Improvement of the outcoupling efficiency of an organic light-emitting device by attaching microstructured films Hoang-Yan Lin a , Jiun-Haw Lee a , Mao-Kuo Wei b, * , Ching-Liang Dai c , Chia-Fang Wu a , Yu-Hsuan Ho a , Hung-Yi Lin d , Tung-Chuan Wu d a Graduate Institute of Electro-Optical Engineering and Department of Electrical Engineering, National Taiwan University, Taipei 106, Taiwan, ROC b Institute of Opto-Electronic Engineering and Department of Materials Science and Engineering, National Dong Hwa University, Hualien 974, Taiwan, ROC c Department of Mechanical Engineering, National Chung Hsing University, Taichung 402, Taiwan, ROC d Mechanical and Systems Research Laboratories, Industrial Technology Research Institute, Hsinchu 310, Taiwan, ROC Received 19 October 2006; received in revised form 26 February 2007; accepted 16 March 2007 Abstract In this paper, we present and analyse the optical characteristics, such as spectral shift, CIE coordinates, viewing angle dependence, luminous current efficiency and luminous power efficiency, of an organic light-emitting device (OLED) with a commercial diffuser film or a brightness-enhancement film (BEF) attached. Compared to a planar green OLED, the luminous current efficiencies of the OLED with an attached diffuser film or BEF increase by 29% and 23%, respectively. The overall luminous power efficiencies are enhanced by 28% and 7%. Compared to the planar green device, we observe blue shifts at different viewing angles when microstructured films are attached, which is the evidence that the waveguiding modes are being extracted. In our planar OLED, the peak wavelength blue shifts and the full width at the half maximum (FWHM) decrease with increasing viewing angles due to the microcavity effect. When the diffuser is attached, the spectral peak has a constant blue shift (6 nm) compared to that of the planar OLED. On the other hand, in the BEF case, the spectral shift depends on the viewing angle (2–12 nm blue shifts from 0 to 80°). This is due to the different operating principles (scat- tering and redirected light) of the diffuser and BEF. Since the transmittance spectra of both the diffuser film and the BEF are flat over the visible range, it is suitable for lighting applications by using white OLED. When attaching the films on a commercial white OLED, the luminous current efficiencies of the OLED with an attached diffuser film or BEF increase by 34% and 31%, respectively. The overall luminous power efficiencies are enhanced by 42% and 8%. Ó 2007 Elsevier B.V. All rights reserved. PACS: 42.79.Bh; 85.60.Jb; 85.85.+j Keywords: Luminous current efficiency; Luminous power efficiency; Organic light-emitting device; Diffuser film; Brightness-enhancement film 1. Introduction Organic light-emitting devices (OLEDs) are one of the most promising technologies for display and lighting appli- cations due to their advantages of low power consumption, large viewing angle, high contrast ratio, fast response and flexible-substrate capability [1,2]. However, the outcou- pling efficiency of a conventional OLED is typically less than 30%, which means that most of the light is trapped in the glass substrate [3]. To improve the OLED’s outcou- pling efficiency, many techniques based on eliminating the waveguiding phenomena between the glass substrate and the air have been studied [4–10]. For example, in 2000, Yamasaki et al. fabricated an OLED consisting of silica microspheres to couple out the waveguide mode [4]. Shiang 0030-4018/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2007.03.028 * Corresponding author. Tel.: +886 3 8634221; fax: +886 3 8634200. E-mail address: mkwei@mail.ndhu.edu.tw (M.-K. Wei). www.elsevier.com/locate/optcom Optics Communications 275 (2007) 464–469