Organic semiconductors can be used in devices like light-emitting diodes, solar cells, lasers and transistors. They have the advantage of being cheap to process and can be built on flexible substrates. However, the disadvantage is that these materials generally have much lower charge carrier mobilities than their inorganic counterparts.

Researchers already know that introducing strain into the crystal lattice helps increase charge carrier mobilities in inorganic semiconductors, such as silicon and germanium. Zhenan Bao of Stanford University in California and colleagues have now shown that the technique works in organic materials too.

Introducing strain
The team employed a technique that involves placing a solution containing the organic semiconductor TIPS-pentacene between two plates, where the bottom one is heated. Sliding the top plate exposes the solution to the air and the heat from the bottom plate quickly evaporates the solvent. This technique has the effect of introducing strain into the crystal lattice of the semiconductor. The strain can increase charge carrier mobilities in the material by allowing greater overlap between electron orbitals in the component molecules.

The researchers succeeded in decreasing the so-called π-π stacking in TIPS-pentacene from 3.33 to 3.08 Angstroms. This value appears to be the shortest π-π stacking distance ever achieved in an organic semiconductor crystal lattice. Positive charge carrier (hole) mobility in transistors made of the material consequently increases from just 0.8 cm2V–1s–1 in unstrained films to as high as 4.6 cm2V–1s–1 in strained ones.

"Charge transport mobility is exponentially dependent on the distance between molecules in organic semiconductors," explained team member Gino Giri. "Making molecules pack closer together is thus an important way to increase charge carrier mobilities," he told Generating strain in organic semiconductors is also an alternative way (other than doping) to achieve unprecedented charge transport mobilities in these materials, he adds.

The team, which includes researchers from the SLAC National Accelerator Lab at Stanford, Harvard University and the Samsung Advanced Institute of Technology in Kyunggi-Do in South Korea, would now like to better understand how strain is formed in organic semiconductors. "Does this apply to other organic semiconductor materials too and can we achieve strain in a controlled fashion?" asked Bao.

The work was published in Nature.