An effective approach of vapour assisted morphological tailoring for reducing metal defect sites in lead-free, (CH3NH3)3Bi2I9 bismuth-based perovskite solar cells for improved performance and long-term stability

Highlights: • For the first time a two-step vapour assisted solution process (VASP) technique was used to systematically react films of BiI₃ with MAI vapours to form perovskite at different reaction time. • On detailed investigation the stepwise formation mechanism of (CH₃NH₃)₃Bi₂I₉ and the effect of metallic content on performance of solar cells was revealed. • World record hysteresis-free, power conversion efficiency of 3.17% obtained for (CH₃NH₃)₃Bi₂I₉ solar cells. • The devices shown breakthrough in long-term stability, proving stable performance for more than 1440 h on continuous 1 sun illumination. We present a controlled, stepwise formation of methylammonium bismuth iodide (CH₃NH₃)₃Bi₂I₉ perovskite films prepared via the vapour assisted solution process (VASP) by exposing BiI₃ films to CH₃NH₃I (MAI) vapours for different reaction times, (CH₃NH₃)₃Bi₂I₉ semiconductor films with tunable optoelectronic properties are obtained. Solar cells prepared on mesoporous TiO₂ substrates yielded hysteresis-free efficiencies upto 3.17% with good reproducibility. The good performance is attributed mainly to the homogeneous surface coverage, improved stoichiometry, reduced metallic content in the bulk, and desired optoelectronic properties of the absorbing material. In addition, solar cells prepared using pure BiI₃ films without MAI exposure achieved a power conversion efficiency of 0.34%. The non-encapsulated (CH₃NH₃)₃Bi₂I₉ devices were found to be stable for as long as 60 days with only 0.1% drop in efficiency. This controlled formation of (CH₃NH₃)₃Bi₂I₉ perovskite films highlights the benefit of the VASP technique to optimize material stoichiometry, morphology, solar cell performance, and long-term durability.