Organic
Electronics:
New uses for plastic in computers
Robert D. Hunkins
CENG 6332 High Performance Computer Architecture
Dr. L. Shih
University of Houston, Clear Lake
Fall 2005
Abstract
The last few years have shown an increased interest in Organic Electronics in the computing industry. Organic electronics are electronic components made using carbon-based polymers and shorter oligomers instead of more traditional inorganic semiconductors.
In this paper, I discuss the advantages and disadvantages of using Organic materials, some of the materials being used, the processing methods being explored to create computer components, and some of the applications envisioned using this new technology.
Introduction
Organic compounds have been used for many years in computer components such as Integrated Circuits (ICs), but until recently, their use has been limited to providing protection for the inorganic semiconducting materials that are the heart of the IC.
In 1977, Alan Heeger, Alan MacDiarmid and Hideki Shirakawa first created organic compounds with the capability to conduct charges, for which they were awarded the Nobel Prize in 2000[i]. Since then, research has been conducted in producing Organic Light Emitting Devices (OLED), Organic Thin Film Transistors (OTFT), and organic Photocells. While none of the compounds yet created can match the performance of Inorganic Silicon and Gallium Arsenide, organic compounds do have some interesting characteristics that make them valuable for use as active computer components.
The Nature of Organic Semiconductors
The organic compounds that are able to conduct charge have “conjugated backbones”, that is, the molecular formations in the molecules have alternating double and single p-electron bonds, which allow charges to move along the backbone and through the crystal, much like charge moves through Silicon.
Like inorganic semiconductors, organic semiconductors (OS) can be doped to produce conduction of positive charges (holes) or negative charges (electrons). A hole can most easily be thought of as an absence of an electron in the valence band of an atom. Electrons from neighboring atoms jump from atom to atom, effectively moving the hole.
The doping of organic semiconductors is different from their inorganic counterparts in that organic compounds are oxidized to produce a scarcity of electrons (p-type) or reduced to an excess of electrons (n-type).
There has been difficulty with developing n-type organic semiconductors that are air stable, because the oxidizing nature of the atmosphere tends to de-dope the electron rich n-type semiconductor, that is, atmospheric oxygen combines with the electrons in the semiconductor and reduces the doping level.
However, there has been some recent success in creating air stable n-type semiconducting materials[ii],[iii]. Both types of semiconductors used together will allow for creation of CMOS-type circuits, with all the low power advantages of that technology.
Advantages
The crystals in OS are held together by Van Der Waals forces, whereas the crystals in Silicon are held together by covalent bonds. This results in a more flexible structure. For example, French Engineers created transistors from Organic compounds that were bent through angles of approximately 90 degrees and still functioned.[iv]
The OS materials can be deposited on flexible substrates at low temperatures, compared to inorganic semiconductors, which must be fabricated in furnaces, or at high levels of vacuum. The ease of processing is much higher and the cost is necessarily much lower. Substrates available for use are much larger than those available for inorganic semiconductors. Plastic sheeting, which would be destroyed at the temperatures where Silicon is processed, is suitable for use as a substrate in OS integrated circuits.
Like some inorganic compounds, there exist Organic semiconductors that will emit light at different frequencies. This combined with the creation of Organic thin Film Transistors will lead to the production of Polymeric Light Emitting Diode displays, which will consume less power than backlit LCD displays, be cheaper to produce, and will be flexible or transparent.
Disadvantages
The disadvantages of OS have mainly been with achieving high
charge carrier mobility. The conductivity(s) of a material is directly
related to the charge carrier mobility (m) according to the
equation 
, where n is the charge carrier density and Q is the electron
charge. Because the materials are held together with weak Van Der Waals forces,
the mobility is limited by thermal interactions between the molecules.
Improving mobility has been a major focus of research. Some materials such as Pentacene (C22H14) have been
found to exhibit higher mobility, but suffer from difficulty with solubility,
and cannot be as easily processed like other materials. The difficulty between
solubility and mobility seem to be related, i.e., the more soluble a compound
is, the lower the charge carrier mobility. Additionally, the higher the mobility,
the more rigid the material is.
Low mobility means that organic semiconductors will not soon displace Silicon and GaAs as the first choice for materials in integrated circuits. However, where flexibility, low cost and ease of production are of prime importance, organic semiconductors have already found a niche in display technology[v].
Examples of Organic Semiconductors
Polyaniline
Polyaniline (Figure 1) was used in the first all-polymer transistor, produced in 1994 by the French CNRS Laboratory of molecular materials[vi]. Philips then produced a complete integrated circuit in 2000.[vii] This compound is both melt and solution processable and can be doped so that it is as conducting as Silicon or Germanium or as insulating as glass[viii] It is therefore one of the most commonly used Organic semiconductors today.
Polythiophene
Figure 2- Polythiophene
Polythiophene (Figure 2) can be made to be either conducting or semiconducting. It is one of the most extensively studied organic semiconductors. It exhibits stability and can be made to emit light. In additions to its use in OLEDs, It has been studied for use in Field Effect Transistors, and Schottky diodes. Polythiophene is insoluble, so a soluble Polythiophene precursor must be used during processing.[ix] Thermal reactions then convert the precursor to Polythiophene.
Some forms of Polythiophene can produce current when exposed to light, so it is being investigated for use in photodiodes, photodetectors, and photovoltaic cells. Other applications being investigated are for its use in current controlled electrical switching and in Metal-Insulator-Semiconductor Field Effect Transistors[x]
Pentacene
Figure 3 - Pentacene
Pentacene (C22H14) is an aromatic hydrocarbon oligomer that has been investigated for use in Thin Film transistors. Pentacene has the advantage that it forms crystals more easily than other organic semiconducting materials and thus has higher charge carrier mobility.
Pentacene is not easily soluble and therefore it is difficult to use solution-based processes such as inkjet printing for fabrication. Volkman, et.al[xi], described a method where a Pentacene precursor could be used in an inkjet printing process and the samples were then annealed to produce the Pentacene layer that could perform the necessary semiconducting. Additionally it is important to produce high quality crystalline Pentacene to achieve high carrier mobilities. Research has been conducted in the past several years on how to grow highly ordered Pentacene crystals[xii]. Pentacene holds the greatest promise for use as a TFT in Active matrix OLED displays.
Processing methods
One of the greatest advantages organic electronics has is the ease of processing compared to Silicon. Where Silicon must be processed at high vacuums or temperatures, Inorganic electronics can also be produced at near room temperatures and pressures using processes familiar to the semiconducting and printing industries. These processes include spin coating, ink-jet printing, Spin coating is done by placing an excess amount of liquid material on a substrate and rotating the substrate at high revolutions. The liquid spreads out due to centrifugal force to thicknesses of less than 10nm.
Traditionally, layers in Transistors, Photodiodes and LED’s have been laid down using micro lithography techniques. These techniques involve the deposition of materials, laying down masks, and removing the unmasked areas with acids or other solvents, and doping unmasked areas as required. It is now possible to use the same technology found in inkjet printers to lay down organic materials and effectively print circuits or displays. This has a great advantage in that it can be done far more easily with far less complicated procedures and less expensive equipment. This is the greatest advantage Organic semiconductors have over their inorganic counterparts.
Applications
Radio Frequency Identification (RFID) Tags
Philips Corp. has produced a circuit that generates a unique 15-bit identification code Radio Frequency identification tag. Part of this circuit makes use of Organic semiconductors. One use of these tags is as an anti-theft sticker for retail goods[xiii]. The tags may also be used for Asset Tracking, Livestock ID, Luggage, Parcel and Rental tracking[xiv]. Since they are inexpensive, and simple to produce, they are cost effective for this application.
Flexible Displays
One of the most promising applications for OLEDs is in electronic displays. Universal Display Corp. of Ewing, NJ is developing flexible displays, transparent displays, as well as Passive and Active Matrix OLED displays. In addition to reducing the costs of displays in applications available today, such as desktop, television and cell phone displays, flexible displays may also someday be incorporated in heads-up displays for aircraft or even in automobiles. Helmets with clear visors may have displays installed to allow the wearer access to data. Another concept idea is to present visual data via a pair of glasses, goggles, or a monocular device worn on a user’s head, providing entertainment, and information on demand.
Photocells
The possibilities for large-scale cheap photocells made of organic semiconductors are particularly exciting. Consider the capability to mass-produce large sheets of photovoltaic cells that can be applied to the roofs of buildings or to other surfaces that are exposed to the sun. The ability to cheaply harness solar energy has potential to reduce dependence on other forms of energy. Research in 2000[xv] and 2002[xvi] reported that the efficiencies of polymer solar cells have been reported at ~2.5%. In 2004, Researchers at Georgia Tech[xvii] produced solar cells using Pentacene and C60, a.k.a. “Bucky balls”. Efficiencies of 3.4% were achieved and the researchers hope to achieve conversions efficiencies of 5%. However, in order for polymer solar cells to become commercially feasible, efficiencies of 10% must be achieved, and the efficiencies must be maintained for long periods of time. Organic photovoltaic technology, while holding promise, has some challenges to overcome.
Conclusion
Organic electronics hold promise for computing technology and could revolutionize the way information is presented to human beings. In a few years, this technology could render CRT displays, and flat panel LCD technology as obsolete as vacuum tubes, paper tape drives and punch card readers. Every current means of relaying information, from billboards to paper could be changed. Will we someday have clothing able to change colors and patterns to suit the wearer’s whim? Will we cover the interior walls of our houses with programmable wallpaper that will not only be as easy to change as throwing a light switch, but can also act as television monitors? Will it replace the paintings and prints we decorate walls with today? Will it even replace the windows themselves, making all offices, regardless of their location in a building, corner offices with a view?
If organic compounds can be created with sufficiently high charge carrier mobilities, and processing challenges are met, organic electronics could be a source for cheap, disposable and biodegradable logic devices. Perhaps there someday will be computer boards whose designs are downloaded from the Internet, printed, and then installed in the computer that did the printing?
If we consider the great advances made in computing over the last two decades, then perhaps these ideas are not so implausible.