Person:
Brédas, Jean-Luc

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

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.

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An improved dynamic Monte Carlo model coupled with Poisson equation to simulate the performance of organic photovoltaic devices

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.

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Communication: Orbital instabilities and triplet states from time-dependent density functional theory and long-range corrected functionals

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.

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The nature of singlet excitons in oligoacene molecular crystals

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.

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Theoretical study of substitution effects on molecular reorganization energy in organic semiconductors

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.

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Quantum chemical modeling of electron transfer in amorphous organic films

2010-06-01 , Brédas, Jean-Luc , Li, Hong

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