How intermolecular geometrical disorder affects the molecular doping of donor–acceptor copolymers

Abstract

Molecular doping of conjugated polymers represents an important strategy for improving organic electronic devices. However, the widely reported low efficiency of doping remains a crucial limitation to obtain high performance. Here we investigate how charge transfer between dopant and donor-acceptor copolymers is affected by the spatial arrangement of the dopant molecule with respect to the copolymer repeat unit. We p-dope a donor-acceptor copolymer and probe its charge-sensitive molecular vibrations in films by infrared spectroscopy. We find that, compared with a related homopolymer, a four times higher dopant/ polymer molar ratio is needed to observe signatures of charges. By DFT methods, we simulate the vibrational spectra, moving the dopant along the copolymer backbone and finding that efficient charge transfer occurs only when the dopant is close to the donor moiety. Our results show that the donor-acceptor structure poses an obstacle to efficient doping, with the acceptor moiety being inactive for p-type doping.

High Conductivity in Molecularly p‐Doped Diketopyrrolopyrrole‐Based Polymer: The Impact of a High Dopant Strength and Good Structural Order

“…This is in a sharp contrast with the CT behavior of the previously studied PCPDT-BT:F4TCNQ system for which almost no CT was observed when the dopant was positioned above the benzothiadiazole acceptor unit. [20] Our calculations reveal the interaction energy gain in 60-100 kJ mol −1 range, for different positions of the dopant relative to the polymer fragments ( Figure 6; Figure S14, Supporting Information). However quite surprisingly, the most stable complex is that one which exhibits the lowest CT degree with CN6-CP placed just above DPP (Complex I, Figure 6).…”

“…However, most of high-mobility DA copolymers, such as DPPbased ones, possess relatively low-lying HOMO levels (−5.3 eV and below) which complicates their oxidation by commercially available dopants having LUMOs with similar energy. [20] This may explain why only a modest success in the doping of the DA copolymers has been achieved to date. [21] Indeed, among the molecularly doped DA copolymers, PCPDTTBT:F4TCNQ displays the highest conductivity of 10 −2 S cm −1 , which is two orders of magnitude lower than the conductivity of a conventional P3HT:F4TCNQ system.…”

Electron Localization of Anions Probed by Nitrile Vibrations

“…9 Infrared (IR) spectroscopy has previously been used to discern charge transfer states by means of spectral shifts of specific vibrational bands such as (C=O) bands [10][11][12][13][14][15] and (C=C) bands. 16 In a similar 4 manner, we postulate that we can measure the degree of electron localization by spectral shifts of specific vibrational bands.…”

Modeling organic electronic materials: bridging length and time scales

“…DFT with Gaussian basis sets has been used to study chargetransfer states between dopants and OSCs providing a link between polymer length and the amount of charge transferred from the OSC molecule to the dopant [41,53]. Figure 2(left) demonstrates the effect of molecular geometry on charge transfer between quatro-thiophene and the dopant F4TCNQ.…”