Defect distributions in thin film solar cells deduced from admittance measurements under different bias voltages

The voltage dependence of the derivative of the capacitance to ( the logarithm of) the measurement frequency is investigated. Relations describing this dependence are derived for the influence of carrier freeze out, of a defect distribution, and of a back contact barrier. The validity of these relations is investigated with numerical simulations. Considering the extraction of the defect density from capacitance-frequency measurements, the extension of existing formulas to different bias voltages leads to an improved accuracy and the possibility to investigate spatial non-uniformities while preserving a direct link between the defect level energy and the apparent defect density. This is illustrated with voltage dependent admittance measurements of thin film Cu(In,Ga)Se-2-based solar cell devices. (C) 2011 American Institute of Physics. [doi:10.1063/1.3641987]