Passivation Studies on Single-Junction GaAs Thin Film Solar Cells on Flexible Metal Tapes for Low-Cost Photovoltaics

Despite the high efficiency of III-V solar cells based on gallium arsenide, their usage in large-scale terrestrial application is very limited due to the excessive cost of GaAs and Ge wafers. Herein, we have developed high-quality epitaxial semiconductor thin films on inexpensive flexible metal tapes to overcome the wafer cost as well as to benefit from the lower manufacturing cost of roll-to-roll processing. Metal organic chemical vapor deposition (MOCVD) is used to grow high-quality epitaxial GaAs solar cell structure on "single-crystalline-like" germanium film on epi-ready metal tape. These epitaxial GaAs films exhibit excellent crystalline alignment with high carrier mobility. The fabricated device is further processed for contact deposition via photolithography. Citric acid used for etching GaAs also resulted in effective passivation of the devices. Passivation of GaAs solar cells with various base thicknesses of 1140, 840, and 380 nm and various device sizes has been demonstrated. After passivation, a 2 order of magnitude reduction in leakage current was experimentally observed in solar cells with a thinner base of 380 nm, whereas solar cells with a thicker base of 1140 nm and an intermediate base of 840 nm exhibited reduction by 1 order of magnitude. The leakage current reduction obtained over a range of device sizes was comparable. Single-junction GaAs devices on flexible metal tapes with 6.8% efficiency have been demonstrated after citric acid treatment.