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

Dye-sensitized solar cells（DSSCs）have attracted much attention due to their advantages,such as simple preparation process,low cost,and environmental friendliness.How to improve the photoelectric conversion efficiency of DSSCs is a bottleneck problem for its large-scale application.As one of the core components of DSSCs,development of efficient dyes is one of an effective pathways to improve the photoelectric conversion efficiency of DSSCs.Organic dyes are powerful alternatives to metal dyes,which is attributed to the advantages including rich raw materials,simple synthesis,and high molar absorption coefficient.It has been a hot and difficult topic in the academic and industrial communities to choose the suitable organic dyes from numerous ones.If the method to develop new organic dyes is varied,the development cost and time would be greatly decreased.Organic dye could be firstly designed.Then,their performance is evaluated by theoretical methods,Finally,the organic dyes with acceptable performance are synthesized in experiment.However,previous theoretical studies mostly focused on the static properties of isolated dyes,and only a few studies considered the aggregation behavior and dynamic properties of dyes.As a result,there are large differences between theoretical predictions and experimental results.Therefore,this thesis not only studied isolated dyes but also studied the adsorption behavior of the dye on the semiconductor surface.More importantly,the dynamic behavior of high-order aggregation of organic dye was firstly studied by us.The specific research content is as follows:1.Effect of different types ofπ-linker on the photoelectric properties of D-π-A type organic dyesThe photoelectric properties of four D-π-A type organic dyes with differentπ-linker are studied by density functional theory（DFT）and time-dependent density functional theory（TDDFT）.The electron donors and acceptors of THI-T,THI-Di,THI-2T,and THI-2T-C are all thieno[2,3-b]indole and cyanoacrylic acid,and theπ-linker are thiophene,dithiophene,thienothiophene and cyclopentadithiophene,respectively.Firstly,the photoelectric properties of isolated dyes are analyzed from geometry,charge distribution,and absorption spectrum;then,the electron transfer rate constants（k）between the dyes and the semiconductors are evaluated by the recombination energy(λ_total);finally,a series of parameters,such as,the light harvesting efficiency（LHE）and injection drive force(ΔG_inject)that affecting the open circuit voltage(V_OC)and the short circuit current(J_SC),are employed to evaluate the power conversion efficiency（η）of DSSCs.It has been found that THI-2T-C with cyclopentadithiophene as theπ-linker has the higher LHE,the larger electron injection efficiency(Φ_inject)and k.In order to make the theoretical results more reliable,the configuration of the dye adsorbed on the TiO₂ surface is simulated.And the change of the band gap between the dye and TiO₂ before and after adsorption and electron injection time is further studied.The results show that THI-2T-C has the most stable adsorption configuration,minimum band gap,and the shortest injection time.Combination of all items,the performance of THI-2T-C stands out from the investigated dyes.2.Effect ofπ-linker length on the photoelectric properties of D-π-A type organic dyesBased on the results of the previous chapter,this chapter further explored the effect ofπ-linker length on the performance of D-π-A organic dyes.On the basis of THI-2T-C,other five novel dyes,THI-3T-2C,THI-4T-3C,THI-5T-4C,THI-6T-5C,and THI-7T-6C,are designed by further improvement of theπ-linker extent.If only the isolated dye is considered,the performance of THI-2T-C,THI-4T-3C,and THI-6T-5C are better.However,the performance of THI-4T-3C and THI-5T-4C are better when the dye-TiO₂ system is considered.Combined with the results of isolated dyes and adsorption systems,THI-4T-3C is considered to be more suitable choice for thieno[2,3-b]indole-based dyes rather than other studied dyes.Furthermore,the dye has a moderateπ-linker,which is easier to be synthesized than dyes with the longerπ-linker.3.Effect of different types of anchoring groups on photoelectric properties and aggregation of A-D-A type organic dyes-COOH is the most common anchoring groups used in organic dyes.However,it has been testified that-COOH as an anchoring groups would also result in the aggregation of organic dyes.Based on the A-D-A type organic dye 1 with coplanar heteroaromatics and-COOH as electron donor and anchoring groups,respectively,other four new organic dyes,2-5,are designed with different anchoring groups.The effect of different anchoring groups on the photoelectric properties and aggregation of A-D-A type organic dyes is investigated by combination of first-principles and molecular dynamics（MD）.Compared to other four dyes,2 with-CSSH as the anchoring group has the wider light absorption range and greater light absorption intensity indicating higher light capture capability.More importantly,the dimer of 2 has the smaller or equal electronic coupling as compared with that of 1,which is favorable for the electron injection from dye into TiO₂.In addition to the properties of dimer,this chapter also studied the interfacial properties of the face-to-face dimer and TiO₂.The results showed that dyes with-CSSH instead of-COOH as the anchoring group not only has stronger light trapping ability,but also could better inhibit the aggregation between the dyes.4.Higher order aggregation of D-A-π-A type organic dyes THI-BTZ-2T-COn the basis of the D-π-A type dye THI-2T-C,a benzothiadiazole electron-withdrawing group is added to form the D-A-π-A type dye THI-BTZ-2T-C.Compared to the dye THI-2T-C,its maximum absorption spectrum is red-shifted by 134 nm suggesting that it is a potential organic dye.In this chapter,the high-order aggregation behavior of THI-BTZ-2T-C is studied from both static and dynamic perspectives by first-principles and MD.If only the static properties are considered,the configurations of dyes is not varied greatly when the adsorbed dyes are increased from monomer to hexamer.However,the absorption spectrum varies regularly with the increased aggregation order.In addition,the contribution of different orbitals of various atoms to density of states（DOS）gradually increases with the aggregation order increasing.When dynamics simulation is performed,the torsion between different components of dye is enlarged.Meanwhile,the perpendicular model is altered during the dynamics simulations,the dye would like to bend towards the TiO₂ surface.Moreover,the electronic coupling is greatly improved during the molecular dynamics simulations especially for the H-aggregation.The dye aggregation mainly affects the electrons injection process resulting in the reduction of the photoelectric conversion efficiency for DSSCs.Reduction of perfect face to face stacking would be an efficient pathway to weaken the aggregation.It is necessary to study the high-order aggregation effects by molecular dynamics simulation with the aim to deeply understand the complex factors related with the influence of dye aggregation.