Ah-Mee Hor

Humidity-induced crystallization of tris (8-hydroxyquinoline) aluminum layers in organic light-emitting devices

We report electroluminescence degradation studies of tris (8-hydroxyquinoline) aluminum (Alq3) organic light-emitting devices (OLEDs) under ambient conditions. Alq3 films and organic bilayer anode/naphthyl-substituted benzidine derivative/Alq3/cathode devices are studied via electroluminescence, photoluminescence, polarization microscopy and atomic force microscopy, and via microscopic infrared spectroscopy. Results reveal that humidity induces the formation of crystalline Alq3 structures in originally amorphous films. The same phenomenon is found to occur in OLEDs and causes cathode delamination at the Alq3/cathode interface that results in the formation of black (nonemissive) spots in the devices.

DEGRADATION PROCESSES AT THE CATHODE/ORGANIC INTERFACE IN ORGANIC LIGHT EMITTING DEVICES WITH MG:AG CATHODES

We report electroluminescence degradation studies on tris(8-hydroxyquinoline) aluminum (Alq3)-based organic light emitting devices (OLEDs) with Mg:Ag cathodes in ambient conditions. The nonemissive spots in the OLEDs are studied via optical and fluorescence microscopy and via microscopic infrared spectroscopy. Studies reveal that a majority of the nonemissive spots are caused by the growth of Mg(OH)2 sites at the Alq3/Mg:Ag interface, associated with local degradation of the Alq3 layer. In addition, the growth of elevated cathode bubbles, which also lead to nonemissive spots, is found to be caused by gas evolution from the galvanic corrosion of the Mg/Ag couple as well as from the electrolysis of absorbed moisture.

Novel high Tg hole-transport molecules based on indolo[3,2-b]carbazoles for organic light-emitting devices

Abstract A variety of 5,11-diaryl-5,11-dihydroindolo[3,2-b]carbazole derivatives have been synthesized for use as hole-transport layers in organic light-emitting devices (OLED). The new hole-transport molecules (HTM) possess several salient features: (i) exceptionally high glass transition temperatures; (ii) ready evaporation to form uniform thin films; and (iii) ideal ionization potentials matched to indium–tin oxide (ITO) anode. Cyclic voltammetry (CV) of these molecules exhibits reversible redox behavior. OLEDs utilizing this type of molecules in their hole-transport layers have demonstrated low threshold voltages and excellent current–voltage (J–V) characteristics.

A study of carrier generation mechanism in benzimidazole perylene/tetraphenyldiamine thin film structures

Abstract Using xerographic, pulsed photoconductivity and fluorescence measurements, the carrier generation mechanism has been studied in benzimidazole perylene/tetraphenyldiamine thin film structures. The results show that carrier generation is dominated by an extrinsic mechanism involving exciton dissociation into free carriers at the pigment/transport layer interface.

Life extension of organic LED's by doping of a hole transport layer

Abstract We investigated the influence of a series of aromatic hydrocarbon dopants in the hole transport layer on the life of organic light emitting devices (OLED) with the following configuration: ITO/NPB+dopant/Alq 3 +fluorescent dye/Mg:Ag. The addition of selected dopants to NPB led to an increase in device lifetime by more than an order of magnitude. Half-life, t 1/2 , of the devices was measured at the current density of J =25 mA/cm 2 . Device life, at initial luminance of 100 cd/m 2 , was estimated by using the scaling law established by A. Van Slyke, C.H. Chen, C.W. Tang, Appl. Phys. Lett. 69 (1996) 2160, ( L initial × t 1/2 =constant). Typically extrapolated device life exceeded 10 000 h, with the best devices exceeding 50 000 h. Life extension by use of dopants is consistent with the recently proposed mechanism that OLED degradation is primarily caused by holes injected into Alq 3 .

Long-term degradation mechanism of tris(8-hydroxyquinoline) aluminum-based organic light-emitting devices

Abstract We investigated the cause of the long-term degradation of organic light-emitting devices (OLED) based on tris(8-hydroxyquinoline) aluminum (AlQ 3 ), a widely used electroluminescent small molecule. We found that injection of holes in AlQ 3 is the main factor responsible for device degradation. This was verified by constructing devices where predominantly holes were transported through a 5-nm-thick AlQ 3 layer. These devices showed a significant decrease in the photoluminescence efficiency upon prolonged current flow, showing that AlQ 3 cations are unstable and their degradation products are fluorescence quenchers. This fact naturally explains various approaches, which have been used previously to increase OLED lifetime.

Solvent-induced dimorphic transformation in magnesium phthalocyanine and its effect on the photoactivity

Abstract Sublimed films of α form magnesium phthalocyanine (α-MgPc) can be induced to undergo a dimorphic phase change under the influence of methylene chloride vapours. The vapour-treated film exhibits, in addition to the Q absorption band, an intense near-IR peak at 830 nm whose absorption coefficient was estimated to be 1.0 × 10 5 cm −1 . Electron microscopy observation revealed that the small granular particles of α-MgPc change upon solvent exposure to large slender crystallites. The solvent-induced dimorphic transformation of MgPc is also associated with a dramatic enhancement in photoactivity, shown in a photovoltaic device with a CdS/MgPc heterojunction.

Improving the efficiency and stability of organic light-emitting devices using mixed emitting layers

We studied an organic light emitting device (OLED) involving electroluminescence from a mixed layer consisting of a hole transport material (HTM) and an emitting electron transport material (ETM) and including thin electron and hole injection contacts. A naphthyl-substituted benzidine derivative (NPB) and tris (8-hydroxyquinoline) aluminum (ALQ3) are used as the HTM and the emitting ETM, respectively. Following a control-experiment approach, the efficiency and the operational lifetime of OLEDs adopting the new structure are compared to those of conventional bilayer devices made of the same materials and fabricated under the same conditions. Efficiency is calculated from the luminance-current density-voltage characteristics. Lifetime tests are carried at constant current density in dry air. Photoluminescence is used to detect changes in the quantum efficiency of the ALQ3 on mixing with the HTL. Compared to a conventional bilayer device, the new device structure leads to approximately 50 percent higher efficiency and an order of magnitude increase in the operational lifetime. The higher efficiency is attributed to (i) reduced leakage of charge carrier to the electrodes, (ii) exciton confinement away from the metal cathode, and (iii) higher quantum efficiency of the emitting electron transport material due to mixing with the hole transport material. Possible reasons for the higher stability are also discussed.

Raman spectra of solid films—IV. Pb and Sn phthalocyanine complexes

Abstract Spontaneous and resonance Raman (RR) spectra of lead and tin phthalocyanine (Pc) complexes are reported. The surface enhanced Raman effect by silver island films was used to study the PbPc compound that gives rise to a strong fluorescence under normal laser excitation. Observed spectra are discussed in terms of characteristic group frequencies.

Photovoltaic properties of cadmium sulfide/trivalent‐metal phthalocyanine heterojunction devices

Thin‐film photovoltaic devices consisting of a CdS/trivalent‐metal phthalocyanine heterojunction have been prepared. The devices are fabricated by first electrodepositing a thin film of CdS onto a transparent conducting indium‐tin‐oxide substrate and then depositing phthalocyanine and gold layers sequentially in a vacuum coater. The trivalent‐metal phthalocyanines used are chloroaluminium chlorophthalocyanine (ClAlClPc), chloroaluminium phthalocyanine (ClAlPc), and chloroindium phthalocyanine (ClInPc). Under an AM2 illumination of 75 mW cm−2, these heterojunction devices produce an open‐circuit voltage Voc of 0.70 V and short‐circuit current Jsc of 0.8 mA cm−2. The conversion efficiency is about 0.2%, which represents one of the highest values reported for phthalocyanine photovoltaic devices at high light intensity.