Planar microcavity LEDs

The first term in Eq. (2) is the penetration depth of the electromagnetic field into the dielectric stack, the second term is the sum of optical thicknesses of the layers between the two mirrors, and the last term is the effective penetration depth into the top metal mirror. The phase shift at the metal reflector ( pm) is given by26 

FIGURE 4.3. (a) Schematic structure of a microcavity LED with an Alq emitting layer. The top mirror is the electron-injecting contact and the bottom mirror is a three-period dielectric quarter-wave stack (QWS) with Si02(n = 1.5) and SixNy(n = 2.2). (b) Electroluminescence spectrum from a cavity LED compared with that from a noncavity LED. The noncavity LED possesses the same layer structure as shown in (a), but has no QWS. 

The approximate theoretical spectrum for emission normal to the plane of the device layers can be calculated following the approach of Deppe et al.8 The calculated spectrum can be approximated by 

An inspection of Eq. (4) reveals the importance of the spatial location of the emitting dipoles in determining the nature of the emission spectrum. If the emitting dipoles are located at a node of the standing wave formed between the two end mirrors, then the emission would be greatly suppressed, and, likewise, if the emitting dipoles are located at an antinode, the emission intensity would be a maximum, all other factors being the same. 

Lidzey et al. have demonstrated that, under appropriate conditions, there can be strong coupling between excitons and photons in organic semiconductor microcavities.14 Clear evidence has been found for the formation of polariton 

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Single Mode and Multimode Planar Microcavity LEDs... 

Figure 141 shows the EL spectra from a microcavity (a) and conventional LED (b) based on the emission from an NSD dye forming a thin emitting layer of a three-organic layer device. It is apparent that the half-width of emission spectra from the diode with microcavity is much narrower than those from the diode without cavity. With 0 = 0°, for example, the half-width of the spectrum of the diode with cavity is 24 nm whereas that of the sample without cavity increases to 65 nm. According to Eq. (275), the resonance wavelength, A, decreases with an increase of 0 in agreement with the experimental data of Fig. 141. We note that no unique resonance condition in the planar microcavity is given due to broad-band emission spectrum of the NSD emission layer. Multiple matching of cavity modes with emission wavelengths occurs. Thus, a band emission is observed instead a sharp emission pattern from the microcavity structure as would appear when observed with a monochromator the total polychromic emission pattern is a superposition of a range of monochromatic emission patterns. The EL spectra...

Fig. 4.3b illustrates the modifications to the free-space emission produced by the planar microcavity device whose structure is shown schematically in Fig. 4.3a. The principal effect is a narrowing of the emission spectmm in most directions. The cavity device spectrum is taken along the cavity axis, and the full width at half-maximum (FWHM) is lowered from 100 nm for a noncavity device to 18 nm. With higher- Q cavities, it is possible to get narrower spectra, and Tokito et al. have reported LEDs with a FWHM of 8 nm.28...

The effects produced by a planar microcavity on the electroluminescence characteristics of organic materials have been described. A number of organic and polymeric semiconductors have been employed by various groups in studies on microcavity LEDs. However, for detailed descriptions, three categories of emissive materials have been considered undoped Alq, Alq doped with 0.5% pyrromethene, and Alq+NAPOXA. Alq has a broad free-space emission spectrum spanning the... 

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