1. Introduction

Recently, intensive research and development have been conducted on organic light-emitting diodes (OLEDs) [1] that utilize organic electroluminescent (EL) materials for the realization of high-efficiency, long-lived, and full-color emissions. Regarding development, it is critical to solve the problem of low-light extraction efficiency from the OLED devices (<0.2). For this purpose, several trials have been conducted that introduce new structures for out-coupling schemes, by eliminating wave-guiding phenomena: microstructuring [2, 3, 4], microlenses [5, 6, 7, 8], microcavities [9, 10, 11, 12, 13, 14, 15, 16], nanopattering citenano1, silica aerogels [18], and shaped mesa substrates [19]. Although silica aerogels [18] and shaped mesa substrates[19] increase the out-coupling factor over ~1.8, they have serious drawbacks, such as the complexity and difficulty of the fabrication process. By contrast, the preparation of microlenses or microcavities is simple and the cost is potentially lower. However, the improvements made to the microlens or microcavity structures are still not sufficient; problems remain regarding a low out-coupling factor (~1.5), change in the radiation pattern, and/or undesirable angle dependent emission spectrum [16]. Thus, their successful performance for OLED devices has not yet been demonstrated. In this report, we propose a combined structure of resonant cavity OLEDs together with microlenses. By employing this combined architecture, we have demonstrated suppressed angle-dependent emission spectra and an increase in the out-coupling factor of ~1.8.

The structure of the microcavity OLED with microlens array used in this study is shown schematically in Fig. 1. On a transparent substrate, a stacked one-dimensional photonic crystal (1-D PC) is formed and a transparent indium-tin-oxide (ITO) layer is formed as an anode on the PC mirror. EL organic materials are deposited over the ITO anode regions and then a metal cathode layer is formed. In such architectures [9, 10, 11, 12, 13, 14, 15], single or multi modes of an optical cavity, formed between the reflecting metallic cathode and the PC mirror, overlap the free-space emission spectra of the organic materials such that one or several optical modes can be selected. The resonant cavity mode depends strongly on the photonic band of the PC as well as the angle of propagation direction of emission, which results in sharply directed emissions (λ₁,λ₂ in the glass in Fig. 1). However, because of the microlens system formed on the transparent substrate, the directed emission due to the microcavity is refracted at the surface of the microlenses, as shown in Fig. 1. Thus, the microlens system may reduce the dependence of the direction of emission on the wavelength, which allows us to suppress undesirable angle-dependent characteristics of spectral emission, particularly for color display devices. Moreover, one may expect this structure to be useful for further increasing the out-coupling factor by eliminating the wave-guiding effect.

 

Fig. 1. Schematic illustration of the microcavity OLED structure with irregular microlens arrays (left) and a scanning electron micrograph of the microlens arrays (right).

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2. Experimental methods

For the experiments, a 1-D PC, consisting of four stacks of alternative layers of SiO ₂ and SiN_x, was formed on the glass substrate (0.7 mm thick). The thickness and refractive index of each layer were, respectively, 90±9 nm and 1.48 for SiO ₂ and 70±7 nm and 1.75 for SiN_x to form a resonant cavity mode at 525±15 nm (green). The ITO layer (80 nm, 10–20 ohm/square sheet resistance) was formed on the 1-D PC multilayers by sputtering. After cleaning the substrate by routine procedures, the organic materials were thermally deposited over the ITO anode regions at a rate of 0.1 nm/s under a base pressure of 5×10^-7 Torr. The deposited organic layers consisted of a hole injection layer (HIL) of 4,4’, 4”-tris [N-(3-methylphenyl)-N-phenylamino]-triphenylamine (m-MTDATA: 60 nm), a hole transporting layer (HTL) of N, N’-diphenyl-N, N’-bis (1-naphthyl)-(1,1’-biphenyl)-4,4’-diamine (NPD: 20 nm), a green emitting material layer (EML) and an electron transporting layer (ETL) of tris (8-quinolinolato) aluminum (Alq3: 55 nm). Then, a LiF/Al cathode layer (0.7 nm/150 nm) was formed on the top of the organic material layers via thermal deposition. Separately, a hemispherical microlens film was made of flat polyethyleneterephtalate (PET) film (180 µm thick) using a master mold of Al. The microlenses, 5 µm in diameter and 2.5 µm high, were fabricated randomly over the film surface to avoid undesirable diffraction patterns. Finally, the microlens film was laminated to the glass substrate of the OLED at the air/glass interface. In our experiments, three types of OLED device were fabricated and compared: a microcavity OLED without microlens (device A), a microcavity OLED with microlenses (device B), and a conventional OLED without microcavity and microlens (device C). Note that, in the devices, the organic layer structure and used materials were identical and thus, electrical characteristics (I–V) were identical in every device as shown in Fig. 2. For example, the turn-on voltages of devices A, B, and C were ~3.5 V and the current densities of devices A, B and C at 10 V were ~21.6 mA/cm².

 

Fig. 2. The electrical characteristics (I–V) of the three fabricated devices: black circles for device A, black squares for device B, and blue circles for device C.

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3. Results and discussion

First, we observed the EL spectra from several viewing angles (θ) from devices A and B (Fig. 3). As a reference, the EL spectrum of device C at θ=0° is also shown by dotted curves in Figs. 3 (a) and 3(b). In Fig. 3(a) for device A, the luminance spectra show characteristic sharp peaks (FWHM ~12 nm) at the resonant cavity mode (λ_max). Moreover, the EL spectra change significantly with θ, i.e., dλ_max/dθ~0.9 nm for device A, in comparison with dλ_max/dθ~0.1 nm for device C (FWHM ~50 nm). By contrast, in device B, the EL spectra (Fig. 3(b)) clearly show two distinctive features: (1) The luminance spectra show relatively broad emissions (FWHM ~30 nm). The emission spectra can be decomposed into the spectral peaks shown in Fig. 3(a). (2) The change of emission spectra with viewing angle is suppressed, i.e., dλ_max/dθ~0.5 nm, which suggests improvement of the emission directionality by the microlens. This may suppress undesirable color variation. To confirm this, we measured the color variations as a function of θ, as shown in Fig. 3(c). From the figure, one can clearly see that the color variation of device A is fairly large due to the changes in the condition of the resonant cavity, while that of device B is much suppressed and comparable to that of conventional device C.

Next, we observed the EL characteristics of the devices. Figure 4(a) shows the measured normalized EL intensity as a function of θ from devices A, B, and C. One can see from the figure that the light intensity from device A depends strongly on θ. In particular, just after θ=55°, the intensity from device A falls significantly to below 50% of its maximum. By contrast, the intensity from device B falls gradually, and even at over θ=55°, its intensity remains at ~65% of its maximum value. Thus, we may conclude that device B is more appropriate for lighting applications with wide viewing angles. Figure 4(b) shows the dependence of the external quantum efficiencies (η_ext) on applied voltage (V) at θ=0°. Here, η_ext was determined from the conventional luminance-current characteristics and the EL spectrum, assuming a perfectly diffusive EL emission surface[20]. Of course, the EL emission patterns, particularly for device A, do not follow Lambertian’s law, which yields an error. However, in this report it was assumed for simplicity that deviations of emission patterns for all devices from the law are not serious. In Fig. 4(b), η_ext of the reference device C increases monotonically as the applied voltage V increases, while η_ext of the device A is nearly always 1.25 times higher than that of reference device C. By contrast, for device B, η_ext is superior to devices A and C and is nearly 1.78 times higher than that of reference device C. This indicates that the out-coupling performance of device B is superior to that of device C with respect to an increase in external efficiency by a factor of ~1.8. Note that the factor of ~1.8 of device B is comparable to that of OLED devices with silica aerogels (~1.8) or shaped substrates (~1.9).

 

Fig. 3. The EL spectra from the OLED devices A (a) and B (b) for several different θs at V=10 V. The dotted line shows the reference EL spectrum of the device C. (c) A CIE chromaticity diagram showing the color variations of EL emission from devices A (blue squares), B (red circles), and C (white circles) for several θs with the NTSC standards.

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Fig. 4. (a) Normalized EL intensity as a function of viewing angle θ at V=10 V and (b) external quantum efficiency as a function of applied voltage at θ=0° from the devices A (blue squares), B (red circles), and C (white circles).

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Here, we wish to comment on the microlenses for further increase in extraction efficiency. First, the used hemisphere lens profile can be modified by changing the radius and/or curvature of microlenses. It might reduce Fresnel losses or total internal reflections, resulting in increased extraction efficiency. Second, the used hemisphere lens profile having a fill-factor about 90.4 % can be replaced by square-type microlenses with a high fill-factor over 95 %. It might also help to further improve extraction efficiency.

Finally, photographs of the operating microlens OLEDs with microcavity are shown in Fig. 5. The top of the figure shows operating twin microcavity OLEDs at 10 V and θ~55°. Onto the operating left OLED, a microlens film on a slide glass with index matching oil was attached as shown in the middle figure. From the comparison with operating device without microlens(right), one can clearly see enhanced emission from the active areas of device with microlenses even at θ~55°. In order to verify the enhanced emission, the microlens film was attached onto the right OLED, as shown in the lower figure. One can clearly see higher out-coupling from device with microlens system. This result clearly visualizes that a microlens OLED with a microcavity can be applied to lighting devices when the thicknesses and shapes of the substrate and the lens are optimized to avoid image blurring.

 

Fig. 5. Top: photographs of the twin operating microcavity OLEDs at 10 V and θ~55° Middle: a microlens film with a slide glass was attached onto the left OLED by using index matching oil. Lower: the microlens film was attached onto the right OLED. The active area of each OLED is 3×3 mm².

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4. Conclusions

In summary, we have demonstrated that a microlens OLED with a microcavity promises to improve the out-coupling factor and wide viewing angles without color variation. The combination of the method reported here and the previously reported high-efficiency OLED layers will surely lead to highly efficient OLEDs without any alteration of device configuration and materials design. The microcavity OLED with microlenses is a potential candidate for high out-coupling schemes for OLED devices with wide-viewing angles, in various applications of lighting displays, optoelectronic devices, and 3D displays.