In Operando GISAXS and GIWAXS Stability Study of Organic Solar Cells Based on PffBT4T-2OD:PC₇₁BM with and without Solvent Additive

Solvent additives are known to modify the morphology of bulk heterojunction active layers to achieve high efficiency organic solar cells. However, the knowledge about the influence of solvent additives on the morphology degradation is limited. Hence, in operando grazing-incidence small and wide angle X-ray scattering (GISAXS and GIWAXS) measurements are applied on a series of PffBT4T-2OD:PC₇₁BM-based solar cells prepared without and with solvent additives. The solar cells fabricated without a solvent additive, with 1,8-diiodoctane (DIO), and with o-chlorobenzaldehyde (CBA) additive show differences in the device degradation and changes in the morphology and crystallinity of the active layers. The mesoscale morphology changes are correlated with the decay of the short-circuit current J_sc and the evolution of crystalline grain sizes is codependent with the decay of open-circuit voltage V_oc. Without additive, the loss in J_sc dominates the degradation, whereas with solvent additive (DIO and CBA) the loss in V_oc rules the degradation. CBA addition increases the overall device stability as compared to DIO or absence of additive.