In the n+(p+)−p(n) crystalline Ge-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, 2 and 3, being in good accordance with an important result obtained by Shockley and Queisser (1961), with the use of the second law of thermodynamics, 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 2 and 3, as: 13.05 % (14.82 %), obtained in such n+(p+)−p(n) crystalline Ge-junction solar cells at 300 K, respectively. Then, from the well-known Carnot-efficiency theorem, as given in Eq. (47), being obtained by the second principle of thermodynamics, and from those limiting highest efficiencies, the corresponding highest hot reservoir temperatures, TH, are found to be given by: 345.04 K (352.20 K), respectively. In other words, TH also increases with an increasing IBG, being a new result.