Plasmonic OLED

This is the second article in Bright Side with Brannan, a column by Prof. Alexander Brannan of the University of Manchester. In this column, Prof. Brannan explores the science behind OLED technology — the materials, the physics, and the open questions driving the field forward. You can read the first article in the series, on open-shell radical emitters, here.

In my previous article on radical OLED emitters, I briefly mentioned circularly polarised luminescence, or CPL, as an emerging direction for TTM-based materials.[1] Since then, the first CP-OLEDs based on donor–acceptor-type neutral TTM radicals have been reported, with g_EL reaching 7.2 × 10⁻³.[2] This shows that the area is continuing to grow, but what does that value actually mean for an OLED display? As CPL appears increasingly often in emitter research and OLED-Info coverage, I wanted to give a simple overview of why it is attracting interest, how researchers are trying to produce it, and where it may realistically be useful.

What is CPL?

Light consists of oscillating electric and magnetic fields, but polarisation is usually described using the electric field. In unpolarised light, this field has no fixed orientation. A linear polariser selects one direction, while a correctly aligned quarter-wave plate changes this back-and-forth motion into a rotating electric field. This produces circularly polarised light, often pictured as a left- or right-handed corkscrew. CPL can also be generated directly by a chiral emitting material or organised chiral system, without requiring these external optical components.[3]

In recent years, the main focus of the OLED industry was on reducing power consumption. OLED developers introduced several novel technologies, including new backplanes, materials and device architectures, to increase device and system efficiency.

This article looks into the potential pressure that the acceleration of on-device AI (locally running AI engines as opposed cloud-based AI) will have on OLED developers to reduce the power consumption even further, especially within the laptop segment, and adopt novel next-gen technologies.

OLED displays have been gaining popularity rapidly, and are already the dominant smartphone display technology. OLEDs are also the display technology of choice in the smartwatch market, making inroads into the TV, monitor, laptop and tablet markets. The future of the OLED industry looks bright.

In recent years, the focus of the industry, beyond increasing capacity and reducing production costs, has been improving the performance of OLEDs in the areas of display brightness, efficiency, and lifetime. Brightness is required in many applications - from TVs (for HDR and to view in ambient lighting) through smartphones (outdoor viewing) to automotive, and efficiency is a plus in any scenario (but mostly in mobile displays). Display lifetime is already good enough for many applications, but in some cases (like automotive, and IT displays) it is critical. These three properties usually go together - if you can make more efficient OLED displays, you can drive them at a lower current to achieve the same brightness, and so lifetime increases, or you can achieve higher brightness, etc. 

Universal Display reported its financial results for Q3 2021. Revenues were a record $143.6 million, up from $117.1 million in Q3 2020 and net profit was $46.1 million (up from $40.5 million from Q3 2020). The company has about $789 million in cash and equivalents, the it reiterates its full-year 2021 revenue guidance to be between $530 million and $560 million.

Regarding its blue PHOLED R&D effort, UDC says it is making "excellent progress", and the company believes that a commercial blue material is a question of when and not if.

Researchers from Europe, working with funding from the EU research program at the MOLOKO project, developed a new plasmonic sensor system that integrates an organic light-emitting transistor (OLET), an organic photo-diode (OPD), and a nanostructured plasmonic grating (NPG) all in the same device.

The researchers say that this design enables to create cost-effective and highly miniaturized optical sensors based on the optics-less plasmonic system. The researchers further say that the potential of OLETs, as planar multielectrode devices with inherent micrometer-wide emission areas, offers the incorporation of the light-emitting and light-detection modules together at unprecedented lateral proximity.

A couple of months ago we reported that researchers at UDC developed an OLED device with plasmonic decay rate enhancement that dramatically increase device stability. It turns out that UDC filed for two new trademark applications that seem to hint it is aiming to commercialize this technology.

The two trademarks at PLASMON PHOLED and PLASMONLED, which both of course indicate that these are emitters that adopt plasmon technology.

Researchers at Universal Display, developed an OLED device with plasmonic decay rate enhancement that dramatically increase device stability. By including a nanoparticle-based out-coupling scheme to extract energy from the plasmon mode, the researchers managed to maintain the device efficiency.

The device used an archetypal phosphorescent emitter to achieve a two-fold increase in functional stability at the same brightness as a reference conventional OLED device and extracted 16% of the energy from the plasmon mode as light.

Japan's Oji Paper developed a new microfabrication technology that can be used to enhance the light extraction efficiency of OLEDs. The idea is to use microparticle coating to form a regular pattern that is several-nanometer-wide on the surface of the metal part of the negative electrode in an OLED panel. The nanostructure turns the plasmon energy (that is sometimes released from an OLED) into light.

Oji paper says that this technology was used to improve a red OLED's light extraction efficiency by about 100%. They are now working to apply this to a white OLED.