Studies Of Fluorescence/phosphorescence Hybrid Warm White Organic Light Emitting Diodes

White organic light-emitting diodes?WOLEDs?have unique advantages,such as high efficiency,transparency,flexible display,lightweight and ultra-thin,which make them have very important application in the field of lighting and flat panel display.In recent years,new organic materials and devices for OLEDs,such as thermally activated delayed fluorescence?TADF?,triplet-triplet annihilation?TTA?and local and charge-transfer?HLCT?,have been designed from the aspects of material synthesis and device structure.WOLEDs by using the delayed fluorescence characteristics of exciplex can effectively improve the luminescence efficiency and achieve nearly 100%internal quantum efficiency.In addition,the method of mixing blue fluorescence/with long-band phosphorescence can effectively utilize the good stability of fluorescence and the highly efficiency phosphorescence.It is an ideal strategy for achieving high efficiency and long lifetime WOLEDs.In this paper,high-efficiency yellow phosphorescent and fluorescent-phosphorescent hybrid warm WOLEDs with undoped structures were prepared by interfacial exciplex energy transfer strategy.The research contents are as follows.?1?The photoluminescence properties of 1,3-bis?9H-carbazol-9-yl?benzene?mCP?and 1,3,5-triazine-2,4,6-triyl?benzene-3,1-diyl?tris(diphenyl phosphine oxide?PO-T2T?films and mCP:PO-T2T co-doped films were studied.The PL spectra of mCP:PO-T2T co-doped films show the radiation of exciplex formed between electron donor mCP and electron acceptor PO-T2T.The triplet energy of the exciplex is lower than that of mCP and PO-T2T,which can effectively confine excitons to the emitting layer?EML?.Moreover,mCP and PO-T2T have higher hole and electron mobility,respectively,as well as greater HOMO and LUMO energy levels difference between them,which can promote more carriers to accumulate at the interface of mCP/PO-T2T.In order to investigate the electroluminescent properties of exciplex of mCP and PO-T2T,the devices with mCP:PO-T2T and double-layer structure of mCP/PO-T2T were fabricated.The EL spectra of the devices are consistent with those of the PL spectra of the mCP:PO-T2T films,indicating that the exciplex of mCP and PO-T2T are formed at the mCP:PO-T2T and the mCP/PO-T2T.The ultrathin layer of iridium?III?bis?4-?4-t-butylphenyl?thieno[3,2-c]pyridinato-N,C20?acetylacetonate?PO-01-TB?with a thickness of 0.1 nm was used as exciton detection layer.The exciton distribution in the double-layer structure device of mCP/PO-T2T was studied.The results show that the excitons are distributed at the interface of mCP/PO-T2T,and the excitons are mainly exciplex,accompanied by a small amount of mCP excitons.?2?High performance yellow organic phosphorescent devices with low efficiency roll-off incorporating undoped structure based on interfacial exciplex were fabricated by using NP structure of C₆₀/MoO₃ as hole injection layer?HIL?.Firstly,MoO₃?5 nm?thin layer,C₆₀?1 nm?/MoO₃?4 nm?and C₆₀?1 nm?/MoO₃?3 nm?HIL are inserted between ITO and mCP,respectively.The effect of HIL on the photoelectric performance of the device is studied.The turn-on voltage with the device of C60?1 nm?/MoO3?3nm?HIL layer approaches the thermodynamic limit of 2.46 V.The maximum brightness,current efficiency and power efficiency are 66 340 cd/m²,52.6 cd/A and 67.2 lm/W,respectively.Furthermore,the thickness of C60 in HIL was optimized.The device with C₆₀?2 nm?exhibits the best optoelectronic performance,which maximum brightness,CE and PE are 52 390 cd/m²,56.72 cd/A and 71.57 lm/W,respectively.When the brightness is 1000 cd/m²,the driving voltage is only 3.86 V,the CE is 52.06 cd/A,and the roll-off is only 8.2%.The low efficiency roll-off is related to the balance of carrier injection and transport due to the introduction of C₆₀/MoO₃ in the NP-structured HIL.The NP structure HIL significantly improves the photoelectric performance of the device and provides a new pathway for achieving high performance OLEDs.?3?High-efficiency fluorescent and phosphorescent hybrid complementary-color warm WOLEDs utilizing TADF material 4,5-di?9H-carbazol-9-yl?phthalonitrile?2CzPN?doped into mCP as blue layer and ultrathin yellow phosphorescent material PO-01-TB inserted into the interface of mCP/PO-T2T as yellow layer were fabricated.By well adjusting the thickness of the ultrathin layer,the optimized device shows a low turn-on voltage of 2.5 V,a maximum brightness of 5437cd/m²,a maximum PE of 55.4lm/W and an maximum EQE)of 14.7%.The device remains a PE of 27.8 lm/W and an EQE of 11.0%with the Commission Internationals de L'Eclairage?CIE?coordinates of?0.478,0.498?and CCT of 3056 T even at a brightness of 1000 cd/m².The 2CzPN emission in the device comes from the energy transfer of the exciplex at the mCP/PO-T2T interface.At high doping concentration of 2CzPN,the density of 2CzPN molecule in the doped layer increases,and the contact radius between 2CzPN molecule and PO-01-TB molecule increases,which accelerates the energy transfer from 2CzPN exciton to PO-01-TB,resulting in the decrease of the intensity of 2CzPN.Therefore,appropriate 2CzPN concentration and ultra-thin PO-01-TB thickness are the key to obtain high efficiency fluorescent and phosphorescent hybrid warm WOLEDs.