A Light Scattering Layer for Internal Light Extraction of Organic Light-Emitting Diodes Based on Silver Nanowires

A Light Scattering Layer for Internal Light Extraction of Organic Light-Emitting Diodes Based on Silver Nanowires

Keunsoo Lee,

Jin-Wook Shin,

Jun-Hwan Park,

Jonghee Lee,

Chul Woong Joo,

Jeong-Ik Lee,

Doo-Hee Cho,

Jong Tae Lim,

Min-Cheol Oh,

Byeong-Kwon Ju,*

and Jaehyun Moon*

†Soft I/O Interface Research Section, Electronics and Telecommunications Research Institute, Daejeon 34129, Republic of Korea

‡Display and Nanosystem Laboratory, College of Engineering, Korea University, Seoul 136-713, Republic of Korea

§Research Institute of Electrical Communication, Tohoku University, Sendai, Miyagi 980-8577, Japan

∥School of Electrical Engineering, Pusan National University, Pusan (Busan) 609-735, Republic of Korea

ABSTRACT: We propose and fabricate a random light scattering layer for light extraction in organic light-emitting diodes (OLEDs) with silver nanodots, which were obtained by melting silver nanowires. The OLED with the light scattering layer as an internal light extraction structure was enhanced by 49.1% for the integrated external quantum efficiency (EQE). When a wrinkle structure is simultaneously used for an external light extraction structure, the total enhancement of the integrated EQE was 65.3%. The EQE is maximized to 65.3% at a current level of 2.0 mA/cm². By applying an internal light scattering layer and wrinkle structure to an OLED, the variance in the emission spectra was negligible over a broad viewing angle. Power mode analyses withfinite difference time domain (FDTD) simulations revealed that the use of a scattering layer effectively reduced the waveguiding mode while introducing non-negligible absorption. Our method offers an effective yet simple approach to achieve both efficiency enhancement and spectral stability for a wide range of OLED applications.

KEYWORDS: organic light-emitting diodes, light extraction, silver nanowires, wrinkle structures,finite difference time domain

1. INTRODUCTION

Organic light-emitting diodes (OLEDs) can provide a human- friendly spectrum and wide color gamut. For this reason, OLEDs have been widely used in displays and lighting equipment. Because of advancements in organic materials, device stack design, and electrode optimization, the internal quantum efficiencies of OLEDs have reached nearly 100%.

Recently, the external quantum efficiency (EQE) of a conventional bottom emission type OLED has been reported to reach as high as 30%.¹⁻³However, because of the presence of various intrinsic light confinements, the EQE of conventional OLEDs still has low values.⁴⁻⁶ In practical terms, low efficiencies are detrimental to the lifetime and energy consumption of the device. Various methods have been suggested to improve the efficiencies of OLEDs. Broadly, these methods can be classified as external and internal light extraction methods.⁷⁻¹⁰Other methods, such as internal cavity design,¹¹⁻¹³molecular orientation, and application of low SPP electrodes,¹⁴⁻¹⁶ have also been suggested. External light extraction methods can extract the confined light at the

glass/air interface. However, the light in an organic layer/high refractive index electrode is still confined. Thus, to fully out- couple the confined light, it is technically necessary to develop internal light extraction methods. To this end, we focused on technologically accessible methods to form internal light extraction structures. Existing methods include photolithog- raphy patterning and vacuum depositions, which may have complex processes and high costs.

Recently, as a method to extract internally confined light, we developed a light scattering layer using a fabrication method based on dewetting of thin metalfilms.¹⁷Although the internal light extraction structure formed by the aforementioned method can offer good light extraction and spectral stability, the process itself requires a high vacuum-based process and high-precision thickness control of the metal films. In this paper, for a low cost and simple process, we propose the

Received: March 12, 2016 Accepted: June 17, 2016 Published: June 17, 2016

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fabrication of a light scattering layer fabricated with silver nanowire (AgNW). AgNWs have been widely investigated as an alternative transparent electrode dominantly used with indium tin oxide (ITO).¹⁸⁻²² Unlike the deposition of thin metalfilms by vacuum-based thermal evaporation, AgNWs can be readily coated onto a substrate by various low-cost processes.²³⁻²⁶ Here, we used AgNWs to form a hard mask to fabricate an internal light scattering layer. Because no vacuum deposition process is required in the deposition of the AgNWs, the process time and cost are greatly reduced. We thermally melted the AgNWs to form spatially discrete Ag nanodots, which act as a hard mask. The exposed area between the Ag nanodots was etched away to fabricate an internal scattering layer. On this scattering layer, we fabricated OLEDs and investigated the light extraction of the OLEDs. In addition to the extraction of the internally confined light, we used spontaneously formed organic wrinkles to extract the light confined at the glass/air interface. In this work, we present the process for using AgNWs as a starting material and the

performance of OLEDs equipped with an internal light scattering layer fabricated with the AgNWs.

2. EXPERIMENT SECTION

Figure 1 shows the fabrication process of an OLED device with a scattering layer as an internal light extraction structure. A SiO_xlayer of 400 nm was deposited onto the glass substrate by plasma-enhanced chemical vapor deposition. Silver nanowires (Cambrios Technologies Corporation) were spin-coated onto the SiO_xlayer. The diameter and length of the AgNWs are 40 nm and 30μm, respectively. To form the etching mask, the spin-coated AgNWsfilm was annealed at 400 °C for 2 h in air atmosphere. During the annealing process, AgNWs undergo a melting process producing Ag nanodots on the SiO_xlayer. Various studies and analyses of the melting behavior of thin metal nanowires have been done.²⁷⁻³⁰The diameters of the Ag nanodots varied from 200 to 600 nm, which covers most of the visible light wavelength range. The exposed SiO_x layer was etched with induced coupled plasma reactive ion etching in CF₄ and Ar atmosphere. After the etching, the Ag nanodots were removed with a diluted nitric acid solution. This process yields a light scattering layer on the glass substrate. A planarization layer is coated onto the light scattering layer.

Figure 1.Processingflow of substrate equipped with an internal light extraction layer.

Figure 2.SEM images. (a) Ag NW spin-coated on SiO_xlayer. (b) Ag nanodots formed by Ag NW melting. (c) Internal scattering SiO_xlayer (45° tilt view). (d) Internal scattering SiO_xlayer (cross sectional view). (e) Internal scattering layer with planarization layer. (f) Wrinkles for external light extraction.

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From the perspective of device operation, it is important to electrically stabilize the device. Rough surfaces are prone to induce electrical failure. Additionally, to facilitate light traveling, it is desirable to have a planarization layer with a refractive index comparable to that of a transparent anode. If the refractive index of the planarization layer is much lower than that of the anode, the light emitted from the organic emission layer could be confined in the anode/organic layer. An ITO was used that is used widely as an anode for OLEDs. The refractive index of the ITO is∼1.9. To planarize the scattering layer, we used an ultraviolet (UV) curable resin that contains dispersed zirconia nanocrystals (purchased from Pixelligent). The cured film has a refractive index of 1.81 at a wavelength of 550 nm. The resin was spin coated onto the light scattering layer and subsequently dried briefly in a nitrogen atmosphere. After that, the film was exposed to UV to finalize the planarization layer fabrication process.

The OLED device has the following stack sequence. Our OLED had the following stack structure: ITO (150 nm)/1,4,5,8,9,11- hexaazatriphenylene-hexacarbonitrile [HAT-CN] (10 nm)/4,4′- cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] [TAPC]

(45 nm)/ HAT-CN (10 nm)/TAPC (45 nm)/HAT-CN (10 nm)/

TAPC (45 nm)/2,6-bis-[3-(carbazol-9-yl)phenyl]pyridine [DCzPPy]

doped with 7% of fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] (20 nm)/1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene [BmPyPB] (60 nm)/

LiF (1 nm)/Al (120 nm). All organics were deposited with a thermal evaporation method. To achieve a high hole injection rate and electrical stability, we used a hole transport layer (HTL), in which the HAT-CN and TAPC are alternated. The HAT-CN is a high mobility strong n-type organic semiconductor, which can extract electrons from adjacent organics. Alternately deposited HAT-CN extracts the electrons from highest occupied molecular orbital level to lowest occupied molecular orbital level, which induces hole generation effect.

Thus, the devices can be operated at low voltage with electrical stability. The OLED characteristics with HTL have been reported elsewhere.^31,32To protect the organics from atmospheric degradation, the fabricated OLEDs were glass encapsulated in a glovebox. The emitting area of the devices was 70 mm² (10 mm× 7 mm). To maximize the efficiency, we used a wrinkle structure that has a role in an external light extraction structure of OLED. The wrinkle structure was fabricated on an adhesive plasticfilm and attached to the glass surface of a bottom emission OLED. Additional information and details on the fabrication process of the wrinkle structure is available in our previous report.³³ The current density (J)−voltage (V) and voltage (V)−luminance (L) characteristics of the devices were measured using a current/voltage source/measure unit (Keithley 238) and a spectro-radiometer (CS-2000, Minolta), respectively. Their angular spectra and luminance distributions were measured with a spectro-radiometer (Minolta CS-2000) and a goniometer-equipped sample stage. The efficiencies were measured with an integrating sphere (HM series, Dae Sung Hi-Tech).

3. RESULTS AND DISCUSSION

Figure 2a,b shows the SEM images of the spin-coated AgNW on a glass substrate and Ag nanodots produced by the melting process, respectively. The reported melting point of bulk Ag is 962°C. As the size decreases to nanometric scale, the melting point drops. This is due to the contribution of the surface atoms that are weakly bonded compared to those in the bulk part. In the case of the materials having a large surface-to- volume ratio such as nanowires, the melting point is lowered dramatically with a size decrease.³⁴The size of the Ag nanodots shown in Figure 2b is on the scale of several hundred nanometers, which is due to the agglomeration of the melted liquid Ag NW parts. The Ag nanodots are located randomly on the SiO_x layer, which is desirable for light scattering in the terms of the wavelength dependence,³⁵ and their sizes are similar to the wavelength of visible light.Figures 2c,d shows the internal scattering SiO_x layer, which is obtained after dry etching and stripping of the Ag nanodots. The height and

diameter of the scattering layer is 400 nm and several hundred nanometers (∼200−600 nm), respectively. According to our previous study, the higher structure yields better light extraction effect.³⁶The height of the structure is controllable by varying the etching conditions. However, from processing viewpoint, it is difficult to planarize structures with high aspect ratio. Failure of planarization results in electrical failure or low device reproducibility. In this work, we chose a specific height that yields stable device characteristics. The geometric sizes of the scattering structurefit within the size range for the internal light scattering structure. Figure 2e shows the planarized light scattering layer. Planarization is performed with a UV curable resin. Figure 2e shows the cross-sectional SEM image of the scattering layer planarized with the resin. The thickness of the planarization layer between the top of the scattering layer and the surface of the planarization film is ∼150 nm. Figure 2f shows the wrinkle structure that is used as an external light extraction structure to maximize the device efficiency. More information on the wrinkle was reported in detail in our previous study.³³Briefly, wrinkles were formed by a UV cross- linking liquid prepolymer. To evaluate the light extraction capacity of the light scattering layer and wrinkle, we fabricated three types of devices: First, a planar OLED was fabricated as a reference device; Device A contained an internal light scattering layer planarized with a UV curable resin, and Device B was the same as Device A except for an additional wrinkle structure for external light extraction.

Figure 3shows the (a) J−V and (b) V−L characteristics of the devices. All the J values increase as the applied voltage

increases. The rates gradually decrease, which is typical electrical behavior of OLEDs. Ideally, because the light extraction structure is not an electrical component, the J−V characteristic of the devices should superimpose on each other.

However, the current density of the devices with the light extraction structure is relatively higher than that of the planar device. The planarization layer is not completelyflat but slightly wavy, (Figure 2e) which causes an increase in the surface area and partially thins the organic layer. The increased surface area and partially thinned organic layer are thought to have induced the cause of the higher current density compared to that of the planar device. The V−L characteristic shows a luminance change as a function of the applied voltage. For a given voltage, the L of the OLEDs equipped with light extraction structures show a higher L level than that of the planar OLED.

Figure 4shows the luminance distribution of the devices as a function of the viewing angle at a constant current density level of 2.0 mA/cm². The luminance distributions of OLEDs are dependent on the thickness of organics.³⁷ In this work, to ensure stable device operation, we used OLED devices with thick HTL (165 nm). The distribution is not Lambertian but Figure 3.(a) The J−V characteristics. (b) The L−V characteristics.

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has stronger emission in the high angle range. In the planar device, the luminance in the high angle side is higher than that of the normal direction. Additionally, the luminance distribu- tions of Devices A and B also have higher luminance in the side directions than those of the normal directions. However, as the viewing angle increases, the luminance ratio of the normal direction to the side direction increases gradually. The luminance ratio of the normal direction (988 cd/m²) to an angle of 60° (1518 cd/m²) is 65%. The luminance ratios of Device A and B are 89 and 89.4%, respectively. These results reveal that the scattering layer effectively scatters light uniformly over the whole angle range (0° to ∼70°).

Considering that normal direction luminance increases but still has a lower luminance value than that of the side direction, the scattered light is not concentrated in the normal direction.

The technical implication here is that, by using our structure, it is possible to modify the luminance distribution to be uniform or Lambertian-like. We used a wrinkle structure as the external light extraction structure.³³ The refractive index of the cured wrinkle matches that of a glass substrate at 1.5. Previously, we have shown that our wrinkle is an effective structure for extracting the light confined at the glass/air interface.³³ The OLED (Device B) equipped with an internal scattering layer and external wrinkles had not only the highest luminance level but also the most uniform luminance distribution.

Figure 5shows the EQE and power efficiency of the devices.

We measured these values with an integrating sphere. The

measurements were under a constant current density level of 2.0 mA/cm². In the case of the planar device, the measured EQE was 22.2%. The measured EQEs of Devices A and B were 33.1 and 36.7%, respectively, corresponding to enhancements of 49.1 and 65.3%. The power efficiency of the planar device was 46.5 lm/W. The power efficiency of Devices A and B was 73.4 and 80.9 lm/W, respectively, corresponding to enhance- ments of 57.8 and 74%. Device A had an enhancement of 33.1% and 57.8% for the EQE and power efficiency, respectively. For the planar device, roughly 35% of the generated light is confined at the ITO (∼1.9)/organic layer (∼1.78) interface for a second-order cavity.⁶ Because the refractive index of the planarization layer is comparable to that

of ITO, light can travel to the light scattering layer and then out coupled to the substrate. The waveguided light randomly scatters at the interfaces of the scattering/planarization layers resulting in an enhanced luminance. Because of the randomness in the distribution of the scattering components, the scattered light is not concentrated in a specific direction but rather uniform in all directions (Figure 4). Device B had an enhancement of 65.3% and 74% for the integrated EQE and power efficiency, respectively. The wrinkle has a role in extracting light confined in a glass substrate. Unlike the mechanism of the internal light scattering layer, the wrinkle extracts the light by changing the light path on the boundary between the glass and air. Details on wrinkles have been reported elsewhere.³³

Figure 6 shows the integrated EQEs and PEs versus luminance. The OLEDs equipped with light extraction

structures had higher efficiencies in the whole luminance range. In all cases, the EQEs and PEs decrease as the luminance increases. In the planar OLED case, the rate of decrease is higher than those observed in Devices A and B. The decrease in efficiencies, which is commonly referred as roll-off, can be attributed to resistive losses and various annihilation processes taking place in the light emitting layer.³⁸ Because we used identical organic stacks in all devices, the difference in electrical characteristics (Figure 3a) has a role in the difference in the rate. The results in Figure 6 show that our light extraction structures are applicable to a wide range of luminance levels.

Figure 7a−c shows the normalized electroluminescence (EL) spectra of the OLEDs as a function of the viewing angle. To be Figure 4.Luminance distributions of OLEDs.

Figure 5.Efficiencies. (a) External quantum efficiencies (%) and their enhancements. (b) Power efficiencies (lm/W) and their enhancement.

Figure 6.Efficiencies as a function of current density. (a) External quantum efficiencies. (b) Power efficiencies (lm/W).

Figure 7.EL spectra as a function of viewing angle. (a) Planar OLED (no light extraction). (b) Device A (internal light extraction equipped). (c) Device B (internal and external light extractions equipped). And (d) The CIE coordinates of Planar, Device A, and Device B OLEDs.

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used as a luminescent device, the angular spectral distortion must be minimized. The planar device shows a considerably distorted angular spectrum, and the variance of the full width at half-maximum (fwhm) is 20 nm. The main peak obtained at a viewing angle of 60° is shifted by 32 nm to a longer wavelength.

The spectrum distortion problem arises due to the intrinsic microcavity effect of the OLEDs. Especially when a strong microcavity effect is present in the device, severe spectrum distortion is observed.³⁹⁻⁴¹The variance of the fwhm for both Device A and B is 5 nm. By applying the scattering layer and wrinkle, the variance of the fwhm can be remarkably reduced.

The main peak shifts of Devices A and B are almost negligible.

The internal scattering layer stabilizes the main peak shift

dramatically by scattering the light uniformly over the whole angle range. The wrinkle also contributes to stabilize the main peak shift. Particularly, by using an internal scattering layer and wrinkle, it was possible to keep the main peak position unchanged. Our results show that the structures are very useful in preserving the original EL spectrum without distortion.

Figure 7d shows the 1931 Commission internationale de l′éclairage (CIE) color coordinates, which were extracted from the EL spectra. The standard deviations of the x and y coordinates are 0.022 and 0.021, 0.005 and 0.003, and 0.005 and 0.003 for the planar device and Devices A and B, respectively. The large value of the coordinate deviation of the planar device is a result of a weak microcavity effect. Because of Figure 8.OLED device structures used in FDTD simulation (a) without internal scattering layer and (b) with internal scattering layer. E²of OLED (c) without internal scattering layer and (d) with internal scattering layer.

Figure 9.(a) The EQE and enhancement ratio calculated as a function of planarization layer thickness. The power fraction of each mode as a function of ETL thickness (b) without internal scattering layer and (c) with internal scattering layer.

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the uniform scattering of light, the standard deviation is greatly reduced in the case of Devices A and B.

To elucidate the light out coupling effect of the scattering layer,finite difference time domain (FDTD) simulations were conducted and then compared with experimental results. The OLED device structures used in the FDTD simulation are shown in Figure 8a,b. To build the relevant OLED structure, shown inFigure 8b, nanopillars with a height of 380−420 nm and a radius of 140−260 nm were randomly distributed.

Depending on the position of the source relative to the scattering layer, different results could be obtained. This effect was minimized with a dipole source array and a random initial phase on each point. Our OLED light source is an array of dipoles. On the light source plane xy, we generated 8× 8 or 64 positions. On each position we located three dipoles. The initial phase of each dipole was set randomly. This feature was to mimic the low coherence characteristics of OLED light. The technical details of constructing random scattering layers and light sources are described elsewhere.⁴²To effectively perform the FDTD simulations, we set the spatial grid as 20 nm in each direction and took a calculation volume of 6.0× 6.0 × 5.6 μm³. Mirror boundary conditions and perfect matched layers were set along the x, z directions and y direction. The wavelength used in simulation was 515 nm with a bandwidth of 70 nm.

Figure 8c,d shows the internal electricfield (E²) distributions of the OLEDs. Compared to the E² of the planar OLED case (Figure 9c), the E² of the OLED with the scattering layer (Figure 8d) clearly propagates in an extended manner with a much stronger intensity. This is interpreted as the light out coupling of the trapped light in the ITO−organic layer due to the scattering layer.

As can be seen inFigure 2e, the surface of the planarization layer is slightly wavy. Thus, there is a need to extract an effective planarization layer thickness, tp, which yields an enhancement ratio in agreement with the experimental result shown inFigure 5a. As depicted inFigure 8b, the t_pis defined as the spacing between the top of the SiO_xnanopillar and the terminal of the planarization layer. To obtain the t_p, the EQEs and their enhancement ratios were calculated as a function of the planarization layer thickness shown in Figure 9a. To estimate the effect of thickness of planarization layer on the device efficiency, we simulated the device efficiency as the thickness of planarization layer changes. Because of the change in the microcavity length, the enhancement ratio slightly oscillates, as the t_p changes. At t_p = 140 nm, and an enhancement ratio of 1.5 was obtained. Referring to Figure 5a, this value is very close to the enhancement obtained in the actual OLED device. We used t_p= 140 nm to construct mode power fraction plots of the OLEDs with and without the scattering layer. In the simulation, a maximum EQE enhance- ment ratio of 1.55 times was observed at t_p= 100 nm.Figure 9b,c shows the power fraction of each mode as a function of the electron transport layer (ETL) thickness. Significant change was observed in the wave-guided mode by the introduction of the scattering layer. The loss due to the wave-guided mode decreased over the entire range of the ETL thickness considered. The wave-guided mode decreases from 14% to 4% in thickness of ETL 60 nm. In both results, there was no evident change in the fraction of the surface plasmon resonance (SPR) mode and substrate mode. Additionally, presumably due to the presence of the planarization layer, non-negligible absorption occurs over the entire range of the ETL thickness.

At an ETL thickness of 60 nm, the out coupled light fractions

of the planar and scattering layer equipped OLEDs were 18.80% and 28.54%, respectively. The enhancement ratio was calculated to be 1.52, which is close to that of the experimental result of 1.49 times. The simulations show that our scattering layer can effectively extract the wave-guided mode and enhance the EQE of OLEDs.

4. SUMMARY

In summary, we fabricated an internal light scattering structure that enhances the efficiency and stabilizes the emission spectrum. Out scattering layer was fabricated with silver nanodots, which were obtained by melting silver nanowires.

The light scattering layer equipped with a high refractive index planarization layer remarkably enhanced the device efficiency of OLEDs. The integrated maximum EQE and power efficiency of the device with the scattering layer and wrinkle were 36.7% and 80.9 lm/W, respectively, corresponding to enhancements of 65.3 and 74% compared with the planar device. The internal scattering layer not only enhances the efficiency but also stabilizes the EL spectrum. On the one hand, the planar device has a considerably distorted EL spectrum. On the other hand, the device with the light scattering layer and wrinkle has reduced variance of the fwhm and no main shift as the viewing angle changes. To verify the light extraction capacity of the light scattering layer, an optical simulation was also done with FDTD, and the simulation result showed a similar enhance- ment as the experimental result. The proposed light scattering structure in this paper is not limited to OLEDs. It can be readily applied to various photonics devices to improve their efficiency and stabilize the spectrum.

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*E-mail:bkju@korea.ac.kr. (B.K.J.)

*E-mail:jmoon@etri.re.kr. (J.M.) Notes

The authors declare no competingfinancial interest.

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This work was supported by“Technology Development of Low cost Flexible Lighting Surface”, which is a part of the R&D program of Electronics and Telecommunications Research Institute and by the National Research Foundation grant (2014R1A2A1A10051994) funded by the Korean government.

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