2, University of Science and Technology, Daejeon, , Korea (the Republic of)
Very recently, we have developed a nano-lens array (NLA, lens size: <1 um) fabrication technology using a one-step vacuum deposition.1 The sequence of the dry NLA process was as follows. First, N,N’-Di(1-naphthyl)-N,N’-diphenyl-(1,1’-biphenyl)-4,4’-diamine (NPB, purity : >99.9%) powders were put into a vaporize. Secondly, they were heated to generate organic vapors. Thirdly, the vapors were transferred by means of nitrogen gas and deposited on the samples. In contrast to the two-step fabrication of a micro-lens array (MLA, lens size: several hundred micrometers) which is generally used to improve the efficiency of organic light emitting diodes (OLEDs), our prosess is spontaneous and obviates the need of masking, modling and curing, hence it can be easily applicable to the industry. Additionally, benefited from their small sizes of sub-um, NLAs do not distort the image of the active matrix OLED (AMOLED) displays with a pixel size of same or less than 50 um. The other advantages of NLA technology were size controllable between ~50 nm and ~500 nm, and easily formable on both a rigid and a flexible substrates. The NLA integration in top-emitting OLEDs (TOLEDs) by forming the NLA on a indium zinc oxide (IZO) significantly enhanced a light extraction efficiency and color stability.1,2
Currently, flexible AMOLED market is experincing rapid growth. Moreover, TOLEDs better fit into AMOLEDs, as TOLEDs are structurally unaffected by the number of thin film transistors integrated on a substrate. Therefore, extensive studies on the efficiency and color stability of flexible TOLEDs have become a greater importance. In this talk, the device characteristics of flexible TOLEDs using a polyethylene-naphthalate film (Teonex, DuPont Teijin Films) as a substrate, without and with NLAs, are presented. Prior to the flexible TOLEDs fabrication, rigid TOLEDs using a glass substrate were prepared to investigate the feasibility of a thin film encapsulation layer (Al2O3, 50nm) deposited on the OLED layer. The difference between flexible and rigid TOLEDs is a substrate material. The Al2O3 layer was formed using atomic layer deposition equipment (LUCIDA D100, NCD). Trimethylaluminium and ozone were used as precursors. The Al2O3 thickness was measured using ellipsometry (Nanospec 9100, Nanometricslpitas). Lighting tests using rigid TOLEDs indicate that air exposure does not damage the Al2O3 encapsulated OLED device. Furthermore, NLAs with various sizes are employed in the OLEDs, and the effects of the introduction of NLAs on the OLEDs will be discussed.
1. Y. -S. Park, K. -H. Han, J. Kim, D. -H. Cho, J. Lee, Y. Han, J. T. Lim, N. S. Cho, B. Yu, J. -I. Lee and J. -J. Kim, Nanoscale, 9, 230 (2017)
2. K. -H. Han, Y. -S. Park, D. -H. Cho, Y. Han, J. Lee, B. Yu, N. S. Cho, J. -I. Lee and J. -J. Kim, Nanoscale, submitted (2017)