Research On High-efficiency Organic Light Emitting Diodes Based On Thermally Activated Delayed Fluorescence

Recently,novel fluorescent emitters with thermally activated delayed fluorescence?TADF?characteristics based on organic light emitting diodes?OLEDs?have attracted remarkable attention,promising to be the mainstream technology in the next generation of information display and lighting.However,the practical application of TADF mechanism based on OLEDs is in its infancy,there are still lots of key scientific problems about the application of TADF materials and internal energy transfer mechanism of OLEDs to be solved.In this work,we have focused on these key scientific issues and made some achievements listed as below:1.A full-fluorescence high-efficiency TADF white organic light-emitting devices?WOLED?was constructed by employing CDBP:PO-T2T exciplex as the host,2Cz PN and AnbTPA as blue and red dopants,respectively.By adjusting the carrier transport balance property and exciton distribution of each layer in EMLs,the WOLED with optimized device structure was obtained.As a result,the WOLED not only achieves high efficiency EL emission(maximum current efficiency,power efficiency,external quantum efficiency is 36.7 cdA^-1,46.2 lmW^-1,19.2%,respectively),and get high quality white light emission.At a luminance of 1000 cd m^-2,the stable white emission exhibits a high color-rendering-index of 82,a CIE coordinate of?0.33,0.38?and a small CIE variation value of?0.00,0.02?in the luminance range of 100-3000 cdm^-2.It is the best result of the current white OLEDs based on the TADF mechanism.2.The efficient red TADF exciplexes have been rarely reported,we propose a novel strategy to construct efficient red TADF exciplexes by introducing a phosphor as one component.Due to the strong spin-orbit coupling of heavy metal?e.g.,Ir,Pt,et al?ion cores,the NR decays will be evidently decreased for both singlet and triplet excitons,reducing these undesired excitons waste.Based on this strategy,we fabricate a red exciplex containing PO-T2T and Ir?ppy?₃ as components and realize a red emission with a peak at 604 nm,a CIE coordinate of?0.55,0.44?,and a high maximum external quantum efficiency of 5%and operational time of 24 hour,aging at the same brightness of 1000 cdm^-2.The high performance of red device is far superior than that of the device based on the conventional red exciplex emitter,proving the superiority of our novel strategy to construct red TADF exciplexes with phosphors.3.To address the low efficiency and serious roll-off at high excitation densities of TADF emitter based on organic light emitting devices,we propose a novel strategy for a new type of three-element exciplex TADF emitter by introducing a single-molecule TADF emitter and two tradition materials as one of the three-element exciplex compent.Such a new type of exciplex TADF emitter will have three RISC routes utilizing more triplet excitons,exhibit higher efficiency in the devices.Based on this strategy,we fabricated a high efficiency three-element exciplex CDBP:PO-T2T:DBT-spiro.The CDBP:PO-T2T:DBT-spiro exciplex based on OLEDs exhibits a low turn on voltage 2.4V and a high maximum EQE of 20.5%.Additionally,the EQE is still 14.2%at a high luminance of 1000 cdm^-2,proving the superiority of our novel strategy.The outstanding performance is the best result of the current OLEDs based on the TADF exciplex.