| Many-body Green's function theory(MBGFT)is a state-of-the-art and high-precision first-principles method,which includes GW method and Bethe-Salpeter equation(BSE).It can be used to calculate nonperiodic molecular systems as well as periodic systems.So far,it has been successfully used to study these properties(such as ionization energy,electron affinity energy,absorption spectra,excitation energy and excited-state lifetime)of crystals,clusters,inorganic/organic molecules and nanomaterials.In this paper,we investigate the excited-state properties of water clusters and periodic photovoltaic systems employing many-body Green's function theory.The depletion of fossil energy makes it urgent to find new alternative energy sources Water is the most common substance in our daily life.As a bridge between water monomer and condensed-phase water,the hydrogen bond network of water clusters determines that they have special properties which are different from condensed-phase water.It has always been a topic of great concern.At the same time,water photolysis under ultraviolet light can produce H·,OH·,H+,OH-,and eaq-which can react with other molecules.Hydrogen,another photolysis product of water,is an important clean energy resource.In addition,the inexhaustible solar energy is one of the alternative sources of fossil energy.Solar cell,which can convert solar energy into electricity,is a semiconductor device with photovoltaic effect.Exciton generation,exciton diffusion,exciton separation and exciton recombination are the main concerns of solar cells.Different solar cells have different exciton separation mechanism and charge transfer mechanism,which are closely related to the performance of solar cells,and has always been a problem to be clarified.In this paper,we use many-body Green's function theory to systematically investigate four typical systems.In the first chapter,the research background of water clusters,the structures and exciton transfer mechanism of two solar cells are introduced.In the second chapter,the theoretical framework of GW+BSE method and excited-state dynamics is described in detail.We systematically study the variation of the excited-state properties of water clusters with size in chapter 3.In chapter 4,we explore the different dissociation rates of the singlet and triplet excitons of pentacene,which is a typical singlet fission material.In chapter 5,we study the effects of solvent co-adsorption and defective substrates on charge transfer and separation at the interface.In the sixth chapter,the role of metal ions in metallo-porphyrin dyes in electron injection is preliminarily obtained.The main contents and conclusions in this paper are as follows:(1)Water is deeply involved in our daily life.Ultraviolet radiation can lead to water splitting,creating OH radicals which can react actively with other molecules.The excited states of small water clusters(monomer and dimer)and bulk ice have been studied extensively and clearly understood.Our knowledge on the excited states of large water clusters,as the intermediate between small clusters and bulk ice,is still limited.However,large clusters are the main form of water in the atmosphere.Their excited-state dynamics has profound effects on the chemical reaction in the atmosphere.In this work,for the first time we investigate systematically the excited states of a series of water clusters up to(H2O)48 by the first-principles MBGFT.First,we explore how the optical absorption peak and optical gap change with the cluster size,finding the coexistence of the bulk-like delocalized excited state and the monomer-like highly localized surface excited state in large clusters.This provides new insights on how photolysis proceeds in water clusters.Second,off-diagonal matrix elements of the self-energy operator have pronounced effects on the unoccupied electronic levels and optical absorption for small clusters with n<10 when using density functional theory as the starting point for GW calculations,providing some methodology improvements in MBGFT on how to calculate the Rydberg-state-related electronic and optical properties of water clusters more accurately.This can give very useful hints for future studies on water clusters and other similar clusters by MBGFT and other quantum chemistry approaches.(2)Singlet fission,especially in pentacene,is considered to be a promising technique to improve the solar-to-electric power conversion efficiency of solar cells.However,experiments find that the dissociation rate of the lowest triplet(T1)exciton at the donor/acceptor interface is more than one order of magnitude lower than that of singlet excitons,and in some solar cells T1 even cannot dissociate.The low dissociation rate of T1 would affect the potential applications of singlet fission in solar cells.The reason for the difference in dissociation behavior between singlet and triplet excitons is still unclear.Using the many-body Green's function theory and time-dependent Schrodinger equation,we find that T1 of pentacene cannot dissociate itself at the pentacene/TiO2 interface since its highly localized wave function does not allow it to resonate with the interfacial charge-transfer(CT)and charge-separation(CS)states of solar cells.However,singlet excitons and higher-energy triplet excitons(Tn)could dissociate at the time scale of 100 fs due to their strong coupling with interfacial CT and CS states.We therefore propose that the electron transfer from pentacene T1 to acceptor as observed in experiments might be a two-step process,first,promotion of T1 to Tn,and second,dissociation of Tn via the resonance with interfacial CT and CS states.And the additional promotion process of T1 might result in the low dissociation rate of T1 as observed in experiments.(3)Catechol,which is one of the simplest organic molecules,possesses two neighboring hydroxyl groups which are usually used as anchoring group to attach to TiO2 surface.The catechol/TiO2 interface is a typical model system for investigating the relevant properties in dye-sensitized solar cells(DSSC).So far,a large number of experiments and theoretical calculations have been carried out to study this interface,mainly focusing on the molecular adsorption types,the coverage and adsorbed substrates.However,solvent co-adsorption and defective substrates may affect the performance of solar cells.We calculate four catechol/TiO2 interfaces employing the MBGFT,aiming to explore the influence of coverage,water and defect on electronic level and exciton properties.We find that the adsorption of catechol makes the valence band maximum(VBM)and the conduction band minimum(CBM)of substrate move up by 0.7 eV.Increasing coverage and presence of solvents result in the redshift of the energy of charge transfer(CT)excitons.Using the hydroxylated TiO2 as the adsorption substrate,the highest occupied molecular orbital(HOMO)of molecule appears at 2.51 eV below the CBM,which is consistent with the experiment.The exciton distributions at the four interfaces extend to several unit cells,especially for defect system.Although the existence of defect reduces the open-circuit voltage of the system,its corresponding exciton distribution is more delocalized.Therefore,it is valuable to control the defect concentration of TiO2 to improve the photovoltaic efficiency in solar cell.(4)Porphyrin dyes,a class of commonly used dyes with D-?-A structure,have been extensively investigated due to their strong molar absorptivity and good light stability.The solar-to-electric power conversion efficiency of porphyrin-based DSSC has exceeded 13%.Porphyrin can coordinate with many metals to form metallo-porphyrins,but few studies have explored the role of metal ions in them.Combining the many-body Green's function theory with time-dependent Schrodinger equation,we have explored the electron levels,exciton levels and carriers separation at interfaces.The calculated results show that metallo-porphyrins make HOMO and the lowest unoccupied molecular orbitals(LUMO)upshift.Excited-state dynamics calculations reveal that metallo-porphyrins possess faster electron injection rates and are likely to improve the performance of solar cells. |
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