Despite the excellent merits of lead perovskite solar cells, their instability and toxicity still present a bottleneck for practical applications. Bismuth perovskite has emerged as a candidate for photovoltaic (PV) applications, because it not only has a low toxicity but is also stable in air. However, the power conversion efficiency (PCE) remains an unsolved problem. We performed band gap tuning experiments to improve the efficiency. The absorption of ABi₃I₁₀ structure films was extended within the visible region, and the optical band gap was decreased considerably compared to that for Cs₃Bi₂I₉. Furthermore, we explained the correlation between the structure and the optical properties via a first-principles study. A device employing CsBi₃I₁₀ as a photoactive layer exhibits a PCE of 1.51% and an excellent ambient stability over 30 days.