The rapid advancement of perovskite solar cells (PSCs) has established them as one of the most promising next-generation photovoltaic technologies due to their tunable bandgaps, excellent absorption characteristics, and cost-effective fabrication routes. However, the widespread adoption of conventional lead-halide perovskites is hindered by their toxicity and poor long-term stability, necessitating the exploration of lead-free alternatives. In this regard, Cesium Bismuth Iodide (Cs₃Bi₂I₉) has emerged as a potential candidate owing to its all-inorganic composition, reduced toxicity, superior chemical stability, and attractive optoelectronic properties. Despite these advantages, the reported device efficiencies of Cs₃Bi₂I₉-based solar cells remain relatively low, and comprehensive studies integrating both numerical simulations and experimental validations are limited.

In this work, we introduce a novel all-inorganic device architecture, FTO/TiO₂/Cs₃Bi₂I₉/CuSCN/Al, which, to the best of our knowledge, has not been reported in prior literature. Numerical simulations were first employed to explore and optimize the performance of this architecture using SCAPS software. Critical parameters such as absorber thickness, defect density, and the thicknesses of both the electron transport layer (ETL) and hole transport layer (HTL) were systematically varied to evaluate their influence on power conversion efficiency (PCE). Furthermore, the role of interface defects and charge recombination dynamics was analyzed in detail to identify the dominant loss mechanisms. Through this optimization, a peak simulated efficiency of 12.57% was achieved, demonstrating the theoretical promise of Cs₃Bi₂I₉ in carefully engineered device configurations.

Complementing the simulation study, experimental efforts were undertaken to fabricate the Cs₃Bi₂I₉ absorber layer using a one-step spray coating technique, a scalable and cost-effective method suitable for large-area device fabrication. The structural, optical, and morphological quality of the deposited films was confirmed through X-ray diffraction, ultraviolet–visible absorption spectroscopy, and scanning electron microscopy. The Cs₃Bi₂I₉ films exhibited well-defined crystallinity, strong light absorption in the visible region, and compact morphology, all of which are favorable for photovoltaic applications. To further probe the optoelectronic properties, time-correlated single-photon counting measurements were carried out, revealing the recombination pathways and carrier lifetimes in the absorber layer. Additionally, X-ray photoelectron spectroscopy (XPS) and elemental mapping confirmed the uniform distribution of Cs, Bi, and I elements, highlighting the chemical homogeneity and stability of the films.

Finally, the fabricated devices were evaluated under standard AM 1.5G solar illumination. Although the experimental device efficiency reached only 0.52%, this result provides valuable insight into the challenges associated with translating numerical predictions into real devices. Factors such as interfacial losses, suboptimal film coverage, and higher-than-expected defect densities are likely responsible for the observed performance gap. Nevertheless, the consistency between simulation trends and experimental observations underscores the reliability of the design approach and provides a clear roadmap for further optimization.

Overall, this study presents a comprehensive exploration of Cs₃Bi₂I₉-based perovskite solar cells by combining numerical modeling with experimental fabrication and characterization. The results not only demonstrate the feasibility of a new device structure but also highlight the importance of addressing interface defects and recombination losses in improving performance. By showcasing both the opportunities and challenges of Cs₃Bi₂I₉ as an absorber material, this work contributes to the ongoing development of eco-friendly, stable, and scalable lead-free perovskite photovoltaics, paving the way for next-generation solar cell technologies.