Enhancement of solar cell performance of p-Cu2O/n-ZnO-nanotube and nanorod heterojunction devices

Abstract

Nanostructured thin films are appealing to solar light harvesting and high-efficiency photovoltaic devices. A new Cu2O/ZnO heterojunction solar cell architecture has been developed with ZnO nanotubes (NTs) and nanorods (NRs), on a 500nm thick, n-type ZnO seed layer, covered by a 2.5–3µm thick, p-type Cu2O layer to provide the full built-in potential across the junction area, all done by electrodeposition method. This architecture is used to improve the light harvesting and charge collection efficiency by taking advantage of the high surface area junction and direct charge transfer pathway of these one-dimensional ZnO nanostructures. In addition, we calculate the optimized thickness of p-type Cu2O film of 2.5–3µm to maximize their open-circuit voltages. Post-annealing of the p-type Cu2O nanostructured film at 200°C is found to provide better crystallinity and a higher conductivity for additional improvement in the performance. Further optimization of the Cu2O film thickness according to their charge carrier concentration and absolute permittivity values also assures the full built-in potential across the junction interface. Finally, the electrodeposited tubular ZnO nanostructures are grown on the top of a sufficiently thick ZnO seed layer, which prevents any leakage pathway and increases the junction area, and consequently increases their short-circuit current values. The open-circuit voltages obtained for Cu2O/ZnO-NT (0.66V) and Cu2O/ZnO-NR heterojunction devices (0.71V) represent the highest values reported to date for this type of electrodeposited Cu2O/ZnO solar cells. Interestingly, the short-circuit current density of the Cu2O/ZnO-NT cell (2.40mA/cm2) is twice that of the Cu2O/ZnO-NR cell (1.12mA/cm2) with a photon-to-electron conversion efficiencies of 0.8% and 0.4%, respectively. These results illustrate the advantage of single-step electrodeposition of ZnO nanotubes, which provide a larger interfacial area and a much lower defect density than previously reported nanotubes obtained by etching ZnO nanorods. The new device architecture also offers a minimum leakage path and reduced recombination loss expected in typical nanostructure-based photovoltaics. This study demonstrates a promising approach to fabricating low-cost metal oxide nanostructured thin-film solar cells by a scalable, efficient electrochemical method for cost reduction, process simplification, and performance improvement.