Removing Homocoupling Defects in Alkoxy/Alkyl-PBTTT Enhances Polymer:Fullerene Co-Crystal Formation and Stability

C₁₄-alkoxy/alkyl PBTTT-OR-R, a derivative of the prototype PBTTT polymer, is relevant for optoelectronic applications because it combines PBTTT-like intercalation behavior with enhanced charge-transfer absorption at longer wavelengths. Additionally, fundamental insights are achieved by comparing this alternating conjugated polymer in the presence and absence of homocoupling defects. While the homocoupling-free oxidative variant, PBTTT-OR-R(O), as well as the Stille polymer with 19% homocoupling defects, PBTTT-OR-R(S), can both form co-crystals when stoichiometrically mixed with PC₆₁BM, the co-crystal of the latter shows properties which are highly dependent on the thermal processing conditions because of the presence of these defects. The PBTTT-OR-R(S):PC₆₁BM co-crystal shows a low thermal stability and PC61BM can progressively be expelled upon heating. In cooling from the melt and depending on the cooling rate, mixtures of PBTTT-OR-R(S) and PC₆₁BM show a competition between co-crystallization (fast cooling) and separate crystallization (slow cooling). The PBTTT-OR-R(O):PC₆₁BM co-crystal is much more stable up to at least 260 °C and is not influenced by the applied cooling rate. These findings also translate to the device level. The performance of PBTTT-OR-R:PC₆₁BM photodiodes is severely deteriorated after isothermal annealing in case of PBTTT-OR-R(S), whereas the thermally robust co-crystals of PBTTT-OR-R(O) allow a more flexible design of optimized devices.