Cu2O-based heterojunction solar cells with an Al-doped ZnO/oxide semiconductor/thermally oxidized Cu2O sheet structure

Abstract

This paper introduces the present status and prospects for further development of Al-doped ZnO (AZO)/n-type metal oxide semiconductor/p-type Cu2O hybrid heterojunction (HbH) solar cells that feature a structure that is fabricated by inserting an n-oxide semiconductor thin film between an AZO transparent electrode and a Cu2O sheet. An improvement of photovoltaic properties was achieved by both stabilizing the surface of polycrystalline p-Cu2O sheets that had been prepared by thermal oxidization of Cu sheets and also developing low-temperature and low-damage deposition technology for applying thin films as an n-oxide semiconductor layer. It should be noted that the obtainable photovoltaic properties in AZO/oxide semiconductor/Cu2O HbH solar cells were found to be considerably more affected by the surface condition of the p-Cu2O layer, i.e., the interface at the heterojunction, than the diffusion potential resulting from the difference of work functions between the p-Cu2O and n-oxide semiconductor layers. To achieve a higher efficiency in AZO/n-oxide semiconductor/p-Cu2O HbH solar cells, it was necessary to improve the interface at the heterojunction as well as reduce the series resistance and increase the parallel resistance of the HbH solar cells. The effect of the inserted n-oxide semiconductor thin film on the obtainable photovoltaic properties was investigated in the Cu2O-based HbH solar cells by inserting various kinds of n-oxide semiconductor thin films prepared under various deposition conditions using a pulsed laser deposition (PLD) method. Although either a nondoped ZnO or Ga2O3 thin film deposited at room temperature by PLD is suitable as the n-oxide semiconductor layer, an amorphous Ga2O3 thin film with a high resistivity was found to be the most suitable oxide. The improvement of the p–n junction, as seen in the Ga2O3/Cu2O heterojunction, could be achieved by decreasing the defect levels at the interface, which decreases not only the recombination associated with defects at the interface between the Ga2O3 and Cu2O, but also the conduction band discontinuity. We have achieved a maximum conversion efficiency of 5.38% in an AZO/Ga2O3/Cu2O heterojunction solar cell fabricated by depositing a Ga2O3 thin film on a Cu2O sheet with a resistivity on the order of 102Ωcm.