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Silicon based thin tandem solar cells were fabricated by plasma enhanced chemical vapor deposition (PECVD) in a 30 × 30 cm2 reactor. The layer thicknesses of the amorphous top cells and the microcrystalline bottom cells were significantly reduced compared to standard tandem cells that are optimized for high efficiency (typically with a total absorber layer thickness from 1.5 to 3 µm). The individual absorber layer thicknesses of the top and bottom cells were chosen so that the generated current densities are similar to each other. With such thin cells, having a total absorber layer thickness varying from 0.5 to 1.5 µm, initial efficiencies of 8.6–10.7% were achieved. The effects of thickness variations of both absorber layers on the device properties have been separately investigated. With the help of quantum efficiency (QE) measurements, we could demonstrate that by reducing the bottom cell thickness the top cell current density increased which is addressed to back‐reflected light. Due to a very thin a‐Si:H top cell, the thin tandem cells show a much lower degradation rate under continuous illumination at open circuit conditions compared to standard tandem and a‐Si:H single junction cells. We demonstrate that thin tandem cells of around 550 nm show better stabilized efficiencies than a‐Si:H and µc‐Si:H single junction cells of comparable thickness. The results show the high potential of thin a‐Si/µc‐Si tandem cells for cost‐effective photovoltaics. Copyright © 2010 John Wiley & Sons, Ltd.

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Several approaches that lead to shorter production times of a-Si/ mc-Si tandem cells are combined in this paper: high-rate sputtering of aluminum-doped zinc oxide, high-rate 40 MHz plasma deposition of microcrystalline silicon and reduced ilayer thicknesses. On standard lab-type texture-etched ZnO:Al, 1 cm 2 a-Si:H/mc-Si:H tandem test cells on a deposition area of 30 Â 30 cm 2 were made that showed an initial efficiency of 9.9%, whereas the total effective deposition time of intrinsic layers was only 22 min (15 min for the top cell and 7 min for the bottom cell). The silicon thickness is only 600 nm. On high-rate texture-etched ZnO:Al an efficiency of 9.4% initial was reached. Standard light-induced degradation experiments showed a degradation rate of only 5.5-7.9% after 1000 h. This regime of very short preparation times and relatively highstabilized efficiencies is highly interesting from the production point-of-view.

For the mobility gap of hydrogenated micro-crystalline silicon (μc-Si:H) a value near 1.1 eV is commonly found, similar to the bandgap of crystalline silicon. However, in other studies mobility gap values have been reported to be in the range of 1.48-1.59 eV. Indeed, for accurate modeling of μc-Si:H solar cells it is paramount that key parameters like the mobility gap are accurately determined. In this work we will discuss a method to determine the mobility gap of μc-Si:H using the dark current activation energy of μc-Si:H pin devices, and apply this method to μc-Si:H solar cells with varying crystalline volume fraction. We found the mobility gap is around 1.2 eV to 1.26 eV for μc-Si:H solar cells with a crystalline volume fraction between 50 % and 70 %. For a highly crystalline solar cell we found a mobility gap of 1.07 eV.

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