Investigation of deposition conditions and annealing treatments on sputtered ZnO:Al thin films: Material properties and application to microcristalline silicon solar cells

摘要

The Transparent Conductive Oxide (TCO) thin film used as a front electrode in microcrystalline silicon solar cells is a key material for the improvement of their efficiency. A promising TCO is aluminium doped zinc oxide (ZnO:Al) deposited by RF magnetron sputtering. A first part of the thesis is dedicated to the study of the influence of deposition conditions on microstructure, growth mechanisms, and opto-electronic properties of ZnO:Al thin films. We obtained an optimum in terms of maximal transparency in the visible range, and minimal resistivity, for films realized at a pressure of 0.12 Pa and at a temperature of 325 °C. A second part of the work is focused on the study of post-deposition annealing treatments, such as thermal or laser annealing processes, on the properties of the material deposited at room temperature. The influence of the annealing atmosphere (under vacuum, in N2/H2, or in pure N2) and temperature (from 400 to 500 °C) was studied. The thermal annealing step resulted in an improvement of the optical and electrical properties of the ZnO:Al thin film up to a resistivity of 3.5×10-4 Ohm.cm for a transmittance between 400 and 1000 nm of 81.2%. The laser annealing treatment influences particularly the morphological properties. The finale step of this thesis work is based on the study of the wet chemical etching of the TCO thin film, in order to create an optimal surface morphology for light confinement in the microcrystalline silicon layers without a degradation of the electrical properties. The integration of these ZnO:Al thin films as front electrodes in PIN silicon thin film solar cells was realized and investigated in depth.