Electroluminescence quantum

The synthesis-driven approach towards material science can be applied to create oligomers and polymers with optimized properties, e.g. maximized carrier mobilities and electrical conductivities or high photo- and electroluminescence quantum yields. It becomes obvious, however, that the ability to synthesize structurally defined -architectures is the key to these high performance materials. 

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. 

Poly[2,5-dialkoxy-l,4-phenylene) vinylenejs with long solubilizing alkoxy chains dissolve in conventional organic solvents such as chloroform, toluene, or tetrahydrofuran [21, 28, 32-36]. Their emission and absorption spectra are red-shifted relative to PPV itself, and the polymers fluorescence and electroluminescence quantum yields are greater than parent PPV. This benefit may be a consequence of the long alkyl chains isolating the polymer chains from each other. 

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. 

Electroluminescence from CdSe quantum dot / polymer composite. App/. Phys. Lett, 66,1316-1318. 

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. 

Research on semiconductor nanoparticle technology by chemists, materials scientists, and physicists has already led to the fabrication of a number of devices. Initially, Alivisatos and co-workers developed an electroluminescence device from a dispersion of CdSe nanoparticles capped with a conducting polymer349 and then improved on this by replacing the polymer with a layer of CdS, producing a device with efficiency and lifetime increased by factors of 8 and 10, respectively. 0 Chemical synthetic methods for the assembly of nanocrystal composites, consisting of II-VI quantum dot polymer composite materials,351 represent one important step towards the fabrication of new functional devices that incorporate quantum dots. 

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.)... 

FIGURE 9.13 Optoelectronic characteristics of AM-PLED, (a) Calculated display luminance and estimated display luminance versus /data characteristics, (b) measured Paata evolution with the /data, and (c) PLED and AM-PLED electroluminescent (EL) spectra are shown. The CIE color coordinates of the PLED and AM-PLED are also shown in the inset of this figure. (From Hong, Y., Nahm, J.-Y., and Kanicki, J., IEEE J. Selected Top. Quantum Electron. Org. Light-Emitting Diodes, 10, 1, 2004 With permission.)... 

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

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