Preparation and characterization of CZTS thin films by vacuum-assisted spray pyrolysis and fabrication of Cd-free heterojunction solar cells

The quaternary chalcogenide material Cu2ZnSnS4 (copper zinc tin sulphide [CZTS]) has received much attention owing to its excellent photovoltaic conversion properties. We report the deposition of CZTS thin films by vacuum spray pyrolysis and explore the structural, elemental, optical and electrical properties and its use as an absorber layer in the fabrication of a cadmium-free solar cell device. The thin films were deposited at various substrate temperatures ranging from 325 degrees C to 400 degrees C using the vacuum spray pyrolysis technique. Structural analysis shows a crystalline nature with a CZTS kesterite phase formed at 375 degrees C. FESEM analysis reveals the presence of coral-reef-like morphology. XPS measurements were carried out to confirm the elemental composition. UV-vis measurements indicate that the optical band gap of the as-deposited films is 1.5 eV with a high absorption coefficient of 10(5) cm(-1), which is close to the ideal value for an absorber layer. Hall effect measurement confirms the p-type nature of the samples. A simple heterojunction is fabricated with a device structure using CZTS as the absorber layer and Al:ZnO as the window layer. The estimated solar cell parameters are V (oc) = 0.85 V, I (sc) = 16.68 mA, fill factor = 36.54% and efficiency, eta = 2.87%.