Wednesday, 15 Aug 2018

Researchers demonstrate efficient near-infrared (NIR) electroluminescence from a "metal-free" fluorophore

There is growing interest to adopt NIR OLEDs in a range of applications, such as sensing, bio-imaging, photodynamic therapy, and optogenetics through to Li-Fi communications


12 Jul 2018 | Editor

Researchers from the London Centre for Nanotechnology and the Department of Physics and Astronomy at UCL, alongside with collaborators from Chalmers University of Technology (Sweden), Addis Ababa University (Ethiopia) and Flinders University (Australia), have demonstrated "unprecedented" efficiencies from NIR OLEDs based on a purely organic fluorescent active layer emitting above 800 nm. The researchers have recently published a paper in Advanced Materials.

The authors leveraged a newly designed and synthesised triazolobenzothiadiazole-based NIR emitter (BTT*), which was blended in a novel indacenodithiophene-based transport polymer matrix (PIDT-2TPD). Thanks to the optimal transport properties of the polymer matrix, and the spectral overlap between the emission of the polymer matrix and the absorption of the NIR guest, such a blend exhibits virtually pure NIR electroluminescence peaked at 840 nm, external electroluminescence quantum efficiencies in excess of 1.15 % and turn-on voltages as low as 1.7 V. For this spectral range, such values are the best ever reported from NIR OLEDs with purely organic and solution-processed active layers.

Fluorescent materials such as BTT* are desirable for both bio-medical applications because free from heavy metals, and for Li-Fi applications since intrinsic fluorescence lifetimes (1 ns or so), as they make it theoretically possible to extend the data transmission rate up to the Gb/s regime.

NIR OLED structure, with the PIDT-2TPD:BTT* blend sandwiched between a Ca/Al and ITO/PEDOT:PSS electrodes. The electroluminescence spectrum measured from the PIDT-2TPD:BTT* OLED is also shown. The researchers note the NIR purity of the spectrum, with 99 % of photons emitted at λ > 700 nm and maximum at 840 nm.

Burgeoning interest for near-infrared (NIR) organic light-emitting diodes (OLEDs) is fuelled by the huge prospects for integration in a broad range of applications, spanning from "biomedics" (for sensing, bio-imaging, photodynamic therapy, optogenetics, to name just a few) to Light-Fidelity (Li-Fi) all-optical wireless telecommunications (e.g. luminaire-integrated "last metre" connection stubs), and security/biometrics.

Obtaining high efficiency from NIR fluorophores is much more challenging than for visible ones because of the inherent tendency to aggregation of large conjugated chromophores, and because of the so-called "Energy-gap rule".

To date, the highest efficiencies in the NIR have been obtained from OLEDs incorporating phosphorescent emitters or, more generally, materials exploiting triplet excited states. However, the long triplet lifetime (microseconds at least) limits the OLEDs switching speed (e.g. in Li-Fi). Furthermore, the toxicity of heavy metals in phosphorescent materials also raises biocompatibility concerns for applications in wearable or "skin" electronics.

Source: London Centre for Nanotechnology

The authors gratefully acknowledge funding by the European Community's Seventh Framework Programme (FP7/2007-2013) ITN MSCA action under grant agreement No. 607585 (OSNIRO) and by the H2020 ETN MSCA action under grant agreement 643238 (SYNCHRONICS), and EPSRC (grant EP/P006280/1). W. Mammo and Z. Genene acknowledge financial support from the International Science Programme, Uppsala University, Sweden. F. Cacialli is a Royal Society Wolfson Merit Award holder.

Efficient Near‐Infrared Electroluminescence at 840 nm with "Metal‐Free" Small‐Molecule:Polymer Blends

Alessandro Minotto | Petri Murto | Zewdneh Genene | Andrea Zampetti | Giuseppe Carnicella Wendimagegn Mammo | Mats R. Andersson | Ergang Wang | Franco Cacialli

First published: 10 July 2018 | https://doi.org/10.1002/adma.201706584


Due to the so‐called energy‐gap law and aggregation quenching, the efficiency of organic light‐emitting diodes (OLEDs) emitting above 800 nm is significantly lower than that of visible ones. Successful exploitation of triplet emission in phosphorescent materials containing heavy metals has been reported, with OLEDs achieving remarkable external quantum efficiencies (EQEs) up to 3.8% (peak wavelength > 800 nm). For OLEDs incorporating fluorescent materials free from heavy or toxic metals, however, we are not aware of any report of EQEs over 1% (again for emission peaking at wavelengths > 800 nm), even for devices leveraging thermally activated delayed fluorescence (TADF). Here, the development of polymer light‐emitting diodes (PLEDs) peaking at 840 nm and exhibiting unprecedented EQEs (in excess of 1.15%) and turn‐on voltages as low as 1.7 V is reported. These incorporate a novel triazolobenzothiadiazole‐based emitter and a novel indacenodithiophene‐based transport polymer matrix, affording excellent spectral and transport properties. To the best of knowledge, such values are the best ever reported for electroluminescence at 840 nm with a purely organic and solution‐processed active layer, not leveraging triplet‐assisted emission.

www.chalmers.se    https://www.ucl.ac.uk/cmmp/cmmp-people/group-pages/organic-semiconductors    www.aau.edu.et    www.flinders.edu.au/institute-nanoscale-science-technology   

About London Centre for Nanotechnology

The London Centre for Nanotechnology is a multidisciplinary research centre in physical and biomedical nanotechnology in London, based at UCL's campus in Bloomsbury which houses a state-of-the-art cleanroom and core open access facilities. It brings together institutions that are world leaders in nanotechnology and was conceived from the outset with a management structure allowing for a clear focus on exploitation and commercialisation.

The LCN's work requires it to draw on the combined skills of multiple departments, including medicine, chemistry, physics, electrical and electronic engineering, biochemical engineering, materials and earth sciences, and two leading business centres. The Centre’s experimental research is supported by leading edge modelling, visualisation and theory through its access to the cleanroom, characterisation, fabrication, manipulation and design laboratories.

The LCN’s stated vision is to become Europe’s premier research centre in nanotechnology applied to health care, information technology and the environment.

Source: London Centre for Nanotechnology

About Chalmers University of Technology

The Department of Chemistry and Chemical Engineering at Chalmers University of Technology has a long tradition of encouraging world leading research in an environment where people develop and create new knowledge.

Extensive expertise enable us to take on major challenges ranging from basic research to industrial applications. Our research projects are often cross-border and are carried out in close cooperation with industry, institutes and other higher education institutions.

Our research is conducted at various levels; departmental level, through participation in Chalmers' Areas of Advance and in our competence centers. All operations at Chalmers contribute to achieving the Chalmers vision: Chalmers – for a sustainable future.

Source: Chalmers University of Technology

About Addis Ababa University

The Department of Chemistry at Addis Ababa University, Ethiopia, is premier institution of learning engaged in teaching and research. One of the active research areas in the Department is the synthesis and characterization of conjugated polymers for electronic and photonic applications. Members of the research team work together with scientists in Europe, North America, Asia and Australia to solve scientific problems of common interest.

Source: Addis Ababa University

About Institute for Nanoscale Science and Technology

work across energy, health and other biological processes, environment, and security. Our team works with business and industry, connecting real-world problems with research-driven solutions.

Our advances and discoveries have found their way into many industries – from medicine, to forensics, to solar energy. It is the work of over 100 researchers, students and affiliated researchers who collaborate with people around the world, from regional South Australia to partners in the UK, Italy, France, Thailand and Saudi Arabia.

Source: Institute for Nanoscale Science and Technology

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