Disclosed herein is a method for modifying the surface of a counter electrode. According to the method, the surface modification is achieved by treating the surface of a counter electrode with a polyethylene glycol derivative having a pendant group at one end. Also disclosed is a counter electrode whose surface is modified by the method. The electron transfer rate at the interface between the counter electrode and an electrolyte layer of a photovoltaic device is increased and the affinity of the counter electrode for the electrolyte layer is improved, resulting in an improvement in the power conversion efficiency of the photovoltaic device.