New Phenothiazine Dyes With Benzothiadiazoleacceptor And Corresponding DSSC Study

Dye-sensitized solar cells（DSSCs）have attracted much attention owing to its great potential for applications under both indoor and outdoor illumination.Dye sensitizer is an essential element in DSSC devices,playing important roles of light harvesting,participating in exciton dissociation and dye regeneration,and hence its properties directly affect the performance of the cell.In this paper,based on benzothiadiazole（BTD）acceptor,we designed and synthesized a series of new phenothiazine dyes,which were applied to DSSC devices to study the relationship between their structures and the properties of DSSC devices.In detail:（1）Based on BTD and cyanoacrylic acid（CA）/rhodanine-3-acetic acid（RA）,we designed and synthesized four new phenothiazine dyes（CBPB-CA,CBPBB-CA,CBPB-RA and CBPBB-RA）,and applied them in DSSC devices.Compared with CBPB-CA and CBPB-RA,the insertion of the benzene ring between the auxiliary electron acceptor BTD and the electron acceptor of CA/RA endows CBPBB-CA and CBPBB-RA a larger twist angle between BTD and CA/RA,resulting in an obvious blue shift of the intramolecular charge transfer（ICT）absorption peak and a decrease in light-harvesting ability.On the other hand,the larger twist angle can effectively prevent dye aggregation and back electron transfer,thereby significantly inhibiting charge recombination and improving the photovoltaic performance.From the point of view of the electron acceptor,when the CA in dyes CBPB-CA and CBPBB-CA is replaced by RA with a much stronger electron-withdrawing ability,enhanced intramolecular charge transfer（ICT）effect is realized,which leads to the bathochromically shifted ICT absorption peaks with obvious increased molar extinction coefficients for new dyes CBPB-RA and CBPBB-RA.However,the sp³-C between rhodanine ring and carboxyl group in RA blocks the conjugation between them,which destroys the delocalization of the dye LUMO electrons to the anchored group and reduces the electron injection efficiency from the dye LUMO into the conducting band of the titanium dioxide,and thus resulting in a lower performance of the DSSC device.Among the four dyes,DSSC device with CBPBB-CA exhibited a maximum photoelectric conversion efficiency（PCE）of 5.79%,and the corresponding J_SC and V_OC were 12.66 m A cm^-2 and0.76 V,respectively.（2）We designed and synthesized four phenothiazine dyes with different electron acceptors,and applied them in DSSC devices.Among these dyes,two are new phenothiazine dyes（CBPTZ-3 and CBPTZ-4）with BTD acceptor,and the other two are reported dyes（CBPTZ-1 and CBPTZ-2）.The study shows that CBPTZ-2 with RA electron acceptor possesses a broader and stronger ICT absorption peak,but it has the lowest electron injection efficiency,resulting in the lowest PCE.Theoretical calculations and photophysical measurements disclose that with respect to CBPTZ-3,CBPTZ-4 comprising an additional ethynylene-group between BTD and benzoic acid shows a more coplanar molecular skeleton,which endows CBPTZ-4 with a higher dye loading and a strengthened ICT effect,resulting in a red-shifted ICT absorption with a higher molar extinction coefficient.EIS and OCVD measurements show that compared to CBPTZ-1 and CBPTZ-2,CBPTZ-3 and CBPTZ-4 exhibit a better performance in terms of inhibiting charge recombination,which can effectively restrain charge recombination and improve the performance of DSSC devices.Among the four dyes,DSSC device with CBPTZ-4 shows the best photovoltaic performance:J_SC=14.04 m A cm^-2,V_OC=0.82 V and PCE=7.33%.This performance is among the good DSSC devices based on a newly synthesized dye,showing its potential for DSSC applications.