A brighter future for flat screens: exploring the development of high efficiency blue OLEDs to meet industry demand
Author : Dr Franky So | Chief Technology Officer | NextGen Nano
01 December 2022
Iconic inventor & engineer, Nikola Tesla once said: “Electrical science has revealed to us the true nature of light, has provided us with innumerable appliances & instruments of precision, and has thereby vastly added to the exactness of our knowledge.” The industry has grown its knowledge significantly since, but what happens when devices are required to give even better levels of performance?
This article was originally featured in the December 2022 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.
Here Dr Franky So, Chief Technology Officer of fluorescent organic light-emitting diode (OLED) developer, Nextgen Nano reveals how demand for more efficient smartphones and televisions have heralded a bright innovation in OLEDs...
Because of their excellent ability to render full colour displays, blue OLEDs light up everything from small screens on smartphones and tablets to large flat screen televisions and other displays. However, blue OLEDs are also unstable emitters, due to the limited lifetime of the organic materials from which they are made. When used in a flat screen device, blue OLEDs typically have a lifetime of around 14,000 hours before they dim to half the original brightness — equivalent to continual use for eight hours a day, over five years.
Considerable investments have been made into improving the external quantum efficiency (EQE) — the amount of current a cell produces when exposed to sunlight — of blue OLEDs, and with good reason. Ever since Sony and Panasonic announced a joint venture to create low-cost, mass-produced OLED televisions in 2012, it has been clear that blue OLEDs can be made and upscaled at low cost.
Blue OLEDs are clearly suited to mass production, as evidenced by the abundance of smartphones and tablets in the world today. For a start, blue emitters used in commercial OLED displays are fluorescent emitters with a cost-effective operating voltage typically larger than three volts. Their cost is set to become even more attractive, due to evolving printing technologies allowing OLEDs to be printed to any suitable substrate.
Fabrication of the OLED substrate is currently more expensive than thin-film-transistor liquid-crystal-displays (TFT LCDs) — also used in televisions, car displays, computer monitors and more. But it’s only a matter of time before blue OLED displayers are cheaper to produce than their LCD or plasma equivalents. Nevertheless, improvements to the efficiency and lifecycle of blue OLEDs will be essential to these developments.
The most popular method to improve the lifespan of blue OLEDs is thermally activated delayed fluorescence (TADF). During this process, molecules in a non-emitting excited — or triplet — state incorporate surrounding thermal energy, which causes them to undergo a change in state and thereby achieve fluorescence. TADF compounds are one of the three main light-emitting materials already used in organic OLEDs, so have long been trusted as a mass-produced light source.
TADFs are presently the “heir apparent” to existing OLED technologies. Through the support and efforts of several large global manufacturers, it’s hoped that a stable and highly-efficient blue emitter will be brought to market within the next 18 months — if not sooner.
Approaching a brighter future
TADFs are not the only route to stable, high efficiency blue OLEDs. Ongoing research by Nextgen Nano’s New Fusion division has uncovered a technology that is not only an alternative to TADFs, but may also offer a considerable improvement.
The fluorescent OLEDs have a turn-on voltage that is 50% lower than conventional blue OLEDs. The result is an operating voltage that is only half that of conventional OLEDs, plus two main advantages: far lower power consumption and a longer operational lifetime for the device itself.
The New Fusion division’s approach involves choosing an organic material with the right molecular properties, which make it possible to achieve electroluminescence at voltages below the molecule’s bandgap voltage. A smartphone display illuminated by these fluorescent OLEDs would draw less power from the battery without any loss to the display’s luminosity. This could extend the phone’s battery life, while maximising the operational lifespan of the OLEDs themselves.
The world’s smartphone and TV manufacturers will be interested in the results of tests already conducted; specifically, into the performance of these fluorescent OLEDs compared with TADF blue OLEDs. The tests measured Nextgen Nano’s technology against the best phosphorescent and TADF OLEDs with similar Commission Internationale de l’Elcairage (CIE) colour coordinates, the mapping system for the combination of three colour values close to red, green and blue (RGB).
While the tests found that TADF and phosphorescent OLEDs achieve very high values of EQE and power efficiency (PE) at very low luminance, they showed a significant efficiency roll-off at higher practical luminance ranges over 500 candela per square metre, the derived SI unit of luminance. The optimised fluorescent OLEDs, meanwhile, did not show any efficiency roll-off at a luminance of up to 200 candela per square metre.
The team attributes this to the low operating voltages of its fluorescent OLEDs. The results show higher power efficiency compared with all other TADF and phosphorescent devices with similar spectral colour coordinates. According to our findings, the technology can achieve a maximum EQE of 9.8% and, as a measurement of its luminous efficiency, a PE of 16.4 lumens per watt. At 1,000 candela per square metre, Nextgen Nano’s fluorescent OLEDs emit an EQE of 9.8% and PE of 14.5 lumens per watt.
The New Fusion division’s claim that the turn-on voltage of its fluorescent OLEDs is half that of conventional blue OLEDs is also backed by research. During tests, its blue OLED produced 1,000 candela per square metre at 3.4 volts; indeed, 50% of the voltage required by other tested blue OLEDs.
The tests have also found that its triplet fusion process becomes increasingly efficient at higher current densities. Our team believes that a strong light excitation leads to the highest possible efficiencies that can be realised in a triplet fusion composition. The molecules exhibited fluorescent properties at lower energy values, allowing them to illuminate at lower voltages than phosphorescent OLEDs and produce a stable blue OLED with a higher operating power efficiency compared with existing blue phosphorescent OLEDs.
The next phase of the New Fusion division’s ongoing research involves developing new molecules capable of emitting blue light that meets the BT 2020 standards for Ultra HD displays. This doesn’t require precious metal catalysis or expensive reagents for production, resulting in more reasonable costs for scaling up production.
Current results have shown the ability to tune the emission colour of their dyes towards the blue BT 2020 requirement, while retaining quantum efficiency. Additionally, the team has been able to tune the volatility of their dyes, allowing for less risk of dye decomposition during device production.
A key takeaway from Nextgen Nano’s continuing research & development is that it can be possible for tomorrow’s smartphones, tablets and TVs to achieve higher levels of luminosity using only half the energy. Further studies are underway to tune the EQE of devices produced using the team’s new dyes. This will also extend the operational lifetimes of devices so that, in the words of Nikola Tesla, our devices may continue to “reveal to us the true nature of light.”