Synthesis And Photoelectric Properties Of Copolymers Based On Dibenzothiophene-S,S-dioxide

Dibenzothiophene-S,S-dioxide (SO) is an n-type aromatic heterocycle moiety, whichpocesses high fluorescence efficiency, electron affinity energy and electron-transportingproperties.Green light-emitting polyfluorenes were synthesized by Suzuki polycondensation viaintroducing dibenzothiophene-S,S-dioxide-(di-)tri-phenylamine moieties of D-A-D structureand dibenzothiophene-S,S-dioxide unit into the polyfluorene backbone. The copolymers showa high thermal stability; the efficient energy transfer from fluorene segments to low-band-gapunits occurred in the PL process; and the EL emission peaked at about510nm wasexclusively from the low-band-gap units. The lowest unoccupied molecular orbital (LUMO)levels reduce with increasing the SO unit content in the polymers. Incorporating SO unit intothe polymer backbone makes the device performances improved. The maximal luminousefficiency of9.0cd·A-1with the CIE coordinate of (0.27,0.56) was obtained based on asingle-layered device of ITO/PEDOT:PSS/polymer/CsF/Al. And the polymers exhibiteddramatic LE stability at high current densities. SO unit lowers the LUMO level, balances theinjection and transportation of both electron and hole in the polymers, and therefore improvesthe device performances.Novel white-emitting polyfluorenes were synthesized by intruducing2,7-bis(2-thienyl)fluoren-9-one unit into backbones. Then dibenzothiophene-S,S-dioxide unit was introduced intobackbones to obtain white-emitting polyfluorenes. Dibenzothiophene-S,S-dioxide possessesstrong electron-withdrawing ability and can improve the device performances. The bestdevice performanes are a maximum luminous efficiency (LE) of3.5cd·A1, a maximumluminance of9298cd/m2and a CIE coordinate of (0.34,0.32) with the configuration:ITO/PEDOT/PVK/polymer/CsF/Al.The alternating copolymer of2,8-bis(octyloxy)-dibenzothiophene-S,S-dioxide and3,6-carbazole and homopolymer of2,8-bis(octyloxy)-dibenzothiophene-S,S-dioxide weresynthesized. The alternating polymer exhibited the LEmaxand EQEmaxof0.78cd·A1and0.6%, respectively. And both of the polymers have the potential to act as host materialsbecause of their large band gaps.A series of novel neutral amino-and diethanolamino-functionalized polyfluorenederivatives with various ratios of2,8-bis(octyloxy)-dibenzothiophene-S,S-dioxide ascomonomers were synthesized and utilized as electron-injection layer (EIL) in combinationwith high-work-function Al electrode in polymer light-emitting diodes (PLEDs) and polymer solar cells (PSCs). The UV-vis absorption and photoluminescence properties of these novelpolymers are mainly determined by the conjugated main chains. The polymers possess deeperLUMO/HOMO levels than often-used EIL material poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)](PFN). These acid-assisted alcohol-soluble polymers can be used as EILs in combination with Al electrode to fabricate P-PPVPLEDs with device performances significantly higher than bare Al cathode device,comparable to or exceeding well-known Ba/Al cathode device and even PFN/Al cathodedevice. Among them, the best device performances are74times higher maximum luminousefficiency (LEmax=22.1cd·A1, EQE=7.9%) than that of bare Al cathode device (LEmax=0.3cd·A1, EQE=0.1%),1.9times higher than that of Ba/Al cathode device (LEmax=11.9cd·A1,EQE=4.3%) and1.5times higher than that of PFN/Al cathode device (LEmax=14.5cd·A1,EQE=5.0%). The PFO-DBT35:PCBM PSCs based on Al cathode with these novel polymersas EILs also show excellent device performances higher than Al cathode device, comparableto or exceeding PFN/Al cathode device. Among them, the best power conversion efficiency(PCE) is1.6times higher than that of bare Al cathode device and1.2times higher than that ofPFN/Al cathode device. The enhanced open-circuit voltages (Vocs) clarified that the electroninjection barrier from the Al electrode can be lowered by inserting the novel polymers as EILs.Moreover, the improvement of electron injection in combination with hole-blocking due tothe deep LUMO/HOMO levels of these novel polymers with the introduction of2,8-bis(octyloxy)-dibenzothiophene-S,S-dioxide unit into polymer backbones brings about theflux of charge carriers balanced, leading to an increase in efficiency. The characteristics ofboth main chain structures and pendant polar groups of these polymers have influence on theirelectron-injecting ability. The good device performances, the enhanced electron injectionfrom air-stable high-work-function metal Al in combination with hole-blocking and theunique solubility in environmentally friendly solvents make the novel polymers in this studypromising candidates as a new type of EIL materials for highly efficient PLEDs and PSCswith high-work-function Al electrode.