Rongzhen Cui

Green organic light-emitting devices with external quantum efficiency up to nearly 30% based on an iridium complex with a tetraphenylimidodiphosphinate ligand

In this work, a series of electroluminescent (EL) devices with single- or double-light-emitting layer(s) (EML) were fabricated to further improve the EL performances of green iridium complex (tfmppy)2Ir(tpip) (tfmppy = 4-trifluoromethylphenylpyridine, tpip = tetraphenylimido-diphosphinate). p-Type material 4,4′,4′′-tri-s(carbazole-9-yl)triphenylamine and bipolar material 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine were chosen as host materials of EML1 and EML2, respectively. Experimental results displayed that not only the doping concentration but also the thicknesses of EML and the electron transport layer strongly influence device performances. Finally, a high performance green EL device with maximum brightness, current efficiency, power efficiency and external quantum efficiency (EQE) up to 113 610 cd m−2, 112.30 cd A−1, 97.95 lm W−1 and 29.4%, was realized. Even at the practical brightness of 1000 cd m−2, current efficiency as high as 107.6 cd A−1 (EQE = 28.1%) can still be retained by the same device. To our best knowledge, EL performances of this device were amongst the highest results of the previously reported green devices.

High performance red phosphorescent organic electroluminescent devices with characteristic mechanisms by utilizing terbium or gadolinium complexes as sensitizers

In this work, we demonstrated the efficacy and feasibility of utilizing terbium and gadolinium complexes with low-lying energy levels to sensitize red-emitting iridium complexes in organic light-emitting diodes (OLEDs). Compared with devices without the introduction of a sensitizer, the obtained sensitized devices showed remarkably enhanced electroluminescence performances, which can be attributed to improved carrier balance as well as a wider recombination zone. Moreover, characteristic sensitizer emission was invisible in all sensitized devices due to the inferior hole trapping ability of sensitizer molecules. Finally, the sensitized device co-doped with 0.4 wt% of the terbium complex realized superior electroluminescence performances with maximum brightness, current efficiency, power efficiency and external quantum efficiency as high as 145 071 cd m−2, 64.87 cd A−1, 69.11 lm W−1 and 24.7%, respectively. Meanwhile, even at the practical brightness of 1000 cd m−2 (4.0 V), outstanding external quantum efficiency and current efficiency up to 22.7% and 59.7 cd A−1, respectively, were obtained.

High performance red organic electroluminescent devices based on a trivalent iridium complex with stepwise energy levels

In this work, electroluminescent (EL) devices with double light-emitting layers (EMLs) having stepwise energy levels were designed and fabricated to improve the EL performances of the red light-emitting trivalent iridium complex bis(2-methyldibenzo[f,h]quinoxaline)(acetylacetonate)iridium(III) [Ir(MDQ)2(acac)]. To broaden the recombination zone and facilitate the balance of carriers on emitter molecules, the widely used p-type material 4,4′,4′′-tri(N-carbazolyl)triphenylamine (TcTa) and bipolar material 2,6-bis(3-(9H-carbazol-9-yl)phenyl)pyridine (26DCzPPy) were chosen as host materials of EML1 and EML2, respectively due to their well matched energy levels. Interestingly, slight decomposition of Ir(MDQ)2(acac) molecules was observed during the deposition of EML, which causes the rapidly decreased brightness at relatively high doping concentration. Finally, a high performance red EL device with maximum current efficiency of 44.76 cd A−1, power efficiency of 40.19 lm W−1, and external quantum efficiency (EQE) of 15.5% was obtained by optimizing the doping concentration of Ir(MDQ)2(acac). Even at a high brightness of 1000 cd m−2 (5.2 V), a current efficiency as high as 40.59 cd A−1 (EQE = 14.4%) can still be retained by the same device.

Highly efficient orange-red electroluminescence of iridium complexes with good electron mobility

Two iridium complexes with 1-(2,6-bis(trifluoromethyl)pyridin-4-yl)isoquinoline (tfmpiq) and 4-(2,6-bis(trifluoromethyl)pyridin-4-yl)quinazoline (tfmpqz) main ligands and a tetraphenylimidodiphosphinate (tpip) ancillary ligand were applied in organic light-emitting diodes (OLEDs). The introduction of quinazoline greatly influences the nature of the complex. The quantum yield and the electron mobility of Ir(tfmpqz)2(tpip) are much higher than those of Ir(tfmpiq)2(tpip) (Ir(tfmpiq)2(tpip): Φ: 0.47, μe: 8.93–9.47 × 10−6 cm2 V−1 s−1 under an electric field from 1040 (V cm−1)1/2 to 1300 (V cm−1)1/2; Ir(tfmpqz)2(tpip): Φ: 0.98, μe: 6.44–7.20 × 10−6 cm2 V−1 s−1 under an electric field from 1040 (V cm−1)1/2 to 1300 (V cm−1)1/2). In addition, the Ir(tfmpqz)2(tpip)-based device also displayed better performance than that using Ir(tfmpiq)2(tpip). Furthermore, with a europium complex, Eu(DBM)3phen (DBM = dibenzoylmethide; phen = 1,10-phenanthroline) as a sensitizer, the device based on Ir(tfmpqz)2(tpip) with a double emissive layer structure of ITO/MoO3 (3 nm)/TAPC (50 nm)/Ir(tfmpqz)2(tpip) (5 wt%):TcTa (10 nm)/Eu(DBM)3phen (0.2 wt%):Ir(tfmpqz)2(tpip) (5 wt%):26DCzPPy (10 nm)/TmPyPB (50 nm)/LiF (1 nm)/Al (100 nm) displayed the best performance with a maximum luminance of 129 466 cd m−2, and a maximum current efficiency and a maximum power efficiency of 62.96 cd A−1 and 53.43 lm W−1, respectively, with low efficiency roll-off. The current efficiency still remains as high as 58.84 cd A−1 at a brightness of 1000 cd m−2 and 53.27 cd A−1 at a brightness of 5000 cd m−2. These results suggest that Ir(III) complexes with quinazoline units are potential orange-red phosphorescent materials for OLEDs.