In this paper, we present our investigation into the relationship between the observable microstructure at heterojunction interfaces and the obtainable photovoltaic properties in ZnO/Cu2O heterojunction solar cells fabricated with various structures on thermally oxidized Cu2O sheets. Recently, we reported that high efficiencies of 2.19 and 4.12% as well as open circuit voltages of 0.5 and 0.72 V were obtained in heterojunction solar cells fabricated with an Al-doped ZnO (AZO)/Cu2O or AZO/non-doped ZnO/Cu2O structure, respectively: solar cells fabricated by depositing thin films on thermally oxidized Cu2O sheets at room temperature using a pulsed laser deposition (PLD) method [1,2]. The improvement in obtained photovoltaic properties suggests that it is necessary to stabilize the surface of Cu2O sheets as well as develop low-damage and low-temperature deposition techniques for forming and applying heterojunctions. In addition to improvements in the surface condition of Cu2O sheets, the significant improvement of obtained photovoltaic properties exhibited in AZO/non-doped ZnO/Cu2O heterojunction solar cells is attributable to enhanced potential barrier height and carrier lifetimes near the interface, resulting from the inserted buffer layer functioning as an n-type ZnO layer as well as an active layer in the n-p heterojunction. Cross-sectional views of ZnO/Cu2O heterojunctions obtained by transmission electron microscopy revealed interface microstructures that differed significantly between high-efficiency and low-efficiency solar cells, fabricated with different deposition conditions, irrespective of the device structure used. For example, in AZO/non-doped ZnO/Cu O heterojunction solar cells fabricated with both the AZO and non-doped ZnO thin films prepared by PLD at the same deposition temperatures, either RT or 200oC, the solar cells formed at RT exhibited higher efficiency than those formed at 200oC. A smooth heterojunction was observed near both the interfaces within AZO/non-doped ZnO/Cu2O heterojunction solar cells fabricated at RT; this contrasts with many dislocations observable near both of these interfaces of solar cells fabricated at 200oC, i.e., between the non-doped ZnO layer and the Cu2O and between the AZO and non-doped ZnO layers. Thus, the TEM observations of the heterojunction solar cells fabricated by forming both thin films at RT indicate that the higher efficiency results from an increase of carrier lifetimes, a consequence of suppressing carrier recombination and trapping near the interfaces of the heterojunctions.

[1] Y. Nishi et al., Thin Solid Films, 520 (2012) 3819.

[2] Y. Nishi et al., J. Vac. Sci. Technol. A 30 (2012) 04D103.