Simple method for fabricating scattering layer using random nanoscale rods for improving optical properties of organic light-emitting diodes

www.nature.com/scientificreports OPEN Received: 19 June 2018 Accepted: 5 September 2018 Published: xx xx xxxx Simple method for fabricating scattering layer using random nanoscale rods for improving optical properties of organic lightemitting diodes Jin Ho Kwack1,2, Junhee Choi1, Cheol Hwee Park1, Ha Hwang1, Young Wook Park3 & Byeong-Kwon Ju1 We investigated a low-temperature mask-free process for preparing random nanoscale rods (RNRs) as a scattering layer. The process involves spin coating and dry etching, which are already widely applied in industry. Our film exhibited 17–33% optical haze at 520 nm wavelength and 95% total transmittance in the visible range. Therefore, this film can be used as a scattering layer for improving viewing angle characteristics and decreasing substrate mode loss in organic light-emitting diodes (OLEDs). Specifically, we focussed on varying the height and density of the RNRs to control the optical characteristics. As a result, the OLEDs with RNRs revealed a variation in colour coordinates of Δ(x, y) = (0.007, 0.014) for a change in the viewing angle, which was superior to those without the RNRs that displayed a variation of Δ(x, y) = (0.020, 0.034) in CIE 1931. Moreover, the OLEDs with RNRs exhibited 31% enhanced external quantum efficiency compared to those of the OLEDs with the bare substrate. The flexibility of the polymer used for the RNRs and the plasma treatment suggests that the RNRs can be applied to flexible OLED displays and lighting systems. Organic light-emitting diodes (OLEDs) are being widely applied in displays for mobiles and televisions owing to their self-emitting characteristics, excellent colour gamut, high-speed operation, and applicability in flexible or stretchable devices1. Practical OLEDs function based on phosphorescence owing to their nearly 100% internal phosphorescence efficiency2,3. However, the external quantum efficiency of OLEDs is ~20% because the surface plasmon polariton modes at the metal-organic interface, the waveguide mode in the indium tin oxide (ITO)/organic layer, and the substrate mode in the glass substrate produce light loss4–8. Generally, light loss in the substrate mode is larger than that in the waveguide mode due to small differences in refractive index between the ITO/glass interface and the glass/air interface9. In addition, microcavity top-emitting organic light-emitting diodes (TEOLED) without the loss of the substrate mode and having the advantage of improved colour purity and aperture ratio have been applied to mobile displays. However, microcavity TEOLED originally had a problem associated with viewing angles due to the blue shift that occurred with the variation of the sight angle, even though the light efficiency along the normal direction was amplified. Therefore, researchers have attempted to find solutions for improving the light extraction and viewing angle of OLEDs10. Different approaches have been adopted with the aim of optimizing light extraction and viewing angle, including micro lens array (MLA)5,11–16, dielectric Bragg gratings17–19, surface plasmons20, buckling patterns21, periodic corrugation8, low-index grids22, and photonic crystals23. Methods that modify the internal or external surfaces of the substrate of OLEDs minimize the total internal reflection. Such surface shapes are produced by different technologies which traditionally involve photo-lithography or printing, moulding, and embossing methods24–28. 1 Display and Nanosystem Laboratory, School of Electrical Engineering, Korea University Seoul, Seoul, 136-713, Republic of Korea. 2Samsung Display Co., Samsung St. 181, Tangjeong-Myeon, Asan-City, Chungcheongnam-do, 31454, Republic of Korea. 3School of Mechanical and ICT Convergence Engineering, SUN MOON University, Asan-City, Chungcheongnam-do, 31460, Republic of Korea. Correspondence and requests for materials should be addressed to Y.W.P. (email: ) or B.-K.J. (email: ) Scientific RePortS | (2018) 8:14311 | DOI:10.1038/s41598-018-32538-4 1 www.nature.com/scientificreports/ Figure 1. FDTD simulation of E-field distribution induced by transverse magnetic and electric dipoles: (a) Reference (w/o RNRs), (b) unpatterned SU-8 on a glass structure, and (c) RNRs on the glass structure. Therefore, these technologies employ at least the photo-lithographic step or utilize lithographic templates once. They generally end up being considerably complex or expensive for mass production. On the other hand, other researchers have reported a simple scattering layer synthesized by competitive and scalable methods. In these methods, particles or voids having different refractive indices were embedded in a polymer matrix and utilized for the scattering layer29–36. However, because the particles or voids are micro-sized or the pitch between the nearest particles or voids is micro-sized, these techniques reveal a small effect of diffraction that is difficult to control. In addition, to prevent the loss of the substrate mode, the most representative MLA has larger visibility than a nano-sized array because of its size. Alignment is a problem in the case of high-resolution displays with very small pixels. In this paper, we propose a simple method to fabricate random nanoscale rods (RNRs) as scattering layers for enhancement of light extraction and improvement of viewing angle characteristics in OLEDs. In our device, there is virtually no spectral distortion or shift, which are issues observed in 2D photonic crystal-based OLEDs, and the spectral shift according to the change in viewing angle is reduced. This layer is fabricated by a cost-effective and scalable procedure that consists of coating and etching the polymer at low temperatures (below 100 °C) without photomasks or templates. Additionally, the RNRs can control the diffraction as well as scattering effects in the visible wavelength range owing to the distance between closest rods and are compatible with high-resolution displays for virtual reality because the process of alignment between the individual pixels in the panel and the scattering layer is unnecessary. The RNRs can also be applied in flexible displays and lighting owing to the use of etched polymers. OLEDs equipped with RNRs exhibit superior optical properties such as high external quantum efficiency (EQE), high luminance efficiency (LE), and colour variation with changes in viewing angle, to control OLEDs. Results and Discussion To investigate the optical effect of RNRs on glass, the electrical field distribution was simulated by the finite-difference time domain (FDTD) method. The boundary conditions for the simulation were set for a perfect optically matched layer to avoid the reflection of electromagnetic waves at the edges of the structure on all the sides, except for the metal cathode layer. The simulated structure consisted of an aluminium cathode, N,N’-bis(naphthalen-1-yl)-N,N’-bis(phenyl)-benzidine (NPB; refractive index n = 1.81), tris(8-hydroxy-quino (...truncated)