Abstract
This paper reports on the development of an amorphous silicon cell used in the top cell of Micromorph® tandem solar modules produced in the pilot line of Oerlikon Solar in Trübbach — Switzerland. Tuning of the process parameters used for PECVD deposition of the absorber layers such as process pressure, RF power density, SiH4/H2 ratio, and substrate temperature can result in significant improvement in the material quality of the absorber layer and therefore in the performance and light induced degradation of the a-Si cell. We have measured the single layer properties of different absorber layers by infrared spectroscopy and have found a strong correlation between both the microstructure factor R and the H-content bonded to Si and the stabilized efficiency or relative degradation of the a-Si cells containing the corresponding absorber layers. A combination of absorber layers with superior material quality, adapted p-doped and buffer layers and ZnO front and back contacts with enhanced light trapping have achieved record values for the conversion efficiency of industrial thin a-Si single junction cells and modules. Our results show initial efficiencies on test cells prepared on 1.4m2 substrates of over 11%, an active area efficiency of 10.5% for a champion 1.4m2 a-Si single junction module and an 8.7% stabilized conversion efficiency for an industrial 1.4m2 a-Si single junction champion module.
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