Properties Of Organic Light-Emitting Diodes Based On Iridium Complexes

Three green-emitting heteroleptic iridium complexes with 2-(4,5,6-trifluorophenyl)pyridine (F3,4,5ppy),2-(3,4,6-trifluorophenyl)pyridine (F3-4,6ppy) or 2-(3,4,5-trifluorophenyl)pyridine (F4,5,6ppy) as main ligands and tetraphenylimidodiphosphinate acid (Htpip) as the ancillary ligands were synthesized and characterized. The electron mobilities of these complexes were measured by the means of transient electroluminescence (TEL). The organic light-emitting diodes (OLEDs) (ITO/TAPC (1,1-bis(4-(di-p-tolyl-amino)phenyl)cyclohexane,30 nm)/ Ir-complex (x wt%):mCP (N,N’-dicarbazolyl-3,5-benzene,15 run)/TPBi (1,3,5-tri(1-phenyl-1H-benzo[d]imidazol-2-yl)phenyl,45 nm)/LiF (1 nm)/Al (100 nm)) based on these above phosphorescent emitters exhibited good performances. The device doped Ir(F3,4,6ppy)2tpip with 8 wt% concentration showed superior performances with a peak current efficiency (ηc) of 66.36 cd·A-1 and a peak external quantum efficiency (ηext, EQE) of 25.7%at 5.8 V, a maximum power efficiency (ηp) of 48.20 lm·W-1 at 4.4 V and a maximum luminance (Lmax) of 47627 cd·m-2 at 12.6 V. It is worth noting that the EL efficiency roll-off effects at relatively high current density in all devices are very mild, which are helpful to obtain high efficiency and brightness. The results suggested that the fluorine atoms on phenyl ring and the application of Htpip as the ancillary ligand can improve the electron mobility of the complexes to comparable to that of popular electron transport material of Alq3 under the same electric fields.Three ancillary ligands containing electron-transporting [1,3,4]oxadiazole moieties,2-(5-phenyl-[1,3,4]oxadiazol-yl)-phenol (HPOP),2-[5-(4-fluoro-phenyl)-[1,3,4]oxadiazol-yl]-phenol (HFPOP),2-[5-(4-trifluoromethyl-phenyl)-[1,3,4] oxadiazol-yl]-phenol (HCF3POP) and their corresponding indium complexes with 2-(3,4,6-trifluorophenyl)pyridine (F3,4,6ppy) as main ligand, Ir(F3,4,6ppy)2POP, Ir(F3,4,6ppy)2FPOP and Ir(F3,4,6ppy)2CF3POP were synthesized and characterized. The three complexes have green emission with maximum peaks in the range of 503-521 nm with quantum efficiency yields of 18.04%,12.05% and 12.87%, respectively. The lifetimes of complexes are in the range of microseconds (1.83-2.14 μs in CH2Cl2 solution). The OLEDs (ITO/TAPC (30 nm)/Ir(F3,4,6ppy)2POP (x wt%):host materials (15 nm)/TPBi (45 nm)/LiF (1 nm)/Al (100 nm)) exhibited good performances. SimCP2 (bis(3,5-di(9H-carbazol-9-yl)phenyl)di-phenylsilane) and mCP were employed as the host material, respectively. Particularly, the device with 8 wt% concentration doped in SimCP2 showed superior performances with a peak ηc of 61.49 cd·A-1 and a peak ηp of 46.03 lm·W-1. Furthermore, the efficiency roll-off ratios from the peak values to that at the practical luminance of 100 cd·m-2 and from the brightness of 100 cd·m-2 to 1000 cd·m-2 in these devices are slow, which is helpful to keep high efficiency at relatively high current density and high luminance.