As-deposited undoped microcrystalline silicon (μc-Si:H) has in general a pronounced n-type behavior. Such a material is therefore often not appropriate for use in devices, such as p-i-n diodes, as an active, absorbing i layer or as channel material for thin-film transistors. In recent work, on p-i-n solar cells, this disturbing n-type character had been successfully compensated by the ‘‘microdoping’’ technique. In the present letter, it is shown that this n-type behavior is mainly linked to oxygen impurities; therefore, one can replace the technologically delicate microdoping technique by a purification method, that is much easier to handle. This results in a reduction of oxygen impurities by two orders of magnitude; it has, furthermore a pronounced impact on the electrical properties of μc-Si:H films and on device performance, as well. Additionally, these results prove that the unwanted donor-like states within μc-Si:H are mainly due to extrinsic impurities and not to structural native defects.

Recently the authors have demonstrated that compensated or “midgap” intrinsic hydrogenated microcrystalline silicon (μc-Si:H), as deposited by the Very High Frequency Glow Discharge (VHF-GD) technique, can be used as active layer in p-i-n solar cells. Compared to amorphous silicon (a-Si:H), μc-Si:H was found to have a significantly lower energy bandgap of around 1 eV. The combination of both materials (two absorbers with different gap energies) leads to a “real” tandem cell structure, which was called the “micromorph” cell. Micromorph cells can make better use of the sun's spectrum in contrast to conventional double-stacked a-Si:H / a-Si:H tandems.The present study will show that the compensation technique (involving boron “microdoping”) used sofar for obtaining midgap μc-Si:H can be replaced by the application of a gas purifier. The use of this gas purifier has a beneficial influence on the transport properties of undoped intrinsic μc-Si:H. By this procedure, increased cell efficiencies in both, single microcrystalline silicon p-i-n as well as micromorph cells could be obtained. In the first case 7.7 % stable, and in the second case 13.1% initial efficiency could be achieved under AM1.5 conditions. Preliminary light-soaking experiments performed on the tandem cells indicate that microcrystalline silicon could contribute to an enhancement of the stable efficiency performance. Micromorph cell manufacturing is fully compatible to a-Si:H technology; however, its deposition rate is still too low. With further increase of the rate, a similar cost reduction potential like in a-Si:H technology can be extrapolated.

bstractRecently, we have demonstrated that intrinsic hydrogenated microcrystalline silicon, as deposited by the very high frequency glow-discharge technique, can be used as the active layers of p-i-n solar cells. Our microcrystalline silicon Ž represents a new form of thin film crystalline silicon that can be deposited in contrast to any other approach found in . literature at substrate temperatures as low as 2008C. The combination of amorphous and microcrystalline material leads to a 'real' silicon-based tandem structure, which we label 'micromorph' cell. Meanwhile, stabilised efficiencies of 10.7% have been confirmed. In this paper, we present an improved micromorph tandem cell with 12% stabilised efficiency measured under outdoor conditions. Dark conductivity and combined SIMS measurements performed on intrinsic microcrystalline silicon layers reveal a post-oxidation of the film surface. However, a perfect chemical stability of entire microcrystalline cells as well as micromorph cells is presented. Variations of the pri interface treatment show that an increase of the open circuit voltages from 450 mV up to 568 mV are achievable for microcrystalline cells, but such devices have reduced fill factors.

Recent progress of solar cells based

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