Abstract: Efficiency enhancement of Cs_0.1(CH₃NH₃)_0.9PbI₃ solar cell devices was performed by using iso-butyl ammonium iodide (IBA) passivated on Cs_0.1(CH₃NH₃)_0.9PbI₃ films. The n–i–p structure of perovskite solar cell devices was fabricated with the structure of FTO/SnO₂/Cs_0.1(CH₃NH₃)_0.9PbI₃ (FTO, i.e., fluorine doped tin oxide) and IBA/Spiro-OMeTAD/Ag. The effect of different weights of IBA passivated on Cs-doped perovskite solar cells (PSCs) was systematically investigated and compared with non-passivated devices. It was found that the 5-mg IBA-passivated devices exhibited a high power conversion efficiency (PCE) of 15.49% higher than 12.64% of non-IBA-passivated devices. The improvement of photovoltaic parameters of the 5-mg IBA-passivated device can be clearly observed compared to the Cs-doped device. The better performance of the IBA-passivated device can be confirmed by the reduction of PbI₂ phase in the crystal structure, lower charge recombination rate, lower charge transfer resistance, and improved contact angle of perovskite films. Therefore, IBA passivation on Cs_0.1(CH₃NH)_0.9PbI₃ is a promising technique to improve the efficiency of Cs-doped perovskite solar cells.