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Abstract
In order to suppress the viewing angle dependence of top emission organic light emitting diodes (TEOEDs) on a strong microcavity structure, we prepared multi-layered nano scattering film which was consisted with transparent planarizing layer and hazy crosslinked scattering layer. Through such an approach, we could obtain not only a stable color shift and luminance distribution with viewing angle but also a negligible pixel blur level. Meanwhile, we investigated a black tint level of TEOLEDs after attachment of circular polarizer (CP) on various nano scattering films because nano scattering film deteriorates a black level. We found that the black level could be improved from the black tint by reducing the refractive index difference between planarizing layer and scattering layer.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Since their discovery by Tang and Van Slyke in 1987, organic light emitting diodes (OLEDs) have been investigated in depth by numerous research institutes and companies due to their excellent display performance. The display characteristics of these devices include vivid color, wide color gamut, fast response time, thin display form factor, flexible design ability, and high contrast ratio originating from their self-emitting nature [1–6]. Through extensive investigative effort, AMOLED (Active Matrix OLED) displays for mobile phones and TVs have initiated a rapid transition from LCDs to OLEDs. However, the short lifetime of OLEDs remains an unsolved problem [7, 8]. To increase the lifetime of these devices, a reduction of the required current density to achieve higher brightness levels is crucial. In this regard, many research groups have investigated outcoupling technologies to minimize the light loss originating from total internal reflection (TIR), surface plasmon coupling, internal absorption, etc [9–11]. For example, uneven surface morphology structures, such as micro lens arrays (MLAs), could easily increase the outcoupling efficiency by more than 30% in comparison with pristine OLED devices [12, 13]. Unfortunately, these approaches are not easily applied to display technology because they can cause a pixel blur phenomenon, whereby pixel boundaries are difficult to distinguish [14–17]. OLED panel manufacturers are attempting to increase luminance by redesigning OLED device structures from bottom emission organic light emitting diodes (BEOLEDs) to top emission organic light emitting diodes (TEOLEDs). These TEOLED structures can easily increase luminance and color purity via a strong microcavity effect between a highly reflective bottom electrode and a semi-transparent top electrode, according to the Fabry-Perot resonator Eqs. shown below [18–22].
In Eq. (1), λ denotes the peak emission wavelength, Lcav represents the optical cavity thickness, Lcav = nd; n = the refractive index, d = cavity thickness and, Rt and Rb are the reflectance of the top and bottom electrodes, respectively. The full width at half maximum (FWHM) is associated with the color purity such that this parameter normally increases as the FWHM decreases. In the Eq. (2), I(λ, θ) is the spectral emission intensity which exhibits microcavity effect at wavelength λ, and emission angle θ, which can be calculated as a function of transmittance, reflectance, distance between emitter and electrode, and phase shift. In addition, Tt, denotes the transmittance of the top electrode, z is the distance between emitter and the highly reflecting surface, a ϕb represents the phase shift at the bottom electrode, a ϕt denotes the phase shift at the top electrode, a θorg,EML is the emission angle in the emitting layer, and an I0(λ) denotes the initial spectral emission intensity of molecules, respectively. For the phase shift Eq. (3), ni, di, and θorg,i represent the refractive index, the cavity distance and the light propagation at the i-th layer, respectively. From these mathematical expressions, we expect TEOLEDs to have a narrow angular luminance distribution and a hypsochromic wavelength shift with increasing viewing angle. These properties affect the viewing angle characteristics. In order to suppress the viewing angle dependence of TEOLEDs that exhibit a strong microcavity effect, we fabricated a nanoporous polymer film (NPF), as we reported previously [23–27]. However, the black color level of the TEOLEDs at turn-off condition was faded via the application of an NPF to a circular polarizer (CP). We performed this step because the uneven surface morphology of the NPF increases diffusive reflection in the presence of ambient light. In this paper, we introduce a multi-layered scattering film consisting of a planarizing layer coated onto a nano-scattering layer, to improve the black level. From this approach, diffusive reflection was minimized and the optical characteristics of the TEOLEDs were improved, despite the introduction of the nano-scattering layers.2. Experimental
2.1 Materials
We fabricated green phosphorescent TEOLEDs, witch exhibited a strong microcavity effect due to its second order microcavity structure. These devices were used to investigate the suppression of viewing angle dependence by utilizing a new multi-layered nano scattering film. We used indium tin oxide (ITO) / silver (Ag) / ITO as a highly reflective anode. N,N’-bis(naphthalen-1-yl)-N,N’-bis(phenyl)benzidine (NPB) was used as a hole injection as well as a hole transport layer (HIL and HTL), 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) as a hole generation layer, and 4,4′,4″-tris(carbazol-9-yl) triphenylamine (TCTA) as a hole transport layer (HTL) as well as an electron blocking layer (EBL). Beryllium bis(2-(2′-hydroxyphenyl) pyridine) (Bepp2) was used as a host material for the emission layer, bis(2-phenylpyridine)(acetylacetonato) iridium(III) [Ir(ppy)2(acac)] as a dopant material also for the emission layer (EML), and 4,7-diphenyl-1,10-phenanthroline (BPhen) as an electron transport layer (ETL) as well as a hole blocking layer (HBL). Finally, lithium quinolate (Liq) was used as an electron injection layer (EIL), magnesium (Mg) doped in Ag as a semi-transparent cathode, and NPB was applied to the cathode as a capping layer (CL). All of the materials for the fabrication of the devices were purchased from commercial suppliers including Luminescence Technology Co. (Taiwan), Jilin OLED Material Tech Co. (China), Sigma-Aldrich Co. LLC., etc. The primary material used for the preparation of the NPF was cellulose acetate butyrate (CAB, acetyl content: 16–19 wt. %, butyryl content: 30–35 wt. %, hydroxyl content: 1.0–1.6 wt. %, Mw: 12,000 g /mol) which was purchased from ACROS and dissolved in chloroform. In order to strengthen the solvent resistance of the NPF, it was crosslinked (x-NPF) by adding Desmodur® N3300 (crosslinking agent) and dibutyltin dilaurate (DBTDL, catalyst), which were both purchased from Sigma-Aldrich Co. LLC. We used an SPC-370 based acrylate polymer containing fluorine which was purchased from FOSPIA Co., Ltd. (Republic of Korea), to produce a low refractive index planarizing layer. In addition, we used COHRI-183 consisting of ZrO2 in propylene glycol monomethyl ether acetate (PGMEA) based organic/inorganic hybrid material to produce a high refractive index planarizing material (purchased from ChemOptics Inc. (South Korea)).
2.2 Device fabrication
We fabricated TEOLEDs on the 4 mm2 (2 mm × 2 mm) sized pixels. The device substrates were sequentially cleaned by sonication in acetone and isopropyl alcohol, rinsed in deionized water, and finally irradiated in a UV-ozone chamber. The organic materials were thermally deposited in vacuum chamber under a pressure of around 5 × 10−7 Torr and their deposition rate was in the range of 0.5 and 1 Å/s. Liq as an EIL and Mg and Ag for cathode were deposited at a rate of 0.15 Å/s, 1.8 Å/s and 0.2 Å/s under around 10−6 Torr, respectively. We used 0.5mm thick glass lid (GL) to encapsulate OLED devices.
2.3 Preparation of scattering films
The schematic diagram for fabricating a multi-layered scattering film is shown in Fig. 1.
As we reported previously, the NPF was prepared using a simple spin coating process at 1000 rpm for 90 s in an atmosphere with a continuous supply of water droplets generated by an ultrasonic humidifier [35, 38]. After this process, the NPF was dried for 10 minutes at room temperature. A crosslinkable CAB polymer solution was formulated by the addition of 0.1 g Desmodur® N3300 (crosslinking agent) and 0.1 ml DBTDL (catalyst) to 10 ml of chloroform solution containing 1g of CAB [28–30]. We then utilized a similar procedure to prepare a thin x-NPF (e.g. spin coating process at 1000 rpm for 90 s in a wet atmosphere; over 90% RH controlled by ultrasonic humidifier; dried for 2 hours in 60 °C). Crosslinking of the NPF is critical because without this process, the film is easily deformed during overcoating processes. After deposition of the x-NPFs, two different types of planarizing materials with different refractive indices (SPC-370: 1.37 and COHRI-183: 1.83), were overcoated using spin coating.
2.4 Measurements
The electrical characteristics of the devices such as current, current density, and voltage were measured by power measurement units (Keithley 238). In addition, the optical characteristics such as luminance, color coordinates, and the electroluminescence (EL) spectra were measured using a spectroradiometer (CS-2000A, Minolta). The power efficiency was calculated by integrating the angular luminance using a goniometer with the spectroradiometer. The morphology of the scattering films was investigated by field emission scanning electron microscopy (FE-SEM, S-4700, Hitachi). In addition, the optical haze and transmittance of the scattering films were examined using a haze meter (NDH-5000 and SH-7000, Nippon Denshoku Industries). The refractive index values provided by material suppliers were adopted. Ray tracing simulation was performed using Light Tools software (Synopsys) and the transmittance simulation of the multilayered films was performed using Essential Macleod (Thin Film Center).
3. Results and discussion
3.1 Investigation of scattering films
We compared optical characteristics of various scattering films which were coated on GL for encapsulation including bare encapsulation as shown in Fig. 2 and Table 1.
Fig. 2 (a) Haze-wavelength and (b) parallel transmittance-wavelength characteristics of multilayered films.
Table 1. The optical characteristics of scattering films deposited on the GL for encapsulation
The haze value of both multilayered films planarized with high R.I. and low R.I. materials decreased with increasing wavelength, as shown in Fig. 2(a). In other words, the parallel transmittance values of the multilayered films increased films increased with increasing wavelength, because of the light dispersion characteristics of the film, as shown in Fig. 2(b). Very interestingly, the CAB solution with Desmodur N3300 was initially hazy, as shown in Fig. 3. Hence, the haze of the CAB film formed with the N3300 was about 8%, even though the CAB solution was spin casted without a supply of water mists. The haze of the CAB film only, without any additives, was about 0% as we previously reported. We obtained a haze value of 38% for the spin coated CAB solution without the crosslinking agent, and 85% for the solution formulated for x-NPFs. We could not adopt GLs with excessively high haze values (> 50%) because such these components can induce unacceptably high levels of pixel blurring effect. Therefore, reduction of the optical haze values of GL coated with nano-scattering films is a critical aspect which must be addressed before the proposed technology can be exploited for display applications.
Fig. 3 Photographic image of solutions for coating scattering film; The solution including crosslinker, Desmodur® N3300, shows hazier behavior than the solution without crosslinker.
We performed overcoating of the NPFs using two different types of planarizing materials. As a result, we successfully minimized the high haze of the x-NPF. In this investigation, we obtained optical haze values of 45% and 29% after planarization with the overcoating materials with high refractive index (R. I. = 1.83) and low refractive index (R. I. = 1.37), respectively (see Table 1). From this result, we established that the haze value of the final film can be controlled by the refractive index difference between the planarizing layer (or overcoating layer) and the base materials used for the NPF (in this study, R. I. = 1.48). It follows from Snell’s law in classic optics, that the change in the optical path length becomes smaller as the refractive index difference is reduced. To evaluate changes in the haze values associated with the application of the different types of overcoating materials as planarizing layers, we performed optical simulations and estimated the haze effect for various scattering conditions, as shown in Fig. 4.
Fig. 4 Ray tracing simulation of the parallel incident lights during passing through the glass; R.I. = 1.50 (a) without any scattering film, (b) with 4 μm thickness single layer scattering film; R.I. = 1.48, (c) with multilayered scattering film by coating 3 μm thickness high refractive index material; R.I. = 1.83 as planarizing layer on the scattering layer, (d) with multilayered scattering film by coating 3 μm thickness low refractive index material; R.I. = 1.37 as planarizing layer on the scattering layer. The scattering film was consisted with hexagonal close packed concave hemispherical patterns with a diameter of 1 μm.
As expected, most of the light passed through the bare GL without any refraction or reflection, as shown in Fig. 4(a). In contrast, most of the incident light was scattered during transmission through the NPF as shown in Fig. 4(b). Therefore, the application of NPF as a viewing angle suppression film for TEOLEDs is a viable consideration. However, this scattering effect was essentially diminished when the planarizing layer with an R. I. of about 1.83 was overcoated, as shown in Fig. 4(c). Furthermore, the scattering effect decreased dramatically when the planarizing layer with an R. I. of 1.37 was overcoated, as shown in Fig. 4(d). Thus, it is expected that the multi-layer scattering film with the NPF covered with a high refractive index planarizing layer should effectively suppress the viewing angle dependence, as well as the optical haze effect. Field emission scanning electron microscope (FE-SEM) images of the aforementioned various scattering films are shown in Fig. 5.
Fig. 5 FE-SEM images: (a) the front of NPF and (b) the front of x-NPF, (c) the cross section of the NPF, (d) the cross section of multilayered film by overcoating high refractive index material on the NPF, (e) the cross section of multilayered film by overcoating high refractive index material on the x-NPF, and (f) the cross section of multilayered film by overcoating low refractive index material on the x-NPF, respectively. The insets in (e) and (f) are the magnified images of the boundary between planarizing layer and x-NPF.
Very interestingly, the pore size of the x-NPF increased more than a factor of two (500 – 1500 nm) compared to that of NPF (~500 nm) as shown in Fig. 5(a) and 5(b). The increase of the pore size inside the x-NPF may be caused by a partial crosslinking reaction during the polymer solution formulation, so that the viscosity of the mixed composition of polymer solution increased. The proposed crosslinking mechanism is shown in Fig. 6.
Fig. 6 Chemical structures of (a) CAB and (b) Desmodur® N3300. (c) crosslinking reaction of CAB and Desmodur® N3300; The urethane bonding (-NHCOO-) is formed by reaction of hydroxyl groups in CAB and isocyanate groups in Desmodur® N3300.
The hydroxyl moieties inside the CAB could react with the isocyanate units in the crosslinking agent to give a urethane linkage, although the hydroxyl content inside the CAB is very low (1.0–1.6 wt. %). This led to an enhanced hazy characteristic because the large pore sizes caused a reduction in Mie scattering throughout the x-NPF. Figure 5(c) and 5(d) represent cross sectional images of both the NPF and film overcoated by a high R. I. planarizing layer. As expected, the surface morphology of pristine NPF was not maintained and melted by solvent of polymer solution with high R. I. planarizing material as shown in Fig. 5(d). In contrast, the surface morphology of the x-NPF was well maintained during overcoating with the high R. I. as well as the low R. I. planarizing materials, as shown in Fig. 5(e) and 5(f).
3.2 Investigation of optical characteristics in OLED
We fabricated green phosphorescent TEOLEDs which exhibited a strong microcavity effect (FWHM = 33 nm) to evaluate an approach for the effective suppression of viewing angle dependence as follows:
Device A: ITO (70 nm) / Ag (100 nm) / ITO (70 nm) / NPB (70 nm) / HATCN (7 nm) / NPB (85 nm) / TCTA (15 nm) / Bepp2:Ir(ppy)2(acac) (20 nm, 97:3) / BPhen (40 nm) / Liq (1 nm) / Mg:Ag (14 nm, 9:1) / NPF (60 nm) / bare GL for encapsulation (0.5 mm)
Device B: Device A / NPF (~3 μm)
Device C: Device A / x-NPF (~4 μm)
Device D: Device A / x-NPF (~4 μm) / high R.I. planarizing layer (~3 μm)
Device E: Device A / x-NPF (~4 μm) / low R.I. planarizing layer (~9 μm)
The current density-voltage-luminance (J-V-L) characteristics of the fabricated devices in this study are shown in Fig. 7(a). There was little difference in the J-V characteristics of the devices, which means that the utilized coatings did not have any electrical influence, because the scattering films were coated outside the GL, as shown in Fig. 7(a).
Fig. 7 (a) J-V-L characteristics of devices; (b) luminance-current efficiency characteristics of devices; (c) luminance-power efficiency characteristics of devices; (d) normalized angular luminance distribution of devices.
Each device exhibited different luminance characteristics with the various applied driving voltages because each film has different light scattering characteristics. At the luminance setting of 1 cd/m2, the turn-on voltages were 2.88 V, 2.89 V, 2.92 V, 2.88 V, and 2.88 V for Device A (pristine device), Device B (with NPF), Device C (with x-NPF), Device D (with high R.I. planarizing layer on the x-NPF), and Device E (with low R.I. planarizing layer on the x-NPF), respectively. As can be seen, it was difficult to distinguish between the turn-on voltage values for the various scattering films. The driving voltages required to achieve a luminance of 1000 cd/m2 were 4.19 V, 4.26 V, 4.37 V, 4.23 V, and 4.22 V for Device A, Device B, Device C, Device D, and Device E, respectively. In addition, the current efficiencies and the power efficiencies at 1000 cd/m2 were 123 cd/A and 63 lm/W for Device A, 101 cd/A and 58 lm/W for Device B, 78 cd/A and 47 lm/W for Device C, 103 cd/A and 57 lm/W for Device D, 114 cd/A and 60 lm/W for Device E, as shown in Figs. 7(b) and 7(c). From this data, we can predict that the nano-scattering film with a higher optical haze value, can decrease the total emission in the normal direction. This accounts for the fact that Device B with a pristine NPF film (haze = 38%) had a lower current efficiency (101 cd/A, lower by 17.9%) compared to the control device (Device A, 123 cd/A). The reduction in the current efficiency (by 36.6%) was much more pronounced with the deposition of x-NPFs (Device C, 78 cd/A, haze = 85%). However, we were able to limit the extent of the efficiency reduction by planarization, as we expected. For example, we were able to reduce the efficiency drop to 16.3% (Device D, 103 cd/A) when we deposited the high R. I. planarizing layer on the x-NPF. This effect was most likely due to an improved haze value (haze = 45%). We were also able to suppress the efficiency drop to 7.3% (Device E, 114 cd/A) by modifying the x-NPF with a low R. I. planarizing layer, due to its relatively low haze value (haze = 23%). The current efficiency of Device E was higher than that of Device D because the refractive index difference between the scattering and the planarizing layers was small, and the refractive index value of the outermost planarizing layer was low. These differences resulted in an improved transmittance of the film and can be explained using the Fresnel’s Eqs. which follow.
Equations (4) and (5) describe the transmittance of s-polarized light and p-polarized light between media 1 and 2, respectively. Numbers 1 and 2 are arbitrary notations for distinguishing two materials having different refractive indices. Thus, terms of media 1 and media 2 refer to the spaces of materials with different refractive indices. In the Fresnel Eqs., n1 represents the refractive index of media 1, n2 is the refractive index of media 2, and θ1 and θ2 are the propagation angles of light in media 1 and 2, respectively. Since the total transmittance is the average of the s-polarized and p-polarized transmittance, the denominator of the Fresnel Eq. is the sum of n1 and n2 perpendicular to the media plane (θ1 = 0° and θ2 = 0°). Thus n1 and n2 should be lowered to increase the overall transmittance. Using this Fresnel Eq., the transmittance of the flat multi-layered films can be easily calculated.We demonstrated that transmittance was increased by reducing the refractive index of the outermost planarizing layer, as shown in Fig. 8. Unfortunately, we encountered the difficulty that it was not possible to choose conditions with low haze values and simultaneous minimal efficiency reduction. In other words, the nano-scattering films with high hazy characteristics led to a wider angular luminance distribution as shown in Fig. 7(d), as well as a lower viewing angle dependence. To quantify this dependence, the color shift (Δu’v’, by CIE1976 color system) was calculated as follows;
where u’a and v’a represent the u’ and v’ values at the viewing angle of a degree and u’0 and v’0 represent u’ and v’ values at the viewing angle of 0°, in the perpendicular direction of the encapsulation glass surface. The color shift of the device indicates the maximum value of Δu’v’ from 0° to 60°. As shown in Fig. 9(a), the color shifts were 0.020, 0.012, 0.009, 0.014, and 0.017 for Device A, Device B, Device C, Device D, and Device E, respectively. The EL peak wavelength shifts were 13 nm, 7 nm, 5 nm, 8 nm, and 11 nm for Device A, Device B, Device C, Device D, and Device E, respectively, as shown in Fig. 9(b)-9(f).Fig. 8 Calculation of transmittance of flat multilayered film by applying planarizing layer with various R.I.; Inset image is transmittance calculation condition.
Fig. 9 (a) Color shift (Δu’v’ in CIE1976 color space) of devices with viewing angle; (b) EL wavelength shift of Device A; (c) EL wavelength shift of Device B; (d) EL wavelength shift of Device C; (e) EL wavelength shift of Device D; (f) EL wavelength shift of Device E.
As a result, Device C, with the highest haze value, exhibited the best efficiency for suppression of the viewing angle dependence. However, these films are probably not practically viable for use in the display industry because they cause a significant reduction in the luminance in the normal direction.
Therefore, we can conclude that the composition of Device D or Device E is better suited for display applications. The optical characteristics of the devices are summarized in Table 2.
Table 2. The optical characteristics summary of devices
3.3 Investigation of optical characteristics in OLED including CP
To evaluate the optical performance of the scattering films as display components, they were coated on a circular polarizer (CP) and attached onto TEOLEDs as follows:
Device A’: Device A / CP (160 μm)
Device B’: Device A / CP (160 μm) / NPF (~3 μm)
Device C’: Device A / CP (160 μm) / x-NPF (~4 μm)
Device D’: Device A / CP (160 μm) / x-NPF (~4 μm) / high R.I. planarizing layer (~3 μm)
Device E’: Device A / CP (160 μm) / x-NPF (~4 μm) / low R.I. planarizing layer (~9 μm)
The optical characteristics of the modified devices are shown in Fig. 10. Device A’ exhibited the highest luminance while Device C’ showed the lowest luminance at the same current density condition, as shown in Fig. 10(a). The current efficiencies and power efficiencies at 1000 cd/m2 were 57 cd/A and 32 lm/W for Device A’, 47 cd/A and 29 lm/W for Device B’, 36 cd/A and 24 lm/W for Device C’, 47 cd/A and 28 lm/W for Device D’, and 52 cd/A and 30 lm/W for Device E’, as shown in Figs. 10(b) and 10(c). We found that the transmittance of the CP was 47%, so that all current efficiencies at 1000 cd/m2 decreased by 53% relative to the devices which excluded the CP. The largest and the smallest reduction in the current efficiency and power efficiency were recorded in Device C’ (37% and 25%, respectively) and in Device E’ (8% and 5%, respectively). These results are very similar to those obtained from previous tests without the CP. In addition, the angular luminance distribution characteristics of the devices with the CP, changed slightly from the previous results obtained without the CP, as shown in Fig. 10(d). Thus, the influence of the CP on the scattering film was small, and only the luminance was affected due to the intrinsic transmittance characteristics of the CP.
Fig. 10 (a) J-L characteristics of devices; (b) luminance-current efficiency characteristics of devices; (c) luminance-power efficiency characteristics of devices; (d) normalized angular luminance distribution.
We also investigated the color shift with viewing angle of the devices with the CP, to investigate the influence on EL wavelength mixing by the scattering films. The results are shown in Fig. 11. The color shifts and EL peak wavelength shifts of devices including CP were 0.021 and 13 nm for Device A’, 0.013 and 8 nm for Device B’, 0.009 and 5 nm for Device C’, 0.013 and 8 nm for Device D’, and 0.018 and 11 nm for Device E’, respectively.
Fig. 11 (a) Color shift (Δu’v’ in CIE1976 color space) of devices having CP with viewing angle; (b) EL wavelength shift of Device A’; (c) EL wavelength shift of Device B’; (d) EL wavelength shift of Device C’; (e) EL wavelength shift of Device D’; (f) EL wavelength shift of Device E’.
As these results, we found that the CP had little effect on viewing angle characteristics because differences in color shift and EL peak wavelength shift depending on presence or absence of CP were under 0.001 and 1 nm, respectively. The devices including CP were summarized in Table 3.
Table 3. The optical characteristics summary of devices including CP
3.4 Investigation of visibility of pixels for display application
We evaluated the pixel blur level associated with legibility by applying various scattering films to determine their applicability for use in displays. This is important because display panel makers don’t apply conventional diffuser films or light extraction films such as MLAs because they can induce significant pixel blur. To quantitatively analyze the pixel blur level, we captured light-emitting pixel images using a charge coupled device (CCD) camera and extracted intensity information from the acquired data. To determine the pixel blur level of the multilayered scattering films, additional devices were assembled using MLAs with hemispherical patterns (diameter: 80 μm). The device was named Device F’.
Device F’: Device A/CP (160 μm)/MLAs (150 μm)
The difference in the brightness between the center and periphery of the pixels is shown in Fig. 12. These figures can be used to compare the blur of a pixel. Pixel blurring was the most serious for Device F’ as we observed from visual inspection through photographic images in Fig. 12(a).
Fig. 12 (a) Electroluminescent pixel photographic image of various devices; (b) brightness profile conversion from photographic image; (c) normalized brightness profiles from pixel edge position. The TEOLED pixel size was 4 mm2 (2 mm x2 mm).
In particular, apart from exhibiting blurred pixels, Device F’ also demonstrated a certain pattern (e.g. hemispherical patterns in MLA) because MLAs with large scattering patterns (diameter: 80 μm) were utilized. In contrast, the pixels of Device A’, Device B’, and Device E’ could not be easily identified by visual inspection. Thus, we extracted intensity information from the digital images as shown in Fig. 12(b), which was then normalized based on the intensity of the pixel edge as shown in Fig. 12(c). To further quantify the pixel blur level, the distance which corresponded to one-tenth of the brightness of the pixel edge was measured and referred to as the pixel blur distance. For devices with pixel area of 4 mm2, this distance was 167 μm, 325 μm, 566 μm, 391 μm, 207 μm, and 616 μm nm for Device A’, Device B’, Device C’, Device D’, Device E’, and Device F’, respectively. From these results, we found that the multilayered scattering films could effectively suppress pixel blur in comparison with conventional MLAs. Moreover, we determined that a lower haze scattering film could suppress pixel blur. Finally, we investigated the black color characteristics of various scattering films as shown in Fig. 13, because the dark black color under non-driving conditions was changed to faded black by coating the NPFs on CP, in our previous paper [26].
Fig. 13 The black color (turn off condition) photographical images of Device A’, Device B’, Device C’, Device D’, Device E’, and Device F’.
Since deterioration of the black level can result from an increase in the diffusing reflectance caused by the uneven surface morphology of the NPFs, we coated an additional planarizing layer on the NPFs to reduce the diffusing reflectance. As a result, the black level of the devices was recovered (see black level of Device D’ and Device E’). So, Device D’ and Device E’ can be much darker than the devices without a planarizing layer (e.g. Device B’, Device C’ and Device F’). In particular, Device E’, which had a small difference in the refractive index between the planarizing layer and NPF, exhibited the darkest black property among the devices with different scattering films.
4. Conclusions
We successfully suppressed the viewing angle dependence of TEOLEDs via the application of NPFs and x-NPFs based scattering films. However, they caused significantly hazy optical behavior, which led to a substantial level of pixel blur, as well as a faded black tint response. Thus, we fabricated x-NPFs based on multi-layered scattering films covered with two types of planarizing layers with different refractive indices. As a result, we found that a large difference in the refractive index between the scattering layer and the planarizing layer helps to maintain the suppression of the viewing angle dependence. It also produced a moderate level of black tint as well as a surface hazy behavior. In contrast, a small difference in the refractive index between the two layers did not improve the viewing angle dependence, and produced only minimal black tint as well as a surface hazy behavior. It was determined that the viewing angle dependence, the black tint, and the pixel blur phenomena are in a trade-off relationship with each other. We found that these phenomena can be controlled by manipulating the refractive index difference between the scattering layer and the planarizing layer.
Funding
Ministry of Trade, Industry and Energy (MOTIE) (10051438); Korea Display Research Corporation (KDRC).
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