Recently, two-dimensional phosphorene (2D BP) has become an ideal material for electrocatalysts due to its short charge transport distance, high carrier mobility and fully exposed surface active sites. However, the inadequate adsorption energy of oxygen-containing intermediates results in the sluggish reaction kinetics, and thus limits its practical application. In this paper, granular amorphous Ni2P on the surface of BP to construct Ni2P@BP heterostructure were introduced, in order to improve its electrochemical stability and reaction activity. The results show that, compared with the pure BP and Ni2P catalysts, the Ni2P@BP catalyst exhibits excellent hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalytic activities with an overpotential of 167 and 186 mV at a current density of 10 mA/cm2, respectively. As a bifunctional electrocatalyst, Ni2P@BP requires only 1.54 V applied bias (Vapp) to achieve a current density of 10 mA/cm2. Finally, over 7% solar to hydrogen (STH) coversion efficiency is achieved assisting with a-Si:H/a-SiGe:H/a-SiGe:H triple-junction solar cell, which is 95% higher than that of pure BP as a bifunctional electrocatalyst.