Theoretical Study And Design For P-type To N-type Or Ambipolar Transition Of Organic Semiconductor Materials

With simple low-temperature processing technology and good mechanical flexibility,organic semiconductors provide an ideal solution for realizing low-cost and large-area flexible electronic devices,and also provide a necessary means to enrich the diversity of materials.However,the development of electronic transport(n-type)lags behind that of hole transport(p-type)materials,so improving the electron transport properties is the key to develop and enrich n-type and ambipolar materials.In this paper,we mainly study the microscopic factors that affect the n-type transport properties and improve them to obtain high-performance n-type and ambipolar materials.Some n-type materials were studied by using full quantum charge transfer theory and dynamic Monte Carlo simulation.To find the structural characteristics conducive to the performance improvement of n-type materials,which provides a useful reference for the design of high-performance n-type and ambipolar materials.The p-type materials with herringbone stacking were used as the parent and introducing the substituents to modify them,and then the n-type or ambipolar material with parallel?-?stacking were obtained.We used USPEX program to predict the crystal structure of the constructed molecules,and the symmetry adapted perturbation theory and electrostatic potential analysis were used to reveal the main reason for the change of packing motif from herringbone to?-?stacking.It provides a new idea for the design of high performance ambipolar or n-type materials with?-?stacking.The major results of our thesis are as follows:1.The effect of backbone connection and dicyanomethylene on the charge transport properties--Theoretical study on the charge transport properties of dicyanomethylene quinoidal thiophene derivativesDicyanomethylene quinoidal thiophene derivatives have attracted much attention as a promising n-type organic semiconductor material due to their high planar and low H-L gap.However,the relationship between the molecular structure,the types and the properties of carrier transport is still unclear.It is very important for the development of n-type and ambipolar materials to research the structural factors(the introduction of dicyanomethylene and the connection type of backbone)that are most conductive to the improvement of electron transport properties.Three series of quinoidal thiophene derivatives,oligothiophene,thienothiophene and benzothiophene,are systematically investigated by employing the full quantum charge transfer theory combined with kinetic Monte-Carlo simulation.The results expounded that the introduction of dicyanomethylene not only significantly decreases the electron reorganization energy,resulting in a decrease in local electron-phonon coupling,but also modulates the molecular polarization orientation and shifts the HOMO and LUMO energy levels so that decrease the H-L gaps,and brings low electron injection barriers,allowing the molecules become n-type candidate.More importantly,the regulation of noncovalent intermolecular interactions by introducing dicyanomethylene was revealed.The effect is bidirectional,which significantly reduces the electrostatic and enhances the dispersive percentage.In addition,our theoretical results demonstrate that quinoidal oligothiophene derivatives(n=3?5)with more thiophene rings will have ambipolar transport properties,quinoidal thienothiophene and benzothiophene derivatives should be promising alternatives as n-type organic semiconductor.When we focused only on the electronic transport properties in the three series of molecules,quinoidal benzothiophene derivatives are better than the other two series.2.The effect of substituent modification on solid stacking--The roles of heteroatoms and substituents on the molecular packing motif from herringbone to?-stacking:A theoretical study on electronic structures and intermolecular interaction of Pentacene derivativesMolecular stacking motifs in solid play a pivotal role on the charge transport properties of materials.The relationship between molecular structure and packing motifs in solid remains challenging.In the present work,the single crystal structures of PEN-O,PEN-N,PEN-CF₃ and TPDO were predicted by evolutionary algorithm using the USPEX program.The packing motifs,the electronic structures and the stabilities of pentacene(PEN)derivatives are systematically investigated by employing density functional theory.The results show that the introduction of trifluoromethyl,oxygen and nitrogen atoms alone can not effectively reduce the LUMO energy level.Only by introducing them simultaneously can obtain n-type materials candidate.In addition,the electrostatic potential analyzed and energy decomposition are used to reveal the effects of heteroatoms and substituents on charge distribution and noncovalent intermolecular interactions.Through introducing heteroatoms and substituents into PEN,the charge redistribution leads to non-uniform electrostatic potentials of conjugated molecular skeleton and the ratio of total attraction energy to total repulsive energy changed,which lead to the change of packing motif from herringbone to?-stacking.3.The cooperated effect of substituents on solid stacking and charge transfer types--Rational molecular manipulation on tuning the packing motif and the carrier transport of Dinaphtho-Thieno-Thiophene(DNTT)-based materialMolecular modification plays an important role in tuning the packing motif and charge transport in organic semiconductor materials.Especially,the electron-withdrawing substituents and functional heteroatoms have seen a recent surge of interest.Here,we modeled four crystal structures of dinaphtho-thieno-thiophene(DNTT)derivatives with the trifluoromethyl(–CF₃)and heteroatoms(O-and N-atoms),and elaborately delineated the impact of intermolecular interaction to establish the relationship between microscopic molecular structures and macroscopic solid-state packing.Then,the effect of–CF₃,O-,and N-atoms positions on charge transport properties are systematically investigated by multiscale theoretical simulations.The results show that the reorganization energy and frontier molecular orbital energy levels are sensitive to the position of O-and N-atoms than–CF₃.Significantly,the position of O-and N-atoms in benzene ring is changed from the central to the terminal can realize the transition from unipolar to ambipolar material after introducing–CF₃.Meanwhile,the cooperated effect of–CF₃,O-,and N-atoms can transform the molecular packing from herringbone-stacking to?-stacking.The introduction of–CF₃ in trans-position and O-,and N-atoms in the terminal benzene ring,simultaneously,can bring the most compact packing and more hydrogen bonds.Besides,the transfer integral fluctuation caused by position of–CF₃ is more intense than that of O-and N-atoms,which is caused by the long-and short-axis sliding motion that as a“killer”phonon mode.Based on DNTT,one of excellent ambipolar material candidate,Trans-2,has been found,which needs to be verified by experiments.Our work shows that proper substituents engineering on p-type might simultaneously realize the change of molecular packing and carrier transport,paving the way toward designing higher-performance specific ambipolar transport materials.