With much effort devoted to the improvement of material and electrical designs, high-quality GaAs single-junction solar cell performance is getting close to its theoretical limit. To further improve device performance, it is critical to find the optimal optical designs for single-junction solar cells. In this work, planar single-junction solar cells are investigated using a semi-analytical model, where combinations of smooth, textured, non-reflective, and reflective surfaces are explored. Statistical ray tracing is used to obtain the optical properties of planar structures and the impact of critical design parameters such as junction thickness, together with material quality and solar concentration on the device performance is analyzed. The combination of textured and reflective surfaces shows the best performance by effectively increasing the photon and carrier densities, which leads to higher open-circuit voltages and conversion efficiencies. It is expected that the GaAs single-junction cells can practically achieve ∼30% conversion efficiency under one sun AM1.5G, with optimal optical structures, the state-of-art material quality, and properly designed doping profile. Even higher efficiency of ∼38% is possible via concentration of 1000 suns.