Intrinsic Degradation in Alq3-Based OLEDs Probed by Deep-Level Optical Spectroscopy

We have successfully investigated degradation-induced variations in electronic band-gap states in the emissive region of the Alq3-based OLEDs by a deep-level optical spectroscopy technique. Through the intrinsic degradation, both deep-level traps and near-band-edge transitions in the Alq3 emissive zone are found to be red-shifted significantly towards their corresponding bulk levels of the Alq3 single layer. These variations in the interfacial electronic states are probably induced by the intrinsic degradation and indicate that initial molecular structures characteristic of the Alq3 emissive zone are transformed into the bulk-like relaxed ones through the degradation.

Influence of Device Configuration on External Quantum Efficiency in Organic Light-Emitting Devices

ABSTRACTThe external quantum efficiency in conventional double-layer type organic light-emitting devices with different layer thickness was carefully evaluated. From the measurements of luminance and emission spectra, external quantum efficiency (uncorrected) was obtained.The correction due to the modification of spatial emission distribution was added from the measurements of spatial emission patterns. The external quantum efficiency, both uncorrected and corrected, showed strong dependency on the distance between a metal electrode and an emissive region. This variation was ascribed to the change of coupling-out efficiency. Independent experimental evaluations of apparent couple-out efficiency using an integrating sphere supported this interpretation.

Multilayer Structures and Emissive Regions in Organic Thin-Film Electroluminescent Diodes

ABSTRACTThe relationships between emission quantum efficiency and emissive regions in organic thin-film electroluminescent (EL) devices were studied. As an emissive layer (EML) and an electron transport layer (ETL) material, 9, 10-bis(4-diphenylaminostyryl)anthracene and 1,3- bis(4-tert-butylphenyl-1,3,4-oxadiazolyl)phenylene, respectively, were used. A zone doped with 2,4-bis(4-diethylamino-2-hydroxyphenyl)-1,3-dihydroxycycrobutenediylium dihydroxide was formed in an EML. The relationships between the emission intensities from the dopant and the positions of doped zones gave information on the emissive regions in each EL device. The emissive region in the single-layer (SL) device consisting only of an EML extended over the EML. That in the two-layer device (DL-E) in which an EML was combined with an ETL was located within 10 nm-wide region near the EML/ETL boundary. Moreover, the emission efficiency of the DL-E device was found to be about 20 times as high as that of the SL device. Therefore, it was found that the carrier recombination within the narrow region sufficiently apart from electrodes gave high emission efficiency.

Atomic Emission Characteristics of Laser-Induced Plasmas in an Argon Atmosphere at Reduced Pressure

The emission characteristics of laser-induced plasma, with the use of a Q-switched ruby laser of 1.5 J, were studied in argon atmosphere at reduced pressure. The time- and spatially resolved emission profiles were measured. In argon atmosphere at reduced pressure, the emission period of plasma is elongated to over a hundred microseconds, and the emissive region expands to more than a few tens of millimeters above the sample surface. The emission intensities of atomic lines increase severalfold in an argon atmosphere, in comparison with those obtained in air at the same pressure. Moderate confinement of plasmas and a resultant increase of emission intensities are achieved at 50 Torr. These results are explained by the chemical inertness and the thermal characteristics of the argon atmosphere and the decrease in absorption of the laser pulse by the plasma plume. The re-excitation of emissive species by collisions with metastable argon atoms seems to be less important.

Theoretical Studies of the Line Emission Spectrum

Our knowledge of the physical conditions and of the structure of the broad line emissive region -the BLR- is reviewed. First we derive the model independent constraints on the different zones emitting the broad lines. Then we discuss photoionized models. In a first step the BLR is assumed to be made only of one type of cloud emitting all the broad lines. We show that this model is unable to explain the observed spectrum. We thus assume that the high and low ionization lines are emitted by different clouds, still photoionized. This assumption is also in contradiction with the observations and we are led to the idea of a large variety of emitting clouds. Finally the hypothesis of a purely radiative heating mechanism should also be questioned.