Flexible OLEDs – The state of play
Were we not promised super-thin bendable, flexible and rollable organic LED (OLED) displays?
Phones like Samsung’s Galaxy and HTC’s Desire have shown us that the rigid versions can deliver stunning images. So where are those flexies?
It turns out that they are also here, at least in proof-of-concept form, and they could even be on the shelves next year.
Sony has shown flexible OLED displays at the Society for Information Display conference more than once in the past few years, and Samsung has demonstrated flexible displays and said it will have products on the market in 2014, with rumours suggesting the Samsung is bringing this forward to 2012.
This said, significant hurdles remain in all four parts of a flexible OLED display: active matrix, substrate, OLED and barrier layers.
Like LCDs, a video-speed OLED display needs a matrix of drive transistors, thin-film transistors (TFTs), and flexible OLED displays will need flexible transistors.
Because OLED is a current-driven technology its transistors have to switch a non-trivial current, compared with LCD TFTs that are only required to change the voltage on what is essentially a capacitor.
Dr Jan Genoe is head of the Polymer and Molecular Electronics (PME) group at Belgian research lab Imec. His group works with the Netherlands’ lab TNO on backplanes for rollable AMOLED displays at their Holst Centre in Eindhoven.
Both teams have been in the European Flame (flexible organic active matrix OLED displays for nomadic applications) project.
“Our main goal is transistors on flexible foil for making backplanes for OLED and other flexible displays,” he told Electronics Weekly. “Our focus is truly rollable displays that can be rolled to 7mm diameter 10,000 times.”
For LCDs on glass, amorphous silicon TFTs have been the order of the day or, for better quality displays, higher mobility polycrystalline transistors made by lasering amorphous silicon.
However, as far as anyone can tell, amorphous silicon has reached its slightly-disappointing full potential and will not be good enough for current-driven displays.
The higher-mobility polycrystalline materials also have limitations in current-driven technologies because different crystals have different characteristics leading to variations between neighbouring transistors. With LCDs, this will show up as a slight difference in pixel speed, which is irrelevant. “One really crucial thing for displays is uniformity because eyes are very sensitive to slight differences,” said Genoe. “Voltage-driven displays are very forgiving of transistor-to-transistor variations. In a current-driven display you will see the small differences.”
So OLEDs need materials with higher mobility than amorphous silicon, and better uniformity than polycrystalline silicon.
“Our main focus is organic or oxide transistors on plastic foil,” said Genoe, “and nowadays there is a shift to oxide transistors.”
Oxide transistors are from a class of devices made from compounds that include metal and a non-metal atoms like oxygen. Copper oxide transistors are one of the earliest examples.
Oxides like copper oxide have a habit of becoming polycrystalline, prompting a search for more elaborate oxides that naturally take the lower-performing but consistent amorphous form, but also have intrinsically high performance.
“GaInZnO was originally developed in Japan. It is an oxide that has good current control and is a compromise between good current capability and uniformity,” said Genoe.
“By mixing three metal atoms – as opposed to CuO with one metal atom – the material’s amorphous mobility uniformity is much better. It has been taken up by most people looking at rollability, and is now taking over from amorphous silicon on glass because of its greater mobility,” he added.
GaInZnO is called ‘gizo’ from its initials, or sometimes ‘igzo’ as it can also be written InGaZnO.
“For glass LCDs in mobile phones they will continue to use amorphous silicon and laser crystallise it to make polysilicon,” said Genoe. “I can see gizo taking over for larger displays like LCD TVs because mobility is higher so the displays can be faster, which is important for 3D, and it will also be used for OLEDs.”
Substrates have to be flexible, dimensionally-stable, survive processing temperatures, and be transparent if the OLED layers (stack) are to emit down through it.
“For a truly rollable display it should be thinner than 75µm,” said Genoe. “You start with a 25µm substrate and add a 50µm-75µm stack, then you could make a truly rollable display.”
According to Genoe, substrates that can be processed at high temperature – like polyimides – are easier to use, but tend to be both less flexible and more expensive.
The Holst Centre is aiming to use the polyester PEN (polyethylene naphthalate) which is at the more-flexible low-cost end of the scale.
“If you really want rollable at a radius of 0.7cm, we believe that you need to work on low-temperature substrates,” Genoe said. “They are much more flexible, but much more demanding in process optimisation to reach those low temperatures. Our target is to have process steps at under less than 150°C, and all steps with good uniformity.”
When companies started developing OLED materials, you could hardly stop them talking about intensity, colour co-ordinates and product lifetime.
The successful companies were bought up or signed deals, and the situation has reversed – few will discuss performance.
Although the Holst Centre concentrates on backplanes, it has made a few prototype displays, initially with organic transistors.
“It is easier to demonstrate you have a working backplane if you have some OLED material deposited on top,” said Genoe. “We tend to use green OLEDs. We have also used orange, red and – twice – blue.”
Initially these were made as part of the Flame programme at the Fraunhofer Institute in Dresden, which is expert at depositing ‘small molecule’ OLED materials.
Now Imec can do its own deposition, and has a low -key long-term plan to make a full-colour OLED display, for which it needs to deposit all three colours separately.
With colour displays, there is an option to use one layer of white LED and, borrowing from LCD manufacture, a layer of colour filters on top.
“We don’t want to go white and colour filters, the result would be too thick for truly rollable displays,” said Genoe.
So the lab keeps an eye on OLED material development, where blue emitters have struggled with short lifetime and feeble emission.
“Up to a year ago, blue was still bad, but it is catching up quickly,” said Genoe. “And there is big progress in candelas-per-amp on the blue side.” Poor blue materials can be compensated for by larger blue pixels. “In the first Samsung Galaxy, the blue pixel was much bigger than the green or red. The greens were tiny as it is by far the most efficient,” he said. “In the Galaxy II, all colours are equal, corresponding to the improvement in blue.”
Richard Kirk knows plenty about the availability of OLED materials. He is CEO of Sedgefield-based Polyphotonix, a manufacturer of specialist OLED lighting. Mostly he makes white light products, but there have been requests for colours. “Deep blues do present a challenge. We avoid anything to do with blue, although if you only want a 2,000 hour lifetime, fine, it can be done,” he said.
Universal Display (UDC) of Princeton makes OLED materials and will talk about lifetime. For bottom-emitting structures it can offer 1.4 million hours predicted lifetime (from 1,000cd/m² to 500cd/m² end-of-life) and 72cd/A efficiency in yellow-green. The figures for red, green and light blue are 600,000hr, 400,000hr and 20,000hr, and 2cd/A, 78cd/A and 47cd/A respectively.
The firm invented the light blue emitter to get around the even shorter life of deep blue.
According to Genoe, self-destruction is less of an issue than it once was. “In the beginning, there was some degradation from current. Nowadays you don’t see this degradation and the lifetime of OLED is mainly determined by oxygen that gets in,” he said.
Almost all OLED materials are rapidly damaged by exposure to oxygen or moisture. If an OLED display is built between two sheets of glass, there is no problem as long as the edges are well sealed because glass is impervious to both oxygen and water.
Plastics unfortunately passes both fairly easily, and various coatings – barrier layers – have been proposed to stop this. A technique from the early days of OLEDs, once dismissed as too expensive for production, is now the front-runner in barrier technology – multiple alternating layers of organic (polymers, for example) and inorganic (metal oxides, for example) materials. Another team at the Holst Centre is looking onto barrier foils.
“The work that is done is using an organic layer and an inorganic layer alternating to provide a good barrier,” Genoe said. “Organic layers are not really good barriers, oxygen and water will diffuse through. Inorganic layers can be an excellent barriers, but when you bend and fold and roll it, you can get micro cracks.”
By combining the two, the inorganic layers block ingress except where they crack, and the organic layers prevent cracks propagating, reducing the chance that cracks will line up. The mechanical and diffusion properties of these multi-layer structures are complex and Genoe is not convinced they have yet been adequately modelled to predict durability.
“Long-term testing is still needed because performance is very difficult to predict,” Genoe said. “For a TV, we would like to have life of 20 years. It is not always clear that accelerated test mapping is predicting accurately.”
According to Janice Mahon, vice-president of technology commercialisation at UDC, the development of an effective barrier system is “the critical path element” for durable OLEDs on flexible substrates.
UDC has a single-layer encapsulation technology for plastic substrates using plasma-enhanced chemical vapour deposition that can provide at least 500 hours of OLED life with no degradation at 85ºC with 85% humidity.
Although Polyphotonix can make flexible OLED displays – it has access to custom roll-to-roll manufacturing equipment – the company’s chief executive is pragmatic, and currently favours production on impervious glass substrates.
“Barrier layers have improved,” said Kirk, “but no one is making kilometers and kilometers of film with barrier layers, so it is difficult to enter flexible OLED manufacturing if you cannot get the supply chain in place.” As Kirk’s customers want lighting, and lighting customers generally demand long life, “there has to be a compelling reason for using plastic foil, like needing flexibility or conformability,” he said.