Microcavity structures

M Kashiwabara, K Hanawa, R Asaki, I Kobori, R Matsuura, H Yamada, T Yamamoto, A Ozawa, Y Sato, S Terada, J Yamada, T Sasaoka, S Tamura, and T Urabe, Advanced AM-OLED Display Based on White Emitter with Microcavity Structure. SID 04, Digest, 1017-1019, 2004. 

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... 

H. Becker, S.E. Burns, N. Tessler, and R.H. Friend, Role of optical properties of metallic mirrors in microcavity structures, J. Appl. Phys., 81 2825-2829, 1997. 

Figure 139 Comparison of a microcavity structure (a) and a conventional organic LED (b).

Figure 141 EL spectra from the microcavity (a) and conventional LED (b) measured at different fixed detection angles (0). The microcavity structure comprised a reflective Ag anode (36 nm), a TAD HTL (250 nm), an emission dye layer [15nm-thick naphthostyryla-mine (NSD) film], an ETL [240nm-thick oxadiazole derivative (OXD) film], and a reflective MgAg cathode. Note that a 10% transmittance Ag layer (36 nm) played here a role of a half mirror component in the microcavity structure (cf. Fig. 139a). A transparent ITO film served as the anode in the conventional LED structure (Fig. 139b). After Ref. 550. Copyright 1993 American Institute of Physics.

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 143 illustrates the high spectral selectivity of the microcavity structure. The EL spectrum of the Eu complex-based conventional LED from Fig. 136a is compared with the EL spectrum of the same organic layers system placed in a microcavity formed by the MgAg metal/100% mirror (150 nm) and a dielectric half mirror (a quarter-wave stack... 

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.

Figure 144 (a) An edge emitting microcavity structure with two metal electrode/100% mirrors, based on the Alq3 emitter and PDA as HTL and (b) the EL spectra of two such different thickness structures (1) D = 350 nm and (2) D = 160 nm, detected at 0 = 0 the surface light output spectrum is shown for comparison (broken line). After Ref. 553. Copyright 1993 SPIE, with permission. 

To obtain the EL intensity (either surface or edge) from an EML constituting a part of a microcavity structure, we need to know the intrinsic spectrum of the radiation from the emissive layer along with its angular intensity pattern, elU, )- For a given 1,... 

Thus, no sharp emission pattern can be expected with the overall emission spectrum. Nevertheless, assuming the Lambertian shape of the emission from microcavity structures may lead to an overestimate as large as 30% [571]. An attempt to compare the measured full spectrum external emission as a function of the emitter thickness (Alq3) with theoretical description of microcavity modes has shown substantial disagreement, the theoretical estimates lead to the emission output much below the experimental data, differing by a factor of 2 for a 40nm-thick emitter [567]. The reason for... 

Bjork, G. (1991). Modification of spontaneous emission rate in planar dielectric microcavity structures. Physical Review A, Vol. 44, No. 1, pp. 669—681. 

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. 

Peng H J, Wong M and Kwok H S, "Design and Characterization of Organic Light Emitting Diodes with Microcavity Structure" SID 03 DIGEST 516 78. 

FlG. 13.12. Schematics of the microcavity structure. Reprinted with permission from Agranovich et al. (66). Copyright Elsevier (1997). 

Incorporation of the microcavity structure into OLEDs is demonstrated to narrow emission spectra (and thus improve color saturation for display... 

T. Tsutsui, N. Takada, S. Saito, E. Ogino, Sharply Directed Emission in Organic Electroluminescent Diodes with an Optical-Microcavity Structure. Appl. Phys. Lett. 1994,65,1868. 

---