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

Abstract C 14 ‐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 PC 61 BM, 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):PC 61 BM co‐crystal shows a low thermal stability and PC 61 BM can progressively be expelled upon heating. In cooling from the melt and depending on the cooling rate, mixtures of PBTTT‐OR‐R(S) and PC 61 BM show a competition between co‐crystallization (fast cooling) and separate crystallization (slow cooling). The PBTTT‐OR‐R(O):PC 61 BM 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:PC 61 BM 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.