The Effects of Zn Precursor on Chemical Bath Deposited ZnS​​(O

Abstract

Among the various thin film solar cells, Cu(In,Ga)Se2 (CIGS) solar cells can be the most promising candidate because of their high efficiency, reliability, and stability. A conventional CIGS solar cell contains CdS film as a buffer layer. For the environmental issues of Cd, CdS should be removed. Moreover, the short wavelength response is limited by the bandgap of CdS (~ 2.4 eV). To overcome these advantages of CdS, many alternative buffer materials have been investigated over the past decade. ZnS is an attractive material due to its nontoxicity, earth abundance and cheapness. Furthermore, ZnS films have a wider bandgap (~ 3.7 eV) than that of CdS (~ 2.4 eV), which results in the higher transmission of photons to the absorber layer and can enhance the blue response of CIGS solar cells. Thus, ZnS film as a buffer layer has been studied in many research groups. Chemical bath deposition (CBD) is the most common process for the ZnS film deposition. Zn salts, thiourea and ammonium hydroxide are used as precursor materials for the CBD solution. The ratio of [Zn]/[S] in the solution can affect the reaction velocity and consequently change the properties of chemically deposited ZnS films. It is necessary to optimize the CBD process condition to obtain high efficiency of CIGS solar cells. In this study, we focused on the effect of the solution compositions on the properties of chemically deposited ZnS films. We have found that deposition rate of ZnS film increases with increasing thiourea concentration and sulfur content in the film is dependent on the film thickness. We characterize the properties of chemically deposited ZnS films with the various kinds of Zn salts and thiourea concentration in the CBD solution. Also, we suggest the optimum CBD process condition by fabricating the CIGS solar cells with the ZnS buffer layer and measuring its efficiency.