The effect of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) as a buffer layer was investigated in polymer solar cells (PSCs). Four different types of PEDOT:PSS were used: PH, PHS00 and their DMSO (dimethylsulfoxide)-doped counterparts. The efficiency of PSCs was independent of the electric conductivity of the buffer layer as a bulk property while it was significantly related to interfacial properties between the buffer layer and a bulk-heterojunction (BHJ) layer. The interfacial properties included charge transfer resistance (R(CT)), hole mobility (mu(h)) and contact angle (theta) of the solution of BHJ on the buffer layer. Lower R(C), higher mu(h), and smaller theta led to the higher fill factor (up to 72%), enabling highly efficient PSCs with efficiency (eta)=4.25%. (C) 2010 Elsevier B.V. All rights reserved.