The authors report on carrier transport properties and spectral sensitivities of hydrogenated microcrystalline silicon-germanium (μc-Si1−xGex:H) alloys fabricated by low-temperature (∼200°C) plasma-enhanced chemical vapor deposition over the wide compositional range. Hall-effect and conductivity measurements reveal a change from weak n-type to strong p-type conduction for x>0.75 and a monotonic decrease in photoconductivity upon Ge incorporation. In a p-i-n diode structure, the Ge incorporation into i layer reduces quantum efficiencies in the short wavelengths, indicating an increased photocarrier recombination at p∕i interface. Nevertheless, under reverse biased condition, a 0.9-μm-thick μc-Si0.6Ge0.4:H absorber yields a large photocurrent of >27mA∕cm2 (air mass 1.5 global) with spectral sensitivities extending into infrared wavelengths, offering a potential advantage over conventional microcrystalline silicon solar cells.
Photocarrier transport in hydrogenated microcrystalline Si1−xGex (μc-Si1−xGex:H) p-i-n solar cells (0<x<0.42) is studied using spectral response measurement under various bias light illuminations. The solar cell composed of μc-Si0.8Ge0.2:H i layer reveals an injection-level-independent carrier collection, demonstrating a 6.1% conversion efficiency with infrared sensitivities higher than double-thickness μc-Si:H solar cells due to an enhanced optical absorption. However, the illumination of the solar cells for x>0.35, particularly in the wavelength range of <650nm, induces a strong carrier recombination near the p-i interface and a weak collection enhancement in the bulk, indicative of field distortion by the negative space charge generated near the p-i interface. This finding is consistently explained by the increased acceptorlike states in undoped μc-Si1−xGex:H for large Ge contents.
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