Real-time X-ray scattering studies of film evolution in high performing small-molecule-fullerene organic solar cells

We have studied the influence of the formulation additive 1,8-diiodooctane (DIO) on the structural evolution of bulk heterojunction (BHJ) films based the small molecule donor 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-5-(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh)) and phenyl-C-butyric-acid-methyl ester ([70]PCBM). Real-time, in situ, grazing-incidence X-ray scattering experiments allow us to characterize the development of crystalline order via diffraction and phase separation via small angle scattering. The performance of p-DTS(FBTTh) based solar cells exhibits a distinct optimum with respect to volume fraction of DIO in the coating solution, unlike many polymer-fullerene systems that exhibit plateaus in performance above a certain additive volume fraction. Increasing the DIO volume fraction increases the crystallinity of p-DTS(FBTTh) and dramatically increases the phase separation length scale even at small DIO amounts. These results suggest that the existence of an optimal DIO amount is a consequence of the phase separation length scale and its relationship to the optimal length for exciton dissociation. The effects of DIO on the time evolution of the drying films indicates that it acts as both a solvent and a plasticizer for p-DTS(FBTTh), controlling its nucleation density and promoting its crystal growth.