Relation between sodium addition and excess selenium in Bridgman-grown CuInSe2

Ingots containing cm-sized monocrystals of CuInSe 2 have been grown in quartz ampoules by a vertical-Bridgman method. Included also in the ampoules were varying quantities of elemental Na, as well as excess Se above stoichiometry. Deposits were seen within the ampoules after growth, including a white deposit formed through a reaction of the Na with the quartz, and a reddish-brown deposit mostly containing Na and Se. Thermoelectric power measurements indicate that stoichiometric material grown with no excess Se and no Na is p-type. With the addition of Na above a critical amount to the ampoule, the resulting ingots were always n-type. This p to n conductivity type change is inhibited when excess Se is also present, and more Na is required for the change to occur. The critical amount of Na was found to be approximately twice the atomic amount of excess Se. XRD indicated that the chalcopyrite structure of the material was maintained with additions of up to 3 at. % Na, even as the material was changed from p- to n-type, and no sodium was detected in the bulk. It is suggested that a reaction between the Na and the Se occurs, resulting in the formation of Na 2 Se. The Se used in this reaction is therefore unable to be a part of the CuInSe 2 lattice, thereby increasing the number of donor-like Se vacancies, rendering it n-type. In this model, the addition of excess Se replaces those Se atoms which react with the Na, so that the material remains p-type.