Development of Inexpensive and Flexible Single Junction GaAs Solar Cells for Photovoltaics

Gallium arsenide (GaAs), a III–V compound semiconductor, has superior optoelectronic properties that make GaAs compelling for photovoltaics. Although GaAs solar cells are highly efficient, their application is limited because of the high cost of using single-crystal GaAs or Ge wafers as substrates. Therefore, we have developed two nonmainstream technologies to produce low-cost GaAs solar cells:

1. Epitaxial growth of semiconductor thin films on low-cost, epi-ready flexible metal tapes can replace 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). A single-crystalline-like GaAs film is developed on a low-cost, polycrystalline metal foil, which is also thin and lightweight. Our approach uses Ge, a material with good lattice matching and thermal properties, as a suitable substrate for GaAs growth. A buffer structure with a single-crystalline-like Ge-terminated layer is developed on a biaxially textured buffer. Therefore, the film is aligned in in-plane and out-of-plane directions with a misorientation angle of <1°.
2. Enabling direct reuse of GaAs substrates for solar cells using the H2O-assisted epitaxial liftoff (ELO) technique is commonly implemented for separating III–V device layers from the single-crystal GaAs substrate. This method reclaims the substrate for successive III–V growth, thus reducing the cost of III–V devices. However, the conventional ELO of GaAs requires using concentrated hydrofluoric acid (HF) to etch out the lattice-matched AlA sacrificial layer, which degrades the parent wafer underneath. Consequently, the wafer undergoes expensive chemical mechanical polishing (CMP), costing approximately 25% of the pristine GaAs substrate. In this study, we developed a method to address this issue, eliminating the need for post liftoff CMP processes by using water-assisted GaAs ELO. A three-layer buffer architecture was developed using alkaline earth compounds in which the top and bottom layers were tuned to match the GaAs lattice and the middle layer served as a water-soluble sacrificial film. The fabricated device exhibited an efficiency of 10.3%. Two successful reuse processes of the substrate of GaAs single junction (SJ) solar cell growth with no performance degradation confirm the viability of the process. Moreover, we demonstrated improved performance of the solar cell device to 15.2% power-conversion efficiency using a semiconductor-based germanium interlayer and vicinal GaAs substrates.