The impact of aggregation on the p-doping kinetics of poly(3-hexylthiophene)

Abstract

The morphological effects of regioregular poly(3-hexylthiophene) (P3HT) on its p-doping kinetics with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) in solution are studied using optical absorption spectroscopy and stopped-flow technique. Two morphological forms, solubilized (s-P3HT) and nanowhiskers (nw-P3HT), are investigated. Both P3HT solubilized and aggregated solutions show similar characteristic near-IR absorption bands for integer charge transfer products with F4-TCNQ. Kinetic analysis on p-doping of s-P3HT with F4-TCNQ indicates that the doping reaction proceeds with a single reaction mechanism that is first order in s-P3HT. The doping kinetics of P3HT aggregate solution shows two distinctive reaction mechanisms. The slow mechanism has a reaction rate constant similar to that of solubilized P3HT solution, so it likely results from s-P3HT components that are present in the aggregate solution. The fast one is assigned to the nw-P3HT component, probably due to more efficient charge delocalization in the aggregated P3HT nanostructures. Additionally, the kinetic trends of the p-doping reactions are better fitted with the consideration of a Gaussian-like distribution of reactivities from P3HT, matching the complexity of polymeric systems originating from molecular weight and morphology variations. This study highlights the importance of considering different morphological forms of conjugated polymers on their charge-transfer reaction kinetics. The knowledge gained here should be fundamentally and practically important for future chemical doping applications in organic electronic device fabrications.

Energetics and Escape of Interchain‐Delocalized Ion Pairs in Nonpolar Media

“…Given the relationship between conductivity, σ, charge density, n , and mobility, μ, (σ = neμ ), we obtain a value of μ AC = 0.0154 ± 0.00383 cm 2 V −1 s −1 from a straight line fit to the, above‐threshold, linear region of σ versus n in Figure b. This mobility is the isotropic value (average over three dimensions) although we expect charge motion to mostly confined to lamellae, being fastest along the chain (crystallographic c axis) or in the π–π stacking direction ( b axis), along the long axis of the nanowire; conductive atomic force microscopy studies[19a,22b] have shown the transport along the a axis to be three to four orders of magnitude slower.…”

“…This aggregation lowers the oxidation potential of the polymer, allowing for increased reactivity with dopants . Doping induces aggregation in P3HT both kinetically and thermodynamically, through increased planarity of the polymer cation, and increased hole delocalization, respectively . We seek to understand if P3HT aggregates enable unprecedented ion pair dissociation constants in nonpolar media.…”

“…Figure b shows that while the addition of F 4 TCNQ (0.03 × 10 −3 m ) to solutions of nw‐P3HT immediately led to absorptions known to be that of P3HT cations and F 4 TCNQ anions, there was no such absorption when F 4 TCNQ was added to solubilized chains (s‐P3HT), even at a higher concentration of 0.13 × 10 −3 m . Doping occurs on longer timescales,[22a] but initial or partial aggregate formation seems to be the rate limiting step. Further evidence for the lack of doping of single chains is provided in the Supporting Information.…”

“…We sought to make an independent estimate of the extinction coefficient of these ions as values found in the literature ranged from 31 000 m −1 cm −1 at 770 nm[22a] to 85 000 m −1 cm −1 at 816 nm . However, it was noticed that our estimates of extinction coefficient varied substantially with the concentration of nw‐P3HT, indicating an equilibrium.…”

High thermoelectric power factor from multilayer solution-processed organic films

“…Molecular doping is an effective way to increase the conductivity of organic semiconductors [8][9][10] . Over the years, P3HT doped by F4TCNQ has become the field's fruit fly system, and the doping mechanism and kinetics have been researched by several groups [11][12][13][14][15] . In conventional solution processing the doped layer is deposited from a common solution containing both the semiconductor (P3HT) and the dopant (F4TCNQ) -in the following we shall refer to this as bulk doping.…”