Synthesis And Photoelectric Property Of Dihydroxypropyl-containing Polyfluorene Copolymers

Conjugated polymers have been widely used in optoelectronic devices (such asorganic solar cells, organic light-emitting diodes and organic field effect transistors)for their excellent Photoelectric Properties and mechanical processing performance.Currently，conjugated polymers are mainly used as electron donor materials inphotovoltaic devices (OPVs). Polymer-fullerene bulk-heterojunction solar cells arethe mostly studied polymer solar cell (PSC), in this configuration, donor materialsshould have wide absorption and low band gap which match the energy level offullerene. Compared with other conjugated polymers, polyfluorene exhibit betterthermal stability, chemical stability and electron mobility. Incorporation of strongelectron-withdrawing unit into the polymer backbone was often used to optimize theUV absorption and bandgap of polyfluorene, Introducing polar groups(such asammonium groups, diethanolamino groups, sulfonate groups and phosphonate groups)into the side chain of polyfluorenes results in polymers which can be readily solublein polar solvents, Interfacial effect between polar groups and metal cathode cangreatly enhance the electron extraction at cathode, more and more such polymers arebeing used as cathode interlayers in PSCs.In this paper, dihydroxypropyl group was used to modify polyfluorenes and itsinfluence on the optical and electrical properties of the polymers was also studied,hydrophilic group dihydroxypropyl group was introduced into9-position of fluoreneto get monomer2,7-Dibromo-9,9-bis(2,3-propanediol-1-yl)-fluorene, Suzuki reactionof the monomer with the diboric acid ester of9,9-dioctyl fluorene resulted in polymerPF; polymer PFketral was prepared by Suzuki coupling reaction between2,7-Dibromo-9,9-bis(2,2-dimethyl-1,3-dioxolan-4-yl)-fluorene and the diboric acidester of9,9-dioctyl fluorene, PF’ with same structure as PF was obtained by hydrolysis of PFketal. The structure of monomers and polymers are characterized by1H NMR，FT-IR was also used to characterize the structure of polymers. Their electrochemicaland optical properties was studied by Ultraviolet-Visible absorption spectrum(UV-Vis), Photoluminescent spectral (PL) and cyclic voltammogram (CV)measurements. Structure characterizations of monomer and polymers demonstratesthat chemical compound were obtained as designed. UV-vis and PL spectras showthat the incorporation of hydroxyl groups influence little on the optical propeties ofpolymers. CV spectra indicate that PF and PF’ have relatively low oxidation onsetpotential compared with PFketal. The solubility of polymers in polar solvents can beimproved by incorporation of hydroxyl groups, PF and PF’ are both readily soluble inN,N-Dimethylformamide (DMF) and Dimethyl sulfoxide (DMSO) which are poorsolvents for PFketal.In order to optimize the UV-Vis spectrum, PL spectral and bandgap ofpolyfluorenes, low band gap comonomer benzothiadiazole (BT) and4,7-bis(5-bromo-thiophene-2-yl)-2,1,3-benzothiadiazole (DBT) were introduced into the backbone ofPF. The electrochemical and optical properties of the resulting copolymers were alsoinvestigated. UV-vis spectras show characteristic absorption peak of BT and DBTunits. Photoluminescent spectral studies indicate that there are efficient energytransfer processes from the fluorene segment to low band gap units. The incorporationof BT and DBT units not only greatly changes the optical behavior of copolymers, butalso lowers the onset oxidation potential and bandgap of polymers.