Organic Thin Film Transistors Approach Speed of Polysilicon Cousins

For years now, Zhenan Bao, a chemical engineering and materials science professor at Stanford University, has been coming up with new techniques to speed up the charge carrier mobility of organic transistors, which have labored under painfully slow speeds compared to their crystalline- or polycrystalline silicon cousins.

A little over two years ago, Bao developed a strain technique much like that used in silicon chips to increase the speed of organic semiconductors. At the time, it was believed that the strain technique could increase the frequencies at which organic circuits operate by as much as four times the rate of existing organic devices.

While even that much of an increase still left the organic circuits operating at one-hundredth the speed of crystalline silicon circuits, the hope was that the advance had opened up a path towards cheap, plastic, high-resolution TVs.

Now Bao and colleagues from the University of Nebraska at Lincoln have developed a new technique that they claim can raise organic semiconductors' operating speeds to levels approaching those of the polysilicon-based devices that control the pixels in advanced TVs.

Advanced research-stage organic transistors have achieved carrier mobility speeds between 5 and 15 centimeters squared per volt second (cm2/Vs), according to Bao, with typical organic transistors staying at about 1-2 cm2/Vs range. The organic transistors in these experiments were not uniform in performance, but their carrier mobilities clustered around 43 cm2/Vs (with one high-end outlier at 108cm2/Vs). Polysilicon transistors typically reach 100 cm2/Vs , with the latest research claiming speeds of 135-500cm2/Vs. So, a carrier mobility speed of 108cm2/Vs is certainly in the territory of polysilicon transistors

The technique, which is described in the journal Nature Communications (“Ultra-high mobility transparent organic thin film transistors grown by an off-centre spin-coating method”)  follows most of the traditional method for creating organic thin film transistors—placing a solution of carbon-rich molecules and a polymer on a spinning disk made of glass. The novelty of the new technique is that they spin the disk at a speed that is faster than usual and coat only a small portion of the disk's surface.

The result is a denser concentration and a more regular alignment of the organic molecules. This, in turn, yields much faster carrier mobility in the resulting thin film transistors.

The Stanford-Nebraska method is still highly experimental at this point. The researchers, who have dubbed their technique "off-center spin coating," have yet to gain a high level of control over the alignment of the organic materials or achieve uniform carrier mobility.

Despite these limitations, the researchers claim that the transparent thin film transistors they've created perform at levels comparable to that of polysilicon materials currently used in advanced displays.

Photo: Jinsong Huang and Yongbo Yuan

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