The use of processing additives has emerged as a powerful approach for the optimization of active layer performance in organic photovoltaic devices. However, definitive physical mechanisms explaining the impact of additives have not yet been determined. To elucidate the role of additives, we have studied the time evolution of structure in polymer-fullerene films blade-coated from additive containing solutions using in-situ spectroscopic ellipsometry and UV–vis transmission. Additives that are poor solvents for poly(3-hexylthiophene) (P3HT), such as 1,8-octanedithiol, and additives that are good solvents for P3HT, such as 1-chloronapthalene, both promote improved polymer order, phase segregation, and device performance. Regardless of the presence or type of additive, the polymer order develops under conditions of extreme supersaturation. Additives, regardless of whether they are solvents for P3HT, promote earlier polymer aggregation compared to additive - free solutions presumably by degrading the solvent quality. We find evidence that the details of the final film morphology may be linked to the influence of the substrate and long-time film plasticization in the cases of the non-solvent and solvent respectively.
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