Complete μc-Si:H p-i-n solar cells have been prepared by the very high frequency glow discharge method. Up to now, intrinsic μc-Si:H has never attracted much attention as a photovoltaic active material. However, an efficiency of 4.6% and remarkably high short circuit current densities of up to 21.9 mA/cm2 due to an enhanced absorption in the near-infrared could be obtained. First light-soaking experiments indicate no degradation for the entirely μc-Si:H cells. Voltage-dependent spectral response measurements suggest that the carrier transport in complete μc-Si:H p-i-n cells may possibly be cosupported by diffusion (in addition to drift).

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.

Compensated" microcrystalline silicon is obtained by adding 8 -20 ppm diborane in the plasma gas phase. p-in cells with such i-layers have increased infrared sensitivity, when compared to a-Si:H p-i-n cells. The preparation of the world's first "mixed stacked" a-Si:H/yc-Si:H tandem cell with an initial efficiency of 9.1 % is reported. The pc-Si:H cells showed no degradation of the cell performance under intense light-soaking. Typical properties of the pc-Si:H cell indicate that the electronic transport is dominated by the crystalline phase of the material.

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