Theoretical Calculation Of Charge Transfer And Energy Level Structure Of Organic Small Molecule Semiconductor

In recent years,organic small molecule semiconductor materials have been widely studied by researchers due to their many advantages of customizability,mechanical flexibility,low cost,and large-area fabrication.It is crucial to understand their internal charge transport and energy level structure deeply for designing new materials and highperformance devices.However,the charge transport mechanism affected by intermolecular interactions in organic small molecule semiconductor materials is not well studied yet,and the research on the trap states and energy level structures of minority carriers in the materials is not clear enough.Therefore,this thesis has carried out in-depth research and analysis on the charge transport and energy level structure of organic small molecule semiconductor materials from the perspective of theoretical calculations combined with some experimental analysis.The specific research contents are as follows:1.Research on intermolecular interactions and bipolar transport properties in ternary organic cocrystal T2P1TC2Mobility within organic small molecule semiconductor materials is a measure of the material’s charge transport properties.We have used different computational methods to calculate the intermolecular interactions and the mobilities of electrons and holes for the ternary cocrystal T2P1TC2,respectively.The results show that the ratio of the average mobilities of electrons and holes calculated by the M06-2X functional is similar to the experiment,which are 7.73 cm2 V-1 s-1 and 2.12 cm2 V-1 s-1,respectively.And we find that there are synergistic effects of π-π interactions,C-H…π interactions and C-H…N interactions in the T2P1TC2 cocrystal,resulting in the existence of many charge transport channels in the material.Some channels can facilitate electron transport,and some channels can facilitate hole transport.Since the arrangement of molecular pairs in the T2P1TC2 cocrystal is more close to the b-axis,the mobility along the b-axis is significantly higher than that along the other directions for both electron transport and hole transport,so that there is obvious anisotropic transport behavior in the T2P1TC2 cocrystal.In addition,the mobility mainly depends on the transfer integral and the reorganization energy.Through the calculation and analysis of the transfer integral and the reorganization energy,we find that the transfer integral plays a major role in determining the mobility in the T2P1TC2 cocrystal.2.Research on trap states and energy level structures in organic small molecule semiconductors Dif-TES-ADTThe presence of minority carrier traps in organic field effect transistors can seriously lead to photo-induced instability of the devices.We have performed structural optimization of the solvent model and vertical excited state calculations for the organic small molecule semiconductor material Dif-TES-ADT in a series of systems with water and oxygen solvents.Through the analysis of the energy level structure and orbital distribution of different systems,we found that oxygen molecules are the fundamental cause of the minority carrier traps.The oxygen molecules can adsorb electrons and drop the LUMO of the system from-2.13 eV for Dif-TES-ADT single molecule to-2.66 eV.And the water molecules play an important role in promoting the formation of the minority carrier traps,which induce oxygen molecules to OH radicals that further adsorb electrons,dropping the LUMO of the system to-2.74 eV.We effectively removed the minority carrier traps and guarantee the photostability of organic field effect transistors well by adding molecular additive TCNQ to solve this problem.The TCNQ molecule not only has a strong adsorption to electrons,but also makes the LUMO of the system drop to-4.19 eV,which is close to the HOMO of Dif-TES-ADT single molecule of-5.11 eV,so the electrons on the LUMO of TCNQ molecule can compound with the holes on the HOMO of Dif-TES-ADT single molecule.We also eliminate the effect of solvent molecules on minority carrier traps through the calculations for different solvent systems under the same conditions by replacing different solvent molecules.This thesis provides a systematic study of the charge transport properties and energy level structures in organic small molecule semiconductors based on density functional theory calculations,providing a theoretical guidance and basic support for designing high mobility organic field effect transistors and ensuring their photostability.