Sputtered metal oxide broken gap junctions for tandem solar cells

Broken gap metal oxide tunnel junctions have been created for the first time by sputtering. Using a ceramic ZnO-SnO2 target and a reactively sputtered copper target we deposited ZnSnO3 and Cu2O for the n-type and p-type layers, respectively. The band edges and work functions of these materials are suitable for favorable alignment with the bands of copper indium gallium selenide (CIGS) for a tandem CIGS-based solar cell applications. Total junction specific resistivities under 1 Ω-cm2 have been achieved with Ohmic current-voltage (I-V) characteristics pointing to a broken gap band alignment. Low temperature I-V measurements confirmed the lack of traps at the interface despite other measurements pointing to an interface where bands overlap. Cu2O films contained copper inclusions, but they were shown, by conductive atomic force microscopy, not to be the dominant paths for conduction across the junction. Post-deposition annealing of junctions demonstrated thermal stability up to 300 °C, and the ability to improve conduction and influence device material's electron affinity by rapid thermal anneal (RTA). Optical transmission over 78% below a band gap of 2.4 eV was attained for as-deposited films.