How Do Transparent OLEDs Work?
Transparent OLEDs (TOLEDs) are exciting next-generation electronic devices. The self-lighting pixels form transparent screens that display interactive information. TOLEDs can act as both a window or a panel and an informational or entertainment screen.
The Structure of Transparent OLEDs
Transparent OLEDs have a sandwich structure consisting of seven or eight semiconductor layers that emit light when connected to a power source.
TOLEDs only contain transparent layers, unlike standard OLEDs which include some opaque layers. The transparent layers allow light to pass through the device from all angles but also render parts of the window or display panel opaque when in use.
A TOLED device includes:
- A transparent substrate, such as tempered glass
- A transparent anode
- The hole transport layer (HTL)
- The emissive layer (EML), containing organic materials
- The electron transport layer (ETL)
- A transparent cathode
While a standard OLED has only one transparent electrode to let out light in one direction, TOLEDs are bidirectional in their light emitting capacity.
The transparent OLED is classified according to device architecture. It can be:
- Standard with the anode at the bottom and the cathode at the top.
- Inverted with the cathode at the bottom and the anode at the top.
- Flexible when the device is built on a soft or fleixble substrate.
What are TOLEDs Made Of?
The efficiency of TOLEDs is typically much lower than that of conventional OLEDs. A TOLED stacked on a glass substrate exhibits optical transmittance below 20%. Performance is impacted by two key parameters: the properties of the electrode materials and the light extraction.
Transparent Electrodes
Transparent electrodes (already a feature of the standard OLED design) require visible light to be emitted without absorption or scattering. They are commonly made using transparent conducting oxides (TCOs). These materials have outstanding optical and electrical properties and display high levels of transparency in the visible region.
A frequently used TCO in the anode structure of standard OLEDs is indium tin oxide (ITO). ITO is desirable due to characteristics such as high conductivity, excellent hardness, chemical stability, and high transparency.
Yet, ITO has its limitations. It is brittle and this limits its potential in flexible and large-area devices. It is also a toxic material, which makes it difficult to process and to recycle. Importantly, ITO cannot be used for the top electrode in a TOLED device because the high processing temperatures destroy the underlying organic layers. Opaque electrodes seen in standard OLED design have to be replaced by thin and semi-transparent metal films.
Alternative materials for TOLED electrodes include metal nanoparticles, carbon nanotube (CNT) networks, graphene, and conductive polymers. CNTs and graphene exhibit high levels of chemical stability, are cheaper to produce, and are far more flexible.
Are Transparent OLEDs New?
Transparent OLEDs may seem futuristic, but they are now a reality. In recent years, tech companies have come up with real-world applications for them.
You can find transparent OLED displays on Chinese subway trains in major cities, including Beijing and Shenzhen, or Japanese overground trains. The replacement windows feature specialized tempered glass for increased strength and durability. These screens can can display the weather, subway maps, news, and more.
The Future of Transparent OLEDs
Transparent OLED panels will likely become a common sight in public spaces across North America and Europe. Restaurants could use them to facilitate food ordering or, on a grander scale, large-area TOLED panels in theatres and on stages could showcase theatrical and musical performances.
Technology companies have been showcasing transparent OLED panels for years at annual trade shows. LG is a frontrunner in the development of transparent TVs. In 2023, LG partnered with Microsoft to develop technical features that would accompany the OLED T, a brand-new model that is now in commercial production.
Before long we will not only be living with TOLEDs, but also wearing them. As flexible designs and mass production becomes more feasible, they will become an everyday feature of smart wearables for our clothing.
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Advanced features, material availability, fabrication issues, and the blue pixel problem are among some of the factors that keep the price of OLED technologies so high.
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How Long Do OLEDs Last?
Researchers and consumers are both very interested in how long OLEDs last. Issues like black spots, burn-in, and pixel failure still affect OLED devices.
Read more...Additional Reading
Huseynova, G. (2021). Transparent Organic Light-Emitting Diodes: Advances, Prospects, and Challenges. Advanced Materials, 9(4). doi: 10.1002/adom.202002040
Yoon-Heung, T. (2002) Criteria for ITO (indium–tin-oxide) thin film as the bottom electrode of an organic light emitting diode. Thin Solid Films, 441(1). doi: 10.1016/S0040-6090(02)00165-7.
Welsh, C. (2024). ‘I’ve looked through LG’s new transparent OLED TV and seen something special.’. The Verge. Online.