The roles of Cu impurity states in CdTe thin film solar cells

Despite potential detrimental effects of Cu impurity inclusion in CdTe thin film solar cells, such as degradation caused by Cu diffusion into CdS, or reduced shunt resistance, most devices include Cu as p-dopant in CdTe and/or part of the back contact material [1 - 3]. I-V characterization and time resolved photoluminescence (TRPL) data reveal that, depending on the ZnTe:Cu/Ti back contact deposition condition, Cu may cause dramatic changes in the electrical properties of the CdTe thin film. Very low levels of Cu can reduce the already low conductivity of the CdTe thin film, while increasing the Cu concentration can increase both conductivity and minority carrier lifetime. Specifically, a contacting temperature to 250°C - 300° C can produce high efficiency devices that demonstrate long minority carrier lifetimes. Increase the contacting temperature beyond ∼300°C can yield a CdTe thin film that becomes insulating, or even to n-type. Such a complicated and puzzling effect on the property and quality of the CdTe thin film - turning from a poor p-type, to a poorer p-type, to a better p-type, to insulating, and then to n-type - is all due to different levels of Cu involvement in the CdTe thin film. In this paper, we try to review such dramatic changes of the CdTe behavior through exploring the roles of Cd vacancy (a double and non-shallow acceptor) and Cu impurity (a substitutional non-shallow acceptor), as well as Cu interstitial (a shallow donor). We study the interaction between the Cd vacancy states and the Cu impurity states, as well as the self compensation of the Cu impurity states. Based on the study, we offer a qualitative explanation of the various CdTe behaviors due to different levels of Cu treatment temperature.