Theoretical Study On P-type Triphenylamine-based Sensitizers By Density Functional Theory

The utilization and development of green and renewable solar energy is an important means to solve the environmental problems caused by the burning of fossil fuels and the problem of energy demand for human.As a new generation of solar cells,Dye-sensitized solar cells（DSSCs）have attracted considerable attention,owing to their great potential of being environment-friendly and low-cost.However,the development of tandem dye-sensitized solar cells is limited by the low efficiency of p-type DSSCs.Non-metallic triphenylamine dyes have been found to have better photoelectric conversion efficiency.Triphenylamine is a kind of excellent electron donor.Its nonplanar structure can effectively reduce the polymerization between dye molecules.In this thesis,many series of triphenylamine-based dyes with different molecular structures were designed.Density functional theory（DFT）and time-dependent DFT（TD-DFT）were employed to investigate the molecular structures and the properties of the dyes.The purpose of this paper is to reveal the relationship between the molecular structures and the properties of dyes.We expected that the results can provide theoretical support for further study and boost the development of p-type photosensitizers.The thesis is organized as follows:On the basis of prototype p-type sensitizers E0 and B0,we designed Series E dyes with D-A-π-A structure and Series B dyes with D-（A-π-A）₂ structure to investigate the effect of thiophene derivatives as acceptor linked donor andπ-spacer on the sensitizer characteristics.Molecular geometry,electronic structures and absorption characters were calculated with DFT and TD-DFT methods.The results show that the highest occupied molecular orbital（HOMO）level and the lowest unoccupied molecular orbital（LUMO）level of all dyes match well with the Ni O semiconductor electrode and I₃^–/I₂^·–electrolyte.All the eight kinds of dyes in the study can be used as dye sensitizers for the p-type DSSCs.The introduction of another acceptor groups into the structure of dyes can improve the planarity of dyes and increase the rigidity of dyes.The dyes for E1 and B1 with 2,3-dimethylpyrazine thiophene（DMPZT-1）as acceptor have the narrowest energy gap in Series E and Series B,respectively.Importantly,the dyes for E1 and B1 show the largest red shift and the maximum integral values of the adsorption over the visible light in Series E and Series B,respectively.Therefore,the introduction of DMPZT-1 as the acceptor in the structure of dyes may improve the optical properties of dyes.Furthermore,all the dyes display an excellent light harvesting efficiency（LHE）performance.The range of LHE values of Series E is 0.988 to 0.992,and the range of LHE values of Series B is 0.998 to 0.999.The LHE values of B series dyes are higher than those of E series dyes.Compared with Series E,the LHE value of Series B was improved.On the basis of triphenylamine as an electron donor with attachment of two–COOH anchoring groups and dicyanovinyl as acceptor,ten dyes with D-π-A structures were designed to investigate the effects of differentπ-linker groups on the properties of the sensitizers,especially the influence of theπ-linkers containing nitrogen cation（N⁺）.The results show that all the investigated dyes can be used as dye sensitizers for the p-type DSSCs except one dye which contains two N⁺.By comparing the properties of each dye,it is found that the relative location of N⁺alongπ-conjugation bridge is very important.The suitable introduction of N⁺in the molecular structure can improve the performance of dyes.The N⁺modified dye（named S3-PZL1C）has narrow energy gap（2.02 e V）,the best light harvesting efficiency（LHE,0.9974）and the smallest internal reorganization energy(λ_int=7.00 kcal/mol).Importantly,S3-PZL1C displays the largest red shift of the UV-vis absorption,the maximum integral values of the adsorption-wavelength curves over the visible light（400～800 nm）and the strongest adsorption energy（?66.84 kcal/mol）on Ni O surface.In addition,S3-PZL1C not only enhances the electronic excitation,but also improves the reorganization energy and charge separation.On the basis of triphenylamine as an electron donor with attachment of two–COOH anchoring groups,dicyanovinyl as acceptor,and thiophene,thieno（3,2-b）thiophene and dithieno[3,2-b:2′,3′-d]thiophene asπ-linker,the A,D and S series dyes were designed to further explore the effect of N⁺inπ-linker on the intramolecular charge transfer for dyes.N⁺was introduced into different positions ofπ-linker,and the properties of the dyes with N⁺as an“electron trap”were investigated.The results show that the properties of the dyes are sensitive to the N⁺position inπ-linkers.D series dyes with the electron-withdrawing units located near donor have better properties than the corresponding A series with the electron-withdrawing units located near acceptor.For A and D series,the N⁺modified dye named T2N+1-d displays the largest red shift of the UV-vis absorption,the maximum integral values of the adsorption-wavelength curves over the visible light,the high light harvesting efficiency（LHE,0.996）and the strongest adsorption energy（?44.33 kcal/mol）.In addition,T2N+1-d also has a large driving force of hole injection(ΔG_inj,?0.74 e V),which results in a more efficient hole injection.Bearing a lengthierπ-linker than T2N+1-d,the properties of T2N+1-s are further improved.T2N+1-d moiety or its increasing conjugated derivatives may be a promisingπ-linker.Different from the research on the modification ofπ-linkers for the dyes,based on triphenylamine as an electron donor and thiophene asπ-linker,Series P and Q p-type sensitizers with D-（π-A）₂ structure were designed to investigate the effects of the different acceptors on the properties of the sensitizers.Series Q dyes were designed with P4 dye as the prototype.The results show that the properties of the dyes can be tuned by the introduction of the different electron-withdrawing groups to the N atom in pyridinium acceptor.Compared with the synthesized Series P dyes used in p-type sensitizers,the properties of Series Q dyes are improved by means of modifying pyridinium acceptors except Q1 and Q3 who can not be used as dye sensitizers for the p-type DSSCs.Compared with the other dyes of Series Q dyes,due to the suitable electron-withdrawing ability of the hexafluorodiacetylamino group in its acceptor,Q6 has the narrowest energy gap（1.90 e V）,the largest driving force of hole injection(ΔG_inj,-0.68 e V)and the smallest internal reorganization energy(λ_int,5.05 kcal/mol).Hence,Q6 not only enhances the electronic excitation,but also improves the reorganization energy.Importantly,Q6 shows the largest red shift and the maximum integral values of the adsorption over the visible light,as well as the strongest adsorption energy（?74.80 kcal/mol）on Ni O surface.Thus,Q6 may be a promising sensitizer for the p-type DSSCs,and the acceptor of Q6may provide a new,and easily accessible high performance acceptor for p-type sensitizers.