Themed issue on small molecules and monodisperse oligomers for organic electronics

Guillermo Bazan

Guillermo Bazan obtained his BSc from the University of Ottawa and his PhD from MIT in Inorganic Chemistry working with Professor Richard Schrock. After a postdoctoral appointment at Caltech with Professor John Bercaw, he joined the Department of Chemistry at the University of Rochester in 1992. His move to UCSB came in 1998, where he is now Professor in the Department of Chemistry & Biochemistry and in the Department of Materials. His research interests are in the design and application of conjugated materials. His citations place him in the top 50 materials scientists, as per Thompson and Reuters.

Martin R. Bryce

Martin Bryce received a BSc from Wolverhampton Polytechnic and a doctorate from York University, U.K. After postdoctoral work at the Universities of British Columbia and Bristol, he moved to Durham University where since 1995 he has been a Professor in the Department of Chemistry. He has received several Royal Society of Chemistry awards and he has held visiting scientist positions at universities in the U.S.A. and Europe. His research activities cover the design, synthesis, characterization and applications of organic optoelectronic materials – small molecules, oligomers and polymers.

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Our fascination with molecular species with an electronically delocalised structure dates back several millennia. Tyrian purple, which can be obtained from Mediterranean molluscs and contains a dibromoindigo core, was utilized by the Phoenicians as a much prized cloth dye. Its color came to be associated with imperial courts and nobility.¹ The synthesis of aniline purple (mauveine) by W. H. Perkin² played a critical role in establishing the industrial basis for dye production and stimulated intense international competition to produce other synthetic dyes.³

Compounds with electronically delocalized structures constitute the basis of organic electronics,⁴ which has been a major focus in the fields of chemistry and physics for more than 50 years, initially leading to liquid crystal displays in the 1970s and the first practical demonstration of thin film light emitting diodes in the mid-1980s. The interdisciplinary topic has been driven by the appeal of systematically modifying chemical structures in ways that fundamentally change the materials' electronic properties. During the last decade the practical applications of organic electronics have expanded in leaps and bounds. Organic electronics are now competitive with traditional technologies and are contributing to the needs of industry, the consumer market and society in fields such as semiconductor devices, display technologies, solid-state lighting, photovoltaics, information processing, data storage and printed electronics. Nonetheless, there are still many fundamental issues and challenges, with new materials chemistry at the heart of some of the most exciting recent advances. This themed issue focuses on small molecules and oligomers that are relevant to, or function as, the active components in contemporary organic electronic devices. This collection of articles is a snap-shot of the types of materials and the experimental and theoretical approaches that are currently at the forefront of the field. Many of the articles were submitted by invitation and the issue draws together contributions from experts in a range of disciplines.

The appeal of small molecules and oligomers for electronic applications is the high degree of structural homogeneity and the high level of purity and batch-to-batch reproducibility that can be obtained, facilitating physical characterization and allowing precise structure–activity relationships to be established. The electronic and structural properties of well-defined oligomers frequently provide valuable insights into the corresponding polymers, for example to probe their mean conjugation length or their band gaps and to refine synthetic strategies.⁵

This themed issue spans studies on the molecular engineering, synthesis, characterization, theory and device applications of a range of materials, notably organic photovoltaic cells, organic light-emitting diodes and thin film transistors. The optoelectronic and transport properties of a wide range of molecules are explored in considerable depth. For solar cells, the larger more complex molecular systems that afford the highest conversion efficiencies can be very demanding to synthesise. An acceptable trade-off between conversion efficiency, scalability and cost is needed to facilitate the development of photovoltaic materials to compete with amorphous silicon for large-scale production. Key building blocks reported in this issue for bulk heterojunction or dye sensitized photovoltaic cells include dithienyldiketopyrrolopyrrole, N-annulated perylene functionalized cyclopentadithiophene dyes, porphyrins, bi-fluorenylidene and S,N-heterohexacene-based donor materials. The question of charge delocalization in amorphous and crystalline fullerene solids is addressed using a range of theoretical methods with implications for charge transport and exciton dissociation at donor–fullerene interfaces. New donor–acceptor dyes with appropriate energy level alignment and strong light-harvesting capability are a key topic in achieving highly efficient dye-sensitised solar cells.

Hot topics in OLED research include new donor–acceptor molecules that integrate aggregation induced emission and delayed fluorescence. For thermally activated delayed fluorescence (TADF) the energy levels in the charge transfer state of a donor–acceptor molecule control the singlet–triplet energy gap, which can be finely tuned leading to very efficient device performance. New molecules are exploited in solution-processed emitter layers in OLEDs: for example, oligofluorene co-oligomers afford green devices, whereas cyclometallated iridium complexes and benzobisoxazole derivatives are key building blocks for deep blue OLEDs. Charged copper complexes are used as emitters in light emitting electrochemical cells (LECs) with considerable improvements in device lifetimes compared to previous LECs based on copper, although there is a trade-off between lifetime and device efficiency.

Another important consideration in the OLED field is the development of new host materials with good hole transporting properties and high triplet energy suitable for blue phosphors; new electron transport materials are optimised to facilitate balanced injection and transport of holes and electrons in the emitter layer in multi-layer OLEDs.

The charge carrier mobility of organic field effect transistors (OFETs) currently match, or even exceed, that of amorphous silicon-based FETs. However, to expand the applications of OFETs there is considerable scope to further improve the mobilities by molecular design. For this purpose new donor–acceptor π-conjugated molecules based on oligoheterocyclic frameworks are promising candidates based on theoretical and experimental results. Oligoacenes are important benchmark organic electronic materials and theoretical studies explore the role that the dimer configuration plays in determining the magnitudes of the electronic couplings in these molecules in the solid-state.

New materials chemistry will undoubtedly continue to be a key ingredient in the development of organic electronics for years to come. However, the success of an electronic material as an active component in next-generation high performance devices depends on many requirements beyond the molecular structure – notably, chemical and morphological stability, processability, environmentally sustainable synthesis, fabrication and manufacturing, and the optimisation of device technologies. These are all fascinating challenges for the organic electronics community that will continue to stimulate innovative interdisciplinary developments.

As guest editors we sincerely thank all the authors who have submitted high quality science to J. Mater. Chem. C enabling us to publish this themed issue.

References

1. P. E. McGovern and R. H. Michel, Anal. Chem., 1985, 57, 1514A–1522A.
2. W. H. Perkin, J. Chem. Soc., Trans., 1879, 717–732.
3. http://www.rsc.org/Chemsoc/Activities/Perkin/2006/minisite_perkin_chemical_non_flash.html .
4. (a) Special Issue: Organic Electronics, Chem. Mater., 2004, 16, 4381–4846; (b) Thematic Issue: Organic Electronics and Optoelectronics, Chem. Rev., 2007, 107, 923–2386; (c) Special Issue: Organic Electronics, Israel J. Chem., 2014, 54, 413–835; (d) Special Issue: Organic Electronics, ChemPhysChem, 2015, 16, 1097–1314.
5. Electronic Materials: The Oligomer Approach, ed. K. Müllen and G. Wegner, Wiley-VCH, Weinheim, 1998.

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