This paper studies the impacts of the non-ideal reflective back scattering, non-radiative recombination, and series resistance on the device performance of ultra-thin GaAs single-junction solar cells. The reflectivity of the textured AlInP/Au interface is calculated by averaging the angular reflectivity against the Lambertian distribution, the value of which is 95% at the GaAs absorption edge. The impact of non-ideal scattering on short-circuit current density (Jsc) and external quantum efficiency (EQE) is investigated using Phong's distribution and a Phong exponent m of ∼12 is determined by fitting both simulated Jsc and EQE to their experimental values. The measured open-circuit voltage (Voc) is lower than the best achievable value, and the difference is attributed to the non-radiative recombination in the device. Fitting of the measured Voc gives a lifetime of ∼130 ns. The impact of series resistance on fill factor is also studied using the single diode equivalent circuit model and the specific series resistivity of the device is determined to be ∼1.2 ω·cm2.