<ul><li>Simulation:  Attempts were made to code simulations of OLEDs in MatLab. The complexity of the undertaking, however...
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Capstone Poster


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My Capstone Group Project, May 2007.

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Capstone Poster

  1. 1. <ul><li>Simulation: Attempts were made to code simulations of OLEDs in MatLab. The complexity of the undertaking, however, proved the task worthy of its own capstone project. The initiative was therefore abandoned. </li></ul><ul><li>Equipment revival: The larger vacuum machine in the lab had to have its cooling system repaired. This was a small task that had held up the project. </li></ul><ul><li>Discovering Operational Parameters: Tremendous amounts of experimentation had to be done in order to find parameters suitable for our experiments. These parameters included distances between the crucibles and substrates, the level of current used, duration of evaporation, time required for bell jar evacuation, method of storage of raw materials, finding appropriate crucibles for various materials and method of multilayer depositions. Overcoming this set of challenges took up the bulk of experimentation time during the course of this project. </li></ul>Background: The Energy Crisis According to the Optoelectronics Industry Development Association (OIDA) there will be an energy crises in the next 10-20 years, considering current trends in energy supply and demand. Once way to alleviate the problem is to use more energy efficient lighting devices than those currently available. Organic light emitting diodes (OLEDs), theoretically, can be made to be more energy efficient, and brighter, than even current fluorescent lights. This set the motivation for this project: to create an OLED that can be used for general lighting applications. Organic Light Emitting Diodes Pierre Bouzi, Abdou Diba, Mohammad Faisal Halim, and Hassan M. Oubei Faculty memtor: Prof. Roger Dorsinville Nonlinear Optics Laboratory Department of Electrical Engineering, The City College of New York / CUNY, New York, NY 10031 Abstract We fabricated and characterized an organic light-emitting diode using a tris-(8-hydroxyquinoline) aluminum (Alq3) emissive layer sandwiched between two electrodes: an optically thin indium tin oxide (ITO) film anode, whereas the cathode was in the form a thick aluminum (Al) film. <ul><li>Materials </li></ul><ul><li>Our organic light emitting material was Tris-(8-hydroxyquinoline) aluminum (called Alq3, for short). Our first batch of Alq3 had been obtained from the Kodak Company, and our second batch was obtained from Sigma Aldrich. </li></ul>Introduction to OLEDs OLEDs are solid-state devices composed of thin films of organic molecules that create light with the application of electricity. OLEDs can theoretically provide brighter, crisper displays on electronic devices and use less power than conventional light-emitting diodes (LEDs) or liquid crystal displays (LCDs), or even fluorescent lighting used for general illumination. Project Objectives and Breakdown Current Market State of the Art The Dorsinville Postulates Research was focused (while still aiming to get a broad overview of the field) by checking if the materials and devices in literature found were: 1) polymeric or single molecules 2) luminescent, or phosphorescent 3) what the turn on voltage was 4) what the threshold voltage was (also that we were interested in materials) 5) lifetime 6) structure (layers, crystalline/non-crystalline, I guess) 7) cd/V --> we wanted, for general lighting applications, a brightness of 100 cd at 3-4V. Device Architectures Procedures <ul><li>Place Alq3 in a glass crucible, and place the crucible in a coil that is connected to electrodes inside the bell jar. </li></ul><ul><li>Place the substrate on a holder (behind a mask), and suspend it over the crucible. </li></ul><ul><li>Evacuate the bell jar: </li></ul><ul><ul><li>Lower the bell jar, so as to seal it. </li></ul></ul><ul><ul><li>Close the air release valve. </li></ul></ul><ul><ul><li>Close the valve connecting the mechanical pump to the diffusion pump. </li></ul></ul><ul><ul><li>Open the valve connecting the bell jar directly to the mechanical pump. </li></ul></ul><ul><ul><li>Wait until the pressure in the bell jar goes down to 10 milliTorr or lower. </li></ul></ul><ul><ul><li>Close the valve connecting the bell jar directly to the mechanical pump. </li></ul></ul><ul><ul><li>Open the valve connecting the mechanical pump to the diffusion pump. </li></ul></ul><ul><ul><li>Open the valve connecting the bell jar to the diffusion pump. </li></ul></ul><ul><ul><li>Wait for some hours, until the pressure in the bell jar reaches about 10^-6 Torr (this is measured using the ion gauge). </li></ul></ul><ul><li>Turn on the current, and increase it until the coil around the crucible turns to a slightly reddish color, and patiently wait for the Alq3 to deposit. </li></ul><ul><li>After the deposition, allow some time for the electrodes to cool down, and for evaporated material to settle. </li></ul><ul><li>Re-pressurize the bell jar: </li></ul><ul><ul><li>Close the valve connecting the bell jar directly to the diffusion pump </li></ul></ul><ul><ul><li>Open the air release, thus flooding air only into the bell jar. </li></ul></ul><ul><ul><li>Raise the bell jar, to allow access to its contents. </li></ul></ul><ul><li>Place a piece of calcium (freshly taken out of the nitrogen glove box) onto a tungsten crucible connected between two electrodes in the bell jar. </li></ul><ul><li>Place an aluminum evaporation slug (also, freshly taken out of the nitrogen glove box) into a tungsten coil, connected between two other electrodes inside the bell jar. </li></ul><ul><li>Evacuate the bell jar, using the instructions in step 3. </li></ul><ul><li>Perform the calcium deposition (calcium is not necessary for a crude device) by passing a high current through the crucible. </li></ul><ul><li>Allow the electrodes to cool, and for vapors to settle. </li></ul><ul><li>Perform the aluminum deposition. </li></ul><ul><li>Allow the electrodes to cool, and for vapors to settle. </li></ul><ul><li>Re-pressurize the bell jar, as in step 6. </li></ul><ul><li>Extract the devices and place them in the nitrogen glove box, for testing. </li></ul>Failures and Accomplishments Challenges Discussion References Glass Substrate ITO, Anode Alq3, Organic Emitting Layer Aluminum, or Aluminum atop Calcium, Cathode Schematic of a high efficiency device, OIDA Schematic of a white light emitting device, OIDA Schematic of a very simple device, OIDA Schematic of devices fabricated for this project Deposition Steps Half coated ITO glass, with electrodes of silver based paint Glass square with Alq3 deposited on it Alq3 molecule: A small molecule for use in green emitting OLEDs Step 1: Preparation (cleaning) of Glass Substrates coated with ITO Step 2: Deposition of Alq3 (emissive layer) Step 3: Deposition of Calcium Step 4: Deposition of Aluminum Step 5: Preservation in a nitrogen atmosphere Step 6: Testing OLED Components How do OLEDs Emit Light? Source: Howstuffworks.com <ul><ul><ul><ul><ul><li>Types of OLEDs: Passive and Active Matrix </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Passive-matrix OLED </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Active-matrix OLED </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Transparent OLED </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Top-emitting OLED </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>Foldable OLED </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li>White OLED </li></ul></ul></ul></ul></ul>Currently, OLEDs are used in small-screen devices such as cell phones, PDAs and digital cameras. There is the possibility of using OLEDs as shaped lighting fixtures, and architects are already looking into that. Source: howstuffworks.com Source: OLED Technology Roadmap, 2002, OIDA A number of large and small companies like Corning, GE, Siemens, Dow, Kodak, Samsung, etc. are currently investing in OLED technologies for their upcoming products. GE and Siemens in particular are in stiff competition to bring white light OLED technology for general illumination to the market. The top left picture shows fabricated devices that failed to work, probably owing to problems with Alq3 layer adhering to the ITO and the glass. At other times, the devices failed due to shorts. The picture on the bottom left shows the results of a flawed method of testing the circuit for use in evaporations. The top right picture shows an extremely smooth Alq3 deposition from which we obtained the fluorescence and absorption spectra shown underneath it. The bottom right picture shows our successful OLED device. It consisted of Alq3, sandwiched between ITO and Aluminum. Owing to its non-uniformity it only provided intermittent light. This project has trained this team in the art and science of fabrication and characterization of OLED devices. The background reading and fabrication training has prepared the team members for a career in this field. Besides the engineering aspects of the task, the members have also learned team dynamics, and task divisions. [1]:Hiroshi Kanno, Yiru Sun, and Stephen R. Forrestb, Appl. Phys. Lett. 86, 263502, 2005 [2]:W. D’Andrade, R. J. Holmes, and S. R. Forrest, Adv. Mater. (Weinheim, Ger.) 16 , 624, 2004. [3]: Tokito, T. Iijima, T. Tsuzuki, and F. Sato, Appl. Phys. Lett. 83 , 2459, 2003 [4]: K. Mameno et al ., IDW’02 Proceedings , 2002, p. 235. To design and fabricate a multi layer organic white light emitting diode, for general illumination purposes. The diode is intended to have a lifetime of a few hours, longer in a nitrogen environment. The task was divided as follows: Technology Survey Market Research and Technology Survey Simulation Fabrication Characterization <ul><li>GE had made OLED panels that were large enough for wall illumination, complete with redundancies for the failure of devices over time. </li></ul><ul><li>Siemens was doing similar research. </li></ul><ul><li>Dow-Corning had just started an initiative for silica and OLED based technologies, and was delivering high end materials for research and development. </li></ul>Source: howstuffworks.com CATHODE GLASS SUBSTRATE EMITTING LAYER ANODE