Importance of Bi-O Bonds at the Cs2AgBiBr6 Double-Perovskite/Substrate Interface for Crystal Quality and Photoelectric Performance

Ultrahigh-performance photodetectors based on low-dimensional Cs2AgBiBr6/CdS heterojunction

Lead-free double perovskite Cs2AgBiBr6 has garnered increasing attention in photoelectric applications owing to its good stability and excellent photoelectric properties. However, the poor carrier transport in Cs2AgBiBr6 thin films constraints their further application in photodetection. To overcome this issue, we have developed an innovative low-dimensional Cs2AgBiBr6/CdS heterojunction photodetector with substantially improved performance. The device achieved a high responsivity of 6.66 × 103 A/W, an outstanding specific detectivity of 2.10 × 1014 Jones, and an impressive external quantum efficiency of 1.88 × 106 %. Additionally, the on/off current ratio of the heterojunction device reached an impressive 6.18 × 107. These key parameters are significantly better than those of most previously reported Cs2AgBiBr6-based photodetectors. Furthermore, scanning photocurrent mapping and band arrangement analysis were performed to elucidate the mechanism of photocurrent generation and transport in the low-dimensional Cs2AgBiBr6/CdS heterojunction photodetectors. This study highlights the outstanding performance of Cs2AgBiBr6/CdS heterojunction and provides a simple and effective strategy for developing high-performance Cs2AgBiBr6-based photodetectors.

Ultraviolet-Visible-Near-Infrared Broadband Photodetector Enabled by Cs2AgBiBr6: Sn/Conjugated Polymer Heterojunction

Broadband photodetectors covering ultraviolet (UV) to near-infrared (NIR) wavelengths play an essential role in communications, imaging, and biosensing. Developing a single photodetector with a broadband optical response operating at room temperature can significantly reduce the complexity and cost of receiver systems for multispectral applications. In this work, utilizing the porous structure characteristics of Cs2AgBiBr6:Sn thin films, a self-powered detector with broad spectral response (UV-vis-NIR) was achieved by constructing an effective Cs2AgBiBr6:Sn/PDPP3T heterojunction. This photodetector possesses a broad response spectrum from 350 to 950 nm with an average detection rate exceeding 1011 Jones and maintains excellent photoelectric performance over two months. Sn2+ doping effectively reduces the bandgap of Cs2AgBiBr6, enhancing its near-infrared absorption and optimizing energy level alignment with conjugated polymer (diketopyrrolopyrrole-terthiophene, PDPP3T). More importantly, the porous structure derived from Sn doping significantly improves carrier extraction and transport under a near-infrared light response at the heterojunction interface. Utilizing its broad spectral response characteristics in the UV-vis-NIR range, a novel information transfer and encryption system employing full optical modulation has been realized within a single perovskite photodetector. This work provides a new approach to fabricating lead-free double perovskite broadband photodetectors with potential applications in photonic devices.

Lead-Free Halide Double Perovskite for High-Performance Photodetectors: Progress and Perspective

Lead halide perovskite has become a promising candidate for high-performance photodetectors (PDs) due to its attractive optical and electrical properties, such as high optical absorption coefficient, high carrier mobility, and long carrier diffusion length. However, the presence of highly toxic lead in these devices has limited their practical applications and even hindered their progress toward commercialization. Therefore, the scientific community has been committed to searching for low-toxic and stable perovskite-type alternative materials. Lead-free double perovskite, which is still in the preliminary stage of exploration, has achieved inspiring results in recent years. In this review, we mainly focus on two types of lead-free double perovskite based on different Pb substitution strategies, including A₂M(I)M(III)X₆ and A₂M(IV)X₆. We review the research progress and prospects of lead-free double perovskite photodetectors in the past three years. More importantly, from the perspective of optimizing the inherent defects in materials and improving device performance, we propose some feasible pathways and make an encouraging perspective for the future development of lead-free double perovskite photodetectors.

BiOBr Surface-Functionalized Halide Double-Perovskite Films for Slow Ion Migration and Improved Stability

Surface-tailored lead-free halide double-perovskite (Cs₂AgBiX₆) thin films are utilized for ion migration studies. A thin surface layer of BiOBr/Cl is grown via intentional annealing of the halide films in ambient conditions. Herein, we physically stacked the two films, viz., Cs₂AgBiBr₆ and Cs₂AgBiCl₆, to thermally activate the halide ion migration at different temperatures (room temperature (RT)-150 °C). While annealing, the films' color changes from orange to pale yellow and transparent brown to yellow as a result of the migration of Br^- ions from Cs₂AgBiBr₆ to Cs₂AgBiCl₆ and Cl^- ions from Cs₂AgBiCl₆ to Cs₂AgBiBr₆, respectively. Annealing helps in homogenizing the halide ions throughout the films, consequently leading to a mixed phase, i.e., Cs₂AgBiCl_xBr_6-x/Cs₂AgBiBr_xCl_6-x (x = 0 to 6) formation. The movement of ions is understood by absorption studies performed at regular time intervals. These investigations reveal a redshift (from 366 to 386 nm) and a blueshift (from 435 to 386 nm) in absorption spectra, indicating the migration of Br^- and Cl^- toward Cs₂AgBiCl₆ and Cs₂AgBiBr₆, respectively. The films characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) reveal the presence of a peak at 2θ = 10.90° and binding energy of 158.1 eV, respectively, corresponding to the formation of Bi-O bonds at the film surface. Also, XRD studies show a lower 2θ shift of the diffraction peak in the case of Cs₂AgBiCl₆ films and a higher 2θ shift in the case of Cs₂AgBiB₆ films, which further confirms the migration of Cl^- and Br^- from one film to the other. XPS investigations confirm the compositional change with a gradual increment in the concentration of Br^-/Cl^- with an increase in heating time for Cs₂AgBiCl₆/Cs₂AgBiBr₆ films. All these studies confirm thermal diffusion of halide ions in double-perovskite films. Further, from the exponential decay of the absorption spectra, the rate constant for halide (Br) ion diffusion is calculated, which shows an increment from 1.7 × 10^-6 s^-1 at RT to 12.1 × 10^-3 s^-1 at 150 °C. The temperature-dependent rate constant follows Arrhenius behavior and renders an activation energy of 0.42 eV (0.35 eV) for bromide (chloride) ion mobility. A larger estimated value as compared to the reported values for Cs₂AgBiBr₆ wafers (∼0.20 eV) reveals a slow mobility of halide ions in thin films of Cs₂AgBiBr₆/Cl₆. The formation of a BiOBr passivation layer at the surface of Cs₂AgBiBr₆ thin film might be one of the plausible causes of the slow anion diffusion in the present work. Slow ion migration is an indication that the films are stable and of high-quality.

Defect-Dependent Nonlinear Absorption in the Lead-Free Double-Perovskite Cs2AgBiBr6

Lead-free perovskites have attracted increasing attention because they can address the toxicity and instability problems inherent to lead-halide perovskites. Furthermore, the nonlinear optical (NLO) properties of lead-free perovskites are rarely explored. Herein, we report significant NLO responses and defect-dependent NLO behavior of Cs₂AgBiBr₆. Specifically, a thin film of pristine Cs₂AgBiBr₆ exhibits strong reverse saturable absorption (RSA), while a film of Cs₂AgBiBr₆ with defects (denoted as Cs₂AgBiBr₆(D)) shows saturable absorption (SA). The nonlinear absorption coefficients are ca. 4.0 × 10⁴ cm GW^-1 (515 nm laser excitation) and 2.6 × 10⁴ cm GW^-1 (800 nm laser excitation) for Cs₂AgBiBr₆ and -2.0 × 10⁴ cm GW^-1 (515 nm laser excitation) and -7.1 × 10³ cm GW^-1 (800 nm laser excitation) for Cs₂AgBiBr₆(D). The optical limiting threshold of Cs₂AgBiBr₆ is 8.1 × 10^-4 J cm^-2 (515 nm laser excitation). The samples show excellent long-term performance stability in air. The RSA of pristine Cs₂AgBiBr₆ correlates with excited-state absorption (515 nm laser excitation) and excited-state absorption following two-photon absorption (800 nm laser excitation), while the defects in Cs₂AgBiBr₆(D) strengthen the ground-state depletion and Pauli blocking, resulting in SA.

Application of Nanostructured TiO2 in UV Photodetectors: A Review

As a wide-bandgap semiconductor material, titanium dioxide (TiO₂ ), which possesses three crystal polymorphs (i.e., rutile, anatase, and brookite), has gained tremendous attention as a cutting-edge material for application in the environment and energy fields. Based on the strong attractiveness from its advantages such as high stability, excellent photoelectric properties, and low-cost fabrication, the construction of high-performance photodetectors (PDs) based on TiO₂ nanostructures is being extensively developed. An elaborate microtopography and device configuration is the most widely used strategy to achieve efficient TiO₂ -based PDs with high photoelectric performances; however, a deep understanding of all the key parameters that influence the behavior of photon-generated carriers, is also highly required to achieve improved photoelectric performances, as well as their ultimate functional applications. Herein, an in-depth illustration of the electrical and optical properties of TiO₂ nanostructures in addition to the advances in the technological issues such as preparation, microdefects, p-type doping, bandgap engineering, heterojunctions, and functional applications are presented. Finally, a future outlook for TiO₂ -based PDs, particularly that of further functional applications is provided. This work will systematically illustrate the fundamentals of TiO₂ and shed light on the preparation of more efficient TiO₂ nanostructures and heterojunctions for future photoelectric applications.

Low-Temperature Vapor-Phase Anion-Exchange Strategy for Wide-Bandgap Double-Perovskite Cs2AgBiCl6 Films toward Weak Ultraviolet Light Imaging

Low-temperature synthesis of high-quality, high-stability, wide-bandgap perovskite films by solution methods is still challenging. Herein, large-scale wide-bandgap Cs₂AgBiCl₆ (CABC) double perovskite films are synthesized by a vapor-phase anion-exchange strategy. By dedicatedly designing an ultrathin TiO₂ modification layer between the substrate and double perovskites, high-quality heterojunctions with matched energy band alignment are formed, contributing to a remarkably enhanced ON/OFF ratio of 2.4 × 10⁴ (86 times) and a responsivity of 16 mA W^-1 (12 times). Additionally, the ultraviolet photodetectors (UV PDs) exhibit an excellent UV detection limit of 1.18 μW cm^-2 (20 nW), a broad linear dynamic range of 146 dB, and a high specific detectivity of 2.06 × 10¹¹ Jones, as well as long-term stability. Finally, we further demonstrate a weak UV imaging system using CABC UV PDs as imaging sensors. The system is capable of imaging weak UV signals as low as 2.94 μW cm^-2 (50 nW). Our results provide a feasible approach for low-temperature fabrication of wide-bandgap perovskite UV PDs and explore the promising application for weak UV detection and imaging.

Recent advances in lead-free double perovskites for x-ray and photodetection

Over the last decade, lead halide perovskites have attracted significant research attention in the field of photovoltaics, light-emitting devices, photodetection, ionizing radiation detection, etc, owing to their outstanding optoelectrical properties. However, the commercial applications of lead-based perovskite devices are restricted due to the poor ambient stability and toxicity of lead. The encapsulation of lead-based devices can reduce the possible leakage of lead. However, it is hard to ensure safety during large-scale production and long-term storage. Recently, considerable efforts have been made to design lead-free perovskites for different optoelectronic applications. Metal halide double perovskites with the general formula of A₂M^IM^IIIX₆or A₂M^IVX₆could be potentially considered as green and stable alternatives for different optoelectronic applications. In this review article, we focus on the recent progress and findings on lead-free halide double perovskites for x-ray and UV-vis photodetection applications. Lead-free halide double perovskite has recently drawn a great deal of attention for superior x-ray detection due to its high absorption coefficient, large carrier mobility-lifetime product, and large bulk resistance. In addition, these materials exhibit good performance in photodetection in the UV-vis region due to high photocarrier generation and efficient carrier separation. In this review, first, we define the characteristics of lead-free double perovskite materials. The fundamental characteristics and beneficial properties of halide perovskites for direct and indirect x-ray detection are then discussed. We comprehensively review recent developments and efforts on lead-free double perovskite for x-ray detection and UV-vis photodetection. We bring out the current challenges and opportunities in the field and finally present the future outlook for developing lead-free double perovskite-based x-ray and UV-vis photodetectors for practical applications.

Band Matching Strategy for All-Inorganic Cs2AgBiBr6 Double Perovskite Solar Cells with High Photovoltage

The lead-free double perovskite has been proven to be one of the promising alternatives to solve the stability and toxicity problems of lead-based organic-inorganic hybrid perovskite solar cells. Here, high-quality Cs₂AgBiBr₆ double perovskite films with large grains and smooth surface have been prepared through a sequential-vapor-deposition method, and a low-cost and eco-friendly Cu₂O film with a suitable energy level and good electrical properties was prepared as an efficient hole transport layer by vacuum vapor deposition for the first time. The Cu₂O-based devices achieve a champion power conversion efficiency increasing from 1.03 to 1.52% and an enhancement of photovoltage from 1.083 to 1.198 V compared with their organic counterparts. More importantly, the Cu₂O-based devices have excellent stability; they maintained the initial 96% efficiency under environmental conditions after 33 days of unpackaged storage. These results also point out the direction for the further development of these new promising perovskite solar cells.

A Highly Stable Photodetector Based on a Lead-Free Double Perovskite Operating at Different Temperatures

In recent years, considerable breakthroughs have been achieved in the explored photodetectors with improved performance and stability. However, such devices suffer from the drifting parameters (photoresponsivity, response time, and specific detectivity) in the case of evident operating temperature changes. Here, a double perovskite Cs₂NaBiCl₆-based ultraviolet (UV) photodetector is developed free from thermal disturbance, exhibiting a steady photoresponsivity (≈ 67.98 mA/W) and response time (≈ 16.42 ms) within a wide temperature range (from 273 to 333 K). Further studies demonstrate that the stability of the crystal structure endows the superior photodetection capability. This result unambiguously highlights the great potential of such double perovskite Cs₂NaBiCl₆ compound as an environmentally friendly alternative for UV photodetectors.

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.

Single-Layered MXene Nanosheets Doping TiO2 for Efficient and Stable Double Perovskite Solar Cells

The inorganic lead-free Cs₂AgBiBr₆ double perovskite structure is the promising development direction in perovskite solar cells (PSCs) to solve the problem of the instability of the APbX₃ structure and lead toxicity. However, the low short-circuit current and power conversion efficiency (PCE) caused by the low crystallization of Cs₂AgBiBr₆ greatly limit the optoelectronic application. Herein, we adopt a simple strategy to dope single-layered MXene nanosheets into titania (Ti₃C₂T_x@TiO₂) as a multifunctional electron transport layer for stable and efficient Cs₂AgBiBr₆ double PSCs. The single-layered MXene nanosheets significantly improve the electrical conductivity and electron extraction rate of TiO₂; meanwhile, the single-layered MXene nanosheets change the surface wettability of the electron transport layer and promote the crystallization of the Cs₂AgBiBr₆ double perovskite in solar cell devices. Therefore, the PCE went up by more than 40% to 2.81% compared to that of a TiO₂ based device, and the hysteresis was greatly suppressed. Furthermore, the device based on Ti₃C₂T_x@TiO₂ showed the long-term operating stability. After storing the device for 15 days under ambient air conditions, the PCE still remained a retention rate of 93% of the initial one. Our finding demonstrates the potential of Ti₃C₂T_x@TiO₂ in electron transfer material of high-performance double PSCs.

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.