Journal of Luminescence 128 (2008) 525–530 Implementation of anti-reﬂection coating to enhance light out-coupling in organic light-emitting devices Kanchan Saxena a,Ã , Dalip Singh Mehta b , Virendra Kumar Rai a , Ritu Srivastava a , Gayatri Chauhan a , M.N. Kamalasanan a a Polymeric and Soft Materials Section, National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, India b Laser Applications and Holography Laboratory, Instrument Design Development Centre (IDDC), Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Received 27 June 2007; received in revised form 25 September 2007; accepted 27 September 2007 Available online 9 October 2007 Abstract We report signiﬁcant enhancement of light out-coupling in organic light-emitting devices (OLEDs) by means of anti-reﬂection coating of magnesium ﬂuoride (MgF 2 ) on the backside of glass substrate. OLEDs were fabricated by employing the green electrophosphorescent material fac tris-(2-phenylpyridine) iridium [Ir(ppy) 3 ] doped in 4,4 0 ,8-N,N-8-dicarbazole-biphenyl (CBP) and 0.4 wt% tetraﬂuorote- tracyano-quinodimethane (F4-TCNQ)-doped naphthylphenylbiphenyl diamine (a-NPD) as hole transport layer (HTL). Single-layer MgF 2 with the thickness of l/4 was then vacuum deposited on the backside of glass substrate of OLED. About two-fold enhancement in luminance with anti-reﬂection coating of MgF 2 has been observed. r 2007 Elsevier B.V. All rights reserved. Keywords: Organic light-emitting devices; Light out-coupling; Anti-reﬂection coating technique. 1. Introduction There has been considerable amount of research and development for the improvement of efﬁciency of organic light-emitting devices (OLEDs) because of their potential applications in general illumination, ﬂat panel displays, automotive and outdoor lighting. Numerous efforts have been made to improve their external coupling efﬁciency (Z cp, ext ) by means of improved device concepts, such as electrode modiﬁcations, synthesis of new organic materials and modiﬁcation in device structure [1–7]. It is well understood that the generated light from the active OLED medium propagates via various modes, that is, external modes (escape from the substrate surface), substrate-, and ITO/organic-waveguided modes due to total internal reﬂection (TIR) [6–8]. According to the ray optics theory, about 80% of the generated light is lost in waveguided modes due to glass substrate and ITO/organic material which means that the majority of generated light is either trapped inside the glass substrate and device, or emitted out from the edges of an OLED [6–8]. For the purpose of applications in general illumination and ﬂat panel displays, light emitted from the substrate surface (external modes) is most useful which is only about 20% of the total emitted light from the OLED. To extract the trapped and waveguided light into external modes, various approaches based on light refraction and scattering to reduce TIR at the interfaces have been reported, such as, the use of a shaped substrate [6,7], use of micro-lenses on the backside of substrate surface [8–10], formation of mono-layer of silica micro-spheres as scattering medium [11,12], and use of high refractive index substrate [13]. In another approach, an extremely low refractive index silica-aerogel layer [14] was inserted between the ITO transparent electrode and glass substrate. A 50% light extraction efﬁciency from OLEDs was recently reported by insertion of a two-dimensional photonic crystal structure [15–18], and using nano-porous and nano-patterned ﬁlms [19–21]. More recently, use of diffusive layer lamination [22], ARTICLE IN PRESS www.elsevier.com/locate/jlumin 0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.09.028 Ã Corresponding author. Tel.: +91 11 2659 1865; fax: +91 11 2658 6729. E-mail address: kanchan@mail.nplindia.ernet.in (K. Saxena).