Preparation and properties of radio-frequency-sputtered half-Heusler films for use in solar cells

The class of half-Heusler compounds opens possibilities to find alternatives for II-VI or III-V compound semiconductors. We aim to find suitable substitutes for the cadmium sulphide buffer layer in chalcopyrite-based thin film solar cells, where the buffer layer is located between the p-type chalcopyrite absorber and an n-type transparent window layer. We report here the preparation of radio-frequency-sputtered lithium copper sulphide 'LiCuS' and lithium zinc phosphide 'LiZnP' films. The optical analysis of these films revealed band gaps between 1.8 and 2.5 eV, respectively. Chemical properties of the film surface and both interfaces between the film and a Cu(In,Ga)Se2 layer and between the film and an (Zn,Mg)O layer were investigated by in-situ photoelectron spectroscopy. The valence band offsets to the Cu(In,Ga)Se2 layer were estimated to be (0.4 ± 0.1) eV for 'LiCuS'/Cu(In,Ga)Se2 and (0.5 ± 0.8) eV for 'LiZnP'/Cu(In,Ga)Se2. This leads to positive conduction band offsets of > 1 eV. These rather large offsets are not compatible with efficient solar cell devices. Under atmospheric conditions 'LiCuS' and 'LiZnP' films show rapid decomposition.