Man-kit Leung

Blue phosphorescent organic light-emitting device with double emitting layer

In this paper, we demonstrated a blue phosphorescent organic light-emitting device (OLED) with a higher current-efficiency and a lower driving-voltage using conventional iridium(III)bis[4,6-(di-fluorophenyl)-pyridinato-N,C2′] picolinate (FIrpic) doped in the double emitting layer (DEML), which consists of a hole- and an electron-transporting material, N,N′-dicarbazolyl-3,5-benzene (mCP) and 2,2′-bis[5-phenyl-2-(1,3,4)oxadazolyl]biphenyl (OXD), respectively. Compared to OLEDs with only single mCP- and OXD-EML, current-efficiency of the optimized DEML-OLED increases by 30.82% and 141.37%, combining with a voltage reduction of 0.34 and 0.59 V at 50 mA/cm2, which comes from the better charge balance in DEML.

Excited state luminescence of multi-(5-phenyl-1,3,4-oxadiazo-2-yl)benzenes in an electron-donating matrix: exciplex or electroplex?

Multi-(5-phenyl-1,3,4-oxadiazo-2-yl)benzenes show emission in organic solvents from ultraviolet to blue (339-447 nm). The reduction potentials E(1/2)(red) cover a large range of -2.11 V for 2,5-diphenyl-1,3,4-oxadiazole to -0.76 V for 1,2,3,4,5,6-hexa(5-phenyl-1,3,4-oxadiazo-2-yl)benzene. An unexpectedly wide spectral range of the oxadiazole (OXD) exciplex emissions in PVK is observed, ranging from 406 to 603 nm. The OXDs also exhibit similar electroluminescence (EL) when blended into polyvinylcarbazole (PVK). A linear correlation between the lambda(max) of the electroluminescence and photoluminescence is observed, implying that the emission mechanisms in both processes are similar. In addition, the linear correlation between the E(1/2)(red) versus lambda(max) of EL (eV) reflected that the term of the charge-transfer configuration of the contact electron-hole pair plays a major role in the exciplex emission. The exciplex EL of 1,2,5-tri(5-phenyl-1,3,4-oxadiazo-2-yl)benzene (5) could be as high as 1.0 cd/A. Since the exciplex emission usually has a large Stokes shift, this provides a window for us to generate duo emissions for near white light EL with high efficiency. Among the devices we tried, the device of PVK/2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole/5/2,5,8,11-tetra-tert-butylperylene (100:40:40:4) gave EL with good current efficiency of 1.63 cd/A.

Orthogonally Substituted Benzimidazole-Carbazole Benzene As Universal Hosts for Phosphorescent Organic Light-Emitting Diodes

The novel ambipolar hosts of o-CbzBz and o-DiCbzBz contain carbazole and benzimidazole through an ortho-connection. The orthogonal conformations cause the triplet state to be confined at the carbazole units to secure efficient energy transfer. The phosphorescent organic light-emitting diodes (PhOLEDs) show a high current efficiency, power efficiency, and low efficiency roll-off. o-DiCbzBz can be used as a host for sky-blue, green, and orange-red PhOLEDs, giving 57.5, 78.4, and 60.3 cd/A, respectively.

Novel Benzimidazole Derivatives as Electron-Transporting Type Host To Achieve Highly Efficient Sky-Blue Phosphorescent Organic Light-Emitting Diode (PHOLED) Device

The development of benzimidazole substituted biphenyls as electron-transporting hosts for bis[2-(4,6-difluorophenyl)pyridinato-C(2),N](picolinato)iridium(III) is reported. Under the optimized conditions, the organic light-emitting diode (OLED) achieves the maximum current efficiency of 57.2 cd/A, power efficiency of 50.4 lm/W, and external quantum efficiency 25.7%.

Novel ambipolar orthogonal donor-acceptor host for blue organic light emitting diodes.

Ambipolar triplet hosts comprising 1,2,4-triazole and carbazole in ortho-positions have been developed. The blue PHOLED has a high current efficiency of 47.1 cd A(-1), power efficiency of 41.2 lm W(-1), and low efficiency roll-off. The high efficiency was attributed to the successful control of π-conjugation through orthogonal arrangement of the substituents so that a wide T1-S0 gap could be maintained.

White-light electrofluorescence switching from electrochemically convertible yellow and blue fluorescent conjugated polymers

We report the study of the fluorescence switching properties of the conjugated copolymers containing triphenylamine, fluorene, benzo[2,1,3]thiadiazole, and cyclic urea moieties. The copolymers show excellent thermal stability and good solubility in polar organic solvents. While the electrofluorescent device (EFD) of P1 emits yellow light under UV excitation, fluorescence intensity is switched off upon electrochemical oxidation. In addition, the fluorescent behavior of the EFD of P1 can be reversibly switched between the non-fluorescent (oxidized) state and the fluorescent (neutral) state with a superb contrast ratio (If/If0) of 21.4. Furthermore, a white-light electrochemical fluorescence switching device is achieved by blending of P1 (yellow) with P2 (blue). Since the fluorescent conjugated polymers of P1 and P2 have their emission simultaneously quenched under the low working potential, the EFD could show a white–dark state of fluorescence with a high contrast ratio (If/If0) of 14.6.

1,3,4-Oxadiazole containing silanes as novel hosts for blue phosphorescent organic light emitting diodes.

Five rigid oxadiazole (OXD) containing silanes, denoted 1-5, have been developed with high morphological stability. Disruption of the π-aromatic conjugation by introduction of Si atoms leads to a large band gap and high triplet energy. Among the OXDs we studied, 2,5-bis(triphenylsilylphenyl)-1,3,4-oxadiazole 5 is the best host for FIrpic, with a phosphorescent organic light emitting diode (PHOLED) turn-on voltage of 6.9 V, maximum luminance of 5124 cd/m(2), current efficiency of 39.9 cd/A, and external quantum efficiency of 13.1%. Special molecular stacking in the single crystal of 5 was discussed.

Tunable Electrofluorochromic Device from Electrochemically Controlled Complementary Fluorescent Conjugated Polymer Films

The fluorescent behavior of the electrofluorochromic devices (Type I) of greenish-yellow emitting P1 and blue emitting P2 can be reversibly switched between the nonfluorescent (oxidized) state and the fluorescent (neutral) state with a superb on/off ratio of 23.8 and 21.9, respectively. Moreover, a tunable electrofluorochromic device (Type II) based on two P1 and P2 polymeric layers that are coated individually on two independent ITO electrodes shows switchable blue-white-(greenish-yellow) multifluorescence states.

Phosphorescent organic light-emitting device with an ambipolar oxadiazole host

In this latter, the authors demonstrated a phosphorescent organic light-emitting device (OLED) with a lower driving voltage and a longer operation lifetime based on 2,2′-bis[5-phenyl-2-(1,3,4)oxadazolyl]biphenyl, an oxadiazole (OXD) derivative, as the host of the emitting layer doped with a common green-emitting phosphorescent dopant, fac-tris(2-phenylpyridine) iridium [Ir(ppy)3]. Rather than an electron transporting materials, the OXD exhibits an ambipolar transport characteristic with suitable Ir(ppy)3 concentrations due to its low highest occupied molecular orbital value (5.9eV) and the hole-transporting characteristics of Ir(ppy)3. It results in a 2.4V voltage reduction and a 2.6 times lifetime elongation of the OXD-based device, as compared to the conventional phosphorescent OLED.

Oxadiazole host for a phosphorescent organic light-emitting device

In this paper, we demonstrate a phosphorescent organic light-emitting device (OLED) with enhanced current efficiency (in terms of cd/A) based on an oxadiazole (OXD) derivative as the electron-transporting host of the emitting layer (EML) doped with a phosphorescent dopant, iridium(III)bis[4,6-(di-fluorophenyl)-pyridinato-N, C2′] picolinate (FIrpic). The maximum current efficiency of OXD-based OLEDs was 13.0 cd/A. Compared to the phosphorescent OLED with a conventional hole-transporting host, 1,3-bis(carbazol-9-yl)benzene (mCP) with 11.1 cd/A in maximum current efficiency, 17.2% improvement was achieved. However, in terms of external quantum efficiency (EQE), the OXD- and mCP-based OLEDs were 4.01 and 4.66%, respectively, corresponding to a 13.9% decrease. Such a discrepancy can be understood from the electroluminescence (EL) variation. Contrary to the hole-transporting mCP, OXD exhibited electron transporting characteristics which shifted the recombination zone toward the anode. The optical interference e...