Thin film of Cu2ZnSnS4 (CZTS) has been successfully deposited by sol gel technique on n-type silicon and glass substrates to fabricate a heterojunction photodiode. The structural properties of the film were investigated by atomic force microscopy. The AFM image of the Cu2ZnSnS4 film reveals that the film is a nanostructure material formed from nanoparticles with the particle size of 50-90 nm. The optical band gap, E-g of the Cu2ZnSnS4 film was found to be 1.48 eV and the obtained optical band gap suggests that CZTS is very suitable for photovoltaic and optoelectronic applications. The current voltage characteristics of the Al/n-Si/Cu2ZnSnS4/Al diode exhibit a good rectification behavior with ideality factor of 2.84 and barrier height of 0.738 eV. The interface states of the diode were analyzed by series resistance and conductance-voltage methods. The presence of interface states in series resistance-voltage plots was confirmed by the illumination. The interface state density D-it for the diode was found to be 3.63 x 10(12) eV(-1) cm(-2). The obtained results indicate that the Al/n-Si/Cu2ZnSnS4/Al diode is a photosensor based on controlling of interface states by illumination. (C) 2011 Elsevier Ltd. All rights reserved.