Molecular Structural Modifications And Theoretical Investigation Of N-Organic Sensitizers With 1,2-Benzophenothiazine Moiety

Dye-sensitized solar cells have been widely investigated for their low cost,environmentally friendly and highly efficient conversion of solar energy into electricity.Currently,the efficiency of DSSC still lags behind that of conventional silicon-based solar cells,limiting their large-scale applications.It is worth noting that dye is the most important components of dye-sensitized solar cells,which is responsible for capturing photons and transferring electrons.And appropriate changes in the structure of the dye material can effectively improve the cell efficiency.It is very complicated and time consuming to regulate molecular structure for highly efficient conversion DSSC with normal experimental synthesis.Using quantum chemical calculations can provide insight into the relationship between the structure and energy conversion efficiency of DSSC at the molecular level and are an important complement to experimental studies of DSSC.Theoretical calculations of dye performance can predict and support experiments,reveal the mechanism of action of dye-sensitized solar cells at the atomic level,and describe the important effects of different components on their efficiency.Density functional theory and time-dependent density functional theory（TD-DFT）are important methods used to study the effect of dyes on light capture efficiency and electron injection efficiency.In this paper,we explore the organic dye molecules,explore their microstructures to analyze their effects on cell performance,so as to design new efficient dye sensitizers and provide useful theoretical guidance for experiments.Phenothiazines have been widely used as electron-rich groups in dye-sensitized solar cells,while there are fewer studies on the application of benzophenothiazines to dye molecules.In this paper,we analyzed ten dye molecules with different structures based on 1,2-benzophenothiazine moieties by adding/transforming donor or acceptor groups in detail,as a way to investigate the effects of different linkage configurations of donor and acceptor on the performance of dye molecules,and thus to screen potential high-performance dye molecules.The geometry,front orbitals,energy levels,optical absorption properties,electronic structure,and interfacial electron transfer of the dye molecules have been calculated mainly by using density generalized theory method.The calculation results show that the addition of furan group between 1,2-benzophenothiazine and cyanoacrylic acid can make the dye molecule obtain smaller orbital energy gap and wider spectral absorption range,so the addition of auxiliary acceptor betweenπ-bridge and acceptor is a feasible strategy.Also comparing with carbazole,it is found that the dye molecule has greater electron transfer efficiency,higher electron injection efficiency when choosing triphenylamine as the donor group,and thus it is more likely to bring excellent photoelectric conversion efficiency to the cell.Similarly,we analyzed the difference between the dye molecule acceptor attached to the C-3 position of phenothiazine and the C-7 position,and found that when the acceptor is located at the C-3 position,the dye molecule can achieve higher charge transfer efficiency,wider spectral response,and better coupling between the dye and titanium dioxide,which is more conducive to the injection of excited electrons from the dye into the Ti O₂ conduction band.Therefore,the dye molecule is expected to achieve higher photoelectric conversion efficiency when using triphenylamine as the donor group,1,2-benzophenothiazine as theπ-bridge,introducing furan as the co-acceptor at the C-3 position,and cyanoacrylic acid as the anchoring group.Based on the previous section,we also designed seven different n-type dye molecules by using1,2-benzophenothiazine as aπ-bridge and triphenylamine as a donor group,while adding phenyl as an auxiliary acceptor,and also connecting one or two methoxy,fluorine,and chlorine atoms to phenyl,respectively,in order to investigate the effects of atoms with different electronegativities on the dye properties.According to the data analysis,the molecules with two atomic groups attached have relatively better optical properties,orbital energy levels,and charge transfer efficiency than the sensitizers without additional atoms attached to the phenyl group,or with one atom attached.Also,when the substituent group is two fluorine atoms（DFBP）,the sensitizer has a smaller energy gap and a significant red-shift in the absorption spectrum compared to other structures.The analysis of its electron-hole transfer properties revealed that the dye DFBP has a significant electron-hole separation and has a high electron transfer amount.Similarly,analysis of its adsorption system after anchoring with Ti O₂ reveals a significant reduction in the orbital energy gap and a red-shift in the absorption spectrum,as well as an upward shift in the edges of the conduction band of Ti O₂semiconductors,which would therefore further increase the short-circuit current and open-circuit voltage.