Effect of Sb and Na Incorporation in Cu2ZnSnS4 Solar Cells

Cu2ZnSnS4-based solar cells suffer from limited power conversion efficiency
(PCE) and relative small grain size compared to selenium containing
absorbers. Introduction of Na in Cu2ZnSnS4 absorbers either during the
synthesis or after this step is used to improve device performances and to
determine whether its effect is based on structural properties improvement
(grain size enhancement, better crystallization) or on opto-electronic properties
improvement (defect passivation). In both cases, presence of Na in the
absorber notably improves current and voltage of the solar cells, but the
effect is more pronounced when Na is present during synthesis. Quantum
efficiency analysis shows that these improvements can be related to longer
minority carrier diffusion length and reduced absorber/buffer interface
recombination. Introducing Na in the process mostly leads to preferential
(112) orientation of the crystal which is clearly correlated with better device
performances. Otherwise, the performance limitation due to small grain size
is discarded by the joint use of Sb and Na, which has a significant impact on
grain size but does not affect solar cells efficiency.