Design And Synthesis Of Non-Fused Electron Acceptors With An A-D-A’-D-A Framework For Organic Solar Cells

As a new clean energy technology,organic solar cells（OSCs）have the advantages of light weight,flexibility,solution processing and roll to roll large-area processing.Especially,non-fused ring acceptors（NFRAs）,featuring simple synthesis,flexible regulation and low cost,have been attracted the attention of researchers in recent years.To improve the power conversion efficiency（PCE）of NFRA-based OSCs,we designed and synthesized a series of new NFRAs through delicate molecular engineering.Furthermore,in this thesis,we investigated the relationships between molecular structure and optoelectronic properties,molecular stacking mode and PCE of devices.The specific contents are as follows.In Chapter 2,we synthesized two A-D-A’-D-A-type NFRAs,BTCIC and BTCIC-4Cl,employing electron-deficient benzothiadiazole（BT,A’）as the central cores and 4H-cyclopenta[2,1-b:3,4-b’]dithiophene（CPDT,D）as theπ-bridges for electron donors for the first time.The differences were terminal groups that 2-（3-oxo-2,3-dihydro-1H-inden-1-ylidene）-malononitrile（IC,A）is for BTCIC and 2-（5,6-dichloro-3-oxo-2,3-dihydro-1H-inden-1-ylidene）-malononitrile（IC-2Cl,A）is for BTCIC-4Cl.Compared with the fused ring acceptors,the NFRAs own the advantages of shorter synthesis route,less toxic coupling method and simple purification.Upon chlorination of end groups,BTCIC-4Cl exhibited higher crystallinity,wider spectral absorption and lower energy level than BTCIC.We selected PBDB-T and PBDB-T-2Cl as donors.BTCIC-4Cl-based OSC obtained the highest PCE of 10.5%,which was attributed to the better phase separation morphology of the blend film.Although BTCIC-based OSC achieved higher open-circuit voltage(V_oc),excessive mixed morphology of BTCIC-based blend films was not conducive to charge separation and transport,limiting the short-circuit current density(J_sc)and fill factor（FF）improvements.In this chapter,it is proved that A-D-A’-D-A type NFRAs with electron-deficient core,featuring the facile synthesis and satisfied device performance,are promising electron acceptors for OSCs.In order to further improve V_oc of the devices,in Chapter 3,we synthesized two small molecules BTZ-HD and ffBTZ-HD using benzotriazole（BTz）and difluorobenzotriazole（ffBTz）,respectively,which are less electronegative,as the center core of NFRAs,for studying the effect of fluorination of central core on photovoltaic performance.Compared with BTz-HD,ffBTz-HD with fluorinated center core showed stronger crystallinity and improved absorption coefficient.These properties show that fluorination effect on packing modes of the molecules in solid state,which was revealed by analyzing the single crystals of model compounds.The fluorinated model compound exhibits more planar molecular skeleton and forms a slip-stack stacking owing to the existence of F···S non-covalent interactions.These results greatly favored the understanding on the enhanced charge carrier mobility and device performance of the fluorinated acceptor.Therefore,when BTz-HD and ffBTz-HD were blended with donor PM6,PM6:ffBTz-HD-based OSC showed a higher J_sc and FF,increasing the PCE from 8.50%（PM6:BTz-HD）to 10.56%（PM6:ffBTz-HD）.The work in this chapter indicated that fluorination of central core is an effective method to improve the photovoltaic performance of NFRAs.In order to further study the effects of fluorinated and chlorinated end groups on NFRAs,we synthesized two small molecules ffBTz C4-4F and ffBTz C4-4Cl with different end groups using ffBTz as center core based on the previous chapter.We reported the single crystal structure of A-D-A’-D-A-type NFRAs for the first time.The single-crystal analysis showed that fluorinated end groups create too many non-covalent interactions between adjacent molecules,which is detrimental to the ordered stacking of ffBTz C4-4F,while the chlorinated end-group ffBTz C4-4Cl can form more compactπ-πstacking,which is conducive to the red shifted absorption spectrum of ffBTz C4-4Cl and the enhancement of crystallinity.When PM6was used as the donor,ffBTz C4-4Cl-based OSC achieved 11.74%PCE,which was higher than the PCE of 10.30%offered by ffBTz C4-4F.These results were attributed to more ordered molecular packing,smaller and more suitable phase separation morphology of ffBTz C4-4Cl blend film,which endowed PM6:ffBTz C4-4Cl-based OSC higher and more balanced charge mobility and weaker charge recombination.In this chapter,we systematically studied the relationship between molecular packing and photovoltaic performance of NFRAs,and confirmed that chlorinated end group is a low-cost and effective regulation strategy for NFRAs.In chapter 5,we focused on the side chains engineering on the central core of NFRAs.With the shortening of side chains on the central core,the crystallinity gradually increased and the absorption spectra redshifted in the order of ffBTz-BO,ffBTz-EH and ffBTz-C4.When blended with polymer donor PM6,ffBTz-EH blend film showed a better face-on orientation and the optimal fibrillary interpenetrating network morphology.Thus,the OSC based on ffBTz-EH obtained the highest J_sc(21.4 m A cm^-2),FF（0.70）and PCE（12.96%）.In this chapter,we systematically studied the effects of side chain engineering on the molecular stacking orientation of NFRAs and the morphology of blend films,which provided design guidance for improving the photovoltaic performance of NFRAs.