Novel photovoltaic devices based on donor-acceptor molecular and conducting polymer systems

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Abstract
Interest in organic photocells was significantly renewed a few years ago due to discovery of extremely fast photo-induced charge transfer (CT) from conducting polymer (CP) to fullerene C/sub 60/ molecules in CP(C/sub 60/) composites. This allowed creation of interfacial junction photocells with relatively high efficiencies. It has been recognized that those are not conventional p/n type semiconducting junction devices, but are so called donor-acceptor (D A) photocells, reminiscent of reaction centers of natural photosynthetic systems. Even more efficient photovoltaic devices have recently been fabricated with interpenetrating D-A networks which allow electron-hole separation to take place throughout the bulk of the device. In the present paper we demonstrate how these photocells can be further improved; we describe new experimental results, give general discussion of the efficiency of such devices, and analyze strategies for design of better D-A type photocells. Our recent experimental results on sensitization by inserting an excitonic layer between donor and acceptor layers multilayered structures, and by doping the D-layer by A-molecules in D(A)/A cells are briefly discussed. The organic photocell, which consists of the multilayer structure: ITO/poly(2,5-dioctyloxy-phenylenvinylene) (OOPPV)/octaethylporphine (OEP)/C/sub 60//Al, has been fabricated. Photocurrent yield spectra are interpreted by light absorption at OEP layer, exciton migration, and charge generation at both organic heterojunctions. That is, double heterojunctions of OOPPV/OEP and OEP/C/sub 60/ contribute to the charge generation by excitonic dissociation. We also report new type of "fractal network" photovoltaic devices made of two conducting polymers, particularly D and A derivatives of poly(phenylene vinylene). Conceptual theoretical work and modeling have been carried out to understand the ways for improvement of the device performance. Strategies to improve each step involving selective doping have been suggested to improve each step. Selective doping can provide low serial resistance and create internal electric fields for the collection of charges, while in the undoped parts excitons are effectively photogenerated. Ways to increase the effective use of absorbed photon energy and filling factor are also suggested.