Synthesis And Photovoltaic Properties Research Of The Polymers Based On Difluorobenzo [c] Cinnoline

Polymer solar cells(PSCs)have attracted tremendous interests for their facile solution processability and potential low cost.Although power conversion efficiencies(PCEs)of fullerene-based and fullerene-free PSCs both have exceeded 10 % recently,it is necessary that new high performance photovoltaic materials should be continually developed to realize the commercial purpose in the future.To exploit high-performance polymer photovoltaic materials,three design strategies have been usually carried out,such as exploiting new building blocks,designing the structure of polymeric backbones,optimizing side chains.Many efforts have been devoted to design the polymeric backbones.The copolymers based on weak electron-donating(D)moieties and strong electron-withdrawing(A)moieties have been widely studied because of their low bandgap and appropriate energy levels.Moreover,the terpolymers with A-D1-A-D2 or D-A1-D-A2 structures have been proposed in order to broaden the absorption spectra of D-A polymers.In this study,2,9-difluorobenzo[c]cinnoline(DFBC)has been designed and synthesized as a kind of weak electron-withdrawing unit,and a series of DFBC-based conjugated polymers have been designed and synthesized.The effects of the polymeric structure on the photophysical and electrochemical properties and photovoltaic performance have been investigated systematically.1.A novel electron-deficient unit DFBC was designed and synthesized for the first time.Subsequently,two 3-(2-octyldodecyl)-thiophene were introduced to both terminals of DFBC units in order to improve the solubility of the polymers.Finally,three new polymers based on DFBC as the electron-withdrawing building blocks,PDFBC-T、PDFBC-T-T and PDFBC-BDTT,were designed and synthesized,in which thiophene,2,2’-bithiophene and 4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b: 4,5-b’]dithiophene(BDTT)were used as the electron-donating building blocks,respectively.The effects of the structure on the photophysical,electrochemical and photovoltaic properties of the polymers have been fully investigated.The results indicate that the polymer PDFBC-BDTT based on BDTT as electron-donating moieties possesses the deepest HOMO(highest occupied molecular orbital)energy level,which leads to the largest open circuit voltages(Voc).The PSC devices based onPDFBC-T,PDFBC-T-T and PDFBC-BDTT only exhibited the PCE values of 0.92%,0.37% and 0.88% because of the wide bandgap,respectively.2.A copolymer with A1-A2 structure is rationally designed and synthesized by using difluorobenzo[c]-cinnoline(DFBC)as weak electron-withdrawing(A)moieties and 1,4-diketo-pyrrolo[3,4-c]pyrrole(DPP)as strong A moieties.Simultaneously,3-(2-hexyldecyl)thiophene was introduced between DFBC and DPP units in order to improve the solubility of the polymer.The synthesized copolymer shows a low bandgap of 1.53 eV and a low HOMO energy level(-5.35 eV),which is lower than most DPP-based polymers with typical values above-5.25 eV.Consequently,the PSCs fabricated with PDFBC-DPP and fullerene derivatives exhibited high open circuit voltages(Voc)(0.78-0.80 V),which are higher than those of most DPP-based polymer PSCs(<0.70 V).Moreover,the PSC device fabricated with PDFBC-DPP and(6,6)-phenyl-C71-butyric acid methyl ester(PC71BM)as the photoactive layer shows a promising PCE of 7.92%,which is one of the highest PCE values for copolymers with A–A structure.This work not only brings a novel electron-withdrawing building block for PSCs,but also provides a novel strategy for designing effective polymer photovoltaic materials by combining weak A units and strong A units.