Devices microcavity-structured

Figure 6.14 illustrates an OLED microcavity structure that comprises a stack of organic layers for providing EL, an upper electrode, and a bottom bilayer electrode of metal transparent conductive layer. The thickness of the transparent conductive layer (e.g., ITO) in the OLED structures can be varied across the substrate surface so as to achieve color tuning. One typical structure of the devices is glass/Ag/ITO (with a graded film... 

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...

Figure 142 Normalized EL spectra recorded normally to the surface of an Au/PPV(rf)/Al microcavity structure with different thickness (d) of PPV layer (a). Variation of the EL spectra with detection angle (0) for an Au/PPV (400nm)/Al device (b). After Ref. 348. Copyright 1996 American Institute of Physics.

Figure 143 Comparison of EL spectra of an Eu complex forming an emitter layer in a conventional organic LED from Fig. 135a (a) with the same system placed in a microcavity (Fig. 139a) with a MgAg electrode/100% mirror and a stack of Si02/Ti02 layers/half mirror (b). Note the disappearance of the small features of the spectrum in device (a) in the spectrum from the microcavity structure (b). After Ref. 425. Copyright 1998 Taylor Francis.

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...

Poitras, D. Dalacu, D. Liu, X. Lefebvre, J. Poole, P.J. Williams, R. L. (2003). Luminescent devices with symmetrical and asymmetrical microcavity structures. Proceedings of the 46th Annual Tech. Conf. of Society of Vacuum Coaters, pp. 317— 322, Philadelphia, May 2003, ISSN 0737-5921, SVC Publication, Albuquerque. 

Another very new development in CP applications has been CP-based semiconductor lasers. The Friend group at Cambridge, England first noted [867] that one to study whether the photoexcited states in electroluminescent CPs such P(PV) are fundamentally non-emitting interchain species or emitting intrachain species was to confine solid films of the CP in a microcavity, where spontaneous and stimulated emission of the CP could be studied. What resulted in an early test of such a device [867] was the observation that upon excitation at 355 nm, stimulated emission, i.e. lasing activity (predominantly at ca. 550 nm), was observed. Fig. 18-7a shows a schematic of the microcavity structure used by these authors. Fig. 18-7b shows characterization data for such P(PV)-based microcavity (laser) devices [867, 868]. 

Among other specific applications of PTs as light-emitting materials, it is necessary to mention microcavity LEDs prepared with PTs 422 and 416 [525,526] and nano-LEDs demonstrated for a device with patterned contact structure, and PT 422 blended in a PMMA matrix that emits from phase-separated nanodomains (50-200 nm) [527,528]. 

Vertical emission can also be achieved by the application of dielectric Bragg mirrors layers, which is in principle the DBR structure applied to the direction of the him normal. Such microcavities have been shown to alter the (electroluminescence spectrum of devices as well as the angular radiation characteristics [200-204], Normally, the angular dependence of the emission from a thin him follows Lambert s law [205]. 

Microcavity studies have employed a number of optically active materials inorganic semiconductors,6,9 organic liquids,2 and organic thin-film structures capable of charge transport.10,19 In this chapter, we shall be concerned with the latter class of materials, which include evaporated small molecules and conjugated polymers. These materials are used in device configurations that often include electron- and... 

The EL spectrum has multiple peaks when more than one mode of the cavity overlaps the free-space emission spectrum. It is possible to realize a white LED with a single electroluminescent material such as Alq by employing a two-mode microcavity device structure in which one of the modes is centered near 480 nm and the other near 650 nm. Such an electroluminescence spectrum, for which the CIE coordinates are (0.34, 0.386),13 is shown in Fig. 4.6. The approximate spectrum calculated with Eq. (4) is also shown in Fig. 4.6. With very minor changes in the device design, it is easy to achieve (0,33, 0.33). For comparison, the CIE coordinates of a noncavity Alq LED are (0.39, 0.56). 

Fig. 12 (a) SEM image of a typical 2D PEG structure. Note the smaller defect hole at the center of the device, and waveguides to the left and right of the 2D PEG array, (b) Normalized transmission spectra of the PhC microcavity (A) after oxidation and silanization (B) after treatment with glutaraldehyde, and (C) after infiltration and covalent capture of ESA. Adapted from [32]... 

Although the basic concepts described concerning the microcavity effect have been applied in the present work to bottom-emission OLEDs and specific materials only, they are general and will remain true whatever the materials used in the device (i.e. pwlymer-based), and for other device structures (such as top-emitting-OLED, tandem-OLED, etc.). 

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