State Diagrams of PBTTT Derivatives Mixed with PC ₆₁ BM and Their Relevance for Organic Electronics: Influence of Incongruently Melting Co‐crystals and Homocoupling Defects

Abstract 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 PC ₆₁ BM, 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:PC ₆₁ BM 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 PC ₆₁ BM 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 PC ₆₁ BM perfectioning for fullerene‐rich PBTTT:PC ₆₁ BM and PBTTT‐OR‐R:PC ₆₁ BM mixtures, whereas PBTTT‐(OR) ₂ :PC ₆₁ BM, as well as Stille‐polymerized PBTTT‐OR‐R:PC ₆₁ BM with homocouplings, show suppression of co‐crystallization with the formation of separate crystals of polymer and PC ₆₁ BM. These opposing effects are explained by the state diagrams, showcasing their substantial value in selecting efficient annealing conditions.