Quantum efficiency photoluminescence

Fluorescent small molecules are used as dopants in either electron- or hole-transporting binders. These emitters are selected for their high photoluminescent quantum efficiency and for the color of their emission. Typical examples include perylene and its derivatives 44], quinacridones [45, penlaphenylcyclopenlcne [46], dicyanomethylene pyrans [47, 48], and rubrene [3(3, 49]. The emissive dopant is chosen to have a lower excited state energy than the host, such that if an exciton forms on a host molecule it will spontaneously transfer to the dopant. Relatively small concentrations of dopant are used, typically in the order of 1%, in order to avoid concentration quenching of their luminescence. 

In addition to the photoluminescence red shifts, broadening of photoluminescence spectra and decrease in the photoluminescence quantum efficiency are reported with increasing temperature. The spectral broadening is due to scattering by coupling of excitons with acoustic and LO phonons [22]. The decrease in the photoluminescence quantum efficiency is due to non-radiative relaxation from the thermally activated state. The Stark effect also produces photoluminescence spectral shifts in CdSe quantum dots [23]. Large red shifts up to 75 meV are reported in the photoluminescence spectra of CdSe quantum dots under an applied electric field of 350 kVcm . Here, the applied electric field decreases or cancels a component in the excited state dipole that is parallel to the applied field the excited state dipole is contributed by the charge carriers present on the surface of the quantum dots. 

Through change in the thickness of the ZnS layer, the two quantum systems were controlled to be either electronically coupled or decoupled. The photoluminescence quantum efficiency was as high as 30% at room temperature.72... 

NC Greenham, IDW Samuel, GR Hayes, RT Phillips, YARR Kessener, SC Moratti, AB Holmes, and RH Friend, Measurement of absolute photoluminescence quantum efficiencies in conjugated polymers, Chem. Phys. Lett., 241 89-96, 1995. 

Z. Peng, J. Zhang, and B. Xu, New poly(p-phenylene vinylene) derivatives exhibiting high photoluminescence quantum efficiencies, Macromolecules, 32 5162-5164, 1999. 

M. Ariu, D.G. Lidzey, M. Sims, A.J. Cadby, P.A. Lane, and D.D.C. Bradley, The effect of morphology on the temperature-dependent photoluminescence quantum efficiency of the conjugated polymer poly(9,9-dioctylfluorene), J. Phys. Condens. Matter, 14 9975-9986, 2002. 

SCHEME 3.25 Chemical structures and optical properties of Alq3 derivatives (maximum emission wavelength, photoluminescent quantum efficiency in CH2C12 and the band gap are listed). 

For vacuum sublimed thin films, Grabuzov et al. [138] reported a photoluminescence quantum efficiency of 32 2%. In the same paper, data on the absorption coefficient at the maximum, a = (4.4 0.1) x 104 cm 1, and the refractive index at 633 nm (n = 1.73 0.05) can be found. Other reported values for the photoluminescence quantum efficiency that can be found in the literature are 30 5% [124] and 25 5% [139]. Naito et al. [109] reported a quantum yield of 5% in the amorphous film compared to 35% in the crystalline state. The fluorescence lifetime is reported to be biexponential with x = 3.4 and 8.4 ns, which is much shorter than in the crystal (17.0 ns). In the amorphous state, the larger free volume allows more vibrations and rotations to take place, which favors nonradiative decay. 

Depending on the kind of synthesis, these quantum dots can be prepared or size separated into batches covering almost the entire visible spectral range from 400 to 750 nm with, in part, high photoluminescence quantum efficiencies (some stable in air [106], others not [107]). Weller et al. reported on a very efficient synthesis for hydrophilic, thiol-capped CdTe quantum dots [108,109], which can be transformed to lipophilic, alkanethiol-stabilized CdTe quantum dots using a place exchange reaction similar to that for metal nanoparticles described above [110]. A related strategy has also been successfully employed to produce hydrophobic or otherwise functionalized CdS [111] or CdSe quantum dots [112] (Fig. 1). 

The Cu ions have a weak absorption spectrum that partially overlaps with the emission band of Cu, resulting in resonant energy transfer. In fact the time course of oxygen chemisorption could be followed by monitoring the Cu1 photoluminescence quantum efficiency with the time of exposure of Cu Y to oxygen. 

Combined with photoemission, DRS provides quantitative data on excitation-luminescence behavior of powdered specimens which can be used to determine photoluminescence quantum efficiencies and the extent of resonant energy transfer among the bulk and surface activators and sensitizers. 

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) ... 

Interest surged in studies of fluorescence after the discovery of electroluminescence from PPV (Burroughes et al., 1990), because of its potential for practical application in light emitting devices (LEDs). Electroluminescence is fluorescent emission produced by the recombination of electrons and holes injected into a thin film of conjugated polymer, and will be discussed in the next section. If photoluminescent emission from a polymer is weak, then the electroluminescence is unlikely to be of practical significance, and consequently studies of photoluminescence and photoluminescent quantum efficiency have been used as a means of selecting polymers likely to be useful in LEDs. 

Using this method, stable aqueous NCs with low Cd content exhibiting the photoluminescence quantum efficiency (PL QE) of 20-30% were obtained. 

The radiative decay of singlet excitons is clearly an important process in the operation of polymer LEDs. This rate is denoted by kr, where for PPV, (k, rl 1200 ns.19 Radiative decay competes with various nonradiative decay processes, such as quenching of excitons by defects, exciton dissociation, and intersystem crossing to form triplet states. Assuming that both radiative and nonradiative decays are monoexponential, the photoluminescence quantum efficiency, PLeff, defined as the number of photons emitted per photon absorbed, is given by... 

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