Effects of the Au/CdTe back contact on IV and CV characteristics of Au/CdTe/CdS/TCO solar cells

A simple analytical theory is presented to explain the measured roll over and cross over behaviour of the $\mathrm{IV}$ characteristics of thin film CdTe solar cells. It involves a classical description of the CdS/CdTe junction and the CdTe/back contact structure and is extended with a new description of minority carrier current in the CdTe contact region. This extension is crucial in describing the light dependence of the forward $\mathrm{IV}$ curves, and hence cross over. The same model also explains the measured $\mathrm{CV}$ curves. It is shown that analysis of the capacitance measurement can yield additional information about the doping density of CdTe in the vicinity of the contact. A relationship between the fill factor of the solar cell and the barrier height of the back contact is derived; this relation is useful as a new, practical criterion for the quality of the back contact. The results of this simple analytical model are confirmed by full numerical calculations of the dc and ac characteristics.