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Solar cells from III-V materials offer outstanding light conversion efficiency and power densities and have a proven reliability record. Nevertheless, the utilization of III-V devices has been hindered by high production costs that partially stem from expensive substrates for the growth of III-V materials. Here, we present an ultrathin epitaxially ready single-crystal Ge membrane, formed by germanium-on-nothing (GON) technology, which employs morphological evolution of an arrayed porous Ge during hydrogen annealing. This new process, inspired by a silicon-on-nothing (SON) process, significantly improves the reformed Ge surface compared with previous porous Ge studies such that low-defect-density heteroepitaxy of GaAs is achievable. We demonstrated the growth of a 14.44% efficient GaAs solar cell on GON with nearly identical open circuit voltage to a control cell grown on a bulk Ge and successfully transferred …