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Advanced properties of Al-doped ZnO films with a seed layer approach for industrial thin film photovoltaic application

: Dewald, W.; Sittinger, V.; Szyszka, B.; Säuberlich, F.; Stannowski, B.; Köhl, D.; Ries, P.; Wuttig, M.


Thin solid films 534 (2013), S.474-481
ISSN: 0040-6090
Fraunhofer IST ()
transparent conducting oxide; light trapping; seed layer; thin film silicon solar cell

Currently sputtered Al-doped ZnO films are transferred to industry for the application in thin film silicon solar modules. These films are known to easily form light trapping structures upon etching which are necessary for absorbers with low absorbance such as µc-Si. Up to now the best structures for high efficiency thin film silicon solar cells were obtained by low rate radio frequency (r.f.) sputtering of ceramic targets. However, for industrial application a high rate process is essential. Therefore a seed layer approach was developed to increase the deposition rate while keeping the desired etch morphology and electrical properties. Aluminum doped ZnO films were deposited dynamically by direct current (d.c.) magnetron sputtering from a ceramic ZnO:Al2O3 target (1 wt.%) onto an additional seed layer prepared by r.f. sputtering. ZnO:Al films were investigated with respect to their optical and electrical properties as well as the morphology created after etching for a-Si/µc-Si solar cells. Additionally atomic force microscopy, scanning electron microscopy, X-ray diffraction and Hall measurements were performed, comparing purely r.f. or d.c. sputtered films with d.c. sputtered films on seed layers. With the seed layer approach it was possible to deposit ZnO:Al films with a visual transmittance of 83.5%, resistivity of 295 µ omega cm, electron mobility of 48.9 cm2/Vs and electron density of 4.3 · 1020 cm-3 from a ceramic target at 330 °C. Etch morphologies with 1 µm lateral structure size were achieved.