Stanford engineers boost electrical efficiency in organic semiconductors
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 your iPad. But there is one serious drawback: Organic semiconductors do not conduct electricity very well.
In a paper to be published in the journal Nature, researchers at Stanford led by chemical engineer Zhenan Bao have changed that equation by improving the ability of the electrons to move through organic semiconductors. The secret is in packing the molecules closer together as the semiconductor crystals form, a technique engineers describe as “straining the lattice.”
Bao and her colleagues 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.
“Strained lattices are no secret. We’ve known about their favorable electrical properties for decades and they are in use in today’s silicon computer chips, but no one has been successful in creating a stable strained lattice organic semiconductor with a very short distance between molecules, until now,” said Bao.
In the past, engineers have tried to compress the lattices in these materials by synthetically growing the crystals under great pressure. “But, as soon as you release the pressure, the crystal just goes back to its natural, unstrained state,” said Bao. “We’ve been able to stabilize these crystals in tighter formations than ever before.”
Bao’s team used a solution-shearing technique similar to a coating process well known in the semiconductor industry. Solution shearing involves a thin liquid layer of the semiconductor 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.
“Using a process so similar to current industry technology is important, as it could speed these new semiconductors to market,” said Bao.
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. These differences are clearly evident in photographs. At slow speeds, the crystals form in long, straight structures, in line with the direction the top plate is moving. At higher speeds, the crystals form wildly irregular patterns, and at other speeds the patterns resemble tiny snowflakes.
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.
“In comparing the photographs of the crystals, it is not the longest, straightest structures that result in the best electrical characteristics,” said Bao, “but the one with a shorter, yet highly consistent pattern.”
New structures, new analyses