Investigation of charge carriers in doped thiophene oligomers through theoretical modeling of their UV/Vis spectra
Author
Salzner, U.
Date
2008Source Title
Journal of Physical Chemistry A
Print ISSN
1089-5639
Volume
112
Issue
24
Pages
5458 - 5466
Language
English
Type
ArticleItem Usage Stats
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Abstract
The nature of the charge carriers in conducting organic polymers (COPs) is a long standing problem. Polythiophene is one of the prototypes of COPs and intensively studied. Because doping leads to changes in UV/vis spectra that are characteristic of the absorbing species, UV/vis spectra of charged thiophene oligomers with up to 25 rings were calculated with time-dependent density functional theory. The credibility of the method was established by comparing the results with a variety of theoretical levels and with experiment. Effects due to counterions (Cl3-) and solvent (CH 2Cl2) were examined. It was found that TDDFT employing hybrid functionals is accurate enough to distinguish the absorbing species. The findings offer an explanation for the experimentally observed difference in UV-spectra of medium-sized and long oligomers upon doping. As chain lengths of the oligomers increase and energy levels get closer, configuration interaction leads to additional absorption peaks in the high energy sub-band region (at around 1.5-2.5 eV). Thus, long oligomers do not behave differently from medium-sized ones upon doping, only their spectra are different. At low doping levels radical cations (polarons) are produced. At higher doping levels, dications that harbor weakly interacting polaron pairs are formed. Bipolarons are predicted only on very short chains or at high doping levels. There is no bipolaron binding energy and disproportionation of monocations into dications and neutral species is energetically unfavorable. © 2008 American Chemical Society.
Keywords
ABS resinsCivil aviation
Conducting polymers
Density functional theory
Oligomers
Organic polymers
Polymers
Probability density function
Sulfur compounds
Thiophene
Conducting organic polymers
Counter-ions
Long standing
Poly-thiophene
Thiophene oligomers
Time-dependent density functional theory
UV/Vis spectra
Charge carriers