Cu₂O Heterojunction Photovoltaics

Citation

Tolstova, Yulia (2016) Cu₂O Heterojunction Photovoltaics. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z92V2D22. https://resolver.caltech.edu/CaltechTHESIS:06012016-163813213

Abstract

Cuprous oxide (Cu₂O) is an earth abundant semiconductor that has several promising photovoltaic properties, including high absorption in the visible range, high minority carrier diffusion length, and high majority carrier mobility. Cu₂O can be easily synthesized by oxidation of copper foils in air. One important advantage that makes Cu₂O highly relevant to today's solar cell markets dominated by crystalline silicon is its wide bandgap of 1.9 eV at room temperature, which makes it an ideal candidate for a top cell in tandem with a crystalline silicon bottom cell. The detailed balance efficiency of such a device exceeds 44%. In this work we aim to understand and address several issues that have limited Cu₂O solar cell efficiency. We address the intrinsic p-type nature and chemical instability of Cu₂O by pairing it with an appropriate n-type heterojunction partner Zn(O,S), which allows us to achieve devices with open circuit voltages exceeding 1 V. We identify presence of a current blocking layer and reduce it, which results in more than doubling the short circuit current to exceed 5 mA/cm². Light beam induced current measurements highlight some of the issues inherent to polycrystalline Cu₂O solar cells, including grain dependent collection and current losses due to presence of grain boundaries. In order to address the issues affecting Cu₂O made by thermal oxidation we also develop thin film growth of Cu₂O by molecular beam epitaxy on several substrates including MgO and heteroepitaxial noble metal templates that act as ohmic back contacts. These studies culminate in achievement of the first Cu₂O/Zn(O,S) solar cells incorporating an absorber layer grown by molecular beam epitaxy.

Thesis Files