In single n+(p+) − p(n) [XX(xx) ≡ GaAs1−xTex]-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, 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. Here, one first remarks that, with increasing x=(0, 0.5,1), (i)- from Table 3, for the single n+ − p X(x)-alloy junction solar cell and for given rSn(Cd)-radius, for example, ηImax. (↗)= 31.14%, 31.55%, 32.11%, according to TH(K) = 435.7, 438.3, 4444 . 99, at VocI(V) = 1.07, 1.07, 11. 00 , respectively, while, (ii)- from Table 5, for the single p+ − n X(x)-alloy junction solar cell and for givenrCd(Sn)-radius, for example, ηIImax. (↘)= 33.04%, 32.65%, 32.51%, according to TH(K) = 444444. 00, 445.4, 444.5, at VocII(V)[> VocI(V)] = 1.20, 1.19, 1.18, respectively, suggesting that suchηImax.(IImax.)-and-TH variations depend on the VocII(V)[> VocI(V)] − values. Then, as given in Table3, for x = 0 and for rd(a) = rTe(Mg), one gets: ηI =23.48 % and 29.71 % at Voc = 0.98 V and 1.1272 V, respectively, which can also be compared with the corresponding results, obtained for the single-junction GaAs thin-film solar cell, 22.08 % and 29.71 %, with relative deviations in absolute values, 6.34 % and 2.1 %, and given respectively by Moon et al. (2016) and Green et al. (2022). As a result, in orderto obtain the highest efficiencies, the single GaAs1−xTex-alloy junction solar cells could be chosen ratherthan the single crystalline GaAs-junction solar cell.