Three materials had highly twisted molecular structures. In film state, excimer emission can be interpreted as result of orientation effect. Wavelength of excimer was controlled through the change of center position of triple core chromophore, and the CIE of white light could be controlled.
Small-molecule OLEDs, deposited by thermal evaporation, allow for precise control over layer thicknesses. This enables optimisation of the optical behaviour of the stack which ultimately determines the outcoupling efficiency. In terms of optical outcoupling there are limits to the efficiency by which the generated electromagnetic radiation can be extracted from the stack. These limitations are linked to the refractive indices of the individual layers. Values for maximum outcoupling efficiency are sometimes calculated under the implicit assumptions that the OLED stack is planar, that all layers are isotropic with a certain refractive index and that the emitters are not preferentially oriented. In reality it is known that these assumptions are not always valid, be it intentional or unintentional. In our work we transcend these limiting assumptions and look at different forms of anisotropy in OLEDs. Anisotropy in OLEDs comes in three distinct flavours; 1. Geometrical anisotropy, as for example in gratings, lenses or other internal or external scattering centres, 2. Anisotropic emitters, where the orientation significantly influences the direction in which radiation is emitted and 3. Anisotropic optical materials, where their anisotropic nature breaks the customary assumption of isotropic OLED materials. We investigate the effect of these anisotropic features on the outcoupling efficiency and ultimately, on the external quantum efficiency (EQE).
To realize low-cost fabrication processes for high-performance OLED displays and lighting panels, the understanding of solution-processed films and devices is becoming more important nowadays. However, differences between vacuum- and solution-processed films have not been sufficiently discussed, and they are sometimes regarded as identical. In this presentation, we show and discuss the important differences between physical properties of vacuum-deposited and spin-coated films of small-molecule OLED materials, especially focusing on the differences in film densities and molecular orientation. Since they are fundamental factors affecting both electrical and optical properties of amorphous films used in OLEDs, we should consider their differences carefully when discussing device performances in detail.
We have developed an apparatus (Hamamatsu C9920-02) for measuring the absolute photoluminescence quantum yield.
This system consists of an excitation light source, a sample holder mounted in an integrating sphere and a multi-channel
CCD spectrometer. Using this apparatus, the absolute phosphorescence quantum yields were measured for several
iridium complexes doped in organic thin films or dissolved in deaerated solutions. The iridium complexes <i>fac</i>-tris
(2-phenylpyridine)iridium(III) (Ir(ppy)<sub>3</sub>) and its derivatives, showed high phosphorescence quantum yields (>90 %), while
bis(2-(2'-benzo(4,5-a)thienyl)pyridinato-N,C<sup>3'</sup>)iridium(III)(acetylacetonate) (Btp<sub>2</sub>Ir(acac)) gave a lower
phosphorescence quantum yield (less than 40 %). To reveal the mechanism of nonradiative decay of the excited iridium
complexes, we made time-resolved photoacoustic measurements. It was found that all of the iridium complexes undergo
S<sub>1</sub>-T<sub>1</sub> intersystem crossing with efficiencies of close to 100 % after photoexcitation. This indicates that the lower
phosphorescence quantum yield for Btp<sub>2</sub>Ir(acac) is due to involvement of the T<sub>1</sub>-S<sub>0</sub> intersystem crossing process.