Thin film p–i–n junction solar cells incorporating hydrogenated microcrystalline Si1-xGex (µc-Si1-xGex:H) absorber i layers (1 µm) have been fabricated by plasma-enhanced chemical vapor deposition in the composition range of 0≤x≤0.35. By increasing Ge content from x=0 to 0.15–0.2, short-circuit current density increases by ∼5 mA/cm2 with spectral sensitivities extending into the infrared wavelengths (>600 nm). However, solar cell parameters for larger Ge contents (x>0.2) are lowered by the increased charge carrier recombination in the µc-Si1-xGex:H i layer. As a result, a 6.3% efficient solar cell is obtained at x=0.2, exhibiting infrared response even higher than that of double-thickness µc-Si:H solar cells. The solar cell shows excellent performance stability under prolonged light soaking.
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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