Density Functional Theory Studies On Ionization Energies,Electron Affinities,and Polarization Energies Of Organic Semiconductors

Organic semiconductor materials have received much attention and have become research focus recently,because of their significant advantages such as lightweight,good plasticity,flexible processing and low cost.An understanding of related mechanisms and design of novel organic semiconductor materials require the accurate prediction of energy levels(i.e.ionization potential and electronic affinity).From the perspective of theoretical calculations,a main challenge comes from a lack of reliable method which can not only provide qualitative but also quantitative prediction,accompanying with an acceptable computational cost.Herein,we develop an approach combining the polarizable continuum model(PCM)and an optimally-tuned range-separated(RS)functional,and demonstrate the approach provides ionization energies(IPs),electron affinities(EAs),and polarization energies of a series of molecular semiconductors in good agreement with the available experimental values.Importantly,the tuning method can enforce the calculated frontier molecular orbital energies(-εHOMO and-εLUMO)that are very close to the corresponding IPs and EAs.The success of this tuning method can be further attributed to the fact that the tuned functional can provide a good balance for the description of electronic localization and delocalization effects according to various molecular systems or the same molecule in different phases(i.e.gas and solid).As a comparison,other conventional functionals cannot give reliable prediction due to the functionals themselves including too low(i.e.PBE)or too high(i.e.M06HF and non-tuned RS functionals)HF%.Therefore,we believe that this PCM-tuned approach represents an easily applicable and computationally efficient theoretical tool to study the issues of energy levels of more complex organic electronic materials.The thesis is organized as follows:In Chapter 1,the development of theoretical calculation is simply introduced,and the concept and classification of organic semiconductor materials is reviewed.In addition,the necessity of theoretical calculation in predicting the IPs and EAs of organic semiconductor materials is demonstrated by analyzing the drawbacks in practical experimental measurements.In Chapter 2,an introduction to the development of density functional theory(DFT)is presented,including Thomas-Fermi model,Hohenberg-Kohn theorem,Kohn-Sham equations and other conventional functonals.Next we introduce the concept of an optimally-tuned range-separated(RS)functional approach,and the theoretical background of the method and its process of the development.In Chapter 3,the frontier molecular orbitals(-εHOMO and-εLUMO)and IPs and EAs of isolated molecule in gas phase and solid state were calculated.Next the mean absolute deviation(MAD)values were collected and the related error analysis were performed using various density functionals compared to the experimental values.We found the tuning method can provide reliable and excellent prediction while other conventional functionals possess different levels of deviations.In Chapter 4,the theoretical studies on the prediction of frontier molecular orbitals(-εHOMO and-εLUMO)IPs and EAs of organic semiconductor materials is summarized and the outlook of following research work is also mentioned.