| Role of high series resistance in admittance spectroscopy of kesterite solar cells |
| Thomas P Weiss, Alex Redinger, Jennifer Luckas, Marina Mousel, Susanne Siebentritt Laboratory for Photovoltaics, University of Luxembourg, Belvaux, Luxembourg |
Over the last decades admittance spectroscopy (AS) has been proven to be a powerful tool to characterize defect states in solar cell devices. AS is based on ac-capacitance profiling at different frequencies and temperatures assuming an equivalent circuit for the device under test. Highly efficient CIGSe solar cells demonstrate very low series resistances of the order of 1 Ohmcm2 down to 100 K. In contrast devices based on Cu2ZnSn(S,Se)4 (CZTSSe) show much higher series resistances (>100 Ohmcm2) at 100 K. This work studies the influence of high series resistances on the common interpretation of admittance data measured on Cu2ZnSnSe4 (CZTSe) solar cells. Generally the presence of localized defect states is expected to produce a step in a C-f profile measured at sufficiently low temperature. Applying a simple circuit model this study points out that an increasing series resistance with decreasing temperature results in a capacitance step within the C-f profile. Consequently, a critical frequency exists above which the measured admittance data is strongly perturbed by the circuit response. In conclusion the observed capacitance step cannot be solely attributed to a defect state but is strongly influenced by the network response. |