Solution coating of organic semiconductors is an excellent method for making large-area and flexible electronic materials. However, it is not at all good for making aligned single-crystalline thin films – the ideal form for organic semiconductors and that have the best electronic properties. Aligned crystals are preferred in these materials because charge carrier transport through these structures depends on the crystal orientation.

A team led by Zhenan Bao at Stanford University and Stefan Mannsfeld of the SLAC National Accelerator Laboratory is now reporting on a new solution-coating method that can produce high-quality, millimetre-wide and centimetre-long highly aligned single-crystalline organic semiconductor thin films. The essence of FLUENCE is that we are able to control the flow of liquid in which the organic semiconductor is dissolved, explains team member Ying Diao. During fast printing, this “ink” often distributes itself unevenly – something that leads to defects and other structural imperfections quickly appearing in the semiconducting crystals.

FLUENCE tackles this problem from two angles, she says. First, it works using a microstructured printing blade containing tiny pillars that mixes the ink uniformly. Second, specially designed chemical patterns on the substrate prevent the crystals from aligning randomly or “stochastically” in a direction that would be the opposite to that in which printing is taking place. These two methods combined lead to large-area highly aligned single crystalline films that are much more structurally perfect.

To prove that their technique works, the researchers fabricated an organic semiconductor made from TIPS-pentacene, a routinely used and much studied organic semiconductor material, and found a charge carrier mobility of as high as 11 cm2 V−1 s−1. This is the first time a mobility of greater than 10 cm2 V−1 s−1 has been reported for TIPS-pentacene.

“The concepts we have developed in FLUENCE could easily be scaled up and applied to commercial printing methods,” said Diao. “The significant improvement in structural quality and electrical performance of the thin films printed with our method could allow to make higher performance, lower power, small and inexpensive organic circuitry,” she told nanotechweb.org. “We hope that our work will help advance such a morphology-by-design approach to make organic semiconductors for high-performance, large-area printed electronics.”

The team, which includes researchers from Nanjing University in China, says that it will now look at pattering aligned crystals at length scales suitable for making sub-micron devices.

The present work is reported in Nature Materials.