Blue emitter

Another important class of electron-transporting emitter is the distyryl-arylenes. These have been explored most extensively by workers at Idemitsu Kosan [31, 32], with the bulk of the published data focusing on a compound designated as DPVBi (see Table 13-2). This class of materials may be considered as small molecule analogs of the PPV polymers, with distyrylbenzene and its derivatives ]33] as prototypical examples. Because of the short conjugation length they tend to be blue 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 —... 

Full Color Displays - The Search for Blue Emitters... 

P-5 CaW04 W CaC03, W03, NaCl, PbC03. Deep blue emitter, CRTs... 

The simulation discussed above was based on full color produced by individual red, green, and blue emitters. Other full-color reproduction approaches have been proposed for OLED displays including color from blue emitter by means of energy down conversion fluorescent filter [177], and color from white emitters by means of transmission color filter sets similar to that used in LCD industry [178,179]. Table 1.5 compares the EL efficiency of equivalent white... 

Color from red, green, and blue emitters Color from blue emitter with phorsphor filters Color from white emitter with transmission filters... 

C. Ego, A.C. Grimsdale, F. Uckert, G. Yu, G. Srdanov, and K. Mullen, Triphenylamine-substi-tuted polyfluorene — a stable blue-emitter with improved charge injection for light-emitting diodes, Adv. Mater., 14 809-811, 2002. 

A dendritic molecular species l,3,5-tris(A-phenylbenzimidizol-2-yl)benzene (TPBI, 89) and its derivatives were patented by Kodak as blue emitters and ETMs [141,142]. The reason to add TPBI as a key ETM is because this material has received much attention recently and... 

For the blue pixel, the standard CIE 1931 color chromaticity coordinates are (0.14-0.16, 0.11-0.15). Since a relatively large band gap is required for blue emitters, the appropriate blue host materials with even larger band gap are needed to optimize the energy transfer requirements. The main challenge in designing the blue emitter or its host is the device stability. 

The most efficient fluorescent blue emitters yet reported belong to the distyrylarylene (DSA) series (Scheme 3.56) and it is likely that the first generation of commercial blue OLEDs will use these materials as emitters or hosts. 

These DSA hosts and DSA amine blue emitter dopants were patented by Idemitsu Kosan Co. Ltd., Japan (Scheme 3.57), and are likely to become a commercial blue emitter candidate [231]. 

SCHEME 3.61 Chemical structures of oligo-phenyl vinyl blue emitters. 

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. 

A blue OLED with a very low turn-on voltage of 2.7 V and a luminance of 500 cd/m2 at 5 V with structure ITO/Spiro-TAD/Spiro-PBD/Al Mg has been reported. The robust and morphologically stable spirobifluorene-cored pyrimidine oligoaryl blue emitter... 

SCHEME 3.68 Chemical structures of spiro-linked blue emitters and transporters. 

Qiu s group investigated the spirofluorene linked dihydroanthracene compound di-Spiro-9, 9 -di-fluorene-9",9" -(9,10-dihydro-anthracene) (DSFA), originally developed in the 1930s, as a blue emitter in ITO/m-MTDATA/NPD/DSFA/Mg Ag [258]. The device exhibited a... 

SCHEME 3.69 Chemical structures of aromatic amines blue emitters as well as hosts. 

SCHEME 3.70 Chemical structures of organosilicon blue emitter and HTMs. 

Replacing the metal Al by a boron atom as the metal chelate center, Tao et al. reported lithium tetra-(2-methyl-8-hydroxy-quinolinato) boron (LiB(qm)4, 240) (Scheme 3.73) quantitatively prepared by reaction of lithium borohydride (LiBH4) with four equivalents of 2-methyl-8-hydroxy-quinoline in ethanol at room temperature [266]. LiB(qm)4 is a pure blue emitter with a maximum peak emission at 470 nm with FWHM of 75 nm. Devices of... 

SCHEME 3.74 Chemical structures of metal chelates as blue emitters. 

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. 

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