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AC 2011-79: FABRICATION OF ORGANIC EMITTING 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 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

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 () represent the state-of-the-art in electronic display technology. Their use ranges from general 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 made from 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 -coated onto glass substrates containing a transparent conductive coating. The 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 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 substrates.

Introduction

The invention of the 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 -assisted vapor deposition; ablation; vacuum evaporation, hot- 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 Page 22.696.2 the door to an entirely new generation of electronic devices. In recognition of their contribution to the emerging field of , the Nobel Prize in Chemistry for 2000 was awarded to Heeger, MacDiarmid, and Shirakawa “for the discovery and development of electrically conductive .” 1 In the ensuing years the field has experienced tremendous growth.

Electroluminescence was reported in large (millimeter scale) crystals, under the application of several thousand by Helfrich and Schneider. 2 The first rectifying devices were developed at Eastman in 1987 by Tang and VanSlyke. 3 These organic structures exhibited high external quantum efficiency (10 -2 /), luminous efficiency (1.5 lumens/watt), and brightness (10 3 candelas/m 2). The first rectifying devices that exhibited 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 (/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 () 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. , 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 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

A representation of a single layer OLED is shown below in Figure 1. The device is a layered structure consisting of an , 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 Page 22.696.3 of the cathode. The sheet resistance of the polymer is high enough to limit electrical contact between adjacent ; 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 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 that the electrons and holes are recombining under the influence of a large electric field. Under typical operating conditions, 4-5V of forward bias dropped across a 100nm thick polymer film, the internal electric field is on the order of 50MV/m. Page 22.696.4

Lowest Unoccupied Molecular Orbital (LUMO)

Cathode (-) Anode (+) ∆ϕ holes electrons e

Highest Occupied Molecular Orbital (HOMO)

∆ϕ h

Figure 2. Simplified energy level diagram for an OLED. The energy levels corresponding to the Lowest Unoccupied Molecular Orbital (LUMO), and the Highest Occupied Molecular Orbital (HOMO) of the polymer are shown along with the positions of the Fermi energy in the anode and cathode. The energy barriers for electron and hole injection at the anode and cathode interfaces

are represented by ∆ϕ e and ∆ϕ h, respectively. On this diagram the energy of an electron is measured positively upward and positively downward for a hole.

Polymer Synthesis

Students constructed OLEDs from two different polymers. Each polymer solution requires some organic chemical synthesis. The synthesis of MEH:PPV, Poly[2-methoxy-5-(2-ethyl-hexyloxy)- 1,4-phenylenevinylene] with the suspended C 60 was described in earlier work. The procedures described here have been adapted from several sources (see Refs. 13,14, and 15). The other polymer, Tris(2,2’-bipyridine)ruthenium(II) tetrafluoroborate ([Ru(bpy) 3](BF 4)2) has previously been synthesized for the use of OLEDs. 13,21 Sodium Hypophosphite was prepared from a 50% hypophosphorous acid solution (Sigma-Aldrich #214906). 10 mL of 50% H 3PO 2 solution was added to 6 mL of water while stirring. Pellets of sodium hydroxide were slowly added until the solution had a pH of 6-8. The final solution was approximately 6 M NaH 2PO 2.

The ruthenium complex was prepared by dissolving 0.1046 grams (0.40 mmol) of ruthenium(III) chloride hydrate (Sigma-Aldrich #206229) in 8 mL of water while stirring. A mortar and pestle was used to grind ~0.3 grams of 2,2’-dipyridyl (Sigma-Aldrich #D216305). 0.188 grams (1.204 mmol) of the 2,2’-dipyridyl powder and 0.44 mL of NaH 2PO 2 were added to the RuCl 3 solution. The stirring was stopped and the beaker was covered with a glass and refluxed for 30 minutes. Additional water was added if necessary to maintain the initial volume. After 30 minutes, 0.333 grams of sodium tetrafluoroborate (Sigma-Aldrich #202215) dissolved in 1.5 mL of water was added to the beaker. The solution was removed from the heat and cooled to room temperature before placing in an ice bath. Crystals started to form immediately. The precipitate was collected using vacuum filtration. The [Ru(bpy) 3](BF 4)2 product was rinsed with cold ethanol and air-dried.

A stock solution of polyvinyl alcohol (PVA) was prepared by bringing 30 mL of water to a boil. PVA (Sigma-Aldrich #363162) was sifted in a No. 200 mesh sieve. 90 grams of the fine PVA Page 22.696.5 powder were slowly dissolved in the boiling water. 0.035 grams of the [Ru(bpy) 3](BF 4)2 complex were dissolved in 3 mL of PVA solution. The solution was magnetically stirred to ensure it was homogenous enough to coat the ITO slides.

Organic Light-Emitting Diode Fabrication

The OLED fabrication process mirrors what was used to produce thin film organic solar cells and has been previously reported (see Refs. 14-17 for more details). The structure begins with a glass substrate coated with ITO (Sigma-Aldrich #703192). The coated glass slides were 25 x 25 x 1.1 mm and had a sheet resistance of about 10 / □. The slide was attached to the central hub of a computer power supply cooling fan, ITO side up, with double-sided sticky tape. One or two drops of polymer solution were applied to the center of the slide with a glass pipette and rubber bulb. The polymers included: MEH:PPV with 2% by weight of the fullerene C 60 and the ruthenium complex [Ru(bpy) 3](BF 4)2 suspended in poly-vinyl alcohol (PVA).

The fan was covered with a splatter shield (an old gallon milk jug with the bottom removed) and brought up to speed for several seconds. Upon removal from the fan, the polymer was allowed to dry for several minutes either on an 80°C hotplate or under the influence of a laboratory heat gun. It seems to be important to drive the remaining solvent from the polymer before the application of the metallic cathode.

The two different metal alloys were used to form the cathodes: Wood’s Metal (Bi 0.50 Pb 0.25 Cd 0.125 Sn 0.125 ) with a melting point of about 80°C and a eutectic alloy of gallium and indium (Ga 0.75 In 0.25) (Sigma-Aldrich #495425-25G) that is liquid at room temperature. The liquid metal is applied either with a pipette or with the wooden handle of a cotton swab. The cathodes students formed were typically about 1mm in diameter.

Students fabricated at least four devices over the course of the three day laboratory activity. They used all combinations of the two polymer solutions and the two cathode materials. If the device that a student produced did not work the polymer was dissolved in the appropriate solvent and the ITO coated glass slide was reused. It should be noted that Wood’s Metal cathodes are easily removed while the GaIn eutectic alloy is quite difficult to displace from a coated slide. It was readily apparent that the best results were obtained with the combination that utilized the ruthenium complex [Ru(bpy) 3](BF 4)2 suspended in PVA in conjunction with the GaIn eutectic alloy cathode.

Experimental Results

Figure 3 below shows the orange electroluminescence that occurs as a result of a student applying a 4V forward bias to an OLED. The device utilizes the ruthenium complex [Ru(bpy) 3](BF 4)2 suspended in poly-vinyl alcohol as the polymer and GaIn as the cathode.

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Figure 3. A student produced OLED under forward bias of 4V. The device utilizes the ruthenium complex [Ru(bpy) 3](BF 4)2 suspended in poly-vinyl alcohol as the polymer and GaIn as the cathode.

The current-voltage characteristics of the OLEDs were measured with a Keithley 2400 source meter controlled by a computer via the GPIB (general purpose instrument bus). Software to control the Keithley source meter is available online. 22 The current-voltage characteristic for the above device is shown in Figures 4 and 5 below. The active area of the OLED is about 0.01cm 2. Note that the rectifying behavior of the device is clearly demonstrated in Figure 4.

15 ) 2

10

5 Current DensityCurrent (mA/cm

0 0 1 2 3 4 Voltage (V)

Figure 4. Current-voltage characteristic of an OLED. The device utilizes the ruthenium complex [Ru(bpy) 3](BF 4)2 suspended in poly-vinyl alcohol as the polymer and GaIn as the cathode.

Page 22.696.7

1.E+01 1.E+01 ) ) 2 2 1.E+00 1.E+00

1.E-01 1.E-01

1.E-02 1.E-02

1.E-03 1.E-03 CurrentDensity (mA/cm CurrentDensity (mA/cm

1.E-04 1.E-04 0 1 2 3 4 0.01 0.1 1 10 Voltage (V) Voltage (V)

Figure 5. Current-voltage characteristics of an OLED displayed in semi-log and log-log form. This is the same data as that of Figure 4 plotted in a different format.

Figure 5 shows the same data as the previous figure, but plotted in semi-log and log-log forms. Plotting the data in these (semi-)logarithmic forms is a useful activity for students. It allows them to glean useful information regarding the functional dependence of the current on the voltage. If the current depends exponentially on the voltage, I= Ae BV with A and B constants, then a semi-log plot should yield a straight line with slope B. If the current has a power-law dependence on the voltage, I= CV n with C and n constants, then a log-log plot yields a straight line with slope n. We observe from the semi-log graph that at lower voltages a significant region of the characteristic is linear; we interpret this to mean the current depends exponentially on the voltage. This type of behavior is expected if carriers are injected over a barrier and diffuse away from the contact. From the log-log plot on the right we observe that at high voltages, the characteristic is linear; we interpret this to mean that the current has a power-law dependence on voltage. This behavior is expected in a situation where carriers are injected from opposite sides (double-injection) of the device and are influenced by their own space-charge. It should be noted that many of the working devices are not stable; they tend to fail after several tens of minutes. 23

The Course: Solid State Device Physics

Solid State Device Physics (PHY 3680) is a 2 credit hour course offered each summer during a condensed seven week term. It has three weekly 75 minute meetings and is arranged in a lecture format. The course is required for all electrical and computer engineering students and is an elective for other science and engineering majors. Enrollment in PHY 3680 over the past five years averaged 11 students, one-third of those being either female or a member of an underrepresented minority group. This enrollment distribution is typical for courses taught in the College of Engineering & Science.

Page 22.696.8 Solid State Device Physics includes topics such as: crystal structure, introductory band theory, carrier transport, junction diodes, and transistors. The photovoltaic effect and light-emitting diodes are introduced in the section on the junction diode. During the summer of 2010, faculty decided to eliminate the coverage of bipolar junction transistors and substitute two related laboratory activities: thin film organic solar cells and OLEDs. Students spent three meeting periods working in the laboratory fabricating and characterizing devices. Students were engaged and interested in the activities and indicated favorable responses on course evaluations.

We plan to continue the laboratory activities during subsequent offerings of the Solid State Device Physics course. Future plans include the use of different polymer compounds and substrate materials. Development of new polymers for thin film organic solar cells in addition to MEH:PPV such as Poly(3-hexylthiophene-2,5-diyl) (P3HT, Sigma-Aldrich #698997) and the fullerene compound [6,6]-phenyl-C61 -butyric acid methyl ester (PC 61 BM, Sigma-Aldrich #684457). This combination has been shown to improve the conversion efficiency of thin film polymer devices.24,25 In addition, we have identified vendors to supply ITO coated plastic substrates. These substrates have the ITO deposited in a striped pattern with lines 1mm wide. The plastic substrates are flexible and transparent and could be used to produce rudimentary addressable displays. We also plan to pursue other metal-organic complexes for electroluminescence in addition to Ru.

Conclusions

The curriculum for Solid State Device Physics (PHY 3680) was modified during the summer of 2010 to include an innovative laboratory activity associated with the fabrication and characterization organic light-emitting diodes. The upper-level laboratory activity was adapted from published results in the areas of physics and chemistry education research. It was our experience that students were interested and engaged throughout the fabrication and characterization processes. This activity seems appropriate for a variety of upper-level courses. It is interdisciplinary in nature; requiring some content knowledge of physics, chemistry, and electrical engineering. As a result, using inexpensive deposition equipment, students were successful at constructing working OLEDs.

Bibliography

1 For details see The Royal Swedish Academy of Sciences website, www.kva.se. 2 Recombination radiation in anthracene crystals, W. Helfrich and W. G. Schneider, Phys. Rev. Lett., 14 , 229- 231 (1965). 3 Organic Electroluminescent Diodes, C. Tang, and S. VanSlyke, App. Phys. Lett., 51 , 913-915 (1987). 4 Light-emitting diodes based on conjugated polymers, J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, Nature, 347, 539–541 (1990). 5 Bringing science policy into the optics classroom: Solid state lighting and United States lighting standards . S. K. Mayer, Am. J. Phys. 78( 12 ), 1258-1264 (2010). 6 Solid state lighting: A system engineering approach . I. Ashdown, Opt. News, 18 , 24-30, (2007). 7 Is the light emitting diode (LED) an ultimate lamp? N. Holonyak, Jr., Am. J. Phys. 68( 9), 864-868 (2000). 8 See for example: http://electronics.howstuffworks.com/oled3.htm. 9

See: http://www.samsung.com/sg/consumer/mobile-phone/mobile-phone/infotainment/GT- Page 22.696.9 I8910DKBXSO/index.idx?pagetype=prd_detail, or go the www.samsung.com and search “AMOLED”.

10 A laboratory experiment with blue light-emitting diodes . E. Redondo, A. Ojeda, G. Gonzalez D íaz, and I. Mártil, Am. J. Phys. 65( 5), 371-376 (1997). 11 Demonstrating the light-emitting diode . D.A. Johnson, Am. J. Phys. 63(8), 761-762 (1995). 12 Solid-State Organic Light-Emitting Diodes Based on Tris(2,2'-bipyridine)ruthenium(II) Complexes . F.G. Gao, and A.J. Bard, Journal of the American Chemical Society, 122 (30), 7426-7427 (2000). 13 The Interdisciplinary Education Group at the University of Wisconsin-Madison has a website with a variety of activities related to nanotechnology. For example see: http://mrsec.wisc.edu/Edetc/nanolab/oLED/ and references therein. 14 A Nanotechnology Application for Physics Laboratory Courses, R. Ross, in Proceedings of the 2010 American Society for Engineering Education Annual Conference & Exposition, Louisville, KY, (2010). 15 Materials physics: A new contemporary undergraduate laboratory. H. Jaeger, M.J. Pechan, and D.K. Lottis, Am. J. Phys. 66(8), 724-730 (1998). 16 Using Organic Light-emitting Electrochemical Thim-Film Devices to Teach Materials Science . H. Sevian, S. Muller, H. Rudmann, and M.F. Rubner, Journ. Of Chem. Ed., 81(11), 1620-1623, (2004). 17 Two examples of organic opto-electronic devices: Light emitting diodes and solar cells . J.L. Maldonado, G. Ramos-Ortíz, M.L. Miranda, S Vázquez-Córdova, M.A. Meneses-Nava O. Barbosa-García, M. Ortíz-Gutiérrez, Am. J. Phys. 76( 12 ), 1130-1136 (2008). 18 Absence of Diffusion in Certain Random Lattices. P.W. Anderson, Phys. Rev., 109 , 1492-1505, (1958). 19 Electronic Processes in Non-Crystalline Materials . N.F. Mott, and E.A. Davis, Clarendon Press, Oxford, (1979). 20 See for example: Physics of Organic Semiconductors , W. Bruetting (ed.), Wiley-VCH, Berlin, (2005); or Highly Efficient OLEDs with Phosphorescent Materials , H. Yersin (ed.), Wiley-VCH, Berlin, (2007). 21 Tris(2,2'-bipyridine)ruthenium(II) Dichloride Hexahydrate , J. A. Broomhead and C. G. Young, Inorg. Syn., 28 , 338-339, (1990). 22 IV-Stat is free to download from http://www.greenmountainengineering.com. It is designed for the measurement of solar cells but is flexible enough to measure a variety of devices. 23 Degradation and failure of MEH-PPV light-emitting diodes , J.C. Scott, J.H. Kaufman, P.J. Brock, R. DiPietro, J. Salem, J.A. Goitia, Journ. of App. Phys., 79 , 2745-2751, (1996). 24 On the use of Woods metal for fabricating and testing polymeric organic solar cells: An easy and fast method . J. Salinas, J. Maldonado, G. Ramos- Ortíz, M. Rodríuez, M.A. Meneses-Nava, O. Barbosa-García, R. Santillan, and N. Farfán, Solar Energy Materials & Solar Cells, 95 , 595–601, (2011). 25 Development of New Semiconducting Polymers for High Performance Solar Cells . Y. Liang, Y. Wu, D. Feng, S.T. Tsai, H.J. Son, G. Li, and L. Yu, J. Am. Chem. Soc. , 131 (1), 56–57, (2009). Page 22.696.10