Bandgap reduction of bismuth tri-iodide via cesium tin iodide addition aimed toward lead-free solar cell applications

Bismuth-based perovskites are promising materials for non-toxic solar cell applications owing to their air stability and high absorption coefficients. However, the wide bandgaps of bismuth-based perovskites limit their performance, rendering them unsuitable for single-junction solar cells. In this study, we successfully demonstrate that the bandgap of bismuth tri-iodide (BiI3) can be reduced by adding a mixture of CsI and SnI2 (CTI: cesium tin iodide). The addition of CTI changes the chemical states, surface morphology, optical properties, and energy-level structure of BiI3. The bandgap energy decreases from 1.75 eV (pure BiI3) to 1.41 eV (CTI added) as evidenced by UV-Vis spectrophotometry measurements. The valence band maximum (VBM) energy was estimated using photoemission yield spectroscopy (PYS). PYS measurements revealed that adding CTI to BiI3 lowered the VBM position. The bandgap narrowing mechanism was further explained by the band alignment, where the addition of CTI effectively adjusted the VBM and conduction band minimum positions to lower the energy levels. The device performance of the (CTI:BiI3) solar cells was investigated, and an improvement in power conversion efficiency was observed, mainly due to an enhancement in the open-circuit voltage and short-circuit current density.