OLED TECHNOLOGY Engr.Sohaib Jamil(1) Dr.Shahzad Hussain(1) Department of Electrical Engineering National University of Sciences & Technology (NUST) Islamabad, Pakistan. szmalik1621@yahoo.com; s.hussain@ceme.nust.edu.pk Abstract This paper presents a review of one of the most emerging technology in today's world i.e. OLED.It is a different type of device and is formed when a organic emissive layer is sandwiched between the two electrodes. When a voltage is applied across the electrodes, light is emitted through the organic layer following the electroluminescence phenomena. Recent developments on OLED and its different types following the comparison with LCD and plasma have been discussed. Due to its self illuminating characteristic and various advantages like high contrast ratio, wide viewing angle, low power consumption, color gamut, it is remarkably becoming popular in the market. The only downside of this technology is its lifetime where a considerable work is still in progress. Besides this drawback, LED is still the winner when compared with LCD,LED and Plasma. Keywords Organic Light Emitting Diode (OLED), flexible paper-thin OLED, transparent OLED, white OLED 1. HISTORY OF OLED The phenomena of electroluminescence was first observed in 1950 by André Bernanos and co-workers at the Nancy- University in France. The mechanism proposed was either dye molecule direct excitation or electrons excitation. In 1960, electrode contacts to organic material was developed by Martin Pope in New York University. By using hole and electron injecting electrodes, double injection recombination electroluminescence was produces by Helfrich and Schneider in Canada. A high conductivity in iodine doped polyacetylene was discovered by Heegar in 1977.While the first diode was proposed by Ching W. Tang and Steven Van Slyke at Eastman Kodak in 1985.J. H. Burroughes working at the Cavendish laboratory proposed a light emitting polymer device in 1990.Later in 1998 Kodak, SANYO depicts AMO(Active Matrix Organic) display. And it was SONY who elaborated a OLED panel in 2003.Following this, first OLED TV was sold out in Japan in 2007.The interesting and flexible transparent OLED's and 40 inch high definition AMOLED was presented by Samsung in 2008.Samsung start showing interest in this field by presenting a 7 inch AMOLED display in 2010.Later on, OLED TV's were presented by Panasonic and LG in 2011 and 2012 respectively. Engr.M.Usman Farooq(1) Engr.Faheem Ahma(1) Department of Electrical Engineering Institute of Southern Punjab (ISP) Multan, Pakistan. engrusmanfarooq@gmail.com; mianfaheem_soomro@yahoo.com 2. INTRODUCTION OLED is one of the most emerging technology in today's world. This technology covers many advantages over the obsolete technology of CRT,Pas ma and LCD.Unlike LCD,OLED does not depend upon backlight. LED is getting popular in market due to its extra emerging features like high contrast ratio, greater viewing angle, low power consumption, color gamut and lighter weight. The structure of OLED is shown in Fig 1 which consists of a organic emissive layer sandwiched between two metal electrodes i.e.; anode and cathode which are made of indium tin oxide(ito) and aluminum(al) respectively. Moreover, organic emissive layer is established between the hole injection layer and electron transport layer. When is voltage is applied across the ends of anode and cathode, electroluminescence affect dominates thereby emitting light from the organic material as shown in Figure 2. Figure 1: OLED Structure It can seen that a transparent glass substrate is used on one side of organic material while a metal electrode is used on the other side so that light can pass through the transparent area.
4. OLED TYPES OLED offers great advantages in this era, therefore is getting popular day by day. It is usually operated at voltages between 2 to 10 Volts. It consists of many thin organics films which emits light when a voltage is applied to it. Based upon its performance, we will discuss here the main types of OLED. A. Passive Matrix OLED(PMOLED) Figure 2: Emission Of Light from OLED A. Electroluminescence Phenomena When a voltage is applied across the ends of anode and cathode, electrons start injecting from the cathode while holes from the anode. When these electrons and holes recombine, they form excitons thereby producing light. It's the same phenomena as electrons from cathode and holes from anode form electron hole pairs which is usually termed as recombination process in semiconductors In this type of OLED,an organic layer is sandwiched between the anode rows and cathode columns. It is to be noted that cathode strips are orthogonal to anode strips. The points where the anode and cathode strips are making contact with each other is the area of pixels formation. So when a current is applied, pixels turned on and off depend upon the external circuitry. This type of OLED is very simple to design but the major drawback is its high power consumption. Therefore its use is limited to smaller displays(2 to 4 inch) like in cell phones and MP3 players. Figure 3: OLED Energy Diagram It can be seen in figure 3 that the whole process takes place in the emissive layer where electrons and holes recombining, thereby releasing energy in the form of a light. Highest Occupied molecular orbit(homo) and Lowest unoccupied molecular orbit(l) are very similar to the inorganic semiconductors valence band and conduction band respectively. It is important to note that which color of light will emit and it mainly depends upon the composition of organic emissive layer. 3. OLED EQUIVALENT CIRCUIT Figure 6: PMO LED B. Active Matrix OLED(AMOLED) AMOLED uses thin film transistors(tft) that acts as a switching source between the anode and cathode.tft's are responsible for controlling the amount of current. As brightness is proportional to the amount of current, hence brightness is also controlled by TFT's. Figure 4: OLED Circuit Figure 5: OLED Circuit Figure 7: Acti ve Matrix OLED (AMO LED) If we look at the circuit of AMOLED in Fig 6, where it is shown that AMOLED consists of two thin film transistors along with a capacitor.t1 is used to charge the capacitor while
T2 acts as constant current source to illuminate the pixel. As AMOLED uses TFT's which generally consumes less power thereby making it suitable for larger size displays. Hence AMOLED find uses in Television, monitors and billboards. Figure 8: AMOLED Circuit C. Transparent OLED This type of OLED emits light in both directions as it consists of transparent cathode, anode and emissive layer. It generally employs the same electroluminescence phenomena but is capable of emitting light in both directions and can be active or passive. Its size varies from smaller d isplays to larger displays thereby making it suitable for tablets and head up displays in the vehicles. E. Flexible OLED Figure 10: Top Emitting OLED These types of OLED's are usually made from glass, plastic and metal. This OLED has several pros and cons as it is stronger and lighter in weight. At the same time, it has lower efficiency and brightness due to the stresses involved in the organic layer. Today it is used in mobiles, electronic paper and other curved surfaces. Figure 11: Flexible OLED Figure 9: Transparent OLED D. Top Emitting OLED In this type OLED, top surface is made transparent while the bottom surface is opaque so that light can pass through the top surface. Hence it is named as top emitting OLED and is best suited to active matrix OLED.It is mainly used in smart cards. 5. CHARACTERISTICS OF OLED OLED offers many advantages like high contrast ratio, greater viewing angle, better color gamut, fast response time, low weight and low power consumption. We will discuss these in details. A. Response time Response time is the amalgam of rise time and delay time. In fig 10, it is shown that black light is in the middle while white light is on sideways. So rise time is the time required for display to make transition from 90% white to 10% black while delay time is the time required for the display to make transition from 10% black to 90% white.
D. Color Gamut Measurement OLED offer variety of color ranges as compared to LCD's.Therefore it has greater color gamut as shown in Fig 12.. Figure 12: Video signal for measuring response time B. Contrast Ratio Contrast ratio can be measured as the brightness on 100% white region divided by the brightness on 0% black region. These optical characteristics can be measured at room temperature with a zero viewing angle after the display has been ON and remains stable for approximately an hour in a dark environment. Generally, OLED has a contrast ratio which makes it suitable in cell phones and adds aesthetic to the material. Figure 15: Color Gamut measurement 6. COMPARISON BETWEEN LCD,PLAS MA and OLED Table 1 is showing the comparison between LCD, plasma and OLED.As we can see, OLED is superior over LCD,Plasma and has my advantages like high contrast ratio, high brightness, low power consumption, greater viewing angle and color gamut. Power consumption for OLED is the least followinglcd and Plasma. Also the contrast ratio for plasma is 5000:1 while for OLED it is up to 1000000:1. TABLE 1. Comparison between LCD,Plasma & OLED Technology LCD TV Plasma OLED Figure 13: Contrast ratio measurement test C. Viewing Angle Generally OLED has greater viewing angle as compared to LCD because light is emitted directly from OLED.It is the angle where contrast ratio exceeds 10:1. Power Consumption 60-300 300-660 24-150 Resolution 1920x1080 1920x1080 1920x1080 Colours 16.7 milion 16.7 milion 16.7 milion Brightness 350-500 700-1000 1000 Contrast 350:1-1000:1 1000:1-5000:1 1000000:1 Response Time 8-12ms 0.01ms 0.05ms Viewing Angle 170/170 178/178 178/178 Lifetime(hrs) 50,000-60,000 50,000-60,000 10,000 Figure 14: Measuring viewing angle Color Gamut Better then EBU Better then EBU Better then EBU
The viewing angle for Plasma and OLED is 178/178 while it is 170/170 for LCD.The main reason for this is that OLED and Plasma consists of pixels of emissive layer that produced light. Generally OLED has greater color gamut then plasma and LCD because organic compounds can be broken easily then gases in plasma. The major disadvantage of plasma is that its screen burned if we keep the picture constant for few hours. Traces of pictures can be seen if we change the picture. If we compare the lifetime of three devices, plasma is leading with lifetime of 50000-6000 but when we compare the power consumption, OLED is the winner. 7. CONCLUSION The future is likely for OLED's.Due to extremely remarkable advantages like high contrast ratio, low power consumption, wide viewing angle and faster response time, OLED is getting fame in the market. The only disadvantages are its lifetime and cost. As this technology is new, so it is quite expensive. People are using LCD and plasma since long, So OLED will capture market industry in near future. Currently Samsung is using AMOLED in smart phones which we are using now a days. [8]. http://www.ledtransformations.com/page5.html, accessed: Mart 2012 [9]. http://www.docstoc.com/docs/74013021/an-overviewof-oled-.display-technology, accessed: April 2012 [10]. J. Żmija, M.J. Małachowski, Organic Light Emitting Diodes operation and application in displays, Archivesof Materials Science and Engineering 40/1 (2009) 5-12 [11]. J. Kalinowski, Electroluminescence in organics, Journal ofphysics D: Applied Physics 32 (1999) 179-250 [12]. J. Kalinowski, Organic Light Emitting Diodes: Principles,Characteristics and Processes, Marcel Dekker, New York 2005. REFERENCES [1]. S. Kunic and Z. Sego, OLED Technology and Displays, in Proceedings of 2012 ELMAR, Sept. 2012, pp. 31-35. [13]. S. Miyata, H.S. Nalwa, Organic Electroluminescent Materialsand Devices, Gordon and Breach, Amsterdam, 1997. [2]. http://thetechjournal.com/electronics/lg-brings-worldslargest-55-incholed-hdtv-panel-in-ces2012.xhtml, accessed: February 2012 [3]. http://www.konicaminolta.co m/about/research/oled/devel pment/indexhtml, accessed: February 2012 [14]. J. Shinar, Organic Light-emitting Devices, Springer, Berlin2004. [15]. J. Kalinowski, Emission Mechanisms in Organic Light-Emitting Diodes, Organic Electroluminescence, Taylor &Francis, Boca Raton, 2005. [4]. http://www.agasonia.com/index.php?prm=40200, accessed: Mart 2012 [5]. http://tech.ebu.ch/events/electro-optic Displays for Television /MichaelE. Backer /display- metrology.com, accessed: Mart 2012 [6]. http://www.oled.si/tv/oled-history/, accessed: April 2012 [7]. http://www.oled -info.com/history, accessed: April 2012 Biography Sohaib Jamil was born in Multan, Pakistan. He did B.Sc Electronics Engineering from NFC Institute of Engineering and Technology, Multan (Pakistan). And is the student of M.Sc Electrical Engineering from College of Electrical and Mechanical Engineering NUST, Islamabad, Pakistan. Email ID:szmalik1621@yahoo.com
Muhmmad Us man Farooq was born in Multan, Pakistan. He did B.Sc Electronics Engineering from NFC Institute of Engineering and Technology, Multan (Pakistan). And is the student of M.Sc Electrical Engineering from College of Electrical and Mechanical Engineering NUST, Islamabad, Pakistan. Email ID:engrusmanfarooq@gmail.com Faheem Ahmad was born in Multan, Pakistan. He did B.Sc Electrical Engineering from University of Engineering & Technology (UET), Lahore (Pakistan). And is the student of M.Sc Electrical Engineering from School of Electrical Engineering Institute of Southern Punjab (ISP), Multan (Pakistan). He was the author and co-author of more than four international conference and journal papers. Email ID: mianfaheem_soomro@yahoo.com Cell : 0092 301 7489150 Dr. Shahzad Hussain Did his Phd in Numerical Modeling of Microwave semiconductor Devices(HEMTS) from Institute of Microwave & photonics, Leeds, UK Currently working as Associate Professor at National University of sciences and Technology, Rawalpindi Email ID: s.hussain@ceme. nust.edu.pk