Tailoring Bi to boost CuAgBi2I8 solar cells

Abstract

Considering sustainable development factors such as element abundance, cost, environmental friendliness, and stability, the research and development of novel inorganic non-lead perovskites are very significant. Copper-silver-bismuth iodide (CABI) is a promising solar cell material with halide perovskite genes, possessing eco-friendly, element-rich, and cost-effective characteristics. The fabrication of high-quality CABI films with tailored compositions still poses a substantial hurdle. We developed a CuAgBi₂I₈ material that effectively reduced the bandgap to 1.69 eV by optimizing Bi distribution to create an environment conducive to in-situ redox reactions of Bi with I₂, Cu, and Ag via vapor-phase synthesis. This strategy proved highly effective in synthesizing high-quality CuAgBi₂I₈ compound, accompanied by significant improvements in film quality, including enhanced crystallinity, minimized defects, and reduced non-radiative recombination. The crystal structure of CuAgBi₂I₈ and mechanisms of elemental reactions and diffusion are discussed. Devices featuring the structure FTO/c-TiO₂/m-TiO₂/CuAgBi₂I₈/CuI/Spiro-OMeTAD/carbon achieved a champion efficiency of 3.21%, the highest for CABI solar cells. This work provides a novel idea and approach to governing the gas–solid element diffusion and reaction for high-quality CABI and related halide perovskite films.