Design And Properties Of Fluorene-based Small Molecule Photovoltaic Donor Materials

Small-molecule organic solar cells（SM-OSCs）possess the advantages of light weight,facile fabrication,good flexibility and semitransparency,which is a promising photovoltaic technology.In recent years,in order to improve photovoltaic properties,tremendous efforts have been made to design and synthesize novel organic photovoltaic materials.Among them,end-capped effects and modulation ofπ-bridge are important ways to modify the structures and properties of organic photovoltaic materials.In this paper,five A-D-A type small-molecule donor（SMD）materials were synthesized by employing the design strategies of end-capped effects and modulation ofπ-bridge to investigate the influence of structural modulation on photoelectric properties.In order to investigate end-capped effects on photoelectric properties,four small molecules based on fluorene have been designed and synthesizd.The novel small molecules are denoted as flu（3TRD）₂,flu（3TCN）₂,flu（3TIN）₂ and flu（3TINCN）₂,with dioctyl-substituted fluorene（flu）as core,3,3’’-dioctyl-2,2’:5’,2’’-terthiophene（3T）asπ-bridge,terminated by rhodanine（RD）,malononitrile（CN）,1,3-indanedione（IN）,and3-（dicyanomethylidene）indan-1-dione（INCN）,respectively.The influence of electron-withdrawing intensity,conjugation length and planarity of different terminal units on photoelectric properties have been fully investigated and analyzed.Subsequently,flu（3TRD）₂was used as a reference molecule in which theπ-bridge was replaced by diketopyrrolopyrrole（DPP）to synthesize small molecule flu（DPPRD）₂,and the effect ofπ-bridge regulation onπ-conjugation and photoelectric properties have been explored.As a result,the different terminal units have little effect on planarity,and all of the molecules possess moderate planarity.Furthermore,flu（3TRD）₂,flu（3TCN）₂,flu（3TIN）₂ and flu（3TINCN）₂ show increased optical absorption properties and narrowed band gap successively,which are attributed to a combination of electron-withdrawing intensity and conjugation length of terminal units.By virtue of weak electron-donating character of fluorene-core,all the four compounds equip with low HOMO level,which could contribute to possessing high V_OC in photovoltaic devices.In addtion,the modulation ofπ-bridge extends the length ofπ-conjugated backbone of flu（DPPRD）₂,thus enhancing the optical absorption properties.Solution processed bulk-heterojunction（BHJ）devices with synthetic small molecules as electron donor and PC₇₁BM as electron acceptor have been fabricated and investigated.Among them,improvements of photovoltaic properties have been successfully realized via tuning terminal electron-withdrawing units.The power conversion efficiency（PCE）of the devices as-cast prepared from flu（3TRD）₂,flu（3TCN）₂ and flu（3TIN）₂ increases in turn,which is ascribed to incremental optical absorption thereby promoting short-circuit current density(J_SC).Particularly,flu（3TCN）₂ shows deeper LUMO and HOMO energy levels than flu（3TRD）₂ and flu（3TIN）₂ due to the strong electron-withdrawing character of malononitrile.As expected,the devices as-cast based on flu（3TCN）₂ exhibit a reasonable high V_OC of 1.07 V.In addition,a satisfactory PCE of 7.06%has been obtained for flu（3TCN）₂-based device with dichloromethane vapor-treatment.