Quantitative Analysis of Doping-Induced Polarons and Charge-Transfer Complexes of Poly(3-hexylthiophene) in Solution

Abstract

The mechanism and the nature of the species formed by molecular doping of the model polymer poly(3hexylthiophene) (P3HT) in its regioregular (rre-) and regiorandom (rra-) forms in solution are investigated for three different dopants: the prototypical π-electron acceptor 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ), the strong Lewis acid tris(pentafluorophenyl)borane (BCF), and the strongly oxidizing complex molybdenum tris[1-(methoxycarbonyl)-2-(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd-CO 2 Me) 3 ). In a combined optical and electron paramagnetic resonance study, we show that the doping of rreP3HT in solution occurs by integer charge transfer, resulting in formation of P3HT radical cations (polarons) for all of the dopants considered here. Remarkably, despite the different chemical nature of the dopants and dopant−polymer interaction, the formed polarons exhibit essentially identical optical absorption spectra. The situation is very different for the doping of rraP3HT, where we observe formation of a charge-transfer complex with F 4 TCNQ and of a "localized" P3HT polaron on nonaggregated chains upon doping with BCF, while there is no indication of dopant-induced species in the case of Mo(tfd-CO 2 Me) 3 . We estimate the ionization efficiency of the respective dopants for the two polymers in solution and report the molar extinction coefficient spectra of the three different species. Finally, we observe increased spin delocalization in regioregular compared to regiorandom P3HT by electron nuclear double resonance, suggesting that the ability of the charge to delocalize on aggregates of planarized polymer backbones plays a significant role in determining the doping mechanism.

Understanding how Lewis acids dope organic semiconductors: a “complex” story

“…However, although the structureless feature is consistent with that expected for the "protonated radical", it could also in principle arise from dynamic effects leading to loss of the structure expected for the polaron signal, or even from other radicals formed through side reactions. We also reckon that, as observed elsewhere in the literature, 24,[49][50][51] the polymer conjugation length plays a paramount role in the context of molecular doping, since different mechanisms might occur depending on the extension of the polymer backbone. To address this point, Table S6 in ESI ‡ reports the computed DH 0 elec values pertaining to Scheme 1 and Scheme 2, using either a PCPDTBT tetramer or an octamer as representative model.…”

“…In a recent study by Arvind et al 24 on P3HT, EPR measurements performed on BCF-doped samples revealed the formation of free radical cations on the polymer backbone, yet showing no indication for the presence of another radical species (i.e., associated with the "protonated radical"). If BCF doping of PCPDTBT proceeds in analogous fashion to that proposed for the BCF-induced doping of P3HT by Arvind et al the overall reaction would be that shown in Scheme 2:…”

“…Our calculations demonstrate that the observed red-shied optical absorption in the adducts results from a complex interplay between hybridization, partial CT and changes in the polymer conformation. In assessing the potential of BCF to induce molecular doping in PCPDTBT based on calculated Gibbs free energies of different proposed reactions, we came to the conclusions that both the overall processes proposed by Yurash et al 16 and by Arvind et al 24 are highly endergonic, mostly because of the thermodynamically unfavourable protonation by BCF(OH 2 ). Reconciling theory with experiment requires considering complexation of the [BCF(OH)] À with another BCF or BCF(OH 2 ) moiety to form more stable anions of the stoichiometry and structure observed crystallographically by Doerrer and Green; 14 these offer a dramatic reduction in the DG 0 penalty for forming the protonated intermediates.…”

“…Instead, adding BCF to a PCPDTBT based lm leads to an increase in electrical conductivity and to the formation of positive polarons, i.e., molecular pdoping. 16,24,48 As in the mechanism proposed by Doerrer and Green for oxidation of metallocenes, 14 Yurash et al suggested that the rst step of this p-doping was the protonation by the highly Brønsted acidic complex BCF(OH 2 ) of the CPDT moiety of the polymer backbone. 16 They further proposed that protonation would increase the EA sufficiently that a nearby neutral chain segment would be able to transfer an electron to the (positively charged) protonated segment (with the segments belonging either to the same or different physical polymer chains, if the process is intrachain or interchain, respectively).…”

“…However, in a later work on p-doping of poly(3-hexylthiophene) (P3HT), Arvind et al could observe only the radical cation using high-resolution electron paramagnetic resonance (EPR) spectroscopy, suggesting either the "protonated radical" does not form or that it is unstable against further chemical reactions. 24 In particular, H 2 elimination, as previously invoked in the contexts of both metallocene oxidation by BCF(OH 2 ) and spiro-OMeTAD p-doping by HN(SO 2 CF 3 ) 2 (another strong Brønsted acid), 25 has been suggested to play a paramount role, but to our knowledge formation of H2 has yet to be observed directly.…”