Sputtered Metal Oxide Broken Gap Junctions for Tandem Solar Cells

Abstract

Broken gap metal oxide junctions have been created for the first time by sputtering using ZnSnO3 for the n-type material and Cu2O or CuAlO2 for the p-type material. Films were sputtered from either ceramic or metallic targets at room temperature from 10nm to 220nm thick. The band structure of the respective materials have theoretical work functions which line up with the band structure for tandem CIAGS/CIGS solar cell applications. Multiple characterization methods demonstrated consistent ohmic I-V profiles for devices on rough surfaces such as ITO/glass and a CIAGS cell. Devices with total junction specific contact resistance of under 0.001 Ohm-cm2 have been achieved with optical transmission close to 100% using 10nm films. Devices showed excellent stability up to 600oC anneals over 1hr using ZnSnO3 and CuAlO2. These films were also amorphous -a great diffusion barrier during top cell growth at high temperatures. Rapid Thermal Anneal (RTA) demonstrated the ability to shift the band structure of the whole device, allowing for tuning it to align with adjacent solar layers. These results remove a key barrier for mass production of multi-junction thin film solar cells.