Person:
Brédas, Jean-Luc

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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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Three-photon absorption in anthracene-porphyrin-anthracene triads: A quantum-chemical study

2004-12 , Zhu, Lingyun , Yang, Xia , Yi, Yuanping , Xuan, Pengfei , Shuai, Zhigang , Chen, Dezhan , Zojer, Egbert , Brédas, Jean-Luc , Beljonne, David

We have applied correlated quantum-chemical methods to investigate the three-photon absorption (3PA) response of a porphyrin triad derivative, where the central macrocycle is linked in mesopositions to two anthracene units via acetylenic bridges. The 3PA frequency-dependent spectrum of this derivative is dominated by a single resonance feature in the transparent region, associated with charge-transfer states between porphyrin and anthracene. The calculations indicate a two order of magnitude enhancement in the 3PA cross section in the triad molecule with respect to the individual entities, which is attributed to close one-, two-, and three-photon resonances together with strong electronic couplings among the units.

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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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Optical properties of singly charged conjugated oligomers: A coupled-cluster equation of motion study

2004-09 , Ye, A. , Shuai, Zhigang , Kwon, O. , Brédas, Jean-Luc , Beljonne, David

We have implemented a coupled-cluster equation of motion approach combined with the intermediate neglect of differential overlap parametrization and applied it to study the excited states and optical absorptions in positively and negatively charged conjugated oligomers. The method is found to be both reliable and efficient. The theoretical results are in very good agreement with experiments and confirm that there appear two subgap absorption peaks upon polaron formation. Interestingly, the relative intensities of the polaron-induced subgap absorptions can be related to the extent of the lattice geometry relaxations.

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Structure-property relationships for three-photon absorption in stilbene-based dipolar and quadrupolar chromophores

2006-07 , Zhu, Lingyun , Yi, Yuanping , Shuai, Zhigang , Brédas, Jean-Luc , Beljonne, David , Zojer, Egbert

Based on essential-state models for three-photon absorption (3PA), we have investigated the structure-property relationships for stilbene-based dipolar and quadrupolar chromophores. The emphasis lies on the evolution of the 3PA cross section with the degree of ground-state polarization. For dipolar systems, we find a dominant role played by Δμ, which expresses the change in dipole moment between the ground state and the 3PA active excited state. Thus, the strategies usually applied to maximize the second-order polarizability β are also applicable to optimize the 3PA cross section. For quadrupolar systems, the 3PA response is dominated by contributions from channels including various low-lying two-photon allowed states, which limits the applicability of essential-state models. Optimization strategies can be proposed but vary for different ranges of ground-state polarization.

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Singlet-triplet splitting of geminate electron-hole pairs in conjugated polymers

2004-08 , Kadashchuk, A. , Vakhnin, A. , Blonski, I. , Beljonne, David , Shuai, Zhigang , Arkhipov, V. I. , Heremans, P. , Emelianova, E. V. , Bassler, H. , Brédas, Jean-Luc

The singlet-triplet splitting of geminate polaron pairs in a ladder-type conjugated polymer has been studied by the thermally stimulated luminescence technique. The energy gap separating the singlet and triplet states of the geminate pairs is measured to be in the range of 3–6 meV, depending on the polymer morphology. The results of correlated quantum-chemical calculations on a long ladder-type oligomer are fully consistent with the observed values of the geminate polaron pair singlet-triplet gap. Such low splitting values have important implications for the spin-dependent exciton formation in conjugated polymers.

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