Electronic structure of conducting organic polymers: insights from time‐dependent density functional theory

Abstract

Cataloged from PDF version of article.Conducting organic polymers (COPs) became an active field of research after it was discovered how thin films rather than insoluble infusible powders can be produced. The combination of the properties of plastics with those of semiconductors opened the research field of organic electronics. COPs share many electronic properties with inorganic semiconductors, but there are also major differences, e. g., the nature of the charge carriers and the amount of the exciton binding energy. Theoretical analysis has been used to interpret experimental observations early on. The polaron model that was developed from one-electron theories is still the most widely used concept. In the 1990s, time-dependent density functional theory (TDDFT) became available for routine calculations. Using TDDFT, electronic states of long oligomers can be calculated. Now UV spectra of neutral and oxidized or reduced species can be compared with in situ UV spectra recorded during doping. Likewise states of cations can be used to model photoelectron spectra. Analysis of states has resolved several puzzles which cannot be understood with the polaron model, e. g., the origin of the dual absorption band of green polymers and the origin of a 'vestigial neutral band' upon doping of long oligomers. DFT calculations also established that defect localization is not crucial for spectral changes observed during doping and that there are no bound bipolarons in COPs. (C) 2014 John Wiley & Sons, Ltd

Spontaneous Charge Carrier Localization in Extended One-Dimensional Systems

“…Charge localization in conjugated systems can occur in several ways: Attachment by point defects [9], lattice disorder effects [5,10], and formation of self-bound charged polarons and neutral solitons by local distortion of the nuclear lattice [11][12][13][14]. However, it still remains an open question whether localization can occur in disorderfree transitionally invariant systems.…”

“…We focus on two representative conjugated polymers, trans-polyacetylene (tPA) and polythiophene (PT), with increasing lengths L = M 1 up to M = 70 and M = 20, respectively ( 1 is the length of the repeat unit). Besides their practical significance [6], they also exhibit interesting physical phenomena, in which polarons, bipolarons and solitons affect charge mobility and localization [4,12,[20][21][22].…”

Molecular modeling of low bandgap diblock co-oligomers with π-bridges for applications in photovoltaics

“…In fact, there is a multitude of functionals available in the literature. However, in the case of oligomers and polymers it is known that their performance are quite dependent of the property under consideration, which suggests the necessity of employing distinct approaches to reproduce the structural, electrical and optical properties of these systems in a appropriate way [30][31][32]29,[33][34][35][36]. Since in the present report we are more interested in evaluating tendencies than providing precise electronic parameters, the DFT/B3LYP approach have been chosen because, in average, it provides a reasonable description of both optical and electronic properties of non-extended oligomeric structures [29,34,36], being then appropriated to our systems.…”

Modeling angle-resolved photoemission of graphene and black phosphorus nano structures

“…This process was necessary because DFT calculation suffers from the approximation of electronic relaxation, and from correlation effects, so it cannot estimate energy levels to accurate absolute values. 15 If comparable experimental data were not available, all energies were shifted so that the midpoint between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) was equivalent to zero binding energy.…”