Phosphorescence emission efficiency

In addition to MEF, Metal - Enhanced Phosphorescence (MEP) at low temperature (18,19) has also been reported, whereby non-radiative energy transfer is thought to occur from excited distal triplet-state luminophores to surface plasinons in non-continuous silver films, which in turn, radiate fluorophore/lumophore phosphorescence emission efficiently (Fig. 10.2-Middle). This observation suggests that photon-induced electronic excited states can both induce and couple to surface plasmons (mirror dipole effect) facilitating both enhanced S fluorescence and... 

Changing the chemical structure of the cyclometalating (C N) ligand allows easy control of the color of phosphorescent emission. Efficient phosphorescent OLEDs with various lr(in)-based dopants emitting in a wide range of colors from deep red to deep blue have been demonstrated and will be discussed in the following sections. 

Standardizing the Method Equations 10.32 and 10.33 show that the intensity of fluorescent or phosphorescent emission is proportional to the concentration of the photoluminescent species, provided that the absorbance of radiation from the excitation source (A = ebC) is less than approximately 0.01. Quantitative methods are usually standardized using a set of external standards. Calibration curves are linear over as much as four to six orders of magnitude for fluorescence and two to four orders of magnitude for phosphorescence. Calibration curves become nonlinear for high concentrations of the photoluminescent species at which the intensity of emission is given by equation 10.31. Nonlinearity also may be observed at low concentrations due to the presence of fluorescent or phosphorescent contaminants. As discussed earlier, the quantum efficiency for emission is sensitive to temperature and sample matrix, both of which must be controlled if external standards are to be used. In addition, emission intensity depends on the molar absorptivity of the photoluminescent species, which is sensitive to the sample matrix. 

MA Baldo, DF O Brien, Y You, A Shoustikov, S Sibley, ME Thompson, and SR Forrest, Highly efficient phosphorescent emission from organic electroluminescent devices, Nature, 395 151-154, 1998. 

C. Adachi, R.C. Kwong, P. Djurovich, V. Adamovich, M.A. Baldo, M.E. Thompson, and S.R. Forrest, Endothermic energy transfer a mechanism for generating very efficient high-energy phosphorescent emission in organic materials, Appl. Phys. Lett., 79 2062-2064 (2001). 

S Tokito, M Suzuki, and F Sato, Improvement of emission efficiency in polymer light-emitting devices based on phosphorescent polymers, Thin Solid Films, 445 353-357, 2003. 

Phosphorescence most commonly follows population of Ti via ISC from Si, itself excited by absorption of light. The Ti state is usually of lower energy than Si, and the long-lived (phosphorescent) emission is almost always of longer wavelength than the short-lived (fluorescent) emission. The relative importance of fluorescence and phosphorescence depends on the rates of radiation and ISC from Si the absolute efficiency depends also on intermolecu-lar and intramolecular energy-loss processes, and phosphorescent emission competes not only with collisional quenching of Ti but also with ISC to So-... 

Figure 50 Quenching efficiency (<5) as a function of dc electric field applied to the electrophosphorescent (EPH) and phosphorescent system. The curves are fits to the Poole—Frenkel (see lower inset) and Onsager (see upper inset) models for charge pair dissociation in external electric fields. The quenching efficiency is defined as a relative difference between the emission efficiency at a given field F[0(F)] and at a field F0[4>(F0)] where a decrease in the EPH efficiency becomes observed (<) (F0) (7 1)]/

A significant step in the development of highly efficient OLEDs consists of the addition of phosphorescent dopants to generate both the singlet and triplet excitons as pho-tons °. This innovation has led to near-quantitative internal emission efficiencies in OLEDs that are based on small organic molecules. 

Extensive research has been carried out on the luminescence of the Alqs complex which is used as an electron transport emitting layer in OLEDs, with strong green emission at 532 nm and phosphorescence quantum efficiency around 32% . ... 

Chloroaniline (91a) and 4-chloro-iV,iV-dimethylaniline (91b) are weakly fluorescent with quantum yields less than 0.03 and lifetimes less than 1.0 nanosecond, and the latter is not very solvent-dependent184. The main deactivation process is intersystem crossing (e.g. 4>isc = 0.66), and it appears to be more efficient in nonpolar hexane solvent than in polar solvents. Both 91a and 91b show well-detectable phosphorescence in a glassy matrix with a spectral shape and emission efficiency similar to the nonhalogenated anilines. 

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