Real-Time Study on Structure Formation and the Intercalation Process of Polymer: Fullerene Bulk Heterojunction Thin Films

Abstract

Fullerene intercalation between the side chains of conjugated polymers has a detrimental impact on both charge separation and charge transport processes in bulk heterojunction (BHJ) organic photovoltaic cells (OPVs). In situ grazing incidence X-ray scattering experiments allow to characterize the structure formation, drying kinetics, and intercalation in blends of phenyl-c61-butyric acid methyl ester (PC60BM) and poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) named (pBTTT-C14) from their 1,2-orthodichlorobenzene (oDCB) solutions with different volume fractions of dodecanoic acid methyl ester (Me12) as a solvent additive. The structure formation process during evaporation of the solvent:additive mixture can be described by five periods, which are correlated to a multistep contraction of the lamellar stacking of the bimolecular crystals. The onset of crystallization is delayed by increasing the additive volume fraction in the coating solution leading to a promoted crystallinity. A conclusive picture of fullerene intercalation and additive-tuned structural evolution during the drying of thin films of the polymer:fullerene BHJ blends will be presented.