Person:

Brédas, Jean-Luc

Permanent Link

https://hdl.handle.net/1853/71119

Associated Organization(s)

Organizational Unit

School of Chemistry and Biochemistry

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Now showing 1 - 10 of 35

Long-range corrected hybrid functionals for pi-conjugated systems: Dependence of the range-separation parameter on conjugation length

(Georgia Institute of Technology, 2011-11) Koerzdoerfer, Thomas ; Sears, John S. ; Sutton, Christopher ; Brédas, Jean-Luc

Long-range corrected hybrids represent an increasingly popular class of functionals for density functional theory (DFT) that have proven to be very successful for a wide range of chemical applications. In this Communication, we examine the performance of these functionals for time-dependent (TD)DFT descriptions of triplet excited states. Our results reveal that the triplet energies are particularly sensitive to the range-separation parameter; this sensitivity can be traced back to triplet instabilities in the ground state coming from the large effective amounts of Hartree-Fock exchange included in these functionals. As such, the use of standard long-range corrected functionals for the description of triplet states at the TDDFT level is not recommended.
Communication: Orbital instabilities and triplet states from time-dependent density functional theory and long-range corrected functionals

(Georgia Institute of Technology, 2011-10) Sears, John S. ; Koerzdoerfer, Thomas ; Zhang, Cai-Rong ; Brédas, Jean-Luc

Long-range corrected hybrids represent an increasingly popular class of functionals for density functional theory (DFT) that have proven to be very successful for a wide range of chemical applications. In this Communication, we examine the performance of these functionals for time-dependent (TD)DFT descriptions of triplet excited states. Our results reveal that the triplet energies are particularly sensitive to the range-separation parameter; this sensitivity can be traced back to triplet instabilities in the ground state coming from the large effective amounts of Hartree-Fock exchange included in these functionals. As such, the use of standard long-range corrected functionals for the description of triplet states at the TDDFT level is not recommended.
Theoretical study of substitution effects on molecular reorganization energy in organic semiconductors

(Georgia Institute of Technology, 2011-09) Geng, Hua ; Niu, Yingli ; Peng, Qian ; Shuai, Zhigang ; Coropceanu, Veaceslav ; Brédas, Jean-Luc

Chemical substitutions are powerful molecular design tools to enhance the performance of organic semiconductors, for instance, to improve solubility, intermolecular stacking, or film quality. However, at the microscopic level, substitutions in general tend to increase the molecular reorganization energy and thus decrease the intrinsic charge-carrier mobility. Through density functional theory calculations, we elucidate strategies that could be followed to reduce the reorganization energy upon chemical substitution. Specific examples are given here for hole-transport materials including indolo-carbazoles and several triarylamine derivatives. Through decomposition of the total reorganization energy into the internal coordinate space, we are able to identify the molecular segment that provides the most important contributions to the reorganization energy. It is found that when substitution reduces (enhances) the amplitude of the relevant frontier molecular orbital in that segment, the total reorganization energy decreases (increases). In particular, chlorination at appropriate positions can significantly reduce the reorganization energy. Several other substituents are shown to play a similar role, to a greater or lesser extent.
An improved dynamic Monte Carlo model coupled with Poisson equation to simulate the performance of organic photovoltaic devices

(Georgia Institute of Technology, 2011-03) Meng, Lingyi ; Wang, Dong ; Li, Qikai ; Yi, Yuanping ; Brédas, Jean-Luc ; Shuai, Zhigang

We describe a new dynamic Monte Carlo model to simulate the operation of a polymer-blend solar cell; this model provides major improvements with respect to the one we developed earlier [J. Phys. Chem. B 114, 36 (2010)] by incorporating the Poisson equation and a charge thermoactivation mechanism. The advantage of the present approach is its capacity to deal with a nonuniform electrostatic potential that dynamically depends on the charge distribution. In this way, the unbalance in electron and hole mobilities and the space-charge induced potential distribution can be treated explicitly. Simulations reproduce well the experimental I-V curve in the dark and the open-circuit voltage under illumination of a polymer-blend solar cell. The dependence of the photovoltaic performance on the difference in electron and hole mobilities is discussed.
The nature of singlet excitons in oligoacene molecular crystals

(Georgia Institute of Technology, 2011-03) Yamagata, H. ; Norton, J. ; Hontz, E. ; Olivier, Y. ; Beljonne, David ; Brédas, Jean-Luc ; Silbey, R. J. ; Spano, F. C.

A theory for polarized absorption in crystalline oligoacenes is presented, which includes Frenkel exciton coupling, the coupling between Frenkel and charge-transfer (CT) excitons, and the coupling of all neutral and ionic excited states to the dominant ring-breathing vibrational mode. For tetracene, spectra calculated using all Frenkel couplings among the five lowest energy molecular singlet states predict a Davydov splitting (DS) of the lowest energy (0–0) vibronic band of only −32 cm ⁻¹, far smaller than the measured value of 631 cm ⁻¹ and of the wrong sign—a negative sign indicating that the polarizations of the lower and upper Davydov components are reversed from experiment. Inclusion of Frenkel-CT coupling dramatically improves the agreement with experiment, yielding a 0–0 DS of 601 cm ⁻¹ and a nearly quantitative reproduction of the relative spectral intensities of the 0–n vibronic components. Our analysis also shows that CT mixing increases with the size of the oligoacenes. We discuss the implications of these results on exciton dissociation and transport.
Simulations of the emission spectra of fac-tris(2-phenylpyridine) iridium and Duschinsky rotation effects using the Herman-Kluk semiclassical initial value representation method

(Georgia Institute of Technology, 2008-12-07) Wu, Yinghua ; Brédas, Jean-Luc

The phosphorescent emission spectra of fac-tris(2-phenylpyridine) iridium [fac-Ir(ppy)₃] due to the lowest triplet T1 and T2 states are simulated using the harmonic oscillator approximation for the S0, T₁, and T₂potential energy surfaces (PESs) and taking the Duschinsky rotation into account. The simulations involve the propagation of 177-dimensional wave packets on the coupled PES according to the Herman–Kluk (HK) semiclassical (SC) initial value representation (IVR) method. The HK SC-IVR method is employed because of its accuracy for the PES with mode mixing and its efficiency in dealing with coupled degrees of freedom for large systems. The simulated emission spectrum due to T₁ reproduces the structures of the emission spectra observed experimentally, while T₂ is found very unlikely to participate in the phosphorescent emission. Although the effect of the Duschinsky mode mixing is small for the T₁ state, neglecting it blueshifts the spectrum due to the T₂ state by 800 cm−1 and changes the relative intensities, indicating that the importance of the Duschinsky rotation is rather unpredictable and should not be overlooked. The present simulations demonstrate that the simple harmonic oscillator approximation combined with the Duschinsky rotation can adequately describe the photophysics of fac-Ir(ppy)₃ and that the HK SC-IVR method is a powerful tool in studies of this kind.
Electronic and vibronic interactions at weakly bound organic interfaces: The case of pentacene on graphite

(Georgia Institute of Technology, 2008-07) Paramonov, Pavel B. ; Coropceanu, Veaceslav ; Brédas, Jean-Luc

The electronic and vibronic processes at the interface between a pentacene monolayer and a graphite surface were characterized using a combination of density-functional theory (DFT) and dynamic vibronic coupling simulations. The electronic interactions were evaluated at the DFT level first between the highest occupied states of pentacene and the graphite surface, as well as among the pentacene molecules within a monolayer. The former are found to be ca. four times stronger than the latter for a parallel molecule/surface geometry. A dynamic vibronic model was used to analyze the interplay between the electronic and electron-vibration couplings and their effects on spectroscopic characteristics. The agreement between the simulated and experimental photoelectron spectra underlines the importance of weak electronic interactions on the vibronic coupling at the interface.
First-principles study of the geometric and electronic structure of Au₁₃ clusters: Importance of the prism motif

(Georgia Institute of Technology, 2008-04) Gruber, Mathis ; Heimel, Georg ; Romaner, Lorenz ; Brédas, Jean-Luc ; Zojer, Egbert

The geometric structure, symmetry, and spin of Au13 clusters are investigated in the framework of densityfunctional theory, with particular attention paid to the correlation among these properties. Several computational approaches are carefully tested on previously proposed cluster configurations. Complications and possible pitfalls in electronic-structure calculations on these systems are highlighted. Using molecular dynamics with quantum mechanically calculated forces, a set of favored high binding energy geometric structures, where a trigonal prism acts as the central building block, is discussed.
Optimizing organic photovoltaics using tailored heterojunctions: A photoinduced absorption study of oligothiophenes with low band gaps

(Georgia Institute of Technology, 2008-02) Schueppel, R. ; Schmidt, Karin ; Uhrich, C. ; Schulze, K. ; Wynands, D. ; Brier, E. ; Reinold, E. ; Bu, H.-B. ; Baeuerle, P. ; Maennig, B. ; Pfeiffer, M. ; Leo, K. ; Brédas, Jean-Luc

A power conversion efficiency of 3.4% with an open-circuit voltage of 1 V was recently demonstrated in a thin film solar cell utilizing fullerene C60 as acceptor and a new acceptor-substituted oligothiophene with an optical gap of 1.77 eV as donor [ K. Schulze et al. Adv. Mater. (Weinheim, Ger.) 18 2872 (2006)]. This prompted us to systematically study the energy- and electron transfer processes at the oligothiophene:fullerene heterojunction for a homologous series of these oligothiophenes. Cyclic voltammetry and ultraviolet photoelectron spectroscopy data show that the heterojunction is modified due to tuning of the highest occupied molecular orbital energy for different oligothiophene chain lengths, while the lowest unoccupied molecular orbital energy remains essentially fixed due to the presence of electron-withdrawing end groups (dicyanovinyl) attached to the oligothiophene. Use of photoinduced absorption (PA) allows the study of the electron transfer process at the heterojunction to C60. Quantum-chemical calculations performed at the density functional theory and/or time-dependent density functional theory level and cation absorption spectra of diluted DCVnT provide an unambiguous identification of the transitions observed in the PA spectra. Upon increasing the effective energy gap of the donor-acceptor pair by increasing the ionization energy of the donor, photoinduced electron transfer is eventually replaced with energy transfer, which alters the photovoltaic operation conditions. The optimum open-circuit voltage of a solar cell is thus a trade-off between efficient charge separation at the interface and maximized effective gap. It appears that the open-circuit voltages of 1.0–1.1 V in our solar cell devices have reached an optimum since higher voltages result in a loss in charge separation efficiency.
Experimental and theoretical study of temperature dependent exciton delocalization and relaxation in anthracene thin films

(Georgia Institute of Technology, 2008-02) Ahn, Tai-Sang ; Mueller, Astrid M. ; Al-Kaysi, Rabih O. ; Spano, Frank C. ; Norton, Joseph E. ; Beljonne, David ; Brédas, Jean-Luc ; Bardeen, Christopher J.

The spectroscopy of solid anthracene is examined both experimentally and theoretically. To avoid experimental complications such as self-absorption and polariton effects, ultrathin polycrystalline films deposited on transparent substrates are studied. To To separate the contributions from different emitting species, the emission is resolved in both time and wavelength. The spectroscopic data are interpreted in terms of a three-state kinetic model, where two excited states, a high energy state 1 and a low energy state 2, both contribute to the luminescence and are kinetically coupled. Using this model, we analyze the spectral lineshape, relative quantum yield, and relaxation rates as a function of temperature. For state 1, we find that the ratio of the 0-0 vibronic peak to the 0-1 peak is enhanced by roughly a factor of 3.5 at low temperature, while the quantum yield and decay rates also increase by a similar factor. These observations are explained using a theoretical model previously developed for herringbone polyacene crystals. The early-time emission lineshape is consistent with that expected for a linear aggregate corresponding to an edge-dislocation defect. The results of experiment and theory are quantitatively compared at different temperatures in order to estimate that the singlet exciton in our polycrystalline films is delocalized over about ten molecules. Within these domains, the exciton’s coherence length steadily increases as the temperature drops, until it reaches the limits of the domain, whereupon it saturates and remains constant as the temperature is lowered further. While the theoretical modeling correctly reproduces the temperature dependence of the fluorescence spectral lineshape, the decay of the singlet exciton appears to be determined by a trapping process that becomes more rapid as the temperature is lowered. This more rapid decay is consistent with accelerated trapping due to increased delocalization of the exciton at lower temperatures. These observations suggest that exciton coherence can play an important role in both radiative and nonradiative decay channels in these materials. Our results show that the spectroscopy of polyacene solids can be analyzed in a self-consistent fashion to obtain information about electronic delocalization and domain sizes.