Impedance model for CdTe solar cells exhibiting constant phase element behaviour

Different equivalent circuit models were used to fit the impedance spectra of CdTe solar cells at fixed d.c.-voltages and the resulting C-V (capacitan ce-voltage)-curves and C-S (capacitance-frequency)-curves were analysed. A simplified equivalent circuit model, which consists of a parallel resistor R-p and capacitor C in series with a resistor R-s, does not give a good fit to the experimental data and is not capable of simulating the: dispersive trend. Also the commonly assumed model for CdTe, which consist of two sub-c ircuits (R-p and C) does not allow the simulation of the measured impedance spectra. As both models are only consisting of frequency-independent circu it elements they can not be used to describe the frequency dispersion of th in film CdTe solar cells. A phenomenological description of the capacitance behaviour was obtained by replacing the capacitor of the first model by a frequency-dependent non-ideal capacitor, a so-called constant phase element (CPE). This element is described by the two CPE-parameters (T and P). A co rrelation of these parameters to the capacitance dispersion and to the real cell capacitance are presented and the physical phenomena responsible for the capacitance dispersion are discussed. (C) 2000 Elsevier Science S.A. Al l rights reserved.