In this paper, we study the performance of 2.0 eV Al0.12Ga0.39In0.49P and 1.4 eV GaAs solar cells over a temperature range of 25–400 °C. The temperature-dependent ${J_{01}}$ and ${J_{02}}$ dark currents are extracted by fitting current–voltage measurements to a two-diode model. We find that the intrinsic carrier concentration ${n_i}$ dominates the temperature dependence of the dark currents, open-circuit voltage, and cell efficiency. To study the impact of temperature on the photocurrent and bandgap of the solar cells, we measure the quantum efficiency and illuminated current–voltage characteristics of the devices up to 400 °C. As the temperature is increased, we observe no degradation to the internal quantum efficiency and a decrease in the bandgap. These two factors drive an increase in the short-circuit current density at high temperatures. Finally, we measure the devices at concentrations ranging from ∼30 to 1500 suns and observe n = 1 recombination characteristics across the entire temperature range. These findings should be a valuable guide to the design of any system that requires high-temperature solar cell operation.