Engineered Cu(InGa)Se2 thin films through CaF2 post-deposition treatment for enhancing solar cell performance

Abstract

It is considered that alkaline elements (such as Na, K, Rb) introduced through post-deposition treatment are able to modify the near-surface region of Cu(InGa)Se2 thin films. However, research on the effects of Ca element on properties of Cu(InGa)Se2 thin films is hardly reported. In this paper, Cu(InGa)Se2 thin films were prepared on quartz glass by e-beam evaporation via two-step process. Study of structural property showed that a certain amount of CaF2 through post-deposition treatment was beneficial to the uniform composition distribution of Cu(InGa)Se2 thin films. Near-surface region of Cu(InGa)Se2 thin films was more Cu-depleted by increasing introduced amount of Ca element. It was inferred that Ca diffused into the Cu(InGa)Se2 thin films substituted the In sites as CaIn so as to increase the carrier concentration. By calculation, the spike barrier of CdS/Cu(InGa)Se2 heterojunction was shown to decrease with increased thickness of CaF2 layers. As a result, Cu(InGa)Se2 solar cell subjected to 20 nm CaF2 post-deposition treatment, which possessed higher fill factor, open circuit voltage and short circuit current density, achieved the optimal power conversion efficiency. An efficiency of Cu(InGa)Se2 solar cell was enhanced by 2.9% after 20 nm CaF2 post-deposition treatment.