Ga-Doped ZnO Film as a Transparent Electrode for Phthalocyanine/Perylene Heterojunction Solar Cell

The photovoltaic properties of Zn–phthalocyanine (ZnPc)/3,4,9,10-perylene-tetracarboxyl-bis-benzimidazole (PTCBI) heterojunction solar cells using Ga-doped ZnO (GZO) film as a transparent electrode are studied. When GZO is the electrode in contact with the donor layer, i.e., ZnPc, the energy conversion efficiency η is only 1/4 of that for the cell using indium–tin-oxide (ITO) as the electrode. When GZO is the electrode in contact with the acceptor layer, i.e., PTCBI, on the other hand, the cell has a three times larger η than the cell using ITO. These results are explained by the work function of GZO being lower than that of ITO. When GZO, or ITO, is used as an electrode for PTCBI, an interesting aging effect is observed. For example, by keeping the GZO-based cell in open air for 6 days in the dark, the energy conversion efficiency and the short-circuit current are increased by factors of 1.67 and 1.36, respectively.