Research into efficient synthesis, fundamental properties and potential applications of phosphorene is currently the subject of intense investigation. Herein, solution processed phosphorene or few-layer black phosphorus (FL-BP) sheets are prepared using microwave-exfoliation method and used in photoelectrochemical cells. Based on our experimental and theoretical (DFT) studies, we found that the FL-BP sheets can act as catalytically active sites and show excellent electrocatalytic activity for triiodide reduction in solar cells. Importantly, the device fabricated based on our newly designed cobalt sulfide (CoSx) decorated nitrogen, sulfur co-doped carbon nanotubes heteroelectrocatalyst coated with FL-BP (FL-BP@N, S-doped CNTs-CoSx) displayed an impressive photovoltaic efficiency of 8.31%, outperforming expensive platinum based cells. This work paves the way for advancing phosphorene research and using phosphorene based electrocatalysts for next-generation energy-storage systems.