Organic semiconductors: Healing contact.


Electronic devices based on π-conjugated molecular or polymer semiconductors have moved out of the realm of academic curiosity and are now regarded as high-value commercial technologies. Displays derived from organic light-emitting diodes have made a tremendous impact in mobile devices and are now being brought to the television market; moreover, organic photovoltaics and field-effect transistors (OFET) are also starting to show promise. However, the presence of chemical and physical imperfections — such as chemical defects and reaction byproducts, environmental impurities, or energetic and structural disorder — still hampers the performance of these soft materials and limits the understanding of their intrinsic electronic and optical properties. Because these imperfections usually escape precise characterization, they generally go by one name: traps. Writing in Nature Materials, Lee and co-workers1 demonstrate that the simple application of a polymer oil based on a perfluoropolyether (PFPE) to the surface of an organic semiconducting crystal markedly reduces the influence of traps and results in the ability to measure the chargecarrier transport properties of π-conjugated materials with high accuracy. The efficiency of hole or electron transport in π-conjugated organic semiconductors is dependent on the conduction pathways present in these materials and on the existence of scattering centres and traps. In such systems, where non-covalent interactions between the organic units drive structural packing, the strength of the electronic communication (electronic coupling) among neighbouring molecules or polymer chains, and the interactions between charge carriers and vibrations play crucial roles in defining the charge-carrier mobilities. Current understanding suggests two extremes to describe how charges move through these media. In the case where electronic coupling is weak, the charge carriers (often referred to as polarons) localize on a molecule or chain segment and diffuse through the material by means of a series of incoherent hops assisted by electron–vibration interactions. When the electronic coupling is strong and significantly larger than the electron– vibration interactions, the charges move through delocalized electronic states (that is, band regime), in a fashion similar to what is known in inorganic semiconductors. Numerous experimental studies, including angle-resolved ultraviolet photoelectron spectroscopy2, electron spin resonance3 and Hall effect measurements4, as well as theoretical developments5 have recently increased our understanding of how thinfilm or crystalline packing impacts the charge-carrier transport characteristics. However, realizing trap-free, perfectly ordered materials where the intrinsic transport properties can be measured remains a difficult task. It is in this context that the work of Lee et al.1 provides an important step forward. These researchers investigated the hole mobility in single crystals of rubrene and tetracene, two of the highest-performing hole transporters in OFETs. They deposited on the surface of the crystals a thin layer of a non-conjugated polymer, PFPE — an inert, insulating lubricant typically used in strongly oxidizing or reactive environments ORGANIC SEMICONDUCTORS

DOI: 10.1038/nmat3808

1 Figure or Table

Cite this paper

@article{Risko2013OrganicSH, title={Organic semiconductors: Healing contact.}, author={Chad Risko and Jean-Luc Br{\'e}das}, journal={Nature materials}, year={2013}, volume={12 12}, pages={1084-5} }