Prof. Clifford Champness (Electrical and Computer Engineering Dept., McGill University)
The addition of sodium compounds in the fabrication of thin film CuInSe2-based solar cells has substantially increased their photovoltaic solar conversion efficiencies. However, how this improvement comes about, is still not completely understood. To help find the mechanism involved, experiments have been made in this laboratory to grow Bridgman bulk monocrystalline CuInSe2+x ingots, with various amounts of elemental sodium in the melt for study . Bulk solid material is more convenient to work with than thin films for measurements, has better quantitative control for additions and is free of microscopic grain boundaries. As a result of previous work, it was found that the conductivity type was changed from p- to n-type with the addition of a critical amount of Na added to the melt, [Nacrit], given by 2x+ δ, where x is the stoichiometric excess of Se in the formula CuInSe2+x and δ is the residual amount of Na needed for the conversion of stoichiometric material (i.e. x=0). These results were obtained previously with transport, EDX/SEM and XRD studies. The present work, however, was carried out with XPS measurements. It was confirmed that Na was detected by XPS in the exterior surface and in the last-zone-to-freeze of the ingot but not detectable in its interior. The chemical form of the Na, such as Na2Se and Na2SeO3, is being investigated. As a result of argon ion surface etch-profiling, the ternary composition at the surface of the ingot appears to be similar to the β-phase (CuIn3Se5) with a low relative concentration of Cu, while the structure in the deep interior corresponds only to the α-phase (chalcopyrite CuInSe2), as indicated by XRD.
Ms Sunyoung Park (Electrical and Computer Engineering Dept., McGill University)