Improvement of Photovoltaic Characteristics by MoO3 Doping of Thick Hole-Transporting Films

Thick heterojunction cells composed of fullerene and p-type hole-transporting materials (HTMs) doped with molybdenum oxide (MoO3) were fabricated. The Fermi level (E F) of HTMs shifted toward the positive direction and close to the upper edge of the valence band following MoO3 doping. These E F shifts indicate that intrinsic HTMs changed to be of the p-type. The introduction of p-type HTMs to the cells increased photocurrent density and fill factor. The increase in photocurrent density can be explained by the formation of built-in potential at the interface between p-type HTMs and C60. On the other hand, the increase in fill factor can be explained by the drastic decrease in the resistance of 300-nm-thick HTM films, which reached a very small value of 2 Ω.