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 of?-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 of?-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?as?-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 the?-bridge was replaced by diketopyrrolopyrrole?DPP?to synthesize small molecule flu?DPPRD?₂,and the effect of?-bridge regulation on?-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 of?-bridge extends the length of?-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.