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
Abstract
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.