The ‘straining’ process essentially involves packing the molecules closer together as the semiconductor crystals form — a technique currently used in silicon electronics, which has so far proved difficult in organics.
Wide-scale adoption of organic electronics is hoped to lead to devices that are cheaper, lighter, more robust and more flexible compared with their silicon counterparts.
However, it is believed they can never compete with silicon in terms of performance because the organic materials fundamentally limit the mobility of electrons and thus the speed of operation.
Nevertheless, it may be possible to considerably speed up their action through fabrication modifications such as straining.
‘Strained lattices are no secret. We’ve known about their favourable 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 project lead Prof Zhenan Bao of Stanford University.
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 stabilise these crystals in tighter formations than ever before.’
Bao’s team used a technique called solution shearing, which involves a thin liquid layer of the semiconductor sandwiched between two metal plates. The lower plate is heated and the upper plate floats on top of the liquid, gliding across it. As the top plate moves, the trailing edge exposes the solution to a vaporised solvent and — heated by the lower plate — the crystals form into a thin film.
The crystals form in differing structures based on the speed at which the top plate moves. The engineers next tested the various crystalline patterns for their electrical properties and found that optimal electrical conductivity was achieved when the top plate moved at 2.8mm/sec.
With this, they more than doubled the record for electrical conductivity of an organic semiconductor and showed an 11-fold improvement over unstrained lattices of the same semiconductor.
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