Organic semiconductors: prototyping the future

Organic semiconductors: prototyping the future
For Toby Cull, prototyping describes how the researchers at Merck “take materials from chemistry and make them do something exciting,” © iStock

Dr Toby Cull explained Merck Chemicals’ prototyping activities in the field of organic semiconductors to the IDTechEx Show in Berlin.

In April, ScTech Europa attended the 2018 IDTechEx Show in Berlin, and there listened to an interesting presentation on organic semiconductors by Dr Toby Cull, head of device prototyping at Merck Chemicals Ltd.’s UK office.

Cull’s address, made as a part of the session entitled ‘New Material Advances’, was called ‘Merck Organic Semiconductors – From Chemistry To Prototyping’,  and focused on how the maturity and confidence in organic thin film transistor (OTFT) technology continues to grow with properties which further differentiate from traditional a-Si solutions. Cull thus discusses ‘the pipeline of OSC development in Merck, and how, through collaboration with integrators and production partners, Merck’s Lisicon® organic semiconductor materials can enable flexible displays with novel additional functionality.’

For Cull, prototyping describes how the researchers at Merck “take materials from chemistry and make them do something exciting,” and he went on to explain that curiosity is a key driver of the innovative work that is done in the area of prototyping at the company, which has recently celebrated its 350th anniversary (making it the oldest chemical and pharmaceutical company in the world) as it looks to the future.

Hybrid electronics

In the area of hybrid electronics, he told his audience, Merck is active in a number of areas, working on materials for, for example, next generation printable organic and inorganic electronics such as sensors (photodetectors and pressure sensors), circuits (smart tags) and flexible/robust displays, and he explained that his talk would focus first on the molecular engineering organic semiconductors for OTFT and some of the applications that can be enabled.

“Starting in the lab,” he went on, the chemists start with the basic building blocks, where monomers are used to make polymers. “These start with two molecular subunits which are selected to synthesise a family of conjugated polymers.” Here, Cull said, 1-Bromo-4-hexadecylbenzene has been used by Merck as a side group introduced by bridging, with the tailor made properties (such as solubility and electrical tuning) being introduced through the addition of other side groups. “We can also improve the packing of the materials by stacking them,” he added, which enables a charge transfer across the chains.

The polymerisation then goes on to be achieved – and while, he explained, this is just one example, there are others, and he briefly discussed the aromatic polymerisation of indacenodithieno[3,2-b]thiophene (IDTT) and dibenzocyclopentathieno[3,2-b]thiophene (DBCTT) monomers, which can be extended, resulting in “different materials that our chemists can then take and analyse for things like band gap and molecular weight. These are then sent to be tested and screened for things like mobility.”

Mobilities

“As a result of this,” Cull continued, “as we start playing with the structures and the chemistries, we start seeing different mobilities. Our reference point here for organic semiconductors is often amorphous silicon,” which has a mobility of around cm2/Vs (<150˚C). “And we can see now that organic polymers are achieving performances far in excess of this,” he added.

Cull also explained that stacking the materials is crucial to the innovations taking place at Merck, with the different layers of the stack maximising the performance that the company’s customers can get from their materials.

The stacks, however, are slightly different depending on whether the focus is being placed on lithography or printing, with the former, he explained, typically using traditional mask-based microfabrication techniques for the enabling of flexible thin film technology (TFT)-sheet-fed production. Lithography will also, he added, have a faster integration route to market, enabling display makers to ‘fill’ existing LCD lines with minimal investment. Furthermore, he said, organic TFT’s can be made in-house with Merck OTFT and photo-resist materials.

Regarding printing, Cull explained that he sees this as “the ultimate approach to cost effective manufacturing of large and small scale electronics.” Printing enables flexible electronics “from the roll,” while also meaning that layers are patterned additively with no subtractive etching, photolithography or vacuum processing steps required.

He went on to argue that printing represents a promising future for OTFT as it is a technology naturally suited to additive manufacturing and, indeed, it is already being deployed for frontplates.

Moving to prototyping

“The confidence and maturity of this organic transistor technology means that we can take this forwards and start thinking of prototyping,” Cull continued, “And we do this in different ways. One of which is through public projects.” Here, he mentioned Merck’s involvement with the ATLASS (‘Advanced High-Resolution Printing of Organic Transistors for Large Area Smart Surfaces’) project, which aims to ‘help achieve the breakthrough of printed electronics in various domains such as smart packaging, safety, tagging, robotics and mobility,’ and which, Cull said, is exploring new applications for printed active-matrix backplanes on foil, targeting application demonstrators.

“The output of this project is for demonstrators which are planned for next year,” Cull said, adding that “TFTs are being developed for pressure sensors for crash testing,” as are sensing skins for robotics, and temperature sensors, and intelligent labels.”

Prototyping with partners

Alongside its work in projects such as ATLASS, Merck is also prototyping by working with its partners, by taking some of their technologies to potentially unlock new form factors, use cases, and user experiences.

Here, Merck works with partners including FlexEnable and Peratech, and Cull highlighted work that has been taking place to develop an OTFT backplane for QTC force-touch arrays. Here, the OTFT back plane no a PEN substrate was designed by Peratech and produced at Flexenable, and uses Merck OSC materials to enable mobility ˃ a-Si.

Cull said: “This material can be discrete but also if we put this in combination with again an OTFT back plane to create a force array matrix.” He went on: “It is a 25ppi display, and 10x5cm.”

Alongside this, a free form LCD display has also been developed. Merck’s liquid crystal technology allows the front plane of the display to flex or confirm. The display has a 130ppi resolution.

It is once the front and back planes are combined, Cull said, that the challenges really begin. He explained that “the pressure sensing is underneath the entire stack for the liquid crystal, so when you press on the display you have to go through the display, through the back light, and then into the active matrix force sensor.

“Because of that, the sensitivity of this prototype isn’t fantastic but it does work; the minimum sensing is about 100g and it can go up to 1kg. We get some quite unique things with this technology too, such as a joystick effect.” This, he said, means that when the screen is pressed, “you can roam your finger like a joystick and because of the sensitivity of the array, you can see where your finger is pointing, and so this could lead to some interesting user interfaces.”

Applications and use cases

According to Cull, this technology can find applications in smartphones, while the ‘joystick effect’ means that it could also have industrial uses – foe users with dirty hands, gloved, or wet hands, for example. “There is no digitiser, it is just force,” Cull said, and so there is also scope for automotive applications, as he explained that often when driving and attempting to operate a touch screen device, the user can experience false touches as they are moved slightly with the motion of the vehicle. With the new technology, the user can touch the screen, and maybe experience some haptic feedback that they have made contact with the screen, and then they will need to use force to actually control what they want. “There would be none of this false touching,” he said. “And you can also keep your eyes on the road.”

Bringing his presentation to a close, Cull highlighted once more the advantages of curiosity-driven prototyping. He said: “We can enable new prototypes and we can really look at new use cases and see if this is possible.” Merck’s prototyping activities also means that it is becoming more familiar with the technical insights related to device integration.

Cull went on: “Organic electronics has advanced a long way, and the materials are getting a lot better; but there is still a lot of complexity involved in bringing all these things together into a working system, and then making that reproducible. Through our prototyping activities we are learning a lot about this.”

The curiosity-driven prototyping activities being overseen by Cull at Merck are thus bearing fruit, and as the company looks to the next 350 years of operations, there is no sign of this innovative approach slowing down.

This article will appear in SciTech Europa Quarterly issue 27, which will be published in June, 2018.

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