Tailoring of the Band Gap of MA₃Bi₂I₉ through Doping at A as well as X Sites (of ABX₃ Structure): Futuristic Material for Multijunction Solar Cells

The reliable use of lead-based organic-inorganic perovskite materials in optoelectronic devices is restricted due to their toxic as well as degrading behavior in an ambient atmosphere. Hence, the bismuth-based perovskites have shown potential applications in multijunction perovskite-perovskite-silicon tandem solar cells due to their less toxic and environmentally friendly behavior. Nevertheless, the power conversion efficiency of Bi-related perovskite solar cells is very small compared with solar cells based on Pb-based perovskite, motivating researchers for the fundamental understanding of film formation of Bi-based perovskites as well as energy-level pinning in devices, which is of prime importance. Band-gap pinning can be achieved by tailoring through various types of doping. Herein, MA3Bi2I9 is doped with various concentrations of CsBr. The doped films of MA3Bi2I9 were characterized with the help of XRD, FESEM, UV–visible absorption, and Raman spectroscopic techniques. The observations reveal that doping through CsBr changes the band gap of pristine MA3Bi2I9 from ∼2.05 to ∼2.24 eV with the variation in concentration of dopant from ∼0.05 to ∼0.15%.