The role of sodium as a surfactant and suppressor of non-radiative recombination at internal surfaces in Cu₂ZnSnS₄

It is well-known that sodium improves the performance of Cu2ZnSnS4 (CZTS) devices, yet the mechanism of the enhancement is still not fully understood. This work aims to present a unified account of the relationships between grain boundaries in CZTS, sodium content at these boundaries, non-radiative recombination, and surfactant effects that produce large microstructural changes. Using temperature-dependent photoluminescence measurements, it is demonstrated that samples containing dramatically different grain sizes display identical radiative and non-radiative decay characteristics when sufficient sodium is present in the film. It is also shown that the sodium concentration needed to efficiently passivate non-radiative defects is significantly less that the quantity needed to obtain micrometer-sized CZTS grains. Finally, the high densities of donor-acceptor pairs that are observed in CZTS films appear to reside within the grains themselves, rather than at grain boundaries.