Electronic Excited States Of Organic Molecule Systems Studied By Many-body Green's Function Theory

As one kind of important photoelectric conversion materials,organic photovoltaic cells materials have become a hot research topic around the world for a long time.However,organic photovoltaic cells are still far away from large-scale applications at present.Although a vast of experimental and theoretical studies have been performed on organic photovoltaic cells,there are still many controversies on the mechanism of photoelectric conversion,the factors that influence their conversion efficiency.There are still many open questions on how excitons diffuse in the donor,how electron-hole pairs dissociate at the donor/acceptor interface,how intermediate states such as charge-transfer excited states and triplet states regulate carriers separation and energy transfer and so on.Among them,linear? conjugated organic semiconductors play an important role in the electronics industry,solar cells and spintronic devices.In recent years,more and more experiments at home and abroad are devoted to the study of the electrical and optical properties of these organic solar cells materials.It is a necessary means to study these problems through theoretical calculation.However,as far as theoretical research is concerned,some problems exist in practical calculations,e.g.the models are too simple,the accuracy is low,etc.Recently,many-body Green's function method which including the GW method and Bethe-Salpeter equation has become an important theoretical tool to study excited-state problems.The development of methodology and software over the past thirty years has made the GW method and the Bethe-Salpeter equation better to solve the actual systems.Many-body Green's function method has exhibited great advantages than other first-principle approaches on both accuracy and efficiency for the study of electronic band,ionization potential,electron affinity and absorption spectra.They can be applied to a variety of systems,including crystals,metals,nanomaterials,chemical and biological systems,etc.We can study the electronic levels,excitonic levels,energy transfer and carriers separation in organic photovoltaic cells materials accurately.Many-body Green's function method has been an important method to calculate the optical and electrical properties of various materials.The aim of this study is to provide some accurate results for the electrical and optical properties of the organic solar cells material that can not be measured in the experiment;We can obtain the exact band structure,spectral properties,electronic excited states,charge transfer and energy transfer properties of organic photovoltaic cells materials through the calculation;We can learn the excited-state dynamics process at the interface in organic photovoltaic cells such as that formed by pentacene and fullerene using first-principles many-body Green's function method,and explore how the interface configurations influence excited-state levels and carriers separation;We hope we can provide some theoretical guidelines for the preparation of organic photovoltaic cell materials.The main contents and conclusions of this study are as follows:(1)Sexithiophene is a kind of organic semiconductor material with good optical and electrical properties.Although many researchers devote to study the electrical and spectral properties of oligothiophene,there are still many controversies,such as the Davydov splitting and charge transfer.The varieties of theoretical calculations are needed to solve these controversies.At present,the progress of the theoretical calculation of the first principle is still limited in this respect.We studied the band structure,absorption spectra and exciton distribution of sexithiophene molecule and crystal with many-body Green's function method.We find that for the gas-phase molecule,different optimization methods can lead to a large difference in the structure of sexithiophene molecule.Then the excitation energies of different structures are very different.In the sexithiophene crystal,the excitons of the first singlet state and the lowest triplet state are localized.With the increase of energies,the charge transfer state appears.(2)We developed a methodology within many-body Green's function method to calculate the singlet and triplet energies of acene molecules and crystals accurately.This is very helpful for future theoretical research on acenes based solar cells and other optoelectronic devices.Singlet fission is considered to be a promising technique to improve the power conversion efficiency of organic solar cells.There is still much controversy on the mechanism of singlet fission.Accurate determination of the singlet and triplet energies is the prerequisite to uncover the mechanism.In this work,we found the significant effects of the resonant-antiresonant transitions coupling on the electron-hole interaction and the large errors of Tamm-Dancoff approximation in the singlet and triplet energies of acene molecules.We also explored the sensitivity of singlet and triplet energies on the theoretical approaches for the optimization of ground-state structures.The possibility for singlet fission to occur in acene molecules and crystals is also discussed.Based on these,we provided a methodology for future research on acenes based solar cells and other optoelectronic devices.(3)Forster-Dexter theory is the fundamental method to study excitation energy transfer from a donor to an acceptor.Many-body Green's function theory is a state-of-the-art ab initio method to compute electronic excitations.In this work,we combine these two methods together to develop a new scheme which can be used to study excitation energy transfer in both isolated and periodic systems.Although excitation energy transfer has been studied by the Forster-Dexter theory for several decades,excitation energy transfer in the periodic system has never been really studied from first principle yet.Our work is a very useful extension of the Forster-Dexter theory.Our method has been tested through calculations on some molecular dimers,exhibiting excellent agreement with other high-level quantum chemistry approaches and the somewhat exact supermolecule scheme.Calculations on the periodic carbon nanotubes reveal some new phenomena that are distinct from the general views on excitation energy transfer,such as the much more important role of dark states in excitation energy transfer than the bright states,and the much faster decay(?R-12)of excitation energy transfer rate with respect to the inter-nanotube distance(R)than expected according to the Forster model(?R-6).(4)Recently,one of the most interesting solar cells is based on pentacene and fullerene heterojunctions which have demonstrated remarkable achievement.The solar cells consist of two constituents,an electron-rich donor and an electron-deficient acceptor.Though increasing research interests have been focused on organic solar cells,the study is still under debate.For example,what is the energy of the charge-transfer states in organic photovoltaic devices?What is the mechanism of the charge carriers transfer process?How are the singlet and triplet excitons transferred to the acceptor?Are the triplet excitons transferred in pairs or independently?How long time are the singlet and triplet excitons transferred from donor to acceptor?We studied the electronic excitation and spectra of pentacene-fullerene heterojunction by GW+BSE method.With the increase of pentacene layers,the energy gaps of the complexes decrease.