photodetectors embedded with chemical vapor deposited lead-free bismuth.

What is it about?

The emerging revolution in the field of Internet of Things (IoT) necessitates the development of flexible, self-powered, and cost-effective photodetectors such as metal halide-based perovskite photodetectors. Lead-free ternary metal halides have emerged as a promising option having no technical barrier with regard to toxicity. However, the performance metrics of lead-free ternary metal-halide-based optoelectronic devices lags behind those of conventional lead-halide-based devices. In this study, we demonstrate that chemical-vapor-deposited methylammonium bismuth iodide (MA3Bi2I9 (MBI)) films and their mixed halide analogues (MA3Bi2I6Br3 (MBIB)) and MA3Bi2I6Cl3 (MBIC)) enhance the performance metrics and stability of photodetectors by enabling the growth of films with smooth and compact morphology. MBIC-integrated devices achieved a responsivity of ∼ 0.92 A/W and specific detectivity of ∼ 2.9 × 1013 Jones under no bias and UV (382 nm) illumination, which is approximately-three times higher than MBI counterparts. They also exhibit outstanding operational stability by maintaining 94 % of the photoresponse after illumination for 500 h (382 nm wavelength at light intensity of 1.0 mW/cm2) in ambient air. Furthermore, these devices show satisfactory mechanical stability, with the attenuation being as low as 15 % of initial photoresponse after bending for 5,000 cycles at 5 % tensile strain. Our findings indicate that MBIC film-integrated devices have immense potential in optoelectronics.

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Why is it important?

Ultraviolet photodetectors (UV PDs) with an ability to convert incident UV (<400 nm) radiation into electrical signals are the essential components in the photodetector family . UV PDs have gathered tremendous research attention because of their wide range of promising applications including advanced communications, environmental monitoring, astronomy, air purification, and medicine [. Commercial photodetectors, e.g., silicon and III–V or II–VI compound semiconductors, are fabricated onto rigid substrates at high temperature and in vacuum, thereby suffering poor mechanical stability and high manufacturing cost . Furthermore, existing conventional photodetectors in the market are not suitable for applications such as flexile image sensors, wearable devices, smart textiles, and e-skin, which are required for recent advances in IoT [9], [10]. Lead iodide-based optoelectronic devices have recently become popular owing to their superior properties and characteristics such as being easy to fabricate at low temperatures in economical process. However, for these devices, there exists a trade-off between high performance with environmental instability and Pb toxicity . In this context, the development of the eco-friendly and stable lead-free perovskite materials has become an attractive research area in the field of optoelectronic applications. The lead-free metal halide perovskites such as ternary bismuth halide perovskites have attracted considerable interest because of their excellent environmental stability, low toxicity, and competitive levels of absorption coefficient . Various strategies have been adopted to fabricate high-quality bismuth halide perovskite films. However, these films exhibit poor surface coverage, weak compactness, and low-crystallinity owing to immature fabrication technology, which results in a poor charge carrier mobility and a high level of trap state density. Although the solution process has many advantages such as a low temperature process, an easy fabrication, and a low cost, the limited solubility levels of precursors in various solvents make it difficult to realize a uniformity and a full-surface coverage with a satisfactory crystalline nature, which results in poor environmental and operational stability. In the case of the vapor deposition process, the substantial difference in the vapor pressure of precursors must be accounted for a fabrication of high-quality films. Specifically, in the conventional chemical vapor deposition (CVD) process, maintaining the balance of metal halide composition might be difficult because of large differences in precursor vapor pressure levels, which results in films with poor surface coverage and low compactness with compositional imbalance . However, solution- and vapor-processed bismuth halide perovskite films in reported literature exhibit both poor surface coverage and compactness with open grain boundaries limiting the overall quality of films. In this regard, our CVD approach with modified source delivery system can create high-performing lead and bismuth halide perovskite-based optoelectronic devices, e.g., solar cells and photodetectors. These devices exhibit excellent moisture and operator stability as they feature high-quality films with smooth and compact morphology . Therefore, we assumed that lead-free bismuth halide photodetectors that were achieved through compositional engineering, such as mixed halide films, would have enhanced performance metrics and operational stability. At present, there is no study that has focused on ternary bismuth halide mixtures created via both solution-processed and vapor-deposited films. Through the creation of high-quality lead-free metal mixed halide films, our unique CVD approach can succeed in manufacturing flexible and stable photodetectors.

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