High efficiency GaAs/Ge monolithic tandem solar cells

Two-terminal monolithic tandem cells consisting of a GaAs solar cell grown epitaxially on a Ge solar cell substrate are very attractive for space power applications. Tandem cells of GaAs grown by metal-organic chemical vapor deposition on thin Ge were investigated to address both higher efficiency and reduced weight. Two materials growth issues associated with this heteroepitaxial system, autodoping of the GaAs layers by Ge and diffusion of Ga and As into the Ge substrate, were addressed. The latter appears to result in information of an unintentional p-n junction in the Ge. Early simulator measurements gave efficiencies as high as 21.7% for 4 cm² GaAs/Ge cells, but recent high-altitude testing has given efficiencies of 18%. Sources of errors in simulator measurements of two-terminal tandem cells are discussed. A limiting efficiency of about 36% for the tandem cell at AMO was calculated. Ways to improve the performance of present cells, primarily by increasing the I_sc and V_oc of the Ge cell, are proposed.