Realization of Highly Efficient Blue-/White-Emission of Polyfluorenes by Adjusting Charge Transmissions

Author:Liang Jun Fei

Supervisor:ye xuan li


Degree Year:2018





Polymer light-emitting diodes(PLEDs)have attracted great attention from academia and industry due to their unique characteristics,such as low cost,large area device and uninterrupted manufacturing.Compared with organic light-emitting diodes based on vapor-deposted,device performance of PLEDs is still far lagged behind,which is unsatisfied for practical application.In particular,the device efficiency and stability of blue lighting device is much poorer than those of green and red light-emitting device,which is come from its big bandgap and hole injected barrier.Therefor,developing excellent performance blue light-emitting PLEDs is the main challenge for employing PLEDs in display and soild-state lighting.In present,most of the reported blue light-emitting device and white light-emitting device shown rather poorer efficiency and stability.To address these problems,this work is focued on developing high performance blue-and exciplex white light-emitting PLEDs.Through contolled the morphology of emissive layer,balanced carrier injection and transportation,developed exciplex white-lighting polymer,we provide several simple and efficient methods to real high efficiency,stability of blue-and white-lighting PLEDs.Firstly,we systematically studied that how the excellent solvent toluene and chloroform affects the photoelectric performance of blue light-emitting polymer PFSO.After toluene and chloroform solvent vapor annealing(SVA),the PFSO film can efficiently formβ-phase which can obviously improve the unbalance charge transportation of PFSO.Additionally,the optimized PLEDs obtained significantly improvement of device performance.The maximun luminous efficiency(LEmax)of PFSO-based device is reached to 4.8 cd/A from 2.77 cd/A.The stability of EL spectrum also become much better after SVA,the EL spectrum of PFSO is near unchanged with increasing the applied voltage from 5 V to 15 V.Secondly,an excellent hole transport material BCFN was dopped into PVK to improve the holr mobility and hole injected barrier of PVK.The device performance of PFSO realized significantly improvement after employing PVK:BCFN as hole transport layer(HTL),because hole injected barrier and exciton quenning can be efficient decreased.After optimized the blending ratio of PVK and BCFN,the LEmaxax of PFSO-based device was improved from 2.77cd/A to 7.5 cd/A after using PVK:BCFN=3:7 as HTL.In addition,the LE was still maintained6.8 cd/A at 1000 cd/m2,which indicated that PVK:BCFN can efficiently avoid LE roll-off of PFSO-based device.After introducing PVK:BCFN,the EL spectrum of blue light-emitting device is very excellent,there is near unchanged with increasing applied voltage from 5 V to15 V。Thirdly,hole transport material BCFN was used to improve the morphology and charge transport characteristic of blue light-emitting polymer PFO.Additionally,the exciton quenching and hole injected barrier of PFO-based PLEDs were efficiently improve after introducing PVK:BCFN as HTL.The LEmax of optimized device was increased from 0.86 cd/A to 4.27 cd/A.And the EL spectrum of PFO also realized obvious improvement,which is almost unchanged with changing the applied votage from 5 V to 12 V.Fourth,we developed a new eciplex polymer(PFTS)by introducing strong electron acceptor SO and donor TPA-Cz in backbone and side chain,respectively.It can be realize emissive spectrum of exciplex polymer through tuning the ratio of electron donor TPA-Cz.Through uncomplete energy transfer from the backbone to the low energy state exciplex,we obtain the first white light-emitting polymer from exciplex.The optimized exciplex-based device realized the LEmax of 2.34 cd/A,the maximum luminance(Lmax)of 12410 cd/m2,the CIE of(0.27,0.39).Additionally,the exciplex polymer PFTS displayed excellent stability,both the LE roll-off and EL spectrum were significantly improved.With increasing applied voltage from 5 V to 15 V,the EL spectrum is near identical.Fifth,we investigate the photoelectric performance of PFTS after blending suitable ratio of PVK which has high triplet energy level.The nonradiative transition of exciplex PFTS was efficiently decreased after blending PVK.Furthermore,the charge transport balance of PFTS was also obviously improved after introducing PVK.The optimized device realized significant improvement of device performance,the LEmaxax was reached to 9.12 cd/A from 2.33 cd/A.Through simplely tuning the thickness of emissive layer,the emissive spectrum of PFTS-based device was changed to near ideal white lighting emission.When the thickness of emissive layer is 150 nm,the CIE of PFTS-based device is come to(0.33,0.32).After further optimizing the device,the device performance of high quality white lighting emission device also obtained obvious improvement.The LEmax is reached to 7.37 cd/A with the CIE of(0.30,0.43).