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Shedding Light On Organic Transistors

Insights could lead to brighter and cheaper displays

4 min read

The first single-crystal organic transistor that can be switched on and off by light is giving physicists a unique peek into the way photons interact with organic semiconductors. The new device could have a major impact on the way organic light-emitting displays are manufactured and may lead to a new generation of high-quality flexible color displays.

The development is emblematic of a renaissance in the field of single-crystal organic-transistor research, which almost collapsed in September 2002, when it hit the headlines for all the wrong reasons. One of the field's rising stars, a young researcher at Bell Laboratories in Murray Hill, N.J., named Jan Hendrik Schon, was found to have falsified the results of groundbreaking organic transistor experiments.

Physicists had toyed with poor-quality transistors made out of amorphous or polycrystalline organic materials for almost 20 years. But nobody had been able to make the jump to much higher-quality single-crystal devices.

Schon claimed to have produced the world's first transistor made of a single-crystal organic semiconductor--a huge step forward that astounded colleagues. But when others could not repeat his work, they became puzzled. The confusion turned to anger when evidence emerged of probable fraud. Many people left the field in disgust, says Vitaly Podzorov, a physicist at Rutgers University, in New Brunswick, N.J.

A few months before the affair broke, Podzorov and colleague Michael Gershenson had had their own breakthrough. In early 2002, they had made their own single-crystal organic transistor. Their method was somewhat different from Schon's, but it worked.

Both their design and the one proposed by Schon consist of an organic semiconductor called rubrene attached to two electrodes. To switch the transistor on, an electric field supplied by a gate has to be applied to the crystal to draw charge to the surface, where it can flow between the two electrodes. Remove the voltage from the gate and the charge sinks back into the material, preventing any further flow of current.

The electrode that applies the gate field must be insulated from the crystal. Schon claimed that he had used aluminum oxide as an insulator. But physicists now know that the high-energy sputtering process necessary to deposit the aluminum oxide actually destroys the organic crystal. Instead of aluminum, Podzorov and Gershenson used parylene, an organic polymer insulator that can be deposited as a vapor at room temperature. Podzorov didn't know it at the time, but with Schon's work about to be discredited, the world's first organic single-crystal transistor belonged to him.

In most amorphous or polycrystalline organic transistors, the mobility drops as the temperature falls. How can that be?

Now Podzorov has gone a step further. "We have known for a long time that these organic semiconductors are optically active," he says. "I was playing in the lab one day, exposing one of my transistors to light, when I discovered a dramatic effect." It turns out that photons free up charge within the crystal, allowing the charge to migrate. By applying an electric field to the crystal at the same time as the light, Podzorov was able to move this charge nearer to the surface or away from it.

In effect, he had discovered that his transistor can be switched on and off by light. "The persistent and well-controlled switching of organic transistors with light is done for the first time," he says of the work, to be published in Physical Review Letters on 8 July, as this issue went to press [see photograph, ].

Podzorov's work is of more than passing interest to many physicists. Organic transistors have been made using amorphous or polycrystalline semiconductors since the mid-1980s. They are of relatively low quality with curious properties that researchers have puzzled over. For example, the mobility of charge within a transistor is an important indicator of its performance. In silicon transistors, mobility increases as the material is cooled because electrons move more easily at low temperatures. But in most amorphous or polycrystalline organic transistors, the mobility drops as the temperature falls. How can that be?

"It turns out that what you are measuring is a property not of the material but of the defects and impurities within it," says Alberto Morpurgo, a physicist at Delft University of Technology, in the Netherlands, who works in the field.

Single-crystal organic transistors are changing all that. Free of defects and impurities, these transistors allow physicists to study the underlying electronic properties of all organic transistors for the first time--leading to some remarkable insights.

In silicon, the current is a flow of positive or negative charge. But in organic materials, the charge generates polarons--moving charges that pull crystal lattice distortions along behind them. The way polarons cross grain boundaries where there are crystal discontinuities and how they are influenced by impurities is hugely complex. That's what makes the behavior of polycrystalline and amorphous organic transistors so hard to fathom.

And it's not just charge that puzzles physicists. The way light induces current within organic transistors is also poorly understood, because it interacts just as easily with defects, grain boundaries, and impurities as with the material itself.

For the makers of the organic light-emitting diode (OLED) displays, which are used in everything from cameras to mobile phones, that poor understanding of light's interactions with material properties is a problem. The OLED pixels are switched on and off by thin-film silicon transistors that are held on a sheet of glass. Today's amorphous or polycrystalline organic transistors are just not up to the job because bathing them in light generates unwanted photo-induced currents. This is one big reason, says Morpurgo, why Podzorov's new light-sensitive transistor is so important.

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