A novel strategy for fabricating hybrid thin film solar cell devices based on poly(3-hexylthiophene) (P3HT)/indium sulfide (In2S3) nanoflake array films is presented in this paper. We take In2S3 as an example to demonstrate that a novel chalcalgenide nanoflake array structure could be in situ fabricated on indium-tin oxide (ITO) glass substrates through a one-step solvothermal treatment of an indium nanocrystal layer and sulfur powder in the presence of absolute ethanol. The resulting products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and UV-vis spectrophotometry. A preliminary photovoltaic performance measurement of the fabricated device (ITO/PEDOT:PSS/In2S3:P3HT/Al) shows a short circuit current density (Jsc) of 0.68 mA cm−2 and a power conversion efficiency (η) of 0.04% under an illumination of 100 mW cm−2. The in situ-fabricated special nanoflake array structure may provide a large interfacial surface area for the In2S3 nanocrystals and the poly(3-hexylthiophene) in such hybrid solar cell devices. This novel wet chemical strategy may remove the need for insulated surfactants, improve the stability of both the thin film and the fabricated device, and is expected to be expanded to fabricate other conjugated polymer/charcalgenide hybrid thin film solar cell devices.