Organic materials for printed electronics

• Organic materials can offer a low-cost alternative for printed electronics and flexible displays. However, research in these systems must exploit the differences — via molecular-level control of functionality — compared with inorganic electronics if they are to become commercially viable. Introduction Conducting and semiconducting organic materials, both polymers and molecules, are being considered for a vast array of electronic applications. The first examples, such as displays in mobile appliances, have found their way to market as replacements for traditional components in existing products. Organic electronics distinguishes itself from traditional electronics because one can define functionality at the molecular level, process the materials from solution, and make displays and circuits that are completely flexible. So far, very little of the uniqueness of organic electronics is expressed in the products promoted as manufacturable; why? One important opportunity for organic electronics is the area of radiofrequency identification (RFID) manufactured using an all-in-line printing process. This technology comprises fast-switching transistors, antennas operating at frequencies above 100 kHz, memory, and so on, all integrated into a plastic foil. The present target in many organic electronics labs around the world is to develop the high-speed (>10 kHz) transistors critical for such devices. The use of organic transistors instead of their inorganic equivalents is motivated by cost. So far, little effort has been devoted to exploring organic electronics in terms of its true unique electronic functionality and the possibility to add electronics to surfaces previously considered electronically inactive. For instance, paper is produced at speeds exceeding 100 km h-1 and is converted into packages and printed media at manufacturing flows typically above 100 m min-1. Adding organic electronics onto, for instance, the paper surface during the paper conversion process would demonstrate the true uniqueness of organic electronics, both from a manufacturing and an application point of view. Retail chains and transportation companies desperately seek a printed electronic technology to provide better safety and security features on packages and automatically track and trace products all the way from the manufacturer to the end customer. The financial losses related to counterfeiting, failure in transportation and damaged packages is comparable to the overall profits made on the product contained in the package. In addition, printed electronics could potentially guide the end-user to properly use the product and to guarantee brand authenticity, for example through an interactive user's guide, and electronic features to replace existing security devices such as the holographic stickers commonly used in packages and bank notes today. It turns out that, for many of these applications, high-frequency signal-processing is not required; 10 ms to 1 s response times are appropriate. These are goals that a very simple printed electronics technology may be able to fill. Silicon-based RFID devices are currently used in high-end products, but are prohibitively expensive for commodities such as food at the consumer package level. Thus, the potential value for printed organic electronics seems to exist if the expense can be kept down. For instance, TetraPak, who produces more than 100 billion packages every year, estimates that the costs for additional security and safety features cannot exceed about 0.2 Eurocents per package (Istvan Ulvros, TetraPak, private communication). Much of the research in organic electronics aims to optimise inherent charge transport and efficiency characteristics of the materials already in use in individual devices. This work has pushed the solar energy-to-electricity power-conversion efficiency in organic solar cells close to 5% (ref. 1) and the luminous efficiency of plastic luminescent devices to around 25 cd A-1 (ref. 2). Organic electrochromic displays now perform extraordinarily well in terms of colour contrast, memory and stability3, and polymer transistors easily run at speeds beyond 100 kHz (ref.4). These results have been achieved by improving the performance at the individual device level. Rarely are integrated circuits or high-volume manufacturing conditions considered in the research. Typically, a series of more than ten patterning, material deposition and post-processing steps are required to make one kind of device. The tradition has been to develop specific materials that exclusively function well in only one device type. RFID circuits (for example) typically require rectifiers, antennas, powering devices, transistors for signal processing, encapsulation layers and in some cases also displays. Merging today's efforts conducted at the organic electronics device level would then result in a production route that would include perhaps 50 (or even more) discrete manufacturing steps. Unfortunately, the cost for a label requiring several tens of patterning steps including exotic organic electronic materials is not compatible with the value and costs of packages. In traditional printers, typically five to ten printing stations are available in series (Fig. 1). Each station also includes one or two convection, infrared or ultraviolet curing steps. At ordinary printing speeds (10 to 200 m min-1) the substrate spends on the order of a tenth to several seconds in each printing station. During this time, registration, material deposition and post-processing must take place. The value structure in printing technology means that the cost for printing scales at least linearly with the number of printing steps. The yield and systematic errors in printing technology becomes a nightmare beyond ten printing steps. The cost for materials such as inks, substrates and coatings is a considerable part of the entire product value. Our own calculations indicate that each individual RFID label would cost more than 10 Eurocents (Lars-Olov Hennerdal, Acreo, private communication).

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