A physics-based analytical model is important to understand the working mechanism through process parameters of any innovative material heterostructure. We present an analytical model to calculate the power conversion efficiency of solar cells based on graphene and III-V direct bandgap semiconductors. The model is comprehensively developed by incorporating several current densities obtained from both the generation and recombination processes. Moreover, to obtain a highly efficient Schottky junction solar cell, we propose an optimized structure of graphene/GaAs with lattice-matched passivation and carrier selective layers. The structure has the advantage of surface passivation and photon recycling that reduces interface recombination and ensures more electron–hole pair generation, respectively. It exhibits a theoretical efficiency of >18  %   from the analytical model simulation which is later verified by numerical simulation using SCAPS 1D software. The analytical model will provide not only a better understanding of the solar cells’ operation but also a comparative study among them to achieve better efficiency in the future. In addition, the enhanced efficiency of the proposed structure will encourage further research in this field of study.