Person:

Atahan, Sule

Recently, an optimum mobility of (8.3 cm^2/(Vs)) has been measured for single-crystal organic field-effect transistors based on the dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]-thiophene (DNTT) molecule. Here, on the basis of quantum chemistry calculations and molecular dynamics simulations, we investigate the microscopic charge transport parameters of the DNTT molecule and crystal. Our findings confirm that the moderate anisotropy of the hole mobility in DNTT is highly dependent on the presence of in-plane herringbonelike intermolecular interactions with large electronic coupling (transfer integral) values (ca. 0.1 eV). Also, we demonstrate that the π-extended heteroaromatic structure leads to strong electronic coupling interactions among neighboring molecules and to a decrease of the intramolecular reorganization energy. In DNTT, thermal modulations of the electronic couplings at 300 K remain small when compared to those exhibited by the pentacene single crystal. This theoretical study suggests that heteroacenes are a promising route toward high-mobility organic semiconductor materials. Charge transport is discussed in the framework of both band and hopping models.

A multiple coherent states implementation of the semiclassical approximation is introduced and employed to obtain the power spectra with a few classical trajectories. The method is integrated with the time-averaging semiclassical initial value representation to successfully reproduce anharmonicity and Fermi resonance splittings at a level of accuracy comparable to semiclassical simulations of thousands of trajectories. The method is tested on two different model systems with analytical potentials and implemented in conjunction with the first-principles molecular dynamics scheme to obtain the power spectrum for the carbon dioxide molecule.

Circuits based on organic semiconductors are being actively explored for flexible, transparent and low-cost electronic applications. But to realize such applications, the charge carrier mobilities of solution-processed organic semiconductors must be improved. For inorganic semiconductors, a general method of increasing charge carrier mobility is to introduce strain within the crystal lattice. Here we describe a solution-processing technique for organic semiconductors in which lattice strain is used to increase charge carrier mobilities by introducing greater electron orbital overlap between the component molecules. For organic semiconductors, the spacing between cofacially stacked, conjugated backbones (the π–π stacking distance) greatly influences electron orbital overlap and therefore mobility. Using our method to incrementally introduce lattice strain, we alter the π–π stacking distance of 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) from 3.33Å to 3.08 Å. We believe that 3.08Å is the shortest π–π stacking distance that has been achieved in an organic semiconductor crystal lattice (although a π–π distance of 3.04Å has been achieved through intramolecular bonding). The positive charge carrier (hole) mobility in TIPS-pentacene transistors increased from (0.8 cm^2 V^{−1} s^{−1}) for unstrained films to a high mobility of (4.6 cm^2 V^{−1} s^{−1}) for a strained film. Using solution processing to modify molecular packing through lattice strain should aid the development of high-performance, low-cost organic semiconducting devices.

In this work, we explore the use of the semiclassical initial value representation (SC-IVR) method with first-principles electronic structure approaches to carry out classical molecular dynamics. The proposed approach can extract the vibrational power spectrum of carbon dioxide from a single trajectory providing numerical results that agree with experiment and quantum calculations. The computational demands of the method are comparable to those of classical single-trajectory calculations, while describing uniquely quantum features such as the zero-point energy and Fermi resonances. The method can also be used to identify symmetry properties of given vibrational peaks and investigate vibrational couplings by selected classical trajectories. The accuracy of the method degrades for the reproduction of anharmonic shifts for high-energy vibrational levels.