Study On Structure Design And Efficiency Roll-off Of Highly Efficient Organic Light-emitting Diodes

Organic light-emitting diodes（OLEDs）have drawn much attention due to their potential applications in full-color display and solid-state lighting.Nowadays,the efficiencies and stabilities of green and red OLEDs have been generally improved by the use of new phosphors and thermally activated delayed fluorescence（TADF）emitters.Nevertheless,there are several issues remains unsolved such as the efficiency stability（so called efficiency roll-off）of blue phosphorescent device,yellow TADF device at high brightness level.Although the efficiency roll-off in blue and yellow obtained some certain degree of progresses with the help of new materials,these are still fewer effective strategies in device design for reducing the efficiency roll-off in blue and yellow emission devices.Thus,in this work we developed a series of effective strategies in device structures to reduce the efficiency roll-off at high brightness in blue and yellow OLEDs,while considering high maximum efficiencies.Detailed works are as follows:1.High-efficiency blue phosphorescent devices with extremely high efficiency and stable device stability are achieved by using iridium（III）bis（4’,6’-difluorophenylpyridinato）tetrakis（1-pyrazolyl）borate（FIr6）as emitter.To balance carrier distribution on emitter molecules and reduce device operation voltage,devices with double emission layers（EMLs）are designed and fabricated.By optimizing the doping concentration of FIr6 and the device structure,balanced carrier distribution and wider exciton recombination zone are obtained,thus leading to a reduction of efficiency roll-off at high brightness.Finally,the optimal device obtained maximum current efficiency of 56.83 cd/A at 700 cd/m²（external quantum efficiency 28.35%）.And the same device can also remain high current efficiency of 56.33and 51.45 cd/A at 1000（3.9 V）and 5000 cd/m²（4.8 V）,respectively.2.A series of high-efficiency yellow OLEDs fabricated by using the designed double EMLs system with bis（2-phenylquinoline）（2,2,6,6-tetramethylheptane-3,5-dionate）iridium（III）（PQ₂Ir（dpm））and tris[2-（p-tolyl）pyridine]iridium（III）（Ir（mppy）₃）as red and green emission sources,respectively.By adjusting the thicknesses of red and green EMLs,we detailed researched the dependence of device performances on different EML thicknesses and excellent device EL performances are obtained due to the double EMLs system distributes the recombination zone all over the EMLs and reduces the triplet exciton quenching processes.Finally,the optimized device showed a maximum current efficiency up to 65.51 cd/A,a low roll-off of efficiency with a current efficiency up to 63.54 cd/A at5000 cd/m²,and good yellow emission with CIE coordinate of（0.464,0.496）at 10m A/cm².3.To our best knowledge,the most discovered yellow TADF emitters showed serious efficiency roll-off and worse device stability due to the narrow energy gap of yellow TADF emitters,which are easy to form carrier trapping and exciton quenching centers,thus resulting in the decrease of device efficiencies and device stability.To obtain excellent yellow emission TADF devices,in this work,we designed a new double EMLs system by using a phosphorescent dye as host materials for the assistant EML.Through utilizing the heavy-atom effects of the phosphor,the radiative rate of excitons within emitters are enhanced and then improved the device EL performances not only efficiencies but also device stability.Finally,the optimal yellow emission device obtained maximum current efficiency of 44.61 cd/A,with a maximum external quantum efficiency of 13.60%.At 2000cd/m²,the same device can also remain high current efficiency and external quantum efficiency of 36.51 cd/A and 11.40%,respectively.The used yellow TADF emitter is named2-[4-（diphenylamino）phenyl]-10,10-dioxide-9H-thioxanthen-9-one（TXO-TPA）.4.The EL performances of devices using TXO-TPA as emitter can further improved by reducing the electron density within EML due to the deep LUMO level of TXO-TPA molecule,leading to unbalanced carrier distribution on emitter molecules and narrow exciton recombination zone,which are harmful for the device performances.To reduce the electron density and charge accumulation within EML,we use another deeper LOMO phosphorescent dye doped within electron transport layer as electron trapper.With the help of the electron trapper doped within electron transport layer,reduced electron density and charge accumulation are obtained,and thus resulting in a decrease of device operation voltage and an enhancement of device performance.Finally,the optimal yellow device exhibits maximum current efficiency,power efficiency and external quantum efficiency of74.52 cd/A,75.81 lm/W and 23.70%,respectively.At 2000 cd/m²,the same device can remain high current efficiency and external quantum efficiency of 70.86 cd/A and 22.62%.The smart device performances have been closed to the best yellow phosphorescent devices in the lecture.