Photovoltaic cells made from bi-layer thin-film heterojunctions having poly(3-octylthiophene) (P3OT) as the electron donor and poly(phenylquinoxaline) (PPQ) as the electron acceptor have been investigated. The photovoltaic characteristics of the heterojunction device are considerably enhanced from those in a single-layer device. The photocurrent is improved by almost two orders of magnitude, and the open-circuit voltage corresponds to the estimated energy level difference of the two polymers. The observed enhancement of the heterojunction device has been discussed in terms of photoinduced charge transfer between P3OT and PPQ. The results showed that PPQ can be regarded as an efficient electron acceptor material. (c) 2005 Elsevier B.V. All rights reserved.