Cu2ZnSn(S,Se)4 solar cells processed by rapid thermal processing of stacked elemental layer precursors

In this contribution we report on the development of a two-step process for the formation of Cu2ZnSn(S,Se)4 thin films for solar cells. The two-step formation process of the pentanary kesterite consists of (i) sputter deposition of the metals Cu, Zn and Sn followed by thermal evaporation of chalcogen and (ii) rapid thermal processing of the metal/chalcogen precursors in chalcogen containing ambient. After the absorber formation process, solar cells were processed by deposition of CdS buffer, window layer and metal grid. We evaluated different metal precursor compositions in the ternary Cu–Zn–Sn metal systems regarding their behavior as appropriate precursors for the crystallization of Cu2ZnSn(S,Se)4 absorbers. X-ray diffraction analyses show the presence of secondary chalcogenide phases in absorbers with Cu-poor composition. In combination with Raman spectroscopy, the efficient sulfoselenization could be demonstrated. A broad compositional region is found giving cell efficiencies above 6% via this process route and the potentials for further improvements are discussed. The best solar cell measured so far reached 6.6% efficiency on 1.34 cm2 cell size.