High quality amorphous Si solar cells for large area mass production Micromorph tandem cells

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 1.4 m² substrates of over 11%, an active area efficiency of 10.5% for a champion 1.4 m² a-Si single junction module and an 8.7% stabilized conversion efficiency for an industrial 1.4 m² a-Si single junction champion module.