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Center for Nonlinear Studies |
Structure and mechanisms of energy transduction in novel optical materials will be discussed with a focus on electro-optic polymers and Gratzel solar cell. Macroscopic order of dipolar chromophores in a polymer matrix in the presence of an external electric poling field is investigated to characterize the electro-optic (EO) coefficient. The theory explains the observed non-linear concentration dependence of the EO coefficient at high chromophore concentration by a phase transition in the dipolar system from para- to antiferroelectric state. Slab shaped samples that are common in practice are least efficient for the poling process. Sample elongation in the direction of the poling field radically increases the maximum value of the EO coefficient. The theory is applied to chromophores that are typical of the EO materials leading to a fine agreement with experiment. [Phys. Rev. E 62, 8324 (2000); Chem. Phys. Lett. 240 328-335 (2001); Phys. Rev. B 65 052104 (2002); Adv. Materials 14 597 (2002); J. Chem. Phys. 117 3354 (2002); Chem. Phys. Lett. 373 207-212 (2003)] A non-adiabatic molecular dynamics (NAMD) simulation of the photoinduced electron transfer (ET) in the Gratzel solar cell from molecular chromophores to TiO2 will be discussed. The electronic structure and adiabatic dynamics are simulated by ab initio density functional theory, while the NA effects are incorporated by the quantum-classical mean-field approach. The ET occurs on a 30 fs time scale by the NA mechanism at low temperatures and within 5 fs by the adiabatic mechanism at room temperature, in agreement with the recent ultrafast experimental data. The electron acceptor states are localized within the first 3-4 layers of the surface with about 20% of the acceptor state density due to a single Ti atom. The simulation predicts a complex non-single-exponential time dependence of the ET process and suggests a mechanism for increasing the solar cell voltage [J. Phys. Chem. B 106 8047 (2002) p.8047; Isr. J. Chem. 42 213 (2003); J. Mol. Struct.-Theochem 630 33 (2003); Adv. Mater. in press].