Photovoltaics of photoactive protein/polypeptide LB films

Two types of photovoltaic cells prepared by LB films of photoactive protein and/or polypeptide were investigated; i.e., the one is of bacteriorhodopsin (bR) which has an "intrinsic" proton pump (Cell-1), and the other is of polypeptide with carbazoyl group which has a high efficiency in photocarrier generation (Cell-2). In case of Cell-1 the monolayer of purple membrane (2-D crystal of bR) was transferred onto the ITO glass substrate by a horizontal transfer technique. The photovoltaic current I_pv flows from Al to ITO electrodes and depends on the depolarization due to the photophysical cycle of bR. The quantum efficiency of bR is quite high compared to the commodity polymers. In case of Cell-2 the formation of Schottky barrier between Al electrode and polypeptide multi-layer and the photocarrier (hole) injection from Al electrode are essential to the photovoltaic effect. Since I_pv strongly depends on the conductivity of polymer layer, the alignment of carbazole groups in 1-D and/or the formation of interdigited column structure with acceptor moety (CT complex) are desirable for the conduction enhancement. For the comparison with protein photoactive cells, phthalocyanine derivatives were also investigated as insulator layers of MIM type cells: The one is an LB film of alkoxy phthalocyanine (APc) and the other is a Ni-Pc thin film formed by molecular beam epitaxy technique (MBE-Pc).