Electronic properties and morphology of copper oxide/n-type silicon heterostructures

Silicon-based tandem heterojunction solar cells utilizing cuprous oxide (Cu₂O) as the top absorber layer show promise for high-efficiency conversion and low production cost. In the present study, single phase Cu₂O films have been realized on n-type Si substrates by reactive magnetron sputtering at 400 °C. The obtained Cu₂O/Si heterostructures have subsequently been heat treated at temperatures in the 400-700 °C range in Ar flow and extensively characterized by x-ray diffraction (XRD) measurements, transmission electron microscopy (TEM) imaging and electrical techniques. The Cu₂O/Si heterojunction exhibits a current rectification of ~5 orders of magnitude between forward and reverse bias voltages. High resolution cross-sectional TEM-images show the presence of a ~2 nm thick interfacial SiO₂ layer between Cu₂O and the Si substrate. Heat treatments below 550 °C result in gradual improvement of crystallinity, indicated by XRD. At and above 550 °C, partial phase transition to cupric oxide (CuO) occurs followed by a complete transition at 700 °C. No increase or decrease of the SiO₂ layer is observed after the heat treatment at 550 °C. Finally, a thin Cu-silicide layer (Cu₃Si) emerges below the SiO₂ layer upon annealing at 550 °C. This silicide layer influences the lateral current and voltage distributions, as evidenced by an increasing effective area of the heterojunction diodes.