Electroluminescence microcavity device

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). 

S Tokito, Y Taga, and T Tsutsui, Strongly modified emission from organic electroluminescent device with a microcavity, Synth. Met., 91 49-52, 1997. 

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]. 

FIGURE 4.8. Electroluminescence spectrum from a three-mode microcavity LED, in which the three peaks are at 488, 543, and 610 nm. The EL spectrum from a noncavity device is shown for comparison. 

The relative ease of inducing alignment of discrete oligomer chains, rather than their polymeric counterparts may yet render the use of oligomer films particularly advantageous in certain applications, such as waveguided or microcavity electroluminescent devices. 

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]. 

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