Significantly improved crystallinity in Cu2O films enabled by heteroepitaxy on abnormally-grown Ag templates

Cuprous oxide (Cu₂O) is a promising p-type absorber in thin film heterojunction solar cells due to its non-toxic, low-cost processing and high theoretical power conversion efficiency (PCE). However, conventional synthesis methods yield Cu₂O films with small grains and numerous grain boundaries that act as charge recombination centers, severely limiting device performance. Strategies to enlarge grains require high-temperature processes (>600 °C) with increased production costs, or involve complex epitaxial growth of Cu₂O films on non-conductive substrates. In this work, abnormally-grown Ag films with large grains were used as the templates for epitaxial growth of Cu₂O films. Grain size as large as ∼10 μm can be obtained in Cu₂O films epitaxially deposited on Ag templates. It is revealed that a composition-transition buffer layer formed at the Cu₂O/Ag interface reduces lattice mismatch and facilitates epitaxy. The photoluminescence spectra (PL) of epitaxially grown Cu₂O films showed extremely high near band-edge luminescence intensity, demonstrating the significantly improved crystallinity for epitaxially grown Cu₂O films on Ag templates. This work suggests that epitaxial growth of Cu₂O films on abnormally-grown Ag templates is an effective method to improve the crystallinity of Cu₂O thin films with a low thermal budget.