We report the first observation of the internal photoemission of holes into the valence band of hydrogenated amorphous silicon from metal contacts. This, together with the corresponding photoemission of electrons into the conduction band, is used to directly determine the room-temperature mobility gap of a-Si:H. The effective mobility gap is found to be 1.89±0.03 eV which is 0.16 eV larger than its Tauc optical gap of 1.73 eV. The important implications of the results on the understanding of a-Si:H band structure, electronic properties, and devices are outlined.

• Received 16 June 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.1420