Two-Stage Bipolaron Formation in Molecularly Doped Conjugated Polymers

The formation and dynamics of bipolarons are crucial in determining the electrical properties of molecularly doped conjugated polymers. Traditionally, bipolarons are known to form at very high doping levels through the combination of two adjacent polarons, a process that is generally accompanied by structural disorder and impaired carrier transport. Here, it is demonstrated that bipolaron formation can occur at both the early stage with low doping levels and the late stage with high doping levels in 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F₄TCNQ) dip-doped conjugated polymer films with glycol sidechains. Bipolaron formation at the early stage is discovered to be mainly associated with double doping, which is uncommon in conventional doped polymer systems. In contrast, bipolaron formation at the late stage is dominated by combining two polarons. Furthermore, these bipolarons are observed to behave differently: early-stage bipolarons generated through double doping enhance both the molecular ordering and carrier transport, whereas late-stage bipolarons resulting from polaron combination occur alongside detrimental effects in structural and transport properties. These findings provide new insights into the mechanisms of bipolaron formation across different doping levels and underscore the potential for optimizing doping strategies. A deeper understanding of bipolarons can guide the design of next-generation molecularly doped conjugated polymers with improved performance.