Close packing: Flexible boost for electronics

Flexing the organic

US researchers have found that packing organic semiconductors closer together can boost the materials' electrical conductivity. X-ray diffractions studies reveal structural details that take us one step closer to more efficient solar energy panels, better television and computer screens and perhaps even flexible electronic gadgets.

The idea of flexible electronics has been touted time and again, and rightly so, imagine devices that you can sit on without cracking the screen or a gadget that can be moulded to fit an object whether seat cover or lower limb. Underlying the development of such flexible electronics are organic semiconductors. They could usher in flexible devices akin to the electronic book reader, or inexpensive high-resolution display technology. But, flexibility aside, organic semiconductors are not as good as their inorganic counterparts. That might change if work published in Nature from chemical engineers at Stanford University fulfils its promise.

Straining the lattice

Zhenan Bao have discovered that packing more tightly together the molecules from which their organic semiconductor is made can boost its conductivity by "straining the lattice." Using this approach, Bao and her colleagues, Guarav Giri, Eric Verploegen, Hector Becerril, have beaten the record by more than twofold for electrical conductivity of an organic semiconductor - 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) - and shown an eleven-fold improvement over unstrained lattices of the same semiconductor.

"Strained lattices are no secret," Bao explains. "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."

Previously, researchers have tried to induce strain in a lattice by synthesising the crystals under high pressures. However, once the pressure is released the crystal reverts to a less strained polymorphic form. Bao and colleagues have taken a different approach to stabilize strained crystals with tighter formations than ever before. The researchers used a solution-shearing technique similar to a coating process well known in the semiconductor industry. Solution shearing involves sandwiching a thin liquid layer of the semiconductor between two metal plates. The lower plate is heated and the upper plate floats on top of the liquid film. The movement of the upper plate exposes the liquid behind the plate's trailing edge to a vaporized solvent, which causes crystals to form as a thin film on the heated plate.

The team can fine 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, so that they can optimise the process. The team explains that different crystal structures are possible by controlling the speed at which the top plate moves. At lower 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 millimetres per second, a speed in the middle of the range they tested.

Scattered pictures

Bao worked with Stefan Mannsfeld and Michael Toney at the Stanford Synchrotron Radiation Lightsource to carry out X-ray scattering experiments to validate the technique. "We have been able to improve how we analyse the relative brightness of the peaks we can see in X-ray diffraction images," explains Mannsfeld. "Previously this was only possible when analysing relatively big single crystals, but we have, for the first time, been able to duplicate this for very thin films of these crystals." Mannsfeld adds that, "Our analysis made it possible not only to see the impact of the strain on the lattice geometry, but also to determine the exact way in which the molecules pack in the lattice. As a result, we obtained a better understanding of why such structures improve the molecule-to-molecule electrical coupling that improves the electrical efficiency."

By adapting and exploiting current industrial technology in this way, Bao hopes that the process could accelerate the development of organic semiconductors and bring them to market more quickly.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.