This is a special issue dedicated to the fascinating field of flexible electronics, and it stems from one of the Symposia held at the European Materials Research Society (E-MRS) Spring Meeting from May 14th to 18th 2012 in Strasbourg (France): Symposium H “Organic and Hybrid Materials for Flexible Electronics: Properties and Applications”. The symposium aimed at bringing together key researchers in the above mentioned field in order to stimulate discussions regarding the main challenges still to be faced towards the widespread application of flexible electronics, with contributions covering both fundamental properties of novel semiconducting organic and hybrid materials and their efficient integration in functional devices mainly by means of, but not limited to, solution-based processes. The symposium was organized by Dr. Mario Caironi (Istituto Italiano di Tecnologia, Milan, Italy), Prof. Thomas D. Anthopoulos (Imperial College London, U.K.), Prof. Jana Zaumseil (Friedrich-Alexander-Universität, Erlangen, Germany) and Prof. Yong-Young Noh (Dongguk University, Seoul, Korea). Seventeen renown invited speakers, the main contributors to this special issue, introduced an equivalent number of topical sessions, covering synthesis of novel organic compounds, morphology control and characterization, interfaces, charge transport and photophysical investigations, molecular electronics, solution-processed transistors and integrated circuits, light-emitting organic devices, hybrid opto-electronic devices, comprising photovoltaics, and printing processes.
The timeliness and relevance of this issue derives from the fact that as the number of functional materials that can be processed at low temperature and/or from solution is continuously expanding, flexible electronics widens their possibilities for an increasing range of novel applications: from low-cost microelectronics, portable biomedical applications and conformable chemical/physical sensors, to lightweight active matrix mobile displays and solar cells. Conjugated organic materials, which have inspired researchers to develop the first “soft” solid-state electronics and led to the commercialization of organic light emitting diode (OLEDs) displays, are still playing an important role in advancing this field. For instance, the dramatic improvement of key device performance metrics such as charge carriers mobility in organic transistors and power conversion efficiencies in organic solar cells over time, provide a clear example of the great opportunities, which are still to be fully explored and potentially deployed in real-life optoelectronic applications. Even more exciting is the possible merging of the field of organic semiconductors with solution processable inorganic systems that include, but not limited to, metal-oxides, quantum dots, nanocrystals, carbon nanotubes and even graphene. The combination of these systems could enable further improvement in performance of a wide range of opto-electronic devices. Such developments are already taking place and affect many applications, examples of which include organic solar cells where precursors or nano-particles based metal conductors, metal-oxides and carbon based semiconductors, are combined to improve the overall device efficiency. It is common practice to refer to this emerging approach and the resulting devices as “hybrid” electronics.
There is no doubt that in parallel to the development of next generation functional materials, deposition and patterning techniques and for manufacturing processes will act as enablers in exploring and deploying the full potential of hybrid flexible electronics. Such systems would require, depending on the application, precise and reliable tools capable of controlling layer thicknesses, lateral features and material interfaces over large-areas to achieve the figure of merits required for real applications (charge carrier mobility in printed transistors, light emission efficiency in OLEDs, and quantum yield in detectors etc.). To this end, the recent development of advanced, yet simple, printing techniques and “smart” device architectures, which can cope with limitations imposed by the deposition and patterning tools, are fostering the transfer of lab-scale devices, produced in a handicraft fashion by highly skilled students and researchers, to the reproducible manufacturing of large-volume, large-area products.
The contributions collected in this special issue aim to cover both fundamental and applicative aspects in the field of flexible electronics, from which clear trends emerge. For example, the critical role of interfaces, dictating and dominating key opto-electronic processes at the boundary between two domains of the same organic material (“grain boundaries”) or between two different organic systems, has become obvious in many fundamental studies. This is even more evident in hybrid systems where organic and inorganic materials, with different electronic structures, come into contact (“hybrid interfaces”). It is therefore expected that an improved understanding of the fundamental processes occurring at such interfaces would enable us to control, and potentially manipulate, their formation and allow for the next generation of opto-electronic devices to be developed.
From an application point of view the possibility of flexible electronic devices and systems to penetrate markets will be primarily dictated by the ability to mass-manufacture with tools (e.g. printing) capable of depositing and patterning electronic materials, devices and systems, with high uniformity over large area substrates. The availability of such tools will undoubtedly play a catalytic role, as it would enable reliable manufacturing of low-cost, high-volume ubiquitous plastic electronics for numerous applications to be developed. In addition to manufacturing, material stability, which has improved greatly in the recent years - for example in the case of electron-transporting (n-type) polymer semiconductors and device based upon them–may also dictate the fate of many of these fascinating envisioned applications of flexible electronics. If such materials, devices and production methods can be developed and successfully combined, the promise of flexible electronics could well be within reach in the near future: the “flexible revolution”, with all its exciting scientific and technological opportunities, appears to be just around the corner.