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  Bombsch, Jakob: Surface and interface properties of chalcopyrite-based solar cell structures as revealed by photoelectron spectroscopy. , FAU Erlangen-Nürnberg, 2021

Abstract:
This thesis presents an investigation of the surface and interface structures in chalcopyrite-based solar cell stacks with a particular focus on the impact of the novel and efficiency increasing RbF treatment on the chalcopyrite related interfaces and on the investigation of a wide bandgap chalcopyrite based stack to track performance limiting factors. Laboratory-based x-ray photoelectron spectroscopy (XPS) and synchrotron-based hard x-ray photoelectron spectroscopy (HAXPES) are used to examine the frontside and backside surface regions of different Cu(In,Ga)Se2 thin film solar cell absorbers, which were deposited at low temperatures (i.e. substrate temperature during CIGSe deposition <500°C, compatible with the use of flexible substrates like polyamide) and underwent NaF or combined NaF/RbF post-deposition treatment (PDT), respectively. A Cu deficient surface region is found at both surfaces of all absorbers and is modelled as a [Cu]:([In]+[Ga]):[Se]=1:5:8 surface compound covering stoichiometric 1:1:2 CIGSe. The application of NaF/RbF leads an even more pronounced Cu depletion and to the incorporation of Rb in the depleted region at the frontside and backside of the absorbers. Additionally, strong indications are found for the NaF/RbF PDT induced formation of a Rb- In-Se-type (RISe) compound with a 1:1:2 stoichiometry, partially covering the absorber frontside surface, while not being present at the backside. The determined valence band maximum shows a shift away from the Fermi level towards the surface on both sides of the CIGSe absorber, which is reinforced by increased RbF PDT. Complementary, a RbInSe2 co-evaporation PDT on high temperature deposited absorbers was investigated in comparison to an untreated and a RbF PDT absorber using HAXPES. The deposited layer was found to be significantly Rb deficient compared to a [Rb]:[In]:[Se] = 1:1:2 stoichiometry, which is in agreement with pronounced Rb diffusion into the CIGSe absorber. While the application of two different fit models suggests that the formation of stoichiometric RbInSe2 and a separate In-Se phase seems more likely, the formation of a strongly Rb poor Rb- In-Se phase cannot be excluded. To elaborate on the impact of the absorber/buffer interface, a thickness series of CdS layer on those absorbers was investigated, revealing a pronounced S deficiency in the early stages of the CdS growth which is explained by the formation of Cd-Se bonds. This S deficiency is increased for higher Rb contents on the absorber and might also be related to an initial phase of very limited CdS growth. Additionally, the wide-bandgap CuInS2/Zn(O,S) interface was investigated using Cu-poor and Cu-rich CuInS2 absorbers to identify factors limiting device efficiency. Using XPS and HAXPES a rather inhomogeneous growth of the Zn(O,S) with respect to its thickness was found. The conduction band alignment, which was derived using inverse photoemission spectroscopy (IPES), was found to show no significant offset, excluding it as a performance limiting factor.