Alkali Metal Ion-Mediated Augmented Carrier Extraction in Iodobismuth Ternary Perovskite-Based Photovoltaic Device

The introduction of iodobismuth ternary perovskites has solved the stability and toxicity issues raised by lead trihalide perovskites. However, their wide band gaps and low structural dimensionalities have limited the power conversion efficiency in solar cell applications. In this work, we have partially incorporated Rb⁺ into Cs₃Bi₂I₉ perovskite and tuned its band gap for better optoelectronic results. The structural and optical characterizations reveal that the monovalent alkali cation Rb⁺ partially replaces the larger Cs⁺ cation in the Cs₃Bi₂I₉ structure and lowers the band gap from 2.01 to 1.94 eV. The reduction in optical band gap is consistent with the electronic band structure calculated from density functional theory. Moreover, this compositional engineering method has reduced the nonradiative carrier recombination in the perovskite material. The solar cell device based on a Rb-incorporated perovskite photoabsorber has resulted in an enormous enhancement of photoconversion efficiency up to 1.17%. The champion cell also possesses better moisture and thermal stabilities than the pristine Cs₃Bi₂I₉-based device. This work demonstrates the partial incorporation technique to tune the band gap of an iodobismuth ternary perovskite for better carrier extraction in solar cells.