State Diagrams of PBTTT Derivatives Mixed with PC₆₁BM and Their Relevance for Organic Electronics: Influence of Incongruently Melting Co-crystals and Homocoupling Defects

Double binary state diagrams of the benchmark semi-crystalline conjugated polymer PBTTT and its alkoxy derivatives PBTTT-OR-R and PBTTT-(OR)₂, mixed with PC₆₁BM, are constructed using Rapid Heat-Cool Differential Scanning Calorimetry and T-resolved synchrotron X-Ray Diffraction. The polymerization method is adapted to ensure the absence of homocouplings and obtain reliable state diagrams, supported by Flory-Huggins calculations. Co-crystallization always occurs at a 45:55 w/w% polymer:PC₆₁BM mixing ratio. All co-crystals remain stable up to 260‒280 °C in a wide composition range, and for the first time, it is proven that they show incongruent (peritectic) melting. The three state diagrams show one eutectic, i.e., between polymer and co-crystal in the case of PBTTT and PBTTT-OR-R, and between polymer and PC₆₁BM for PBTTT-(OR)₂. The latter eutectic beyond 280 °C leads to a reversible “solid-solid” transformation with the corresponding co-crystal. Isothermal treatments, at the onset temperature of co-crystallization during non-isothermal cooling, show no loss of co-crystal quality in combination with PC₆₁BM perfectioning for fullerene-rich PBTTT:PC₆₁BM and PBTTT-OR-R:PC₆₁BM mixtures, whereas PBTTT-(OR)₂:PC₆₁BM, as well as Stille-polymerized PBTTT-OR-R:PC₆₁BM with homocouplings, show suppression of co-crystallization with the formation of separate crystals of polymer and PC₆₁BM. These opposing effects are explained by the state diagrams, showcasing their substantial value in selecting efficient annealing conditions.