Thickness dependence electroluminescence

Fig. 4.21 shows the thickness dependence of the external quantum efficiency for electroluminescence, QFext(FL) as a function of the film thickness for two closely related PPV derivatives, OCIClO-PPVand MEH-PPV. The quantum efficiencies of these devices were measured using an integrating sphere. 

The field effect mobility may also be decreased by interface states. This is supported by the increase of the field effect mobility by using different insulators [250, 350]. Also drift mobilities extracted from electroluminescence measurements show a thickness dependence pointing to an influence of the interface [351],... 

Figure 160 Voltage (a) and current (b) dependence of the time-resolved EL in tetracene single crystals, (a) Reading from bottom to top are the bias voltage Uo, the rectangular voltage pulses (U— U0), the relaxation curves of electroluminescence (F) referring to the steady-stale EL level F0, the analyzing voltage pulses with varying duration time to and delay time t(. (b) The EL decay for two different thickness (d) tetracene crystals (crystal I d 16.5 pm pm crystal II d = 118 pm) under different steady-state current conditions (1 j 63 pA/cirr 2 j 23 pA/cm2 3 j = 0.7 pA/cm2 4 / 28 pA/cirr 5 6.1 pA/cm2). Adapted from Ref. [415],...

The oscillatory dependence on the thickness (d) of the polymer layer arises from the proximity of the metallic mirror electrode (the cathode) the emitting oscillator interacts with the virtual image oscillator behind the mirror [36, 37]. Because the radiation from the emitter and the retarded radiation from the image oscillator interfere, the PL decay rate is an oscillatory function of the distance from the mirror. Consequently, for a thin film, the quantum yields for photoluminescence and electroluminescence are oscillatory functions of d. 

The current below the peak shows the I oc dependence which is characteristic of space charge limited currents (Figure 27). The current is a function of (voltage), not of (voltage/total thickness of oxide). It is nearly independent of temperature from room temperature to 3°K. Electron emission into vacuum and electroluminescence have been observed and indicate energetic electrons. 

In organic LEDs, the electroluminescence spectrum is quite similar to the photoluminescence spectrum of the polymer used in the device (Fig. 12), so that the light emission is attributed to the singlet exciton decay. The excitons are formed by electrons and holes injected into the polymer by the cathode and anode, respectively. The injection current density depends on several parameters such as the potential barriers for charge injection at the polymer-electrode interfaces, the mobility of the charge carriers in the polymer layer, the polymer layer thickness, and the applied voltage. The efficiency of these devices is intrinsically limited by the balance between injected electrons and holes and by the fraction of excitons that decay radiatively. Further efficiency limits are imposed by reflections that occur in the interfaces between the materials that compose the device, including the interface formed... 

In a very early report of direct determination of stabilisers in polymers by luminescence techniques by Drushel et al. [13] the fluorescence of EPR/Age Rite D (trimethyldihydroquinoline) and of EPR/Santonox R were examined. Lack of interference by other polymer additives and polymerisation catalyst residues was emphasised. Age Rite D concentrations can be measured directly in pressed EPR films (<0.01 cm thickness) by fluorescence at levels below 0.1-0.2 wt.% in order to prevent concentration quenching. In the fluorescence emission spectra of Irgafos 168 the fluorescence quantum yield of the phosphate is much greater than that of the phosphite [516]. This difference enables quantification of the phosphate concentration. Although direct quantitative determination of UV stabilisers in extruded polyolefins by means of fluorescence spectroscopy (ex, 370 nm em, 390-550 nm) has been described [517], this is certainly not a universally applicable technique (being additive and matrix dependent). The effects of additives (AOs and cross-linking agent by-products) on electroluminescence... 

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