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This thesis focuses on the investigation and characterization of the surfaces and interfaces of chalcopyrite-based Cu(In,Ga)Se2 (CIGSe) thin film solar cells using various x-ray and electron spectroscopies. In particular, the impact of alkali post deposition treatments (PDT) on the chemical and electronic surface and interface structure of CdS/CIGSe absorbers is studied.
The structure of “real world” CdS/CIGSe interfaces and how they are impacted by different alkali PDTs was investigated by a combination of different x-ray spectroscopies. The interface formation is characterized by studying sample sets with different CdS thicknesses. The chemical environment for indium and cadmium is revealed by deriving the modified Auger parameter α'(In) and α'(Cd) using the kinetic energy of most prominent Auger line together with the binding energy of the chosen core level. A more complex situation is found for CdS/CIGSe samples that underwent NaF+KF PDT, where a K-In-Se compound is initially present on top of the chalcopyrite absorber. The conversion of the K-In-Se type species into a Cd-In-(O,OH,S,Se) interface compound is recorded at short CBD-CdS deposition times. It appears the majority of K that is present at the surface of the NaF+KF PDT CIGSe absorber is dissolved in the CBD and partially re-deposited as K-O type species. The Cd/S ratio clearly deviates from the stoichiometry expected for CdS, and a Cd(O,OH,S)-like compound is likely formed. The electronic structure of CdS/CIGSe interface is similarly more complex for the NaF+KF PDT compared to the NaF PDT case, where only Cd(O,OH,S) buffer was formed.
In an attempt to shed more light into this complex situation, the impact of evaporated alkali metals (K, Rb, Cs) on the surface structure of CIGSe was studied in-system by synchrotron-based hard x- ray photoelectron spectroscopy (HAXPES), aiming at understanding the underlying mechanism of the interfacial effect of alkalis on the performance of CIGSe devices. In the case of K deposition, two K species are observed by x-ray absorption near-edge structure (XANES) and HAXPES, one of which species disappears at high annealing temperature. Furthermore, three new In contributions (In-O and K-In-Se, metallic In species) can be observed after K evaporation. The evolution of chemical contribution supports the formation of a K-In-Se and Cu-poor CIGSe (1:3:5) bilayer structure that is similar to what was reported for “real world” NaF+KF PDTs. Deposition of heavy alkali metals (Rb, Cs) induced the formation of alkali selenide phases after alkali evaporation and during low temperature annealing. Similar chemical changes as seen for the K composition (i.e. presence of metallic In, In-O, and alkali-O) are observed. However, detailed analysis of the Alk/Se ratio and composition provide direct evidence for the formation of a Alk-(In)-Se and (Cu,Alk)(In, Ga)Se2 bilayer.
The insights from these studies promise to provide crucial aid to fully exploit alkali pre-treatments in scientific and industrial CIGSe production, and will deliberate use of this means of surface/interface tailoring to push efficiencies even further.
Heutzutage wird im Bereich der Prozessentwicklung und der Prozessführung die Simulation als ein strategisches Werkzeug angesehen. Sie dient zur modellhaften Darstellung oder Nachbildung bestimmter Aspekte eines vorhandenen oder eines noch zu entwickelnden Systems. Sie erlaubt die Untersuchung von Systemen, an denen die probeweise Durchführung zu gefährlich, zu teuer oder sogar unmöglich ist. Grundlegend für jede Simulation ist die Verfügbarkeit eines allgemeinen oder eingeschränkten Prozessmodells, das mathematisch genau die wesentlichen Prozesseigenschaften beschreibt. Solche Prozessmodelle sind häufig nicht vorhanden oder besitzen noch nicht die Entwicklungsreife, die auch Nichtspezialisten eine problemlose Anwendung erlauben würde. Die vorrangige Aufgabenstellung für den Einsatz von allgemeinen und eingeschränkten Prozessmodellen ist zum einen die Erlangung eines grundlegenden Verständnisses des statischen und dynamischen Verhaltens eines Systems, zum anderen die Vorausbestimmung seines Verhaltens unter definierten Bedingungen. Dies lässt eine Vorhersage zu, wie sich das System unter normalen Bedingungen verhält bzw. steuern lässt. Ferner sind Prognosen möglich, wie sich das System unter extremen oder fehlerhaften Bedingungen verhält.