In the n+(p+)−p(n) crystalline (X≡ CdTe or CdSe)-junction solar cells at 300K, due to the effects of impurity size, temperature, heavy doping, and photovoltaic conversion, we show that, with an increasing donor (acceptor)-radius rd(a), both the relative dielectric constant and photovoltaic conversion factor decrease, and the intrinsic band gap (IBG) increases, according to the increase in photovoltaic efficiency, as observed in Tables 1-5, being in good accordance with an important result obtained by Shockley and Queisser (1961), stating that for an increasing IBG the photovoltaic efficiency increases. Further, for highest values of rd(a), the limiting highest efficiencies are found to be given in Tables 4, 6, as: 26.55 %, and 23.69 %, obtained in such n+(p+)−p(n) crystalline (CdTe, or CdSe)-junction solar cells at the open circuit voltage Voc=0.82 V, and 0.89 V, respectively, and at T=300 K. Furthermore, from the well-known Carnot-efficiency theorem, as given in Eq. (46), being obtained from the second principle of the thermodynamics, and from the above results of limiting highest efficiencies, the corresponding highest hot reservoir temperatures, TH=408.4 K, and 393.1 K, respectively. Thus, as noted above, ηmax. and TH both increase with an increasing IBG, for each (X≡ CdTe, or CdSe)- crystal at T=300 K≡TC.