Excitonic solar cells (XSCs) have attracted tremendous attentions due to their high solar-to-electric power conversion efficiency (PCE). However, to further improve the PCE of XSC, finding an efficient donor material with both suitable direct bandgap and high carrier mobility is still a great challenge. Here, we report a black arsenic–phosphorus monolayer as highly efficient donor for XSCs based on first-principle calculations. Firstly, monolayer arsenic-phosphorus polymorphs with α, β, γ, δ, and ε phases were built, among which α–AsP and β–AsP have been verified to be thermodynamically stable. Significantly, monolayer α–AsP possesses a direct bandgap with energy of 1.54 eV, which covers the main energy of solar spectrum. Moreover, its electronic mobility is as high as 14,380 cm2 V−1 s−1, which is much higher than silicon. These two crucial merits made it a promising candidate as donor materials for XSC device and the theoretical simulations demonstrate a maximum PCE of 22.1% for the primarily designed α–AsP/GaN XSC. Interestingly, the suitable electronic structure of α–AsP enables a formation of perfect type-II semiconductor heterojunction with GaN, which will boost the separation and transport of photogenerated carriers with the assistance of built-in field and high mobility. Particularly, α phase few-layer material of arsenic-phosphorus alloy has been experimentally synthesized recently, which paves the way for experimental realization of black arsenic–phosphorus monolayer donor.