Intensity and Angular Dependence in Planar Microcavities

In these experiments, two different substrates were employed one quartz substrate with a three-period Si02/Si3N4 quarter-wave stack and the other plain quartz. Thirty nanometers of ITO was deposited on both substrates by pulsed-laser deposition of a commercial ITO target in 1 mtorr of O2 at a substrate temperature of 250° C.32 This was followed by depositing 60 nm of TAD and 60 run of Alq, 

FIGURE 4.9. Electroluminescence spectra (along the cavity axis) from three representative cavity devices from the same substrate but with a terraced filler (as in Fig. 4.2). The anode is a 55-nm-thick ITO layer. The three spectra have major peaks at (a) 625 run, (b) 512 nm, and (c) 545 nm. The measured external quantum efficiencies (in units of photons/electron) are also indicated in each case. 

FIGURE 4.10. Electroluminescence spectra (along the normal to the plane of the substrate) from a cavity LED and a reference noncavity LED excited by the same injection current. The active materials for both devices were deposited simultaneously. 

The enhancement of the emission intensity along the cavity axis (at the resonance wavelength) is given by 9 

We have indicated how a modest (1.3) enhancement in angular intensity can be obtained in cavity devices with Alq emissive layers. Further enhancements in angular intensity are possible by choosing emissive layers with narrower free-space emission spectra than Alq. Alq doped with small quantities of the laser dye pyrromethene 580 (PM) results in the emission spectrum of the system becoming narrower than that of Alq. This is result of resonance energy transfer33 from the excited states of Alq to the excited states of PM580. The full width at half-maximum of the luminescence drops from 100 nm to 45 nm. The spectra are shown in Fig. 4.12. The external quantum efficiency of noncavity devices is enhanced in comparison with devices with an undoped Alq emissive layer. For a device with a ITO/TAD/Alq+0.5%PM (20 nm)/Alq/Li (1 nm)/Al(200 m) structure, an external quantum efficiency of 1.8-2% photons/electron was measured. For comparison, equivalent LEDs without the pyromethene dye had an external quantum efficiency (with Li/Al cathodes) of 0.8%. 

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