Fabrication and Optimization of Single-Junction GaAs Thin Film Solar Cells on Epi-ready Flexible Metal Tapes for Low-cost Photovoltaics
Properties like high efficiency, flexibility and light weight, resistance to UV radiation and moisture and a low temperature coefficient make gallium arsenide (GaAs) more favorable than the ubiquitously-used silicon for solar cells. In spite of their high efficiency, GaAs solar cells have found limited use in various application due to high cost of the GaAs or Ge wafer used. In an approach to produce low-cost GaAs solar cells, we have developed a technology to grow epitaxial semiconductor thin films on low-cost flexible epi-ready metal tapes that can replace the expensive wafers. The template layers on the epi-ready metal tapes are grown via a roll-to-roll process using Ion Beam Assisted Deposition (IBAD). Metal organic chemical vapor deposition (MOCVD) is used to epitaxially grow GaAs solar cells structure on ‘single-crystalline-like’ germanium film on epi-ready metal foils. These epitaxial GaAs films exhibit excellent crystalline alignment with high carrier mobility, excellent crystalline alignment and optoelectronic properties. These grown thin films were processed via photo-lithography, etching and contact deposition to fabricate single-junction (1J) GaAs solar cell devices. The fabricated solar cell underwent a process of cap layer removal (passivation) and anti-reflection coating. Efficiency greater than 6% with open circuit voltage (VOC) of 566mV, fill factor (FF) of 68%, short circuit current density (JSC) of 17.4mA/cm2 was obtained. Steps were taken to improve the quality of p-n junction by improving the quality of Ge template and incorporating an intrinsic layer with p-i-n solar cell structures. A device efficiency of 11.5 % with VOC of 566mV, FF of 68%, JSC of 17.4mA/cm2 was obtained at 1 Sun on improved Ge template, using CVD germanium instead of sputtered germanium used before. Solar cells fabricated with the improved p-i-n structure showed a device efficiency of 13.2% at 1 sun with VOC of 650 mV, JSC of 28 mA cm−2, and FF of 72 %. These thin film GaAs photovoltaics, with further improvement in quality, can potentially lead to light-weight, inexpensive and scalable solar cell manufacturing.