This is the Second Edition of the Advanced Materials Organic Electronics Special Issues, focussing on a review and perspective of the progress, challenges, and opportunities of the research driving the current and future applications for this field. We have solicited a broad range of invited contributions that highlight the excitement and diversity of this multidisciplinary area. We showcase the spectrum of research and development offering a state-of-the-art review and current status.
The field of organic electronics has recently experienced hugely impressive progress in the performance of both materials and devices. For example, it is now highly conceivable that transistors fabricated using organic semiconductors can be employed to drive even high-resolution OLED pixels. Both small-molecule and even polymer semiconductor thin films, processed from solution, have demonstrated charge carrier mobilities in excess of 5 cm2V−1s−1 in the last year or so. This is particularly remarkable in the case of polymer semiconductors, where the conventional paradigm of thin-film microstructures comprising highly crystalline π-stacked lamellar domains, optimally oriented and co-aligned with respect to the transport direction, has been challenged by new classes of less-ordered, dipolar polymers. In this case a one-dimensional transport mode along the polymer backbone is speculated to be preferred, leading to very high charge-carrier mobilities. An excellent example of this is a series of polymers based on the diketopyrrolopyrrole repeat unit, as reviewed in this special edition. Here the off axis dipole may facilitate close intermolecular interactions, while side chain functionality ensures the polymers have sufficient solubility for processing.
The combination of polymer conformation modelling, synthesis, and microstructure characterisation has been compellingly employed on the morphology and charge transport properties of a well-known donor-acceptor copolymer, cyclopentabithiophene-co-benzothiadiazole (CDT-BTZ). The supramolecular structure of CDT-BTZ was modeled and compared with grazing incidence wide angle X-ray scattering (GIWAXS) measurements of thin films from the synthesised polymer. Subtle but important differences in the intermolecular packing as well as the side chain orientation arrangements were revealed and discussed, with significant implications for general semiconducting polymer molecular design principles. The concept of molecular and longer length-scale dimensionality and its critical influence on the charge transport properties of semiconducting materials is explicitly explored by Skabara, Garlin, and Geerts in this issue. Drawing from examples illustrating non-covalent intermolecular bonding and the role of molecular conformations in achieving higher order architectures, it is clear to see the influence on electrical properties. The choice of dielectric layer employed in the transistor architecture remains essential to high device performance.
One exciting class of dielectric materials that have been demonstrated, particularly in low voltage applications, are electrolytes and have been reviewed in this edition by Frisbie and co-workers. It is also clear that conventional materials design and multistep, batch synthetic processing may not ultimately be feasible or scalable for the next generation of organic electronics devices and applications. With this in mind, the report by deMello and Nightingale demonstrates the possibility of new nanomaterials production from segmented flow microreactors. These systems, where the reagent phase is dispersed in an immiscible fluid non-solvent, have potential applications in the controlled synthesis of nanocrystals, and the challenges associated with mainstream production are addressed.
Organic photovoltaic devices continue to improve rapidly, with efficiencies of single cell devices now routinely reported in excess of 7% and up to 10% in some instances. Janssen and Nelson dissect the fundamental theoretical limitations of organic solar cells, and evaluate the empirical approaches from both a thermodynamic and kinetic perspective that are employed to estimate the maximum efficiencies that can be expected to be achieved in practical devices. They optimistically suggest that limits of 20–24% may be theoretically reachable in single junction cells, thus raising the bar considerably for materials performance expectations. Measurements of transient current or voltage in dye sensitized solar cells following an optical perturbation gives useful information about charge concentration, transport and recombination in the devices. The theory and practice of a wide range of these measurements illustrated with numerous examples in the report by Barnes, O'Regan, and co-workers in this issue.
The use of cutting edge metrology accelerates improvements in devices. For example, the observation of phase separation and crystallization during the spin-coating process of the bulk heterojunction layer of an organic solar cell has now been demonstrated by Amassian and co-workers in this issue. In particular, the role of polymer crystallization in driving phase separation during film formation is evaluated. Understanding the photophysical processes that determine charge separation and polaron pair recombination in organic solar cells is critical to optimize free carrier formation and boost device efficiency. A copper phthalocyanine small-molecule-based system was studied by Monkman and co-workers and concluded that intersystem crossing is the rate-limiting step for polaron-pair recombination and is driven by spin-orbit coupling.