By Jeremy Ward and Oana Jurchescu
Access to personal electronic devices, such as laptops and mobile phones, has revolutionized the way we work and live. This fundamental transformation to an ever more connected and mobile society was made possible by the optimization and miniaturization of electronic devices composed of silicon, which is at the heart of traditional modern electronics.
However, silicon’s processing relies on high temperatures and vacuum environments, which makes its applications expensive and compatible only with rigid substrates (e.g., glass). These limitations are motivating the development of the next generation of electronics. The vast array of applications predicted to emerge from organic electronics includes flexible and rollable displays, electronic newspapers, smart bandages, conformal solar cells and batteries, disposable single-use sensors, and electronics embedded into clothes.
Such applications are not only new―and unavailable using current materials and technologies―but they come at a fraction of the cost because processing carbon-based (organic) materials is not restricted to complex evaporation processes. On the contrary, processing organic electronic materials takes place from solution inks at high speed and in large volumes using ink-jet printing, roll-to-roll coating, or spray-deposition. Groundbreaking discoveries fueled a spectacular growth in this field and enabled first-generation products. Organic light-emitting diodes (OLEDs) have already grown into a mature technology, which entered the market several years ago. Organic photovoltaics (OPVs) and organic field-effect transistors (OFETs) are rapidly catching up.
The extraordinary versatility of carbon chemistry enables an infinite variety of modifications to generate diverse optical and electronic properties: organic materials range from conductors such as Tetrathiafulvalene 7,7,8,8-Tetracyanoquinodimethane (TTF-TCNQ) to semiconductors (e.g., pentacene and polyhexithiophene) and insulators (e.g., benzocyclobutene and CYTOP). Organic semiconductors are available in two primary varieties: conjugated polymers and conjugated small molecules. Several small-molecule organic semiconductors have matched the performance of amorphous silicon (a-Si), yet these materials have low solubility, which precludes their low-cost manufacture.
The first solution-processing of organic small-molecule semiconductors was developed in 1999 by P. Herwig and K. Müllen. The process consisted of spin-coating the solution of a soluble precursor, followed by thermal treatment to convert to an organic semiconductor. This idea was further developed by J. Anthony, who engineered a variety of organic small-molecule semiconductors that were stable not only in air, but also in several solvents. The onset of such materials has led to the viability of fabricating organic devices with electronic properties compatible with many perceived applications, without compromising the ease of processing.
The discovery of materials that are stable in solution brings forth a vast array of deposition techniques. Ink-jet printing, screen printing, spin casting, doctor blading, and spray coating are among the novel deposition techniques available for soluble organic semiconductors.
While the organic semiconductors are in solution, they remain mobile and maintain the ability to reorganize. Recent results have shown that the molecules within solutions can “see” the surface the fluid is in contact with and anchor themselves accordingly. On surfaces that promote certain interactions with the semiconductor, highly ordered films can be developed. In contrast, when deposited on less-compatible surfaces, where interactions are inhibited, films of a mixture of orientations form and the overall performance is less desirable.
This concept has been used to pattern many devices, effectively isolating individual field-effect transistors (FETs) to prevent “cross-talk” between neighboring devices. By comparison, current techniques to accomplish patterning in a-Si devices require numerous steps that include harsh chemicals and vacuum-processing environments, further adding to production costs. The ability to judiciously control the molecules at the nanometer scale during the film formation process is unique to solution deposition and presents opportunities for new techniques for fabrication and miniaturization of organic devices. Through material engineering and a thorough understanding of the impact that processing has on device performance, organic electronics applications will continue to grow and produce many new opportunities, often foreign to silicon-based technologies.
Jeremy Ward is a physics doctoral student at Wake Forest University in Winston-Salem, NC, where he performs research in the Organic Electronics Group. His research focuses on interface interactions within organic devices and understanding their impact on film structure and performance. He has been a Graduate Research Fellow of the National Science Foundation since 2012.
Oana Jurchescu, a Wake Forest University physics professor since 2009, is especially interested in organic materials and devices for nano and macroelectronics. She earned her PhD from the from the University of Groningen, The Netherlands. She received the National Science Foundation CAREER award, ORAU Ralph E. Powe Junior Faculty Enhancement award, and the Reid-Doyle Prize for Excellence in Teaching.
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