Electronics Go Organic

Electronics Go Organic

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engineering + technology

by Colin Reese

magine rolling your PDA out of a pen, having a sheet of paper containing all the information within the Library of Congress, or tucking your laptop under your arm like a newspaper. Sound impossible? According to researchers in Stanford’s advanced materials laboratories, these technologies may be closer than you think. A rapidly developing class of ‘organic’ technologies promises to enable a host of applications never before thought possible. While the field is in its infancy, the demonstration of cutting-edge devices, such as flexible circuits and displays, using simple production methods have transformed it into a showcase for materials research. The potential for incorporation into a variety of high-profile devices – including photovoltaic cells, light-emitting diodes, RFID tags, sensors and non-volatile memory – has many seeing green.

Pesticide-free processors? What differentiates “organic” technologies from their traditional “inorganic” counterparts are the active electronic materials that make them work. Compared to traditional semiconductors, such as silicon and gallium-arsenide, organic materials are very different in terms of chemical makeup and solid-state nanostructure. Elemental semiconductors such as silicon are crystalline, possessing a well-ordered structure held together by strong, covalent bonds. Organic materials, on the other hand, are comprised of collections of atoms grouped into molecules, which are held together loosely by weak, longrange van der Waals forces. The organic solids can vary from crystalline to completely amorphous, depending on both chemical composition and preparation. Their distinguishing feature – and for which they bear the name “organic” – is that they generally contain hydrocarbons. While most are a far cry from Webster’s “of animal or plant origin,” organic building blocks are much closer to biology than the materials in current solid-state electronics, such as boron, gallium and arsenic.

We don’t eat electronics. Who cares? Integrated circuits, computer processors, memory and solid-state electronics launched the information age. These components are all built atop silicon technologies, which have been incredibly well-developed and characterized. The next technology node allows patterning features in silicon as small as 45 nanometers, enabling billions of logic elements to be crammed onto each processor chip. So why develop a whole new set of organic technologies if silicon meets the industry’s needs? The answer depends on who you ask, but the following are a few

Single crystals of organic electronic materials grown in the lab of Zhenan Bao in Chemical Engineering. These crystals offer the ultimate in performance, and are an ideal tool for the study of intrinsic transport in organic materials.

key reasons device and material designers have set their sights on exploiting organic materials for electronics: • Easier Processing Current solid-state circuitry requires high temperatures, pressures, and otherwise harsh processing conditions. Organic materials, however, can be processed from solution, which offers the possibility of deposition on unconventional substrates, such as flexible plastics that cannot tolerate the rigors of silicon processing. Solution-based fabrication methods, such as inkjet printing, are envisioned as a high-throughput solution for rollto-roll fabrication of organic circuits, displays and photovoltaic cells. • Lower Cost Processing advantages are not only desirable for their simplicity and flexibility, but also for their associated low cost. High throughput and the elimination of complex vacuum

Organic electronics are not meant to compete with silicon devices in the same applications. They will likely be used for niche applications where low cost, flexibility and light weight are desirable. layout design:Jason Shen

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equipment targets the most costly steps of silicon processing. In addition, while the source material for silicon – sand – is hardly a scarcity, the wafers used to make processor chips must be extensively purified, implanted with ions, oxidized and polished

Seeing Green at the Farm Research in organic electronics is highly interdisciplinary. From material design and synthesis to device design, application integration and optimization, experts from the fields of chemistry, chemical engineering, electrical engineering, applied physics and material science are all on board to make organic technologies viable. Stanford, with its emphasis on such collaboration and large number of shared resources and research centers, is in a unique position to capitalize on this rapidly growing field. Professor Zhenan Bao, Department of Chemical Engineering, hosts a number of projects aimed at developing high-performance organic semiconductors and incorporating them into basic devices such as transistors and inverters. Many of her students and researchers are devoted to chemical synthesis of the next class of engineered materials for circuits, photovoltaics, and optics. The development of novel materials will not only boost device performance but also develop the fundamental material/ property relationships necessary for optimized design. Single crystals of these materials allow researchers to explore the upper limit for material performance and to uncover the underlying transport mechanisms. These single crystals are characterized using the transistor, a key logic unit that serves as a metric for material performance. The physics of devices are also studied to optimize their integration. Bao’s group is also working on various patterning techniques. The goal is to allow large-area solution deposition of semiconductors, nanowires and other circuitry elements, which

One of the most exciting prospects of organic electronics is the rational design of materials for specific applications. before they are suitable for fabrication. Organic materials, on the other hand, are generally products of chemical synthesis from much smaller, cheaper, commercially available building blocks. Furthermore, while silicon needs to be extremely pure (and is the purest material available at 99.999999999%), organic materials are not anticipated to require a similar level of purity for efficient device operation. • Functionality By Design Elemental semiconductors possess particular electronic properties, which can then be altered by “doping” them with ions such as boron and arsenic. While doping changes properties important for device operation, it does not change characteristics, such as optical properties, which are specific to a given material. There is, therefore, a fundamental limitation on their use in many types of devices, such as light-emitting diodes and photovoltaic cells, where differences in these properties are crucial to operation. In other words, inorganic semiconductors are limited by how elements arrange themselves into the solid phase. Organics, on the other hand, are limited only by a chemist’s imagination and the synthetic techniques available. As illustrated by the recent explosion in organic semiconductor development, both of these aspects are vast. One of the most exciting prospects of organic electronics is the rational design of materials for specific applications. Although the engineering of specific characteristics and the development of structure/property relationships is still primitive, a large toolbox of organic materials is already being employed in organic devices.

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would be a significant step toward large-scale production of these devices. The basic concept of these techniques is the harnessing of chemical and morphological selectivity to effect preferential deposition of organic materials. These projects offer a look toward hands-free nanoscale circuit assembly.

Organic circuit material, such as nanowires and semiconductors, may be patterned using chemical and morphological selectivity. These optical micrographs of crystalline organics deposited by various means display the potential of these techniques, and suggest that the self-assembly of nanocircuits is indeed possible

Professor Mike McGehee, Department of Materials Science, focuses on the development of highperformance organic solar cells. Employing solution-processible organic materials, his students are exploring new fabrication techniques and device architectures in order to produce high-efficiency cells that would offer a cost-effective alternative energy source. Organics have already surpassed the performance of amorphous silicon and are rapidly improving. Professor Peter Peumans, DepartmentofElectricalEngineering, is exploring two cutting-edge areas for organics: light-emitting diodes (LEDs) and molecular electronics. LEDs harness the tunable properties of organic materials to produce highintensity light, and are anticipated to replace conventional household lighting in the near future. Molecular electronics embody the ultimate scaling of functional materials. As technology advancements drive device size toward the nanometer regime, the critical dimensions will approach that of single molecules. The concept of molecular electronics is realized as a device – a transistor, for example – that consists of a single, semiconducting molecule.

engineering These images showcase a few of the many exciting applications that organic electronics have to offer: a. + Flexible, microscale transistors patterned on a plastic substrate. b. The first organic SRAM module, capable technology of storing 16kb in a small, plastic package, produced by Epson. c. A low-cost black and white display technology, pioneered by E-Ink, seeks to capitalize on markets for flexible newspapers and books, where high frame rate is not required. d. A flexible display developed in joint collaboration between the Army and Universal Display Corporation showcases one of the most exciting applications envisioned for organic circuits -- roll-up displays.

Outlook on Organics Organics outshine their inorganic counterparts in simple deposition techniques, low cost and tunable properties. Despite all of these advantages, experts in the field agree that organics will supplement, not supplant, inorganic materials and technologies. While the increases in performance are dramatic, organics are not yet on the same playing field as silicon as far as absolute speed is concerned. “Organic electronics are not meant to compete with silicon devices in the same applications. They will likely be used for niche applications where low cost, flexibility and light weight are desirable,” Bao says. Organic electronics therefore offer the prospect of improving current technologies, as well as the development of those that were previously impossible. In the future, Bao expects to see dramatic improvement in device performance with organic materials. She also expects more demonstrations of new products with organic electronics. “Last year, Samsung Electronics already demonstrated a 17” SXGA LCD display with OTFT backplane,” she says. “In the next few years, I expect to see such displays demonstrated on plastic substrates.” While you may not expect the next AMD CPU to be named the Orgon, flexible displays driven by organics may be just around the corner. S

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Colin Reese is a third-year graduate student in the group of Zhenan Bao in the Department of Chemical Engineering, performing research in the area of organic single crystals and charge transport. He enjoys mountain biking, playing guitar, and gardening.

To Learn More: More information on current research at Stanford in the area of organic electronics may be found at the Bao group website: http://baogroup. stanford.edu, Peumans group website and home of the Stanford Organic Electronics Lab: http://peumans-pc.stanford.edu/people/peterpeumans, and at the McGehee group web site: http://mse.stanford.edu/ faculty/mcgehee.html. There are many excellent reviews on the exciting prospects of organic electronics: C.D. Dimitrakopoulos and P.R.L. Malenfant. “Organic Thin Film Transistors for Large Area Electronics” Adv. Mater., 14, No. 2, (January 16 2002) “The path to ubiquitous and low-cost organic electronic appliances on plastic.” Stephen R. Forrest. Nature 428, 911-918 (29 April 2004) “Organic thin film transistors.” Reese, C; Roberts, M; Ling, M-M; Bao, Z Materials Today. Vol. 7, no. 9, pp. 20-27. (Sept. 2004)

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