factor of 7. This shows that solution-based silicon is a highly promising candidate for industrial-grade applications of solutionbased semiconductors. Evaluation of Precursors NPS and CPSIn literature, most groups reporting silicon fi lms fabricated from a liquid precursor use a cyclic hydridosilane, namely cyclopentasilane (CPS). We decided to use a branched molecule instead, namely neopentasilane (NPS). The molecular structures of CPS and NPS, as well as the process charts for obtaining solid amorphous silicon (a-Si) layers, are shown in Figure 1 . We characterized the NPS used in our process chain by NMR and by mass spectroscopy, showing the expected fi ngerprints mentioned in literature. [ 4 ] Employing NPS over CPS yields major advantages in processing effi ciency as well as in material quality. In general, branched molecules have a considerably better solubility in organic solvents, because the branches act as spacers, preventing strong interactions between the molecules and enabling better intercalation of solvent molecules. [ 5 ] The NPS material is therefore better soluble than CPS, which leads to improved fi lm homogeneity and uniformity. Moreover, in NMR measurements, we found that the NPS-oligomer bears 70% SiH 3 end groups, in contrast to 1.0% for the CPS-oligomer. Such end groups facilitate the cross-linking of the material to a solid network. Since this process is responsible for the formation of silicon-silicon bonds, we expect a positive effect on the coordination of silicon atoms, resulting in less dangling bonds and improved electronic properties. Until now, we have however not been able to demonstrate differences in nanoscopic amorphous silicon structure between CPS and NPS.Another major advantage of employing NPS instead of CPS lies in the differences in material synthesis. The synthesis of the CPS monomer involves a coupling reaction and subsequent chlorination of diphenyldichlorosilane to obtain decachlorocyclopentasilane. This process produces a large amount of various by-products, which are diffi cult to separate and recycle. However, in the synthesis of NPS, we use catalytic rearrangement of octachlorotrisilane to obtain dodecachloroneopentasilane,

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The influence of oxygen and nitrogen impurities on the performance of thin-film solar cells based on microcrystalline silicon (mu c-Si:H) has been systematically investigated. Single mu c-Si:H layers and complete mu c-Si:H solar cells have been prepared with intentional contamination by admitting oxygen and/or nitrogen during the deposition process. The conversion efficiency of similar to 1.2 mu m thick mu c-Si: H solar cells is deteriorated if the oxygen content in absorber layers exceeds the range from 1.2 x 10(19) to 2 x 10(19) cm(-3); in the case of nitrogen contamination the critical impurity level is lower ([N](critical)=6 x 10(18)-8 x 10(18) cm(-3)). It was revealed that both oxygen and nitrogen impurities thereby modify structural and electrical properties of mu c-Si:H films. It was observed that the both contaminant types act as donors. Efficiency losses due to oxygen or nitrogen impurities are attributed to fill factor decreases and to a reduced external quantum efficiency at wavelengths of >500 nm. In the case of an air leak during the mu c-Si:H deposition process, the cell performance drops at an air leak fraction from 140 to 200 ppm compared to the total gas flow during i-layer deposition. It is demonstrated that oxygen and nitrogen impurities close to the p/i-interface have a stronger effect on the cell performance compared to impurities close to the n/i-interface. Moreover, thick mu c-Si:H solar cells are found to be more impurity-sensitive than thinner cells

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