Theoretical Study For The Influence Of ? Group On Organic Dyes Properties

As the representative of the third generation solar cells,dye-sensitized solar cells(DSSCs)have attracted extensive attentions from academic communities due to the following attractive advantages,such as,low cost for manufacture,reasonable photoelectric conversion efficiency(PCE)and environmental friendliness.No doubt,the dye is one of the critical components in the DSSCs,which plays the key role in determining the overall performance of DSSCs.Inorganic dyes include the rare metal element,which results in the expensive price,difficult synthetic and purified process,and severe environmental problem.In contrast,organic dyes are promising alternatives with tunable structures,simple synthetic route,and high molar absorption coefficient.Donor-?-Acceptor(D-?-A)is the most popular configuration for organic dyes.Among them,?group plays more important role.Lots of?groups have been developed in past decades.How to select the suitable one or synthesize the new?group is an interesting problem.If every?group is synthesized and the corresponding properties are measured,it is almost an impossible task.If the properties of dyes would be predicted before the following experimental measurements,it would greatly enhance the efficiency to develop new organic dyes.Although there are numerous theoretical studies about organic dyes,the accuracy could not be regarded as reasonable.How to improve the accuracy of theoretical study is a critical problem deserved to be considered.Therefore,the effect of?group is investigated by combination of Density Functional Theory(DFT)and Molecule Dynamics(MD)simulations.Besides the absorption spectrum,frontier molecular orbitals(FMO),and electrochemical parameters,the short-circuit current density(J_SC)and open-circuit voltage(V_OC)are quantitatively determined to distinguish the various dyes with minor differences.Moreover,the effect of aggregation on the properties is considered.Our ultimate goals are to improve the accuracy of theoretical prediction and to accomplish the developed pathway from theoretical screening to experimental synthesis for organic dyes.Following contents are studied in the thesis:(1)On the basis of two symmetric carbazole-based dyes,six new carbazole-based organic dyes with“non-symmetry”butterfly structure are designed.In the left“wing”,benzene and thiophene are employed as the?group.And the left wing is fixed.For respective left“wing”,three different right“wings”are constructed with varied?groups with the aim to explore the influence of?group on the properties of carbazole-based dyes.First,the structures of the isolated dyes are optimized at the B3LYP/6-31G(d,p)level of theory.On the basis of optimized geometries,transferred charge(q _CT),the absorption spectrum,J_SC,V_OC,and reorganization energy(?_total)are compared to distinguish the designed dyes.Besides the isolated dyes,the dye-(Ti O₂)₃₈adsorbed structures are optimized by the PBE-GGA method.On the basis of the optimized structures,adsorption energy and the density of states(DOS)are calculated to further investigate their properties.According to the adsorbed energy and the energy difference between lowest unoccupied molecular orbital(LUMO)of dyes and conduction band(CB)of Ti O₂,the overall performance of CBZ-B-BB is the best with one phenyl group as?group in left wing and two phenyl groups in right wing.It testifies that the phenyl group is more suitable to be?group in both wings.(2)Two new indoline-based D-A-?-A dyes,D3F and D3F2,are developed on the basis of the reported D3 by insertion of one or two F atoms on benzothiadiazole group.Among them,indole is donor,benzo[c][1,2,5]thiadiazole is auxiliary acceptor,benzoic acid is acceptor,and alkynyl is?group.The performance of two designed organic dyes,D3F and D3F2,is compared with that of D3 from various aspects including absorption spectrum,light harvesting efficiency(LHE),driving force,and open-circuit voltage.Besides the isolated dye,the interfacial property between dye and Ti O₂surface is also studied.D3F and D3F2 do not show absolute superiority than D3 not only for the isolated dyes but also for the monomeric adsorption system.However,D3F and D3F2 would effectively reduce the influence of aggregation resulting in the much smaller intermolecular electronic coupling,especially for D3F2.The inclusion of F atom would reduce the negative effect of aggregations leading to the improvement of overall performance of DSSCs.For three aforementioned organic dyes,the difference among them is too small to distinguish them,which is also the problem for the qualitative comparison.It is necessary to perform the quantitative investigations to judge the performance of organic dyes,which would be performed in the following two sections.(3)On the basis of phenothiazine-based D-?-A dye(PCT1),three new organic dyes,PBT2,PCP3,and PBP4,are designed with phenothiazine as donor,cyanoacrylic acid as acceptor,and4,4-dimethyl-4H-cyclopenta[2,1-b:3,4-b']dithiophene(PCT1),benzo[2,1-b:3,4-b']dithiophene(PBT2),4,4-dimethyl-4,7-dihydro-1H-cyclopenta[2,1-b:3,4-b']dipyrrole(PCP3),and1,8-dihydropyrrolo[3,2-g]indole as?group,respectively.The properties of isolated dyes are studied at the PBE0/6-31G(d,p)level.When the frontier molecular orbitals(FMO)and absorption spectrum properties are studied,the performance of PCP3 and PBP4 is not better than that of PCT1 and PBT2.Even when the dye-Ti O₂adsorbed system is considered,the differences among four organic dyes are still small with similar adsorbed energy,band gap,and injecting time of excited electrons,which is also perplex for most of theoretical studies.To have the reliable and accurate results,the values of J_SC,V_OC,and PCE for DSSCs based on four organic dyes are accurately determined.Although the J_SCof PCP3 is the smaller than that of PCT1,the PCE of PCP3 is comparable with that of PCT1 due to the larger V_OC.The quantitative calculation is an effective method to make a distinction among every designed dyes.The influence of aggregation is also included to evaluate the properties of dyes.(4)Two new organic dyes,3 and 4 are designed on the basis of reported aniline-based dyes(1 and 2)with different?group.The structures of isolated dyes are optimized at the B3LYP/6-31G(d,p)level.On the basis of the optimized structures,the absorption spectra and reorganization energy are explored.In addition,the dye-Ti O₂adsorbed system are explored by the PBE-GGA method.More important,the J_SC,V_OC,and PCE are calculated,which is helpful to determine the effect of?group on the D-A-?-A dyes.The negative influence of aggregation for the dye-Ti O₂adsorbed system is also considered to evaluate the overall photovoltaic performance.The molecular simulations are carried out to explore the dynamic properties of aggregation.The performance of 3 is better than that of 2 when the effect of aggregation is included,which would be promising candidate for organic dyes in DSSCs.