Al0.2Ga0.8Asp+‐njunction solar cells were fabricated by molecular beam epitaxy (MBE) and the relationship between cell properties and growth conditions was examined. It was found that growth temperature strongly influenced the minority carrier diffusion length in cell layers. At a growth temperature of 700 °C, minority carrier diffusion length was much improved and a high conversion efficiency of 12.9% (1 sun AM1.5, for an active area) was obtained.