High Performance Sensitized Blue And White Organic Light-Emitting Diodes

Due to the characteristics of self-luminescence,light weight and flexibility,organic light-emitting diode(OLED)has brought a qualitative leap in the development of display and lighting technology,and has been commercially applied.For the realization of full color display and white lighting technology,three colors of red,green and blue are necessary.Compared with the mature and stable commercial performance of red and green OLEDs,blue OLED still has low efficiency,poor stability and other defects,resulting in yellowing or burn-in screens of terminal devices.In addition,the relatively lagging development of blue OLED has also reduced the competitiveness of white OLED products in the market.Therefore,the research of high performance blue and white OLEDs has been the focus and difficulty of OLED development.In this paper,we take the design and fabrication of high performance blue and white OLEDs as the main line.According to theoretical guidance,several sensitized strategies have been proposed.Through the targeted control of carriers and excitons behavior,a series of high performance sensitized blue and white OLEDs have been obtained,and the operating principles of sensitized dvices were systematically studied.The main research contents of this paper are as follows:1.For the emission phenomenon of host material and hole transport material in devices,a strategy of sensitized electron transport layer(ETL)was designed.The transition metal complex bis[4-tert-butyl-2’,6’-difluoro-2,3’-bipyridyl](acetylacetone)iridium(Ⅲ)(FK306),which has a low lowest unoccupied molecular orbital(LUMO)energy level,was used as sensitizer and doped into ETL to regulate electrons’ transport.Experimental results demonstrated that FK306 molecules played the role of electrons’ trapping centre,thus reducing electrons’ accumulation in EMLs and boosting carriers’ recombination probability.Through the study of carriers’ recombination position,it was found that electrons’ trapping effect also broadened carriers’ recombination zone,thereby reducing excitons density and improving radiation efficiency.Finally,based on sensitized ETL structure,sensitized devices displayed better electroluminescence(EL)performance.2.To suppress serious excitons annihilation in blue devices,based on the aforementioned sensitized ETL structure,a double-sensitized structure was designed.In double-sensitized devices,electrons’ accumulation around emitter molecules was reduced by utilizing sensitized EML structure,thus weakening triplet exciton-polaron quenching(TPQ)effect.In addition,sensitized ETL structure reduced electrons’transport into EMLs,causing the shift and effective broadening of carriers’recombination zone,thus achieving the purpose of suppressing triplet exciton-tripet exciton annihilation(TTA)effect.Finally,the EL performance of double-sensitized blue devices has been greatly improved.Based on double-sensitized blue devices,a series of high performance white devices were obtained.Meanwhile,it was also verified that double-sensitized structure also produced positive impact on other color devices.3.For improving color stability of white devices,a gradient-doping rare earth sensitized structure was designed.Through the study of EL mechanism,it was found that unbalanced carriers’ distribution and the shift of recombination zone in EMLs were the main reasons for poor color stability of white devices.Based on this conclusion,rare earth complex tris(thiophenyltrifluoroacetone)(1,10-phenanthroline)europium(Ⅲ)(Eu(TTA)3phen)was used as sensitizer and gradiently doped into EML to construct gradient-doping EML structure.Research results demonstrated that Eu(TTA)3phen molecules produced electrons’ trapping effect and adjusted the distribution of carriers’recombination zone,thus improving white light quality.On the basis of electrons’trapping effect,gradient Eu(TTA)3phen molecules distribution brought concentration difference of electrons’ distribution,thus forming built-in electric field.The generation of built-in electric field not only reduced operation voltage of white devices,but also weakened the dependence of electrons on electric field,thus achieving the purpose of enhancing color stability of white devices.4.For improving EL performance of narrow emission blue devices,a hot exciton sensitized thermally activated delayed fluorescence(TADF)device structure was designed.For narrow emission TADF material 2,12-di-tert-butyl-5,9-bis(4-(tert-butyl)phenyl)-5,9-dihydro-5,9-diaza-13b-boranaphth o[3,2,l-de]anthracene(t-DABNA),due to large energy level difference between lowest singlet state and triplet state(ΔES1-T1),low reverse intersystem crossing(RISC)rate led to triplet excitons’ accumulation on t-DABNA molecules.Therefore,there were a large number of triplet excitons annihilation in t-DABNA based devices,resulting in poor EL performance and serious efficiency roll off.In order to reduce triplet excitons’ accumulation on t-DABNA molecules,the hot exciton sensitized TADF structure was designed by using hot exciton material 9,9-dimethyl-2-(1-(pyren-1-yl)-9H-carbazol-9-yl)-9H-thioxanthene 10,10-dioxide(P-Cz-TXO2)as sensitizer.It was proved that the sensitization effect of P-Cz-TXO2 reduced carriers’ recombination probability on t-DABNA molecules,and triplet excitons formed on P-Cz-TXO2 molecules were rapidly converted into singlet excitons through high energy level RISC(h-RISC)process.Then,converted singlet excitons together with singlet excitons formed by carriers’ recombination were transferred to t-DABNA molecules through Forster energy transfer,finally achieveing efficient fluorescence radiation decay on t-DABNA molecules.As a result,the hot exciton sensitized TADF devices not only obtained relatively higher EL performance,but also maintained pure blue emission characteristic with narrow spectra.