High stability near-broken gap junction for multijunction photovoltaics

High performance tunnel junctions were made from sputtered and annealed p-type CuAlO ₂ and n-type ZnSnO ₃ with suitable band alignment for both low resistance and alignment to typical inorganic materials needed for a tandem solar cell. The devices not only exhibit low resistance, they are also thermally stable, capable of sustaining postdeposition temperatures up to 600 °C. This is a key requirement for many high performance multijunction thin film inorganic solar cells. The CuAlO ₂ top-layer remains amorphous, providing a diffusion barrier for top cell stack processing. The materials' stack gives a negligible voltage drop, and the visible-spectrum transparency is near 100%. XPS measurements show that unannealed Cu in the Cu-Al-O films is in the +2 oxidation state, while in the films annealed at 500 °C and above, Cu is in the +1 oxidation state. This suggests that annealing is necessary to form CuAlO ₂ . A near-broken gap alignment provides a low resistance contact with band alignment that is nearly ideal for a tandem device.