Relationship between the electrical properties of the n-oxide and p-Cu2O layers and the photovoltaic properties of Cu2O-based heterojunction solar cells

Abstract We investigated the relationship between the electrical properties (of the n-oxide semiconductor layer and the p-Cu 2 O sheet) and the obtainable photovoltaic properties of Al-doped ZnO (AZO)/n-oxide/p-Cu 2 O heterojunction solar cells that were fabricated using either p-type non-doped Cu 2 O or Na-doped Cu 2 O (Cu 2 O: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-Cu 2 O (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 3×10 19  cm −3 ; the values remained at these maximum levels as N was increased to about 8×10 19  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-Cu 2 O layers. In addition, we found that the hole concentration ( P ) in Cu 2 O:Na sheets (i.e., Cu 2 O sheets postannealed 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-(Ga 0.975 Al 0.025 ) 2 O 3 thin films on p-Cu 2 O:Na sheets by PLD remained constant as P was increased to approximately 1×10 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 Cu 2 O:Na sheets with a P above about 1.4×10 16  cm −3 was attributed mainly to decreases of both the depletion layer width and the diffusion length.