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A stack design for carbon-doped GaAs single junction solar microcells grown in triple-layer epitaxial assemblies is presented. As-grown materials exhibit improved uniformity of photovoltaic performance compared to zinc-doped systems due to the lack of mobile dopants while a slight degradation exists in middle and bottom devices. Detailed electrical and optical characterizations of devices together with systematic studies of acceptor reactivation reveal carbon-related defects accompanied by carrier compensation, and associated scattering and recombination centers are primarily responsible for the degraded contact properties and photovoltaic performance, resulting from prolonged thermal treatments of early-grown materials during the multilayer epitaxial growth. [ABSTRACT FROM AUTHOR]