Sodium — Excess selenium interaction in Bridgman-grown CuInSe2

In otherwise stoichiometric Bridgman-grown CuInSe₂, it has been found, in this laboratory, that the conductivity type changes from p- to n-type when more than 0.25 at. % of elemental sodium is added to the melt. For melts with excess selenium over stoichiometry, x, corresponding to the formula CuInSe_2+x + Na, no conversion was observed up to at least 3 at. % of added Na for x = 0.2. However, for smaller x-values (x ≤ 0.05), the inversion continued to take place, where the percentage of Na causing the inversion ([Na]_crit) increased with x. From a relationship emerging between [Na]_crit and x, it appears that one Se atom reacts with two Na atoms in the inversion process, suggesting the formation of the binary Na₂Se. While this compound has not been detected by XRD in the residue from growth runs with a few percent of added sodium, it was detected in a special run where the element addition was greatly increased. XRD has also confirmed that the Na-added n-type material remains chalcopyrite, even with large Na additions of 3 at. %.