Intedigitated p-n junction: a route to improve the efficiency in organic photovoltaic cells

The short mean free path of excitons photogenerated in real organic photovo ltaic (O-PV) cell strongly reduces the effective active volume of a device. In fact, in a p-n junction O-PV structure, only those excitons that are fo rmed close to the interface between the n-type and the p-type materials are able to dissociate giving free carriers. One of the possible routes to inc rease the efficiency of a O-PV, is to extend the surface area of the juncti on. Within this perspective, an interdigitated structure has been obtained by controlled differential co-sublimation in UHV of C-60 (n-type) and Zn-ph thalocyanine (ZnPc, p-type). Spectroscopic investigation evidenced that the increase of the photovoltaic efficiency is related also to the existence o f a weak charge transfer (CT) absorption state in the near infrared region.