Spray-on process yields quality organic semiconductors

UK scientists have grown high-quality mono-crystals of organic semiconductors, which are large enough to construct FETs on. Such FETs show good clean characteristic curves, although have comparatively low mobility.

A common way to create organic semiconductor crystals is to dissolve the material in a solvent, then deposit the solution onto a surface. Subsequent evaporation of the solvent leaves behind crystals of the semiconductor, with slower evaporation favouring the creation of larger crystals.
However, said a team of researchers from the University of Surrey and the National Physical Laboratory, these crystals are still comparatively small and generally yield only poly-crystalline transistors.
The technique invented by Surrey and NPL to grow larger crystals involves an ‘anti-solvent’ process.
In this, the organic semiconductor is dissolved in a volatile (easily-evaporated) solvent while, separately, the substrate is coated with a second solvent – dubbed the anti-solvent.
Anti-solvent
Solvent and anti-solvent are selected so that the organic semiconductor is less-soluble (or sometimes insoluble) in the anti-solvent.
And, in this case, the anti-solvent is selected to have a higher boiling point (be less volatile) than the solvent, and have a higher surface tension than the solvent.
Spraying the dissolved semiconductor gently onto the anti-solvent-coated substrate, results in a process that yields crystals of organic semiconductor floating in the anti-solvent as the solvent evaporates away.
Because the spraying is gentle, all this happens in the upper part of the anti-solvent layer, away from any disruptive substrate effects. As such, the crystals grow with few defects – confirmed by polarised optical microscopy, scanning electron microscopy, x-ray diffraction and polarised Raman spectroscopy, emphasised the University of Surrey.
Under the right conditions, these crystals are regular – a similar shape to a microscope slide – and over 20μm along the short side.
Subsequent evaporation of the anti-solvent lands these crystals on the substrate.
Solution shearing
The substrate has little effect on crystal formation, but – through a process called ‘solution shearing’ – the angle at which the spray hits the anti-solvent, and the distance from spray nozzle to anti-solvent surface, have a large effect and the size, shape and orientation of resulting crystals, said the University, and solution shearing can be used to control these attributes.
“The trick is to cover the surface with a non-solvent so that semiconductor molecules float on top and self-assemble into highly ordered crystals,” said Dr Maxim Shkunov of the the University of Surrey’s advanced technology institute. “This method is a powerful, new approach for manufacturing organic semiconductor single crystals and controlling their shape and dimensions.”
It works with many organic semiconductors, including anthracene, pentacene, tetracene, anthradithiophene and benzothiphene derivatives, said Surrey.
Most of the research was done with ‘TIPS-PEN’ – a soluble pentacene – dissolved in the volatile solvent toluene. DMF (N,N-dimethylformamide) was used as the anti-solvent as TIPS-PEN hardly dissolves in it.
A slower-evaporating solvent with similar surface tension to DMF yielded non-uniform crystals.
Bottom-contact bottom-gate transistors and the top-contact bottom-gate transistors were fabricated using the crystals, yielding devices with clear FET characteristics and 0.4cm²/Vs mobility, which is low for TIPS-PEN transistors operating in the linear region, said Surrey.
The reasons for low mobility are now under investigation, and improvements are expected.
Full results of the Surrey/NPL research are available in the Nature Communications paper: ‘Spray printing of organic semiconducting single crystals’.