Perovskite solar cells (PSCs) have attracted considerable attention because of their low fabrication cost and impressive energy conversion efficiency. The perovskite layer of planar PSCs is usually prepared by coating a solution of perovskite precursors, that is, PbI2, PbCl2 and methylammonium iodide (MAI), on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS, Clevios P VP Al 4083). Here, the deposition of the perovskite layer from its precursor solution saliently affects the electronic structure and properties of PEDOT:PSS films and thus the photovoltaic performance of planar PSCs. The conductivity of PEDOT:PSS is significantly enhanced from 10−3 to 101 S cm−1. The conductivity enhancement is not due to the solvent but mainly MAI. Even more significant conductivity enhancement occurs for PEDOT:PSS films after being coated with a dimethylformamide (DMF) solution of MAI, while pure DMF only slightly increases the conductivity of PEDOT:PSS by a factor of 2–3. PEDOT:PSS films become rougher after the deposition of a perovskite or MAI layer. The conductivity enhancements are attributed to the phase segregation of PSSH chains from PEDOT:PSS and the conformational change of PEDOT chains. The treatment of PEDOT:PSS with the organic solutions of MAI and solvents of perovskite precursor solutions also affects the photovoltaic performance of the planar PSCs.