Carrier collection characteristics of microcrystalline silicon-germanium p-i-n junction solar cells

We report on the carrier collection characteristics of hydrogenated microcrystalline silicon-germanium (μc-Si1−xGex:H) p-i-n junction solar cells fabricated by low-temperature (∼200 °C) plasma-enhanced chemical vapor deposition. Spectral response measurements reveal that the Ge incorporation into absorber i-layer reduces the quantum efficiencies at short wavelengths. Furthermore, the illumination of the solar cells for x ⩾ 0.35, particularly in the wavelength range of <650 nm, induces a strong injection-level-dependent p-i interface recombination and a weak collection enhancement in the bulk. These results indicate that space charges near the p-i interface are largely negative, which gives rise to an electric field distortion in the i-layer. We attribute the negative space charges to the presence of the acceptor-like states that are responsible for the strong p-type conduction observed in undoped μc-Si1−xGex:H films for large Ge contents.