Polypyridyl Ligands: Challenges and possibilities in SS-LEDs - Part 1

Solid-state lighting (SSL) is a type of lighting that uses semiconductor light-emitting diodes (LEDs), organic light-emitting diodes (OLED), or polymer light-emitting diodes (PLED) as sources of illumination rather than electrical filaments, plasma (used in arc lamps such as fluorescent lamps), or gas.

Prof. Hashem Shahroosvand from Molecular Engineering of Advanced Functional Materials (GMA), University of Zanjan, Iran and his research team has discovered polypyridyl ligands as a versatile platform for solid-state light-emitting devices (SSLED). One of the major scientific breakthroughs that have impacted on everyday human lives on a global scale was the discovery of solid-state lighting (SSL). SSL is based on the concept of electroluminescence, where light is produced by the radioactive deactivation of excitons generated by efficient electron-hole recombination in the bulk of semiconducting materials [1, 2]. Hence, in contrast to traditional lighting devices where light is a by-product, SSL allows significantly and reduces the heat generation, as well as enhanced luminous efficiency. In addition, SSL provides excellent stability, brightness, and light point sources of different colors [3]. Traditionally, its technology is divided into light-emitting diodes (LEDs), organic light-emitting diodes (OLEDs), and light-emitting electrochemical cells (LECs).

Color convertors include the inorganic phosphors doped with rare-earth metals like cerium, yttrium, or organic down-converters. The developments of organic/inorganic materials are more commonly referred to as hybrid LEDs [4]. LEDs are now routinely implemented for automotive, traffic signaling, home illumination, screen backlighting, and advertising as well as for decorative applications, owing to their high efficiency, luminescent properties, and stability [5]. LEDs also showed a subsequent increase in their stabilities and promised with an exciting opportunity with respect to the future globalization.

Current compact fluorescent lamps (CFLs) usually contain an average of 5 mg of mercury per bulb which is an environmental concern. This shows that SSL, not only offers increased power efficiency, but also an attraction for the replacement of older, less ‘green’ sources of lighting [6]. However, organic semiconductors such as polymers, small molecules, and/or coordination complexes are responsible for carrier transport, carrier injection, and emission. Hence, in this context, LECs and OLEDs (Figure 1) are found to be emerging as well as interesting alternatives to LEDs. While efficient OLEDs comprise multilayered architectures prepared via rigorous encapsulation and chemical vapor deposition [7].

Figure 1. LED lamps require less power than older light sources for emitting light [8].

Royal Swedish Academy of Sciences awarded the 2014 Nobel Prize for Physics to three scientists, this prediction was made when they acknowledged for the invention of efficient white LEDs. LEDs have a significant potential to improve the quality of human life on a global scale by extending cheap and affordable lighting sources [9]. For example, in the world, about 1.5 billion people currently do not have access to electricity grids, a problem which can be solved via the large-scale commercialization of LEDs. LEDs operate more efficiently than any other lighting source (Figure 1), reaching to 300 lm W^-1, in comparison to 70 lm W^-1 for their fluorescence. Due to these main key factors of LEDs that can be contributed to large-scale commercial production in industries. In fact, around the world, many homes and vehicles now contain LED technology which has now flooded at the market as the microelectronic components of displays for computers, laptops, smart televisions, mobile phones, and cameras [2, 3].

A windowless plane covered with OLED panels is shown in Figure 2. An article published in the UK’s independent newspaper in 2014 reported that ‘‘those minuscule windows on airplanes could soon become a thing of the past, with a UK developer working on windowless fuselages that house giant, flexible OLED screens’’ [11].

Despite these advances, the progress rates of OLEDs used in mobile phones are much faster than TV displays. In general, it is believed that the fabrication costs can be reduced, then the whole display industry will be ultimately shifted to OLED technology in the near future, although OLEDs displays are lighter, thinner, more flexible, and robust than LCD counterparts. From an academic perspective, it can be seen that, in the field of OLED, there has been an exponential growth of interest. In fact, a recent literature report with the title of ‘‘Organic Light Emitting Diode’’ shows, a total of 27,229 articles including 1,207 reviews. This highlights the importance of the global research field which includes significant contributions from both academics and industries.

Figure 2. A windowless aero plane fabricated by OLEDs which could be a reality in under a decade [11].

In recent years, several reviews are highlighting the most important concepts relating to OLED and LEC performance [12]. A valuable reviews concerning the application of organometallic complexes in optoelectronic devices including of OLED, solar cell, water splinting and hydrogen production [13].Moreover, several comprehensive reviews have been published on solid-state LEDs based on ionic transition metal complexes (ⁱTMCs) including Ir(III) complexes [2]. However, none of them have correlated with the influence of the ancillary ligand in the transition metal emitter with the electroluminescence performance of the resulting complex in a solid-state LED.

Our SNB Team recommended this research review to enrich our reader’s knowledge about the development of SSLEDs. They have mainly focused on the discovery, optimization, and importance of TADF in polypyridyl transition metal complexes. The motivation behind this research work stems from the urgent need for the rational design of TADF emitters to enhance the efficiencies and decrease the processing costs for commercial applications. Further discussion on correlation of polypyridyl ligand with transition metal emitter in solid-state LEDs, and the continuation of this research content will be posted (in our SNB) in the upcoming days.

References

1. B.Pashaei et al., Chem. Soc. Rev.,(2019) DOI: 10.1039/c8cs00075a.
2. E. Fresta and R. D. Costa, J. Mater. Chem. C, 5, 5643 (2017).
3. N. Armaroli and V. Balzani, Energy Environ. Sci., 4, 3193 (2011).
4. P. Schlotter, et al., Appl. Phys. A: Mater. Sci. Process., 64, 417 (1997).
5. A. Nardelli et al., Renewable Sustainable Energy Rev., 75, 368 (2017).
6. J. Graffion, et al, J. Mater. Chem., 22, 6711 (2012).
7. N. Miller and F. Leon, OLED Lighting Products: Capabilities, Challenges, Potential, Pacific Northwest National Lab.(PNNL), Richland, WA (USA),(2016).
8. H. S. Virk, History of luminescence from ancient to modern times, presented in part at the Defect and Diffusion Forum, Switzerland (2015).
9. I. Akasaki, et al., Nobel Prize Lecture, The Nobel Foundation, Stockholm, available at http://www.nobelprize.org/nobel_prizes/physics/laureates/2014/popular -physicsprize2014.pdf, The Royal Swedish Academy of Sciences (2015).
10. E. F. Schubert and J. K. Kim, Science, 308, 1274 (2005).
11. C. Hooton, Windowless planes could be a reality in less than 10 years, Independent,https://www.independent.co.uk/travel/news-and-advice/windowless-planes-could-be-areality-in-less-than-10-years-9820947.html.
12. H. Xu, et al., Coord. Chem. Rev., 293, 228 (2015).
13. H. Xu, et al., Chem. Soc. Rev., 43, 3259 (2014).

Blog Written By

Dr. S. Thirumurugan

Assistant Professor

National College, Tiruchirappalli

Tamil Nadu, India

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Dr. A. S. Ganeshraja

Dr. S. Chandrasekar

Dr. K. Rajkumar

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Dr. K. Vaithinathan