External quantum efficiency, light-emitting

I Schnitzer, E Yablonovitch, C Caneau, TJ Gmitter, and A Scherer, 30% External quantum efficiency from surface textured, thin-film light-emitting diodes, Appl. Phys. Lett., 63 2174—2176, 1993. 

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... 

Nanocrystals of Si trapped in some matrix form an attractive system for device fabrication when compared with Jt-Si, because of the increased surface stability and material rigidity. Visible EL has been observed, for example, from Si nanocrystals embedded in films of a-Si H78 and from an electrochemically-formed nanocrystalline Si thin film deposited on SnCL.79 In the latter case the p-i-n LED at room temperature emitted orange-red light (1.8 eV) that was readily visible to the eye. The light emission is ascribed variously to near surface states78 and the quantum size effect.79 Also, infrared emission near 1.1 eV has been obtained from a room-temperature EL device comprised of Si nanocrystals embedded within a Si-rich Si02-x matrix.80 The PL from this structure has an external quantum efficiency of 10 3. Substantial progress in the development of such nanocrystalline-Si EL structures can be expected over the next few years. 

Table 3 shows that the ITO/CuPc/P-6/Alq3/Al Li device exhibits an external quantum efficiency of 2000 times the ITO/PPV/Al device and 40 times the ITO/P-6/A1 device. The maximum emitted light intensity presented as maximum luminance in the table is 1600 cd/m2 for the best device. Because CuPc absorbs over 500-800 nm, a part of emitted light from the emitting layers is reabsorbed by the CuPc. This is the reason why the external quantum efficiency is not improved even when the CuPc layer was coated onto the ITO electrode... 

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. 

Introducing red-light emitting Ir(III) complex and BT into the main-chain of polyfluorene (polymer 23) can realize white-light emission. A WPLED with a structure of ITO/PEDOT PSS/PVK/polymer 23/CsF/Al showed a maximum external quantum efficiency of 3.7% and the maximum luminous efficiency of 3.9 cd/A at the current density of 1.6 mA/cm2 with the CIE coordinates of (0.33, 0.34). The maximum luminance of 4180 cd/m2 is achieved at the current density of 268 mA/cm2 with the CIE coordinates of (0.31,... 

Another way to introduce Ir(III) complexes into the main-chain of polyfluorenes was realized by Suzuki polycondensation of fluorene segments and /3-diketone ligand chelated with Ir(III) chloride-bridged dimmer (polymer 26 and 27) [34,35]. A saturated red-emitting polymer light-emitting diode was achieved from the device ITO/PEDOT/polymer 27 + PBD (40%)/Ba/Al with the maximum external quantum efficiency of 0.6% at the current density (J) of 38.5 mA/cm2 and the maximum luminance of 541 cd/m2 at 15.8 V. 

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