Phase segregation in Cu2ZnSnSe4 thin films for photovoltaic applications. The effects of precursor microstructure and selenium activity during selenization of electrodeposited metallic precursors.

This thesis develops the synthesis of thin film copper-zinc-tin-selenide kesterite absorber layers, Cu2ZnSnSe4 (CZTSe), suitable for solar cell applications. The kesterites are highly sensitive to stoichiometry deviations and most of the kesterite growth technologies suffer from the segregation of secondary phases, which can be detrimental to the performance of the solar cell. This thesis examines the phase segregation occurring during CZTSe absorber growth during a two-step electrodeposition – annealing process. A method towards reducing phase segregation in the CZTSe absorber is developed by manipulating the microstructure of Cu-Sn-Zn precursors and the annealing conditions. A power conversion efficiency improvement from 0 % to 5.94 % was possible by changing the microstructure of the metallic precursor from a bi-layered with Zn-rich alloy on the top to a matrix-type. This is explained by the higher reactivity of Zn with selenium vapour than Cu or Sn. The synthesis route used in this study consisted of three stages: sequential electrodeposition of Cu, Sn and Zn layers, soft pre-alloying of the resulting metallic stacks, and thermal annealing in the presence of selenium vapour. The pre-alloying step is introduced in order to develop uniform microstructure to the precursors by formation of Cu-Sn and Cu-Zn alloys. Depending on the stacking order of the metals in the electrodeposited precursors, two types of microstructure are developed during the pre-alloying process: a bi-layered structure with the Cu-Zn alloy on the top and the Cu-Sn alloy on the bottom and a matrix-type microstructure with the Zn-rich phase in the Sn-rich matrix. It is shown that the precursor microstructure determines the further segregation of phases during CZTSe growth. Using a rapid thermal annealing system (RTP) it is possible to resolve the mechanism of CZTSe formation at short time intervals. Identification and location of secondary phases is studied by four characterization techniques: XRD, Raman spectroscopy, SEM/EDX and SIMS. The influence of the partial pressure of selenium above the precursor on the CZTSe growth is investigated by theoretical computation and compared to experimental results. It is shown that the rate-limiting step of kesterite formation is the arrival of selenium species from the source to the precursor sample. This is correlated to the thermodynamic feasibility of the metal- vapour phase selenium reactions, since the selenium partial pressure above the sample will determine which phases are prone to form. The studies of the CZTSe formation mechanism show that Zn is the first metal to react with selenium vapour. This explains why the presence of Zn on the precursor surface enhances the ZnSe phase segregation during the CZTSe growth.