25 Years of Light‐Emitting Electrochemical Cells
2020; Wiley; Volume: 30; Issue: 33 Linguagem: Inglês
10.1002/adfm.202002879
ISSN1616-3028
Autores Tópico(s)Catalytic Cross-Coupling Reactions
ResumoSince the pioneering work on the electroluminescence response of anthracene crystals by P. Vouaux in 1953, organic-based thin-film lighting devices have made it all the way from laboratory prototypes to commercial products. Over these decades, research activities have been focused on i) understanding the electroluminescence phenomena in organic semiconductors, ii) reducing the applied bias to enhance device efficiency and stability, and iii) designing new materials and device architectures. Broadly speaking, two approaches have been followed: i) multi-layered devices with a manifold of thin layers tailoring the charge injection, transport, and emission processes, and ii) ionic-based devices, in which the presence of mobile ions in the active layer assists charge injection and transport processes in a single-layered device. The former refers to organic light-emitting diodes (OLEDs), which have significantly advanced since the first report by C. W. Tang in 1987. The latter is the light-emitting electrochemical cell (LEC) that were discovered by Q. Pei et al. in 1995. LECs are considered the simplest thin-film lighting device up to date. In particular, the active layer consists of a myriad of electroluminescent emitters—e.g., conjugated polymers (CPs), ionic transition-metal complexes (iTMCs), small molecule (SMs), quantum dots (QDs), and nanoparticles (NPs)—that are blended with ionic additives such as ion polyelectrolytes (IPs), ionic liquids (ILs), and inorganic salts (ISs). Since the discovery of the LEC concept in 1995, the impact of the ions on the device mechanism has thoughtfully been studied using several techniques. However, it took around 15 years to reach a consensus on the device mechanism. During this period, other works focused on i) designing new CPs and ruthenium(II)/iridium(III) iTMCs featuring different emission colors, ii) determining the best electrolyte for each emitter, and iii) studying the emitter degradation under device operation conditions. Since 2010, the research community started to search for new electroluminescent materials, such as d10 iTMCs, SMs, QDs, and NPs, to achieve highly efficient and stable devices spanning the whole visible range. Furthermore, new opportunities appeared with respect to new ionic additives, such as inorganic salts, binary complexes, and electroactive electrolytes. These contributions have led to new horizons in terms of elucidating i) the best combination of emitter and ionic additives, ii) the impact of the type of emitters and ionic electrolytes on the device mechanism, and iii) new low-cost fabrication methods for large-area devices. In parallel, many groups have turned their attention to i) enhancing the device efficiency by balancing the charge transport and electron-hole recombination; ii) designing frozen junctions; iii) optical engineering using color-converting layers, cascade- and tandem-like devices, light-emitting gels, plasmonic effects, and waveguide coupling; iv) establishing up-scalable solvent-based deposition techniques like spray-coating, gravure, roll-to-roll, and inject printing, etc; v) demonstrating the LEC potential with respect to low-cost fabrication of displays and easy-to-do devices prepared on any kind of substrates. In light of all of the aforementioned, it is safe to state that the last 25 years have been a successful test-bed time for LEC technology, ranging from a mature understanding of the device mechanism, to the limitless design of emitters and ionic additives, to novel device architectures, and to industrial relevant fabrication methods. This special issue honors the efforts of a growing lighting community, providing an excellent platform to showcase the most recent advances in the LEC field after 25 years. In particular, this special issue can be divided into three sections, namely i) design of the third generation of emitters and ionic additives, ii) recent advances in device chromaticity, and iii) new insights into device mechanism and characterization techniques. Concerning the design of new materials, LECs require emitters with excellent ambipolar carrier transport and high luminescence in thin films, as well as ionic electrolytes with high ion conductivity and a wide electrochemical window. These aspects are needed to stabilize an efficient emission from the p-i-n junction of LECs. As a general introduction, T. Takenobu and co-workers (article number 1908641) have comprehensively summarized the most desired electroluminescent features of non-polymeric emitters. In particular, Y. Choe et al. (article number 1907126) focused on the recent advances and limitations in LECs using SMs as emitters, disclosing the most promising material design for each color region as well as other important aspects, such as ionic versus neutral SMs, multifunctional materials including molecular host-guest and dyads. To complement this, E. Zysman-Colman and colleagues (article number 1908677) have provided an excellent review concerning the synthesis and application of the multi-resonant thermally activated delayed fluorescence emitters based on heteroatom doped nanographenes. The latter is an emerging class of emitters for both, OLEDs and LECs, featuring interesting temperature- and electric field-induced photo- and electro-luminescent behaviors. As an example, R. D. Costa and co-workers (article number 1906830) have provided an original report that deciphers the origin of one of the most stable and efficient white LECs using a blue-emitting BN doped nanographene. In short, they described how self-heating activates new exciton radiative mechanisms, leading to efficient white photo- and electro-luminescence responses featuring a ternary emission mechanism. In this line, L. Edman and co-workers (article number 1908649) have prepared an excellent review of the self-heating process in LECs. On a different note, E. Nannen et al. (article number 1907349) have summarized the recent progress in LECs based on inorganic QDs and perovskite NPs. They have highlighted the impact of these materials on the device mechanism and the different device architectures towards highly performing devices. Finally, J. Slinker and colleagues (article number 1906715) focused on strategies to bypass traditional IPs and ILs, such as coupling electrolytes and emitters, ISs as new electrolytes, and easily polarized neutral emitters like perovskites and nanoparticles. Strongly linked to the material design is the device chromaticity. Highly efficient and stable white-emitting LECs is one of the major milestones of the field. R. D. Costa's and K.-T. Wong's groups (article numbers 1908176 and 1906898, respectively) have summarized the status of white LECs, highlighting material designs (multiemissive mechanism, multifluorophoric emitters), active layer designs (host-guest, multilayers, exciplex and electroplex emissions, etc.), and architecture designs (photoactive filters, tandem, etc.). The white LEC milestone is flanked by the challenges to realize stable and efficient blue/green and red LECs. L. Duan and co-workers (article number 1907156) have provided a timely review of the current understanding of the molecular design of traditional and new emitters, identifying the guidelines and limitation towards efficient and stable blue LECs. In addition, they also focused on device structures with respect to color tuning and enchancement of the efficiency and stability. In addition, L. He et al. (article number 1907169) have emphasized the design of blue-emitting iridium(III) iTMCs and their implementation in LECs. Thus far, they are still the best performing blue emitters, although they are much worse performing (stability and efficiency) than green, yellow, and orange LECs. Here, A.V. Mudring and co-workers (article number 1909809) fabricated one of the most efficient and stable green LECs combining the rational ligand design of iridium(III) complexes in concert with an optimized driving mode. Finally, H. Shahroosvand and colleagues (article number 1908103) have provided an excellent review on deep-red and NIR emitters applied to LECs, highlighting the differences in performance and device mechanism between CPs, iTMCs, SMs, and QDs. As the last section of this special issue, the reader will find a fine collection of reviews and original papers focused on new insights into device architectures, mechanism, and characterization techniques. H.-C. Su's group (article number 1906788) has summarized optical strategies to manipulate light-extraction towards enhancing device efficiency and controlling device chromaticity. In addition, they also described optical techniques to study the dynamic evolution of the p-i-n junction in terms of balanced carrier transport and electron-hole recombination as a tool to optimize device performance. This is nicely accompanied by i) the progress report on exciplex emission and light out-coupling techniques provided by Y. Nishikitani's group (article number 1907309), and ii) the original work of R. Hany (article number 1906803) that describes numerical simulations that nicely correlates the ion motion and movement of the intrinsic zone in the p-i-n junction. The industrial relevance of LECs was highlighted by J. Gao's and K.-H. Lee's groups (article number 1907003 and 1907936, respectively). The former summarizes the recent progress using bipolar electrodes that leads to the wireless formation of many localized emitting p-i-n junctions in the bulk of the device. The latter describes a new type of quasi-solid-state LECs using gel electrolyte composites featuring high-strength and high-conductivity features that are of high relevance for low-cost printable commercial devices. Finally, Li, Pei, et al. (article number 1909102) reviewed the dynamic formation of p-i-n and how this is applied for LEC materials selection, junction stabilization, and the demonstration of stretchable LECs. Overall, this special issue illustrates the excellence past and current research of leading scientists loving the LEC technology platform. We are heartily grateful to all the authors, the referees, the editor (Muxian Shen), and the whole team at Wiley-VCH for their efforts to carry this special issue forward. Qibing Pei is Professor of Materials Science and Engineering and Professor of Mechanical Engineering at the University of California, Los Angeles. He specializes in synthetic polymers and composites for electronic, electromechanical, and photonic applications. He received his B.S. degree from Nanjing University, PhD degree from the Institute of Chemistry, Chinese Academy of Sciences, and carried out postdoctoral research Linköping University. He worked at UNIAX Corporation (now DuPont Display) and SRI International, Menlo Park, California before joining the UCLA faculty in 2004. Rubén D. Costa completed both his B.Sc/M.Sc studies (2006) and PhD (2010) in chemistry at the University of Valencia (Spain). From 2011–2013, he was a Humboldt postdoc at the Friedrich-Alexander-University Erlangen-Nürnberg (Germany). Here, he started the Hybrid Optoelectronic Materials and Devices lab as Liebig junior group leader in 2014. In 2017, he moved part of his group to IMDEA Materials Institute (Spain) as senior program leader. In 2018, he expanded his labs to the U. Waseda (Japan) as Associate Professor. His research encompasses the design and preparation of new hybrid materials and their use in optoelectronic devices for energy- and medical-related applications.
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