Nature © Macmillan Publishers Ltd 1998 8 letters to nature 906 NATURE | VOL 392 | 30 APRIL 1998 peak I can be used for the fabrication of efficient polymer photovoltaic cells 4 .In excited states that have their origin in a transition from a delocalized orbital to a delocalized orbital, both carriers, electron and hole, are similarly extended, and therefore spatially overlapped. The same is true for excited states that involve transitions from a localized orbital to another localized orbital. However, in an excited state formed by a transition from a localized orbital to a delocalized orbital, the hole is confined to the phenylene ring, while the electron is delocalized over the chain (or vice versa). We consider that this is the reason for the higher photocurrent yields of these states.These results, which have antecedents in early work on molecular crystals (refs 25-27 and refs therein), have more general implications for the understanding of electronic excitations in low-dimensional systems 10,11 . The presence of charge-transfer states that separate more readily into free electron and holes can provide a natural description for the progression from molecular to semiconductor models of electronic excitations. Ⅺ