Infrared OLEDs Reach New World Record

You can’t see it with the naked eye, but a new fluores­cent organic light-emitting diode could shed light on the development of innovative appli­cations in devices such as smartphone and television displays using near-infrared light. Created through the combined work of engineers from Poly­technique Montréal and chemists from Université de Montréal, this fluorescent OLED is 300 % more efficient than existing OLEDs in its category.

Engineering physics PhD student at Polytechnique Montréal Alexandre Malinge, holds a glass substrate containing six large infrared OLEDs, each one 1.5 mm per side. (Source: O. Ortiz)

In contrast to conven­tional light-emitting diodes OLEDs emit light through the use of organic molecules. Already in use in smartphone displays and high-end tele­visions, OLED technology is already well established. Yet despite adoption by industry, important challenges still need to be overcome to push this technology forward. On such example is that blue OLEDs face stability issues, which leads to much faster degra­dation than their green and red counter­parts. On the other side of the spectrum, infrared OLEDs tend to be very inefficient – instead of emitting photons at infrared wavelengths, excited molecules prefer to lose their energy through vibration.

“As emission wavelength is pushed further out into what’s considered infrared, it becomes harder to develop efficient emitters, explains Stéphane Kéna-Cohen from Poly­technique Montréal’s department of engi­neering physics. “Very few organic materials efficiently emit in this region (infrared) of the spectrum.” Kéna-Cohen and his team managed to find a way to reduce the wasted energy in infrared OLEDs composed of purely organic molecules. William Skene of  Montréal University developed two new organic compounds to create this new OLED. The near-infrared emitter was inspired by a class of molecules previously used for biomedical imaging – which now makes it possible to design an all-organic OLED with unparal­leled pro­perties.

When an organic molecule is excited by an electrical current, it finds itself in one of two quantum states: a singlet or a triplet. For most organic molecules, only the singlet state will produce useable light. For triplets to effi­ciently generate photons, heavy metal atoms need to be introduced within the molecular structure, increasing the production cost of OLEDs. Kéna-Cohen and his team found a way to harness triplet energy without relying on metal atoms. Their inno­vative solution? They designed an organic molecule where singlet and triplet states have very similar energy levels, allowing the triplets to be transformed into emissive singlets through thermally-activated delayed fluores­cence (TDAF).

With its emission peak at a wavelength of 840 nm, the OLED designed by the research team showed a quantum effi­ciency of 3.8 %. The latter corresponds to the per­centage of electrons circu­lating throughout the device, electrons which are then converted into useable light. It’s a new world record for all-organic OLEDs emitting above 800 nm – exceeding the efficiency of the best fluorescent OLEDs by over 300 % – and reaching values comparable to those of OLEDs containing platinum-based molecules.

The new OLED’s exceptional effi­ciency makes it feasible to finally consider inte­grating infrared OLEDs within existing display techno­logies such as smartphones. “One distin­guishing feature of OLEDs is the ability to manu­facture devices directly on glass or plastic, and over large areas – in stark contrast to conven­tional LEDs. This allows OLEDs to be used in appli­cations that would be otherwise impossible for LEDs,” explains Kéna-Cohen.

“One of the greatest advan­tages of OLEDs is their low manu­facturing cost,” continues Kéna-Cohen. “However, most OLEDs still contain expensive metals such as platinum or iridium, which is problematic for cost and in terms of sustainability. Our device uses purely organic molecules.” He noted that the absence of visible light emission from the infrared OLEDs created by his research team would also permit their use in light-based wireless communi­cation (Li-Fi). Kéna-Cohen also highlights that these world-record breaking OLEDs could poten­tially be used for biomedical applications, for facial recog­nition, or for night-time photo­graphy. “iPhones already use infrared lasers for some facial recog­nition and autofocus functions. These are the types of appli­cations where infrared OLEDs could be useful,” notes Kéna-Cohen. (Source: PT Montréal)

Reference: A. Shahalizad et al.: Efficient Solution‐Processed Hyperfluorescent OLEDs with Spectrally Narrow Emission at 840 nm, Adv. Func. Mat., online 10 November 2020; DOI: 10.1002/adfm.202007119

Link: Dept. of Engineering Physics, École Polytechnique de Montréal, Montreal, Canada

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