In this work we present an investigation of the photoexcited states in conjugated polymer (donor) - fullerene (acceptor) interpenetrating networks embedded into conventional polymer hosts like polystyrene (PS), polyvinylcarbazole (PVK) or polyvinylbenzylchloride (PVBC) (guest - host approach), using photoinduced FT-IR spectroscopy. We discuss the influence of the specific host polymer matrices on the photoexcited states of the photoactive guests and investigate the photoinduced electron transfer by analysis of the infrared activated vibrational (IRAV) bands of poly-3-octylthiophene (P3OT) in comparative studies. Solar cells based on mixtures of poly [2-methoxy, 5-(3＇,7＇-dimethyl-octyloxy)]-p-phenylene vinylene (MDMO-PPV), a highly soluble fullerene derivative [6,6]-phenyl C-61-butyric acid methyl ester (PCBM) and a conventional polymer (PS, PVK or PVBC) are characterized. We studied the current-voltage characteristics of devices and determined the energy conversion and electron/photon conversion efficiencies. (C) 2000 Elsevier Science B.V. All rights reserved.