Phase Transformations and Crystalline Quality of CuInS2 Thin Films

In recent years, due to the growing interest in alternative energies, photovoltaics has become an active field for research. Economically, thin film solar cells offer an interesting alternative. Cu-chalcopyrite semiconductor materials are used as absorber layers in heterojunction solar cell devices. These materials exhibit a direct band gap and absorb the sunlight within a layer of a few microns thickness due to their high absorption coefficients. Compared to the knowledge accumulated on classical semiconductors such as silicon, the understanding of the physical properties of chalcopyrite materials is rather limited. The chalcopyrite semiconductor CuInS2 (CIS) is particularly promising for photovoltaic applications as it is Se-free and its band gap (1.5 eV) is well adapted to the solar spectrum. Conversion efficiencies above 12 % on a laboratory scale have been demonstrated. In CuInS2-based thin film solar cells the photo collection and charge carrier generation mainly occurs in the p-absorber (CuInS2), whereas the n-type layer is transparent and only needed to form the pn-junction. The performance of CuInS2-based photovoltaic devices is currently limited by the open circuit voltage Voc. This value is significantly below the theoretical limit calculated for an ideal solar cell with a direct band gap of 1.5 eV. In order to overcome this limitation recent research has focused on modifications, such as the preparation of CuInS2 thin films with different Cu/In-ratios and with different doping elements. In the course of these investigations it has been found that both, different Cu/In-ratios as well as the presence of doping elements, have an influence on the structural and electronic properties. Theoretical calculations on polytypes and defect structures of chalcopyrites motivated the investigation of those crystalline phases in order to explain the dependence of the device performance on the composition. Previously, it has been found that in Cu-poor prepared CuInS2 films the so-called CuAu-phase is highly dominating. At present the understanding of this material with respect to these variations, in particular the role of sodium during the growth, is far from being complete. Detailed knowledge of the structural properties and their dependence on the different process parameters is required to explain the functionality of CuInS2-based thin film solar cells. The main focus of this thesis is on the structural analysis of CuInS2 films. Real-time experiments during the sulfurization of precursor layers with different Cu/In-ratios have been performed as well as studies on the influence of sodium on the growth of Cu-poor prepared Cu/In precursor layers. Energy dispersive X-ray diffraction (EDXRD) as a bulk sensitive method and Raman spectroscopy as a surface sensitive method have been combined in in-situ experiments. While real-time X-ray diffraction (XRD) has been introduced before, it was one goal of this work to introduce Raman spectroscopy as an additional in-situ investigation method for chalcopyrite growth studies. One advantage of Raman spectroscopy is that the different crystalline phases of CuInS2, such as the chalcopyrite- and the CuAu-phase are observable. Therefore, the growth of CuInS2 films especially with respect to these phases has been investigated in detail. Electronic defects in the absorbing layer determine the energy conversion efficiency of a heterojunction solar cell. For an industrial production application it is important to relate defect densities to specific process parameters and to the performance of the solar cells. Raman spectroscopy is well suited to assess the crystal quality of semiconductor materials. The line shape of a Raman band has been related to the defect density within the films. This defect density manifests itself in a coherent volume of scatterers, which gives rise to an activation of non-centered phonons in the films and leads to an asymmetric line broadening. In this work not only single CuInS2 layers but also completed solar cells have been investigated ex-situ with Raman spectroscopy. It will be shown that the spectroscopic line shape of an intense CuInS2-Raman band correlates with the electric properties of the completed devices. These results demonstrate the possibility to introduce Raman spectroscopy as an in-line process control for solar cell fabrication. Raman spectroscopy allows the investigation of the structural quality of the absorber layers directly after their preparation without the need of completing the whole device in order to assess its quality. Furthermore, as with Raman spectroscopy the relevant regions of the absorber layers can be investigated, it will be shown, how the structural quality is related to the recombination mechanism of the devices.