The major trend of the display market is shifting from the existing high-efficiency and high-resolution-oriented display, to the emotional image-quality display aiming at realizing a high color purity for demonstration of natural colors. In this respect, organic light-emitter-based organic light emitting diode (OLED) devices have remarkably developed, inorganic quantum dot LEDs with the improved color purity have been actively researched and developed as alternatives. However, in the viewpoint of emitting materials, both the organic light-emitters and the inorganic quantum dot light-emitters have intrinsic limitations.
The existing organic light-emitters have an advantage of high efficiency, but the existing organic light-emitters have a wide spectrum and poor color purity. Although the inorganic quantum dot light-emitters have been known to have good color purity because the luminescence occurs by quantum size effects, there is a problem that it is difficult to uniformly control the sizes of the quantum dots as the color approaches the blue color, and thereby the size distribution deteriorates the color purity. Furthermore, because the inorganic quantum dots have a very deep valence band, there is a problem that it is difficult to inject holes because a hole injection barrier from an organic hole injection layer or an anode is too large. Also, the two light-emitter are disadvantageously expensive. Thus, there is a need for new types of hybrid light-emitters that compensate for the disadvantages of the organic light-emitters and inorganic quantum dot emitters and maintains their merits.
Since the hybrid light materials have advantages of low manufacturing costs and simple manufacturing and device manufacturing processes and also have all advantages of organic emitting materials, which are easy to control optical and electrical properties, and inorganic emitting materials having high charge mobility and mechanical and thermal stability, the hybrid emitting materials are attracting attention academically and industrially.
Among them, since the hybrid perovskite materials (hereafter, hybrid perovskite) have high color purity (full width at half maximum (FWHM)≈20 nm), simple color control, and low synthesis costs, the hybrid perovskite materials are very likely to be developed as the light-emitter. Since the high color purity from these materials can be realized because they have a layered structure in which a two-dimensional (2D) plane made of the inorganic material is sandwiched between 2D planes made of the organic material, and a large difference in dielectric constant between the inorganic material and the organic material is large (εorganic≈2.4, εinorganic≈6.1) so that the electron-hole pairs (or excitons) are bound to the inorganic 2D layer.
A material having the conventional perovskite structure (ABX3) is inorganic metal oxide.
In general, the inorganic metal oxides are oxides, for example, materials in which metal (alkali metals, alkali earth metals, lanthanides, etc) cations such as Ti, Sr, Ca, Cs, Ba, Y, Gd, La, Fe, and Mn, which have sizes different from each other, are located in A and B sites, oxygen anions are located in an X site, and the metal cations in the B site are bonded to the oxygen anions in the X site in the corner-sharing octahedron form with the 6-fold coordination. Examples of the inorganic metal oxides include SrFeO3, LaMnO3, CaFeO3, and the like.
On the other hand, since the hybrid perovskite has the ABX3 in which organic ammonium (RNH3) cations (or “A site cation” in perovskite crystals) are located in the A site, and halides (Cl, Br, I) are located in the X site to form the organic metal halide perovskite material, the hybrid perovskite are completely different from the inorganic metal oxide perovskite material in composition.
In addition, the materials vary in characteristics due to a difference in composition of the materials. The inorganic metal oxide perovskite typically has characteristics of superconductivity, ferroelectricity, colossal magnetoresistance, and the like, and thus has been generally conducted to be applied for sensors, fuel cells, memory devices, and the like. For example, yttrium barium copper oxides have superconducting or insulating properties according to oxygen contents.
On the other hand, since the hybrid perovskite (or inorganic metal halide perovskite) has a structure in which the organic planes ((or “A site cation” plane in the perovskite crystal structure)) and the inorganic planes are alternately stacked and thus has a structure similar to a lamellar structure so that the excitons are bound in the inorganic plane, it may be an ideal light-emitter that generally emits light having very high purity by the crystal structure itself rather than the quantum size effect of the material.
If the hybrid perovskite has a chromophore (mainly including a conjugated structure) in which organic ammonium has a bandgap less than that of a crystal structure composed of a central metal and a halogen crystal structure (BX6), the luminescence occurs in the organic ammonium. Thus, since light having high color purity is not emitted, a full width at half maximum of the luminescence spectrum becomes wider than 50 nm. Therefore, the hybrid perovskite are unsuitable for a light emitting layer. Thus, in this case, it is not very suitable for the light-emitter having the high color purity, which is highlighted in this patent. Therefore, in order to produce the light-emitter having the highcolor purity, it is important that the luminescence occurs in an inorganic lattice composed of the central metal-halogen elements without the organic ammonium which contain the chromophore. That is, this patent focuses on the development of the light-emitter having high color purity and high efficiency in the inorganic lattice. For example, although an electroluminescent device in which a dye-containing hybrid material is formed in the form of a thin film rather than that of a particle and used as a light emitting layer, the emission originated from the emitting dye itself, not the intrinsic crystal structure as disclosed in Korean Patent Publication No. 10-2001-0015084 (Feb. 26, 2001), light is not emitted from the perovskite lattice structure.
However, since the hybrid perovskite has small exciton binding energy, there is a fundamental problem that the luminescence occurs at a low temperature, but the excitons do not efficiently emit light at room temperature due to thermal ionization and delocalization of charge carriers and thus are separated into free charge carriers and then annihilated. Also, there is a problem in that the excitons are annihilated by the layer having high conductivity in the vicinity of the excitons when the free charges are recombined again to form excitons. Therefore, to improve light emission efficiency and brightness of the hybrid or metal halide perovskite-based LED, it is necessary to prevent the excitons from being quenched.