Perovskite solar cells (PSCs) have attracted much attention due to their high power conversion efficiency (PCE) and low cost fabrication. Since the PCE is critically related to the perovskite crystal morphology and quality, controlling the perovskite crystal formation is becoming more important. In this work, we chose PDDA poly electrolyte as the crystal capping agent to modulate the perovskite thin film formation. The PDDA capping agent significantly enhanced the short-circuit current (J(sc)) and fill factor (FF) of the corresponding PSCs, which produced an optimal PCE of 14.1% compared to 12.5% of control device without the modifying agent. The effect of PDDA agent is examined by various techniques. It was found that improved surface morphology, enhanced crystallinity, and decreased grain boundary related defects are contributed to the performance and stability enhancement.