Conductivity and Structure of Sputtered ZnO:Al on Flat and Textured Substrates for Thin-Film Solar Cells

摘要

Aluminum-doped zinc oxide (ZnO:Al) is a prominent representative of the materialclass denoted as transparent conductive oxides (TCO). TCOs feature electricalconductivity while being transparent in the visible range. These unique propertiesconstitute the wide application of TCOs in opto-electronic devices. This work targetsthe application of TCOs for thin-film silicon and chalcopyrite-based solar cells.Generally, TCOs are deposited onto at substrates. However, TCO growth ontextured, light scattering substrates for thin-film silicon solar cells and on the roughchalcopyrite absorber also call for the optimization of TCO deposition on texturedsubstrates. Therefore, the deposition of sputtered ZnO:Al on at as well as ontextured substrates is elaborated. The focus is the understanding and optimizationof electrical conductivity accompanied by a detailed investigation of the material'sstructural properties.On at substrates, I propose a conductivity model that comprises three scatteringmechanisms, namely ionized-impurity, electron-phonon, and grain boundary scattering.The prominent feature of the model is the analytical description of grainboundary scattering by feld emission, i.e. quantum mechanical tunneling of electronsthrough potential barriers at grain boundaries. For this purpose, a theory of Stratton(R. Stratton, Theory of Field Emission from Semiconductors, Phys. Rev. 125(1962), 67 - 82) is adapted to double Schottky barriers at grain boundaries. Theconductivity model is applied to a wide range of literature data to show its applicabilityand explanatory power.