Aluminum-induced degradation and failure mechanisms of a-Si:H solar cells

Low temperature annealing effects on a-Si:H solar cells with Al contacts have been investigated. A reduction in shunt resistance upon annealing in the temperature range of 150-180 degrees C has been observed which clearly indicates degradation of a-Si:H solar cells at these low temperatures. The activation energy for Al diffusion in Si was measured to be about 1.2 eV which may be due to diffusion of Al into the a-Si:H through high diffusivity microstructural grain boundaries, and defects such as microvoids. Thirty minute annealing at 180 degrees C decreases the shunt resistance by a factor of 50 and the solar cells show a considerable decrease in efficiency and fill factor. Annealing at 200 degrees C degrades the solar cell to the point that no conversion is observed. Visible surface morphological changes of the aluminum pad surface and the a-Si:H film underneath initiate at approximately 170 degrees C. Al contact metal-induced crystallization of a-Si:H material underneath the contacts has been observed. Raman spectroscopy and X-ray diffraction techniques have been used to detect the crystallization of the interacted a-Si:H. This crystallization is observed to initiate at temperatures as low as 180 degrees C. The degradation and failure mechanisms of solar cells are explained using a simple model involving aluminum diffusion into the silicon followed by crystallization and type conversion of the n(+) a-Si:H due to aluminum counterdoping.