Theoretical Study On The Novel Molecular Design And Photoelectric Properties Of Organic Dyes

Dye-sensitized solar cells?DSSCs? have attracted extensive attentions due to the following advantages,such as,simple fabrication,environmental friendliness,and easy to accomplish the larger scale applications as well as flexible and transparent.Normally,DSSCs are composed by sensitizers,counter electrodes,redox electrolytes,and semiconductor electrodes,in which the sensitizer is the critical component.Sensitizer plays a vital role in harvesting the sunlight and injecting electron,which greatly affects the photoelectric conversion efficiency of the device.As compared with metal dyes,organic dyes have the high molar extinction coefficient,simple synthesis and purification,abundant and cheap raw materials and tunable structures.The organic dyes would be better than other dyes because of the lower cost and less pollution.However,the overall photoelectric conversion efficiency?PCE? of DSSCs based on organic dyes is still not good enough to meet the commercial applied criterion.Development of new organic dyes are attracting and hot topics.There are numerous organic dyes with diverse configuration and substituted groups leading to different properties.It is an interesting thing to screen out the suitable organic dyes with the desired properties.The pathway starts from evaluation of properties by theoretical method to experimental synthesis following the device fabrication.To evaluate the properties accurately is an insurance for the successful application of aforementioned method.Although lots of theoretical studied have been performed for organic dyes,there are still following problems.Most of theoretical studies are qualitative comparison rather than quantitative calculations.In addition,the absorption spectrum,frontier molecular orbital?FMO? and electronic structure are compared.However,the influence of aggregation on the PCE is rarely included to evaluate the overall performance.As a result,there is great deviation between theoretical and experimental results.We performed the following theoretical calculations with the goal to improve the accuracy of theoretical studies.The theoretical studies are performed for both isolated dyes and dye-TiO₂ adsorbed system to include the interfacial properties between dye and TiO₂ surface.The influence of molecular configuration and substituted group on the properties is investigated,especially for the dyes with the same group but different configuration.The effect of aggregation is included to evaluate the performance of dyes including static and dynamic properties.More important,the values of short-circuit current density(J_SC),open-circuit voltage(V_OC),and PCE are calculated,which is favorable to distinguish the different dyes.The specific research content is as follows:1.Effect of different auxiliary acceptor or donor groups on the properties of A-D-A-D-A type organic dyesFour A-D-A-D-A type organic dyes,2CZ-BT,2PZ-BT,2PZ-PP,and 2PZ-SP,with different auxiliary acceptor or donor groups have been theoretically investigated by first principle.2CZ-BT and 2PZ-BT have been synthesized experimentally,in which donor is carbazole or phenothiazine,auxiliary acceptor is benzothiadiazole,and anchoring group is cyanoacrylic acid.2PZ-PP and 2PZ-SP are theoretically designed on the basis of 2PZ-BT by employment of diphenyl-substituted pyrido[3,4-b]pyrazine and diphenyl-substituted pyrido[3,4-b]pyrazine-2,5-thiophene as auxiliary acceptor,respectively.The frontier molecular orbitals,charge distributions,and absorption spectra are investigated by LC-BLYP/6-31G?d,p? level of theory.It has been found that 2PZ-SP is not suitable to be organic dye in DSSCs because of the inappropriate electron density distribution.For other three dyes,the recombination energy(?_total),the light harvesting efficiency?LHE? and injection drive force(?G_inject) related with J_SC and V_OC are compared to evaluate the PCE qualitatively.However,it is difficult to distinguish the performance of other three dyes since no one has all superiors.After the dye is adsorbed on TiO₂ surface,2PZ-PP is the better than other two dyes because of the smallest energy gap and the shortest injection time(t_inj).In addition,a new D-?-A dye?PZ-BT? with the identical donor,?group?auxiliary acceptor?,and acceptor groups is designed to be compared with 2PZ-BT to investigate the influence of molecular configuration on the properties of dyes.It is found that 2PZ-BT has the absolute advantage to be potential dye for DSSCs.Besides the suitable groups,the appropriate configuration is more critical aspect to determine the performance.2.Effect of different?groups on the photoelectric properties of D-?-D-A type organic dyesOn the basis of the experimental synthetic D-?-D-A type dye 1,other two organic dyes,2,and 3,are theoretically designed.They employ dithiafulvenyl as donor,phenothiazine as auxiliary donor,cyanoacrylic acid as acceptor,and diphenyl/dithienyl/thiopheneyl-benzothiadiazole as ? groups,respectively.First,the FMO,absorption spectra and electronic structure are investigated at the CAM-B3LYP/6–31G?d,p? level of theory.There is no distinct difference among three dyes on the basis of only isolated dyes.When the dye-TiO₂ adsorbed system is considered,3 is excluded because of the smallest energy gap between the lowest unoccupied molecular orbital?LUMO? of adsorbed dye and the conduction band of TiO₂,and the longert_inj.However,it is still a difficult task to differentiate 1 and 2.In this chapter,the aggregation effect is studied including both static and dynamic models.As compared with 2,the electronic coupling between dimer of 1 is the larger leading to the poor electron injection capability.Therefore,it can be judged that 2 is the most excellent of the three molecules.Additionally,a new D-?-A dye 4 with the identical donor,?-bridge and acceptor is designed to be compared with 2 to investigate the influence of molecular configuration on the properties of dyes.However,its performance is much less than2.The suitable combination of configuration and?group would improve the overall performance.3.Quantitative calculation of J_SC and V_OC for T shaped?D?₂-A-?-A organic dyesBased on of the reported dye,1,three new T-shaped?D?₂-A-?-A organic dyes 2,3,and 4 are developed with different auxiliary acceptors.The donor is triphenylamine,the?group is furan,the acceptor is cyanoacrylic acid,and the auxiliary acceptors are 6-methyl-6H-indolo[3,2-b]azaquinoxaline,5-methyl-5H-[1,2,5]thiadiazolo[3,4-b]-carbazol,and5-methyl-5H-[1,2,5]oxadiazolo[3,4-b]carbazole,respectively.On the basis of the isolated dyes,the F?rster resonance energy transfer?FRET? is studied at the PBE0/6–31G?d,p? level of theory.4 has the largest FRET,suggesting the faster intramolecular energy transfer.Dye-TiO₂ adsorbed system is considered by first principle.Even if the dye-TiO₂ adsorbed system is considered,it is difficult to distinguish them.On the basis of the adsorbed system,the accurate values of J_SC,V_OC,and PCE are calculated.Additionally,the aggregation effect is also considered to evaluate the performance of the dyes.The results show that 4 has the largest PCE and smaller aggregation effect.Combination of all items,the performance of 4 stands out from the investigated dyes.The D-?-A dye?5? is designed with the same donor,?group,and acceptor with dye 4.It is established that the insertion of the ancillary acceptor is important to refine the overall performance since all evaluated items of 4 are better than that of 5.