| In the last decades, organic photoelectron materials have been widely used in microelectronic devices for several obvious advantages, such as lightweight, low-cost, flexible adjustion of properties, and convenient large-area fabrication. Numerous theoretical and experimental investigations have demonstrated clearly that the charge carrier transport effeciency is the most important factor that determines the performance of photoelectron devices. Hence, it is very significant to theoretically investigate the charge carrier mobility of organic photoelectron materials for designing and developing new organic semiconductor materials with some special functions. In this dissertation, based upon the systematical study on the the molecular design and charge carrier mobility for several newly-synthesized small molecules containing sulfur and nitrogen heteroatoms, we have analyzed in detail the influences of molecular geometries (such as substituents), electronic properties (such as frontier molecular orbital levels, ionization potentials, and electron affinities), and molecular packing patterns in solid phases (such as crystal structures) on the charge carrier mobility at the molecular and crystal levels. In addition, the substituent effects on the light absorption and emission properties of these compounds have also been discussed. Our study may provide several valuable references for designing and synthesizing new photoelectronic materials with the high carrier mobility and air stability. The whole dissertation mainly includes the following four sections,1. Based on anthra[2,3-c]thiophene unit (AcTH), a set of5,10-disubstituted anthra [2,3-c]thiophene derivatives have been designed, and their molecular structures, electronic properties, optical stability, inner reorganization energies, and hole mobilities have been investigated. Our calculations reveal that cyano group substitution and ethinylation, which can enhance the molecular rigidity of parent compound, are efficient strategies designing organic semiconductor materials with the low inner reorganization energy. In addition, upon the simple one dimensional charge transport model and semiempirical Marcus-Hush electron transfer theory, the hole mobilities of AcTH, DCHC-AcTH, and DCN-AcTH have been estimated and compared with that of Pentacene. The result shows that these three compounds have higher hole mobilities under the same condition than Pentacene, which indicates AcTH, DCHC-AcTH, and DCN-AcTH may be promising p-channel OFET materials and worthy of being studied further in experiments.2. The molecular geometries, electronic properties, spectral properties, and electron mobilities have been studied for7,8,15,16-tetraazaterrylene (TAT) and its three tetrasubstituted derivates with the electron-withdrawing groups (-F,-Cl,-CN). Our calculation shows that the introduction of strong electron-withdrawing groups can remarkably lower the frontier molecular orbital energy levels and the electron injection potential barrier, and enhance the oxido-reduction stability. More important, introducing these groups can also increase the hole and electron transfer integrals in the dominant charge transfer channel, and then the electron transport ability. Especially,4CN-TAT possesses a quite large adiabatic electron affinity of3.599eV, and so it is very stability as n-channel OFET materials exposed to water and oxygen. Lastly, upon the quantum-corrected Marcus-Levich-Jortner (MLJ) rate model coupled with the random-walk simulation of diffusion coefficient and the Einstein equation, the electron mobility for TAT molecular crystal is predicted to be as high as3.404×10-2cm2·V-1·s-1, which suggestes that TAT crystal may be a promising n-channel OFET material and is worthy of being studied further in experiments. In addition, the simulation for the spectral properties indicates that the strongest absorption and emission peaks red shift with the introduction of electron-withdrawing groups and these peaks are dominated by the transition between HOMO and LUMO.3. Based on two newly-synthesized small molecular compounds with the dicyanovinyl group, including BTMN and BCMN, the hole and electron mobilities have been theoretically investigated with the quantum-corrected Marcus-Levich-Jortner (MLJ) electron transfer rate formulation and the Einstein equation. The results show that the hole and electron mobilities at room temperature (T=300K) reach6.387×10-2cm2·V and1.936×10-2cm2·V-1·-s-1for BTMN crystal,2.404×10-1cm·V-1·-s-1and1.418×10-1'cm2·V-1·s-1for BCMN crystal. Our prediction reveals that BTMN and BCMN should be potential ambipolar transport OFET materials for their close hole and electron mobilities. More important, BCMN crystal displays large carrier mobilities, which are even more than the threshold value of0.1cm2·V-1·s-1, that is enough high for applying in practical OFET devices. Hence, BCMN is expected to be promising ambiploar transport materials and deeply be studied in experiments. In addition, the simulation for the light absorption and emission properties indicates that the strongest absorption and emission peaks are mainly dominated by the transition between HOMO-1and LUMO. Practically, the light absorption and emission in the studied compounds are the intramolecular electron transfer process induced by light between the fused ring and the dicyanovinyl group.4. In this section, the molecular structures, electronic properties, crystal structures, and electron transport parameters for four novel nitrogen-rich pentacene derivatives with two cyano groups (PBD1, PBD2, PBD3, and PBD4) have been investigated at the molecular and crystal levels by means of density functional theory (DFT) calculations coupled with the prediction of crystal structures and the incoherent charge-hopping model. Calculations reveal that the nitrogen doping and cyano group substitution can lower remarkably the HOMO and LUMO energy levels, and do not break the parent's planar structure, then which are viewed as the efficient strategies designing organic electron transport materials with the high air-stability. The prediction of crystal.--structures indicates that these compounds in crystals can stack the close face-to-face style with the short interplanar distance along the crystal axis direction. In addition, based on the crystal structures obtained with the molecular mechanics (MM) method coupled with the Marcus-Hush charge transfer model and the Einstein equation, the electron mobility of these molecular crystals have been studied. Our calculations show that these crystals may be potential n-channl OFET materials for their high electron mobility (0.518?1.052cm2·V-1·s-1), and worthy of being investigated further in experiments. Furthermore, we find the electron transport in these crystals shows remarkable anisotropic, and the maximum ?e value appears along a certain crystal axis direction. |
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