High-Performance Red-Light Photodetector Based on Lead-Free Bismuth Halide Perovskite Film

Highly Efficient Flexible Photodetectors Based on Pb-Free CsBi3I10 Perovskites

Perovskites have made remarkable advancements in optoelectronics owing to their high light absorption coefficient, tunable bandgap, and long charge diffusion. Nonetheless, the practical applications of Pb-based perovskites have been hindered by the instability and toxicity of Pb, especially in flexible electronics, which require high biosecurity and low toxicity. Hence, the development of stable Pb-free perovskite materials has gained increasing attention. In this study, we synthesized stable CsBi3I10 Pb-free perovskites outside the glovebox and improved the optoelectronic and mechanical performances of the CsBi3I10-based flexible devices through polyvinylcarbazole (PVK) doping. Flexible photodetectors with the device structure of PET/ITO/PEDOT:PSS/CsBi3I10:PVK/Au was fabricated. The results indicated that the introduction of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) reduced the surface roughness of the flexible PET substrate, while PVK doping further improved the surface smoothness of CsBi3I10 thin films, thereby enhancing the interfacial charge transportation. Moreover, PEDOT:PSS and PVK acted as stepwise hole transport layers in the photodetectors. The device demonstrated a maximum responsivity of 0.3 A/W, detectivity of 2.6 × 1011 Jones, and a response time of 102 μs at 650 nm. After subjecting it to 1000 bending tests, the light current retained 80% of its initial value. This study presents a universally applicable method for controlling the surface morphology of a flexible perovskite thin film.

Efficient Carrier Transport in 2D Bi2O2Se/CsBi3I10 Perovskite Heterojunction Enables Highly-Sensitive Broadband Photodetection

2D Bi2O2Se has recently garnered significant attention in the electronics and optoelectronics fields due to its remarkable photosensitivity, broad spectral absorption, and excellent long-term environmental stability. However, the development of integrated Bi2O2Se photodetector with high performance and low-power consumption is limited by material synthesis method and the inherent high carrier concentration of Bi2O2Se. Here, a type-I heterojunction is presented, comprising 2D Bi2O2Se and lead-free bismuth perovskite CsBi3I10, for fast response and broadband detection. Through effective charge transfer and strong coupling effect at the interfaces of Bi2O2Se and CsBi3I10, the response time is accelerated to 4.1 µs, and the detection range is expanded from ultraviolet to near-infrared spectral regions (365-1500 nm). The as-fabricated photodetector exhibits a responsivity of 48.63 AW-1 and a detectivity of 1.22×1012 Jones at 808 nm. Moreover, efficient modulation of the dominant photocurrent generation mechanism from photoconductive to photogating effect leads to sensitive response exceeding 103 AW-1 for heterojunction-based photo field effect transistor (photo-FETs). Utilizing the large-scale growth of both Bi2O2Se and CsBi3I10, the as-fabricated integrated photodetector array demonstrates outstanding homogeneity and stability of photo-response performance. The proposed 2D Bi2O2Se/CsBi3I10 perovskite heterojunction holds promising prospects for the future-generation photodetector arrays and integrated optoelectronic systems.

Synthesis of different organic ammonium-based bismuth iodide perovskites for photodetection application

Bismuth-based perovskites are promising candidates for highly stable halide perovskites with low toxicity. Here, we report the synthesis of a series of bismuth iodide-based perovskites with different primary, secondary, and tertiary ammonium cations and study their structural, thermal, and optical properties, and the likelihood of photodetection. Interestingly, the variation of A-site organic ammonium cations, with different interlayer spacings between adjacent bismuth iodide monolayers, has exotic effects on the diffraction patterns and morphological structures of the perovskite crystals. Thermogravimetric analysis reveals the highest thermal stability of tertiary ammonium-based bismuth perovskite with a decomposition temperature of 385 °C. The branched primary ammonium-based photodetector has photo-responsivity roughly two and four times faster than that of secondary and tertiary ammonium-based devices, respectively. These findings provide insight into the importance of A-site cation engineering for structural modulation and tailoring the optoelectronic properties of bismuth-based perovskites for emerging optoelectronic devices.

Fast and Reversible Quasi-Solid-State Anion Exchange in Highly Luminescent CsPbX3 Perovskite Nanocrystals for Dual-Mode Encryption-Decryption

Solid-state anion exchange method is easy to handle and beneficial to improve stability of CsPbX3 (X = Cl, Br, I) perovskites nanocrystals (NCs) with respect to anion exchange in liquid phase. However, the corresponding exchange rate is rather slow due to the limited diffusion rate of anions from solid phases, resulting in mixed-halide perovskite NCs. Herein,  a fast and reversible post-synthetic quasi-solid-state anion exchange method in CsPbX3 NCs with inorganic potassium halide KX salts/polyvinylpyrrolidone (PVP) thin film is firstly reported. Original morphology of the exchanged NCs is well-preserved for all samples. Complete anion exchange from Br- to Cl- or I- is successfully achieved in CsPbX3 NCs within ≈20 min through possible vacancies-assisted ion exchange mechanism, under ambient conditions and vice versa. Particularly, Br- -exchanged CsPbCl3 and CsPbI3 NCs exhibit improved optical properties. Encouraged by the attractive fluorescence and persistent luminescence as well as good stability of the resulted CsPbX3 NCs, an effective dual-mode information storage-reading application is demonstrated.  It is believed that this method can open a new avenue for the synthesis of other direct-synthesis challenging quantum-confined perovskite NCs/nanoplates/nanodisks or CsSnX3 NCs/thin film and provide an opportunity for advanced information storage compatible for practical applications.

Enhanced Photoluminescence and Random Lasing Emission in TiO2-Decorated FAPbBr3 Thin Films

Herein, titanium-dioxide-decorated organic formamidinium lead bromide perovskite thin films grown by the one-step spin-coating method are studied. TiO₂ nanoparticles are widespread in FAPbBr₃ thin films, which changes the optical properties of the perovskite thin films effectively. Obvious reductions in the absorption and enhancements in the intensity of the photoluminescence spectra are observed. Over 6 nm, a blueshift of the photoluminescence emission peaks is observed due to 5.0 mg/mL TiO₂ nanoparticle decoration in the thin films, which originates from the variation in the grain sizes of the perovskite thin films. Light intensity redistributions in perovskite thin films are measured by using a home-built confocal microscope, and the multiple scattering and weak localization of light are analyzed based on the scattering center of TiO₂ nanoparticle clusters. Furthermore, random lasing emission with sharp emission peaks is achieved in the scattering perovskite thin films with a full width at the half maximum of 2.1 nm. The multiple scattering of light, the random reflection and reabsorption of light, and the coherent interaction of light within the TiO₂ nanoparticle clusters play important roles in random lasing. This work could be used to improve the efficiency of photoluminescence and random lasing emissions, and it is promising in high-performance optoelectrical devices.

Lead-Free Metal Halide Perovskite Nanocrystals: From Fundamentals to Applications

Lead (Pb) halide perovskite nanocrystals, with the general formula APbX₃ , where A=CH₃ NH³⁺ , CH(NH₂ )²⁺ , or Cs⁺ and X=Cl^- , Br^- , or I^- , have emerged as a class of materials with promising properties due to their remarkable optical properties and solar cell performance. However, important issues still need to be addressed to enable practical applications of these materials, such as instability, mass production, and Pb toxicity. Recent studies have carried out the replacement of Pb by various less-toxic cations as Sn, Ge, Sb, and Bi. This variety of chemical compositions provide Pb-free perovskite and metal halide nanostructures with a wide spectral range, in addition to being considered less toxic, therefore having greater practical applicability. Highlighting the necessity to address and solve the toxicity problems related to Pb-containing perovskite, this review considers the prospects of the Pb-free perovskite, involving synthesis methods, and properties of them, including advantages, disadvantages, and applications.

Enhanced Optoelectronic Performance Induced by Ion Migration in Lead-Free CsCu2I3 Single-Crystal Microrods

Lead-free perovskite has attracted great attention in realizing high-performance optoelectronic devices due to their excellent atmospheric stability and nontoxic characteristics. Although a pronounced ion migration effect has been observed in this new class of materials, its potential in enhancing the overall device performance is yet to be fully explored. In this work, we studied the effect of ion migrations on the carrier transport behavior and found that the recoverable migration process can contribute to enhancing the on/off ratio in a lead-free CsCu2I3 single-crystal microrod-based photodetector. In detail, we synthesized CsCu2I3 single-crystal microrods via an in-plane self-assembly supersaturated crystallization approach. These microrods with well-defined morphologies were then used to construct ultraviolet (UV)-band photodetectors, which outperform most reported lead-free perovskite photodetectors based on individual single crystals. Simultaneously, ion migration can result in asymmetric band bending in the two-terminal device, as confirmed by surface potential profiling with Kelvin probe force microscopy (KPFM). Such an effect can be harnessed to increase the on/off ratio by almost an order of magnitude. Furthermore, the lead-free CsCu2I3 single crystal exhibits excellent thermal and air stabilities. These findings demonstrate that the CsCu2I3 single-crystal microrods can be used in stable and efficient photodetection, and the ion migration effect can potentially be utilized for improving the optoelectronic performance of lead-free devices.

Cs4CuSb2Cl12-xIx(x = 0-10) nanocrystals for visible light photodetection

Lead-free layered double perovskite nanocrystals (NCs) with tunable visible range emission, high carrier mobility and low trap density are the need of the hour to make them applicable for optoelectronic and photovoltaic devices. Introduction of Cu²⁺in the high band gap Cs₃Sb₂Cl₉lattice transforms it to the monoclinic Cs₄CuSb₂Cl₁₂(CCSC) NCs having a direct band gap of 1.96 eV. The replacement of 50% Cl^-by I^-ions generates <5 nm Cs₄CuSb₂Cl₆I₆(C6I6) monodispersed NCs with an unchanged crystal system but with further lowering of the band gap to 1.92 eV. Thep-type C6I6 NCs exhibit emission spectra, lower trap density, appreciable hole mobility and most importantly a lower exciton binding energy of only 50.8 ± 1.3 meV. The temperature dependent photoluminescence (PL) spectra of the C6I6 NCs show a decrease in non-radiative recombination from 300 K down to 78 K. When applied as the photoactive layer in out-of-plane photodetector devices, C6I6 NC devices exhibit an appreciable responsivity of 0.67 A W^-1at 5 V, detectivity of 4.55 × 10⁸Jones (2.5 V), and fast photoresponse with rise and fall time of 126 and 94 ms, respectively. On the other hand, higher I^-substitution in Cs₄CuSb₂Cl₂I₁₀NCs (C2I10) degrades the lattice into a mixture of monoclinic and trigonal crystal phases, which also lowers the device performance.

A Study on UVC Photodetector Using Mixed-Cation Perovskite with High Detection Rate as Light-Absorption Layer

In this study, a mixed-cation perovskite ultraviolet (UV) C photodetector was fabricated using a simple formamidinium iodide (FAI) post-treatment process. The fabricated device uses FA_xMA_1−xPbI₃ perovskite as a light-absorption layer and SnO₂, which has high transmittance in the UVC wavelength region, as an electron-transport layer. The fabricated device exhibited a response of 50.8 mA/W, detectability of 4.47 × 10¹³ Jones, and external quantum efficiency of 53%. Therefore, the approach used in this study is promising for many applications in the UVC wavelength region.

Toward High-Performance Self-Driven Photodetectors via Multistacking Van der Waals Heterostructures

The unique optoelectronic properties of layered van der Waals (vdW) heterostructures open up exciting opportunities for high-performance photodetectors. Self-driven photodetectors are desirable for reducing power consumption and minimizing the device size. Here, a semiconductor-insulator-semiconductor-type multistacking WSe₂/graphene/h-BN/MoS₂ vdW heterostructure is demonstrated to realize an enhanced self-powered photodetector with a high on-off current ratio of about 1.2 × 10⁵ and a high photoresponsivity of 3.6 A/W without applying bias, which is the highest photoresponsivity ever reported for self-powered photodetectors. Because of the difference in the Fermi level, a built-in electrical field is formed at the WSe₂/graphene junction, where the photoexcited electrons and holes can be efficiently separated and the carriers can easily tunnel through the MoS₂/h-BN junction driven by the enhanced potential. Therefore, the enhanced self-powered photodetection is attributable to highly efficient carrier tunneling through large h-BN electron barriers. By comparison, when the stacking sequence is changed to make WSe₂/MoS₂ p-n heterojunctions lay on graphene/h-BN, the self-powered photocurrent is still generated because of the type-II band alignment, which exhibits lower but still relevant values with a light on/off ratio of ∼8 × 10³ and a photoresponsivity of ∼2.39 A/W. The efficient enhancement demonstrates that multistacking heterostructures significantly elevate the performance of self-powered photodetectors, providing a feasible route to develop high-performance self-powered optoelectronic devices and extend their applications in integrated optoelectronic systems.

Metal-Assisted Chemical Etching for Anisotropic Deep Trenching of GaN Array

Realizing the anisotropic deep trenching of GaN without surface damage is essential for the fabrication of GaN-based devices. However, traditional dry etching technologies introduce irreversible damage to GaN and degrade the performance of the device. In this paper, we demonstrate a damage-free, rapid metal-assisted chemical etching (MacEtch) method and perform an anisotropic, deep trenching of a GaN array. Regular GaN microarrays are fabricated based on the proposed method, in which CuSO₄ and HF are adopted as etchants while ultraviolet light and Ni/Ag mask are applied to catalyze the etching process of GaN, reaching an etching rate of 100 nm/min. We comprehensively explore the etching mechanism by adopting three different patterns, comparing a Ni/Ag mask with a SiN mask, and adjusting the etchant proportion. Under the catalytic role of Ni/Ag, the GaN etching rate nearby the metal mask is much faster than that of other parts, which contributes to the formation of deep trenches. Furthermore, an optimized etchant is studied to restrain the disorder accumulation of excessive Cu particles and guarantee a continuous etching result. Notably, our work presents a novel low-cost MacEtch method to achieve GaN deep etching at room temperature, which may promote the evolution of GaN-based device fabrication.

Perovskite photodetectors and their application in artificial photonic synapses

Organic-inorganic hybrid perovskites exhibit superior optoelectrical properties and have been widely used in photodetectors. Perovskite photodetectors with excellent detectivity have great potential for developing artificial photonic synapses which can merge data transmission and storage. They are highly desired for next generation neuromorphic computing. The recent progress of perovskite photodetectors and their application in artificial photonic synapses are summarized in this review. Firstly, the key performance parameters of photodetectors are briefly introduced. Secondly, the recent research progress of photodetectors including photoconductors, photodiodes, and phototransistors is summarized. Finally, the applications of perovskite photodetectors in artificial photonic synapses in recent years are highlighted. All these demonstrate the great potential of perovskite photonic synapses for the development of artificial intelligence.

Sensing of ultraviolet light: a transition from conventional to self-powered photodetector

Clouds in the sky pass almost 80% of ultraviolet (UV) radiation to the earth's surface, which has a significant impact on humankind. Conventional UV photodetectors (PDs) require an external battery, which not only increases the device size but also has a limited life span and maintenance costs can be prohibitively expensive. An alternative and more technically-sound solution would be the use of self-powered UV PDs that can operate independently, eliminating the need for an external source. Although many exciting studies have been done and state-of-the-art research is underway to successfully fabricate self-powered UV PDs, periodic reviews on this topic are deemed essential so that the technology's readiness can be properly evaluated and critical challenges can be addressed in a timely manner. In this article, the key issues and most exciting developments made in recent years on built-in electric field assisted self-powered UV PDs based on p-n homojunctions, p-n heterojunctions, and Schottky junctions followed by energy harvester integrated UV PDs are extensively reviewed. Finally, a summary and comparison of different types of self-powered UV PDs as well as future challenges that need to be addressed are discussed. This review sets a foundation providing essential insights into the present status of self-powered UV PDs with which researchers can engage and deal with the major challenges.

Artificial Synapses Based on Lead-Free Perovskite Floating-Gate Organic Field-Effect Transistors for Supervised and Unsupervised Learning

Synaptic devices are expected to overcome von Neumann's bottleneck and served as one of the foundations for future neuromorphic computing. Lead halide perovskites are considered as promising photoactive materials but limited by the toxicity of lead. Herein, lead-free perovskite CsBi₃I₁₀ is utilized as a photoactive material to fabricate organic synaptic transistors with a floating-gate structure for the first time. The devices can maintain the I_light/I_dark ratio of 10³ for 4 h and have excellent stability within the 30 days test even without encapsulation. Synaptic functions are successfully simulated. Notably, by combining the decent charge transport property of the organic semiconductor and the excellent photoelectronic property of CsBi₃I₁₀, synaptic performance can be realized even with an operating voltage as low as -0.01 V, which is rare among floating-gate synaptic transistors. Furthermore, artificial neural networks are constructed. We propose a new method that can simulate the synaptic weight value in multiple digit form to achieve complete gradient descent. The image recognition test exhibits thrilling recognition accuracy for both supervised (91%) and unsupervised (81%) classifications. These results demonstrate the great potential of floating-gate organic synaptic transistors in neuromorphic computing.

Lead-Free Perovskite Photodetectors: Progress, Challenges, and Opportunities

State-of-the-art photodetectors which apply hybrid perovskite materials have emerged as powerful candidates for next-generation light sensing. Among them, lead-based ones are the most popular beyond doubt on account of their unique and superior optoelectronic properties. Nevertheless, trade-off toward commercialization exists between nontoxicity and high performance, with the poor stability of lead-based perovskites, indicating that it is indispensable to substitute lead with nontoxic element meanwhile bringing about a comparable figure of merit of photodetectors and relatively long-term stability. Herein, recent advances in lead-free perovskite photodetectors are reviewed, analyzing the principle while designing new materials and highlighting some remarkable progress, which are comparable, even superior, to lead-based photodetectors. Furthermore, their potential strategy in optical communication, image sensing, narrowband photodetection, etc., is examined and a perspective on developing new materials and photodetectors with superior properties for more practical applications is provided.

Recent Progress on Electrical and Optical Manipulations of Perovskite Photodetectors

Photodetectors built from conventional bulk materials such as silicon, III-V or II-VI compound semiconductors are one of the most ubiquitous types of technology in use today. The past decade has witnessed a dramatic increase in interest in emerging photodetectors based on perovskite materials driven by the growing demands for uncooled, low-cost, lightweight, and even flexible photodetection technology. Though perovskite has good electrical and optical properties, perovskite-based photodetectors always suffer from nonideal quantum efficiency and high-power consumption. Joint manipulation of electrons and photons in perovskite photodetectors is a promising strategy to improve detection efficiency. In this review, electrical and optical characteristics of typical types of perovskite photodetectors are first summarized. Electrical manipulations of electrons in perovskite photodetectors are discussed. Then, artificial photonic nanostructures for photon manipulations are detailed to improve light absorption efficiency. By reviewing the manipulation of electrons and photons in perovskite photodetectors, this review aims to provide strategies to achieve high-performance photodetectors.

High-Performance Flexible Self-Powered Photodetectors Utilizing Spontaneous Electron and Hole Separation in Quasi-2D Halide Perovskites

Although there are recent advances in many areas of quasi-2D halide perovskites, photodetectors based on these materials still cannot achieve satisfactory performance for practical applications where high responsivity, fast response, self-powered nature, and excellent mechanical flexibility are urgently desired. Herein, utilizing one-step spin-coating method, self-assemble quasi-2D perovskite films with graded phase distribution in the order of increasing number of metal halide octahedral layers are successfully prepared. Gradient type-II band alignments along the out-of-plane direction of perovskites with spontaneous separation of photo-generated electrons and holes are obtained and then employed to construct self-powered vertical-structure photodetectors for the first time. Without any driving voltage, the device exhibits impressive performance with the responsivity up to 444 mA W^-1 and ultrashort response time down to 52 µs. With a bias voltage of 1.5 V, the device responsivity becomes 3463 mA W^-1 with the response speed as fast as 24 µs. Importantly, the device's mechanical flexibility is greatly enhanced since the photocurrent prefers flowing through the metal halide octahedral layers between the top and bottom contact electrodes in the vertical device structure, being more tolerant to film damage. These results evidently indicate the potential of graded quasi-2D perovskite phases for next-generation optoelectronic devices.

Recent advances toward environment-friendly photodetectors based on lead-free metal halide perovskites and perovskite derivatives

Recently, metal-halide perovskites have emerged as promising materials for photodetector (PD) applications owing to their superior optoelectronic properties, such as ambipolar charge transport characteristics, high carrier mobility, and so on. In the past few years, rapid progress in lead-based perovskite PDs has been witnessed. However, the critical environmental instability and lead-toxicity seriously hinder their further applications and commercialization. Therefore, searching for environmentally stable and lead-free halide perovskites (LFHPs) to address the above hurdles is certainly a worthwhile subject. In this review, we present a comprehensive overview of currently explored LFHPs with an emphasis on their crystal structures, optoelectronic properties, synthesis and modification methods, as well as the PD applications. LFHPs are classified into four categories according to the replacement strategies of Pb²⁺, including AB(ii)X₃, A₃B(iii)₂X₉, A₂B(i)B(iii)'X₆, and newly-emerging perovskite derivatives. Then, we give a demonstration of the preliminary achievements and limitations in environment-friendly PDs based on such LFHPs and perovskite derivatives, and also discuss their applications in biological synapses, imaging, and X-ray detection. With the perspective of their properties and current challenges, we provide an outlook for future directions in this rapidly evolving field to achieve high-quality LFHPs and perovskite derivatives for a broader range of fundamental research and practical applications.

Constructing Caesium-Based Lead-Free Perovskite Photodetector Enabling Self-Powered Operation with Extended Spectral Response

Since the discovery of the state-of-the-art hybrid halide perovskites, their application in optoelectronic systems has drawn considerable attention. However, the toxicity from lead (Pb) and the volatility induced by organic constituents hinder their future large-scale market development. Herein, a fully inorganic Pb-free halide perovskite based on robust Cs₃Bi₂I₉ is synthesized and realized its potential in photodetector application. The material property investigation suggests the good crystalline quality with strong absorption coefficient suitable for photodetection. An interesting feature based on the extended absorption is obtained, which is the characteristic of a weak phonon-assisted transition. Additionally, the morphological features display the beautifully grown micrometer-sized crystals of Cs₃Bi₂I₉. The fabricated photodetector demonstrated the self-powered operation (zero-bias state) with a very low dark current of 0.46 pA. Profiting from this, an improved photosensitivity of 1.4 × 10⁴ is achieved. Moreover, along with self-powered photodetection, the photodetector exhibits a broad spectral response (450-950 nm), high detectivity (1.2 × 10¹⁰/1.6 × 10¹² Jones), high responsivity (0.59 μA W^-1/3.8 mA W^-1), and fast response speed (ms) under a weak optical signal of 0.1 mW cm^-2 with a larger active area of 0.25 cm². The photodetector shows high photostability which was well retained for almost 2000 repetitive cycles without degradation. More strikingly, based on the core stability of the perovskite film, an excellent long-term stability of 3 months (90 days) is achieved for the photodetector even after exposure to moist air (75% relative humidity). This study thus highlights one of the few Pb-free all-inorganic perovskite photodetectors employing a simple device architecture with a larger active area that outshines by showing efficient and comparable performance under the self-powered mode under low light conditions.

A highly efficient bilayer graphene/ZnO/silicon nanowire based heterojunction photodetector with broadband spectral response

This paper presents three self-powered photodetectors namely, p⁺-bilayer graphene (BLG)/n⁺-ZnO nanowires (NWs), p⁺-BLG/n⁺-Si NWs/p^--Si and p⁺-BLG/n⁺-ZnO NWs/p^--Si. The Silvaco Atlas TCAD software is utilized to characterize the optoelectronic properties of all the devices and is validated by analytical modeling. The proposed dual-junction photodetectors cover broadband spectral response varying from ultraviolet to near-infrared wavelengths. The dual-heterojunction broadband photodetector exhibits photocurrent switching with the rise and fall time of 1.48 and 1.27 ns, respectively. At -0.5 V bias, the highest external quantum efficiency, photocurrent responsivity, specific detectivity, and the lowest noise equivalent power of 71%, 0.28 A W^-1, 4.2 × 10¹² cmHz^1/2 W^-1, and 2.59 × 10^-17 W, respectively, are found for the dual-heterojunction device with a wavelength of 480 nm at 300 K. The proposed nanowires based photodetectors offer great potential to be utilized as next-generation optoelectronic devices.

A solution-processed ternary copper halide thin films for air-stable and deep-ultraviolet-sensitive photodetector

Recently, the newly emerging lead-halide perovskites have received tremendous attention in the photodetection field because of their intrinsic large light absorption and high well-balanced carrier transport characteristics. Unfortunately, the issue of instability and the existence of toxic lead cations have greatly restricted their practical applications and future commercialization. Furthermore, the previous studies on perovskite photodetectors mainly operate in visible and near-infrared light region, and there are practically no relevant reports aimed at the deep-ultraviolet (DUV) region. In this study, an air-stable and DUV-sensitive photoconductive detector was demonstrated with a solution-processed ternary copper halides Cs₃Cu₂I₅ thin films as the light absorber. The proposed photodetector is very sensitive to wavelengths of light below 320 nm and unresponsive to the visible light. Because of the high material integrity and large surface coverage of the Cs₃Cu₂I₅ thin films, the detector presents an outstanding photodetection performance with a photoresponsivity of ∼17.8 A W^-1, specific detectivity of 1.12 × 10¹² Jones, and fast response speed of 465/897 μs, superior to previously reported DUV photodetectors based on other material systems. Unlike traditional lead-halide perovskites, the lead-free Cs₃Cu₂I₅ shows remarkable stability against heat, UV light, and environmental oxygen/moisture. Thus, the unsealed photodetector demonstrates good operation stability for 11 h of continuous running in open air. Even after 80-day storage in ambient air, its photodetection capability can nearly be maintained. The results suggest that non-toxic Cs₃Cu₂I₅ could be a potential candidate for stable and environment friendly DUV detectors, enabling an assembly of optoelectronic systems in the future.

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.

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.

Self-Powered and Broadband Lead-Free Inorganic Perovskite Photodetector with High Stability

Metal halide perovskite materials have opened up a great opportunity for high-performance optoelectronic devices owing to their extraordinary optoelectronic properties. More than lead halide ones, stable and nontoxic bismuth halide perovskites exhibit more promise in their future commercialization. In this work, we developed for the first time photodetectors based on full-inorganic Cs₃Bi₂I_9-xBr_x perovskites and modulate their performance by varying x in the composition systematically. Among those self-powered photodetectors, those based on Cs₃Bi₂I₆Br₃ shows the best performance with excellent photosensitivity of 4.1 × 10⁴ at zero bias as well as the responsivity and detectivity reaching 15 mA/W and 4.6 × 10¹¹ Jones, respectively. More strikingly, the full-inorganic perovskite photodetectors exhibit excellent stability in the ambient environment and can maintain over 96% of the initial value after 100 days owing to the high stability of the core perovskite film. The paper definitely paves an alternative and promising strategy for the design of future commercial photodetectors that are self-powered, stable, nontoxic, etc.

A Potential Checkmate to Lead: Bismuth in Organometal Halide Perovskites, Structure, Properties, and Applications

The remarkable optoelectronic properties and considerable performance of the organo lead-halide perovskites (PVKs) in various optoelectronic applications grasp tremendous scientific attention. However, the existence of the toxic lead in these compounds is threatening human health and remains a major concern in the way of their commercialization. To address this issue, numerous nontoxic alternatives have been reported. Among these alternatives, bismuth-based PVKs have emerged as a promising substitute because of similar optoelectronic properties and extended environmental stability. This work communicates briefly about the possible lead-alternatives and explores bismuth-based perovskites comprehensively, in terms of their structures, optoelectronic properties, and applications. A brief description of lead-toxification is provided and the possible Pb-alternatives from the periodic table are scrutinized. Then, the classification and crystal structures of various Bi-based perovskites are elaborated on. Detailed optoelectronic properties of Bi-based perovskites are also described and their optoelectronic applications are abridged. The overall photovoltaic applications along with device characteristics (i.e., V OC, J SC, fill factor, FF, and power conversion efficiency, PCE), fabrication method, device architecture, and operational stability are also summarized. Finally, a conclusion is drawn where a brief outlook highlights the challenges that hamper the future progress of Bi-based optoelectronic devices and suggestions for future directions are provided.

Substantially Improving Device Performance of All-Inorganic Perovskite-Based Phototransistors via Indium Tin Oxide Nanowire Incorporation

All-inorganic halide perovskites (IHPs) have attracted enormous attention due to their intrinsically high optical absorption coefficient and superior ambient stabilities. However, the photosensitivity of IHP-based photodetectors is still restricted by their poor conductivities. Here, a facile design of hybrid phototransistors based on the CsPbBr₃ thin film and indium tin oxide (ITO) nanowires (NWs) integrated into a InGaZnO channel in order to achieve both high photoresponsivity and fast response is reported. The metallic ITO NWs are employed as electron pumps and expressways to efficiently extract photocarriers from CsPbBr₃ and inject electrons into InGaZnO. The obtained device exhibits the outstanding responsivity of 4.9 × 10⁶ A W^-1 , which is about 100-fold better than the previous best results of CsPbBr₃ -based photodetectors, together with the fast response (0.45/0.55 s), long-term stability (200 h in ambient), and excellent mechanical flexibility. By operating the phototransistor in the depletion regime, an ultrahigh specific detectivity up to 7.6 × 10¹³ Jones is achieved. More importantly, the optimized spin-coating manufacturing process is highly beneficial for achieving uniform InGaZnO-ITO/perovskite hybrid films for high-performance flexible detector arrays. All these results can not only indicate the potential of these hybrid phototransistors but also provide a valuable insight into the design of hybrid material systems for high-performance photodetection.

All-Inorganic Halide Perovskite Alloy Nanowire Network Photodetectors with High Performance

Organic-inorganic hybrid lead halide perovskites have attracted much attention in the photoelectric field due to their excellent characteristics, such as a tunable band gap, simple fabrication process, and high photoelectric conversion efficiency. However, the commercialization of the perovskite-based devices still faces many challenges, one of which is the inclusion of the toxic lead. Herein, we demonstrated a two-step solution method for synthesizing tin-based perovskite nanowires (NWs) with their application in photodetectors (PDs). By changing the halide exchange time and the Sn content in the precursor, the dark current of the CsPb_xSn_1-x(Br_yI_1-y)₃ perovskite NW PDs increased with increasing content of tin and decreased with increasing Br concentration, and the lowest dark current with a value of 0.672 nA at 1 V was achieved for the perovskite alloy NW PDs synthesized with 0.5 mg mL^-1 SnI₂. Our optimized perovskite alloy NW PDs showed high performance with a linear dynamic range of up to 120 dB, a rising/falling time of 4.25/4.82 ms, and a detectivity of 2 × 10¹⁰ Jones. In addition, our Sn-based perovskite NW devices could maintain good performance after storing in air for 30 days. These results demonstrated good practical application for the Sn-based perovskite NW devices.

CsPbI3 Nanotube Photodetectors with High Detectivity

In the present work, the exploration of photodetectors (PDs) based on CsPbI₃ nanotubes are reported. The as-prepared CsPbI₃ nanotubes can be stable for more than 2 months under air conditions. It is found that, in comparison to the nanowires, nanobelts, and nanosheets, the nanotubes can be advantageous to be used as the functional units for PDs, which is mainly attributed to the enhanced light absorption ability induced by the light trapping effect within the tube cavity. As a proof of concept, the as-constructed PDs based on CsPbI₃ nanotube present an overall excellent performance with a responsivity (R_λ ), external quantum efficiency (EQE) and detectivity of 1.84 × 10³ A W^-1 , 5.65 × 10⁵ % and 9.99 × 10¹³ Jones, respectively, which are all comparable to state-of-the-art ones for all-inorganic perovskite PDs.

Lead-Free Perovskites for Lighting and Lasing Applications: A Minireview

Research on materials with perovskite crystal symmetry for photonics applications represent a rapidly growing area of the photonics development due to their unique optical and electrical properties. Among them are high charge carrier mobility, high photoluminescence quantum yield, and high extinction coefficients, which can be tuned through all visible range by a controllable change in chemical composition. To date, most of such materials contain lead atoms, which is one of the obstacles for their large-scale implementation. This disadvantage can be overcome via the substitution of lead with less toxic chemical elements, such as Sn, Bi, Yb, etc., and their mixtures. Herein, we summarized the scientific works from 2016 related to the lead-free perovskite materials with stress on the lasing and lighting applications. The synthetic approaches, chemical composition, and morphology of materials, together with the optimal device configurations depending on the material parameters are summarized with a focus on future challenges.

Lead-Free Perovskites: Metals Substitution towards Environmentally Benign Solar Cell Fabrication

Perovskite solar cells have attracted significant attention during the current decade owing to their efficacy and photovoltaics performance, which has reached a new milestone in the thin-film category. Perovskite solar cells have witnessed a remarkable 25.2 % light-to-electricity conversion efficiency; however, the toxicity of the commonly employed Pb counterpart towards humans as well as the environment, in addition to material instability, are current bottlenecks towards commercial application. The scientific community has explored other metal ions as substitutions for Pb, while preserving the unique properties of the material, to produce environment-friendly perovskites. In this Review, we highlight the recent developments and challenges of Pb-free halide perovskite-based light harvesters for solar cell applications. This summary is intended to aid in the further development of a materials library for this sustainable technology.

Sn-Based Perovskite for Highly Sensitive Photodetectors

Organic-inorganic hybrid perovskites have emerged as promising functional materials for high-performance photodetectors. However, the toxicity of Pb and the lack of internal gain mechanism in typical perovskites significantly hinder their practical applications. Herein, a low-voltage and high-performance photodetector based on a single layer of lead-free Sn-based perovskite film is reported. The device shows broadband response from ultraviolet to near-infrared light with a responsivity up to 105 A W-1 and a high gain at a low operating voltage. The outstanding performance is attributed to the high hole mobility, p-doping nature, and excellent optoelectronic properties of the Sn-based perovskite. Moreover, the device is assembled on a flexible substrate and demonstrates both high sensitivity and good bending stability. This work demonstrates a route for realizing nontoxic, low-cost, and high-performance perovskite photodetectors with a simple device structure.

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

The development of organic-inorganic hybrid perovskite materials has been rapid in recent years; but their applications are limited by the toxicity and stability of the materials. To address these issues in the context of resistive switching devices, an inorganic lead-free perovskite namely CsBi₃I₁₀ is developed. Uniform and pinhole-free CsBi₃I₁₀ thin films can be fabricated by using CsI-rich precursor solution via a facile antisolvent-assisted spin-coating method. The nonvolatile resistive switching devices based on CsBi₃I₁₀ demonstrate a large on/off ratio (10³), reliable retention properties (10⁴ s), and endurance (150 cycles). Conductive atomic force microscopy reveals that the high- and low-resistance states are formed by breaking and formation of conductive filaments in the perovskite thin film. Because of the excellent stability of the CsBi₃I₁₀ perovskite, the devices exhibit no obvious change in resistive switching behavior even after over 2 month storage in an ambient (60% relative humidity) environment. Our work suggests that the all-inorganic lead-free CsBi₃I₁₀ perovskite has great potential in resistive switching memory as well as in other optoelectronic devices where toxicity and stability are a concern.

High-Performance Photoresistors Based on Perovskite Thin Film with a High PbI₂ Doping Level

We prepared high-performance photoresistors based on CH₃NH₃PbI₃ films with a high PbI₂ doping level. The role of PbI₂ in CH₃NH₃PbI₃ perovskite thin film was systematically investigated using scanning electron microscopy, X-ray diffraction, time-resolved photoluminescence spectroscopy, and photoconductive atomic force microscope. Laterally-structured photodetectors have been fabricated based on CH₃NH₃PbI₃ perovskite thin films deposited using precursor solution with various CH₃NH₃I:PbI₂ ratios. Remarkably, the introduction of a suitable amount of PbI₂ can significantly improve the performance and stability of perovskite-based photoresistors, optoelectronic devices with ultrahigh photo-sensitivity, high current on/off ratio, fast photo response speed, and retarded decay. Specifically, a highest responsivity of 7.8 A/W and a specific detectivity of 2.1 × 10¹³ Jones with a rise time of 0.86 ms and a decay time of 1.5 ms have been achieved. In addition, the local dependence of photocurrent generation in perovskite thin films was revealed by photoconductive atomic force microscopy, which provides direct evidence that the presence of PbI₂ can effectively passivate the grain boundaries of CH₃NH₃PbI₃ and assist the photocurrent transport more effectively.

Air-stable lead-free hybrid perovskite employing self-powered photodetection with an electron/hole-conductor-free device geometry

In this study, we have demonstrated the fabrication of one of the few lead-free perovskite photodetectors reported to date. In particular, we fabricated the photodetector based on a methylammonium bismuth iodide, MA3Bi2I9 (MABI), structure that works under a self-powered mode of operation. The optoelectronic properties of the photodetector were investigated systematically. The photoresponse of the photodetector was carefully studied and compared with the literature, which demonstrated the capability of the MABI structure for detecting light at a very low incident irradiance of 10 μW cm-2. Also, it exhibited a fast and reproducible response speed along with high photosensitivity (∼105) and detectivity (∼1012 Jones) at low operating voltages (0 V and 0.8 V) through the large effective device area. It was also revealed that the MABI photodetector had good stability after storage for two months at ambient conditions. It is therefore concluded that our photodetector fabricated with a MABI perovskite structure using a simple geometry has great potential to further improve the optoelectronic properties when proper electron/hole-transporting layers are fused for proper charge extraction. This will have significant benefits for next-generation cost-competitive optoelectronic technology to address the scalability issue.

Lead-free hybrid perovskites for photovoltaics

This review covers the state-of-the-art in organo-inorganic lead-free hybrid perovskites (HPs) and applications of these exciting materials as light harvesters in photovoltaic systems. Special emphasis is placed on the influence of the spatial organization of HP materials both on the micro- and nanometer scale on the performance and stability of perovskite-based solar light converters. This review also discusses HP materials produced by isovalent lead(II) substitution with Sn2+ and other metal(II) ions, perovskite materials formed on the basis of M3+ cations (Sb3+, Bi3+) as well as on combinations of M+/M3+ ions aliovalent to 2Pb2+ (Ag+/Bi3+, Ag+/Sb3+, etc.). The survey is concluded with an outlook highlighting the most promising strategies for future progress of photovoltaic systems based on lead-free perovskite compounds.

Synthesis of Easily Transferred 2D Layered BiI3 Nanoplates for Flexible Visible-Light Photodetectors

Bismuth triiodide, BiI₃, is one of the promising 2D layered materials from the family of metal halides. The unique electronic structure and properties make it an attractive material for the room-temperature gamma/X-ray detectors, high-efficiency photovoltaic absorbers, and Bi-based organic-inorganic hybrid perovskites. Other possibilities including optoelectronic devices and optical circuits are envisioned but rarely experimentally confirmed yet. Here, we report the synthesis of vertical 2D BiI₃ nanoplates using the physical vapor deposition mechanism. The obtained products were found easy to be separated and transferred to other substrates. Photodetectors employing such 2D nanoplates on polyethylene terephthalate substrate are demonstrated to be quite sensitive to red light (635 nm) with good responsivity (2.8 A W^-1), fast stable photoresponse (3/9 ms for raise/decay times), and remarkable specific detectivity (1.2 × 10¹² jones), which attest to high comparability of the assembled components with many latest 2D nanostructured light sensors. In addition, such photodetectors exhibit outstanding mechanical stability and durability under different bending strains within the theoretically affordable levels, suggesting a variety of potential applications of 2D BiI₃ for flexible devices.

Constructing Sensitive and Fast Lead-Free Single-Crystalline Perovskite Photodetectors

We developed a high-performance photodetector based on (CH₃NH₃)₃Sb₂I₉ (MA₃Sb₂I₉) microsingle crystals (MSCs). The MA₃Sb₂I₉ single crystals exhibit a low-trap state density of ∼10¹⁰ cm^-3 and a long carrier diffusion length reaching 3.0 μm, suggesting its great potential for optoelectronic applications. However, the centimeter single crystal (CSC)-based photodetector exhibits low responsivity (10^-6 A/W under 1 sun illumination) due to low charge-carrier collection efficiency. By constructing the MSC photodetector with efficient charge-carrier collection, the responsivity can be improved by three orders of magnitude (under 1 sun illumination) and reach 40 A/W with monochromatic light (460 nm). Furthermore, the MSC photodetectors exhibit fast response speed of <1 ms, resulting in a high gain of 108 and a gain-bandwidth product of 10⁵ Hz. These numbers are comparable to the lead-perovskite single-crystal-based photodetectors.

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

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

Hybrid Organic-Inorganic Perovskite Photodetectors

Hybrid organic-inorganic perovskite materials garner enormous attention for a wide range of optoelectronic devices. Due to their attractive optical and electrical properties including high optical absorption coefficient, high carrier mobility, and long carrier diffusion length, perovskites have opened up a great opportunity for high performance photodetectors. This review aims to give a comprehensive summary of the significant results on perovskite-based photodetectors, focusing on the relationship among the perovskite structures, device configurations, and photodetecting performances. An introduction of recent progress in various perovskite structure-based photodetectors is provided. The emphasis is placed on the correlation between the perovskite structure and the device performance. Next, recent developments of bandgap-tunable perovskite and hybrid photodetectors built from perovskite heterostructures are highlighted. Then, effective approaches to enhance the stability of perovskite photodetector are presented, followed by the introduction of flexible and self-powered perovskite photodetectors. Finally, a summary of the previous results is given, and the major challenges that need to be addressed in the future are outlined. A comprehensive summary of the research status on perovskite photodetectors is hoped to push forward the development of this field.