Tailoring the film morphology and interface band offset of caesium bismuth iodide-based Pb-free perovskite solar cells

Bismuth-based halide perovskites (Bi-HaP) are low toxicity and air-stable materials with promising photo-absorber properties. In this study, we fabricated Bi-HaP (Cs3Bi2I9 and CsBi3I10) films by a solution process followed by solvent annealing and investigated the crystal growth and optoelectronic properties of these materials. A compact and large grain morphology of the Bi-HaP films was realized by annealing under ambient solvent vapor conditions. Collective analysis of XRD patterns, and Raman spectra, absorption and PL spectra of the fabricated films corroborates that the Cs3Bi2I9 film (E-g similar to 2.08 eV) with a hexagonal crystal phase is more stable under annealing conditions in a wide temperature range and ambient solvent vapor annealing conditions as compared to the other CsBi3I10 thin film, having the narrower E-g similar to 1.8 eV, of the Bi-HaP family. We have achieved the best power conversion efficiency as high as similar to 1.26% with the open circuit voltage of 0.74 V for the device fabricated with Cs3Bi2I9. The analysis of material properties and device characteristics indicates that morphology tailoring, surface chemistry control, and interface band offset engineering are important for the further improvement of Bi-HaP-based devices.