Defect Engineering in Kesterite Materials for Thin Film Solar Cells

Abstract: Cu2ZnSnS4 has great potential to be applied as an earth abundant and non-toxic absorber material in thin film solar cells, based on its suitable optical properties. However, several challenges have prevented the achievable efficiencies from exceeding 12.6 %, which is well below marketable efficiencies compared to competing solar cell technologies. One of the struggles in the development of Cu2ZnSnS4 solar cells is the high number of harmful defects leading to severe potential fluctuations. This thesis investigates different strategies of defect engineering in Cu2ZnSnS4, in particular to reduce Cu-Zn disorder.Cu2ZnSnS4 thin films are produced by a two-step process, where Cu-Zn-Sn-S precursors are deposited by co-sputtering and then annealed at high temperature to yield crystalline films. The material properties are investigated with Raman spectroscopy, photoluminescence and spectrophotometry.In the scope of this thesis, the following approaches to defect engineering are investigated: thermal treatments, varying partial pressures during the annealing step, and cation exchange to form the compound Cu2MnSnS4. Thermal treatments substantially enhance the degree of order in Cu2ZnSnS4. However, for the first time the severe limitations of such treatments are shown, indicating their insufficiency to reduce cation disorder to a level where potential fluctuations no longer affect Cu2ZnSnS4 solar cells. Furthermore, the stannite Cu2MnSnS4 suffers from cation disorder just like kesterite Cu2ZnSnS4 demonstrating that cation disorder is not restricted to the kesterite crystal structure and posing new challenges for finding new solar cell materials.On the other hand, the presented results demonstrate a strong composition dependence of the ordering kinetics. Compositions with high densities of vacancies or interstitials significantly enhance the ordering rate by reducing the activation energy while the critical temperature is constant for the investigated compositions. Furthermore, the important effect of S2 and SnS partial pressures during the annealing step of the fabrication is predicted from chemical models and experimentally verified by investigation of composition-spread Cu2ZnSnS4 thin films. Increasing both partial pressures leads to higher solubility of vacancies in Sn-rich Cu2ZnSnS4 further amplifying the positive effect of composition on the order-disorder transition. Investigation of composition-spread thin films further revealed the interplay between material properties and composition as well as secondary phases. In particular, the photoluminescence yield was drastically enhanced in the presence of SnSx secondary phases. This thesis discusses these results in the context of the current understanding of Cu2ZnSnS4.