Compared to heterojunction solar cells, homojunction solar cells have better lattice and band edge matching, which can effectively reduce the loss of open-circuit voltage and enhance the fill factor. AgInSe<inf>2</inf> is a stable chalcopyrite semiconductor with a direct-type band structure, and its band edge positions are in the range of the empirical limits for allowing both n-type and p-type doping, making it an ideal absorber for homojunction solar cells. Here, the possibility of a AgInSe<inf>2</inf> p-n junction as an absorber for homojunction solar cells is explored by using first-principles calculations and device simulations. Firstly, the electronic structure, defect properties, and corresponding material property parameters of AgInSe<inf>2</inf> are determined by calculations. The results show that p-type and n-type AgInSe<inf>2</inf> semiconductors can be prepared under Ag-poor, In-poor and Se-rich, and non-Ag-poor environments, respectively, and that their corresponding defect and carrier concentrations can be selected and optimized to the requirements for use as photovoltaic absorbers. Subsequently, the material's property parameters from first-principles calculations were used as input data for SCAPS-1D device simulations. The results demonstrate that homojunction solar cells exhibit an open circuit voltage of 0.74 V, a short circuit current of 36.34 mA cm⁻², and a power conversion efficiency (PCE) of 22.48%. Finally, the PCE is still 21.62% after optimizing the thickness of the p-n junction to reduce the cost, which is higher than the experimentally reported PCE of AgInSe<inf>2</inf> heterojunction solar cells and close to the PCE record of chalcopyrite-based heterojunction solar cells. Our theoretical work provides a concrete research scheme for further experimental study of AgInSe<inf>2</inf> homojunction solar cells.