Effect of Na presence during CuInSe2 growth on stacking fault annihilation and electronic properties

While Na presence is essential for the performance of high efficiency Cu In,Ga Se2 thin film solar cells, the reasons why Na addition by post deposition treatment is superior to pre deposition Na supply particularly at low growth temperatures are not yet fully understood. Here we show by X ray diffraction and electron microscopy that Na impedes annihilation of stacking faults during the Cu poor Cu rich transition of low temperature 3 stage co evaporation and prevents Cu homogeneity on a microscopic level. Lower charge carrier mobilities are found by optical pump terahertz probe spectroscopy for samples with remaining high stacking fault density, indicating a detrimental e ect on electronic properties if Na is present during growt