Understanding how Lewis acids dope organic semiconductors: a "complex" story.

Pablo Simón Marqués,Brett Yurash,Thuc-Quyen Nguyen,David Beljonne,Seth R Marder,Giacomo Londi,Stephen Barlow

doi:10.1039/d1sc01268a

Abstract

We report on computational studies of the potential of three borane Lewis acids (LAs) (B(C6F5)3 (BCF), BF3, and BBr3) to form stable adducts and/or to generate positive polarons with three different semiconducting π-conjugated polymers (PFPT, PCPDTPT and PCPDTBT). Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations based on range-separated hybrid (RSH) functionals provide insight into changes in the electronic structure and optical properties upon adduct formation between LAs and the two polymers containing pyridine moieties, PFPT and PCPDTPT, unravelling the complex interplay between partial hybridization, charge transfer and changes in the polymer backbone conformation. We then assess the potential of BCF to induce p-doping in PCPDTBT, which does not contain pyridine groups, by computing the energetics of various reaction mechanisms proposed in the literature. We find that reaction of BCF(OH2) to form protonated PCPDTBT and [BCF(OH)]−, followed by electron transfer from a pristine to a protonated PCPDTBT chain is highly endergonic, and thus unlikely at low doping concentration. The theoretical and experimental data can, however, be reconciled if one considers the formation of [BCF(OH)BCF]− or [BCF(OH)(OH2)BCF]− counterions rather than [BCF(OH)]− and invokes subsequent reactions resulting in the elimination of H2.

Highlights

Molecular doping[1,2,3,4,5,6] is a paramount topic in the organic semiconductor community, where it can enhance charge-carrier density and electrical conductivity, improve charge injection and lower contact resistance, or increase charge mobility thanks by lling traps
Gas-phase Lewis Acid–Base (LAB) adduct binding energies were estimated for the three Lewis acids (LAs) as total energy differences between the adduct coordinated with a LA and the sum of the isolated neat oligomer and LA molecule
We modelled the interactions between three boron-based LAs and different semiconducting p-conjugated polymers, performing detailed quantum-chemical calculations of the structural, energetics and optical signatures for ground-state LAB adducts between LAs and either PFPT or PCPDTPT

Summary

Introduction

Molecular doping[1,2,3,4,5,6] is a paramount topic in the organic semiconductor community, where it can enhance charge-carrier density and electrical conductivity, improve charge injection and lower contact resistance, or increase charge mobility thanks by lling traps. Depending on the nature of the semiconducting polymers, LAs either effectively act as p-dopants or form Lewis Acid–Base (LAB) adducts.[7] The aim of this computational study is to give insight into these two types of reactivity. The formation of a new stable covalent bond yields a LAB adduct with a speci c ngerprint in optical absorption[9] and increased charge carrier density with respect to the unbound polymer,[10,11,12] representing a means of postsynthetic engineering.[13] More speci cally, alternating donor– acceptor conjugated copolymers, where the acceptor moiety is pyridylthiadiazole (PT), are able to strongly coordinate LAs, such as BCF, likely resulting in partial ground-state charge transfer (CT). The interaction with BCF has been shown to translate into a red-shi ed onset in optical absorption of the organic semiconductor by $0.3 eV, a shi primarily due to the effect of the electron-withdrawing LA moiety on the electron affinity in presence of the LA itself.[13]

Methods

Results

Conclusion