Controlling the Formation of Charge Transfer Complexes in Chemically Doped Semiconducting Polymers
journal contributionposted on 05.03.2021, 19:09 by Dane A. Stanfield, Yutong Wu, Sarah H. Tolbert, Benjamin J. Schwartz
Chemical doping of semiconducting polymers predominantly takes place via integer charge transfer (ICT), where an electron is entirely removed from the host conjugated polymer and transferred to reside on the dopant guest species. In contrast, chemical doping of small conjugated molecules and oligomers often leads to the formation of charge transfer complexes (CTCs), which have significant orbital overlap and shared electron density between the host and guest species. To date, the observation of fractional charge transfer in doped conjugated polymers is relatively rare, occurring only under extreme processing conditions that can be difficult to achieve, which is fortunate given that CTC formation generally yields fewer mobile carriers per dopant. In this work, we use the classic conjugated polymer/dopant pair of P3HT and F4TCNQ to demonstrate how simply adjusting the casting solvent for the dopant in sequential processing can fundamentally alter the nature of doping in this well-studied system, leading to tunable production of CTCs. Using solvent blends of dichloromethane and chloroform, selected for their low and high solubility toward P3HT, respectively, we show that the relative amount of polymer-dopant CTCs can be readily controlled over an order of magnitude. Increasing the amount of chloroform in the dopant solvent blend favors the creation of CTCs, while increasing the dichloromethane content results in doping by the more standard ICT; the results allow us to explain why CTC formation is common in charge-transfer salts but generally less so in doped conjugated polymers. We also explore the role of the doping method and the crystallinity of P3HT films in controlling the relative amounts of ICT and CTC formation. We find that the use of evaporation doping and higher-crystallinity material discourages CTC formation, but that even in the most favorable case of evaporation doping with high polymer crystallinity, fractional charge transfer always occurs to some extent. Finally, we show that brief thermal annealing can convert CTCs to integer charge transfer species, indicating that ICT is the thermodynamically preferred doping mechanism in conjugated polymers, and that fractional charge transfer is the result of kinetic trapping. With this understanding, we offer guidelines for limiting the occurrence of charge transfer complexes during sequential doping of conjugated polymers, thus avoiding the deleterious effects of CTCs on charge transport.