The effects of solvent-modulated poly (3,4-ethylenedioxythiophene) : poly (styrenesulfonate) (PEDOT: PSS) on the performance of organic photovoltaic devices (OPVs) were investigated. The PEDOT: PSS buffer layers were modulated by two different methods: blending and surface treatment. Both types of buffer layer modulation improved the power conversion efficiency (PCE) by reducing the series and contact resistances of OPVs. Compared with blending, which requires a carefully controlled doping level, surface treatment can be processed simply and exhibits much more obvious improvements after modulation. In short, efficient charge transport was possible after modulation, and high PCE values, 4.17% for blending and 5.14% for surface treatment, were obtained. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3493492] All rights reserved.