Electroluminescence quantum efficiency

The enormous progress in the field of electroluminescent conjugated polymers has led to performances of oiganic light-emitting devices (LEDs) that are comparable and in some aspects superior to their inorganic counterparts 11). Quantum efficiencies in excess of 5% have been demonstrated [2] and show that a high fraction of the injected carriers in a polymeric electroluminescence (EL) device form electronic excitations which recombine radiatively. 

In electroluminescence devices (LEDs) ionized traps form space charges, which govern the charge carrier injection from metal electrodes into the active material [21]. The same states that trap charge carriers may also act as a recombination center for the non-radiative decay of excitons. Therefore, the luminescence efficiency as well as charge earner transport in LEDs are influenced by traps. Both factors determine the quantum efficiency of LEDs. 

Many of the linear conjugated tricyclic systems have interesting fluorescence or other electrophysical properties. Bis-pyrazolepyridines such as compound 30 have been incorporated into polymers as fluorescent chromophores <1999JMC339>, and used in doped polymer matrices <1997JMC2323>. They are electroluminescent at 425 nm and photoluminescent at 427 and 430 nm in a poly(vinylcarbazole) matrix with a quantum efficiency of 0.8. 

In electroluminescent applications, electrons and holes are injected from opposite electrodes into the conjugated polymers to form excitons. Due to the spin symmetry, only the antisymmetric excitons known as singlets could induce fluorescent emission. The spin-symmetric excitons known as triplets could not decay radiatively to the ground state in most organic molecules [65], Spin statistics predicts that the maximum internal quantum efficiency for EL cannot exceed 25% of the PL efficiency, since the ratio of triplets to singlets is 3 1. This was confirmed by the performance data obtained from OLEDs made with fluorescent organic... 

Y Cao, ID Parker, G Yu, C Zhang, and AJ Heeger, Improved quantum efficiency for electroluminescence in semiconducting polymers, Nature, 397 414—417, 1999. 

J.H. Park, Y.T. Lim, O.O. Park, J.K. Kim, J.-W. Yu, and Y.C. Kim, Polymer/gold nanoparticle nanocomposite light-emitting diodes enhancement of electroluminescence stability and quantum efficiency of blue-light-emitting polymers, Chem. Mater., 16 688-692, 2004. 

J. Stampfil, S. Tasch, G. Leising, and U. Scherf, Quantum efficiencies of electroluminescent poly(p-phenylenes), Synth. Met., 71 2125-2128, 1995. 

Spiro-FPAl/TPBI/Bphen Cs/Al. A very low operating voltage of 3.4 V at luminance of 1000 cd/m2 was obtained, which is the lowest value reported for either small-molecule or polymer blue electroluminescent devices. Pure blue color with CIE coordinates (0.14, 0.14) have been measured with very high current (4.5 cd/A) and quantum efficiencies (3.0% at 100 cd/m2 at 3.15 V) [245]. In another paper, Spiro-FPA2 (126) was used as a host material with an OLED device structure of ITO/CuPc/NPD/spiro-FPA2 l%TBP/Alq3/LiF that produces a high luminescent efficiency of 4.9 cd/A [246]. 

FIGURE 3.23 External quantum efficiencies of PtOEP/CBP and PtOEP/Alq3 devices as a function of current with and without a BCP blocking layer (left). Emission spectra of CBP-based electroluminescent devices with and without a BCP exciton blocking layer (right). (From O Brien, D.F., Baldo, M.A., Thompson, M.E., and Forrest, S.R., Appl. Phys. Lett., 74, 442, 1999. With permission.)... 

Rau U (2007) Reciprocity relation between photovoltaic quantum efficiency and electroluminescent emission of solar cells. Phys Rev B 76 085303... 

The /3-diketonate [Nd(dbm)3bath] (see figs. 41 and 117) has a photoluminescence quantum efficiency of 0.33% in dmso-7r, solution at a 1 mM concentration. It has been introduced as the active 20-nm thick layer into an OLED having an ITO electrode with a sheet resistance of 40 il cm-2, TPD as hole transporting layer with a thickness of 40 nm, and bathocuproine (BCP) (40 nm) as the electron injection and transporting layer (see fig. 117). The electroluminescence spectrum is identical to the photoluminescence emission the luminescence intensity at 1.07 pm versus current density curve deviates from linearity from approximately 10 mA cm-2 on, due to triplet-triplet annihilation. Near-IR electroluminescent efficiency <2el has been determined by comparison with [Eu(dbm)3bath] for which the total photoluminescence quantum yield in dmso-tig at a concentration of 1 mM is Dpi, = 6% upon ligand excitation, while its external electroluminescence efficiency is 0.14% (3.2 cdm-2 at 1 mAcm-2) ... 

Stampfl, J., Tasch, S., Leising, G. and Scherf, U. (1995) Quantum efficiencies of electroluminescent poly (para)-... 

A first demonstration of phosphorescence-doped OVPD-OLEDs with identical VTE performance was achieved at Universal Display Corporation (UDC) and at TU Braunschweig [48] by use of PtOEP. The electroluminescence spectrum, with emission at 651.1 nm, and the structure of the device are shown in Fig. 9.10. The external quantum efficiency of this OVPD device reached 3.88% at 3.7 V forward potential (Fig. 9.11) this is identical to the external quantum efficiency of the nearly identical VTE device reported by Baldo and Forrest [49]. 

Keywords. Light-emitting diodes, Conjugated polymers, Energy transfer, Electroluminescence, Quantum efficiency... 

Table 6. Photoluminescence and Electroluminescence Quantum Efficiencies of MEH-PPV, DSiPV, and Blended Polymers...

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