Methylammonium (MA) and formamidinium (FA) mixed perovskites have been widely utilized in invert planar perovskite solar cell (PSC) applications due to their great potentials such as solution processing, high power conversion efficiency, high absorption, and low cost. To commercialize the PSCs, further improving the device performance without detrimentally changing the device configuration is important at present. Herein, small amount (5%) of CsI is introduced to form a pinhole-free mixed perovsldte light absorption layer in a flexible photovoltaic system with enhanced main photovoltaic parameters of the device, resulting in a similar to 36% enhancement of power conversion efficiency (PCE) from 8.9% to 12.1% (from 11.3% to 14.6% for rigid device). The origins of the enhancement have been studied by exploring the crystal structures, optical absorption, and crystal morphlogies of the synthesised perovskite layers and charge carrier behaviors of the devices. Through small perturbation transient photovoltage (TPV) measurements, the lifetime of the charge carriers are found remarkably increased after the small amount of CsI doping. These characteristics make cation doping of the mixed perovskite layer a promising method in facile tuning of the film quality of the perovskite layer for efficient charge carriers transporting and further boosting the PCE of the devices.