Traditional electronic devices are made from single inorganic crystals, usually silicon, typically grown from a melt at very high temperatures in chambers with inert gases. This takes time and energy, requiring temperatures as high as 1500°C.
Organic semiconductors, by contrast, can be grown as crystals at room temperature and could open the door to large-scale manufacture of cheaper electronics. Researchers have now reported spraying such semiconductor crystals onto a surface to generate transistors using a cheap high-street spray nozzle – the first time that a low-cost, scalable spray-printing process has been demonstrated with such high-quality organic crystals, the researchers report (Nature Communications, doi: 10.1038/ncomms13531).
‘We used different techniques in the past, such as inkjet printing, dip casting and spin coating,’ says senior author Maxim Shkunov at Imperial College London, UK. ‘But this new technique is probably the simplest we can think of to deposit a layer of crystalline material.’
Important properties such as charge transport depend on having crystals with high molecular order, he points out. ‘The difference with charge carrier mobility, or how quickly charges can move within a film, can differ by eight orders of magnitude, just by going from amorphous film to really single crystal films,’ says Shkunov. ‘If we make transistors, their switch and speed are defined by the mobility of charge carriers. Improve this mobility by six orders of magnitude, we can improve the transistor the same.’
They sprayed the crystals in solvent solution on top of an ‘anti-solvent’ coating already present on the substrate. Crystal growth began once the solvent started to evaporate on the anti-solvent surface. The size, shape and orientation of the crystals were controlled by the spray angle and distance to the substrate.
‘For the spray coating, we used an artist’s brush, which costs £5 to £10,’ says Shkunov, adding that crystals generated by the technique may have applications in making cheaper transistors, bendable electronics, LEDs, lasers and polymer sensors for clothing.
‘Their technique is a clever way of quickly fabricating lots of relatively uniform crystals of organic semiconductor. These kinds of scalable approaches are important because without them the use of crystalline organic semiconductors won’t be feasible for industrial-scale manufacturing,’ comments Stuart Higgins at the Optoelectronics Group of the Cavendish Laboratory, University of Cambridge, UK.
‘The next step would be to show how their method can be combined with a quick, automated process for aligning and positioning the crystals where they’re needed - something which could potentially slow down a future manufacturing process.’