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

Abstract

The ability of opposite charges to escape their mutual coulombic attraction in molecular films has long been a puzzle in the field of organic electronics. In media with dielectric constants of ε r = 2-3, point charges, even initially separated by a generous 10 Å, would be bound with energies of 0.72-0.48 eV, respectively, which is many times the thermal energy available at room temperature (k B T = 0.026 eV). In nonpolar solutions, ion concentrations would have to drop to <pm-nm for entropy to start favoring free ions over ion pairs. Yet, in both organic photovoltaics (OPVs) and chemically doped conjugated polymer films, charges do escape, as evidenced by efficient OPVs [1] and highly conductive doped films. [2] There are many theories to explain this escape, although a common aspect in both cases is the delocalized The electron acceptor F4TCNQ p-dopes aggregates "nanowires" of poly(3hexylthiophene) in nonpolar solvents but does not dope unaggregated chains. The standard free energy change for the charge transfer to form an ion pair is ΔG°e t = -0.21 eV. The dissociation constant to produce free ions in toluene by DC conductivity is K°d = 1 × 10 -8 ± 50% (ΔG°d = 0.48 ± 0.05 eV). This remarkably large K°d, for ions in such a low polarity medium, may reflect interchain delocalization of the hole. The particular characteristics of this material system enables determination of both ΔG°e t and ΔG°d, to find the overall free energy change from the two neutral species to completely separated ions in nonpolar media. It is endergonic by +0.27 ± 0.05 eV in contrast to -0.6 eV estimated from reported HOMO LUMO differences, illustrating the challenges that persist in determining such energetics. Steady state microwave conductivity experiments on doped aggregates confirm that holes in the aggregates cannot easily escape their dopant counterion, but at higher dopant concentrations, holes become mobile. These results provide insight into the mechanisms of charge separation involving intermolecularly delocalized charges in nonpolar media, an integral process in organic photovoltaic devices and doped molecular films.

High‐Efficiency Ion‐Exchange Doping of Conducting Polymers

“…It describes the efficiency of the ion‐exchange process at equilibrium in terms of the concentration of each ion in solution and the selectivity coefficient. Interestingly, when = 1, Equation () is equivalent to the Langmuir isotherm, which was previously found to describe the charge‐transfer equilibrium, k ct , in P3HT:F4TCNQ films [ 2,33–35 ] (Section , Supporting Information).…”

Tunable Dopants with Intrinsic Counterion Separation Reveal the Effects of Electron Affinity on Dopant Intercalation and Free Carrier Production in Sequentially Doped Conjugated Polymer Films

“…Indeed, only the high‐redox‐potential DDB‐F 72 is able to dope P3HT aggregates in solution, as described in more detail in Section S8, Supporting Information. [ 77 ] Thus, the higher the redox potential of the dopant, the more ability to create carriers in different regions of a semicrystalline P3HT film. When this oxidizing power is combined with sequestration of the anion yielding minimal Coulomb trapping, it is not surprising that 100% doping efficiency can be achieved.…”

Overcoming Coulomb Interaction Improves Free-Charge Generation and Thermoelectric Properties for n-Doped Conjugated Polymers

“…Typically, conjugated polymers have low dielectric constants (ε r ≈ 3), 15 which are expected to lead to binding energies of the CTC on the order of several hundred millielectron-volts. 14,16 Such strong Coulomb interaction reduces the number of free charges and subsequently the doping efficiency. 14 Moreover, as noted by several theoretical studies, the counterions are likely to broaden the density of states (DOS) and act as Coulomb traps to reduce the carrier mobility.…”