Searching to Distinguish Defects and the Presence of Negative Capacitance

In the search for solar cells with lower manufacturing costs, thin film technology was developed. These thin films are only micrometers thick and are grown at relatively low temperatures, resulting in films with imperfections known as defects. Defects can cause thin film solar cells to have lower efficiencies than their single crystalline counterparts. In order to create more efficient thin film solar cells the physical mechanisms behind defects need to be investigated by sensitive techniques. Capacitance measurements of solar cells detect minute changes in charge in the material. For that reason, capacitance is used to electrically characterize the solar cell. Standard interpretations of capacitance rely on assumptions, which, if wrong, can skew the results. In some solar cells where a back contact barrier is suspected, measurements at high forward bias can be used. However at high forward bias there is an injection of minority carriers that can have an effect on the results, such as causing a negative capacitance response. We have seen that apparent signatures of a back contact barrier in Cu(Inx Gax-1)Se2 might actually be the first signs of a negative contribution to capacitance. This paper will discuss the observation of negative capacitance, the possible implications of such a measurement, and its relationship to other electronic characteristics of the device