The discovery of novel two-dimensional (2D) materials with excellent electronic and optoelectronic properties have attracted much scientific attention. Based on the first-principles calculations, we predict an unexplored 2D W4PCl11 monolayer, which is potentially strippable from its bulk counterpart with the exfoliation energy of only 0.16 J m-2. The dynamical, thermal, and mechanical stabilities have also been confirmed. Remarkably, W4PCl11 monolayer is direct semiconductor with a bandgap of 1.25 eV, which endows the monolayer with very strong visible-light absorption in the magnitude of 105 cm-1. Meanwhile, the calculated carrier mobilities of W4PCl11 monolayer can reach to 103 cm2 V-1 s-1. Considering the moderate direct bandgap and high carrier mobility, W4PCl11 monolayer should be a superior candidate for the donor material of excitonic solar cells. The estimated power conversion efficiency of the fabricated W4PCl11/Bi2WO6 heterojunction reaches as high as 21.64%, which much superior to those of most recently reported 2D heterojunction. All these outstanding properties accompanied with its experimental feasibility endows W4PCl11 monolayer with promising photovoltaic applications.