Side Chain Molecular Engineering Of Oganic Functional Dye And Its Effect On Property Modulation

In recent years,pure organic sensitizers have attracted more and more attention in the field of dye-sensitized solar cells（DSSCs）due to their advantages of low cost,superior photostability,and controllable properties.Thus,in one sense,the molecular design presents crucial influence on its photovoltaic performance.In our previous work,it’s found that the planarization of the donor group can selectively raise the HOMO energy level,while the LUMO energy level is almost unaffected.However,the planar molecular structure also brings some problems,such as poor solubility and serious molecular aggregation.These problems will also affect the photovoltaic performance of DSSCs to certain extent.Therefore,in this paper,a series of D-A-π-A sensitizers are designed and synthesized by introducing side chains with different steric effects at different positions of the molecule to regulate the molecular properties.Through the detailed characterization of the photophysical,electrochemical and photovoltaic properties of the dyes,the structure-activity relationship between the side chain and multiple molecular properties can be revealed,which will provide the theoretical basis for the molecular design of organic sensitizers in the future.In chapter 1,the development,characteristics and classification of organic sensitizers are briefly described,as well as the means of the regulation of the energy level.The research progress of organic sensitizers based on different donor groups in recent years is also reviewed in detail.Then,the ideas and main content of this thesis are proposed.In Chapter 2,with different long alkyl chains,dyes CS-38-40 based on carbazole donor units are designed and synthesized.The chemical structures of all intermediates and target compounds are characterized by ¹H NMR,¹³C NMR,HR-MS,and FT-IT,etc.In Chapter 3,various properties of the dyes are carefully evaluated.Although the photophysical and electrochemical properties of dyes with different side chains are barely affected by the side chains,the effect of side chains on interfacial charge transfer properties are quite different:1)the charge injection efficiency increased to the highest value of 90.89%when the ethyl groups were used as the alkyl chain,then slightly decreased to 89.50%;2)the dye regeneration efficiency increased to the highest value of 89.67%with the butyl groups as the alkyl chains;3)as the alkyl chains were extended,the charge recombination process was effectively suppressed and the charge collection efficiency efficiently increased.Therefore,with the suitable alkyl chain of butyl group,dye CS-39 presents the best photovoltaic performance among four dyes,with a photocurrent density of 15.70 m A cm^-2,a photovoltage of 0.765 V,a fillfactor of 0.698,and a power conversion efficiency of 8.39%.In Chapter 4,a series of organic sensitizers,coded as CS-60-62 were designed and synthesized by inserting carboxyl groups into the D-A-π-A framework as side chains.The chemical structures of all intermediates and target compounds are characterized by ¹H NMR,¹³C NMR,and FT-IT,etc.In Chapter 5,through the photophysical and electrochemical measurements of the dyes synthesized in Chapter 4,it is found that as the increase of the number of side chain carboxyl groups,the absorption band gradually blue shifts,and the molar extinction coefficient is greatly reduced,which will reduce the light-harvesting capability of the dyes.Meanwhile,the LUMO energy levels are significantly upshifted.The adsorption behavior of multiple carboxyl dyes CS-61 and 62 are characterized by FT-IR spectra.The absent of carbonyl stretching vibration peak around 1700 cm^-1suggests that all the carboxyl groups have been adsorbed on Ti O₂surface.Considering the chemical structure of the dyes,CS-61 and 62should lie on the Ti O₂surface,which will separate the Ti O₂from redox couple,thus resulting in the suppression of charge recombination.In Chapter 6,conclusion,by inserting different side chains at different positions of the planar dye molecules,it is found that the side chain molecular engineering presents obvious regulation effect on the properties of the molecules and the prepared DSSC devices,especially for the interfacial charge transfer properties,providing us an effective strategy for the optimization of DSSCs performance based on planar organic dyes.