Spray-printed crystals could replace silicon

Scientists from the University of Surrey have demonstrated for the first time a low-cost, scalable spray-printing process to fabricate high-quality isolated organic single crystals. The method is suitable for a wide variety of semiconducting small molecules, suggesting that it could be used to replace silicon with printable organic semiconductor inks.

Writing in the journal Nature Communications, the researchers explained that single-crystal semiconductors, such as silicon, serve as the backbone of several electronic devices. Unfortunately, inorganic single crystals tend to be grown from a melt at very high temperatures in special chambers filled with inert gas, using time-consuming and energy-intensive processes.

“Organic semiconductor single crystals, however, can be grown using solution-based methods at room temperature in air, opening up the possibility of large-scale production of inexpensive electronics targeting applications ranging from field-effect transistors and light-emitting diodes to medical X-ray detectors,” the scientists said.

In collaboration with the UK National Physical Laboratory (NPL), the Surrey researchers showed how a variety of semiconducting small molecules can be dissolved in solvents to make semiconducting inks and then be deposited on virtually any substrate. This ‘spray-printing’ process combines the advantages of antisolvent crystallisation and solution shearing with the crystals’ size, shape and orientation controlled by “the sheer force generated by the spray droplets’ impact onto the antisolvent’s surface”, the researchers said.

Images of spray-printed single crystals generated under different spraying angles and distances.

“If we look at silicon, it takes almost 1500°C to grow semiconductor-grade crystals… and it will fetch a very hefty electric bill for just 1 kg of silicon — same as for running a tea kettle for over two days non-stop. And then, you would need to cut and polish those silicon ‘boules’ into wafers,” said Grigorios Rigas, a first author on the study from the university’s Advanced Technology Institute (ATI) and NPL.

“We can make single crystals in a much simpler way, entirely at room temperature, with a £5 artist spray brush,” continued Rigas. “With a new class of organic semiconductors based on carbon atoms, we can spray-coat organic inks onto anything and get more or less the right size of crystals for our devices right away.

“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. We can also beat silicon by using light-emitting molecules to make lasers, for example — something you can’t do with traditional silicon.”

The crystals have been found to be high-quality structures, as confirmed by a combination of characterisation techniques. The method thus offers a powerful approach for controlling the shape and dimensions of single crystals, opening up “amazing capabilities for printable organic electronics”, according to Rigas.

Top: Images of a single crystal captured under different angles.