Electrical effect of introducing elemental sodium into the Bridgman melt of CuInSe2+x crystals

• Bulk CuInSe2 was grown by a Bridgman-method with varying amounts of elemental Na and excess Se.
• Na changed the conductivity of the CuInSe2 from p- to n-type by Seebeck and Hall measurements.
• This type change was mitigated with excess Se, so that more Na was needed for the change to occur.
• A Se-starvation interaction model between the Na and Se to explain the mechanism responsible.

Transport and other measurements have been made on CuInSe2+x samples obtained from Bridgman ingots grown from melts containing controlled amounts of elemental sodium, where x represents the excess of Se over stoichiometry. Thermoelectric power and Hall coefficient at room temperature show a conductivity sign change from p- to n-type in otherwise stoichiometric CuInSe2 (x=0), with added Na between 0.2 and 0.3 at%. It is further found that the critical amount of Na required to change the conductivity type, denoted by [Na]crit, increases with x, almost linearly, with an initial slope of 2, corresponding to an approximate formula of [Na]crit=2x+δ, where δ is found to be 0.25 at% Na. This behavior can be accounted for quantitatively using a ‘selenium starvation’ model, whereby at the p-to-n type change, two atoms of sodium have reacted with one atom of Se to form a molecule like Na2Se, with an atom-to-atom ratio [Na]/[Se]=2. Such molecules were detected in growth run residues. However, no Na or Na compounds were detected in the interior of bulk crystals. In stoichiometric material (x=0), the crystal structure remained chalcopyrite with up to at least 3 at% Na in the melt, despite the conductivity type changes. Further, no β-phase was detected in the bulk material by XRD but it was found at the surface of samples by XPS.