Theoretical Investigation On Charge Carrier Mobility Of Heterocycles Aromatics As N-type Organic Semiconductors

The organic semiconductor devices have some advantages like simple preparation,low cost,low consumption and flexible in comparison with the inorganic semiconductor devices.Recently,the research on the application of organic electronic materials such as organic semiconductor,organic solar device,organic light-emitting device,organic sensor and organic memory device has updated quickly and shown a promising potential application.Organic semiconductor can be divided into p-type and n-type organic semiconductors,corresponding to the electron transport and hole transport respectively.They construct the organic complementary circuit.However,the performance of the organic semiconductors,especially the n-type ones cannot exceed the inorganic semiconductors yet.Thus,searching for the n-type organic semiconductors with high performance is a big problem to overcome.High performance n-type organic semiconductor should meet the requirements of highπ-conjugation,high electron affinity,large overlap of electron wave function between adjacent molecules.This paper adopted the density functional theory（DFT）combined with Marcus-Hush hopping model to explore the charge transport properties of fused aromatics with electron withdraw substitution as organic semiconductor candidates.Our work mainly includes the following three parts:1.The charge transport properties of perylene diimide（PDI）and its fluorinated derivatives were explored by density functional theory（DFT）coupled with the incoherent charge-hopping model.The geometric structure,reorganization energy,frontier molecule orbital,electron affinity,ionization potential,transfer integral as well as the anisotropic mobility were discussed.By attaching fluorine atoms to the bay region of PDI,the p-type material converts to the n-type or ambipolar ones（DF-PDI and TF-PDI）.The electron mobility of DF-PDI(0.33 cm2·V^-1·s^-1)is much larger than its corresponding hole mobility(0.0008 cm2·V^-1·s^-1)due to its lower LUMO energy and more efficient pack-stacking,hence it could be a candidate of n-type organic semiconductor.The introduction of strong electron-withdrawing substituents（such as fluorine）to the perylene-based organic semiconductor materials is a promising strategy for the high-performance n-type organic semiconductors.Besides,these three molecules exhibit remarkable anisotropic behaviors.Both the hole and electron maximal mobilities occur in the parallel π-π stacking dimers.2.DFT method combined the charge hopping model was adopted to calculate the charge transport properties of DPTTA and the derivations,the relationship between substitutions and charge transport mobility was clarified at molecular and crystal levels.The research has found that,the delocalized and low-lying LUMO and large EA are in favor of the stability when used as the n-type organic semiconductor.The transfer integral and reorganization have crucial effects to the charge transport mobility.Among these three compounds,DPTTACH3 possesses the largest mobility(0.18 cm2·V^-1·s^-1)due to its small reorganization energy and large transfer integral deriving from the face to face π-π arrangement.From the perspective of the transfer integral,the transfer integral of dominant hopping path indicates that the charge transport occurred between dimers with n-n interaction.As for the study of mobility anisotropy,we found that the charge transport mobility of DPTTACH3 shows the remarkable angle resolution anisotropy.3.The relationship between molecular geometries,crystal structures and charge mobilities of HTP and its three derivatives（2Se-HTP,4Se-HTP,6Se-HTP）were studied and discussed with DFT theory combined with hopping mechanism in the molecular and crystal level.The effect of Se substituent number on charge mobility was also be discussed.The calculated result showed that the derivatives exhibit good planarity and the molecular geometries have little variation in charge transfer process.The electron mobility is 1.20 cm2·V^-1·s^-1 for HTP and 2.30 cm2·V^-1·s^-1 for 6Se-HTP,respectively,much more larger than corresponding hole ones,which indicates that HTP and 6Se-HTP are good candidates for n-type materials.While 2Se-HTP and 4Se-HTP have comparative hole and electron mobilities,are suitable for ambipolar materials.