On the basis of density functional theory computations, we demonstrated that two-dimensional (2D) α- and β-Sb2TeSe2 monolayers are promising candidates for constructing high-efficiency heterojunction excitonic solar cells. These two 2D materials possess moderate band gaps (∼1.1 eV), which can be flexibly tuned by applying external strains. They possess high carrier mobility (∼3000 cm2 V-1 s-1) and can absorb sunlight over the whole range of the solar spectrum. Remarkably, the α- and β-Sb2TeSe2 monolayers can form desirable type II heterostructures with HfSe2 and BiOI monolayers, respectively. The power conversion efficiencies of α-Sb2TeSe2/HfSe2 and β-Sb2TeSe2/BiOI heterojunction excitonic solar cells can reach 22.5 and 20.3%, respectively. Since α-Sb2TeSe2 and β-Sb2TeSe2 monolayers have good stability and high synthesis feasibility, they have important applications in photovoltaic solar cell devices.