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Optoelectronics/Displays??

For flexible future, think organic

Posted: 18 Jul 2005 ?? ?Print Version ?Bookmark and Share

Keywords:rfid? flexible display? electronic paper? dodabalapur? organicid?

Dodabalapur: Startup companies tied to universities are one of the main engines of innovation these days.

Electronic paper. Flexible displays. RFID tags that sell for a penny apiece. These are among the promises of organic electronics that Ananth Dodabalapur has been exploring since his first organic-electronics program at bell labs in 1991. Now a professor of electrical and chemical engineering at the University of Texas in Austin, Dodabalapur is also a co-founder of startup OrganicID, which is pursuing the development of cheap organic RFID tags. It all started, he tells EE Times, with a picture he saw in a magazine.

EE Times: How did you get interested in organic electronics?
Ananth Dodabalapur: I was working on inorganic semiconductors at Bell Labs, and in 1991 I saw a photograph on the cover of Nature showing a flexible polymer led. I looked at it and said to myself, "This is a rich gold mine for device physics."

What are some applications for organic semiconductors?
Electronic paper is still one of the three promising applications for organic electronics, with RFID tags and chemical sensing, and I wouldn't say that one is better or further along than another.

Chemical sensing is one of my main research activities in Texas, using organic semiconductors for organic and biological sensing. It has something in common with RFID and display applications in that they all require low-cost substrates.

Sensing makes use of the fact that these organic semiconductors are chemical and interact with other chemicals, with transduction from either the vapor or solution phases. The way sensing devices work is very natural; the molecules work together to transport charge.

There is a change in the sensing material, but these are usually reversible. We have shown that they can cycle as much as 70 times, though it may be less with ketones and other strong chemicals.

The interactions are weakstrong enough to sense, but not so strong to make the changes irreversible. We decorate these organic semiconductors with organic receptors and increase the selectivity of the sensors. But the reusability of these sensors is not easy. In most cases, we don't care and just throw it away after extracting the information. In fact, that may be a better way of building sensors for one-time use. Then we are absolutely sure that we have a clean sheet when we start out.

Tell us about the Bell Labs work.
I made a proposal to the management at Bell Labs and it was well-received. There was a core group of half a dozen of us that stayed on the program throughout the 1990s until the early years of this decade. It was the first organic-electronics program in the United States, though others had been established in Europe. Most of my work at Bell Labs was in polymer transistors, but we also worked on OLEDs that emit red, green and blue.

What are polymer transistors made of?
The highest-mobility transistors are based on pentacene. But today, its stability is not the best, and processing is difficult. Pentacene is not easy to depositit has to be in a vacuum. Some of the new materials may be longer-lasting. For different projects, we have different agreements with companies and universities. I see myself as a consumer of materials that come from other places. For instance, Northwestern University is developing n-channel materials and we have collaborative relationships with them. We work with various university chemistry groups and some chemical companies. What we do is focus on device engineering.

What are some characteristics of these molecules?
These are conjugated organics, with alternating single and double bonds that allow charges to move easily. They are well-ordered, as crystalline as possible, to allow high mobility.

In all conjugated organics, there is one limiting factor: the interaction of carriers with phonons. These are soft materials, and charge carriers interact with the lattice. At room temperature, mobility is limited by this coupling problem. The mobility for organics is limited to about 10cm?/V-s, which compares with a few hundred for silicon and a few thousand for GaAs. But that's enough for many applications. The performance is more than enough for e-paper and just sufficient for RFID tags.

Typically, we might see between 0.1cm?/Vs and 1cm?/Vs for certain materials. That matches with amorphous silicon, which has 20 years of development behind it.

These organic molecules are larger than silicon atoms. Pentacene molecules are about 20, while silicon lattice spacing is a few angstroms. Pentacene is one of the materials we talk about a lot in public. Others are procured through various arrangements, so we don't like to talk about their characteristics much.

What about the startup with which you are involved, OrganicID?

Klaus Dimmler is the CEO and he has experience in the RFID field. The university, a strategic intellectual-property partner, encouraged us to set it up. The goal is to use organic-electronics technology to create low-cost electronics for one-cent RFID tags, a cost target that silicon-based electronics presumably will not be able to match.

Are you the CTO?
I am an adviser, not an employee, because I am a full-time employee at the university.

I've been doing transistor research for 10 years, so I have certain insights on how to solve problems and cross bridges.

It keeps me interested in the sense that it is not an academic exercise. It's not publishing a paper; it's about doing something real.

Startup companies tied to universities are one of the main engines of innovation these days. And in this sense the United States is well ahead of any other country. I've had friends, in the United Kingdom, for instance, tell me how advanced the whole entrepreneurial structure is here, the knowledge base and skill level of the venture-capital community.

Where is the company at now?
We have enough funding for our needs at this moment. We have four people in Colorado and four in Austin, though we probably will coalesce to one place soon. Our circuit design team is based in Colorado, so they can be anywhere and still be very effective.

To get to one-cent RFID tags, the demands placed on transistors are higher than for displays. We need relatively high mobility. We need very small channel dimensions, ways to make structures small and practical, with design rules of a few microns. With 4,000-5,000 transistors, we can create an antenna, an RF front-end, the processor and memory.

What about packaging? In some cases, packaging costs more than the bare die itself.
That has been a concern, but it's also one of our advantages. With a modest amount of packaging, organic semiconductors can last a long time. So we're looking at a couple of low-cost packaging options.

You worked on e-paper at Bell Labs. What is the latest on that?
Most of the e-paper IP that we've developed at Bell Labs has been licensed to E Ink Corp., which is continuing to work on it. Xerox, Philips and E Ink are the main companies, though there are others, in the field.

E-paper has the look and feel of paper, but the information can be changed electronically. In the future, we'll be downloading books and using this kind of display to read. There's a lot of expense from paper. And the environmental aspects are there, so reusable e-paper is quite attractive. We need a flexible dense array of transistors with adequate performance to be the enabling backplane.

One thing about print is that it has high resolution, so you can read a newspaper for an hour or so. What about e-paper?
Well, 150dpi should be good enough for reading. But even 300dpi is doable. This will give consumers a new way to handle data and publishers a new way to market information.

What needs to happen to get to 150dpi?
There's a need for integrated row and column drivers, shift registers and decoders that also need to be organic. It's not practical to have a silicon chip to drive the OLED.

We have demonstrated 1,000-transistor CMOS circuits with organics that should be able to do the job. For most reading, organic electronics can efficiently be used as drivers. At a clock rate of 1kHz, we can change all of the information in a 1,000-by-1,000 element in 1s, or a fraction of a second if we are clever about the electronics. That's plenty for reading. It takes much longer to change a Webpage on a high-speed computer.

With e-paper, you are holding it, bending it and not worrying about dropping it. You can sit back and relax. Notebook LCDs are getting better with time, but you'll never be as comfortable with them as with holding something in your hands. Cost is another factor. E-paper is much cheaper.

- David Lammers
EE Times




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