Yuanzuo Li

Insight into external electric field dependent photoinduced intermolecular charge transport in BHJ solar cell materials

External electric field dependent photoinduced intermolecular charge transfer in BHJ solar cell materials was theoretically investigated, including the influence of the external electric field on the electronic coupling, the reorganization energy and free energy change of the electron transfer. The theoretical approach not only promotes a deeper understanding of the connection between the external electric field, chemical structure and optical and electronic properties of the donor⋯acceptor system, but also can be developed for the rational design of a novel conjugated system for organic solar cells.

Density functional theory study on Herzberg–Teller contribution in Raman scattering from 4-aminothiophenol-metal complex and metal-4-aminothiophenol-metal junction

Density functional theory (DFT) and time-dependent DFT calculations have been performed to investigate the Raman scattering spectra of metal-molecule complex and metal-molecule-metal junction architectures interconnected with 4-aminothiophenol (PATP) molecule. The simulated profiles of normal Raman scattering (NRS) spectra for the two complexes (Ag(2)-PATP and PATP-Au(2)) and the two junctions (Ag(2)-PATP-Au(2) and Au(2)-PATP-Ag(2)) are similar to each other, but exhibit obviously different Raman intensities. Due to the lager static polarizabilities of the two junctions, which directly influence the ground state chemical enhancement in NRS spectra, the calculated normal Raman intensities of them are stronger than those of two complexes by the factor of 10(2). We calculate preresonance Raman scattering (RRS) spectra with incident light at 1064 nm, which is much lower than the S(1) electronic transition energy of complexes and junctions. Ag(2)-PATP-Au(2) and Au(2)-PATP-Ag(2) junctions yield higher Raman intensities than those of Ag(2)-PATP and PATP-Au(2) complexes, especially for b(2) modes. This effect is mainly attributed to charge transfer (CT) between the metal gap and the PAPT molecule which results in the occurrence of CT resonance enhancement. The calculated pre-RRS spectra strongly depend on the electronic transition state produced by new structures. With excitation at 514.5 nm, the calculated pre-RRS spectra of two complexes and two junctions are stronger than those of with excitation at 1064 nm. A charge difference densities methodology has been used to visually describe chemical enhancement mechanism of RRS spectrum. This methodology aims at visualizing intermolecular CT which provides direct evidence of the Herzberg-Teller mechanism.

Photoinduced Electron Transfer in Organic Solar Cells.

Electron transfer (ET) is the key process in light-driven charge separation reactions in organic solar cells. The current review summarizes the progress in theoretical modelling of ET in these materials. First we give an account of ET, with a description originating from Marcus theory. We systematically go through all the relevant parameters and show how they depend on different material properties, and discuss the consequences such dependencies have for the performance of the devices. Finally, we present a set of visualization methods which have proven to be very useful in analyzing the elementary processes in absorption and charge separation events. Such visualization tools help us to understand the properties of the photochemical and photobiological systems in solar cells.

Local and Remote Charge-Transfer-Enhanced Raman Scattering on One-Dimensional Transition-Metal Oxides

The one-dimensional (1D) transition-metal oxide MoO(3) belt is synthesized and characterized with X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. Charge-transfer-(CT) enhanced Raman scattering of 4-mercaptobenzoic acid (4-MBA) on a 1D MoO(3) belt was investigated experimentally and theoretically. The chemical enhancement of surface-enhanced Raman scattering (SERS) of 4-MBA on the MoO(3) belt by CT is in the order of 10(3). The SERS of 4-MBA was investigated theoretically by using a quantum chemical method. The remote SERS of 4-MBA along the 1D MoO(3) belt (the light excitation to one side of the MoO(3) belt, and the SERS spectrum is collected on the other side of the MoO(3) belt) is also shown experimentally, which provides potential applications of SERS. The incident polarization dependence of remote SERS spectra has also been investigated experimentally.

Electronic Structure and Optical Properties of Dianionic and Dicationic pi-Dimers

The absorption spectra of dianionic tetrocyanoethylene and dicationic tetrathiafulvalene dimers have been studied theoretically with the time-dependent density functional theory and the recently proposed Coulomb-attenuated model. The nature of the excited states was further explored by means of the two-dimensional (2D) site (transition density matrix) and three-dimensional (3D) cube (transition density and charge difference density) representations. By use of the 3D transition density and charge difference density, we visualized the orientation of transition dipole moment and also explained charge-transfer characteristics occurring in the dianionic/dicationic pi-dimers system. It is found that for the dianionic/dicationic pi-dimers system there exist two kinds of charge-transfer patterns for the mainly excited states, the intermolecular charge transfer and the mixture of intramolecular charge transfer coupled with intermolecular charge transfer. Meanwhile, the coupling effect of excitation and the oscillation of electron-hole pairs between the monomers have been revealed with 2D site representation of transition density matrix, which also indicates the electron-hole coherence upon photon excitation.

Fullerene-Based Photoactive Layers for Heterojunction Solar Cells: Structure, Absorption Spectra and Charge Transfer Process

The electronic structure and optical absorption spectra of polymer APFO3, [70]PCBM/APFO3 and [60]PCBM/APFO3, were studied with density functional theory (DFT), and the vertical excitation energies were calculated within the framework of the time-dependent DFT (TD-DFT). Visualized charge difference density analysis can be used to label the charge density redistribution for individual fullerene and fullerene/polymer complexes. The results of current work indicate that there is a difference between [60]PCBM and [70]PCBM, and a new charge transfer process is observed. Meanwhile, for the fullerene/polymer complex, all calculations of the twenty excited states were analyzed to reveal all possible charge transfer processes in depth. We also estimated the electronic coupling matrix, reorganization and Gibbs free energy to further calculate the rates of the charge transfer and the recombination. Our results give a clear picture of the structure, absorption spectra, charge transfer (CT) process and its influencing factors, and provide a theoretical guideline for designing further photoactive layers of solar cells.

Effect of aqueous and ambient atmospheric environments on plasmon-driven selective reduction reactions

We successfully realised plasmon-driven selective reduction reactions of 2-amino-5-nitrobenzenethiol (2A-5-NBT) to 3,3’-dimercapto-4,4’-diaminoazobenzene , an azobenzene derivative, using surface-enhanced Raman scattering (SERS) spectroscopy, and supported by the theoretical calculations. The SERS spectra demonstrated that two 5-nitro groups of 2A-5-NBTs were selectively reduced to the –N=N– chemical bond of 3,3’-dimercapto-4,4’-diaminoazobenzene, whereas the 2-amine group of 2A-5-NBT remained unchanged. Our experimental results revealed that aqueous environments were preferable to ambient atmospheric environments for this selective reduction reaction. The product is very stable in aqueous environments. However, in ambient atmosphere environments, the product is not stable and can revert back to 2A-5-NBT, where the –N=N– chemical bond can be broken by plasmon scissors. The plasmon-induced catalytic reactions in aqueous environments could be used for the efficient synthesis of aromatic azobenzene derivative compounds, which are valuable chemicals that are widely used in the chemical industry as dyes, food additives and drugs.

DFT study of adsorption site effect on surface-enhanced Raman scattering of neutral and charged pyridine-Ag4 complexes.

Density functional theory (DFT) and time-dependent DFT (TDDFT) methods have been used to investigate the adsorption site effect of Raman scattering for neutral and charged pyridine-Ag4 complexes. The calculated results show that the SERS spectra are strongly dependent on adsorption site and the configuration of new complexes. The normal Raman spectra of neutral and charged pyridine-Ag4 complexes are similar with that of isolated pyridine but with an enhancement factor below 10 times. This enhancement is ascribed to ground state chemical enhancement. The pre-surface-enhanced Raman scattering (SERS) spectra were calculated at 1256 nm, 769 nm and 744.3 nm, which are nearly resonant with the charge transfer excited states S2 for neutral and charged pyridine-Ag4 complexes, respectively. We obtain the enhancement factor about 10(4) to 10(5) in pre-SERS spectra which is mainly caused by charge transfer resonance Raman enhancement. The three-dimensional cube representation is also applied to describe the photoinduced CT, which are considered as direct evidence of chemical enhancement, between pyridine and two isomers of Ag4 clusters.

Selective plasmon-driven catalysis for para-nitroaniline in aqueous environments.

The plasmon-driven oxidation of amine (−NH2) groups and the reduction of nitro (−NO2) groups on a nanostructured metal surface in an aqueous environment have been reported experimentally and theoretically. The question of which process occurs first in the aqueous environment is an interesting question in the field of plasmon-related photochemistry. Para-nitroaniline (PNA), with both nitro (−NO2) and amine (−NH2) groups, is the best candidate for studying the priority of the plasmon-driven oxidation and the reduction reactions in an aqueous environment. Using surface-enhanced Raman scattering (SERS) spectroscopy, our experimental results and theoretical simulations reveal that PNA is selectively catalyzed to 4,4′-diaminoazobenzene (DAAB) through the plasmon-assisted dimerization of the nitro (−NO2) group into an azo group in an aqueous environment. This indicates that the plasmon-driven reduction of the nitro (−NO2) group clearly occurs before the oxidation of the amine (−NH2) group in an aqueous environment. The plasmon-driven reduction of PNA to DAAB is a selective surface catalytic reduced reaction in aqueous environment.

Effects of different functional groups on the optical and charge transport properties of copolymers for polymer solar cells

The ground state properties of polymers BSeTT, QTT, BDT-DTBTBPz (Pz), BDT-DTBTBQx (Qx) and their derivatives D1, D2, D3, including frontier molecular orbital, energy gaps, ionization potentials (IPs), electron affinities (EAs) and electron or hole reorganization energies, have been investigated with DFT-B3LYP/6-31G(d) method. Based on optimized structures, the absorption and fluorescence spectra of the seven oligomers are simulated with TD-DFT-Cam-B3LYP/6-31G(d) method. The external electric field is taken into account in view of the practical application. Moreover, developed visualized charge difference density is applied to present the charge density redistribution for the fullerene/polymer complexes. More deeply, the electronic coupling matrix, reorganization and Gibbs free energy are estimated to calculate the rates of the charge transfer and recombination for the fullerene/polymer complexes. Results show that the lengths of conjugated chains obviously affect the energy levels of HOMO and LUMO, and the ionization potentials and electron affinities. By evaluating the rates of the charge transfer and recombination for the fullerene/polymer complexes, reorganization energy and spectrum nature, it is found that the molecule BDT-DTBTBPz and designed molecule (D2) have the best optical and electronic properties among the investigated system, indicating that introducing appropriate functional group can effectively improve the photoelectric characteristics.