Reaction between sodium and selenium in Bridgman-grown CuInSe2

In the fabrication of CIGS-based solar cells, the incorporation of sodium to increase performance is now standard practice. Because grain boundaries may play a role in the Na action, in this laboratory, studies have been made of the effects of introducing Na into a melt of composition CuInSe_2+x to obtain Bridgman-grown monocrystals. It was found that if the added Na exceeds a critical value, [Na]_crit, the resulting material changes from p- to n-type. The quantity [Na]_crit is almost proportional to x, the excess of Se over stoichiometry according to the formula [Na]_crit= (2x+δ)/(1+2x+δ). This can be explained by the strong affinity of Na for Se to form Na₂Se-like compounds, starving the ternary of Se to produce a Se-deficient and therefore n-type chalcopyrite. This conclusion is based on measurements of transport properties, along with XRD, SEM/EDX measurements. On the monocrystal surfaces, CuIn₃Se₅ was detected by XPS.