In single n+(p+)−p(n) [X(x)≡Si1−xGex]-alloy junction solar cells at 300 K, 0≤x≤1, by basing on the same physical model and the same treatment method, as those used in our recent work (Van Cong et al., 2023; Van Cong, 2023), we will investigate the highest (or maximal) efficiencies, ηImax.(IImax.), obtained at the open circuit voltage Voc(=VocI(ocII)), according to highest hot reservoir temperatures TH(K), obtained from the Carnot efficiency theorem, being proved by entropy law, in the following. (i)-First, in the single