Wenming Su

Emission mechanism in organic light-emitting devices comprising a europium complex as emitter and an electron transporting material as host

The emission mechanism in organic light-emitting devices, where the emission layer is composed of Eu(DBM)3pyzphen (DBM=Dibenzoylmethane, pyzphen=pyrazino-[2,3-f][1,10]-phenanthroline) doped into electron transporting/hole blocking material BPhen (4,7-diphenyl-1, 10-phenanthroline), is investigated. Energy transfer and carrier trapping simultaneously exist in the luminescence process, and carrier trapping is a main process. Direct carrier trapping by Eu(DBM)3pyzphen molecules is confirmed by the difference of electroluminescence and photoluminescence spectra as well as J-V characteristics. Efficient Foster and Dexter energy transfer from BPhen to Eu(DBM)3pyzphen molecules were speculated in terms of analysis of photoluminescence spectra of fixed solutions, triplet energies, and phosphorescent lifetimes. Based on these mechanisms, the overall performances of these devices were improved. High efficiencies were obtained under carrier trapping by Eu(DBM)3pyzphen molecules, and the emission of BPhen was elimina...

Performance improvement of organic light emitting diode with aluminum oxide buffer layer for anode modification

A thin Al2O3 insulating buffer layer deposited on indium tin oxide (ITO) anode by atomic layer deposition has been investigated for organic light-emitting diodes (OLEDs). With an optimal thickness of 1.4u2009nm and low density of structural defects of the Al2O3 film, the OLEDs current efficiency and power efficiency were simultaneously improved by 12.5% and 23.4%, respectively. The improvements in both current and power efficiency mean lower energy loss during holes injection process and better balanced charge injection. To understand the mechanism behind the enhanced performance of OLED by the buffer layer, a series of Al2O3 films of different thicknesses were deposited on ITO anode and characterized. The roughness, sheet resistance, and surface potential (EF′) of the Al2O3 modified ITO were characterized. Also, the properties of Al2O3 films were investigated at the device level. It is believed that the block of holes injection by the Al2O3 buffer layer makes more balanced carrier density in the emitting lay...

Novel ternary bipolar host material with carbazole, triazole and phosphine oxide moieties for high efficiency sky-blue OLEDs

A novel bipolar host material 9-(3-(5-(4-(diphenylphosphoryl)phenyl)-4-phenyl-4H-1,2,4-triazol-3-yl)phenyl)-9H-carbazole (CTPO) with carbazole, triazole and phosphine oxide moieties was designed and synthesized. CTPO was found to exhibit a high glass transition temperature (Tg = 127 °C), suitable HOMO and LUMO levels (5.65 and 2.42 eV), a high triplet energy (3.06 eV) and excellent bipolar properties. A device with 15 wt% doping concentration showed a low turn-on voltage of 2.5 V and maximum current and power efficiencies of 41.6 cd A−1 and 43.0 lm W−1, respectively. A high efficiency of 40.1 cd A−1 was achieved at the brightness of 100 cd m−2. Even at a high luminance of 1000 cd m−2, the efficiency remained as high as 35.2 cd A−1.

Enhanced light extraction of organic light emitting diodes by embedding printed polymethyl methacrylate dot array

Inkjet-printed dot array structure embedded at the organic layer/ITO interface for enhancing out-coupling of OLED has been demonstrated. Polymethyl methacrylate (PMMA) which is transparent, cheaper and has refractive index (n= 1.47) similar to SiO2 (n= 1.45), is selected as the dot array material. At the optimum period of 100 μm, the PMMA dot array improved the OLEDs current efficiency and power efficiency by the factors of 1.37 and 1.30, respectively. The current density and luminance of the device and light transmittance of different structure modified substrates have also been characterized to understand the improvement. The enhancement can be attributed to the light out-coupling of the printed dot array and the lower refractive index of PMMA material.

Tunneling effect on enhanced OLED performance using Al 2 O 3 buffer layer

It has been found that introduction of buffer layers between organic holes transport layer and anode layer plays an important role in improving device stability and hole injection efficiency of organic light-emitting diodes (OLED). As for the mechanism of the improvement due to the buffer layer, it is still controversial. To understand the mechanism behind the enhanced performance of OLED by the buffer layer, a new model is therefore proposed which combines classical model and quantum tunneling model to explain the OLED performance improvement. A thin Al2O3 insulating buffer layer deposited on indium tin oxide (ITO) anode by atomic layer deposition has been investigated for OLED. The observed power efficiency and current efficiency improvement at the optimal thickness of 1.4 nm are well explained by the model. A series of Al2O3 films of different thicknesses were deposited on ITO anode and characterized. Their roughness, sheet resistance, surface potential, and resulted OLED current density were investigated. It is believed that the blocking of hole inject by the Al2O3 buffer layer makes more balanced carrier density in emission layer, thus enhances the current efficiency. Though less number of holes are injected in OLED due to the insertion of Al2O3 layer, quantum tunneling through the ultra-thin buffer layer play an important role to contribute to the hole injection, which avoids crossing the interface barrier, resulting in less energy consumed and power efficiency enhanced.