| Organic semiconductors(OSCs)are widely used in various optoelectronic devices because of their low price,light weight,good flexibility,easy processing and high integration.Among them,the organic field effect transistor(OFET)is the core functional unit of organic electronic devices.At present,the preparation and application of OFET have achieved great success,but with the in-depth understanding of its application and large-scale production,new problems have gradually emerged in the process of device operation and application,such as low mobility,high power consumption caused by high device drive voltage,and device instability.These problems have become the Achilles’Heel of current organic small molecule semiconductor devices and the main obstacle restricting the further development and application of OFET.The solution of these problems involves the intrinsic properties of OSCs,such as molecular structure and intermolecular packing in crystals.Among the many methods to regulate the effect of molecular intrinsic properties on charge transport,it is noted that heteratom modification of dense ring aromatic compounds is an important and effective method to change the intrinsic properties of OSCs.The introduction of heteroatoms can not only regulate the electronic structure,but also regulate the packing mode by introducing different intermolecular interactions,which is a kind of multiple regulation means.In addition to the nature of the molecule itself,some external factors,such as electric field,morphology,etc.,have not yet had a clear guiding principle for the regulatory mechanism of charge transport properties,and the lack of understanding of the regulation of charge transport properties of OSCs has not yet attracted enough attention from the industry.Theoretical chemical calculation is an effective means to explore the charge transport properties of different materials from the molecular level.Using theoretical chemical calculation,we can systematically understand the influence of different factors on the charge transport properties of OSC materials from the source.Therefore,under the premise of heteroatom substitution,we use theoretical chemical calculation to analyze the regulation principle of electric field on heteroatom substitution of OSCs and the reasons for the formation of different crystal morphologies.In this paper,the typical organic molecular systems of benzobenzene,thiophene,biphenyl and bithiophene and N,O and S hybrid anthracene derivatives are selected as the research objects.Density functional theory and charge hopping transport model are used to study their intrinsic charge transport properties and the effect of applied electric field on their geometric and electronic structures and charge transport properties.In addition,the growth morphologies of different heteroatom-substituted anthracene derivatives are predicted by the adhesion energy model,and the effect of these morphologies on the charge transport properties is revealed.By comparing different heteroatom-substituted organic aromatic systems,the relationship between electric field and growth morphology and charge transport properties was explored,which provided valuable theoretical reference for a wide variety of OFET materials.The specific work content and main conclusions are summarized as follows:1.The regulation of external electric field(EEF)on charge transport properties when heteroatoms are located in the skeleton-Theoretical study on the regulation of charge transport properties of OSCs by EEFThe effect of EEF on the charge transport properties of the cohesive aromatic and non-cohesive aromatic systems and the cohesive thiophene and non-cohesive thiophene systems containing heteroatom S was investigated from the typical conjugated structures of acenes,oligothienoacenes,oligobenzenes,oligothiophenes.At the same time,the dimer model is used to study the different regulation ways of EEF on intermolecular electron couplings of different systems.The regularity between the"structure(electronic structure+geometric structur)-property"of OSC molecules regulated by EEF is sought.The results show that the longer theπ-conjugated molecular skeleton,the more sensitive the electronic structure to EEF.In addition,the geometry of non-cohesive systems(oligobenzenes and oligothiophenes)is more susceptible to EEF than that of cohesive aromatic conjugated oligomers(acenes and oligothienoacenes).The intrinsic mobility of OSCs is greatly affected by the EEF,which will lead to the mobility deviation of these common OSC molecules by 0.3 to 20 times,so it is a factor that cannot be ignored.At the same time,a heterogeneous structure-TH-BTz was designed basing on the above research.Like the oligothiophenes-series,the reorganization energy of TH-BTz can be significantly reduced under the EEF,while the change of frontier molecular orbital energy level can withstand high EEF regulation and always maintain good stability and small injection barrier,and finally,the mobility is effectively improved.2.The influence of crystal morphology on charge transport properties when heteroatoms are located at different positions of aryl substituents-Theoretical study on charge transport properties of pyridyl substituted anthracene derivativesThe star molecule of small OSC-2,6-diphenylanthracene(2,6-DPA)has both high luminescence and excellent charge transport properties,and its mobility is as high as 34 cm~2/V·s.However,the 2,6-dipyridinanthrene(2,6-DPy A)molecule replaced by heteroatom N,although the geometric and electronic structure of the molecule is very similar to that of 2,6-DPA,the experimental results indicate that its mobility is much lower than that of 2,6-DPA.Moreover,when the substitution position of N-atom on phenyl group is different,the properties also show obvious difference.The intrinsic charge transport properties and thermal disorder effects of three kinds of 2,6-DPy As and 2,6-DPA molecules were studied by using the full quantum formula that considering nuclear tunneling and molecular dynamics.The results show that the intrinsic mobility of 2,6-DPy A-b is better than that of 2,6-DPA molecule when N atom is in the meta-substitution.The results show that the intrinsic mobility of N atom is better than that of 2,6-DPA molecule when N atom is in the intermediate substitution(2,6-DPy A-b),and the thermal disorder effect is excluded as the reason for the low mobility observed by 2,6-DPy A-b.The subsequent theoretical prediction of crystal growth morphology(based on periodic bond chain(PBC)theory,considering the attachment energy of crystal system to predict crystal morphology)also proved that 2,6-DPy As has more growth crystal faces and poor crystallinity.This result highlights the importance of crystal morphology for charge transport property.3.The influence of crystal morphology on charge transport properties when heteroatoms are located on alkyl chains-Theoretical study on charge transport properties of substituted anthracene derivatives with different heteroatomic alkyl chainsAfter exploring the effect of heteroatom N substitution on the charge transport properties of 2,6-DPA molecules,the heteroatom substitution exploration on the alkyl chain substituents of 2,6-DPA was continued,and it was found that three structures,BEPAnt,BOPAnt,and BSPAnt,were obtained by modifying the phenyl group of2,6-DPA through O and S hybrid alkyl chains(ethyl,methoxyl,methyl sulfur)in the experiment.The morphology and charge transport properties of these three structures are different at different substrate temperatures.The intrinsic charge transport and photophysical properties of the three structures were investigated by using DFT and TDDFT theory.The calculated results were in agreement with the experimental measurements.The introduction of heteroatoms can significantly change the vibration mode and intensity of the low frequency region during charge transport and transfer.At the same time,the introduction of O/S atoms leads to the formation of O/S…H bond which makes the intermolecular packing close,but it will cause the intermolecular slip to different degrees,and then affect the charge transport rate.In addition,the growth morphology of crystals is predicted by the PBC theory combined with the AE model.Combined with the intermolecular interaction,the formation reasons of the crystal/film morphology at different substrate temperatures were analyzed,and the morphology prediction results perfectly reproduced the interfacial isotropic growth pattern of BEPAnt and BOPAnt crystals found in the experiment.In summary,a systematic theoretical study on the charge transport properties of heteroatom-replaced organic small molecule semiconductor system was carried out by using quantum chemistry calculation method.In addition to the changes in parameters related to the intrinsic mobility of molecular crystals brought about by chemical modifications(such as reorganization energy,electron coupling,etc.).Meanwhile,the important influence and role of electric field and crystal growth morphology on charge transport properties(mobility)are also fully considered.It is hoped that this work can provide new design ideas and theoretical references for promoting the development of novel and efficient multi-functional OSC materials. |
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