Electroluminescent polymer blends

The use of electroluminescent polymer blends as an active material has provided a new route to the fabrication of highly efficient organic devices [80-89]. In particular, polymer LEDs fabricated with F8BT blended with TFB as the emissive layer have shown significantly improved device performances (see Figure 18.4) [53,90-92]. 

Luminescent and electroluminescent polymer blends are interesting alternatives for production of efficient organic devices [5,13,244,245,247,250-256]. Kim and coworkers fabricated polymer LEDs based on poly(2,7-(9,9-di- -octylfluorene-a/t-benzothiadiazole)) (F8BT) and poly(2,7-(9,9-di-n-octylfluorene)-a/t-(l,4-phenylene-((4-iec-butylphenyl)imino)-l,4-phenylene)) (TFB) polymer blends as the emissive layer [252,257], Their studies on the phase separation of these blends [258] evidenced a micron-scale lateral phase separation with a nanoscale vertical phase segregation due to the enrichment of the lower surface energy component (TFB) at both air and substrate interfaces. The use of this blend as an emissive layer enhanced the device performance. 

G Yu, N Nishino, AJ Heeger, T-A Chen, and RD Rieke, Enhanced electroluminescence from semiconducting polymer blends, Synth. Met., 72 249-252, 1995. 

J. Huang, H. Zhang, W. Tian, J. Hou, Y. Ma, J. Shen, and S. Liu, Violet-blue electroluminescent diodes utilizing conjugated polymer blends, Synth. Met., 87 105-108, 1997. 

An alternative approach is to dissolve an efficient electroluminescent chro-mophore in an inert polymer host in a guest-host system or polymer blend. Unfortunately, phase separation and demixing often limits the amount of low-molar-mass electroluminescent chromophores that can be dissolved in polymer hosts. Therefore, the relative brightness is lower for such guest-host systems. This problem can be overcome by fixing the chromophore chemically to the polymer itself as a pendent group on a side-chain polymer separated by spacer units. Bulk phase separation is then impossible, although microphase separation may still take place. This is illustrated by the structures collated in Table 6.10 for some typical side-chain polymers 53-55. ... 

Hu B., F Karasz.E., Blue, green, red, and white electroluminescence from multichromophore polymer blends, /, Appl. Phys. 93, (November 2003)1995, doi 10.1063/ 1.1536018... 

We have shown an extreme enhancement of the exciplex emission in bilayer EL as compared to PL and the appearance of weak exciton EL only through thermal activation from the exciplex at higher temperatures. At low temperatures, the exciton contribution is frozen out completely and only exciplex electroluminescence is seen. This demonstrates unambiguously that the only source of bulk excitons during electrical excitation is endothermic energy transfer from exciplex states that are generated via barrier-free electron-hole capture and confirms the work presented in Section 2.2.1 that was based on room-temperature emission from polymer blend LEDs and time-resolved PL. 

Jenekhe. Electroluminescence of multi-component conjugated polymers. 2. photophysics and enhancement of electroluminescence from blends of polyquinolines. Macromolecules, 35(2) 382—393,... 

Bruttig, W., Berleb, S., Egerer, G., Schwoerer, M., Wehrmann, R., and Elschner, A. 1997. Full color electroluminescence using dye-dispersed polymer blends. Synth. Met. 91 325-327. 

Another fluorene copolymer containing the luminescent dye [4-dicyanomethylene-2-methyl-6-4//-pyran (DCM) as acceptor compound was irradiatiated with UV light in the presence of gaseous triaUcylsilanes. This reagent selectively saturates the C = C bonds in the DCM comonomer units while leaving the fluorene units essentially unaffected. As a result of the photochemical process, the red electroluminescence of the acceptor compound vanishes, and the blue-green electroluminescence from the polyfluorene units is recovered. Compared with previous processes based on polymer blends, this copolymer approach avoids problems associated with phase-separation phenomena in the active layer of OLEDs [256]. 

Shim, H.-K., Kang, 1. N., and Zyung, T., Electroluminescence of blend polymers composed on main and side chain chromophores, 96 International Conference on Synthetic Metals, Salt Lake City, July 28-August 3, 1996. 

Vestwever, H., Sander, R., Greiner, A., Heitz, W., Mahrt, R. F., and Bassler, H., Electroluminescence from polymer blends and molecularly doped polymers, Synth. Met., 64, 141-145 (1994). 

The most studied of the PPV compounds are those containing at least one solubilizing alkoxy group (271-273) poly[(2-methoxy-5-(2-ethylhexyl)oxy-l,4-phenylenejvinylene (MEH-PPV) is most commonly used (274). The ability to fine-tune the color to produce red, green, and blue PLEDs has been demonstrated by appropriate functionalization of polymers (275-279), copolymerization (280-282), and blending (283-285). Several representative electroluminescent polymers covering the visible spectrum are shown in Table 3 (286-290). 

Voltage-Tunable Multicolor Electroluminescence from Single-Layer Polymer Blends and Bilayer Polymer Films... 

Although PPPs and its derivatives reveal extraordinarily high thermal and oxidative stabilities, corresponding single-layer OLEDs exhibit only low electroluminescence efficiencies. Higher efficiencies have been achieved by preparing polymer blends or by virtue of two-layer OLED-constructions. External efficiencies up to 3% were determined for an ITO/PVK/poly(2-decyloxy-l,4-phenylene)/Ca — OLED [86,87]. 

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