Thin films are gaining ground in photonics and optoelectronics because of promising improvements in their efficiency and functionality, as well as decreased material usage compared with bulk technologies. However, the proliferation of thin films would benefit not only from continuous improvements in their fabrication, but also from a unified and accurate theoretical framework of the interplay of photons and charge carriers. In particular, such a framework would need to account quantitatively and self-consistently for photon recycling and interference effects. To this end, here, we combine the drift-diffusion formalism of charge-carrier dynamics and the fluctuational electrodynamics of photon transport self-consistently using the recently introduced interference-extended radiative-transfer equations. The resulting equation system can be solved numerically using standard simulation tools and, as an example, here, we apply it to study well-known Ga As thin-film solar cells. In addition to obtaining the expected device characteristics, we analyze the underlying complex photon-transport and recombination-generation processes, demonstrating the physical insight provided for unevenly excited structures through the direct and self-consistent description of photons and charge carriers. The methodology proposed here is general and can be used to obtain an accurate physical insight into a wide range of planar optoelectronic devices, of which the thin-film single-junction solar cells studied here are only one example.