Electrochemical deposition of Cu-Zn-Sn-S films

• Main Author: Tay, En
• Other Authors: Ryan, Mary P. ; Heutz, Sandrine E. M.
• Published: Imperial College London 2015
• Subjects: 621.47
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.682074

CZTS (Cu2ZnSnS4) solar cells offer an earth-abundant and non-toxic alternative to CIGS (CuIn1 xGaxSe2) and CdTe technologies. In a one-bath electrodeposition approach for CZTS, the wide deposition potential difference between Cu and Zn would result in dendritic morphology with poor adhesion to the substrate. This challenge was explored in Chapter 4 with three different additives (citric acid, thiosulfate, and thiourea) in pH 1.5 - 2.0, where Sn2+ is stable. Cyclic voltammetry was used to study reduction and oxidation peaks in electrolyte baths containing the metal ion and additive. Citric acid did not show significant complexing effect while thiosulfate and thiourea exhibited a negative shift in Cu deposition potential such that the deposition window is defined by Sn and Zn instead of Cu and Zn. In the pH range investigated, thiourea was found to be much more stable than thiosulfate, which decomposed to sulfur particles. With a suitable additive (thiourea) identified, a one-bath deposition of Cu-Zn-Sn was explored in Chapter 5. Electrolyte baths with Cu2+, Zn2+, Sn2+, and an additive was used for electrodeposition of films at -0.2 V, -0.5 V, -0.8 V, -1.1 V, and -1.3 V. Films deposited using thiosulfate showed weak adhesion to the substrate, and flaked off easily. Nanostructures obtained from citric acid and no additive exhibited were similar, highlighting an inactivation of citric acid at low pH values as suggested in literature. Nanostructures obtained from thiourea were different. In terms of film homogeneity, electrodeposition with thiourea resulted in films with improved surface coverage and less pinholes than the case with no additive and citric acid. The composition of the films exhibited a Sn incorporation from -0.5 V and below, and Zn incorporation from -1.1 V and below for citric acid and thiourea. Once films of Cu-Zn-Sn were obtained, sulfur was incorporated using co-deposition of metal ions and sulfur particles in Chapter 6. A comparison of continuous and pulsed co-deposition was explored and films obtained from pulsed co-deposition were found to exhibit significantly better film homogeneity, hence pulsed co-deposition was used for subsequent studies. Sulfur loading was increased from 0 g/L to 0.32 g/L and 0.64 g/L. The morphologies obtained from these films were similar, with sulfur incorporation increasing from 0 g/L to 0.32 g/L, with a slight increase from 0.32 g/L to 0.64 g/L. The results obtained from this work will be advantageous towards an economical one-bath electrodeposition approach for earth-abundant and non-toxic CZTS solar cells. In addition, this study is helpful for future possibilities of a multi-metal electrodeposition in a one-bath approach.