By packing molecules closer together, Stanford University chemical engineers have improved the electrical conductivity of organic semiconductors. The advance could herald flexible electronics, more efficient solar panels, and perhaps even better television screens.

While organic semiconductors could usher in an era of foldable smartphones, better high-definition television screens and solar clothing that turns sunlight into electricity for recharging devices, current organic semiconductors do not conduct electricity very well.

So the researchers improved the ability of electrons to move through organic semiconductors by packing the molecules closer together as the semiconductor crystals form, a technique engineers describe as “straining the lattice.”

They have more than doubled the record for electrical conductivity of an organic semiconductor and shown an eleven-fold improvement over unstrained lattices of the same semiconductor.

Solution-shearing

Bao’s team used a solution-shearing technique similar to that used in current chip industry technology: a thin liquid layer of the semiconductor is sandwiched between two metal plates. The lower plate is heated and the upper plate floats atop the liquid, gliding across it like a barge. As the top plate moves, the trailing edge exposes the liquid to a vaporized solvent. Crystals form into a thin film on the heated plate.

The engineers can then “tune” the speed at which the top plate moves, as well as altering the thickness of the solution layer, the temperature of the lower plate and other engineering factors, to achieve optimal results.

The crystals form in differing structures based on the speed at which the top plate moves. The engineers tested the various crystalline patterns for their electrical properties. They found that optimal electrical conductivity was achieved when the top plate moved at 2.8 millimeters per second, a speed in the middle of the range they tested.

Ref.: Gaurav Giri et al., Tuning charge transport in solution-sheared organic semiconductors using lattice strain, Nature, 2011 [doi: 10.1038/nature10683]