Organic Electronics 11 (2010) Contents lists available at ScienceDirect. Organic Electronics. journal homepage:

Organic Electronics 11 (2010) 137 145 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel Deep blue, efficient, moderate microcavity organic light-emitting diodes Hyoung Kun Kim a,1, Sang-Hwan Cho b,1, Jeong Rok Oh a, Yong-Hee Lee b, Jun-Ho Lee c, Jae-Gab Lee c, Soo-Kang Kim d, Young-Il Park d, Jong-Wook Park d, Young Rag Do a, * a Department of Chemistry, Kookmin University, Seoul 136-702, Republic of Korea b Department of Physics, Korea Advanced Institute of Science and Technology, Daejeon 350-710, Republic of Korea c School of Advanced Materials Engineering, Kookmin University, Seoul 136-702, Republic of Korea d Department of Chemistry/Display Research Center, The Catholic University of Korea, Bucheon 420-743, Republic of Korea article info abstract Article history: Received 14 August 2009 Received in revised form 1 October 2009 Accepted 17 October 2009 Available online 23 October 2009 Keywords: Organic light-emitting diodes Microcavity Bragg mirror Blue Two types of microcavity blue organic light-emitting diodes (OLEDs) with a dielectric Bragg mirror (two different center wavelengths, type-a 465, type-b 470 nm) were designed to achieve the color coordinates of NTSC blue standard and enhance the quantum efficiency in the normal direction. A moderate microcavity OLED was defined as a microcavity OLED with a single pair of TiO 2 /SiO 2 high/low dielectric layers inserted between an indium tin oxide (ITO) layer and a glass substrate. The moderate microcavity blue OLED doped with 9,10-bis(3 0,5 0 -diphenylphenyl)-10-(3 000,5 000 -diphenylbiphenyl-4 00 -yl)anthracene (TAT) exhibited excellent color coordinates (type-a; x = 0.143, y = 0.068, type-b; x = 0.139, y = 0.081), which were better than the color coordinates of the NTSC standard (0.140, 0.080) and the TAT-doped conventional noncavity OLED (0.156, 0.094). There were approximately 60% and 54% improvement in the relative quantum efficiency of the type-b TAT-doped moderate microcavity OLED, respectively, compared to those of the conventional noncavity reference OLED (ITO = 150 nm) and other reference type-ii with an identical ITO layer thickness (ITO = 85 nm). These improvements in color coordinates and the relative quantum efficiency were attributed to the optimization of narrowed spectrum bandwidth and enhanced integrated spectrum intensity in the TAT-doped blue OLED, resulting from the effective microcavity effect. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Organic light-emitting diodes (OLEDs) are used widely as ideal emissive devices in lighting and flat panel displays on account of their low driving voltage, low power consumption, high color gamut, high contrast and rapid response [1 4]. For full color display applications, it is essential to develop high performance red-, green-, and blue-emitting OLEDs with high EL efficiency, good thermal properties, and long device lifetime as well as pure * Corresponding author. E-mail address: yrdo@kookmin.ac.kr (Y.R. Do). 1 These authors contributed equally to this work. color coordinates (1931 Commission Internationale de l Eclairage (CIE) x, y coordinates). To develop stable, efficient and saturated red to green and blue OLEDs, the general approach is to design and synthesize new organic small molecules as emitters in OLEDs. Many organic small molecules for red [5 7], green [8,9] and blue emitters [10 19] have been synthesized. Recently, high efficient and pure red fluorescent and green OLEDs have been developed with CIE x, y coordinates and electrical efficiencies of (0.67, 0.23) and 11 cd/a and (0.29, 0.64) and 21 cd/a, respectively. However, the color purity and efficiency of the blue-light emitters are still lower than the requirements for balancing with red and green emitters for full color displays. Therefore, improvements in the blue emitting performance are necessary, particularly for large display 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.10.011

138 H.K. Kim et al. / Organic Electronics 11 (2010) 137 145 applications, such as digital video displays (DVD) and laptop computer screens, which require a larger color gamut. In particular, blue emitters require the CIE x, y coordinates of the National Television System Committee (NTSC) blue color standard (0.14, 0.08). With this respect, synthesizing new blue emitters with a narrow band and saturated color might help in obtaining a deep blue color. Many types of blue emitters have been studied extensively, and considerable effort has been made to improve their blue EL performance [10 19]. Thus far, the best results of a deep bluelight emitter are anthracene-cored derivatives with excellent color coordinates (0.156, 0.088) and good electrical efficiency [19]. The color coordinates of anthracene-cored derivatives are still inferior to the NTSC blue color standard. Therefore, further improvements in blue color and luminescence efficiency are essential for applications to large display devices, such as televisions and computer screens. However, the broad emission spectra due to both vibronic transitions and strong inhomogeneous broadening of the transitions in various organic molecules make it difficult to develop deep blue color OLEDs using a new type of blue emitter only. Another way of overcoming the problem of limited color purity of blue OLEDs is the optical modification of the OLED structure and filtering out the unwanted long wavelength emission. With this respect, a simple modification of the OLED structure, i.e. the insertion of organic layers into a microcavity structure has been investigated extensively in recent years owing to the enhanced luminescence, narrowing bandwidth, and tunable emission color [20 32]. Microcavity OLEDs can be divided into two categories: one is OLEDs embedded in a weak cavity structure [20], and the other is insertion into a strong cavity structure [21 32]. The former is a conventional OLED structure, in which the central organic layers are sandwiched between an aluminum (Al) metal cathode and an indium tin oxide (ITO) anode. On the other hand, the latter uses a conventional Al electrode and semitransparent metal mirror [21 24] or an ITO anode coated onto a dielectric Bragg mirror [25 32]. As previously reported, the weak microcavity has relatively minor effects on the electroluminescence (EL) performance [20]. However, the strong microcavity modifies the photonic mode density within the OLED significantly, improving the EL performance and tuning the color purity from a homogeneously broadened emitter at the microcavity resonance [21 32]. The use of metal mirrors enhances the EL efficiency and luminance in the top- or bottom-emitting OLEDs significantly but their narrowing capability of the emission spectrum from the metal mirror still does not satisfy the NTSC pure blue. Recently, Mudler et al. [21] obtained a high efficient and deep blue color (0.116, 0.136) OLED from a sky blue phosphorescent OLED using a strong microcavity structure with a semitransparent Ag anode. Although the color purity of the phosphorescent OLED was improved significantly using a strong metal mirror-based microcavity, it is necessary to control the detailed peak position and bandwidth of the emission spectrum in order to approach the NTSC blue color standard. On the other hand, dielectric mirrors reduce the bandwidth of the EL spectrum considerably but the improvement in OELD efficiency is not as high as the metal-based strong microcavity. In the early stages of dielectric-based microcavity OLEDs, Tokito et al. [28,29] reported that the purity of red, green and blue obtained in microcavity OLEDs was superior to the NTSC standard. However, the total enhancement of their device by integration over all angles and wavelengths was not reported. Recently, Zhang et al. [30] and Jung et al. [31] reported the tuning of the emission color and improvement in EL efficiency simultaneously by simply adjusting the cavity length and carefully designing the device structures. They narrowed the emission spectrum of blue, green and red OLEDs significantly, but improved the green and red efficiency only. Quite recently, Cho et al. [32] reported that moderate microcavity formed by an Al cathode and one pair of high- and low-index (SiN x /SiO 2 ) dielectric layers enhanced the efficiency of sky blue OLEDs (0.110, 0.216) by more than 26%. Although there are many papers on the enhanced output of the emission peaks from microcavity OLEDs, there are few reports on the high efficient and saturated blue OLEDs, which are more efficient than noncavity conventional OLEDs and have a deeper color than the NTSC blue color standard (0.14, 0.08). This paper reports microcavity OLEDs containing dielectric Bragg mirrors with a finely tunable peak position and a bandwidth of EL emission that achieves enhanced external quantum efficiency and a deep blue color (y 6 0.08). The significant change in bandwidth, external quantum efficiency and color of the microcavity OLEDs with three different pairs and two lattice constants of highand low-index dielectric layers was examined in order to gain a better understanding of the effect of the degree of microcavity TiO 2 /SiO 2 layers on the efficiency and color purity of 9,10-bis(3 0,5 0 -diphenylphenyl)-10-(3 000,5 000 -diphenylbiphenyl-4 00 -yl)anthracene (TAT)-doped OLEDs. Among the various microcavity OLEDs, a moderate microcavity OLED was defined as a microcavity OLED with only a single pair of TiO 2 /SiO 2 layers inserted between the indium tin oxide (ITO) layer and a glass substrate to achieve the NTSC blue standard and enhance the normally directed quantum efficiency. 2. Experimental Fig. 1 shows a schematic diagram of the microcavity OLED with one, two and four pairs of high- and low-index pair of dielectric Bragg mirrors. Two types of reference blue OLEDs were fabricated with a commercialized emitter in the following configurations: ITO (150 nm or 85 nm)/4, 4 0,4 00 -tris(n-(2-naphthyl)-n-phenylamino)triphenylamine [2-TNATA] (49 nm)/n,n 0 -bis(naphthalen-1-yl)-n,n 0 -bis (phenyl)benzidine [NPB] (12 nm)/9,10-bis(3 0,5 0 -diphenylphenyl)-10-(3 000,5 000 -diphenylbiphenyl-4 00 -yl)anthracene [TAT] (30 nm)/8-hydroxyquinolinealuminium [Alq 3 ] (24 nm)/lif (1 nm)/al (150 nm). The optical lengths of the type-a and type-b TAT-doped microcavity blue OLEDs were attributed to the different optimized structures summarized in Table 1. In the type-a microcavity blue OLEDs, one, two and four pairs of high-(tio 2 ) and low-index (SiO 2 ) layers were placed between the ITO layer and glass substrate to control the top reflectivity of the reflection band.

H.K. Kim et al. / Organic Electronics 11 (2010) 137 145 139 Fig. 1. Schematic diagram of a microcavity OLED device and chemical structure of TAT. Fig. 1 also shows the chemical structures of the blue emitting material (TAT) in this experiment. As reported previously [19], TAT was purified employing a silica column and recrystallization, was characterized with nuclear magnetic resonance (NMR), Fourier transform infrared (FT-IR), and fast atom bombardment mass (FAB-MS) analysis. Boration and Suzuki aryl aryl coupling reactions were used in all syntheses. The detailed synthesis route and characterization results of TAT are reported elsewhere [19]. To examine the effect of the dielectric Bragg mirrors experimentally, OLEDs were fabricated on two reference substrates (conventional reference (ITO = 150 nm) and reference type-ii (ITO = 85 nm) and two types (type-a one pair and type-b one pair) of dielectric Bragg mirrors covering the substrates. Table 1 summarizes the thicknesses and sheet resistances of each sample. Mono-color OLED samples were fabricated with an active area of 2.0 2.0 mm 2. High-index TiO 2 (n = 2.3) and low-index SiO 2 (n = 1.46) thin films were deposited alternatively on glass substrates in a single chamber using an e-beam evaporation technique [33]. The ITO layer was then deposited by magnetron sputtering with no intentional heating, and organic layers corresponding to 2-TNATA, NPB, TAT, and Alq 3 as well as a LiF film (as EIL) and aluminum metal cathode were evaporated sequentially to fabricate both the reference and microcavity devices. Finally the OLEDs were encapsulated with 0.7 mm Corning cover glass. The current voltage (I V) characteristics of the electroluminescence (EL) devices fabricated were obtained using a Keithley 2611 electrometer. The EL spectra, EL brightness and color coordinates were obtained with a spectrophotometer (PSI Co., Ltd.) and a Minolta CS-1000A. The enhancement ratio of the quantum efficiency was obtained by comparing the normally directed light output of microcavity OLEDs at 10 ma/cm 2 to that of a conventional reference OLED (ITO = 150 nm). 3. Results and discussion As reported previously [29 32], the electrical efficiency, relative quantum efficiency and color purity of the microcavity OLEDs depend on the thickness of the ITO, organic, low-index, and the high-index layers, respectively. The finite-difference time-domain (FDTD) simulations were used to determine the resonance wavelength of the cavity by varying the thickness of the ITO, low-index (SiO 2 ), and high-index layers (TiO 2 ). Dielectric Bragg mirrors with two different lattice constants were also inserted into the blue OLEDs in an attempt to tune the position of the blue emission peaks. Here, two types of quarter-wave stacks were designed to have center wavelengths of 465 (type-a) and 470 nm (type-b). The thicknesses of the ITO, low-index, and high-index layers are defined as the values when a deep blue color was realized. The reflectance (R) and transmittance (T) of the two types of quarter-wave stacks consisting of alternating low-index SiO 2 and highindex TiO 2 were measured as a function of the number of periods. As shown in Fig. 2, the reflectance of the type-a Bragg mirrors at the blue region increased with increasing periodic number of stacks. The central wavelength of the reflectance band shifted toward a bluish color with increasing number of high/low stacks, which is consistent with those in the calculation. The inset in Fig. 2 shows a side-view scanning electron microscopy (SEM) image of a real fabricated type-a dielectric multilayer comprised of alternate TiO 2 and SiO 2 quarter-wave films of the fourth periods. As shown in Table 1 and Fig. 2, the thicknesses Table 1 The designed and fabricated thicknesses of the ITO, TiO 2, and SiO 2 films for the two types of reference OLEDs and two types of microcavity OLEDs along with the sheet resistances of ITO for each sample. Noncavity OLEDs Microcavity OLEDs Type-A Type-B Conventional reference Reference type-ii One pair Two pairs Four pairs One pair Thickness (nm) ITO 150 85 80 85 SiO 2 80 82 TiO 2 50 51 Sheet resistance (±2 X/h) 13 47 34 44 41 36

140 H.K. Kim et al. / Organic Electronics 11 (2010) 137 145 Fig. 2. The measured diffusive reflectance spectra of two and four pairs of TiO 2 /SiO 2 high/low stacks coated on glass substrate. The inset shows a side-view scanning electron microscopy (SEM) image of the fourth periods of [TiO 2 /SiO 2 ] films coated with the ITO film. of the TiO 2 /SiO 2 films match those of the dielectric films in the designed Bragg mirrors satisfactorily. In order to compare the performance of the conventional noncavity OLED and the type-a microcavity OLEDs, the color coordinates, brightness, electrical efficiency and power efficiency should be considered. The normalized EL spectra of both type-a devices and the conventional reference (ITO = 150 nm) device (Fig. 3a) indicates that an increase in the periodic number of stacks decreases the full width at half maximum (FWHM) of the emission spectrum up to 9 nm (four pairs) [29,30]. Single-mode blue emissions with peak wavelength of 460 466 nm with reduced FWHMs were realized by simply increasing the periodic number of SiO 2 /TiO 2 stacks. Compared to the conventional noncavity devices with a broader EL spectrum (55 nm FWHM), controlling both the narrowness and position of the emissive spectrum will be of great benefit in optimizing the color purity of blue OLED for applications to display devices [32]. Fig. 3b shows the relative EL spectra of the conventional OLED and type-a OLEDs with the periodic number of stacks at a viewing angle of 0. The intensity of the EL spectra increased with increasing periodic number of stacks up to two pairs and then decreased with further increases. This decrease in EL intensity in the four pairs of TiO 2 /SiO 2 stacks was attributed to the increased reflectance and reduced transmittance of blue light, as shown in Fig. 2. The figure compares the normally directed emission spectra recorded at a constant current density (10 ma/cm 2 ) for the conventional noncavity OLED and type-a microcavity OLEDs with a periodic number of stacks. The results show that at the main peak wavelength, the emission intensity of the type-a microcavity OLEDs with one, two, and four pair stacks was 2.3, 3.3, and 2.2 higher than that of the conventional reference OLED, respectively. Moreover, the EL spectra of the microcavity OLEDs were narrowed and shifted, indicating the microcavity effect. Therefore, the thicknesses of the ITO, lowand high-index layer affect the spectral characteristics [30]. Our FDTD calculations were also used to determine Fig. 3. (a) Normalized EL spectra and (b) relative EL spectra of the conventional reference OLED (ITO = 150 nm) and type-a microcavity OLEDs with the periodic number of high/low stacks at viewing angle of 0. The inset in (a) shows the relationship between the full width at half maximum (FWHM) of the EL spectrum and the periodic number of high/ low stacks, and the inset in (b) shows the relationship between the EL spectrum intensity and the periodic number of high/low stacks. (c) Calculated EL spectra of the conventional reference OLED (ITO = 150 nm) and type-a microcavity OLEDs with the periodic number of high/low stacks at viewing angle of 0. The inset in (c) shows the relationship between the calculated and measured electrical efficiency along normal direction and the periodic number of high/low stacks.

H.K. Kim et al. / Organic Electronics 11 (2010) 137 145 141 the normally directed emission spectrum and electrical efficiency of type-a microcavity OLEDs with one, two and four pair stacks, indicating an improvement in the relative electrical efficiency of this device of 23%. Fig. 3c shows the calculated emission spectra and relative electrical efficiency of the type-a microcavity OLEDs. Fig. 3b and c confirm that the calculated shape of the emission spectra and the enhancement of the electrical efficiency are very similar to those of the measured spectra and electrical efficiency in the normal direction. Fig. 4a and b shows the current density (J) normal direction luminance (L) voltage (V) characteristics of the conventional noncavity reference OLEDs and type-a microcavity OLEDs with the periodic number of stacks. The J of the conventional reference OLED increased faster with increasing voltage than those of all microcavity OLEDs. The superior J V characteristics was attributed to the enhanced hole injection from the thicker ITO anode of the conventional OLED with a lower resistance, as summarized in Table 1. On the other hand, the dependence of J for all type-a microcavity OLEDs showed similar trends due to the similar sheet resistance of the ITO anodes, irrespective of the periodic number of stacks. Fig. 4b shows that the difference between the L V curves of the conventional OLED Fig. 4. (a) Current density bias voltage characteristics, (b) current density normal direction luminance characteristics, (c) current density electrical efficiency characteristics, (d) current density power efficiency characteristics (e) enhancement ratio of the quantum efficiency in the normal direction and (f) CIE color coordinates of the conventional reference OLED (ITO = 150 nm) and type-a microcavity OLEDs with a periodic number of high/low stacks at a viewing angle of 0. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

142 H.K. Kim et al. / Organic Electronics 11 (2010) 137 145 and the microcavity OLEDs with the periodic number of stacks is larger than that between the J V curves. This figure also shows the different trends of the L V curves of the microcavity OLEDs as a function of the periodic number of stacks. The microcavity OLEDs with increasing number of quarter-wave stacks showed decreased brightness due to the different dependences of the microcavity effect on the periodic number of stacks. Although higher color purity was achieved by increasing the microcavity effect, the luminous efficiency was reduced despite the enhanced peak intensity due to the loss of light around the shoulder of the peak at 480 nm, which has high eye sensitivity. It appears that when dielectric mirrors are used, there is some trade-off between the color purity and luminance for blueemitting materials. Fig. 4c and d shows the relationship between the current efficiency current density and power efficiency current density of the conventional reference OLED and type-a microcavity OLEDs. The current efficiency at 10 ma/cm 2 in the normal direction for the conventional noncavity reference OLED was 2.8 cd/a, whereas the efficiencies for the type-a microcavity OLEDs with one, two and four pairs of quarter-wave stacks were 3.2, 2.4, and 1.0 cd/a, indicating that the current efficiency of conventional reference OLED in the normal direction is slightly lower than that in the type-a microcavity OLED containing a single pair of quarter-wave stacks. Fig. 4d also shows a similar relationship of power efficiency current density with the number of quarter-wave stacks. Although the current and power efficiencies of the moderate microcavity blue OLED with one pair of high-low stacks is slightly higher than that of the noncavity blue OLED, the photons emitted from the microcavity blue OLEDs was significantly enhanced by the microcavity effect, as illustrated by the emission spectra in Fig. 3b. For a systematic comparison of these OLEDs, the enhancement ratio of the quantum efficiency was defined as the ratio of the normally directed light output of microcavity OLEDs or other reference OLEDs to that of a conventional reference OLED (ITO = 150 nm). This enhancement ratio only shows how much the EL quantum efficiency at 10 ma/cm 2 of microcavity OLEDs was improved relative to that of a conventional noncavity reference OLED. Fig. 4e shows the enhancement ratio of the quantum efficiencies of each OLED. In contrast to the trends in current efficiency, the quantum efficiency of type-a microcavity OLEDs with a single pair of quarter-wave stacks was superior to that of the conventional reference OLED. However, the microcavity OLEDs with a higher number of pairs of quarter-wave stacks showed lower quantum efficiency. The normally directed quantum efficiency of the type-a microcavity OLEDs with a single pair of high and low-index layers was 1.57 higher than that of the conventional reference OLED. As expected, with the resonant wavelength set to 465 nm (19 nm longer than the PL peak of TAT), the type-a moderate microcavity device with a single pair of quarter-wave stacks showed the significantly enhanced quantum efficiency compared to the conventional reference OLED device. Even with brightness slightly higher than the conventional OLEDs, the quantum efficiency of the microcavity OLEDs with a single pair of quarter-wave stacks was significantly higher than that of the conventional OLED device. This was attributed to the lower eye sensitivity of more pure blue moderate microcavity OLEDs. In addition, Fig. 4f shows that the CIE color coordinates of the type-a microcavity OLEDs with a different number of quarter-wave stacks were purer than those of the conventional noncavity OLED. The increased color purity with the increasing number of high/low stacks was attributed to the blue shift and narrowing of the emission spectrum due to the enhanced microcavity effect. In particular, the moderate microcavity OLED with a single pair of high/low stacks showed a deeper blue and higher quantum efficiency than the conventional noncavity OLED. The angular dependence of the emission spectrum was also examined because changes in color with changing viewing angle are undesirable for display applications [28,29]. Fig. 5a and b shows the angle dependence of the emission spectra of a conventional reference OLED (ITO = 150 nm) and a type-a microcavity OLED with a single pair of high/low stacks at room temperature, respectively. The figures show negligible angular dependence in the peak shape of the conventional reference OLED but a blue shift in the emission spectrum of the microcavity OLED with a single pair of high/low stacks. The CIE coordinates were used to assess the color changes in the OLEDs. Fig. 6 shows the angular dependence of the CIE color coordinates for the conventional reference OLED and type-a microcavity OLED with a single pair of high/low stack on a chromaticity diagram. When viewed from the normal direction, the CIE color coordinates for the conventional reference OLED and moderate microcavity OLED were (0.156, 0.094) and (0.143, 0.068), respectively. This suggests that the light emitted by the moderate microcavity OLED has deeper color purity than the conventional reference standard. The results also show that the color change ratio of the moderate microcavity OLED (Dx = 13%, Dy = 15%) became slightly worse than that of the conventional reference OLED (Dx = 1%, Dy = 31%) as the viewing angle was changed up to 60 of the normal. In contrast to the color change in the conventional reference OLED (0.156, 0.094? 0.157, 0.123) and the angular dependence of color purity of the moderate microcavity OLED (0.143, 0.068? 0.161, 0.058) showed improved color purity with increased viewing angle. The improved color purity and comparable angular dependence of the color purity are another advantage that the moderate microcavity OLEDs have over conventional reference OLEDs. The far-field pattern of the integrated spectrum intensity was measured to determine the physical characteristics underlying the angular dependence of a single EL emission intensity. The far-field photon radiation from the OLED devices was measured directly with a photodiode, 1 mm 2 in area, which was located 10 cm from the radiation source. Fig. 7 compares the far-field profiles of the conventional reference and moderate microcavity OLEDs cut along the horizontal line. It shows that the noncavity OLED radiates into air in a manner consistent with a Lambertian source. It also suggests that the moderate microcavity OLEDs show only small variations in their angular radiation patterns over a large angular span. Therefore, the far-field profiles of the moderate microcav-

H.K. Kim et al. / Organic Electronics 11 (2010) 137 145 143 Fig. 5. Viewing angle dependence of the EL spectra of (a) conventional reference OLED (ITO = 150 nm) and (b) type-a moderate microcavity OLED with a single pair of high/low stacks. Fig. 7. The viewing angle dependence of integrated EL intensity profiles of conventional reference OLED (ITO = 150 nm) and type-a moderate microcavity OLED with a single pair of high/low stacks under 10 ma/cm 2 of current density. Fig. 6. Viewing angle dependence of the Commission Internationale de l Eclairage (CIE) color coordinates of the conventional reference OLED (ITO = 150 nm) and type-a moderate microcavity OLED with a single pair of high/low stacks under 10 ma/cm 2 of current density. ity OLEDs with a single pair of high/low stacks were similar to that of the conventional reference OLEDs. To achieve the NTSC blue color from TAT-doped blue OLED, the tunable effects of the microcavity (type-a one pair and type-b one pair) on OLEDs were examined by controlling the central wavelength of the dielectric Bragg

144 H.K. Kim et al. / Organic Electronics 11 (2010) 137 145 mirror. OLEDs were fabricated on two reference substrates (conventional reference (ITO = 150 nm) and reference type-ii (ITO = 85 nm)) and two moderate microcavity substrates (type-a: k cen = 465 nm, type-b: k cen = 470 nm) deposited with a single pair of high/low stacks, side by side for comparison. Consistent with the simulated results, Fig. 8 suggests that the emission spectrum of the type-b microcavity OLED was shifted slightly to a longer wavelength compared to that of the type-a microcavity OLEDs. This indicates that the emission spectra of two types of moderate microcavity OLEDs were narrower than those of the two reference OLEDs. Fig. 8 also shows the CIE color coordinates in the normal direction for the reference OLEDs and moderate microcavity OLEDs. The color coordinates of a thin ITO (85 nm) coated reference type-ii OLED was slightly higher than that of the conventional reference OLED (ITO = 150 nm). This means that the reduced ITO thickness of the noncavity OLED have a minor effect in controlling the color purity and EL performance. As mentioned Fig. 8. The normalized EL spectra and CIE color coordinates of the conventional reference OLED (ITO = 150 nm), reference type-ii OLED, type-a moderate microcavity, and type-b moderate microcavity OLED under a current density of 10 ma/cm 2. above, the light emitted from the moderate microcavity OLEDs had deeper color purity than that emitted by both types of reference OLEDs. The color coordinates (x = 0.139, y = 0.081) of the type-b moderate microcavity OLED with a single pair of high/low stacks was very close to the color coordinates of NTSC blue (x = 0.140, y = 0.080). Therefore, a pure blue color was obtained from TAT-doped blue OLEDs simply by introducing a single pair of TiO 2 /SiO 2 high/low dielectric stacks on the substrate. The electroluminescence measurements showed the current density luminance characteristics of the four types of OLEDs with and without the microcavity high/ low stack. The luminance values of the two types of moderate microcavity OLEDs and reference OLEDs measured in the normal direction (h = 0 ) under dc excitation at 10 ma/cm 2 were 320 (type-a one pair, ITO thickness of 80 nm), 340 (type-b one pair, ITO thickness of 85 nm), 280 (conventional, ITO thickness of 150 nm), and 290 cd/m 2 (conventional, ITO thickness of 85 nm). Fig. 9 shows the current efficiency and enhancement ratio of the normally directed quantum efficiency for two conventional OLEDs and two types of moderate microcavity OLEDs. For the conventional OLEDs, the current efficiencies and relative quantum efficiencies at 10 ma/ cm 2 in the normal direction were 2.8 cd/a and 1.00 (conventional reference) and 2.9 cd/a and 1.04 (reference type-ii). By comparison, the current efficiencies and quantum efficiencies of the type-a and type-b moderate microcavity OLEDs were 3.2 cd/a, 1.57 and 3.4 cd/a, 1.60, respectively. This indicates that the current efficiencies of the moderate microcavity OLEDs in the normal direction are higher than those of the corresponding conventional OLEDs. Moreover, the relative quantum efficiencies of the type-a and type-b moderate microcavity OLEDs in the normal direction were 57% and 60% higher than those of the corresponding conventional reference OLEDs. This also suggests that the type-b moderate microcavity OLED with color purity close to the NTSC blue standard had higher efficiency than both conventional reference OLEDs and reference type-ii with an ITO layer of identical thickness. Fig. 9. Electrical efficiency and the enhancement ratio of the normally directed quantum efficiency of the conventional reference OLED (ITO = 150 nm), reference type-ii OLED, type-a moderate microcavity, and type-b moderate microcavity OLED under current density of 10 ma/cm 2.

H.K. Kim et al. / Organic Electronics 11 (2010) 137 145 145 4. Conclusion This study examined the effects of inserting dielectric Bragg mirrors in an OLED with the aim of achieving NTSC pure blue and enhancing its relative quantum efficiency. First the FDTD method was used to calculate the resonance wavelength of the cavity while varying the thickness of the organic and high/low dielectric layers. The introduction of a single pair of TiO 2 /SiO 2 high/low stacks into the OLED structure (denoted as a moderate microcavity OLED) is an effective way of enhancing the quantum efficiency and achieving a NTSC pure blue color from a TAT-based blue OLED. It was demonstrated experimentally that the incorporation of two types of moderate microcavity OLEDs with a single pair of high/low stacks achieved pure blue CIE color coordinates (type-a; x = 0.143, y = 0.068, type-b; x = 0.139, y = 0.081). The type-a and type-b moderate microcavity OLED showed more than 57% and 60% improvement in relative quantum efficiency in the normal direction, respectively, compared to the conventional reference OLED (ITO = 150 nm) in the normal direction. They also improved the relative quantum efficiency by more than 51% and 54%, respectively, compared to the reference type-ii (ITO = 85 nm). In addition, the moderate microcavity OLED is more favorable for the angular dependence of the color purity and EL intensity. This simple technique of introducing a dielectric Bragg mirror with a single pair of high/low dielectric stacks into a TAT-doped blue OLED can be generalized for the development of efficient and saturated blue OLEDs from the standpoint of realizing a deep blue color in computer and television screens, and for full color display applications. Acknowledgments This study was supported by grant number 2008-03573 of the Nano R&D Program and grant number R11-2005- 048-00000-0 of the ERC program through the Korea Science and Engineering Foundation, funded by the Ministry of Education, Science and Technology. This study was partly supported by the IT R&D program of Ministry of Knowledge Economy/Institute for Information Technology Advancement (2009-F-020-01). This work was also supported by the faculty research program 2009 of Kookmin University of Korea. H.K. Kim and S.-H. Cho contributed equally to this work. References [1] C.W. Tang, S.A. Van Slyke, C.H. Chen, J. Appl. Phys. 65 (1989) 3610. [2] C. Adachi, T. Tsutsui, S. Saito, Appl. Phys. Lett. 55 (1989) 1489. [3] M.A. Baldo, M.E. Thompson, S.R. Forrest, Nature 403 (2000) 750. [4] C.C. Wu, Y.T. Lin, K.T. Wong, R.T. Chen, Y.Y. Chien, Adv. Mater. 16 (2004) 61. [5] Y.S. Yao, Q.X. Zhou, X.S. Wang, Y. Wang, B.W. Zhang, J. Mater. Chem. 16 (2006) 3512. [6] B.J. Jung, J.I. Lee, H.Y. 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