1. Introduction

One of the principal reasons that organic light-emitting diode (OLED) technology [1, 2] has attracted great interests is its potential for use in full-color displays that might replace active-matrix liquid crystal displays (LCDs). Patterning the organic light emitting materials by using conventional photolithographic technique usually seriously deteriorates their fluorescence performance. In order to realize full color displays of organic light-emitting diodes (OLEDs), several approaches [3, 4] have been proposed without directly photolithographic patterning these organic fluorescent materials. The first method is to dispose red, green and blue pixels side by side with shadow mask, which is the most adapted technology. However, shadow masking has the disadvantage of requiring frequent mask replacement or cleaning to maintain high dimensional control of the display panel. The second technology is to prepare a continuous white electroluminescent layer without patterning and get the different color pixels through color filters. This method suffers very low efficiency though the color purity could be improved. The third approach is to convert blue light of common layer into green and red light through photoluminescence color arrays which are patterned before the preparation of the blue light emitting layer. This method has reasonable efficiency but the complex of the color conversion pixels preparation. In addition, there is another method, which is to employ the color selectivity of microcavity by the different optical cavity length for different pixels without patterning the organic light emitting layers. The technique usually has good efficiency and excellent color purity but shows view angle dependent colors. [5–11] Recently, the approach to achieving a full color display is to use the microcavity structure together with color filter, where microcavity is formed by optimizing the organic film thickness for the different primary color OLEDs. [12] It achieved very good color purity and reduced the view angle dependence of color purity.

In this letter, we proposed a combination of the color filter, microcavity structures, and a common white polymeric OLED, where the three primary colors were originally realized by the different optical microcavity and then purified by passing through the color filters. The accurate patterning of three different organic layers is not required in this kind of structure. In the microcavity structure, the optical microcavity length was optimized by the different thickness of patterned ITO electrode. We fabricated a full color 128×128 passive matrix polymeric OLED by using this structure and discussed the effect of color filter on improving the color purity in the display panel.

2. Experimental result and discuss

Figure 1 shows the schematic cross section of the full color organic light-emitting devices based on the combination of microcavity structure, color filter and a common white polymeric OLED. The color filter consisting of 70μm×210μm red, green and blue pixels was prepared by the photolithography of dyeable photo-polymer on glass substrates. The color filter was over-coated by a protection resin layer to reduce the thickness variation. On the overcoat layer, a reflection Distributed Brag Reflector (DBR) structure consisting of three periods of alternative layers of SiO₂ and TiO₂ was formed by sputtering. The ITO layer with a thickness of 180nm was formed on the DBR. The strip patterned ITO on the DBR was prepared by photolithography and wet etching. The ITO line and space widths were 60μm and 10μm, respectively. Then the ITO cavity spacers were made by photolithography and anisotropic wet etching. The thicknesses of ITO terraces were 180nm for red (without etching), 130nm for green and 80nm for blue, respectively.

The prepared substrates were solvent cleaned and oxygen plasma treated, followed by a polymeric OLED fabrication process. We used a 100-nm-thick 3, 4-polyethylenedioxythiophenepolystyrenesulfonate (PEDOT) [13] as the hole injection layer as well as protection layer to reduce the roughness resulted from wet etching of ITO cavity spacer in this study. The white light emitting layer was a blend of Poly[9,9-bis(octyl)-fluorene-2,7-diyl] (PF) and its red and green co-polymers with a thickness of 70 nm. [13] After deposition of these polymer layers by spin coating, the top cathode was prepared by sequential deposition of a 3nm Ba layer and a 150nm Al overlayer without breaking the vacuum. In this study, three kinds of devices as the reference samples were fabricated to compare with: a microcavity OLED without color filter (Ref. 1), a white OLED fabricated on 30 nm ITO coated color filter substrate (Ref2), a conventional white OLED fabricated on 30 nm ITO coated glass (Ref. 3). In these devices, the structures and materials of the organic layers, cathodes, color filters and microcavity structures were identical.

 

Fig. 1. The schematic structure of full color OLED device with the microcavity structure and color filter.

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Table 1 summarizes color coordinates of the three primary colors in the microcavity OLED without color filter (Ref1), white OLED with color filter (Ref2), and full color OLED with microcavity and color filter for several view angles in this study. Compared to the view angle dependence of color purity in the three primary color microcavity OLED, that of full color OLED with microcavity and color filter is reduced. This kind of effect of color filter on the microcavity OLED was also described in the previous paper. [12] Figure 2(a) shows the microscopic photograph of DBR and patterned ITO cavity spacers on a glass substrate. The dim red, green and blue strips are observed, which means that the three weak resonance peaks at the red, green and blue wavelengths can be extracted though the white light source of microscope is far from the DBR. Figure 2(b) shows the microscopic photograph of full color OLED device based on the combination of the microcavity structure, color filter and a common white polymeric OLED in this study.

Table 1. Color coordinates of the three primary colors in the microcavity OLED without color filter (Ref. 1), white OLED with color filter (Ref. 2), and full color OLED with microcavity and color filter.

View Table

 

Fig. 2. (a). The microscopic photograph of DBR and patterned ITO cavity spacers on the glass substrate. (b) The microscopic photograph of full color OLED device with the microcavity structure and color filter.

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Figure 3 shows the electroluminescent spectra of the microcavity OLEDs without color filter (Ref. 1) for three primary colors and convention white OLED (Ref. 3). It indicates that each microcavity has the second resonant mode, which degrades the color purity. For example, the blue pixel has two emitting peaks located at 440 nm and 570 nm, the green pixel has two at 430 nm and 530 nm, and the red pixel has two at 460 nm and 605 nm. It is evident that the two resonant modes in a microcavity OLEDs for the primary colors are not suitable for wide color gamut of the full color displays.

 

Fig. 3. The electroluminescent spectra of the microcavity OLEDs for red (red line), green (green line) and blue (blue line) and that of convention white OLED (black line).

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The relationship between the optical thickness and mode positions was described in some previous papers [5–7]. The total optical thickness of the cavity, L, is give by:

Where ^△ n is the index difference between the layers that constitute the DBR, λ is resonance wavelength of cavity, n is the average index, and φ_m is the phase shift metal reflector. The first term in Eq. (1) is the penetration depth of the electromagnetic field into the DBR, the second term is the sum of optical thickness of the layers between the two mirrors, and the last term is the effective penetration depth into the top metal mirror. The positions of the cavity modes are given by the relation mλ = 2L(λ), where m is the mode index. Although the structure of single-mode microcavity devices is almost the same as that of multi-mode microcavity devices, the thickness of the single-mode microcavity is smaller than that of multi-mode so that it has one resonant mode within the emission spectrum. However, it is difficult to fabricate single-mode microcavity for each primary color OLEDs, since the roughness resulted from patterned ITO terraces as cavity spacer in this study. For example, for λ = 450 nm, the calculated total thickness of polymeric layers in single-mode microcavity should be less than 47 nm, even if the penetration depth of the electromagnetic field into the DBR is assumed to be negligible. In the calculation, the thickness of ITO spacer is 80 nm, and the respective refractive indices of polymeric layers and ITO spacer are 1.7 and 1.8, respectively. It is evident that the total thickness with 100 nm of PEDOT used as protective layer and 70 nm of white light emitting layer is much more than the calculated thickness. The same kind of the second peak was also shown in the previous paper. [5]

Figure 4 shows the electroluminescence spectra of the white OLED device with color filter (Ref. 2). The emitting peaks of red, green and blue are 610 nm, 535 nm and 450 nm, respectively. The half-width of emission spectra of the white OLED device with color filter is wider than those of the microcavity OLED. For example, the half-width of the spectrum of the white device with green filter is 90 nm and that of the first-mode of the microcavity red OLED is 30 nm. However, the color purity (0.65, 0.35) of the OLED white device with red filter is higher than that (0.42, 0.25) of the microcavity red OLED because of the second resonance peak in the electroluminescence spectrum of the microcavity red OLED.

 

Fig. 4. The electroluminescence spectra of the white OLED device with color filter for red (red line), green (green line) and blue (blue line).

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Figure 5 shows the electroluminescent spectra of the microcavity OLEDs with color filter. The half-width of the emitting peaks of the devices are almost same as those of the first mode of microcavity OLEDs, respectively. The second peak corresponding to the second mode is absorbed by color filter, resulting in the color purities are higher in comparison with those of the microcavity OLED as shown in Table 1. For example, the color purity of microcavity red OLED with color filter is (0.65, 0.35) while that of microcavity red OLED is (0.42, 0.25). This is because the second resonance peak at 460 nm of microcavity red OLED is absorbed by the red color filter. Thus, the other effect of color filter is suppressing the undesirable peak resulted from microcavity structure, which leading to the increase of color reproduction.

 

Fig. 5. The electroluminescent spectra of the microcavity OLEDs with color filter for red (red line), green (green line) and blue (blue line).

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Figure 6 shows the luminance-current characteristics of the three primary color devices in the microcavity OLED without color filter (Ref. 1), white OLED with color filter (Ref. 2), and full color OLED with microcavity and color filter in this study. The luminance of white OLED with color filter is approximately equal to 72% of that of the microcavity OLED without color filter at the same drive current (150 mA/cm²). And the luminance of the OLED with microcavity and color filter is approximately equal to 65% of that of the microcavity OLED without color filter at the same drive current (150 mA/cm²).

 

Fig. 5. The luminance-current characteristics of the microcavity OLED without color filter (Ref1) (solid triangle), white OLED with color filter (Ref2) (solid square) and full color OLED with microcavity and color filter in this study (solid circle) for red (red line), green (green line) and blue (blue line).

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3. Conclusion

In conclusion, we demonstrated the fabrication of the full color passive matrix polymeric OLEDs based on the combination of microcavity structure, color filter and a common white OLED, where the optical microcavity length was optimized by the different thickness of patterned ITO electrode spacers. The primary color emitting peaks were originally generated by the different optical microcavity and then the second resonance peak was absorbed by the color filter, resulting in the increase of the color purities.