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OLED R&D challenges: Emitters, light extraction, materials

OLED R&D challenges: Emitters, light extraction, materials

By BizLED Bureau

Feb 21, 2017: OLEDs are based on organic (carbon-based) materials. Unlike LEDs, which are small point sources, OLEDs are made in sheets that provide a diffuse-area light source. While developing rapidly, OLED technology is less mature than LED technology, whose rapid advancement has created a moving target for OLED products in terms of lighting performance and pricing. Still, OLED manufacturers are optimistic that with a few key breakthroughs, OLEDs will offer a value proposition complementary with LED lighting approaches.

Key challenges

Innovations are still needed on multiple fronts to increase the efficiency, lifetime, and light output of OLED devices. The development of device architectures and materials systems (particularly blue) that allow for improved stability and efficiency, and methods to extract the light generated by the OLED, remain key challenges.

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In addition to increasing the performance of devices, OLED costs need to be simultaneously reduced. Manufacturing technology developments and infrastructure investments will be essential to enable price reductions and transition OLED products from the prototype stage to commercial viability. By improving the yield (panel-to-panel color, brightness consistency) and reliability (premature failure rate) through improved manufacturing processes, costs could be lowered considerably. Another possible route to decreasing production costs is the development of roll-to-roll manufacturing capabilities using printing deposition processes, which may allow for high materials usage efficiency and throughput. What’s more, roll-to-roll manufacturing is well-suited for processing on flexible substrates, and a key differentiating feature of OLEDs is their ability to operate as flexible or conformable devices.

Leveraging the flexibility and other distinctive qualities of OLEDs—e.g., thin, lightweight, large-area, diffuse light source—to create novel luminaires is another key challenge. The purpose of such luminaire concepts is to spur the adoption of OLEDs, promoting consumer interest and familiarity with the technology and generating revenue for continued R&D investment.

OLED R&D challenges: Emitters

The thing that actually converts electricity into light is the emitter itself. So there are a number of scientific challenges that are still out there in OLEDs, particularly in emitters. Even though you can go to the cell phone store and buy yourself a cell phone that’s powered by OLED emitters, and active matrix OLEDs are present in cell phones and in televisions, and it leads people to think that all of the issues are solved.

Read Also: OLED growth spurs investments among raw material producers

There’s a lot of questions of the thermodynamics of OLED emitters. What is it that leads to their degradation? There are a lot of questions about the solubility– that this is a mixed system. So how do we keep the OLED emitters dispersed? What controls their solubility in the phases that they’re in?

We spend a lot of time working through different structures for OLED emitters, where we change the ligands that are bound to the central metal. We change the groups that are on the periphery, and it changes the physical behavior of that emitter significantly. And so we spend a lot of time iterating through different structures and then asking ourselves how did that change affect the properties of the emitter? How did it affect the properties of the OLED?

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The lifetime of red OLEDs is very, very long. The lifetime of green OLEDs is very, very long. Very, very long I mean decades. And in fact the extrapolated lifetimes for red OLEDs can be as long as 100 years. The lifetime for blue OLEDs is much shorter, and it largely comes from processes that we only sort of understand.

The real goal here is to minimize that degradation of the emitter and the matrix while the device is running so that the blue OLED– just like the green and the red OLED– will have a lifetime of decades. You’re going to have a solid state lighting panel, which is what the DOE is most interested in right now that will be installed in the house, and it’s going to live as long as the house lives. It’s not– somebody’s going to take it down because they don’t like how it looks, not because the bulb burned out.

OLED R&D challenges: Light extraction

Light extraction is the process of removing light from the device itself. So when the device emits a photon, it gets trapped in the glass, in the organic layers themselves in the metals. And the– what our challenge is, is to get all of that light into the viewing angle. So that when somebody sees the OLED they’re seeing the full efficiency of that device. Rather than having light lost in the various segments.

You think of this device as occupying a plane. And the photons that you want to see are the ones that are coming more or less perpendicular to the plane. But if the light is emitted parallel to the plane then it’s trapped and you have to extract that somehow. Get it into the glass. Once it gets into the glass, then we can take it out with microlens arrays.

We put in a grid of varying index of refraction contrast and it’s sitting below the anode. It’s just in the substrate so it’s not impacting the device structure. And so the wave guided light that’s running parallel to the plane, scatters into the glass modes. If you don’t do anything you get 20% outcoupling. With the micro-lens you get about 40% and with the sub-anode grid, you get another 15%. So you’re about 55%.

And now you’re left with the hardest. And that’s where we are today, at 45%. We have some schemes that we think we can actually get around an 80% external quantum efficiency. If we do that then OLEDs would become a spectacularly efficient light source either for displays or lighting. To have 80% external efficiency is just really quite remarkable and these things work at very low voltage and so on. So this is a challenge worth approaching.

OLED R&D challenges: Solution-based materials

Solution based materials are a different way of processing the conventional OLED materials. So conventional OLED materials are processed by vacuum thermal deposition or heating up a powder in a crucible and evaporating it onto the substrate. Solution process materials are dissolved in a variety of solvents and then they can be coated by spin coating for lab sized experiments, or ink-jet printing, or slot-die coating, or a variety of other printing methods.

Solution based materials can give you some advantages in manufacturing. It could be a little bit easier to apply. They’re easier to handle and they can bring the cost down. Printing equipment is conventionally used in a lot of processes. And the substrate could come in a roll and have continuous processing and this tends to be a slightly cheaper method of manufacturing than a sheet to sheet method that is what’s being done today with vacuum thermal deposition.

Solution based materials are quite challenging. One, because most of the materials used in OLED today are not remarkably soluble. A lot of them are really rocks. So finding a solvent to put them in is quite difficult. And also a solvent that doesn’t degrade the electro-optical properties of the material over time that can be very stable. Once you make an ink, you want to be able to ship it to the manufacturer and have it sit on their shelf also for a few months and still give the same results from printing. So there’s a lot of work going on today about finding the right solvents that are shelf stable that keep the properties but also dissolve these very rock like materials.

And consequently, we’ve also developed new materials that are analogous to what we use for vapor but are slightly more soluble. So we keep the electo-optical core but we put some different moieties around to make it more soluble. So there’s a lot of optimization that goes on with those materials as well. I think we’ll really start to see solution processing of OLED displays in the next one or two years with mass production shortly thereafter. For lighting it’s a little bit more of a challenge because we don’t need to make very fine pixels of RGB for lighting. It’s really a uniform white color. So there hasn’t been a manufacturing push but once it becomes established in display, and those materials are more ubiquitous, and more available, and the cost comes down, those will be more available and attractive to lighting manufacturers as well.

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