The photovoltaic properties of a Ga-doped ZnO (GZO)/3,4,9,10-perylene-tetracarboxyl-bis-benzimidazole (PTCBI)/Zn-phthalocyanine (ZnPc)/Cu heterojunction cell (cell A) and a GZO/ZnPc/Cu Schottky-barrier cell (cell B) were investigated. The energy conversion efficiency η of cell A was only 0.02% immediately after the device preparation but improved to 0.46% after aging for 24 days in air. To elucidate the mechanism of this aging effect, photocurrent action and electro-absorption spectra were measured for cell B. The results reveal that a Schottky barrier exists at the ZnPc/Cu interface which blocks the transport of photogenerated holes to the Cu electrode, and the barrier height is reduced by a white-light illumination of the device after aging. The change in barrier height is attributed to the formation of electron traps at the surface of the ZnPc layer on aging that trap photogenerated electrons.