Theoretical Study On Design And Properties Of Lindqvist-Type Polyoxometalate-Based Sensitizer

The gradual depletion of fossil fuel sources along with the progressive increase in environmental pollution has stimulated an urgent demand for renewable clean energy sources.Solar energy has the largest potential to satisfy the global need for renewable energy sources,and converting solar energy into electricity presents one of the most challenging technological tasks.Dye-sensitized solar cells(DSSCs)are the most representative examples of the third-generation sunlight-to-electricity devices,which have attracted a great deal of attention due to their low cost and easy fabrication.Dye plays the important role in DSSCs,which can affect the efficiency of the light-to-electrical energy conversion.Polyoxometalates(POMs)can accept many electrons without losing their structural integrity,and recently,POMs have been widely used in DSSCs as photosensitizers due to their electronic versatility.POMs can be used as inorganic building blocks for constructing inorganic–organic hybrids,in which POMs act as the electron acceptors and organic segments act as the electron donors.The maximum absorption bands of the hybrids significantly red-shift compared with those of the individual components,resulting from the synergistic effect of the POMs and organic segments,thereby resulting in long-wavelength absorption and high energy conversion.Hence,it is important to explore the relationship between structures and properties in POM-based dyes.A series of POM-based dyes were designed and systematically analyzed by the density functional theory(DFT)and time-dependent DFT(TD-DFT)calculations with the aim of qualitatively(or even quantitatively)describing the properties of dyes and predicting the performance of dyes in DSSCs.Apart from the electronic properties,absorption spectra,intramolecular charge transfer(ICT)characteristics of isolated dyes,short-circuit photocurrent density(JSC),and open-circuit photovoltage(VOC),as well as dye aggregation effects were also taken into consideration.It is expected that the present work can provide a commendable option for further designing high performance dyes.The research content is mainly divided into the following four parts:1.The prototype dye D5(abbreviated as dye 1)was synthesized by Sun and co-workers,in which triphenylamine and cyanoacrylic acid act as an electron donor and electron acceptor,respectively,and 2-vinylthiophene acts as a?linker.Dye 2 is designed by adding a Lindqvist-type POM([Mo6O19]^2–)into the synthesized dye 1 as the electron-withdrawing group.Dyes 3–5 were constructed by replacing the2-vinylthiophene in dye 2 with 2,2'-bithiophen,C atom rigidified bithiophene and N atom rigidified bithiophene as?linkers,respectively.Compared with dye 3containing a 2,2'-bithiophene?linker,its two counterparts(dyes 4 and 5)with rigidified bithiophene?linkers tend to accelerate the IET process,as well as slow the electron recombination,and thus improve the performance of the DSSCs.In particular,dye 4 with a rigidified bithiophene?linker fastening using a C atom obtains a larger J_SC and V_OC compared to that of dye 5 with N atom rigidified bithiophene?linker.2.A series of POM-based porphyrin dyes derived from synthesized dye SM315were designed and analyzed systematically by using DFT and TD-DFT calculations aiming at seeking high performance dyes.The electronic properties,absorption spectra and ICT characteristics,JSC and VOC as the key factors affecting the performance of DSSCs were investigated.Dye 3 with an electron donor D3 exhibits better photovoltaic performance among dyes 1–3 due to its broadest absorption spectra and largest JSC^max,hence,D3 can act as a promising donor candidate for the construction of efficient dyes.Furthermore,the performance can be further improved by replacing the?linker in dye 3 with benzo[1,2-b:4,5-b']difuran-2,6-dione(dye 4)and pyrrolo[2,3-f]indole-2,6(1H,5H)-dione(dye 5).Compared with dye 3,dyes 4 and5 possess broader absorption spectra and light harvesting efficiency curves,larger ICT parameters,J_SC^max and conduction band energy shift?E_CB.In particular,dye 4 might obtain a larger JSC,and dye 5 might obtain a larger VOC.3.A series of POM-based dyes with different configurations resulting from different positions of POM inserted in dyes have been comprehensively investigated with DFT and TD-DFT methods.The configuration effects of dyes on the performance of DSSCs were evaluated.JSC and VOC connected with absorption spectra,ICT parameters,electron injection efficiency,?E_CB,charge recombination and dye aggregation effects were analyzed.The results indicate that dye 3 possesses the largest JSC due to the red-shifted absorption spectra,superior ICT parameters and the largest electron injection efficiency.At the same time,dye 1 might perform well on V_OC because of the smallest?E_CB towards the more negative direction with respect to the pristine CB of Ti O2 and the reduced charge recombination as well as dye aggregation processes due to the smallest?E_tot(–80.63 kcal/mo1)among dyes 1–3.4.Theoretical calculations based on the density functional theory(DFT)and time-dependent DFT(TD-DFT)were employed to screen efficient acceptor group candidates for POM-based dyes.Compared to the commonly used cyanoacrylic acid acceptor group in dye 1,the acceptor groups in dyes 2–4 showed increased adsorption ability onto the Ti O2 surface,which is beneficial for device stability.Dye 3 not only exhibited the broadest absorption spectrum,largest ICT parameters,largest JSC governed by electronic coupling and largest V_OC connected with?E_CB among dyes1–4,but also possessed superior adsorption stability and the lowest interaction energy for the most stable dimer configuration,which can improve the photovoltaic efficiency of DSSCs.Thus,the 5-methylenepyrimidine-2,4,6(1H,3H,5H)-trione acceptor group in dye 3 can be a promising acceptor group candidate for high-performance dyes used in DSSCs.