A new photovoltaic film consisting of monoclinic WO3 semiconductor and conjugated polythiophene (PT) is prepared via an in situ polymerization which is initiated by photoexcited WO3. It is observed that PT grows on the WO3 substrate along with reaction time, leading to uniform and high quality PT-WO3 composite films. Structures of the as-synthesized films are studied by using Raman and X-ray photoelectron spectroscopy (XPS) with the aim of gaining an insight into the interface. The results show that the sulfur sites of PT are bound to the semiconductor through a strong linkage and an acceptor donor complex is formed as a result of the electron transfer from PT to WO3. The cyclic voltamrnetry analysis confirms the charge-transfer reaction. Film devices are fabricated by using the PT-WO3 composite film as the active layer and measured under AM 1.5G illumination for the photocurrents and incident photon-to-current conversion efficiency.