New technique to control crystallization of organic semiconductor thin films

Organic semiconductor (OSC) films offer the promise of flexible, printable electronics, but their use has so far been limited by the low mobility of charges, which determines device speed. In 2011, a Stanford laboratory used a technique called 'solution shearing' to make thin OSC films containing polymorphic crystals with more than double the mobility of existing OSCs.

To figure out how the polymorphic crystals form and how to control them, the researchers joined forces with Aram Amassian's team at the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia. They used novel measurement techniques developed at KAUST to characterize the crystallization process in the thin films. Writing in Nature Communications, they report that confinement of crystallization to a thin sheet of solution only tens of nanometres thick is crucial to the formation of polymorphic crystals.

The research also revealed that changes in the solvent can tune the extent of polymorphism. Other potential tuning factors were already known, but "the solvent-dependent results are really exciting," says Amassian, "because this now shows a practical new knob by which to control and select polymorphism."

The ability to tune polymorphism is a significant advance over current approaches, which rely on trial and error for optimization. "The discovery of polymorphic control brings us one step closer to the dream of plastic electronics," says Amassian.

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