Photodoping of zinc phthalocyanine: formation, transport, and activity of ionized oxygen in phthalocyanine solar cells

摘要

Solar cells composed of thin layers of titanium dioxide (TiO_2) and zinc phthalocyanine (ZnPc) show a large decay of the photocurrent in ambient atmosphere. This decay is caused by the combined presence of oxygen, light, and an external electric field. A two-step mechanism is proposed which involves the formation of a mobile photodopant arising from the present of molecular oxygen in the layers. In the first step, oxygen radical anions are formed under influence of bandgap illumination of ZnPc. Subsequently these negative charged species drift towards the interface with TiO_2 under the influence of an external electric field. They accumulate and quench ππ~*-singlet excitons which depresses the photocurrent dramatically. A concentration of more than 5.10~(18) cm~(-3) ionized oxygen species is found to be present in the ZnPc films under ambient atmosphere and 5-mW 670-nm irradiation, which is two orders of magnitude higher than in the dark.