Research On Mixed-host-structure-based White Organic Light-emitting Diodes

Organic light-emitting diodes(OLEDs)are widely used in display,lighting,medical treatment,optical fiber communication,and other fields.They have attracted a lot of attention from academia and industry because of their excellent characteristics such as high efficiency,lightweight,flexibility,and environmental protection.However,the problems of unbalanced carrier transport,high-efficiency roll-off,poor spectra stability,and short lifetime of non-optimized white OLEDs seriously restrict the development of commercial applications.To solve the above problems,a mixed-host structure is proposed in this paper,which can effectively solve the problem of unbalanced carrier transport by regulating the carrier behavior of the devices.At the same time,with its excellent characteristics,it is possible to prepare white OLEDs with low-efficiency roll-off,stabile spectra,and long lifetime.The schemes we designed to solve the key technical problems faced by white OLED for different focuses.The research work of this paper is summarized as follows:1.Aiming at the poor performance of OLEDs caused by unbalanced carrier transport,a scheme of carrier transport control using a multilayer mixed-host structure without transport layer is proposed.The mixed-host composes of hole-transport material TCTA and electron-transport material Bm Py Ph B.The carrier transport is manipulated by adjusting the doping ratio of the mixed host,and the correctness of the theory is verified by detecting the carrier mobility.The monochromatic blue,red,yellow and green OLEDs with mixed-host structure achieve the maximum efficiency of 38.8 cd/A,36.6 cd/A,69.3 cd/A,and 72.6 cd/A respectively.At the same time,they realize low turn-on voltage and low-efficiency roll-off.Next,based on the multilayer mixed-host structure,red-blue,yellow-blue,and red-yellow-blue white OLEDs are developed.The multilayer mixed-host structure can broaden the exciton recombination region and balance the carrier transport,to effectively suppress the change of spectra with the increase of voltage.The final white OLED achieves the maximum efficiencies of 43.3 cd/A,52.1 cd/A,and 47.3 cd/A respectively,and the devices achieve low-efficiency roll-off at the same time.This research scheme effectively solved the problem of poor device performance caused by the unbalanced carrier transport and provided an effective scheme for realizing the balanced carrier transport of OLEDs.2.Aiming at the problem of unstable spectra caused by exciton quenching and the shift of exciton recombination region in white OLEDs,a research scheme of introducing the mixed spacer into the white OLEDs with the combination of ultrathin emission layer and doped emission layer is proposed.The mixed spacer layer is composed of TCTA and Tm Py PB.The electroluminescent performance and spectra stability of the devices are manipulated by adjusting the doping ratio of the mixed spacer layer.Compared with the white OLEDs with unipolar spacer,the spectra stability of the white OLED with mixed spacer is significantly improved,while the performance of the device still needs to be improved.Then,the mixed-host structure is introduced into the yellow emission layer,and it is detected that the optimal white device achieves a high efficiency of 47.5 cd/A and excellent spectral stability.The CIE coordinate changes only(0.005,0.001)in the brightness range from 1050 cd/m~2 to13760 cd/m~2.By analyzing the exciton recombination region,energy transport mechanism,and carrier transport characteristics of the white OLEDs,it is concluded that the high spectra stability is due to balanced carrier transport and widened exciton recombination region.This technology solves the problem of spectral change caused by exciton quenching effect and the movement of exciton recombination region,providing a new idea for the design of white OLED with stable spectra.3.Aiming at the problems of high-efficiency roll-off of white OLEDs at high luminance,an optimization scheme of efficiency roll-off of white OLEDs based on thermally activated delayed fluorescence(TADF)mixed-host structure is proposed.Firstly,a new TADF mixed host DMAC-DPS:Tm Py PB is constructed and applied to white OLEDs.Then,the performance of blue devices based on the TADF mixed host is explored.Compared with the traditional host,it is found that the TADF mixed host can significantly improve the efficiency and efficiency roll-off of blue devices.Then,white devices are prepared by using the TADF mixed-host structure.In scheme 1,the yellow emission layer adopts the ultrathin layer structure.The white OLED achieves the maximum efficiency of 46.1 cd/A and the efficiency roll-off is only 7.4%at 3000cd/m~2;In scheme 2,the double TADF mixed-host structure is adopted.The white OLED achieves the maximum efficiency of 53.7 cd/A and the efficiency roll-off is only1.5%at 1000 cd/m~2 and 5.2%at 3000 cd/m~2.TADF mixed host structure can balance the carrier transport and make the excitons uniformly distributed in the emission layer.Its efficient reverse intersystem crossing mechanism minimizes the concentration of triplet excitons,thus inhibiting the exciton quenching effect.The scheme solves the technical problem of high-efficiency roll-off of white OLEDs at high luminance and provides a feasible technical scheme for the design of low roll-off white OLEDs.4.To solve the problem that many degradation mechanisms of white OLEDs seriously affect the device lifetime,a research scheme based on TADF mixed-interlayer structure is proposed to improve the lifetime of white OLEDs.Firstly,a novel TADF mixed spacer DMIC-TRZ:TPBi is constructed and applied to fluorescent/phosphorescent hybrid white OLEDs.Importantly,compared with the white OLEDs using the traditional unipolar spacer,the mixed spacer greatly improves the device lifetime,which is up to 3319.2 hours at 1000 cd/m~2,which is in the forefront compared with the previously reported unpacked white devices.Through the detection of the carrier transport characteristics of white OLEDs and the analysis of energy transfer mechanism,it is concluded that the long device lifetime is due to the balanced carrier transport in the spacer layer,the promoted F(?)rster energy transfer mechanism and the adoption of long-lifetime blue fluorescent materials.In addition,the white OLEDs adopt a double ultrathin emission layer structure,which is more suitable for commercial mass production.