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

High Performance of Cs2AgBiBr6 Perovskite-based Photodetectors by Adding DEAC

Perovskite-based photodetectors (PDs) are broadly utilized in optical communication, non-destructive testing, and smart wearable devices due to their ability to convert light into electrical signals. However, toxicity and instability hold back their mass production and commercialization. The lead-free Cs2AgBiBr6 double perovskite film, promised to be an alternative, is fabricated by electrophoretic deposition (EPD), which compromises film quality. Herein, we improved the quality of the Cs2AgBiBr6 films and the performance of PDs by adding N-acetylethylenediamine (DEAC) to Cs2AgBiBr6 perovskite precursor for EPD, in which the ligand DEAC provides a pair of lone electrons to Bi3+, forming coordinate covalent bonds. The optimized PDs have high detectivity, up to 4.4×1012 Jones, and high stability in the air. The high-performance, flexible Cs2AgBiBr6 perovskite PDs were fabricated on this basis, it also achieved the detectivity up to 3.2×1012 Jones with excellent bending cycle stability. These results propose a feasible approach to improving the crystallization of double perovskite thin films by introducing ligands during the EPD process. Furthermore, they demonstrate enhanced optoelectronic performance, indicating that this method is also applicable to halide perovskites, offering an effective strategy to improve their film quality.

Structurally and Electronically Anisotropic Nature of Bridgman-Grown Cs3Sb2Br9 Perovskite Single Crystal toward Efficient Photodetector

Cs3Sb2Br9, as a sort of novel lead-free perovskite single crystal, has the merits of high carrier mobility and a long diffusion length. However, the large-sized and high-crystallized Cs3Sb2Br9 single crystals are not easily obtained. Herein, we apply the vertical Bridgman method to grow centimeter-sized Cs3Sb2Br9 single crystal. The temperature-dependent crystal structure of Cs3Sb2Br9 is in situ characterized in the temperature range of 100-400 K. A novel crystallographic and electronic structure anisotropy of the as-grown Cs3Sb2Br9 single crystal along the transmission directions of [100] and [001] is experimentally and theoretically proved. Owing to the layered two-dimensional (2D) structure of Cs3Sb2Br9, quantum confinement effects prolong the lifetime of hot carriers, leading to their accumulation within the Sb-Br plane along the [100] direction, thereby resulting in a higher density of electronic states. Accordingly, the [100] device exhibits a carrier mobility higher than that of the [001] device, with the [100] device mobility being 4 orders of magnitude higher than that of the [001] device at 423 K, showing a remarkable anisotropy. The [100] device also shows responsivity ∼10 times higher than that of the [001] device.

Enhanced Responsivity and Photostability of Cs3Bi2I9-Based Self-Powered Photodetector via Chemical Vapor Deposition Engineering

Lead-based halide perovskites have gained significant prominence in recent years in optoelectronics and photovoltaics, owing to their exceptional optoelectronic properties. Nonetheless, the toxicity of lead (Pb) and the stability concern pose obstacles to their potential for future large-scale market development. Herein, stable lead-free Cs3Bi2I9 (CBI) films are presented with smooth and compact morphologies synthesized via chemical vapor deposition (CVD), demonstrating their application as an UV photodetector in a self-powered way. The self-powered photodetectors (SPDs) exhibit remarkable characteristics, including a responsivity of 1.57 A W-1 and an impressive specific detectivity of 3.38 × 1013 Jones under the illumination of 365 nm at zero bias. Furthermore, the SPDs exhibit a nominal decline (≈2.2%) in the photocurrent under constant illumination over 500 h, highlighting its impressive long-term operational stability. Finally, the real-time UV-detection capability of the device is demonstrated by measuring the photocurrent under various conditions, including room light and sunlight at different times. These findings offer a new platform for synthesizing stable and high-quality perovskite films, and SPDs for advancing the development of wearable and portable electronics.

Graphene photodetectors integrated with silicon and perovskite quantum dots

Photodetectors (PDs) play a crucial role in imaging, sensing, communication systems, etc. Graphene (Gr), a leading two-dimensional material, has demonstrated significant potential for photodetection in recent years. However, its relatively weak interaction with light poses challenges for practical applications. The integration of silicon (Si) and perovskite quantum dots (PQDs) has opened new avenues for Gr in the realm of next-generation optoelectronics. This review provides a comprehensive investigation of Gr/Si Schottky junction PDs and Gr/PQD hybrid PDs as well as their heterostructures. The operating principles, design, fabrication, optimization strategies, and typical applications of these devices are studied and summarized. Through these discussions, we aim to illuminate the current challenges and offer insights into future directions in this rapidly evolving field.

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.

High performance CsBi3I10/PCBM bulk heterojunction perovskite photodetector

Lead halide perovskites (LHPs) have been extensively studied due to their remarkable optoelectronic performance. However, the toxicity of a lead ion to humans and its instability under ambient conditions render lead-based halide perovskite an unsuitable material for commercialization. Meanwhile, lead-free halide perovskite (LFHP) devices generally exhibit poor performance. Therefore, enhancing photoelectric conversion capacity is the most important issue that needs to be addressed. Here, we propose a photodetector (PD) fabricated using C s B i 3 I 10/p h e n y l-C 61-butyric acid methyl ester (PCBM) bulk heterojunction as the active layer. The PD illuminated under 532 nm can reach a high responsivity (1.54 A/W) at -2V bias, while at 2 V bias, the PD reaches a higher responsivity (224.40 A/W). All of those results suggest that C s B i 3 I 10/P C B M bulk heterojunctions hold enormous potential in substituting for LHPs in optoelectronic 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.

High-performance broadband photoresponse of self-powered Mg2Si/Si photodetectors

Infrared optoelectronic devices are capable of operating in harsh environments with outstanding confidentiality and reliability. Nevertheless, suffering from the large band gap value, most semiconductor materials are difficult to detect infrared light signals. Here, Mg₂Si/Si heterojunction photodetectors (PDs), which possess the advantages of low-cost, easy process, environmental friendliness, and compatibility with silicon CMOS technology, have been reported with a broadband spectral response as tested from 532 to 1550 nm under zero-bias. When the incident light wavelength is 808 nm, the Mg₂Si/Si photodetector (PD) has a responsivity of 1.04 A W^-1and a specific detectivity of 1.51 × 10¹²Jones. Furthermore, we find that the Ag nanoparticles (Ag_NPs) assembled on the Mg₂Si layer can greatly improve the performance of the Mg₂Si/Si PD. The responsivity and specific detectivity of Mg₂Si/Si device with Ag_NPs under 808 nm illumination are 2.55 A W^-1and 2.60 × 10¹²Jones, respectively. These excellent photodetection performances can be attributed to the high-quality of our grown Mg₂Si material and the strong built-in electric field effect in the heterojunction, which can be further enhanced by the local surface plasmon effect and local electromagnetic field induced by Ag_NPs. Our study would provide significant guidance for the development of new self-powered infrared PDs based on silicon materials.

Interfacial Gradient-Energy-Band-Alignment Modulation via a Vapor-Phase Anion-Exchange Reaction toward Lead-Free Perovskite Photodetectors with Excellent UV Imaging Capability

Bi-based inorganic perovskites have attracted great attention in optoelectronics, as they feature similar photoelectric properties but have high stability and lead-free merits. Unfortunately, due to the high exciton binding energy and small Bohr radius, their photodetection performance still largely lags behind that of Pb-based counterparts. Herein, using a vapor-phase chloride ion-substitution strategy, Cs3Bi2Br9 photodetectors (PDs) with gradient energy band alignment were delicately modulated, contributing to a high carrier separation/collection efficiency. The optimized Bi-based perovskite ACCT (Al2O3/Cs3Bi2Br9/Cs3Bi2ClxBr9-x/TiO2) PDs exhibit outstanding performance, the ON/OFF ratio and linear dynamic range (LDR) are significantly improved by 20 and 2.6 times, respectively. Significantly, we further demonstrate the high-SNR (signal-to-noise ratio) UV imaging based on the optimized device, which shows 21.887 dB higher than that of the pristine device. Finally, the vapor-phase anion-exchange modified perovskite PDs show long-term stability and high UV resistance. Vapor-phase ion-substitution is a promising approach for the synergistic effect of matched energy band alignment and interface passivation, which can be applied to other perovskite-based optoelectronic devices.

A nanomesh electrode for self-driven perovskite photodetectors with tunable asymmetric Schottky junctions

Self-driven photodetectors are essential for many applications where it is unpractical to provide or replace power sources. Here, we report a new device architecture for self-driven photodetectors with tunable asymmetric Schottky junctions based on a nanomesh electrode. The vertical-channel nanomesh scaffold is composed of a hexagonally ordered nanoelectrode array fabricated via the nanosphere lithography technique. The top and bottom nanoelectrodes are separated by only 30 nm and the areal ratio of the two nanoelectrodes can be fine-tuned, which effectively modifies the geometric asymmetricity of the Schottky junctions in the photodetector devices. The self-driven photodetectors are fabricated by depositing the (FAPbI₃)_0.97(MAPbBr₃)_0.03 (MA = methylammonium, FA = formamidinium) perovskite films onto the nanomesh electrodes. Under the self-driven mode, the optimized device demonstrates a high detectivity of 1.05 × 10¹¹ Jones and a large on/off ratio of 2.1 × 10³. This nanomesh electrode is very versatile and can be employed to investigate the optoelectronic properties of various semiconducting materials.

Self-Assembled Organic Cations-Assisted Band-Edge Tailoring in Bismuth-Based Perovskites for Enhanced Visible Light Absorption and Photoconductivity

Bismuth-based zero-dimensional perovskites garner high research interest because of their advantages, such as excellent moisture stability and lower toxicity in comparison to lead-based congeners. However, the wide optical bandgap (>2 eV) and poor photoconductivity of these materials are the bottlenecks for their optoelectronic applications. Herein, we report a combined experimental and theoretical study of the structural features and optoelectronic properties of two novel and stable zero-dimensional bismuth perovskites: (biphenyl bis(methylammonium))1.5BiI6·2H2O (BPBI) and (naphthalene diimide bis(ethylammonium))1.5BiI6·2H2O (NDBI). NDBI features a remarkably narrower bandgap (1.82 eV) than BPBI (2.06 eV) because of the significant orbital contribution of self-assembled naphthalene diimide cations at the band edges of NDBI. Further, the FP-TRMC analysis revealed that the photoconductivity of NDBI is about 3.7-fold greater than that of BPBI. DFT calculations showed that the enhanced photoconductivity in NDBI arises from its type-IIa band alignment, whereas type-Ib alignment was seen in BPBI.

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.