Impedance model for CdTe solar cells exhibiting constant phase element behaviour

Friesen, Gabriele and özsar, M.E. and Dunlop, Ewan D. (2000) Impedance model for CdTe solar cells exhibiting constant phase element behaviour. Thin Solid Films, 361.

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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 (capacitance-voltage)-curves and C-f (capacitance-frequency)-curves were analyzed. 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-circuits (R p and C) does not allow the simulation of the measured impedance spectra. As both models are only consisting of frequency-independent circuit elements they can not be used to describe the frequency dispersion of thin 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 correlation 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.

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