Driving Force and Optical Signatures of Bipolaron Formation in Chemically Doped Conjugated Polymers

Abstract

Molecular dopants are often added to semiconducting polymers to improve electrical conductivity. However, the use of such dopants does not always produce mobile charge carriers. In this work, ultrafast spectroscopy is used to explore the nature of the carriers created following doping of conjugated push–pull polymers with both F4TCNQ (2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane) and FeCl3. It is shown that for one particular push–pull material, the charge carriers created by doping are entirely non‐conductive bipolarons and not single polarons, and that transient absorption spectroscopy following excitation in the infrared can readily distinguish the two types of charge carriers. Based on density functional theory calculations and experiments on multiple push–pull conjugated polymers, it is argued that the size of the donor push units determines the relative stabilities of polarons and bipolarons, with larger donor units stabilizing the bipolarons by providing more area for two charges to co‐reside.

Understanding Bipolarons in Conjugated Polymers Using a Multiparticle Holstein Approach

“…15,16,19,23,38 In donor−acceptor polymers, bipolaron stability has also been linked to the conjugation length of the donor units. 18 We further surmise that with very high dopant concentrations tripolarons and higher multipolarons are created due to the smoothing out of the Coulomb binding potential. For example, assuming periodic boundary conditions, a linear lattice of anions will exert uniform binding, independent of the position of the hole.…”

“…Moreover, a peak in the mid-IR spectrum at 0.8 eV in push− pull polymers was also attributed to bipolarons based on pump-probe transient absorption measurements. 18 We are currently investigating whether the 0.8 eV shoulder in P3HT might be related to higher-energy bipolaron absorption signatures. Thus far, our theoretical approach is limited to transitions from bipolarons to higher-energy bipolaronstransitions among the states represented by eq 11, which are responsible for mid-IR absorption.…”

“…Upon doping, polymers like polythiophene (PT), with nondegenerate ground states, are believed to support polarons and bipolarons as the major charge carriers . Early studies supported bipolarons as the primary charge-storage state, − although more recent studies show that polarons also contribute, depending in a complex manner on the doping mechanism (i.e., chemical vs electrochemical), the nature of the dopant, and the morphology of the polymer film. − …”

Li+ and Oxidant Addition To Control Ionic and Electronic Conduction in Ionic Liquid-Functionalized Conjugated Polymers

“…These optical signatures indicate that mobile electronic charge carriers are induced in the P3HT-IM sample upon the addition of NOBF4 oxidant. 66,67 Grazing-incidence wide-angle X-ray scattering (GIWAXS) studies indicate that the addition of lithium salt to the polymer causes some disruption to the crystallinity, however, the polymer still retains crystalline order. (Figure 2b).…”

Microscopic picture of molecular double doping

“…For example further details of the charge formation such as formation of individual polarons and/or bi-polarons upon double-doping are of significant relevance for the transport properties of the resulting doped system. [17][18][19][20][21][22] In this work, we investigate a simultaneous charge transfer between two p(g 4 2T-TT) conjugated polymer (oligomer) chains in a close arrangement and a single CN6-CP p-type dopant molecule as illustrated in Figure 1(a). Our study is firmly based on density functional theory (DFT) calculations taking into account inter-molecular van-der Waals interactions and dielectric environment.…”