Hagen Klauk on the Quest for Organic Transistors
Scientist Interview: June 2010
Did you know about that material before these Stuttgart chemists approached you?
I didn't know. These chemists said, "We have these materials, what do you think? We should do something with it," and so we did.
Are you surprised that this paper is so highly cited?
When we saw how good the transistors were, we expected that this would be an important development in organic electronics. And that's why we sent it to Nature.
At that time it was clear to everyone that the operating voltages of organic transistors had to come down, that we had to come up with a way to bring down the voltage. When we saw how easy it was to do and how good the performance was, we knew that would have a big impact. So it wasn't a surprise that it did.
In the field of materials science itself, your most-cited paper is the 2003 Advanced Materials article on the relationship between structure and performance in an organic transistor (Halik M, et al., "Relationship between molecular structure and electrical performance of oligothiophene organic thin film transistors,"15[11]: 917-+, 5 June 2003). What did that say and how did that research come about?
Marcus Halik, the first author, had established a very fruitful collaboration with two outstanding scientists at a company in Germany who had synthesized a series of organic semiconductors. By a series, I mean these all had the same basic structure but they systematically varied a substituent that was attached to that basic structure.
You could now make a transistor with each semiconductor, measure them, and get what's called a structure-property relationship, if you're lucky. Marcus did that and we saw that as this substituent got longer and longer, the performance of the transistor changed in a systematic manner.
Marcus suggested a structure-property relationship that would explain our observations. In effect, by tuning the length of that substituent, we could tune the performance of the transistor.
So for the first time we had a grasp on what makes an organic semiconductor a good semiconductor for transistors. Until then we had just been doing trial and error, creating organic transistors in different ways and seeing what kind of performance we would get.
Now we had a nice series of a half-dozen substances and we could work out the relationship between the structure of the substituent and the performance of the transistors.
"One reason why almost every electronic device you use in your daily life is based on inorganic semiconductors, like silicon or gallium arsenide, is that they're really stable."
We saw an optimal length of the substituent. If it's too short, you get disorder and poor performance. If it's too long, you get excellent order but the substituent gets in the way of electrons moving vertically through the semiconductor.
The optimal length gives you just enough order without the substituent getting in the way of things. It's like finding a tropical island with the perfect climate—not too hot, not too cold. And I suppose that's why that paper gets cited a lot.
How has your research evolved in the years since these highly cited papers were published?
We still work on organic transistors; we're still trying to find the best materials and still trying to optimize the fabrication processes, trying to squeeze the maximum performance and stability out of these devices for the simplest possible or easiest possible processing. We're trying to get the best bang for the buck, as Tom Jackson would put it.
We don't want the materials to be too complicated. If possible, we want to keep them cheap and easy and environmentally friendly. And we don't want to create really complicated structures. We want to keep it simple, so we've been working on that and on the problem of stability.
What's the problem with stability, and why is that so important?
One reason why almost every electronic device you use in your daily life is based on inorganic semiconductors, like silicon or gallium arsenide, is that they're really stable. It's essentially impossible to destroy silicon. Another reason why silicon is so dominant is that silicon devices are extremely cheap, but that's another story.
Inorganic semiconductor devices last forever. The manufacturer guarantees 10 years, but in reality they'll last far longer. Those Pioneer satellites that left the solar system were launched almost 40 years ago, and I'm sure the silicon circuits are still operational.
Organic semiconductors aren't like that. They're like you and me. We break down. These organic substances are easily changed. They're soft. They degrade.
Our transistors on day one are really nice. A week later they're really nice. If we're lucky, they're still working nicely a month later. But a year later, they're not so nice. So the stability of these devices is one issue we really have to work on. We're trying to better understand this process of degradation.
It turns out that many people have an idea why these transistors degrade, but we really don't understand it completely yet—not 100%. We're still trying to understand why they degrade and which knobs we have to turn to make them more stable. And that's a lot of materials chemistry.
Most people agree that most of the instability comes from the chemical structure of the compound, and we're trying to find ways to make more stable organic compounds. If we do, these transistors will last longer. And some of our more recent papers actually show some progress in that direction. So we're optimistic.
Hagen Klauk, Ph.D.
Max Planck Institute for Solid State Research
Stuttgart, Germany
HAGEN KLAUK'S MOST CURRENT MOST-CITED PAPER IN ESSENTIAL SCIENCE INDICATORS:
Klauk H, et al., "High-mobility polymer gate dielectric pentacene thin film transistors," J. Appl. Phys. 92(9): 5259-63, 1 November 2002 with 453 cites. Source: Essential Science Indicators from Clarivate.
ADDITIONAL INFORMATION:
- Hagen Klauk was a New Entrant in Materials Science in February 2010.
KEYWORDS: THIN FILM TRANSISTORS, ORGANIC ELECTRONICS, ORGANIC SEMICONDUCTORS, LOW TEMPERATURES, STABILITY, SUBSTRATES, DIELECTRIC, CONTACTS, INSULATOR, LOW VOLTAGE, STRUCTURAL PROPERTY RELATIONSHIP.