As displays have become larger in size in recent years, there has been an increasing demand for flat panel display devices that take up little space. Liquid crystal display devices as representative flat panel display devices can be reduced in weight when compared to the prior art cathode ray tubes (CRTs), but have several disadvantages in that the viewing angle is limited, the use of backlight is inevitably required, etc. Organic light-emitting diodes (OLEDs) as a new type of flat panel display devices are self-emissive display devices. Organic light-emitting diodes have the advantages of a large viewing angle, light weight, small thickness, small size and rapid response time over liquid crystal display devices.
Organic electroluminescent devices are characterized by their low driving voltage (e.g., 10V or below), broad viewing angle, rapid response time, high contrast, etc., in comparison with plasma display panels (PDPs) and inorganic electroluminescent display devices. These characteristics allow the use of organic electroluminescent devices as pixels of graphic displays, television image displays and surface light sources. In addition, organic electroluminescent devices can be fabricated on flexible transparent substrates, can be reduced in thickness and weight, and have good color representation. Therefore, organic electroluminescent devices are recognized as promising devices for use in next-generation flat panel displays (FPDs).
Typical organic electroluminescent devices comprise a first electrode as a hole injection electrode (anode), a second electrode as an electron injection electrode (cathode), and an organic light-emitting layer disposed between the anode and the cathode wherein holes injected from the anode and electrons injected from the cathode combine with each other in the organic light-emitting layer to form electron-hole pairs (excitons), and then the excitons fall from the excited state to the ground state and decay to emit light. The applicability of organic electroluminescent devices to full-color displays is expected. To obtain full colors, it is necessary to arrange pixels emitting light of three primary colors of green, red and blue on a panel. Various methods have been suggested to arrange pixels on a panel. Such methods include: i) the arrangement of three types of organic electroluminescent devices emitting blue, green and red light, ii) the separation of light emitted from a white (a mixed color of the primary colors (RGB)) light-emitting device into three primary colors through a color filter, and iii) the use of light emitted from a blue organic light-emitting device as a source of fluorescence emission to convert the light to green and red light. In any case, red light emission is essentially required. Thus, red light emission has been the subject of intense research.
Organic electroluminescent devices consist essentially of a transparent electrode as an anode, an organic light-emitting layer including a light-emitting region and a metal electrode as a cathode. The type of the organic electroluminescent devices (i.e. blue, green and red light-emitting devices) will be determined depending on what kind of material is used for the formation of the light-emitting layer.
The principle of light emission from a light-emitting material is as follows. Electrons and holes injected from respective electrodes combine with each other to form excitons. At this time, singlet excitons and triplet excitons are involved in fluorescence and phosphorescence processes, respectively. Phosphorescent materials using triplet excitons whose probability of formation is 75% exhibit higher luminescence efficiency than fluorescent materials using singlet excitons whose probability of formation is 25%. Molecules that readily undergo phosphorescence emission are metal complexes containing high atomic number metals in which intersystem crossing occurs readily. Particularly, iridium complex compounds have received considerable attention due to their high phosphorescence efficiency.
Korean Unexamined Patent Publication No. 10-2005-81032 suggests a red phosphorescent material using an iridium metal complex represented by the following formula:
wherein M is Ir, Rh, Re or Os; R1, R2, R3, R4, R5 and R6 are independently hydrogen, C1-C20 alkyl, C1-C20 aryl, C1-C20 cycloalkyl, halogen, a linear or branched substituent with at least one halogen atom, a linear or branched substituent with at least one hetero atom, carbonyl, carboxyl, vinyl, acetylenyl or trialkylsilyl, the adjacent groups may be bonded together to form a ring; and L is an ancillary ligand.
As the red color purity of an organic electroluminescent device using the red phosphorescent material increases (i.e. the x values on CIE chromaticity coordinates increase), the lifetime of the organic electroluminescent device is shortened and the luminance, quantum yield and power efficiency of the device decrease.