External quantum efficiency, light-emitting diodes

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... 

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. 

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. 

The operation of organic light-emitting diodes (OLEDs) involves charge injection from electrodes, transport of charge carriers, recombination of holes and electrons to generate electronically excited states or excitons, followed by their deactivation by emission of either fluorescence or phosphorescence. The main factors that determine luminous and external quantum efficiencies are the following ... 

R. Windisch, P. Heremans, A. Knobloch, P. Kiesel, G. H. Dohler, B. Dutta, and G. Borghs, Light-emitting diodes with 31% external quantum efficiency by outcoupling of lateral waveguide modes. Appl. Phys. Lett. 74, 2256 (1999). 

Therefore, the different complexes affect the external quantum efficiency of dye doped organic light-emitting diodes. This arises, because the efficiency of Forster energy transfer from the matrix to the dye is dependent on the degree of overlap between the EL spectrum of the matrix material and the absorption spectrum of the dye. ... 

Figure 4.4 Efficiency values for a CdTe/PDDA light-emitting diode (LED). The device showed an electroluminescence turn on at 2.5 V, and maximum lightoutputwasobtained at3.3 Vand 350mAcm , with a peak radiated power of 141 nW, corresponding to an external quantum efficiency of 0.51%. The luminous efficiencies...

Efficiency The fraction of input energy that is converted into useful output energy. Internal quantum efficiency is the number of photons generated inside the light-emitting diode (LED) per number of input electrons. External quantum efficiency is the number of photons that escape from the device per number of input electrons. Extraction efficiency is the ratio of the external quantum efficiency to the internal quantum efficiency, or the fraction of photons generated in the semiconductor that escape from the device. Wall-plug efficiency is the fraction of electrical power into the LED that is converted to optical power out of the device. Luminous efficiency is the power sensed by the human eye per electrical power input into the device. 

Linnros J, Lalic N (1995) High quantum efficiency for a porous silicon light-emitting diode under pulsed operation. Appl Phys Lett 66(22) 3048-3050 Loni A, Simons AJ, Cox TI, Calcott PDJ, Canham LT (1995) Electroluminescent porous silicon device with an external quantum efficiency greater-than 0.1 -percent under Cw operation. Electron Lett31(15) 1288-1289... 

JEO 11] Jeon S O., Jang S.E., Son H.S. etal., External quantum efficiency above 20% in deep blue phosphorescent organic light-emitting diodes . Advanced Materials, vol. 23, no. 12, pp. 1436-1441, 2011. 

Optocouplers are a class of devices with input and output circuits coupled optically while isolated electrically. Such devices were fabricated using Ca/MEH-PPV/ITO diodes, which emit red-orange light, as the input unit and Au/P30T/ITO as the output unit [247]. The input unit presented approximately 1% photons/electron external quantum efficiency and the output unit a quantum yield of about 35% electrons/photon at 590 nm. This device reached a current transfer ratio of 2 x 10 under -10 V reverse bias, comparable to that of commercially inorganic optocouplers. Using the improved devices (optimized LEDs made from MEH-PPV and photodiodes made from P30T sensitized with Cgo). a current transfer ratio of 10 can be achieved even at low bias (0 to -2 V) [248]. 

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