We investigated the relationship between the electrical properties (of the n-oxide semiconductor layer and the p-Cu2O sheet) and the obtainable photovoltaic properties of Al-doped ZnO (AZO)/n-oxide/p-Cu2O heterojunction solar cells that were fabricated using either p-type non-doped Cu2O or Na-doped Cu2O (Cu2O:Na) sheets (prepared by thermally oxidizing a Cu sheet) functioning as the active layer as well as the substrate. We fabricated some heterojunction solar cells using p-Cu2O (non-doped) sheets and n-ZnO thin films with various electron concentrations (N) in the range of 10(17)-10(20) cm(-3); non doped and Al- or Cu-doped ZnO thin films were deposited under various O-2 or O-3 gas atmosphere pressures using a pulsed laser deposition (PLD) method. The obtained photovoltaic properties exhibited a tendency to increase as N was increased from 10(17) to about 3 x 10(19) cm(-3); the values remained at these maximum levels as N was increased to about 8 x 1019 cm(-3), and then they decreased as N was increased further. With an N on the order of 10(20) cm(-3), the reduction of photovoltaic properties was attributed to an increase in the discontinuity of the conduction band at the interface between the n-ZnO and p-Cu2O layers. In addition, we found that the hole concentration (P) in Cu2O:Na sheets (i.e., Cu2O sheets post annealed with various Na compounds) could be controlled in the range of 10(14)-10(19) cm(-3) by varying the annealing temperature and duration. As another example, the obtained photovoltaic properties in heterojunction solar cells that were fabricated by depositing n-(Ga0.975Al0.025)(2)O-3 thin films on p-Cu2O:Na sheets by PLD remained constant as P was increased to approximately 1 x 10(16) cm(-3), and then they decreased significantly as P was increased further. The reduction of the obtained photovoltaic properties in heterojunction solar cells fabricated using Cu2O:Na sheets with a P above about 1.4 x 10(16) cm(-3) was attributed mainly to decreases of both the depletion layer width and the diffusion length. (C) 2015 Elsevier B.V. All rights reserved.