Electrical influence of sodium in Bridgman-grown copper indium selenide

Sodium is well known to improve the performance of thin-film, polycrystalline CuInSe2-based photovoltaic devices. This has led to extensive research on the effects of this element on the polycrystalline material, with the ultimate objective of identifying the mechanism by which Na acts on the cells. However, much less research has been done on the effects of sodium on the monocrystalline form of this material. Such research could help to differentiate bulk from grain-boundary effects, as well as to identify reactions between the Na and the compound itself, or the individual elements within the compound. Therefore, in the present work, Na was added in varying quantities to quartz ampoules containing Cu, In and Se, in the atomic ratios of 1:1:2. The ampoules were evacuated and sealed before being put through a vertical-Bridgman procedure, resulting in ingots containing large, cm-size crystals. Electrical measurements on the ingot material revealed p-type conductivity for all material grown with stoichiometric proportions of the starting elements, without Na, but n-type conductivity for material grown with Na above a certain critical value. It was discovered that this critical value of Na increased when excess Se, above stoichiometry, was also included in the ampoules. Further experiments confirmed the mechanism responsible for the conductivity type change to be a reaction between the Na and Se, in a 2:1 atomic ratio, corresponding to the chemical formula Na2Se, which starved the CuInSe2 of its share of selenium, rendering it Se-deficient and therefore n-type. Other effects of Na on the material are identified, including no detection of sodium within the ternary itself. As well, some photovoltaic cells were made, the best of which achieved an efficiency of 8.8 %.