Nucleation and subsequent initial growth processes of particles in rf silane plasmas are investigated using a newly developed photoemission method. In this method, electrons produced due to interactions of particles with photons (photodetachment, photoionization) are measured. Since threshold energies for the interactions are dependent on particle size, size ranges of the detected particles are determined by wavelengths of the irradiated light. Using this method, small particles such as SinHx (n≲10), SinHx (n≲200) and SinHx (n≳20) can be detected. The experiments show that even in a very early phase of their growth, particles nucleate and grow principally around the plasma/sheath boundary near the rf electrode, where short lifetime radicals are produced actively. This suggests that the short lifetime radicals are the key species contributing to the particle nucleation and its subsequent initial growth processes. Moreover, it is found that small particles exist even in the bulk plasma. Since some of them are considered to be neutral, they are likely to be transported to the substrate on the grounded electrode.
Fluorescence lifetimes of organic thin films alternately deposited with diamine derivative (TPD) and aluminium quinoline (Alq3) were measured. The alternating deposition structure was shown to enhance the emission from Alq3 in spite of imperfect film structures. Energy transfer from TPD to Alq3 was evidenced by the correlation between lifetime and period number. It was found to be a competitive process with the TPD radiation process, and its rate of TPD radiation was estimated to be 1.2×1010 s-1. The authors suggested that the presence of the region mixed with TPD and Alq3 at the TPD/Alq3 interface plays an important role in the electroluminescence (EL) process. The Alq3 exciton confinement was clarified experimentally since the component of the faster fluorescence lifetime of Alq3 increased in the short-wavelength region. In addition, the authors proposed a possible EL model based on the suppression of the thermal inactivation of Alq3 excitons during their diffusion.
We prepared organic light emitting diodes (LEDs) with alternately deposited dye-doped aluminium quinoline (Alq3) and diamine derivative (TPD), and studied their electroluminescence (EL) and photoluminescence (PL) properties. The alternate deposition structure changed the energetic position and full width at half maximum (FWHM) of the EL spectra for the organic LED with alternately deposited undoped Alq3 layers, but did not change those for the organic LED with alternately deposited dye-doped Alq3 layers. On the other hand, the EL efficiency of the dye-doped specimens, despite the use of sample dyes with high PL quantum yield, was lower than that of the undoped specimen. Although the location of an emitting Alq3 layer depends on electron injection, the emitting region in a 5 nm thick Alq3 layer was estimated to be localized near the TPD layer on the anode side by a partial dye-doping method. To explain the conflict between the electron injection dependence of EL and the location of the emitting region, we propose recombination in the TPD layer and the direct energy transfer from TPD to the sample dye without the excitation of Alq3.
We developed an organic light emitting diode (LED) that emits orange light at the forward bias (ITO anode and Al cathode) and green light at the reverse bias (ITO cathode and Al anode). The organic LED has a triple layer structure where an emission layer with different doped guest dye on each side is interposed between two hole transport layers. Its brightness, maximum 20 cd/m2 was not strong because electrons are blocked by the hole transport layer near the cathode. However, the EL efficiency (lm/W) for the reverse bias was 13 times higher than that for the forward bias.
The EL mechanism of organic light-emitting diodes (organic LEDs) with a multilayer structure was studied by the dye-doping method. The actual emitting region of the specimen was found to be in some, not all, of the emission layers near the cathode. It shifted from the emission layer beside the cathode to one beside the anode depending on applied current, and it extended over several emission layers with the increase of current. In addition, the EL efficiency per current density in the specimen was 1/10 lower than that in the double-layer specimen. We discussed the possibility of the effect of exciton confinement.
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