Theoretical Study Of Lindqvist-type Hexamoltbdate-based Dyes With Different Donors For Photovoltaic Application

Polyoxometalates (POMs) are a rich class of inorganic cluster systems with avariety of molecular structure and unique physical and chemical properties. Recently,with the development and update of density functional theory (DFT), the theoreticalstudy of POMs have greatly improved.Dye-sensitized solar cells (DSSCs) have received significant attention because oftheir low fabrication cost and competitive energy conversion efficiencies. Themajority of works have focused on the characterization and theoretical study of n-typeDSSCs (n-DSSCs) up to date and a high efficiency of over11%has been achieved. Inorder to further improve the energy conversion efficiency, one concept is to constructa tandem DSSC, which combines an n-DSSC photoanode with a p-DSSCphotocathode in a simple sandwich configuration.DFT methods have been performed for the study of different organic groupssubstituted hexamolybdates for p-type dye-sensitized solar cells. This work will laythe foundation for further study of POMs used in DSSCs. The present work containsfollowing three aspects:1. DFT and TDDFT calculations have been carried out to study the electronicstructures, absorption spectra and transition natures of3,4-ethylenedioxythiophene(EDOT) or thienothiophene (TT) unit substituted hexamolybdates based on[Mo6O18(MBTH)]2-. Owing to the long π-conjugated bridge and high delocalization,the designed dyes exhibit both strong and broad absorptions as well as remarkably redshift. Especially for dye6, which contains a biTT unit, it has the largest maximumabsorption wavelength and may show a higher short-circuit current density.2. DFT and TDDFT calculations have been performed to investigate thedistribution and energy diagram of frontier molecular orbital, absorption spectra andthe electronic transition natures of hexamolybdates with different electron-donatingsubstituents and electron-withdrawing substituents. The absorption spectra of the dyesare significantly influenced by the introduction of electron-donating substituents andelectron-withdrawing substituents. The systems with electron-donating substituents onthe phenyl display a slight red-shifted absorption wavelength with respect to thosewith electron-withdrawing substituents due to the increase of the electron-densitydonor moiety. Our calculated results point out that JSCof the dyes mainly depend on LHE except for dye3. And dye1, in which the substituent is CH3, has larger LHE andcomparable driving force lead to higher Jsc.3. The electronic structures, absorption spectra and transition natures have beeninvestigated with DFT and TDDFT calculations of hexamolybdates dyes, in whichtriarylamine, carbazole and POMs take the roles of electron donor and acceptor unit,respectively. It is found that introduction of triarylamine and carbazole units exhibitbetter photovoltaic performance due to the increase of the electron-density donormoiety and resulted in a red-shift and broadening of the absorption spectra. Inparticular, dye4shows the best photovoltaic performance: the largest LHE, energiesof regeneration reaction and charge recombination, as well as comparable drivingforce, which lead to a higher Jsc.