External quantum efficiency copolymer

Due to the large band gap and high triplet energy level of the poly(3, 6-dibenzosilole) 5, the copolymer is an excellent host for the fabrication of blue polymer phosphorescent light-emitting diodes. A high external quantum efficiency (t/el) of 4.8% and a luminance efficiency of 7.2 cd/A at 644 cd/m2 have been achieved for blue phosphorescence devices (emission peak (AEL) at 462 nm, CIE coordinates x = 0.15,y = 0.26). The performances of the devices are much better than those reported for blue phosphorescent devices with poly(A--viny 1 cabarzo 1 e) (PVK) as the host.32... 

For PFs with HTM grafting as side chain, the alternating copolymer 18 with electron-deficient moiety (4-ferf-butylphcnyl-l,3,4-oxadiazole) functionalized fluorene and monomer of PFO was synthesized by Shu and coworkers [31]. The device with the configuration ITO/PEDOT PSS/18/Ca/Ag showed improved performance turn-on voltage of 5.3 V (defined as voltage needed for brightness of 1 cdm-2), maximum brightness 2770 cdm-2 at 10.8 V, and maximum external quantum efficiency 0.52% at 537 cdm-2 rela-... 

Subsequently, Yang et al. also prepared a series of fluorene-alt-carbazole copolymers grafted with cyclometalated Ir(III) complexes (polymer 8-12) [24]. The red-light emitting devices were realized and showed the highest external quantum efficiency of 4.9% ph/el, the luminous efficiency of 4.0 cd/A, and a peak emission of 610 nm. 

Subsequently, Cao et al. [30,31] designed and synthesized polymer 20-22 by similar method and the highly efficient saturated red-phosphorescent polymer light-emitting diodes (PLEDs) were achieved on the basis of copolymer 20. The best device performances are observed with an external quantum efficiency of 6.5% photon/electron (ph/el) at the current density of 38 mA/cm2, with the emission peak at 630 nm (x = 0.65, y = 0.31) and the luminance of 926 cd/m2. 

Figure 30 Block copolymer OLEDs are spun-coated on glass/ITO/PEDOT PSS and then capped with LIF/AI electrodes (a). The block copolymer demonstrates superior external quantum efficiency (b), brightness as a function of current density (c), and /-I/characteristics (d) than comparable pure homopolymer PPV and PPV/POX blends. Reprinted with permission from Tao, Y. Ma, B. Segalman, R. A. Macromolecules 200S, 41,7152-7159, Copyright 2008 American Chemical Society. ...

Polymer LEDs with copolymers containing dibenzosiloles have been reported in the literature [39,42]. By varying the 3,6-dibenzosilole content in a fluorene-based polymer, superior colour purity and optimum external quantum and luminous efficiencies were obtained (Fig. 5) [39]. Compared to polyfluorene, these copolymers are also stable to thermal annealing. Similar results were reported for a 2,7-dibenzosilole-co-3,6-dibenzosilole polymer [42]. 

It has been shown that insertion of phenylene group in the PT backbone improves the PL efficiency of the polymer. A green OLED based on 191 gave an external EL quantum efficiency of 0.1% [336]. Thienylene-phenylene copolymers such as 192 and 193 with ethylene oxide side chains also exhibit electroluminescent properties [337,338]. In the case of the paracyclophane derivative 193, a color variable light-emitting device generating two independent colors was demonstrated red under forward bias and green under reverse bias [338]. 

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