**3.3. Full Color of organic light emitting diodes (OLEDs)**

There are five potential methods to make an OLED emit in red (R), green (G) and blue (B) color spectral regions [35]: 1). side by side patterning of red (R), blue (B) and green (G) OLED's, 2). absorptive filtering of white OLED, 3). fluorescent down-conversion of blue OLED's, 4). microcavity filtered OLED's and 5). color-tunable OLED's.

*Method (1)* This method is employed a precisely positioned shadow mask to selectively deposit the R、G and B OLEDs with individual pixels of R、G and B emission.

*Method (2)* A white OLED device can be made using materials with very broad emission spectra, or using two or more sequentially deposited light emitting layers and then color filters on white light OLEDs are used to change the emission into R, G and B colors.

*Method (3)* The full color pixel can be using a single blue OLED to pump wavelength downconverters, which efficiently absorb blue light and re-emit the energy as green or red light.

*Method (4)* The emission from a white OLED is filtered by a microcavity, which is composed of a dielectric quarter wavelength stack as the bottom mirror, the metal contact as the top mirror and an inactive material as a filler layer to adjust the cavity thickness [26]. However, the microcavity resonance causes strong viewing angle dependence of emitted colors, limiting this method to applications which need small viewing angle. In this method about ±15**0** viewing angle can be achieved [27].

*Method (5)* The color variation is achieved by voltage and/or polarity tuning. Only molecular OLEDs are capable of three color tuning. This method shows low efficiency and/ or requires high driving voltage. Hence, the color variable devices based on the polarity and/ or voltage-tuning are still far from applications. White light emission OLEDs can also serve as backlight panels of LCDs. White is the most important color in the lighting industry. A number of device structure concepts have been proposed to achieve white emission. These include the mixing of three primary colors from respective layers in a multilayer structure [28], the doping of appropriate amount of red, green, and blue dopants in the same host [29], the microcavity effect of one emission layer [30], use of exciplex formation, etc.

OLEDs have become viable now for flat panel displays after intensive research and progress in the past decade. Through proper material design/choice and device fabrication, various OLEDs with colors of high brightness have been developed for use in single- or full-color applications. As the operation of an OLED depends on the carrier transport in HTL and ETL, hole and electron confinement in EML and then their recombination to emission light. In most cases the number of injected holes in an OLED is more than electrons. Therefore, improving efficient electron injection is essential for efficient and stable OLEDs.
