Realization And Mechanism Of High-performance White Organic Light-emitting Device

Organic light-emitting devices?OLEDs?which serve as the new generation of plat panel displays?FPDs?and solid state lighting,exhibit fascinating advantages such as wide view angle,ultrathin and light-weight structure,low drive voltage,vibration and impact resistance,homogeneous large area emission and high compatibility with transparent or flexible substrates.White OLEDs?WOLEDs?which have been widely applied in full-color FPD,solid state lighting and backlight source for liquid-crystal displays,have been paid sustained attention as research focus in academy and industry.Nowadays,high-performance WOLEDs suffer from bottleneck problems,such as complicated device structure and the challenged adjustment and control of host-guest doping ratio,which result in the difficulty in device fabrication,the high cost of equipments and the low yield.To solve the above bottleneck problems,in this work,conventional fluorescent materials,thermally activated delayed fluorescence?TADF?materials and TADF exciplexes have been adopted as host,and self-quenching-resistant phosphors have been employed as guest to build varied host-guest luminous systems.Meanwhile,the electroluminescence?EL?mechanisms and processes in these host-guest luminous systems have been systematically studied.Afterwards,hybrid complementary WOLEDs based on an orange ultrathin non-doped phosphorescent layer?UNPL?and blue heavily doped fluorescent layers have been fabricated.As a result,the devices have simultaneously achieved high efficiencies and excellent color stability without inserting a charge blocking layer at the interface of emitting layers?EMLs?.The main research contents are as follows:1.Orange-red phosphorescent OLEDs?PHOLEDs?have been fabricated,which are based on a heavily doped phosphorescent layer employing a phosphor bis[2-?biphenyl-4-yl?benzothiazole-N,C^2?]iridium?III??acetylacetonate?[?4Phbt?₂Ir?acac?]with self-quenching-resistant property.The devices maintain relatively high efficiencies in an easily controlled doping concentration range from 12 wt%to 24 wt%.A power efficiency?PE?,a luminance efficiency?LE?and an external quantum efficiency?EQE?are nearly 29 lm/W,above 33 cd/A and above 16.5%,respectively.In the heavily doped EML,sufficient?4Phbt?₂Ir?acac?molecules provide extra paths for charge recombination via facilitating hole injection and trapping electrons,which broadens the charge recombination region,alleviates exciton concentration quenching,and keeps the dynamic balance between the radiative recombination and quenching of excitons on the dopant.Meanwhile,?4Phbt?₂Ir?acac?molecules with steric-hindrance structure could suppress molecule aggregation and concentration quenching,which is beneficial to improve device efficiencies.Therefore,the devices are highly efficient and concentration-insensitive.2.Orange PHOLEDs have been fabricated,which are based on a lightly doped phosphorescent layer adopting a TADF material 4,5-bis?carbazol-9-yl?-1,2-dicyanobenzene?2CzPN?ashostandaphosphorbis(4-tert-butyl-2-phenylbenzothiozolato-N,C^2?)iridium?III??acetylacetonate?[?tbt?₂Ir?acac?]as guest.The devices achieve relatively high efficiencies at a doping concentration of 1 wt%,in which a PE,a LE and an EQE are 42.1 lm/W,77.9 cd/A and 26.8%,respectively.Meanwhile,the EQE maintains 26.6%and 25.8%at the practical luminance values of1000 and 5000 cd/m²,respectively.Moreover,a critical current density reaches 300mA/cm².In the lightly doped EML,the bipolar host 2CzPN facilitates the balance of charge transport,broadens the exciton formation region and alleviates triplet-triplet annihilation?TTA?.Besides,the up conversion of triplet excitons followed by the host-guest long-radius F?rster energy transfer of singlet excitons guarantees the complete energy transfer of both singlet and triplet excitons.Meanwhile,the above process decreases the triplet density on host,and alleviates TTA and singlet-triplet annihilation.Consequently,the devices achieve high efficiencies and low efficiency roll-off simultaneously.3.Orange PHOLEDs have been fabricated,which are based on a host-guest luminous system consisting of interface exciplexes as host and an UNPL using?tbt?₂Ir?acac?as guest at the interface of electron donor and acceptor materials4,4',4''-tris?N-carbazolyl?triphenylamine/bis?4,6-?3,5-di-?3-pyridyl?phenyl??-2-methylpy rimidine?TCTA/B3PYMPM?.The devices with a 0.3 nm-thick UNPL achieve high efficiencies.A PE,a LE and an EQE are 53.1 lm/W,53.3 cd/A and 19.5%,respectively.The interfacial intermolecular interaction between TCTA and B3PYMPM is efficient,so that TADF exciplexes are formed at this interface and leakage current is avoided.Moreover,TCTA and B3PYMPM with high-lying the lowest triplet excited state?T1?prohibit energy leakage from exciplexes to constituting molecules.Meanwhile,TCTA modulates the emission band of exciplex to totally cover the absorption band of?tbt?₂Ir?acac?,which ensures efficient host-guest energy transfer.Besides,TCTA/B3PYMPM interface triplet exciplexes could suppress their own non-radiative recombination via Dexter energy transfer and up conversion followed by F?rster energy transfer.All of them enable the devices reach high efficiencies.4.Hybrid complementary WOLEDs based on easily fabricated multiple EMLs have been reported.The multiple EMLs are composed of blue heavily doped fluorescent EMLs using a TADF material bis[4-?9,9-dimethyl-9,10-dihydroacridine?phenyl]sulfone?DMAC-DPS?as guest and an orange UNPL using?tbt?₂Ir?acac?.WOLEDs with a doping concentration of 50 wt%in blue EMLs and a 0.1 nm-thick UNPL realize relatively high efficiencies.A PE,a LE and an EQE reach 46.4 lm/W,54 cd/A and17.2%,respectively.As the UNPL is located adjacent to the main charge recombination region,the Commission Internationale de l'Eclairage?CIE?coordinates shift is decreased to?0.019,0.004?at a luminance range from 1000 to 10000 cd/m².However,the devices suffer drastic efficiency deterioration.It is noteworthy that,as the excitons in UNPL are mainly from energy transfer,the devices exhibit good color stability with the increase of luminance.Besides,the efficiencies of WOLEDs will be increased by broadening the major charge recombination region,which alleviates the energy loss of triplet excitons.5.Hybrid complementary WOLEDs have been fabricated,which are composed of an orange UNPL using?tbt?₂Ir?acac?and blue heavily doped fluorescent EMLs using DMAC-DPS as guest and a host material bis[2-?diphenylphosphino?phenyl]ether oxide?DPEPO?with high-lying T1 and wide band gap.The devices maintain high efficiencies and exhibit improved color stability,as the UNPL is inserted in the middle of the blue EML.A PE,a LE and an EQE achieve 45.8 lm/W,45.2 cd/A and 15.7%,respectively.Meanwhile,the CIE coordinates shift is only?0.008,0.003?at a luminance range from1000 to 10000 cd/m².In the heavily doped blue EMLs,bipolar DMAC-DPS which serves as the main channel of charge injection and transport,promotes the balance of charge transport and broadens the major charge recombination region.Therefore,the efficiencies of WOLEDs have been improved by decreasing the triplet energy loss.Meanwhile,the EL mechanism of the UNPL located in the broadened main charge recombination region is dominated by energy transfer,so that the excellent color stability of WOLEDs is realized with the increase of luminance.Consequently,the WOLEDs become highly efficient and color stable simultaneously.In summary,this work has been carried out to solve the bottleneck problems of high-performance WOLEDs,such as complicated device structure,hardly controlled host-guest doping ratio and high production cost.Specifically speaking,conventional fluorescent materials,TADF materials and TADF exciplexes have been employed as host,and self-quenching-resistant phosphors have been adopted as guest to build three host-guest luminous systems.Afterwards,the EL mechanisms and processes in these luminous systems have been studied.Meanwhile,the influence factors in the efficiencies and color stability of WOLEDs based on multiple EMLs have also been analyzed.All of them provide theoretical guidance for high-performance WOLEDs,in terms of the material choice of host-guest luminous systems and the design of device structures,and pave the way for the realization of superior display and lighting devices.