Fabrication of a-SiGeC:H solar cells using monomethyl germane by suppressing carbon incorporation for narrowing optical bandgap

Hydrogenated amorphous silicon–germanium–carbide (a-SiGeC:H) thin films have been fabricated by very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) using monomethyl germane (MMG, GeH3CH3) as the germanium source. It was found that C incorporation into the films was considerably suppressed under high hydrogen dilution (RH) and substrate temperature (TS). Under high RH and TS, we were able to narrow the optical band gap (Eopt) of a-SiGeC:H thin films to 1.39 eV, whereas under low RH and TS, Eopt increased up to 1.9 eV with increasing MMG. The best electrical properties were obtained for a sample of a-Si0.68Ge0.29C0.03:H, whose Eopt and photosensitivity were 1.58 eV and 105, respectively. When this film was used as an absorber layer in a p–i–n structured solar cell, significant enhancements were observed in its quantum efficiency for long wavelengths.