Surface Passivation Properties of Stacked Doped PECVD a-Si:H Layers for Hetero-Structure c-Si Solar Cells

Hetero-structures, such as the crystalline silicon (c-Si)/doped plasma-enhanced chemical vapor deposited (PECVD) amorphous silicon (a-Si:H) structure, are an elegant way to avoid recombination losses at the electrical contacts of high conversion efficiency photovoltaic devices. To find a good compromise between limited charge carrier recombination at the surface and a limited resistivity of the contact, sandwich structures such as c-Si (p or n)/a-Si:H(i)/ a-Si:H (p⁺ or n⁺) have been proposed in the past. In this paper we report on the surface passivation properties of such stacked doped a-Si:H layers, with a total thickness of only a few nm. Whereas subsequent deposition of a-Si:H (n⁺) films on thin a-Si:H (i) layers improves the surface passivation quality, a clear passivation degradation is seen when a-Si:H (i) layers are covered by a-Si:H (p⁺) films. The observed trends are irrespective of the wafer-type. This article discusses possible causes for these phenomena and a model for the recombination-mechanism is proposed