Intrinsic microcrystalline silicon prepared by hot-wire chemical vapour deposition for thin film solar cells
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文摘
Microcrystalline silicon (μc-Si:H) prepared by hot-wire chemical vapour deposition (HWCVD) at low substrate temperature TS and low deposition pressure exhibits excellent material quality and performance in solar cells. Prepared at TS below 250 °C, μc-Si:H has very low spin densities, low optical absorption below the band gap, high photosensitivities, high hydrogen content and a compact structure, as evidenced by the low oxygen content and the weak 2100 cm−1 IR absorption mode. Similar to PECVD material, solar cells prepared with HWCVD i-layers show increasing open circuit voltages Voc with increasing silane concentration. The best performance is achieved near the transition to amorphous growth, and such solar cells exhibit very high Voc up to 600 mV. The structural analysis by Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM) shows considerable amorphous volume fractions in the cells with high Voc. Raman spectra show a continuously increasing amorphous peak with increasing Voc. Crystalline fractions XC ranging from 50 % for the highest Voc to 95 % for the lowest Voc were obtained by XRD. XRD-measurements with different incident beam angles, TEM images and electron diffraction patterns indicate a homogeneous distribution of the amorphous material across the i-layer. Nearly no light induced degradation was observed in the cell with the highest XC, but solar cells with high amorphous volume fractions exhibit up to 10 % degradation of the cell efficiency.