Highly efficient separation of tightly bound excitons (photoinduced electron-hole pairs) at an interface is crucial for high-performance two-dimensional (2D) heterostructure solar cells. Here we propose and demonstrate a strategy---dipole engineering---to effectively separate excitons at a 2D interface, which could evidently enhance power-conversion efficiency (PCE) via Heyd-Scuseria-Ernzerhof density functional calculations. The proposed dipole engineering is based on the synergetic effect of the intrinsic dipole in 2D Janus materials and interface dipole caused by charge transfer at the interface of 2D van der Waals heterostructures, in stark contrast to those strategies previously used, such as applying a strain or electric field. Taking the Janus ${\mathrm{Ga}}_{2}\mathrm{Se}\mathrm{Te}/\mathrm{In}\mathrm{S}$ heterostructure as an example, we demonstrate that the PCE can be promoted to a peak value of 23.2% when the intrinsic and interface dipoles are in the same direction, when accompanied by an appropriate applied compression strain. In addition, the Janus heterostructure possesses suitable band-edge positions of redox reaction potentials for full water splitting. Our work may herald an intrinsic yet effective way for enhancing the performance of optoelectronic devices, in contrast to the conventional external stimuli strategy.