Investigations On The Synthesis And Regulations Of Triplet Energy Levels Of Bipolar Host Materials Based Diphenyphosphine Oxide And Carbazole

The phosphorescent organic light-emitting devices?Ph OLEDs?have been applied in full-color displays and solid-state lighting due to the unique characteristics of theoretically producing 100%internal quantum efficiency?IQE?by harvesting both singlet and triplet excitons.However,phosphorescent materials had usually be doped into the suitable host due to the roll-off efficiency at high current density.Therefore,it is especially important to prepare host materials with superior properties.The bipolar host materials can improve the stability and efficiency of the device based on their characteristics of balancing the carrier transport and broadening the exciton complex region.In this paper,a series of bipolar host materials were facilely synthesized by spirobifluorene,spirofluorene and carbazole?Cz?as donor groups,diphenylphosphine oxide?DPPO?and diphenyl sulfone?DPS?as acceptor groups.With various linkages between the donor and acceptor,the triplet energy?E_T?and the carrier transport properties of the molecules were regulated.The molecular structures were characterized and the photophysical and photochemical properties of these compounds have been studied.The red,green and blue Ph OLEDs were applied with these synthesized materials as bipolar hosts and bis?1-phenyl-isoquinoline??acetylacetonato?iridium?III??Ir?piq?₂acac?,fac-tris?2-phenylpyridine?iridium?Ir?ppy?₃?andiridium?III?bis[?4,6-difluorophenyl?-pyridinato-N,C^2']picolinate?FIrpic?as dopants.The electroluminescent characteristics and luminescence mechanism were also discussed in detail.Firstly,two novel host materials were designed and facilely synthesizedbased on the donor DPPO and the acceptor spirobifluorene and spirofluorene,i.e.,?4-?9,9'-spirobi[fluoren]-2-yl?phenyl?diphenylphosphine oxide?SPDPPO?and?9,9'-spirobi[fluorene]-2,7-diylbis?4,1-phenylene??bis?diphenylphosphine oxide??SBPBDPPO?in order to improve the thermal stability of the host materials by changing the conjugated length of the host materials.The host materials showed excellent thermal properties with the glass transition temperature?T_g?of 140 and 172°C of SPDPPO and SBPBDPPO which were obviouslybetterthanthatofthecurrenthostmaterial4,4'-bis?9H-carbazol-9-yl?biphenyl?CBP,T_g=62°C?.The emission wavelength of SBPBDPPO showed 23 nm red-shift than that of SPDPPO with the increase of the conjugated length.The E_T of SPDPPO and SBPBDPPO were 2.52 and2.48 eV,respectively.As increasing the conjugation length,the E_T of SBPBDPPO was lower than that of SPDPPO.The highest occupied molecular orbital?HOMO?/lowest unoccupied molecular orbital?LUMO?levels of SPDPPO and SBPBDPPO were-5.72/-2.1 and-5.76/-2.35 eV.The PhOLEDs were fabricated by doping Ir?ppy?₃ in hosts as light-emitting layer.The PhOLED employed SPDPPO as host exhibited a low turn-on voltage(V_on)of 3.0 V,the maximum luminance(L_max)of 19 440 cd/m²,the maximum current efficiency(?_c,max)of 36.5 cd/A,power efficiency(?_p,max)of 32.8 lm/W and the maximum external quantum efficiency(EQE_max)of 10.5%,respectively.Secondly,the compounds were synthesized with Cz,spirobifluorene and spirofluorene as the donor and the DPS unit as the acceptor,namely9-phenyl-3-?4-?phenylsulfonyl?phenyl?-9H-carbazole?PPSPCz?,2-?4-?phenylsulfonyl?phenyl?-9,9'-spirobi[fluorene]?PSPSF?and9,9-diphenyl-2-?4-?phenylsulfonyl?phenyl?-9H-fluorene?DPPSPF?in order to investigate the effect of different donor on the properties of the host materials.The electron donating ability of different donor groups were in the order of Cz>spirobifluorene>spirofluorene.The photoluminescence spectrum of PPSPCz exhibited red-shift spectra than that of PSPSF and DPPSPF because of thestronger donor ability of Cz.The E_T of PPSPCz,PSPSF and DPPSPF were2.64,2.48 and 2.81 eV,respectively.The green PhOLEDs were prepared by using PPSPCz,PSPSF and DPPSPF as hosts and Ir?ppy?₃ as emitter.The PhOLED based on PPSPCz exhibited the better performance with the?_c,max of53.8 cd/A,?_p,max of 38.2 lm/W and EQE_max of 15.4%,respectively.Employing PPSPCz as host,blue PhOLEDs with FIrpic as an emitter displayed the?_c,max of14.3 cd/A,?_p,max of 13.7 lm/W and EQE_max of 6.4%,respectively.Consequently,three bipolar materials were designed and synthesized using Cz unit as the donor and DPPO as the acceptor,the Cz linked by the N-phenyl as?3'-?9H-carbazol-9-yl?-[1,1'-biphenyl]-4-yl?diphenylphosphineoxide?CzPhPO?,the Cz connected through C2-and C3-phenyl in?4-?9-phenyl-9H-carbazol-2-yl?phenyl?diphenylphosphine oxide?2PhCzPO?and?4-?9-phenyl-9H-carbazol-3-yl?phenyl?diphenylphosphineoxide?3PhCzPO?.The effects of the different linkage between the donor and the acceptor on the photophysical properties were investigated.The emission of2PhCzPO showed blue-shift relative to 3PhCzPO because the inductive effects played a major role for 2PhCzPO with meta-linkage DPPO of N atom.In addition,the conjugated effect took effect on 3PhCzPO with para-linkage DPPO of N atom,resulting in the red-shift emission spectra of that of them.The E_T of CzPhPO,2PhCzPO and 3PhCzPO were 2.79,2.55 and 2.70 eV,respectively.The CzPhPO had higher E_T owing to the larger space hindrance by the Cz linked the benzene ring of N-phenyl.As a result,the blue PhOLED using FIrpic doped in CzPhPO as emiting layer displayed better performance with?_c,max of 34.7 cd/A,?_p,max of 33.7 lm/W,corresponding EQE_max of 15.8%as well as a low efficiency roll-off.Employing CzPhPO,2PhCzPO and 3PhCzPO as hosts,green PhOLEDs with Ir?ppy?₃ as an emitter displayed EQE_max of 19.7%,18.7%and 17.8%,which had increased by 25.5%,19.1%and 13.4%than that of the green PhOLED using CBP as host,respectively.With CzPhPO,2PhCzPO and 3PhCzPO as the hosts and Ir?piq?₂acac as dopant,the EQE_max of red PhOLEDs were 13.5%,13.4%and 10.7%.The EQE_max of red PhOLEDs based on CzPhPO and 2PhCzPO had increased by 16.4%and 15.5%than that of the red PhOLED using CBP as host.