Excimer quantum efficiencies

In the vapor-deposited OLED community, a number of approaches have been employed to produce white light emission. White OLEDs have been demonstrated based on multilayer structures, e.g., stacked backlights [153,168], multidoping of single-layer structures [145], phosphorescent monomer-excimer emission layers [169] and on doping of phosphorescent materials into separate bands within the emission zone, called a tri-junction [170]. The trijunction device has produced the highest white OLED efficiency of 16% external quantum efficiency demonstrated thus far [171]. 

From the stationary state approximation, d[A ]/dt <= 0 and d[A ]/dt = 0, the expressions for the steady state concentration of [A ] and [A ] can be set up. We can further define, 6m, and as quantum efficiencies of emission from dilute solution, from concentrated solution with quenching, and of excimer emission, respectively ... 

Table I. Ratio of Quantum Efficiencies of Excimer and Normal Emission in Acenaphthylene Homopolymer and Equimolar Copolymers Comonomer IdZIM—...

As for the ppy-based systems discussed in Sect 6.2, the majority of the N N-coordinated complexes display very efficient excimer formation, and the excimers themselves are remarkably highly emissive 0.3). By varying the dopant concentration within the host emissive layer, the ratio of mmiomeric (blue) to excimeric (red) emission is varied, and the observed Ught that the device produces is the net combination. At a doping level of 15% in a CBP host a device has been obtained that emits with CIE coordinates of 0.337, 0.384, close to white light, and with a quantum efficiency of 18.1%, making it one of the most efficient WOLEDs reported hitherto [84]. 

The short wavelength X = 337 nm of the nitrogen laser permits pumping of dyes with fluorescence spectra from the near UV up to the near infrared. The high pump power available from this laser source allows sufficient inversion, even in dyes with lower quantum efficiency [5.170-5.174]. At present the most important dye laser pumps are the excimer laser [5.175,5.176], the frequency-doubled or -tripled output of high-power Nd YAG or Nd glass lasers [5.177,5.178], or copper-vapor lasers [5.179]. 

Bokobza et al.(1977) reported the fluorescence spectra of i-TPH and s-TPH. The quantum efficiency of excimer formation ( gp) at 25 in s-TPH equals the of racemic Al. The quantum efficiency of excimer formation in i-TPH is larger than in meso Al. This was explained as the result of a high delocalized excitation energy in i-TPH. Fluorescence spectra as well as fluorescence decay measurements of all three isomers were reported recently (Moens 1985). The fluorescence spectra of s-TPH, i-TPH and h-TPH in isooctane at room temperature are represented in figure 12. The fluorescence intensities of locally excited state and excimer emission of h-TPH are not the average of the intensities in the spectra of i-TPH and s-TPH. 

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