Microcavity OLEDs

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... 

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. 

The top-emitting OLED with a bilayer anode of Ag/CFX and an ultrathin Ag layer used in the upper semitransparent cathode forms an optical microcavity, which can tailor the spectral characteristics of the emitters therein by allowing maximum light emission near the resonance wavelengths of an organic microcavity [80,81], When the mode wavelength of the cavity is fixed at 550 nm, the thickness of the Ph-PPV layer is determined to be about 110 nm [81]. 

Figure 13-15. Emission spectra measured at various angles from the surface normal of a microcavity OLED. The solid line is the noncavity spectrum measured at 0° from an OLED on the same substrate. Reproduced with permission from [161].

The phenomenon known as "microcavity effect" refers to the enhancement or annihilation of the emitted irradiance related to the position of the emitting material relative to this resonance peak of the irradiance. A weak microcavity effect is usually present in conventional OLEDs because internal reflections are caused by the higher refractive index of the ITO anode compared to most organic layers, and the cathode is highly reflective (Bulovic, 1998). This is usually considered a nuisance, but has been exploited in microcavity OLEDs (Jordan, 1996). With Fabry-Perot filters, the phase condition for the appearance of resonance peaks is given by the following equation ... 

A small microcavity effect, as seen in Sec. 2, is necessary for maintaining a good emission of the device. For that purpose, internal reflections Ranode and Rcathode must not be reduced to zero, and the organic layers inside the OLED act as cavity layers, so that the position of the emitting layer (the thin recombination layer) must be at a resonance peak of the electric field. 

As shown in Fig. 1, the combination of good AR coating and small microcavity effect apparently lead to a contradiction of the anode s role it must have simultaneously a low external reflectance when seen from the substrate and a relatively large internal reflectance when seen from the cavity layers. It has been observed for a long time in thin-fUm optics that a thin layer of a material with a large extinction coefficient k can lead to the kind of asymmetric reflectance (Goos, 1937). In our design, such a layer has thus to be introduced on the anode side of the OLED structure. 

The unfiltered OLED shows a deep absorption peak due to the Fabry-Perot resonance of the naturally-occurring weak microcavity, and the filtered OLED shows oscillations in the reflectance due to the same effect. Lower reflectance filters could be designed with more layers in the DBR, at the expense of added complexity. 

We have demonstrated the concept of a multilayer anode comprising an Au/ Ag bilayer and a metal-dielectric AR coating that has both a high internal reflectance and a low outside reflectance. The former property is used to maintain a microcavity effect in the OLED that is tuned to maximize light out-coupling, and the latter to improve the OLED contrast ratio. 

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.). 

Microcavity OLEDs fabricated on distributed Bragg reflectors (quarter wave stacks) [94]. Such OLEDs are fabricated on dielectric layers with significant dielectric contrast, so they narrow the emission spectrum by constructive interference. The narrow emission spectra also result in more efficient and more stable devices than regular OLEDs. The emission spectrum can be tailored to the specific sensor requirements (i.e., the absorption peak of the sensing element). 

---