Fullscreen HTML5Med Res (??MB)HTML4

Organic (and why things conduct electricity) Organic semiconducting materials will enable the fabrication of large, cheap printable solar panels, cheap disposable medical sensors, flexible display screens, ultra-cheap RFID tags and cheap, disposable chemical sensors. by Dr David Maddison A n organic material is carbon-based and organic semiconducting materials are usually in the form of polymers (like polyethlylene, the common packaging plastic). Organic semiconducting materials combine some properties of traditional semiconducting materials such as silicon with properties of plastics such as low density, flexibility and the ability to produce them cheaply in large amounts. For background on this topic it is first necessary to see why some materials are insulators, some semiconductors and some conductors. Why materials are conductors, insulators or semiconductors Materials are traditionally thought of as being electrical insulators (which will not pass any significant electrical current), such as plastics like polyethylene; semiconductors (which will pass some electrical current but with some resistance), such as silicon; and conductors (which will pass electrical current with ease and with limited resistive losses), such as copper. In electronics, materials with the properties of insulator, semiconductor and conductor are all used in nearly all devices. 16  Silicon Chip Take for example a simple solid-state diode: it will have conducting metal leads, an insulating body and the active junction will be made of some combination of semiconductor materials. What makes something an insulator, semiconductor or conductor? It comes down to the “electronic band” structure of a material. In the classical model, all matter is composed of atoms and each atom has a nucleus surrounded by electrons. The electrons have certain energies associated with them and the particular energy a certain electron has will determine which of a range of permitted energy levels or atomic orbitals in the atom it can occupy. The Pauli exclusion principle says that no two electrons in an atom can have the same energy states and as a result of that, energy levels in atoms become occupied with electrons that all have unique energy states. Incidentally, this exclusion principle is one of the most profound observations of nature and governs the make up of the elements and the Periodic Table. A single atom in isolation will have a number of discrete energy levels occupied by its electrons (a bit like planets orbiting a star – see Fig.1) but when a large number of atsiliconchip.com.au Electronics Fig.1: this shows some energy states of an atom and how different elements have different numbers of electrons. An energy level does not have to be filled to capacity with electrons. This diagram shows the first two energy levels. The nucleus contains protons and neutrons, (ie, except for hydrogen which usually contains only one proton unless it is a heavier isotope). The nucleus in reality is about 100,000 times smaller than the atomic radius. oms are bought together to form a solid, these discrete energy levels merge into what are known as bands, so essentially there is a continuous range of energy levels rather than discrete levels. This happens because all the energy levels of an atom start interacting with others, splitting into more and more levels when atoms come close together, forming so many separate levels that it is as though they were continuous bands of energy (see Fig.3). Fig.4 (overleaf) shows the band structure in insulators, semiconductors and conductors. We can see the valence band and the conduction band, along with a “band gap”. For a material to conduct, electrons have to be available in the conduction band where they can move through the solid from atom to atom. For electrons to reach the conduction band they have to come from the valence band by crossing the band gap. In a conductor, the valence and conduction bands overlap, so there are always electrons available to conduct electricity. In a semiconductor there is a band gap to cross. Electrons can’t easily move through the solid from atom to atom, although some will get through due to thermal excitations of the electrons. One way to increase the small number of electrons crosssiliconchip.com.au Fig.2: there are a number of energy levels in an isolated atom. Whilst the previous diagram showed an atom with the first two levels. This shows four energy levels. In fact there are seven main energy levels. Each of the levels except the first is subdivided into a number of sub-levels, which are described with a number and letter as shown. The energy levels are filled first from the lowest to the highest. ing the gap is to heat the semiconductor and as the temperature increases, more and more electrons go into the conduction band so the conductivity of the semiconductor increases. Light can also excite electrons to cross the band gap. Another way is to “dope” the semiconductor which has the effect of generating extra energy levels and reducing the size of the band gap making conduction easier. This doping is either of an “n” or “p” type, a terminology familiar to the description of a transistor as an “NPN” or “PNP”, and referring to whether the charge carriers are either electrons or “holes” (absence of electrons). In an insulator, the band gap is so large that electrons cannot cross it and move from atom to atom so very little or no conduction can occur. History of plastic materials The first synthetic polymer or plastic was introduced in 1862 by Alexander Parkes, who created a mixture of cellulose nitrate and solvent which could be moulded with heat and pressure. Following that, collodion, a cellulose solution in an alcohol-ether mixture was used by John Hyatt in 1868 to coat billiard balls. He obtained a patent in 1870, later ruled inNovember 2015  17 Fig.3: discrete energy levels in a single atom on the left split into more and more levels as more atoms (n) come together, eventually forming a series of what look like continuous bands (although in reality they are a large number of separate energy levels). There may or may not be a gap between the upper and lower bands. (Image by Norbert Heinz.) Fig.4: valence band shown in blue and conduction band show in green for different materials. (Image by Norbert Heinz.) valid, for a horn-like material of cellulose nitrate and camphor which came to be known as celluloid. The next important polymer materials, introduced around 1897, were casein plastics, made from milk protein and formaldehyde. This was followed by phenol-formaldehyde resins in 1899, which were originally used to replace Ebonite in electrical insulation. The phenol-formaldehyde reaction system was extensively studied and by 1907 the reaction could be well-controlled to give products with the desired products. These phenolic resins became commercially successful and are still in use today. After the success of the phenolics came the use of urea and thiourea as a substitute for phenol. This products could be moulded in light colours, not just black as with phenolics. Some of these polymers are still in use today. The period 1930-1940 was when most of today’s important commodity plastics were developed commercially, including polyvinyl chloride (PVC), polyethylene, polysty- Fig.5: a range of conducting polymers showing their molecular structures. Note their pattern of alternating single and double bonds (by author). 18  Silicon Chip The author in his “white coat and safety glasses” days about to measure the temperature dependence of electrical conductivity in a polypyrrole sample (actual specimen not visible). A “four probe” method is used to measure conductivity to eliminate the effects of contact resistance. One vessel (left) contains liquid nitrogen and the other is an ice/water reference junction for the thermocouple. If the log of the conductivity of a specimen is plotted against temperature to the power of minus one quarter, that is evidence that electron conduction in the material occurs due to the hopping process as mentioned in the text (more specifically known as Mott Variable Range Hopping). siliconchip.com.au rene and poly-(methyl methacrylate) (PMMA - commonly known as Perspex). Research was extensively accelerated during World War II. History of organic conductors All of the plastics mentioned above are insulators, although in some applications some could be rendered slightly conductive by the addition of conducting powders or fibres such as carbon or metal. Organic conductors or semiconductors conduct electricity without the addition of a conducting medium. These conductors may be in the form of non-polymeric compounds or in the form of polymers. The first known electrically-conducting organic compound was made in 1862. The substance, later identified as the polymer polyaniline, was not put to use, probably because there were no identifiable uses at the time. Just as polyethylene is the archetypal non-conducting polymer with its simple structure, polyacetylene is the archetypal conducting polymer (see Fig.5). It was first discovered in 1958 but it was not until 1971 that a laboratory mistake led to a silvery, highly-conducting form being discovered. Then in 1977 it was discovered that the conductivity of this material could be greatly varied (by orders of magnitude) by varying the amount of iodine doping. That work led to the Nobel Prize in Chemistry for 2000. Polyacetylene was seen as an organic equivalent to traditional semiconductors such as silicon, which could also have their conductivity greatly varied by doping. The ability to vary or engineer electronic properties marked the beginning of the field of what is now known as organic electronics. Another early conducting polymer was polypyrrole. It was known to be able to conduct electricity since this property was discovered by an Australian group in 1963 when it was synthesised by a chemical method (see www. drproctor.com/os/weisspaper.pdf). It did not become widely studied until 1979, when it was synthesised by an electrochemical process, which made it simple to fabricate in film form and in reasonable quantities. In the most basic form of the electrochemical synthesis process you have two inert electrodes, an anode and cathode, a power supply and a vessel containing the chemical reactants. With the appropriate reactants, a polymer like polypyrrole will grow on the anode and is then peeled off. Incidentally, organic semiconductors are not unknown in nature. Carotene, a form of which gives carrots their orange colour, is also used in the photosynthesis process is an organic semiconductor. There are also substances in the retina which are organic semiconductors. Fig.5 also shows another related polymer, polythiophene. This will be discussed later. Electrical conductivity in organic semiconductors The diagram of the molecular structures of various conducting polymers shows that they all have a system of alternating double and single bonds, which allows electrons to easily move up and down the polymer chains. This allows conduction of electricity in one dimension, when the material is appropriately doped. If the matesiliconchip.com.au Organic electronic research in Australia Organic electronics is a diverse discipline requiring the input of chemists, physicists, materials scientists, engineers and many other specialists. In Australia there are over 100 researchers working in six universities and the CSIRO. There is no formal overriding organisational structure but there are high levels of informal cooperation. The main Australian research organisations are as follows: • Priority Research Centre for Organic Electronics (COE) at the University of Newcastle. www.newcastle. edu.au/research-and-innovation/centre/coe/about-us   See video “Centre for Organic Electronics - University    of Newcastle, Australia” https://youtu.be/kXQKz7YwvyM • The Intelligent Polymer Research Institute at the University of Wollongong http://ipri.uow.edu.au/index.html • Centre for Organic Photonics & Electronics at the University of Queensland www.physics.uq.edu.au/cope/ • The McNeill Research Group at Monash University http://users.monash.edu.au/~cmcneill/wordpress/ • The Ian Wark Institute at the University of South Australia www.unisa.edu.au/Research/Ian-Wark-Research-Institute/ • The Victorian Organic Solar Cell Consortium (VICOSC) www.energy.unimelb.edu.au/victorian-organic-solar-cellconsortium-vicosc of which The University of Melbourne is the lead partner. rial is crystalline, a band structure will be formed as described above. Many organic conductors are not as crystalline as metals or semiconductors like silicon and can display significant irregularity or disorder in their structures. In these types of materials there are not extended bands but a series of localised states which electrons “hop” between in order to conduct electricity. As can be imagined, when electrons have to “hop” from one place to the next instead of moving in a continuous band of energy, they are impeded and so efforts are directed to making more crystalline forms of these materials or otherwise minimising the energy barriers encountered by electrons. Poly(3-hexylthiophene) (a variant of polythiophene), is of particular interest in organic electronics because of high electron mobility (the ability of an electron to move within a structure) due a more ordered crystalline structure. Another organic molecule of interest for organic electronics is pentacene which has an extended long chain structure of alternating single and double bonds similar to but not a polymer. Organic electronic devices The development of organic electronic devices is proceeding quite rapidly and devices such as OLED (organic light emitting diode) displays, which were the first to be developed, have been widely commercialised. Because of the inherent flexibility of organic chemistry, it is possible to customise chemical groups attached to a polNovember 2015  19 An early, largely unrecognised organic electronic device from 1973. A bistable switch based on the skin pigment melanin. Fig.6: structure of a typical bi-layer OLED. 1) Cathode 2) Emissive layer 3) Emission of radiation 4) Conductive layer 5) Anode. Attribution: “OLED schematic” by Rafał Konieczny ymer chain and these groups might be designed to interact with certain molecules in the environment, if one was to engineer a device that was a chemical sensor, for example. Many organic electronic devices are amenable to being printed and can be produced in large sizes. The “Printegrated Circuit” Some readers may recall the “Printegrated Circuit”, an April Fool’s Day stunt in the April 1974 edition of “ Electronics Australia” by who else but Dick Smith. It was supposedly a printed electronic circuit which to be rendered functional merely had to be held up to the light to check it, then dipped in salt water. (Unfortunately, the gag fell a bit flat because the Printegrated Circuit was printed too dark, masking the words “April Fool” printed on the following page)! Who would have thought that around 40 years later we really would have working printable electronics? The first organic electronic device? While not widely acknowledged as such, possibly the first organic electronic device was made in 1973 in the form of a bistable switch was made using the natural skin pigment melanin which has a backbone in its structure like the conducting polymer polyacetylene. A bistable switch is a fundamental element of computers. The device is now held in “The Chip Collection” at the Smithsonian Institution (http://smithsonianchips.si.edu). Organic Light-Emitting Diodes (OLEDs) OLEDS are a form of light emitting diode which are based on organic materials instead of traditional semiconductors. The organic semiconductors they use are either based on small organic molecules or alternatively, polymers. Left: a commercially available 128x64 resolution blue monochrome display OLED with active display area of around 22mm x 11mm. Such a module can be bought online for around $8 – including delivery to Australia. Above: wearable flexible fully organic OLED display prototype by UK company Plastic Logic www.plasticlogic. com This device contains organic transistors and can display 256 grey levels at 30fps. 20  Silicon Chip siliconchip.com.au As most readers would be aware, the main application of OLEDs is in displays, as in cameras and smart phones. As they generate their own light and therefore do not require a back-light they are capable of displaying very deep black levels. They can also achieve higher contrast ratios than liquid crystal displays (LCDs) and are much thinner because they have fewer layers. Other advantages of OLEDs include lower cost than other display technologies because they can be printed by inkjet processes or roll-to-roll; are flexible and lightweight; wide viewing angles are possible; good power efficiency (no backlight) and a fast response time, much faster than LCDs. But a disadvantage of OLEDs is a relatively short life compared to other display types. Experiments with generating light using organic materials started in the 1950s but the first OLED based on small organic molecules was developed at Eastman Kodak in 1987, while the first device to use conducting polymers was developed at Cambridge University in 1990. The first commercial OLED product was in 1997 when Pioneer released a passive matrix display device for car audio systems. Then in 2007 Samsung released the first active matrix display device. In 2010 OSRAM released a lighting panel based on OLED technology. Build your own Organic Light Emitting Diode (OLED) While this has not been tested by SILICON CHIP, there are instructions available online to make your own OLED (that really works!). See website at http://education.mrsec.wisc.edu/nanolab/ oLED/index.html and http://education.mrsec.wisc.edu/299. htm Also see the associated video “Preparation Of An Organic Light Emitting Diode” https://youtu.be/9HIrapHr8C8 While sources for the component parts and chemicals are listed, we suggest you ensure that overseas suppliers will send them to Australia and check the cost to get them here! Organic Field-Effect Transistors (OFETs) Organic Field-Effect Transistors (OFETs) are Field Effect Transistors in which the channel is made from an organic semiconductor. The first OFET was fabricated in 1987, based on a thiophene polymer. These devices are intended for large area, low cost and possibly disposable applications, such as biological and chemical sensors. In 2007, Sony fabricated a video display in which both the transistors and the lightemitters were all organic. Organic Solar Panels Solar panels are an attractive option to be produced with organic electronic materials because of the ability to print them cheaply by high speed processes on thin supporting substrates. This would reduce the cost and weight compared to traditional solar panels which are heavy, inflexible and still rather expensive because of their extensive glass area and aluminium structures (and the inability to print them). Printed thin-film transistors for sensor applications from the Centre for Organic Electronics. siliconchip.com.au The University of Newcastle’s (Australia) Priority Research Centre for Organic Electronics (COE), led by Prof. Paul Dastoor, is working to produce such printed solar panels using high speed printing processes, similar to the process to print newspapers and magazines and with similar equipment. The cells printed in this way have a substrate layer to act as a support structure, a conducting electrode printed onto the substrate, an active layer a few tens of nanometres thick and then another electrode. Such cells currently have a 2% efficiency; much lower than silicon solar cells but research is under way to improve this. Low efficiency might still be acceptable if they can be made extremely cheap, as the cost to generate a certain amount of power would still be less; albeit more panel area would be required than conventional solar panels. The COE has done extensive cost modelling of these printed photovoltaics to determine the economics of large Representation of a commercially available OLED TV showing the fewer number of layers in the display compared to an LCD which enables it to be much thinner. In addition, the flexible nature of OLEDs makes it easier to fabricate a curved display which gives a sense of viewer immersion in the scene. (From Samsung.) November 2015  21 Fuji printer at Centre for Organic Electronics with multiple copies of printed organic transistors with closeup of printed transistor shown in inset. scale production of these cells. A cost figure of $8 per square meter has been suggested for these panels, so at that low cost they could be used anywhere, even in non-optimal locations. These panels can produce power at lower light levels than silicon panels, thus generating power for longer periods. An area of concern with organic solar panels and organic electronics in general is their long-term stability. While this problem needs to be overcome, traditional silicon solar cells outside of their hermetic packaging are not particularly stable either. The problems related to that would be familiar to anyone who has tried to make their own solar panels from the cheap silicon solar cell wafers available on ebay. Fig.7: printed organic thin-film transistors on flexible substrates by a Japanese group. SAM stands for self-assembled monolayer and PEN is polyethylene naphthalate. Note the relatively large size of the devices. (www.nature.com/articles/srep03947/figures/1) Effective encapsulation to exclude moisture and air is the most difficult part and it is vital to get it right. The economic model that Prof. Dastoor’s group has developed is based upon a panel with a 2% efficiency and a service life of two years; figures optimised to give the lowest possible cost. Contractors would replace the panels at regular intervals just as some commercial organisations pay specialised contractors to regularly replace their light globes. In fact, these low cost solar cells should be considered to be a similar paradigm to light globes which people already accept as having to be replaced from time to time. While longer service life might be desirable, it turns out that the extra cost of encapsulation to do that reduces efficiency and increases material costs. Since these panels Left: a printed, flexible organic solar panel at COE in operation. Below: a printed organic solar panel from COE in mounting frame, suitable for attachment to a building. 22  Silicon Chip siliconchip.com.au Stages in the printing of an organic solar cell at COE. are thin and flexible, replacement might even be as simple as disconnecting an old panel and adhering a new panel right on top of it. degradation are those that are exposed to the environment and the more sensitive layers are deep within the structure (shades of the human eye?) – see Fig.8. Inverted device structure organic solar cells Paint-on solar cells One approach to extending the service life of organic solar cells being researched at COE is to invert the structure of the solar cell so that those layers most resistant to In other work from the Centre for Organic Electronics, “paint-on” solar cells are under development. These would be made from three layers: an electrode layer, an active Organic solar cell panels at COE showing “bleaching” of active cell material due to degradation (light coloured strip areas on right side of right panel) compared to other protected areas of the panel. Research is under way to find suitable methods of preventing degradation of these cells. Work has been able to increase the half life of cells from a few hours to 14 days with a hope of increasing cell stability to 2-3 years which economic modelling shows to be a cost-effective life span. Another type of printed flexible solar cell fabricated at COE. siliconchip.com.au November 2015  23 Continuous production of rolls of printed flexible solar cells film at COE. Note the many metres of material coming out of the roll to roll printing machine. Bottle of “solar paint”, precursor chemicals and continuous rolls of printed solar panels at COE in the background. layer and another electrode layer. These could be sprayed onto a building surface as a conformal coating using traditional spray painting equipment. See videos: “Prof. Paul Dastoor’s Solar Paint Technology – ABC New Inventors – Whole Part” https://youtu. be/-O39o_ERtbg and “Solar Power Generated with Paint” http://on.aol.com/video/solar-power-generated-withpaint-517483009 transistors. For video see “Electronic Skin” https://youtu. be/4oqf--GMNrA Pain-free testing for diabetics using bio-sensors Every diabetic will tell you the procedure they hate the most (even more than insulin injections) is the regular, painful “finger prick” to obtain a tiny blood sample to check their blood glucose levels. Even though glucose is also present in saliva, the concentration is about 100 times less than the blood and current equipment cannot easily measure that. At the COE a sensitive glucose sensor has been fabricated in the form of an organic thin film transistor (OTFT). Electronic skins An “electronic skin” or e-skin is under development by Japanese researchers, for medical monitoring or alternatively, as a sensor skin for artificial limbs to monitor pressure etc. The electronics within this skin are flexible and stretchable and it contains printed thin film organic Fabrication methods for organic electronic materials Organic electronic devices can be synthesised and fabricated by a variety of methods such as traditional chemical reaction, electrochemical polymerisation, chemical vapour deposition, printing with ink jet technology or printing by a roll-to-roll process. Another consideration with fabricating organic electronics is the electrode material. Some organic electronic devices require transparent conducting electrodes. In traditional devices, indium tin oxide (ITO) is used however the price of ITO is increasing due to high demand, so alternative transparent conducting electrodes are being sought. Graphene is one possibility for such an electrode material (see article on graphene in SILICON CHIP, September 2013). Conclusion Organic electronics is still in its infancy but expanding rapidly. As more and more items incorporate electronics and sensors, there will an increasing demand for flexible and low-cost devices, including devices cheap enough to be disposable. The ability to use low-cost printing processes means (Fig.8, below): organic solar cell at COE with inverted structure to provide increased longevity. (Right): freshly printed glucose sensor for use by diabetics. In an actual device the support film would be cut away as the device is much smaller than the support sheet. The beauty of this device is that it is sensitive to glucose in the saliva which is at a concentration of about 100 times less than in the blood. 24  Silicon Chip siliconchip.com.au Electronic skin by the Someya Group Organic Transistor Lab in Tokyo. Described as the world’s lightest and thinnest flexible sensor system, this stick-on device is designed as a wearable health monitoring system. large numbers of devices, or large areas of devices such as solar panels, can be made cheaply. The high levels of customisation possible in organic electronics will also enable devices to be made with very specific capabilities such as sensors to detect specific chemical or biological substances. Dr Ben Vaughan (Australian National Fabrication Facility, [ANFF]) with the surface analysis facility at the Centre for Organic Electronics, University of Newcastle. Traditional semiconductors like silicon and gallium arsenide will likely continue to be used for ultra-high speed devices but organics will find particular niches such as those requiring large scale, low cost, flexibility or disposability, SC as well as mainstream uses such as displays. A combination of small, intermediate and roll to roll (R2R) scale equipment at the Centre for Organic electronics. (a) Dimatix ink jet printer, (b) custom-built single head automated slot dye and blade coater, (c) R2R solar coating line from Grafisk Maskinfabrik and (d) R2R sputter coating unit from Semicore Inc. The middle images show samples fabricated with equipment (a)-(d) in order from top to bottom. siliconchip.com.au November 2015  25