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

In recent years,organic solar cells have developed rapidly with their unique advantages such as light,flexible and semitransparency,which exhibits great potential for portable power sources and building-integrated applications.By means of high design flexibility,easy modification and purification and good repeatability,small molecules have shown great development prospects.Therefore,people are mainly committed to design novel small molecular photovoltaic materials to achieve excellent photovoltaic performance of devices.In this dissertation,four design strategie were employed to synthesize eight small-molecule donors,including end-capped modulation,the fluorination on molecular backones,side-chain engineering and the extension of conjugated backones.We fully explored their photoelectric properties by density functional theory and time-dependent density functional theory calculations,UV-Vis absorption spectroscopy,cyclic voltammetry and finally blended with fullerene acceptors(PC₆₁BM/PC₇₁BM)to fabricate OPV devices and their photovoltaic performance were preliminarily explored.The details are as follows:(1)Four novel small molecules with different terminal groups were designed and synthesized successfully,namely TPAFNPI,TPAFNDI,TPAFNBT and TPAFNDPP,which contained triphenylamine(TPA)as electron donating unit,9-fluorenone(FN)as the central electron withdrawing unit and ethynyl as the?-linker.As a consequence,four molecules exhibited low HOMO level due to the strong electron-withdrawing of terminal groups,which could be beneficial to deliver high V_oc in OPV devices.It is worth mentioning that the bandgap decreased gradually with the enhancement of the terminal electron property,and finally TPAFNBT and TPAFNDPP exhibited better optical absorption properties.(2)Owing to the BT unit exhibited better solubility and film-processing ability than DPP unit,we investigated the effect of fluorination on photoelectric properties on the basis of BT units,obtaining TPAFNf BT and TPAFNff BT.We found that the introduction of fluorine atoms could not only decrease both the HOMO and LUMO energy levels,but also improve the intermolecular interaction,thus delivering more excellent photovoltaic properties.(3)In order to further optimize?-?interactions and effectively avoid the excessive self-aggregation resulting in poor photovoltaic properties,side chain modification was employed by using alkoxyl side chains,namely TPAFNDOBT,which caused enhanced the molar extinction coefficient,improved molecular packing and thus facilitating efficient charge transfer.(4)On the basis of TPAFNDOBT containing better solubility and?-?interactions,the conjugated backbone was further extended,giving the molecule(TPAFN)₂DOBT.As a result,the molecule exhibited low-lying LUMO level,and thus possessed a narrow bandgap of 1.67e V.Finally,the(TPAFN)₂DOBT based device achieved best photovoltaic performance among eight kinds of small molecule donors.