Comparison between a-SiOx:H and a-Si:H as Passivation Buffer Layer for Heterojunction Solar Cells

In crystalline silicon based heterojunction solar cells the surface passivation quality is fundamental to obtain high efficiency. Intrinsic a-Si:H, as obtained by PECVD process from silane dissociation, is a good candidate for surface passivation, but UV absorption of this material limits the current generation. Moreover, surface passivation quality can be compromised when fabrication steps, following the a-Si:H deposition, exceed the a-Si:H deposition temperature. Both drawbacks can be overcome introducing a source of oxygen in PECVD process, obtaining a-SiOx:H layer that provides at the same time high quality passivation, wider optical bandgap and less susceptible to temperature steps than a-Si:H. In this work we compared two heterojunction solar cells different only for front side passivation layer, consisting of a-Si:H or a-SiOx:H. We monitored lifetime and implied Voc during fabrication steps and evaluated the final I-V characteristics and quantum efficiency. We found that the UV light soaking together with heating during subsequent sputtering process, increased lifetime of cell with a-SiOx:H more than that with a-Si:H. Likewise the cell having a-SiOx:H as passivation layer showed electrical parameters higher than the a-Si:H counterpart. Indeed Jsc and Voc were 1mA/cm2 and 20mV respectively higher in cell with a-SiOx:H than in cell with a-Si:H. In particular cell with a-SiOx:H layer achieved an encouraging implied Voc of 751mV.