We report an analysis of p-Cu(In,Ga)Se2/n-ZnMgO solar cell through solar cell capacitance simulator. The cell performance has been studied as a function of absorber and buffer layer parameters like thickness, carrier concentrations, mid-gap and interface defect density, sheet resistance and shunt resistance. The efficiency and fill factor of the optimized cell is found to be 21.93% and 81.65% respectively. The optimized device does not exhibit J–V distortion behavior which is common in CIGS-based solar cells. A few device models with donor interface defects, acceptor interface defects and high conduction band offset are proposed, such that cell efficiency is not compromised, but was able to reproduce the experimentally observed J–V distortion behavior in CIGS cell with ZnMgO buffer layer. These proposed cells have been simulated and analyzed for the dark–light J–V crossover and J–V red-kink behavior in detail. The observed J–V distortion behavior is attributed to the reduction in the effective doping of buffer layer which results in increasing acceptor-type mid-gap defect density, persistent photo conductivity of buffer layer and high value of conduction band offset. Dark–light J–V crossover is explained by change in secondary barrier height at the interface between the absorber and buffer layers. Red-kink behavior is due to the lack of free electrons in the buffer layer where these electrons are trapped into acceptor-type mid-gap defect under red-light illumination. © 2019, Springer-Verlag GmbH Germany, part of Springer Nature.