Recent advances in high efficiency, low-cost GaAs solar cells

By using an n⁺/p/p⁺structure, we have previously succeeded in fabricating GaAs solar cells on single-crystal GaAs and Ge substrates, with conversion efficiency of 21% at AM1. Three approaches are being used to lower the cost of such cells, preparation of large-grained Ge sheets on low-cost substrates, preparation of heteroepitaxial Ge films on inexpensive Si sheets, and preparation of thin single-crystal GaAs layers on reusable GaAs substrates. Important advances have been achieved in all three areas. Crystallites 2 µm × 100 µm have been obtained on fused silica substrates by heating amorphous Ge films with a scanned Nd:YAG laser. Heteroepitaxial Ge films have also been obtained on Si substrates by transient heating, and epitaxial GaAs layers have been grown on such films. Single-crystal GaAs layers as thin as 5 µm have been separated from reusable GaAs substrates by a new process named CLEFT. A 15% (AMI) GaAs solar cell, only 8 µm thick and bonded to a glass substrate, has been fabricated. With these developments, low-cost high-efficiency, GaAs cells may well become a reality.