In the present work, p-Cu2ZnSnS4/n-CdS heterojunction solar cells have been analysed through Solar Cell Capacitance Simulator (SCAPS). The effects of various layer parameters like thickness, carrier concentration, defect density, mobility, conduction band off-set, etc. on the cell performance have been studied in detail. The different reasons for current-voltage distortions (cross-over and red-kink) have been investigated. The optimized cell shows 14.57% efficiency with an open circuit voltage of 1.009 V. The photovoltaic cell has been studied further through capacitance-voltage simulations to obtain the net built-in potential and the apparent doping profile. Thermal admittance spectra have been simulated for defect characterization of the Cu2ZnSnS4 absorber layer and to isolate the effect of back contact barrier. The impedance plot at 300 K has been fitted to an equivalent circuit to get an insight into the secondary barriers of the complete device and also to estimate the carrier lifetime for the trap level. In order to have an idea regarding the effect of inhomogeneity in Cu2ZnSnS4 layer on the device performance, further simulations have been carried out for a randomly graded absorber layer. © 2016 Elsevier Ltd