title: Evolution of Na-S(-O) Compounds on the Cu2ZnSnS4 Absorber Surface and Their Effects on CdS Thin Film Growth.
abstract: Formation of Na-containing surface compounds is an important phenomenon in the Cu2ZnSnS4 (CZTS) quaternary material synthesis for solar cell applications. Still, identification of these compounds and the understanding of their potential influence on buffer layer growth and device performance are scarce. In this work, we discovered that the evolution of Na-S(-O) compounds on the CZTS surface substantially affect the solution/CZTS interface during the chemical bath deposition of CdS buffer film. We showed that Na2S negatively affects the growth of CdS, and that this compound is likely to form on the CZTS surface after annealing. It was also demonstrated that the Na2S compound can be oxidized to Na2SO4 by air exposure of the annealed CZTS surface or be removed using water dipping instead of the commonly used KCN etching process, resulting in significantly better quality of the CdS layer. Lastly, 6.5% CZTS solar cells were fabricated with air exposure treatment without incorporation of the KCN etching process. This work provides new insight into the growth of the CdS/CZTS interface for solar cell applications and opens new possibilities for improving likewise Cd-free buffer materials that are grown with a similar chemical bath deposition process.
authors.affiliation: Ångström Solar Center, Solid State Electronics, Uppsala University , Box 534, SE-751 21 Uppsala, Sweden.
authors.forename: Yi
authors.initials: Y
authors.lastname: Ren
authors.affiliation: Ångström Solar Center, Solid State Electronics, Uppsala University , Box 534, SE-751 21 Uppsala, Sweden.
authors.forename: Jonathan J S
authors.initials: JJ
authors.lastname: Scragg
authors.affiliation: Ångström Solar Center, Solid State Electronics, Uppsala University , Box 534, SE-751 21 Uppsala, Sweden.
authors.forename: Marika
authors.initials: M
authors.lastname: Edoff
authors.affiliation: Ångström Solar Center, Solid State Electronics, Uppsala University , Box 534, SE-751 21 Uppsala, Sweden.
authors.forename: Jes K
authors.initials: JK
authors.lastname: Larsen
authors.affiliation: Ångström Solar Center, Solid State Electronics, Uppsala University , Box 534, SE-751 21 Uppsala, Sweden.
authors.forename: Charlotte
authors.initials: C
authors.lastname: Platzer-Björkman
keywords: CdS; Cu2ZnSnS4; kesterite; sodium; surface compounds; thin film solar cell