Phosphorescent materials

Thomsons picture of the atom emerged from his work with cathode ray tubes. It was a milestone on the road to understanding atomic structure. But it was not the only major advance to come out of cathode ray experiments. Almost every television set in existence today is a cathode ray tube. The electrons stream from the cathode and are deflected by electromagnetic coils guided by signals from the television station. When an electron hits the televisions screen, which is coated with a phosphorescent material, it produces a dot of color. The dots form the picture you see on the screen. 

Phosphorescent materials for application to organic light emitting devices... 

Recent progress in phosphorescent materials for organic light emitting diodes S. Tokito Journal of Photopolymer Science and Technology, 17 307-314... 

PLEDs contain polymeric emissive materials that are almost exclusively processed by solution coating (spin coating or inkjetting). This has been discussed in Chapter 2. While most polymer work uses fluorescent emissive materials, there are a few examples of phosphorescent materials being incorporated into a polymer chain and being used as phosphorescent emitters. This part of the materials discussion will be covered in Chapter 4. 

The highest efficiency red emitters belong to the class of phosphorescent materials and are based on iridium organometallic complexes. The best performance achieved in guest-host systems, for example using a carbazole host and an Ir emitter (Ir(piq-F)2acac, has a maximum power efficiency and luminescent efficiency up to 4.73 lm/W and 13.7 cd/A, respectively. An EQE of 6.7% at 20 mA/cm2 with CIE (0.61, 0.36) has been demonstrated (Scheme 3.96) [361]. 

Once again, the most recent developments have been in the area of green phosphorescent materials where phenomenal efficiencies are now beginning to be coupled with good device lifetimes. The prototypical emitter of this type is iridium-tris-2-phenylpyridine (Ir(ppy)3) used as a dopant (Scheme 3.98), usually diluted into a carbazole type host, because it is prone to serious self-quenching problems. 

Exciton decay When an exciton decays radiatively a photon is emitted. When the excitons form in fluorescent materials radiative decay is limited to singlet excitons and emission occurs close to the recombination region [7] of the OLED due to the relatively short lifetime of the excited state (of the order of 10 ns). For phosphorescent materials, emission can occur from triplet excitons. Due to the longer excited state lifetime (of the order of hundreds of nanoseconds), triplet excitons can diffuse further before decaying. 

In the vapor-deposited OLED community, a number of approaches have been employed to produce white light emission. White OLEDs have been demonstrated based on multilayer structures, e.g., stacked backlights [153,168], multidoping of single-layer structures [145], phosphorescent monomer-excimer emission layers [169] and on doping of phosphorescent materials into separate bands within the emission zone, called a tri-junction [170]. The trijunction device has produced the highest white OLED efficiency of 16% external quantum efficiency demonstrated thus far [171]. 

Baldo et al. [ 164] used the platinum complex of 2,3,7,8,12,13,17,18-octaethyl-21 //,23//-porphine (PtOEP, 66) as efficient phosphorescent material. This complex absorbs at 530 nm and exhibits weak fluorescence at 580 nm but strong phosphorescence from the triplet state at 650 nm. Triplet transfer from a host like Alq3 was assumed to follow the Dexter mechanism. Dexter-type excitation transfer is a short-range process involving the exchange of electrons. In contrast to Forster transfer, triplet exciton transfer is allowed. 

Yersin H (2007) Flighly efficient OLEDs with phosphorescent materials. Wiley-VCH, Weinheim... 

Fluorescent substances emit light when light of a different wavelength (a different colour) is shone on them. Phosphorescent materials do the same, but go on emitting for some time even after the illumination is switched off. 

H. Yersin, Ed., Highly Efficient OLEDs with Phosphorescent Materials, Wiley-VCH, Weinheim, 1 (2008). 

The term smart material is now used for a rather wide variety of materials, some that have been known and used for many years and some that have been developed only recently. For example, phosphorescent and fluorescent materials are familiar and widely used materials that are sometimes defined as smart materials because they have the capacity to absorb electromagnetic radiation of short wavelengths (X-rays or ultraviolet rays, for example) and re-emit that radiation in the form of visible light. The difference between phosphorescence and fluorescence is that a phosphorescent material continues to emit light after radiation has ceased, while the emission of fluoresced light ends as soon as the source of radiation is removed. 

The production of flat-panel cathode ray tubes (CRTs) is essentially a fabrication issue. The basic principle of operation is the same as a standard CRT. Electrons are emitted from a hot cathode. These are guided by a magnetic field to the glass screen coated in a layer of phosphorescent material. Upon impact the energy of the electron is transferred to the phosphor and light is emitted. A regular pattern of red, green and blue phosphors creates a dense pattern of... 

Vacuum fluorescent displays (VFDs) are strongly related to flat-panel CRTs. Electrons are ejected from a cathode source, traverse a vacuum and then strike a pattern of triodes with individual anodes covered in red, green and blue phosphorescent material. However, the operating voltages, e.g. 12 V, and power consumption are much lower than those found for CRTs and PDFs. 

Liu Yingliang, Ding Hong, Research developments of long lasting phosphorescent materials, Chinese J. Inorg. Chem. 17 (2001) pp. 181-187. 

A common uses of phosphors (phosphorescent materials) is in televisions and computer monitors small dots of red, green, and blue phosphors are grouped together on the inner surface of a cathode-ray tube. When electrons generated in the back of the tube hit the phosphors, they absorb the energy and then emit light. 

Phosphorescent materials that glow when activated by the impact of fast electrons, as wel 1 as high frequency radiation such as ultraviolet or x-ray... 

Among phosphorescent metal complexes, Ir(III) complexes have been considered as one of the best phosphorescent material candidates because they show intense phosphorescence at room temperature and significantly shorter emission lifetime compared with other heavy-metal complexes, which are crucial for utility of phosphorescent materials. Moreover, the emission colour can be tuned easily over the entire visible region by simply modifying the chemical structures of ligands [21-23]. 

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