Controlling the Formation of Charge Transfer Complexes in Chemically Doped Semiconducting Polymers

Abstract

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.

Microstructure and heteroatom dictate the doping mechanism and thermoelectric properties of poly(alkyl-chalcogenophenes)

“…demonstrated that a poor doping solvent leads to a contraction of P3HT lamellar lattice spacing, suggesting that F4TCNQ molecules coordinate within the  stacking region along the polymer backbone leading to CTC doping. 33 Thomas et al observed a similar effect in P3HT vs. P3EHT, where the branched alkyl sidechains in P3EHT create steric hindrance that inhibits F4TCNQ intercalation into the polymer side chains and promotes F4TCNQ intercalation between the π-stacks, and this was accompanied by a slight decrease of the alkyl stacking distance. 13 Therefore, we postulate that P3RT and P3RSe are doped via a CTC mechanism, evidenced by the contraction in alkyl stacking distance (shift in the alkyl stacking peak to higher Q) from 22.6 nm to 19.7 nm for P3RT, and 22.4 nm to 21.5 nm for P3RSe.…”

Multi-charge transfer from photodoped ITO nanocrystals

“…In addition, this compound is able to react through integer charge transfer and / or form charge transfer complexes. 46,47 The ionized species of F4TCNQ are also particularly reactive and can interact with each other or even dimerize. 48,49 All these transformations can generate new optically allowed transitions that "intervene" in the absorption with new peaks that may appear along with those already mentioned.…”

Single‐Solution Doping Enabling Dominant Integer Charge Transfer for Synergistically Improved Carrier Concentration and Mobility in Donor–Acceptor Polymers

“…[19] Moreover, the formation of CTC states induces trapped charge carriers, which further decreases the doping efficiency as well as the carrier mobility. [20][21][22][23][24] Thus, 2,3,5,6-tetrafluoro-tetracyanoquinodimethane (F4TCNQ)-doped poly(3-hexy-lthiophene-2,5-diyl) (P3HT) or poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno-[3,2-b]thiophene] (PBTTT) displays a relatively low σ of typically about 0.1-10 S cm −1 via SSD method. [9,[25][26][27] A higher σ could be obtained for thiophene-polymers by modifying side chains with volume freed for stronger dopants.…”