In recent years, the development of photovoltaic assisted electrocatalytic (PV-EC) water splitting has emerged as a critical issue in achieving carbon neutrality through green hydrogen evolution. However, traditional electrocatalysts do not meet the high solar-to-hydrogen (STH) conversion efficiency requirements for PV-EC systems. Therefore, it is crucial to obtain electrocatalysts with low cost and reaction overpotential. In this work, transition metal W with high valence states was employed to dope NiFe phosphide, and NiFeW ternary phosphides were prepared by a simple one-step electrodeposition method. The results indicate that NiFeW phosphide has excellent hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities. Furthermore, when used as a bifunctional electrocatalyst, NiFeW phosphide shows a 51 mV reduction in overpotential at a current density of 10 mA/cm2 compared to NiFe phosphide sample. Finally, NiFeW phosphide and a-Si∶H/a-SiGe∶H/a-SiGe∶H solar cell are employed as bifunctional electrocatalyst and driving source, respectively. The PV-EC devices achieve above 7% theoretical STH efficiency, which is significant for practical applications of solar to hydrogen devices.