Phase Behavior in the Active Layer of Small Molecule Organic Photovoltaics: State Diagram of p-DTS(FBTTh₂)₂:PC₇₁BM

A comprehensive study was undertaken to obtain a more fundamental understanding of the phase behavior of the p-DTS(FBTTh₂)₂:PC₇₁BM system, used in small molecule organic solar cells, with a strong focus on the amorphous phase and its influence on crystallinity. Three dedicated thermal protocols were used in combination with advanced thermal analysis, solid-state NMR, and wide-angle X-ray diffraction. Rapid cooling, to avoid structure formation and gain insight in the amorphous phase, and slow cooling, to promote structure formation, were used as limiting cases to explain the intermediate behavior after device processing from solution. A complete state diagram was developed, and the glass transition (Tg)–composition relationship was determined. In the case of slow cooling and the procedure used for device processing, the rapid crystallization of p-DTS(FBTTh₂)₂ leads to an enrichment of the amorphous phase in PC₇₁BM, increasing its Tg and causing vitrification of the mixed amorphous phase before crystallization when the total amount of PC₇₁BM exceeds 70 wt %. The common processing additive 1,8-diiodooctane (DIO) was found to lead to a lower p-DTS(FBTTh₂)₂ crystallinity and smaller average crystal size. More importantly, it acts as a strong plasticizer, lowering Tg significantly and thus reducing the morphological stability of the p-DTS(FBTTh₂)₂:PC₇₁BM mixtures.