The knowledge of ac parameters of solar cells/arrays is important for designing of high performance power conditioning circuits to match the input at operating conditions. A systematic study of crystalline silicon solar cells is done using impedance spectroscopy under forward bias conditions and at different illumination levels. A "negative capacitance" behaviour is observed in the low frequency region under forward bias voltages and at low illumination levels. An electrical equivalent circuit with inductive component is used to fit the experimental data and the pseudo-capacitance behaviour is explained on the basis of an equivalent circuit model. (C) 2016 Elsevier Ltd. All rights reserved.