Observation of band structure on amorphous silicon surface and performance of solar cells with controlled band offsets on hetero-junctions

Control of band offsets on hetero-junctions is important to achieve a higher efficiency amorphous silicon-based solar cell. It is expected to improve cell's properties of Voc and FF by suppressing band offsets on the junction. The configuration of conduction band level (Ec), valence band level (Ev) and Fermi level (Ef) based on vacuum level governs the formation of band offset on the junction. We show that the configuration of the band structure on the surface of amorphous Si thin film is deposited by plasma-CVD equipment with a VHF power supply. Ec and Ev were obtained from ionization energy and energy band gap observed by ultraviolet photoemission spectroscopy and optical measurement, respectively. Ef was measured as work function by means of Kelvin probe method. As a result, it is found that shifts of energy levels originate from change of bonding characteristics in the amorphous Si network. This phenomenon is peculiar to amorphous Si-based materials. The results indicate that the upper shift of Ec and Ev are effective in suppressing band offsets on the hetero-junction. We show the performance of solar cells with controlled band offsets on hetero-junctions and discuss the possibility of higher Voc and FF by employing amorphous Si-based materials.