In recent years, luminosity and lifetime of LED elements have been significantly improved. Wide market deployment of emission substances is in progress.
In light-emitting diode elements (hereinafter referred to as LED elements) using an inorganic emission substance, which are currently mainstream, the luminous efficiency thereof has been remarkably improved. Specifically, white LEDs are considered to exceed fluorescent lamps in luminous efficiency in the future. However, when LEDs are used for illuminating apparatuses, they are required to be excellent in not only luminous efficiency but also color rendering property in many applications. Under the present circumstances, LEDs using only an inorganic emission substance may not satisfy all of these properties.
The concept to use an organic emission substance in an LED is already known (JP-A 2007-1880 KOKAI). However, under the present circumstances, LEDs using an organic emission substance as a luminous substance have not been put into practical use yet for illumination applications due to the following problems.
1) Specifically, in the case when an organic emission substance is used in an LED having a near-ultraviolet LED, which is currently becoming mainstream, as a light source and using luminous substances of R, G and B, organic compounds are significantly deteriorated by ultraviolet ray. This is because organic compounds are generally susceptible to ultraviolet ray. Specifically, the deterioration is accelerated when an absorption based on n-π* transition is present on a near-ultraviolet region.
2) A fluorescence spectrum of an organic emission substance may change according to the concentration of the substance, which makes control of the spectrum difficult. Furthermore, emission intensity also depends on the concentration. Concentration quenching occurs at a higher concentration region.
3) A fluorescence spectrum changes according to the kind of a polymer to which an organic emission substance is to be dispersed.
In general, a emission substance composed of a rare earth complex may have the following advantages as compared to general organic emission substances.
1) It has a luminescence wavelength which is inherent to a rare earth and not affected by the concentration of a pigment and the kind of a polymer to be dispersed. A fluorescence spectrum of rare earth is stable.
2) The ligand is an organic compound. When the ligand is excited by absorbing light, it returns to a ground state by energy transfer to a center element. Whereby an opportunity is decreased which causes an irreversible chemical change from the excitation state. Therefore, durability against ultraviolet ray may be expected.
Rare earth complexes have the above-mentioned advantages which other organic emission substances do not have. However rare earth complexes have a limitation in luminous intensity since they have a weak light absorbance.
Furthermore, rare earth complexes have a disadvantage that they have generally a low solubility and may not be dissolved at a high concentration when dissolved in a polymer.