In this study, we investigated property change in the poly(3,4-ethylenedioxythiophene) poly-styrene sulfonate (PEDOT:PSS) layer by introducing phenylethylammonium iodide (PEAI) in inverted p-i-n perovskite solar cells. After depositing PEAI on the PEDOT:PSS layer, PEA+ induces a screening effect and decreases the interaction between PSS and PEDOT chains, accelerating their phase separation. The separated PSS chains are drawn to the film surface owing to the Coulombic attraction between PEA+ and -SO3- groups. Additionally, the I- ion (-1 value) of PEAI is oxidized to I2 (0 value) after annealing, which contributes to the separation of PEDOT and PSS units. The modified PEDOT:PSS presents a smooth morphology, which contributes to the construction of improved perovskite film quality with enlarged grains and decreased trap defects, accelerating hole extraction and migration efficiency. Moreover, PEAI modification could also realize a favorable energy level match between the PEDOT:PSS layer and perovskite active layer. Consequently, the power conversion efficiency is enhanced from 15.95% to 18.71%, and light and thermal stabilities are improved for the PEAI-modified device.