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  • Title: Secondary Phase Formation Mechanism in the Mo-Back Contact Region during Sulfo-Selenization Using a Metal Precursor: Effect of Wettability between a Liquid Metal and Substrate on Secondary Phase Formation.
    Author: Kim SY, Kim SH, Hong S, Son DH, Kim YI, Kim S, Ahn K, Yang KJ, Kim DH, Kang JK.
    Journal: ACS Appl Mater Interfaces; 2019 Jul 03; 11(26):23160-23167. PubMed ID: 31252489.
    Abstract:
    Recently, highly efficient CZTS solar cells using pure metal precursors have been reported, and our group created a cell with 12.6% efficiency, which is equivalent to the long-lasting world record of IBM. In this study, we report a new secondary phase formation mechanism in the back contact interface. Previously, CZTSSe decomposition with Mo has been proposed to explain the secondary phase and void formation in the Mo-back contact region. In our sulfo-selenization system, the formation of voids and secondary phases is well explained by the unique wetting properties of Mo and the liquid metal above the peritectic reaction (η-Cu6Sn5 → ε-Cu3Sn + liquid Sn) temperature. Good wetting between the liquid Sn and the Mo substrate was observed because of strong metallic bonding between the liquid metal and Mo layer. Thus, some ε-Cu3Sn and liquid Sn likely remained on the Mo layer during the sulfo-selenization process, and Cu-SSe and Cu-Sn-SSe phases formed on the Mo side. When bare soda lime glass (SLG) was used as a substrate, nonwetting adhesion was observed because of weak van der Walls interactions between the liquid metal and substrate. The Cu-Sn alloy did not remain on the SLG surface, and Cu-SSe and Cu-Sn-SSe phases were not observed after the final sulfo-selenization process. Additionally, Mo/SLG substrates coated with a thin Al2O3 layer (1-5 nm) were used to control secondary phase formation by changing the wetting properties between Mo and the liquid metal. A 1 nm Al2O3 layer was enough to control secondary phase formation at the CZTSSe/Mo and void/Mo interfaces, and a 2 nm Al2O3 layer was enough to perfectly control secondary phase formation at the Mo interface and Mo-SSe formation.
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