Na effects on CuInSe2: Distinguishing bulk from surface phenomena

Abstract

The effect of sodium on the performance of ${\mathrm{CuInSe}}_2$ -based solar cells has been under discussion for already a decade. We present experimental evidence using secondary ion mass spectroscopy, x-ray photoelectron spectroscopy (XPS) and other, complementary physical characterization methods, which indicate that, after exposure to an external Na source, no significant amounts of sodium, beyond the residual amount, found in as-grown samples, enter intact crystals, except via defects such as grain boundaries. However, after such exposure, sodium is found in significant concentrations on crystal surfaces, something that is accompanied by an increase in oxygen concentration, as judged by XPS. As expected metallic Na attacks the crystals and can destroy them or at least introduce significant defect densities. Adding ${\mathrm{Se}}^0$ is found, via ${\mathrm{Na}}_2\mathrm{Se}$ formation, to temper Na activity specifically its effects on crystal disintegration. This is different from the effect of ${\mathrm{Se}}^0$ along where annealing (of n-type) crystals results in n to p type conversion by Cu outdiffusion.