Microstructure and photovoltaic performance of polycrystalline silicon thin films on temperature-stable ZnO:Al layers

Polycrystalline silicon (poly-Si) thin films have been prepared by electron-beam evaporation and thermal annealing for the development of thin-film solar cells on glass coated with ZnO:Al as a transparent, conductive layer. The poly-Si microstructure and photovoltaic performance were investigated as functions of the deposition temperature by Raman spectroscopy, scanning and transmission electron microscopies including defect analysis, x-ray diffraction, external quantum efficiency, and open circuit measurements. It is found that two temperature regimes can be distinguished: Poly-Si films fabricated by deposition at low temperatures (T-dep < 400 degrees C) and a subsequent thermal solid phase crystallization step exhibit 1-3 mu m large, randomly oriented grains, but a quite poor photovoltaic performance. However, silicon films deposited at higher temperatures (T-dep > 400 degrees C) directly in crystalline phase reveal columnar, up to 300 nm big crystals with a strong < 110 > orientation and much better solar cell parameters. It can be concluded from the results that the electrical quality of the material, reflected by the open circuit voltage of the solar cell, only marginally depends on crystal size and shape but rather on the intragrain properties of the material. The carrier collection, described by the short circuit current of the cell, seems to be positively influenced by preferential < 110 > orientation of the grains. The correlation between experimental, microstructural, and photovoltaic parameters will be discussed in detail. (C) 2009 American Institute of Physics. [doi:10.1063/1.3240343]