Studies on the effect of sodium in Bridgman-grown CuInSe₂

Ingots containing single crystals were grown from melts of Cu, In and Se in either stoichiometric proportions (CuInSe2) or with an excess of Se (CuInSe2.2). In addition, either sodium selenide (Na 2Se) or elemental sodium (Na0) was introduced to both sets of compositions in concentrations ranging from 0 to 3 at. %. The starting constituents were placed in quartz ampoules, which were evacuated and sealed before undergoing a vertical-Bridgman growth procedure. Analysis of deposits seen on the ampoule walls and on the ingot surface after growth revealed the presence of Na, as well as various forms of the other starting elements; however, no Na was found within the crystals. Electrical measurements revealed trends in the thermoelectric power of the ingots to correspond with additions of Na, as well as the presence of excess Se. A sign conversion from p- to n-type was confirmed with addition of sodium to stoichiometric CuInSe2. A suggested mechanism used to explain the effects of Na on the material, based on these experimental observations, is presented