Crucial processing steps for microcrystalline silicon bottom cells

In the device processing of microcrystalline silicon bottom cells, a variety of processing steps determining not only the material properties but also the interface properties should be taken into account for higher efficiencies. In particular, since microcrystalline silicon is sensitive to impurity incorporation and its variation of the microstructure, processing window can be much narrower than for amorphous silicon based devices. We have found the advantage of the low processing temperature to overcome the oxygen contamination effects and the boron-induced phenomena. We have found that another important factor to influence the device performance particularly under the high rate deposition conditions is the combination of the high deposition pressure and silane depletion conditions. The advantage of the high-pressure depletion technique is discussed in terms of plasma physics and chemistry and its influence on the material properties. Finally, the optical design of the tandem cell is briefly discussed.