This title appears in the Scientific Report : 2014 

Analyse der Lichtstreuung zur Textur-Optimierung von Zinkoxid-Frontkontakten für Silizium-Dünnschichtsolarzellen
Jost, Gabrielle Christina Elisabeth (Corresponding author)
Photovoltaik; IEK-5
Jülich Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag 2014
203 p.
Thin Film Photovoltaics
Reihe Energie & Umwelt / Energy & Environment 223
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Within the scope of this work two aspects of zinc oxide front contact layers in silicon thin film solar cells were investigated: The application of low-cost and industrial relevant production processes and the quality evaluation and control of the front contact surface texture. The surface texture influences the solar cell efficiency as it plays a key role in the solar cell’s light management. To investigate the application of low-cost processes and materials, zinc oxide front contacts were deposited with an industrial relevant, high-rate deposition process on commercially available float glass. In a first step, a silicon oxynitride (SiO$_{x}$N$_{y}$) interlayer was developed and deposited on the float glass substrates. This interlayer was characterized regarding three layer properties: anti-reflective properties, nucleation properties for zinc oxide growth and barrier properties to prevent sodium diffusion from the glass substrate. On top of the interlayer aluminum-doped zinc oxide was sputtered with a high-rate deposition process using two ceramic 0,5 wt % rotatable magnetron targets. A suitable pressure and temperature regime for the deposition process was defined. Different surface textures and their influence on the lightmanagement of a-Si:H/$\mu$c-Si:H tandem solar cells was investigated. The work focused on the two optical effects: light-coupling and light-scattering. Small front contact structures with a diameter of about 400 nm were suitable for light-coupling as long as they provided a significant roughness around 100 nm at the same time. Larger features in the range of ca. 700-900 nm were more suitable for light scattering within the device given that they also provided a high roughness. A model using the lateral structure diameter and the roughness of the front contact surface texture as parameter to describe theability of a surface texture to couple and scatter light was presented. An optimized double-texture that combined a surface texture for light scattering (primary texture) and a superimposing surface texture for light-coupling (secondary texture) showed a significant improvement of the light-trapping compared to the single texture substrate in tandem solar cells. The initial efficiency of the a-Si:H/$\mu$c-Si:H tandem soalr cell on a double-textured front contact was as high as 12,5 % with a total absorber layer thickness of only 1,64 $\mu$m. Angular resolved scattering measurements are known to be highly texture sensitive and were performed to evaluate the surface texture quality. A correlation between the discrete large angle scattering at the front contact layer and the short-circuit current density of $\mu$c-Si:H solar cells as well as a-Si:H solar cells was demonstrated. The above mentioned surface quality parameters (lateral structure diameter and roughness) of front contacts in tandem solar cells were extracted from angular resolved scattering measurements. A correlation between the ratio of the intensities measured in transmission at the discrete angles 15° and 60° and the lateral structure diameter was shown. Additionally, a correlation between the surface texture roughness and the integral of all scattered lightwas demonstrated. To extend the monitoring to a control of the surface texture quality, selected parameters in the zinc oxide production process were evaluated related to their correlations with the surface texture quality parameters. Especially the deposition temperature showed a strong influence on the lateral structure diameter of the texture. A linear functional relationship between the temperature and structure diameter was demonstrated for the first time within the temperature range that wassuitable for the front contact deposition. The lateral structure diameter was determined using angular resolved scattering measurements after the surface texturing step. Applying a feedback loop the angular resolved scattering data was successfully used in a test series to adjust the deposition temperature of a new sputter deposition system to yield the angular resolved scattering data and hence surface topography of a reference texture.