High Triplet Energy Polymer as Host for Electrophosphorescence with High Efficiency

Abstract

We report the conjugated polymer P(tBu-CBP) as a host with high triplet energy (E(T) 2.53 eV) and suitable HOMO (5.3 eV) and LUMO (2.04 eV) energy levels. Upon doping with green and red emission Ir-complexes, it gives devices with high luminous and external quantum efficiencies for green emission (23.7 cd/A, 6.57%) and for red emission (5.1 cd/A, 4.23%), respectively, and low turn-on voltage (3 V). For both devices, the efficiencies are higher than those of the corresponding devices with the same backbone P(3,6-Cz) as a host by a factor of 4, even though the latter has an E(T) (2.6 eV) slightly higher than that of the former. The results reflect that, in phosphorescent devices, the difference in E(T) between the host and guest is not the only factor that determines the device efficiency, and the present side group modification via the 9 position of carbazole also plays an important role, which allows a tuning of HOMO and LUMO levels to provide more balance in electron and hole fluxes and provides prevention from formation of excimer.

Multifunctional Triphenylamine/Oxadiazole Hybrid as Host and Exciton‐Blocking Material: High Efficiency Green Phosphorescent OLEDs Using Easily Available and Common Materials

“…[ 25 , 26 ] It is important to confi ne excitons in EML to increase the recombination effi ciency of holes and electrons by blocking both charge carriers and excitons diffusion out of EML. Since the triplet energy ( E T ) of NPB (2.3 eV) [ 18 , 20 ] is not high enough to block triplet excitons of green emitter (ppy) 2 Ir(acac) ( E T : 2.41 eV), [ 23 ] devices G-I with the confi gurations of ITO/MoO 3 (10 nm)/ NPB (65 nm)/ m -TPA-o -OXD (5, 7.5, and 10 nm for device G, H, I, respectively)/ m -TPA-o -OXD: (ppy) 2 Ir(acac) (9 wt%, 20 nm)/TPBI (40 nm)/LiF (1 nm)/Al(100 nm) were fabricated to further improve the device performance, in which a thin 1000 and 10000 cd m − 2 , the roll-off values are 5% and 25% for device B, whereas those are 14% and 31% for device A, respectively. The better EL performance and lower roll-off in TPBI based device compared to BCP/Alq3 based device, can be elucidated from the three aspects: i) holes can be easily injected into EML since no hole injection barrier from NPB to m -TPAo -OXD host.…”

Singlet Generation from Triplet Excitons in Fluorescent Organic Light-Emitting Diodes

“…For the most common blue (aqua) phosphor, FIrpic [4], vacuum deposition causes partial loss of fluorine substituents from the ligands, and partial decomplexation of the picolinate (pic) ancillary ligand [5,6], this then causes further degradation over a short period of running, leaving the blue pixels with a much shorter lifetime than both the red and green. Secondly, because of the necessity of having such a high triplet energy to yield the correct blue colour, problems arise with finding a host material with a triplet energy higher than that of the emitter but which can also act as a good transport material for electrons and holes [7][8][9][10][11]. Currently, the best state-ofthe-art blue phosphorescent material is that demonstrated by BASF and the group of Kido at Yamagata University [12], who report a "blue" phosphorescent device from a nonfluorine containing Ir complex having peak emission at 454 nm with high quantum efficiency 36 lm/W, 29 cd/A, EQE 18.6%, but which suffers from serious efficiency roll off.…”

Theoretical investigation of the degradation mechanisms in host and guest molecules used in OLED active layers

“…Typically hole-transporting fragments based on carbazole 13,36 or silane 37,38 derivatives have been considered although issues related to device efficiency or lowering of device operating voltage remain. For example, the most commonly used carbazole-based material, 4,4'-bis(N-carbazolyl)-1,1'-biphenyl (CBP) with high triplet energy gaps and excellent hole transporting properties, has been widely used as host material for green and red phosphorescent emitters.…”