Vibrational Stark Effect Mapping of Polaron Delocalization in Chemically Doped Conjugated Polymers

Abstract

The nitrile vibrational modes of the 2,3,5,6-tetrafluoro-7,7,8,8tetracyanoquinodimethane (F 4 TCNQ) anion occur across a range of different frequencies in doped conjugated polymers. We show that these shifts can be understood as resulting from the vibrational Stark effect. F 4 TCNQ − serves as a sensitive vibrational Stark probe, and its nitrile stretches provide a direct readout of the locally experienced electric fields from the nearby polaron on the doped polymer backbone. The shift of the F 4 TCNQ − B 1u mode and broadening of the B 2u mode in doped poly(3-hexylthiophene-2,5-diyl) (P3HT) confirm that the dopant anion's long axis is oriented perpendicular to the P3HT backbone. The magnitude of the vibrational shifts prove that the anion−polaron distance in F 4 TCNQ − -doped P3HT films is ∼6 Å. We also show that the intrachain polaron coherence can be varied over a range spanning 6 to about 7.5 P3HT monomer units by controlling the local polymer order and crystallinity. At the highest degree of local order, polaron delocalization is limited by the minimum intrinsic strength of the anion−polaron Coulombic interaction. This work provides the first direct experimental measurement of polaron delocalization in doped conjugated polymer films and verifies theoretical models relating polaron coherence to the shape of the doped polymer's mid-IR electronic absorption spectrum.

Critical dopant concentrations govern integer and fractional charge-transfer phases in doped P3HT

“…We note, however, that the latter slightly shifts towards lower energy with increasing dopant loading which would indeed be in line with an additional bipolaron contribution. However, a bathochromic (hypsochromic) shift in the low-energy polaron peak has also been attributed to an increase (decrease) in the polaron coherence length [33,67,68], which we cannot assess from our present data, overall making an assignment of bipolaron features too speculative for our data.…”

“…Due to the exposed position of the C≡N bonds at the periphery of the dopants, their vibrations are highly sensitive to the environment [17,[30][31][32][33][34]. This includes strong influence of the countercharge location [30], of hydrogen transfer towards the dopant [17,32], and the vibrational Stark effect from varied degrees of polaron delocalization [33].…”

“…Due to the exposed position of the C≡N bonds at the periphery of the dopants, their vibrations are highly sensitive to the environment [17,[30][31][32][33][34]. This includes strong influence of the countercharge location [30], of hydrogen transfer towards the dopant [17,32], and the vibrational Stark effect from varied degrees of polaron delocalization [33]. Therefore, to deduce IPA, CPX, and neutral dopant content from the C≡N modes despite the convolution of these various influences, correlations with other experimental data such as optical absorption features must be taken into account.…”

“…With our calculations showing that the percentage of IPA-forming dopants concomitantly increases with σ, we can then find the initial σ required to start the aforementioned nucleation effect. Our comprehensive data from multiple experimental techniques allows us, further, to assess the validity of using shifts of the F4TCNQ C≡N stretching-modes in FTIR (see section 3.2) for deriving the degree of charge transfer, which is a common practice in the field but subject of debate [17,[30][31][32][33]. Thereby, we heed the initial advice from the seminal 1986 paper by Meneghetti and Pecile [34] that the ring C=C stretching mode would better serve that purpose-albeit often being masked by host-related peaks-as the C≡N modes are particularly sensitive to the molecular environment.…”

Molecular Dynamics Study of the Thermodynamics of Integer Charge Transfer vs Charge-Transfer Complex Formation in Doped Conjugated Polymers

“…We distributed the positive 0.5e charge on each neighboring polymer chain in a Gaussian manner with a full width (2σ) of six monomer units, with the charge on each monomer unit distributed equally among the heavy atoms of each thiophene ring. We note that our laboratory recently measured the intrachain coherence length of polarons in doped crystalline P3HT using the vibrational Stark effect, 60 and we chose the size of the polaron in our calculations to match what we experimentally measured.…”