Color filters based on enhanced optical transmission of subwavelength-structured metallic film for multicolor organic light-emitting diode display

Transcription

1 Color filters based on enhanced optical transmission of subwavelength-structured metallic film for multicolor organic light-emitting diode display Xiao Hu,* Li Zhan, and Yuxing Xia Institute of Optics and Photonics, Department of Physics, State Key Lab of Advanced Optical Communication Systems and Networks, Shanghai Jiao Tong University, Shanghai , China *Corresponding author: Received 16 May 2008; revised 4 July 2008; accepted 14 July 2008; posted 14 July 2008 (Doc. ID 96299); published 8 August 2008 Using metallic film perforated with a subwavelength periodic structure, a novel concept of a color filter for multicolor organic light-emitting diode (OLED) display is proposed. Based on the phase-matching condition for extraordinary optical transmission, three primary color emissions can be obtained by optimizing the structure s periodicity. Two periodic structures, an array of one-dimensional periodic slits and a two-dimensional periodic hole array, are studied using coupled mode theory. Also, the feasibility of applying these structures as color filters is analyzed. The relative intensity at the unwanted wavelength, which is generated by higher resonant transmission, had been calculated to eliminate its effect on the purity of these filters. It is important that this type of color filter simultaneously solves the low emission efficiency problem for OLEDs with the aid of enhanced transmission of metal film Optical Society of America OCIS codes: , , /08/ $15.00/ Optical Society of America 1. Introduction Organic light-emitting diodes (OLEDs) are the most promising next generation display devices following liquid crystal displays [1]. One of the key problems for commercializing OLEDs is full-color display. One way to achieve multicolor emission is to use multi-emission layer structures, in which the three primary colors are emitted from different organic layers [1]. However, this approach restricts the industrialized processing of OLEDs to a great extent because of high cost, low efficiency, complicated process, and so on. Another way is to develop white OLEDs combined with color filters [2]. A color filter is a pixel array that has red (R), green (G), and blue (B) color elements, and a black matrix is located between the colors to avoid leakage of light. The conventional color filter is fabricated that contains R, G, and B colors from either dyes or pigments [3]. In this paper, a novel color optical filter is proposed for OLED display that is compact and easily fabricated. This type of filter can also serve as a component that helps to extract emission light from OLEDs at the same time. The proposed color filter is based on the phenomenon of extraordinary optical transmission in a periodically perforated metallic film. It had been believed that when light was incident at tiny holes or slits, the transmission would be fourth-order inversely proportional to the wavelength of incident light, and thus no transmission would happen at subwavelength holes or slits [4]. However, since Ebbesen s discovery in 1998 [5], a lot of works about the extraordinary enhanced transmission at certain wavelengths through periodic subwavelength holes or slits in metal dielectric interfaces have been reported [6]. Both experiments and theories have shown that surface plasmon polaritons (SPPs) play a main role in this phenomenon [5,6]. Extraordinary transmission happens at the wavelength where the reciprocal vectors of the structure period help to satisfy the phase-matching condition between SPPs 10 August 2008 / Vol. 47, No. 23 / APPLIED OPTICS 4275

2 and incident light [5,6]. By choosing appropriate periodicities and making the transmission peaks overlap the wavelength of pure tricolors, color filters for pure R, G, and B colors can be obtained. The proposed color filters can also serve as the electrodes of OLEDs, and their enhanced transmission property helps improve the extraction efficiency of light emission in the OLEDs [7]. The proposed color filter is promising for fine-color representation because of its high transmission and sharp transmission resonant peak. 2. Theory and Discussion Figure 1 is the schematic view of the proposed color optical filter in a full-color display OLED system. The system is composed of pure tricolor R, G, and B cells, and each cell includes an active layer of organic semiconductor sandwiched by two metallic films, one of them perforated with a subwavelength structure that not only serves as electrodes but also serves as a color filter and helps in extracting light. Each emitting cell is independently driven by the different bias, which can modulate the emission of the cell. By adjusting the relative intensity of three pure tricolor RGB cells, any required color may be obtained. A piece of organic layer sandwiched by two metallic layers forms thesimplest configuration of OLEDs[8,9]. The main shortcoming of this configuration is that the electrodes are opaque and prevent light emission. The light-extracting efficiency is very low because the relative high refractive index of the organic layer forms a waveguide for light and therefore traps the optical energy, which is eventually coupled into a surface polariton and consequently decays nonradiatively [10,11]. The most common way to improve the extracting efficiency is using a semitransparent electrode such as tin oxide as the hole-injecting anode. However, for the electron-injecting cathode, opaque metallic electrodes are still being used [12]. Recent research shows that perforating the metallic film with periodic subwavelength holes or slits will sharply increase the emission extraction at a certain wavelength due to the abnormal transmission [7]. In our proposal, the cathode is a metallic film perforated by subwavelength slits or holes, which overcomes the problem of low light-extracting efficiency due to the extraordinary transmission on the periodic structured metallic film, and at the same time the cathode acts as an optical filter due to the phase-matching condition. On the smooth metal dielectric interface, light cannot efficiently couple into the SPP mode because the momentum conservation is not satisfied [13]. In the metal film with periodic subwavelength holes or slits, the periodicity allows the grating coupling of the SPPs to light. Indeed, it was found [5,6] that the optical transmission through a subwavelength structured array fabricated on optically thick metallic film is enhanced at resonance wavelengths because light couples to SPP excitations. The transmission maxima are decided by the periodicity of the structure [5,6]. If the maxima are at the wavelengths of CIE primary colors, the metal film plays the roll of color filter, and thus a multicolor display may be realized. For the one-dimensional subwavelength slit arrays illustrated in Fig. 2(a), with periodicity Λ, the phasematching condition is [6] k sp ¼ k mg; ð1þ where jkj ¼ð2π=λÞsin θ is the in-plane wave-vector component of incident light, for normal incidence, θ ¼ 0, G is the reciprocal vectors, jgj ¼2π=Λ, and k sp is the propagation constant of the SPPs, which can be solved by the dispersion relation [8] rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ε k sp ¼ n eff k ¼ m ε d k; ð2þ ε m þ ε d where ε m and ε d are the dielectric constants of the metal film and the OLEDs, respectively, and n eff is the effective index of the SPP modes. If a two-dimensional hole array is considered, the formula for phase-matching condition is k sp ¼ k mg x þ ng y ; ð3þ where jg x j¼2π=λ x and jg y j¼2π=λ y. For simplicity, Λ x ¼ Λ y ¼ Λ is supposed. According to the CIE standard, the wavelengths of pure tricolors are 700 nm (R), 546 nm (G), and 436 nm (B). Here, we suppose that the cathode is an Ag film, whose optical parameters can be obtained from [14], and the index of the active layer and the substrate is By using Eqs. (1) (3)and making the first-order resonant peak equal to the pure tricolor wavelength, we can obtain periodicity of 460 nm for R, 344 nm for G, and 246 nm for B. The light-extracting efficiency is proportional to the transmission intensity, which can be calculated Fig. 1. Schematic view of the proposed color optical filter in the full-color display OLEDs system APPLIED OPTICS / Vol. 47, No. 23 / 10 August 2008

3 Fig. 2. Schematic view of the (a) periodic one-dimensional slit array and (b) two-dimension hole arrays. by the coupled mode theory [15]. The transmission spectrum of the proposed color filter for 700 nm is illustrated in Fig. 3. The resonant peaks are denoted by their orders [(m) for periodic slit arrays, where m is in Eq. (1), and (m, n) for periodic hole arrays, where (m, n) is in Eq. (3)]. The figure shows that, besides the desired 700 nm resonant peak, higher order resonant transmissions peak at 420 nm and 528 nm for the Fig. 3. Spectrum calculated by coupled mode theory: (a) slit arrays, (b) hole arrays (Ag film on the white OLED with refractive index n ¼ 1:45). hole array [corresponding to order (1,1) and (0,2)] and 420 nm for the slit array [corresponding to order (2)] appear in the visible band. Since high color purity is essential for excellent color representation, it is extremely important to eliminate the unwanted wavelength and to retain only the necessary light. Compared with the slit array, the hole array configuration has an additional resonant peak (1,1) between peaks (0,1) and (0,2), which comes from the periodicity in the diagonal line, so it is more difficult to put the hole array configuration into practical application. For the slit array configuration, from Eq. (1), the wavelength of the m order resonant peak can be calculated as λ m ¼ð1=mÞ ½Λ=n eff ðλ m ÞŠ. If one neglects the dispersion, λ m ¼ λ 1 m, the second-order peaks are at 350 nm, 273 nm, and 218 nm for the 700 nm (R), 546 nm (G), and 436 nm (B) filter, respectively. They are all outside the visible band ( nm) and do not influence the properties of the color filters. However, Fig. 3 shows that the dispersion of Ag film makes the second-order resonant peak move into the visible band, which affects the purity of color filter. One way to eliminate the unwanted resonant wavelength in the visible band is to choose less dispersive metal film or make a dispersive active layer to prevent the second-order resonant peak from moving into the visible band. The method proposed in this work is to choose an active layer with a proper fixed index that ensures the second-order resonant transmission peaks outside the visible band or makes their relative intensity very small. The influence of the active-layer refractive index n on the second-order resonant wavelength is calculated using the coupled mode theory for the 700 nm filter, which is shown in Fig. 4(a). The figure demonstrates that, for Ag film, only when n < 1:1, the second-order resonant transmission is outside the visible band and the purity of the 700 nm filter cell is ensured, which is a rigid requirement. Figure 4 (b) shows the influence of n on the intensity of the unwanted second-order resonant peak relative to the 700 nm transmission peak. The figure demonstrates that the intensity of the second-order resonant peak oscillates with the increase of the active-layer index, and it will be very small when the index n is near the valley values (1.2756, , , , , , , , ). Choosing the active layer with the index near the above valley wavelengths will maintain nice color purity for the 700 nm filter. Figure 5 shows the spectra of the three RGB filter cells (the active-layer index is 1.488), which demonstrate the good purity property of the color filter. For each filter cell, the wanted transmission peak is sharp and the unwanted peak in the visible band is very small. The intensity of the transmission peaks of the three RGB filters is calculated and shown in Fig. 4(c). The calculation demonstrates that when the emission spectrum of the white OLED active layer is absolutely flat in the visible band, the transmission 10 August 2008 / Vol. 47, No. 23 / APPLIED OPTICS 4277

4 if the structure parameters such as film thickness and the slit width are optimized[16]. The experiment shows the transmittance could be up to 2 times the fractional aperture area [5] (the area of hole or slit arrays divided by the whole area of the metal film). For example, supposing the periodicity of the red filter cell is 460 nm as calculated above, and the slit width is 150 nm, about 33% fractional aperture area and up to 60% transmittance may be obtained. Fig. 4. Wavelength of the second resonant peak (a), the relative intensity of the second resonant peak to the desired color wavelength (700 nm) (b), and the intensity of the three filter (c) versus the refractive index of the active layer. intensity of the three RGB filter cells varies with the change of the active-layer index, and the intensity of the three filters is approximately equal when the active-layer index is around 1.3. When the activelayer emission spectrum is not absolutely flat, for example, when its spectrum is like that in [2], in which, the emission intensity at 436 nm is about 2 times those at 700 nm and 536 nm, is a nice choice for the active-layer index. Changing the slit widths or adjusting the electric voltage also works for obtaining equal intensity of the three RGB filter cells. Theory demonstrates that near 100% transmittance may be obtained at the resonant wavelength Fig. 5. Transmission spectra of the three RGB filter cells (the active-layer index is 1.488) APPLIED OPTICS / Vol. 47, No. 23 / 10 August 2008

5 Resolution, i.e., the pixel numbers in the width and height directions on the screen, is important for display, and therefore the RGB filter cells must be small enough to realize the required resolution. For the same resolution, the bigger the display screen, the bigger the pixel size. A bigger RGB filter cell also means it is easier to manufacture. The resolution for a standard notebook display includes VGA ð Þ, SVGA ð Þ, and XGA ð Þ, SXGA þð Þ, and UVGA ð Þ. For a 14 inch XGA display, the visual dimension is 14 inches, i.e., 285:7 mm 214:3 mm, the resolution is , and each pixel pitch is 285:7=1024 or 214:3=768 ¼ 0:29 mm. Each pixel is composed of three RGB emitting cells, so the dimension of each RGB filter cell is about 90 μm on average. Supposing the periodicities for RGB filters are 460 nm (R), 344 nm (G), and 246 nm (B) as calculated above, the number of periodicity is 196 (R), 262 (G), and 366 (B). On the contrary, if each filter is required to have more than 100 periods, then the dimension is 46 μm (R), 34 μm (G), and 25 μm (B). On average, each filter is less than 40 μm, and the pixel pitch can be less than 120 μm. Compared with the most expensive UVGA ð Þ, whose visual dimension is commonly 15 inches ð306:1 mm 229:6 mmþ, its pixel s dimension is 191 μm, and the cell of the proposed filter is small enough to realize such high resolution. High resolution requires a small number of periodicity, while good color representation requires a large one, which is a trade-off and must be considered in design. The RGB elements of a traditional color filter s pixels are fabricated with colorants such as dye and pigment [3], and their entire manufacturing process includes the same three development processes for R, G, and B. Generally, there are four developing methods for the three tricolors, which are dyeing, the pigment-dispersed method, printing, and electro-deposition. Compared with those traditional color filters, the proposed filter is suitable for mass production, and it is easy to change the elements sizes; it has good heat stability, light stability and chemical stability; its spectrum can be made very sharp by enlarging its periodicity in one emitting cell, and its resolution is adequate for application in display. 3. Conclusion A novel color filter for OLED display has been proposed that is based on the phenomenon of wavelength dependent extraordinary optical transmission. In the structure, the structure periodicity on the metal film compensates the mismatch between the wave vector of the extracting light and the SPPs and results in wavelength-selective enhanced optical transmission. At the same time, the proposed color filter plays the role of extracting light from OLEDs. 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