovskites is essential for constructing perovskite-based devices. Here, recent advances of perovskite-based eNVMs (memristors and field-effect-transistors) are reviewed in terms of the dimensionality of perovskite materials and their potentialities in storage or neuromorphic computing. The corresponding material preparation methods, device structures, working mechanisms, and unique features are showcased and evaluated in detail. Furthermore, a broad spectrum of advanced technologies (e.g., hardware-based neural networks, in-sensor computing, logic operation, physical unclonable functions, and true random number generator), which are successfully achieved for perovskite-based electronics, are investigated. It is obvious that this review will provide benchmarks for designing high-quality perovskite-based electronics for application in storage, neuromorphic computing, artificial intelligence, information security, etc.

High Temperature CsPbBrxI3-x Memristors Based on Hybrid Electrical and Optical Resistive Switching Effects

The emergence of perovskite-based memristors associated with the migration of ions has attracted attention for use in overcoming the limitations of the von Neumann computing architecture and removing the bottleneck of storage density. However, systematic research on the temperature dependence of halide perovskite-based memristors is still required due to the unavoidable thermal stability limits. In this work, mixed halide CsPbBr_xI_3-x-based (X = 0, 1, 2) memristors with unique electrical and optical resistive switching properties in an ambient atmosphere from room temperature to a 240 °C maximum have been successfully achieved. At room temperature, the CsPbBr_xI_3-x-based memristors exhibit outstanding resistive switching behaviors such as ultralow operating voltage (∼0.81, ∼0.64, and ∼0.54 V for different devices, respectively), moderate ON/OFF ratio (∼10²), stable endurance (10³ cycles), and long retention time (10⁴ s). The CsPbBr_xI_3-x-based memristors maintain excellent repeatability and stability at high temperature. Endurance failures of CsPbI₃, CsPbBrI₂, and CsPbBr₂I memristors occur at 90, 150, and 270 °C, respectively. Finally, nonvolatile imaging employing CsPbBr₂I-based memristor arrays based on the electrical-write and optical-erase operation at 100 °C has been demonstrated. This study provides utilization potentiality in the high temperature scenarios for perovskite wearable and large-scale information devices.

One-Dimensional (NH=CINH3)3PbI5 Perovskite for Ultralow Power Consumption Resistive Memory

Organic-inorganic hybrid perovskites (OIHPs) have proven to be promising active layers for nonvolatile memories because of their rich abundance in earth, mobile ions, and adjustable dimensions. However, there is a lack of investigation on controllable fabrication and storage properties of one-dimensional (1D) OIHPs. Here, the growth of 1D (NH=CINH3)3PbI5 ((IFA)3PbI5) perovskite and related resistive memory properties are reported. The solution-processed 1D (IFA)3PbI5 crystals are of well-defined monoclinic crystal phase and needle-like shape with the length of about 6 mm. They exhibit a wide bandgap of 3 eV and a high decomposition temperature of 206°C. Moreover, the (IFA)3PbI5 films with good uniformity and crystallization were obtained using a dual solvent of N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). To study the intrinsic electric properties of this anisotropic material, we constructed the simplest memory cell composed of only Au/(IFA)3PbI5/ITO, contributing to a high-compacted device with a crossbar array device configuration. The resistive random access memory (ReRAM) devices exhibit bipolar current-voltage (I-V) hysteresis characteristics, showing a record-low power consumption of ~0.2 mW among all OIHP-based memristors. Moreover, our devices own the lowest power consumption and "set" voltage (0.2 V) among the simplest perovskite-based memory devices (inorganic ones are also included), which are no need to require double metal electrodes or any additional insulating layer. They also demonstrate repeatable resistance switching behaviour and excellent retention time. We envision that 1D OIHPs can enrich the low-dimensional hybrid perovskite library and bring new functions to low-power information devices in the fields of memory and other electronics applications.

Halide Perovskites for Memristive Data Storage and Artificial Synapses

Halide perovskites have been noted for their exotic properties such as fast ion migration, tunable composition, facile synthetic routes, and flexibility in addition to large light absorption coefficients, long carrier diffusion lengths, and high defect tolerance. These properties have made halide perovskites promising materials for memristors. Applications in the field of resistive switching memory devices and artificial synapses for neuromorphic computing are especially noteworthy. This Perspective covers state-of-the-art perovskite-based memristive devices. Moreover, the fundamental mechanisms and characteristics of perovskite-based memristors are elucidated. Interesting opportunities to improve the performance of perovskite-based memristors for commercialization are provided, including improving film uniformity and air stability, controlling the stoichiometry, finding new all-inorganic and lead-free halide perovskites, and making perovskites into single crystals or quantum dots. We expect our Perspective to be the foundation of realizing next-generation halide perovskite-based memristors.

Quasi-2D Halide Perovskite Memory Device Formed by Acid-Base Binary Ligand Solution Composed of Oleylamine and Oleic Acid

Organometal halide perovskite materials are receiving significant attention for the fabrication of resistive-switching memory devices based on their high stability, low power consumption, rapid switching, and high ON/OFF ratios. In this study, we synthesized 3D FAPbBr₃ and quasi-2D (RNH₃)₂(FA)₁Pb₂Br₇ films using an acid-base binary ligand solution composed of oleylamine (OlAm) and oleic acid in toluene. The quasi-2D (RNH₃)₂(FA)₁Pb₂Br₇ films were synthesized by controlling the protonated OlAm (RNH₃⁺) solution concentration to replace FA⁺ cations with large organic RNH₃⁺ cations from 3D FAPbBr₃ perovskites. The quasi-2D (RNH₃)₂(FA)₁Pb₂Br₇ devices exhibited nonvolatile write-once read-many (WORM) memory characteristics, whereas the 3D FAPbBr₃ only exhibited hysteresis behavior. Analysis of the 3D FAPbBr₃ device indicated operation in the trap-limited space-charge-limited current region. In contrast, quasi-2D (RNH₃)₂(FA)₁Pb₂Br₇ devices provide low trap density that is completely filled by injected charge carriers and then subsequently form conductive filaments (CFs) to operate as WORM devices. Nanoscale morphology analysis and an associated current mapping study based on conductive atomic force microscopy measurements revealed that perovskite grain boundaries serve as major channels for high current, which may be correlated with the conductive low-resistive-switching behavior and formation of CFs in WORM devices.

Designing zero-dimensional dimer-type all-inorganic perovskites for ultra-fast switching memory

Resistive switching memory that uses halide perovskites (HP) has been considered as next-generation storage devices due to low operation voltage and high on/off ratio. However, the memory still faces challenges for stable operation with fast switching speed, which hinders the practical application. Thus, it should be considered from the stage of designing the HP for memory applications. Here, we design the perovskite memory using a high-throughput screening based on first-principles calculations. Total 696 compositions in four different crystal structures are investigated and essential parameters including stability, vacancy formation, and migration are considered as the descriptor. We select dimer-Cs3Sb2I9 as an optimal HP for memory; the device that uses dimer-Cs3Sb2I9 has ultra-fast switching speed (~20 ns) compared to the device that uses layer-Cs3Sb2I9 (>100 ns). The use of lead-free perovskite avoids environmental problems caused by lead in perovskite. These results demonstrate the feasibility to design the memory with ultra-fast switching speed.

Advances in Halide Perovskite Memristor from Lead-Based to Lead-Free Materials

Memristors have attracted considerable attention as one of the four basic circuit elements besides resistors, capacitors, and inductors. Especially, the nonvolatile memory devices have become a promising candidate for the new-generation information storage, due to their excellent write, read, and erase rates, in addition to the low-energy consumption, multistate storage, and high scalability. Among them, halide perovskite (HP) memristors have great potential to achieve low-cost practical information storage and computing. However, the usual lead-based HP memristors face serious problems of high toxicity and low stability. To alleviate the above issues, great effort has been devoted to develop lead-free HP memristors. Here, we have summarized and discussed the advances in HP memristors from lead-based to lead-free materials including memristive properties, stability, neural network applications, and memristive mechanism. Finally, the challenges and prospects of lead-free HP memristors have been discussed.

Impact of Hydroiodic Acid on Resistive Switching Performance of Lead-Free Cs3Cu2I5 Perovskite Memory

Herein, we employed lead-free Cs₃Cu₂I₅ perovskite films as the functional layers to construct Al/Cs₃Cu₂I₅/ITO memory devices and systematically investigated the impact on the corresponding resistive switching (RS) performance via adding different amounts of hydroiodic acid (HI) in Cs₃Cu₂I₅ precursor solution. The results demonstrated that the crystallinity and morphology of the Cs₃Cu₂I₅ films can be improved and the resistive switching performance can be modulated by adding an appropriate amount of HI. The obtained Cs₃Cu₂I₅ films by adding 5 μL HI exhibit the fewest lattice defects and flattest surface (RMS = 13.3 nm). Besides, the memory device, utilizing the optimized films, has a low electroforming voltage (1.44 V), a large on/off ratio (∼65), and a long retention time (10⁴ s). The RS performance impacted by adding HI, providing a scientific strategy for improving the RS performance of iodine halide perovskite-based memories.

Lead-Free Halide Perovskites for Light Emission: Recent Advances and Perspectives

Lead-based halide perovskites have received great attention in light-emitting applications due to their excellent properties, including high photoluminescence quantum yield (PLQY), tunable emission wavelength, and facile solution preparation. In spite of excellent characteristics, the presence of toxic element lead directly obstructs their further commercial development. Hence, exploiting lead-free halide perovskite materials with superior properties is urgent and necessary. In this review, the deep-seated reasons that benefit light emission for halide perovskites, which help to develop lead-free halide perovskites with excellent performance, are first emphasized. Recent advances in lead-free halide perovskite materials (single crystals, thin films, and nanocrystals with different dimensionalities) from synthesis, crystal structures, optical and optoelectronic properties to applications are then systematically summarized. In particular, phosphor-converted LEDs and electroluminescent LEDs using lead-free halide perovskites are fully examined. Ultimately, based on current development of lead-free halide perovskites, the future directions of lead-free halide perovskites in terms of materials and light-emitting devices are discussed.

Large-Grained All-Inorganic Bismuth-Based Perovskites with Narrow Band Gap via Lewis Acid-Base Adduct Approach

Bismuth halide perovskites have recently been considered a potential alternative to lead halide analogues due to their low toxicity and high stability. However, the layered flake structure and wide band gap limit their applications in perovskite solar cells (PSCs). We herein show that large-grained all-inorganic bismuth-based perovskites with a narrow band gap can be obtained from a Lewis acid-base adduct reaction under ambient conditions. Thiourea (CH₄N₂S) is utilized as a Lewis base to interact with BiI₃, confirmed with infrared (IR) spectra. The strong coordination between thiourea and the Bi³⁺ center could slow down the perovskite crystallization and promote the preferred orientation of the perovskite crystals with a hexagonal phase. The morphology of the perovskite films varies dramatically with an increase of molar ratio of BiI₃ and thiourea in the precursor. The perovskites derived from a BiI₃/thiourea ratio of 1:2 display extrathick grains, higher surface coverage, extended light absorption, higher crystallinity, and similar air stability compared to the pristine sample. The power conversion efficiency (PCE) of the thiourea-induced bismuth perovskite solar cells is significantly enhanced due to the higher surface coverage and the broader absorption of the perovskite film.

Reducing current fluctuation of Cs3Bi2Br9 perovskite photodetectors for diffuse reflection imaging with wide dynamic range

Recently, the newly booming metal halide perovskites have attracted extensive attention worldwide due to their outstanding optoelectronic performance, and are expected to be ideal candidates for photodetectors (PDs). However, there is still lack of perovskite PDs-based imaging devices coming into commercialization stage, due to some practical reasons including toxicity brought by lead-based perovskites and the large light current fluctuations. In this paper, for the first time we fabricate a lead-free Cs₃Bi₂Br₉ perovskite PD, and build a prototype of this perovskite PD-based imaging system with diffuse reflection imaging mode. Moreover, we propose a new parameter F related to light current fluctuation to evaluate imaging performance of a PD especially for weak diffuse light condition, and prove its usability by comparison of unoptimized lead-free Cs₃Bi₂Br₉ perovskite PD and atomic layer deposition (ALD) optimized Cs₃Bi₂Br₉ PD. ALD-optimization can improve the quality of perovskite film and suppress the dark current and current fluctuation. Finally, we obtain satisfactory diffuse reflection images of 2D and 3D objects with wide dynamic range. Therefore, the ALD-optimized Cs₃Bi₂Br₉ PD has addressed two major concerns about perovskite PDs-based imaging devices, that may extend application of perovskite materials and improve imaging quality.