Strong interplay between Na- and O-related defects in Cu-based chalcogenides*

Recent advances in thin-film photovoltaics became possible by controlling the incorporation of impurities. The most notable among them are alkalis in-diffusing from soda-lime glass and oxygen incorporating from various sources during the baseline processing. Herein, we investigate the interaction between Na and O in co-sputtered Cu₂ZnSnS₄(CZTS) by combining theoretical and experimental techniques. First, using secondary ion mass spectrometry (SIMS), we demonstrate that Na and O distributions in the absorber are correlated. Then, employing first-principles methods, we show that the correlation is driven by a strong ionic Na-O bonding that triggers the formation of NaO and Na₂O complexes. The remarkably high binding energies for these complexes are proven to cause Na-O clustering at all temperatures of the baseline processing. Hence, the overall character of the impurity profiles is explained through O immobilizing Na by forming the defect complexes and leading to the Na accumulation near the CZTS surface. This defect interplay paves the way for a more accurate impurity control needed for fabricating high-performance devices.
Reference: S. Grini, K. V. Sopiha, et al., Adv. Energy Mater. 9, 1900740 (2019).