Blue emission/emitters

Electron-Deficient Polymers - Luminescent Transport Layers 16 Other Electron-Deficient PPV Derivatives 19 Electron-Deficient Aromatic Systems 19 Full Color Displays - The Search for Blue Emitters 21 Isolated Chromophores - Towards Blue Emission 21 Comb Polymers with Chromophores on the Side-Chain 22 Chiral PPV - Polarized Emission 23 Poly(thienylene vinylene)s —... 

A number of other up-conversion processes are known. The blue emission from a Yb3+/Tm3+ couple in which the active emitters are defect Tm3+ centers is mainly due to the efficient excitation ET process from Yb3+ centers. Two-frequency up-conversion has been investigated using Pr3+ defects in a fluoride glass matrix. Illumination with one pump wavelength results in GSA, while simultaneous irradiation with a second pump wavelength further excites the GSA centers via ESA. The doubly excited defects emit red light. Up-conversion and visible output only takes place at the intersection of the two beams. 

In realizing the poor film-forming property of 9,10-(diphenyl)anthracene, the Kodak group improved this property by designing a series of blue emitters based on further substituted anthracene derivatives. The chemical structures of these materials were patented in a U.S. Patent in 1999 [239], In their patent, Kodak also reported the EL data using one of these compounds as a host material and using TBP as a blue dopant (Scheme 3.62). The device structures is ITO/CuPc/NPD/anthracene compounds.5%TBP/Alq3/Mg Ag. The EL of the device showed blue emission with CIE color coordinates of (0.144, 0.196). Without the... 

Perylene (199) and its derivative (TBP, 200) have been widely used as blue dopant materials owing to their excellent color purity and stability. Efficient blue emitters with excellent CIE coordinates are found in biaryl compound 2,2 -bistriphenylenyl (BTP, 201) as shown in Scheme 3.62 [145]. A device of structure ITO/TPD/BTP/TPBI/Mg Ag emits bright blue emission with CIE (0.14, 0.11). A maximum brightness of 21,200 cd/m2 at 13.5 V with a maximum EQE of 4.2% (4.0 cd/A) and a power efficiency of 2.5 lm/W have been achieved. 

Bis(dimesitylboryl)-2,2 -bithiophene (BMB-2T, 242) forms a stable amorphous glass and emits pure blue color with a high fluorescence QE of 86% in THF solution [270]. However, an OLED with ITO/m-MTDATA/TPD/BMB-2T/Mg Ag emits with a broad emission due to an exciplex with TPD. The exciplex can be prevented by insertion of a thin layer of 1,3,5-tris(biphenyl-4-yl)benzene (TBB) between TPD and BMB-2T, leading to a pure blue emission. It seems that the boron complex or boron-containing compounds easily form an exciplex with common HTMs. Other similar blue emitter materials also demonstrate such behavior. 

The simplest method yet most complex structure for white OLEDs consists of three primary emission colors blue, green, and red. Kido et al. reported using three emitter layers with different carrier transport properties to produce a white emission [273], The multilayer structure of such an OLED is ITO/TPD/p-EtTAZ/Alq3/Alq3 Nile Red/Alq3/Mg Ag, in which a blue emission from the TPD layer, a green emission from the Alq3 layer, and a red... 

If there is one clear need in the field of OLED materials it continues to be in the area of blue emitters. A blue emissive material with good color coordinates CIE (0.10, <0.10) coupled with long device lifetime (>10,000 h) and high electrical efficiency (>5 cd/A) is the holy grail of materials chemists in this field. A major effort to find such materials continues in many laboratories including our own and the current sets of available materials may be supplanted at any time. However, the current best candidate blue emitters in the SMOLED area compromise many desirable properties — the most troublesome being long lifetime. 

Transparent electrodes also allow the fabrication of stacked LEDs with electrodes within the device structure. By applying voltages between different adjacent pairs of electrodes, it is possible to control the amount of emission from each layer and hence to control the color of the emission (see Fig. 5.22), taking advantage of the fact that blue emitters, for example, are transparent to red, green and blue emission. This strategy was first demonstrated in polymer LEDs by Martens et al.,99 and has also been used in molecular organic LEDs.100... 

Spirobifluorenes have been investigated by Salbeck [100] and found to be promising materials for use in blue LEDs, and so some effort has been made to incorporate these units into polymers (Scheme 30). As poly(diarylfluorene)s have proven to be stable blue emitters it comes as no surprise to find that polymers containing spirobifluorenes such as 66 [101] and 67 [102] also produce stable blue emission. A fluorene-spirobifluorene alternating copolymer 68 has been made and was found to give stabler emission than the fluorene homopolymer 16, but green emission was still observed upon heating in air at 150 °C [103]. 

Unfortunately, as with PDAFs, obtaining stable blue emission in the solid state has presented difficulties. The tetraoctylpolymer 87 was a green emitter in the solid state, with the PL and EL spectrum being dominated by a broad emission band at 560 nm [113]. Films of the polymer 88 with ethylhexyl substituents show blue PL (A.max = 429 450 nm). The EL from 88 was initially... 

The first PLEDs based on 3,6-carbazoles was achieved in 1996, by Zuppiroli and coworkers with a poly(7V-butyl-3,6-carbazole) in a single-layer device with ITO and A1 electrodes [123]. Diodes produced blue emission with a low EL performance (.Vext = 0.07% and a few cd/m2 at 15 V). Aiming at increasing the EL performances, five-layers OLEDs based on small molecules were fabricated using carbazole dimers as emitters. A pure blue light-emitting device has been achieved with CIE coordinates a = 0.158, y = 0.169, Apeak = 456nm, a luminance of 1000 cd/m2, luminance efficiency of 4.7 cd/A at 10 V and a r]ext = 33% [125]. 

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