Untersuchung des reaktiven Sputterprozesses zur Herstellung von aluminiumdotierten Zinkoxid-Schichten für Silizium-Dünnschichtsolarzellen

This thesis addresses the development of aluminum doped zinc oxide (ZnO:Al) films by reactive sputter deposition. The study focuses on the relation between deposition conditions and the resulting electrical, optical and structural film properties. The structure of the ZnO:Al films strongly affects the surface morphology obtained after wet chemical etching. The technological goal was the design of ZnO:Al films with optimized surface texture and thus light scattering properties for the application as front contact in amorphous (a-Si:H) and microcrystalline (μc- Si:H) based thin film solar cells. The comparatively low absorption coefficient of silicon in the long wavelength range of the sun spectrum necessitates additional mechanisms to enhance light absorption within the silicon layers. This can be realized by a combination of rough, light scattering front contacts and highly-reflective rear contacts. Initially smooth, sputter deposited ZnO:Al films can be roughened by post-deposition wet-chemical etching. In the ideal case, these rough layers introduce almost completely diffuse transmission, and back reflector and front contact guide the light through the silicon layers until it is totally absorbed (ideal light trapping). The ZnO:Al films were prepared by reactive mid-frequency (MF) sputtering from metallic Zn:Al targets in a vertical in-line deposition system. Initially, technological aspects like stabilization of the working point and the influence of substrate movement required for dynamic deposition were studied. Doping concentration and deposition conditions were varied to optimize the optical and electrical ZnO:Al properties, leading to resistivities ρ < 3 · 10$^{−4}$ Ωcm and at the same time transmission values exceeding 80 % in the visible and near infrared spectrum. By increasing deposition pressure, decreasing substrate temperature or shifting the working point towards the oxide mode, the surface morphology after etching in diluted hydrochloric acid could be changed from crater-like to granular shapes. The working point during reactive sputtering enables the density of points of etch attack in post-deposition etching steps to be varied over a wide range. In subsequent discussion concerning the etching behavior of sputter deposited ZnO:Al films in acids and bases, known dependencies are identified and some analogies to the etching of ZnO single crystals are drawn. Based on the experimental findings we developed an optimized high rate deposition process including post-deposition etching for surface textured ZnO:Al films. When applied as a front contact in solar cells, initial efficiencies of 9.9 % and 8.2 % were realized for single junction cells with a-Si:H and μc-Si:H absorber layers, respectively. These ZnO:Al front contacts were sputtered at dynamic deposition rates of 100 nm·m/min which correspond to static deposition rates as high as 400 nm/min. For an a-Si:H/μc-Si:H tandem structure solar module, an initial aperture area efficiency of 9.7 % was obtained on 64 cm$^{2}$ aperture area