Fabrication of Organic Light Emitting Diodes in an Undergraduate Physics Course

AC 2011-79: FABRICATION OF ORGANIC LIGHT EMITTING DIODES IN AN UNDERGRADUATE PHYSICS COURSE Robert Ross, University of Detroit Mercy Robert A. Ross is a Professor of Physics in the Department of Chemistry & Biochemistry at the University of Detroit Mercy. His research interests include semiconductor devices and physics pedagogy. Ross received his B.S. and Ph.D. degrees in Physics from Wayne State University in Detroit. Meghann Norah Murray, University of Detroit Mercy Meghann Murray has a position in the department of Chemistry & Biochemistry at University of Detroit Mercy. She received her BS and MS degrees in Chemistry from UDM and is certiﬁed to teach high school chemistry and physics. She has taught in programs such as the Detroit Area Pre-College Engineering Program. She has been a judge with the Science and Engineering Fair of Metropolitan Detroit and FIRST Lego League. She was also a mentor and judge for FIRST high school robotics. She is currently the chair of the Younger Chemists Committee and Treasurer of the Detroit Local Section of the American Chemical Society and is conducting research at UDM. Page 22.696.1 Page c American Society for Engineering Education, 2011 Fabrication of Organic Light-Emitting Diodes in an Undergraduate Physics Course Abstract Thin film organic light-emitting diodes (OLEDs) represent the state-of-the-art in electronic display technology. Their use ranges from general lighting applications to cellular phone displays. The ability to produce flexible and even transparent displays presents an opportunity for a variety of innovative applications. Science and engineering students are familiar with displays but typically lack understanding of the underlying physical principles and device technologies. We believe that OLEDs provide a valuable context in which to engage science and engineering students in the study of electronic devices. Colleges and universities typically do not have the resources available for students to produce working electronic devices like diodes or transistors made from semiconductors like silicon. This paper will describe how science and engineering students, in an upper-level undergraduate physics course, fabricate OLEDs. The active layers of the OLEDs are spin-coated onto glass substrates containing a transparent conductive coating. The cathode is formed by the deposition of an appropriate metal contact layer. The deposition and measurement equipment is relatively inexpensive and can be adopted for use in undergraduate physics or engineering courses; as such we believe the topic will be of broad interest to the physics and engineering community. The paper will discuss the synthesis of the polymer compounds and the associated deposition techniques. Properties of the devices, including current-voltage characteristics, will be presented along with future plans for the development of flexible structures on plastic substrates. Introduction The invention of the transistor and the development of silicon planar technology ushered in a new age of synthetic materials. New materials are produced by a variety of processes including, but not limited to: molecular-beam epitaxy; sputtering (rf, dc and magnetron); chemical, physical and plasma-assisted vapor deposition; laser ablation; vacuum evaporation, hot-wire decomposition, and many others. The materials are combined into innovative structures to produce the better, smaller, faster electronic devices that everyone has become accustomed to. From a pedagogical perspective, the deposition equipment is expensive and requires significant technical skill to operate; as such it is beyond the scope of most undergraduate laboratories. The materials used in modern electronic devices consist almost entirely of inorganic materials. For example, the elemental semiconductors from group IV (Si, Ge) of the periodic table; binary compounds from the III-V or II-VI groups (GaAs, CdS); and ternary and quaternary compounds (GaAlAs, AlGaInP) are used in specialized devices. The metallic interconnects and insulating materials are typically inorganic. The development of conductive organic compounds opened 22.696.2 Page the door to an entirely new generation of electronic devices. In recognition of their contribution to the emerging field of polymer chemistry, the Nobel Prize in Chemistry for 2000 was awarded to Heeger, MacDiarmid, and Shirakawa “for the discovery and development of electrically conductive polymers.” 1 In the ensuing years the field has experienced tremendous growth. Electroluminescence was reported in large (millimeter scale) anthracene crystals, under the application of several thousand volts by Helfrich and Schneider. 2 The first rectifying devices were developed at Eastman Kodak in 1987 by Tang and VanSlyke. 3 These thin film organic structures exhibited high external quantum efficiency (10 -2 photon/electron), luminous efficiency (1.5 lumens/watt), and brightness (10 3 candelas/m 2). The first rectifying devices that exhibited electroluminescence utilizing conjugated polymers were reported by Burroughes, et al. 4 Light-emitting diodes have the potential to significantly impact two somewhat divergent technology applications. The first is in the area of solid-state lighting and the second relates to advanced displays. Recent legislation in the U.S. and Europe will, for the first time, mandate efficiency standards that regulate how living and work spaces are illuminated. 5 One of the effects of the legislation is to phase-out inefficient lighting systems - primarily incandescent lamps. There are several strategies for improving the efficiency of lighting systems and our engineering students will soon be in decision-making positions. One possibility is to utilize solid state lighting. Light-emitting diodes can have very high internal quantum efficiencies (photons/electron) and long lifetimes;6 some authors argue they are the ultimate lamp .7 Active- matrix addressing 8 in conjunction with organic light-emitting diodes (AMOLEDs) represent the current state-of-the-art in display technology. At the present time AMOLED displays are used in cell phones and MP3 players. Active-matrix organic light-emitting diodes can be built on plastic substrates to produce flexible and transparent high-definition displays. Samsung, one of the leading manufacturers of AMOLED displays, claims that “AMOLED is the future of video displays.” 9 Solid state devices, and LEDs in particular, provide a unique opportunity for teaching and learning. 10,11, 12,13 We previously reported a laboratory activity that incorporated various aspects of nanotechnology. In the activity, undergraduate students produced working solar cells by spin- coating a conjugated polymer containing the fullerene C 60 onto indium tin oxide (ITO) coated glass slides and subsequently applying a metallic cathode. 14 Undergraduate laboratory activities on the dielectric and mechanical properties of polymers have been reported. 15 In this paper we present another activity that we believe is suitable for physics and engineering students in undergraduate courses. 16,17 The paper is organized in the following manner. First, we present a brief outline of the physics of OLEDs. Subsequently, we describe the organic synthesis procedures, the device deposition processes, the electrical characterization, and the course content. Device Physics of an Organic Light-Emitting Diode A representation of a single layer OLED is shown below in Figure 1. The device is a layered structure consisting of an anode, a photo-active polymer layer and a cathode. In these devices, since the ITO covers the entire glass slide, the active area of the OLED is determined by the area 22.696.3 Page of the cathode. The sheet resistance of the polymer is high enough to limit electrical contact between adjacent cathodes; as such each metallic cathode forms an independent opto-electronic device. The polymer layer has a thickness of about 100nm. glass ITO polymer cathode light anode Figure 1. Structure of an organic light-emitting diode. Note that the device is not drawn to scale. The mathematical description of the conduction of electrons and holes in a polymer film is complicated; the material is not crystalline so the concept of energy bands loses validity. The theoretical treatment is similar to that used for disordered or amorphous materials; the disorder present in the polymer causes the creation of localized energy states and conduction occurs via a hopping (quantum mechanical tunneling) process. 18,19,20 Electrons in the lowest unoccupied molecular orbital (LUMO) are mobile while holes in the highest occupied molecular orbital (HOMO) are mobile and contribute to electrical conduction. Shown below in Figure 2 is a schematic representation of the energy levels associated with the OLED structure, it is essentially an energy band diagram. Under the application of a forward bias (positive voltage on the anode), electrons from the cathode are forced over the barrier ∆ϕ e and are injected into the polymer. At the same time, holes are forced over the barrier ∆ϕ h from the anode. The emission of light is due to the radiative recombination of electrons and holes in the polymer layer in close proximity to the anode. The motion of electrons and holes is influenced by the applied field, their diffusion lengths and recombination lifetimes, and the space-charge present in the polymer layer. Since the mobility of carriers in disordered media is so low there is significant space-charge due to free and trapped charges. It should also be noted

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