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Title: Theoretical Studies on Conjugated Phenyl-Cored Thiophene Dendrimers for Photovoltaic Applications

Abstract

{pi}-Conjugated dendrimers are an important class of materials for optoelectronic devices, especially for light-harvesting systems. We report here a theoretical investigation of the optical response and of the excited-state properties of three-arm and four-arm phenyl-cored dendrimers for photovoltaic applications. A variety of theoretical methods are used and evaluated against each other to calculate vertical transition energies, absorption and excitation spectra with vibronic structure, charge transport, and excitonic behavior upon photoexcitation and photoemission processes. Photophysical phenomena in these dendrimers are, in general, better explained with ab initio methods rather than with semiempirical techniques. Calculated reorganization energies were found to correlate well with the device photocurrent data where available. The excitons formed during photoexcitation are calculated to be more delocalized than the ones formed after vibrational relaxation in the excited states for fluorescence emission. The localization of excitons in emission processes is a result of geometrical changes in the excited state coupled with vibronic modes. Correlated electron-hole pair diagrams illustrate breaking of {pi}-conjugation in three-arm dendrimers due to meta linkage of arms with the core, whereas four-arm dendrimers are not affected by such breaking due to presence of ortho and para branching. Yet, ortho branching causes large twist angles between the coremore » and the arms that are detrimental to {pi}-electron system delocalization over the structure.« less

Authors:
; ; ; ; ; ;
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
982261
DOE Contract Number:
AC36-08GO28308
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of the American Chemical Society; Journal Volume: 129; Journal Issue: 46, 2007
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 36 MATERIALS SCIENCE; ABSORPTION; ALKYL BENZENESULFONATES; ARMS; BEHAVIOR; CHARGE TRANSPORT; DATA; DIAGRAMS; EMISSION; EQUIPMENT; EXCITATION; EXCITED STATES; EXCITONS; FLUORESCENCE; MATERIALS; PHOTOCURRENTS; PHOTOEMISSION; RELAXATION; SPECTRA; THIOPHENE; WELLS; Computational Sciences; Materials Science and Semiconductors; Chemical and Biosciences

Citation Formats

Kose, M. E., Mitchell, W. J., Kopidakis, N., Chang, C. H., Shaheen, S. E., Kim, K., and Rumbles, G. Theoretical Studies on Conjugated Phenyl-Cored Thiophene Dendrimers for Photovoltaic Applications. United States: N. p., 2007. Web. doi:10.1021/ja073455y.
Kose, M. E., Mitchell, W. J., Kopidakis, N., Chang, C. H., Shaheen, S. E., Kim, K., & Rumbles, G. Theoretical Studies on Conjugated Phenyl-Cored Thiophene Dendrimers for Photovoltaic Applications. United States. doi:10.1021/ja073455y.
Kose, M. E., Mitchell, W. J., Kopidakis, N., Chang, C. H., Shaheen, S. E., Kim, K., and Rumbles, G. Mon . "Theoretical Studies on Conjugated Phenyl-Cored Thiophene Dendrimers for Photovoltaic Applications". United States. doi:10.1021/ja073455y.
@article{osti_982261,
title = {Theoretical Studies on Conjugated Phenyl-Cored Thiophene Dendrimers for Photovoltaic Applications},
author = {Kose, M. E. and Mitchell, W. J. and Kopidakis, N. and Chang, C. H. and Shaheen, S. E. and Kim, K. and Rumbles, G.},
abstractNote = {{pi}-Conjugated dendrimers are an important class of materials for optoelectronic devices, especially for light-harvesting systems. We report here a theoretical investigation of the optical response and of the excited-state properties of three-arm and four-arm phenyl-cored dendrimers for photovoltaic applications. A variety of theoretical methods are used and evaluated against each other to calculate vertical transition energies, absorption and excitation spectra with vibronic structure, charge transport, and excitonic behavior upon photoexcitation and photoemission processes. Photophysical phenomena in these dendrimers are, in general, better explained with ab initio methods rather than with semiempirical techniques. Calculated reorganization energies were found to correlate well with the device photocurrent data where available. The excitons formed during photoexcitation are calculated to be more delocalized than the ones formed after vibrational relaxation in the excited states for fluorescence emission. The localization of excitons in emission processes is a result of geometrical changes in the excited state coupled with vibronic modes. Correlated electron-hole pair diagrams illustrate breaking of {pi}-conjugation in three-arm dendrimers due to meta linkage of arms with the core, whereas four-arm dendrimers are not affected by such breaking due to presence of ortho and para branching. Yet, ortho branching causes large twist angles between the core and the arms that are detrimental to {pi}-electron system delocalization over the structure.},
doi = {10.1021/ja073455y},
journal = {Journal of the American Chemical Society},
number = 46, 2007,
volume = 129,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • No abstract prepared.
  • We have prepared two series of first-generation thiophene-bridge dendrimers, with either three (3G1) or four (4G1) arms attached to a phenyl core, to elucidate their structure-property relationships. Optical properties were investigated with a combination of steady-state and time-resolved spectroscopic techniques. Steady-state spectroscopic data for the 3-arm dendrimers suggests that the exciton is delocalized over the {alpha}-conjugated thiophene segment and the phenyl core, but that the meta-linking of the dendrons prevents their electronic communication. In contrast, conjugation through the core to dendrons in the ortho and para positions is permitted in the 4-arm dendrimers, although the data suggest that the conjugationmore » length does not extend over the full length of the {alpha}-conjugated sections of two coupled dendrons. This observation is due to steric interactions between neighboring arms, which forces the arms to twist and bend out of the plane of the phenyl core, and is particularly prevalent in disrupting the conjugation through the ortho positions. As expected, our results show that an increase in the bridge length results in an increase in the conjugation length for both dendrimers, and a subsequent red-shift of the absorption and emission. In addition, an increase in the dendron length results in an increase in the photoluminescence quantum yield and lifetime, suggesting that the ground and excited-state geometries are very similar and that the electronic transition is coupled to fewer vibrational modes.« less
  • Conjugated organic systems such as thiophene are interesting topics in the field of organic solar cells. We theoretically investigate π-conjugated polymers constituted by n units (n = 1–11) based on the thiophene (Tn) molecule. The computations of the geometries and electronic structures of these compounds are performed using the density functional theory (DFT) at the 6–31 G(d, p) level of theory and the Perdew–Burke–Eenzerhof (PBE) formulation of the generalized gradient approximation with periodic boundary conditions (PBCs) in one (1D) and two (2D) dimensions. Moreover, the electronic properties (HOCO, LUCO, E{sub gap}, V{sub oc}, and V{sub bi}) are determined from 1Dmore » and 2D PBC to understand the effect of the number of rings in polythiophene. The absorption properties—excitation energies (E{sub ex}), the maximal absorption wavelength (λ{sub max}), oscillator strengths, and light harvesting—efficiency are studied using the time-dependent DFT method. Our studies show that changing the number of thiophene units can effectively modulate the electronic and optical properties. On the other hand, our work demonstrates the efficiency of theoretical calculation in the PBCs.« less