New see-through circuitry to power smart windows

Scientists have found a way to make transparent transistors and other essential components of electronic circuitry, an advance that may lead to displays on car windscreens, transparent TV sets and smart windows. Indium tin oxide (ITO) is the current material of choice for electronics because it combines optical transparency with electrical conductivity.

Its use ranges from touch-sensitive smartphone screens to light-harvesting solar panels. Indium is in short supply, and as demand increases for ITO-containing devices, so does the price of indium.

One promising low-cost ITO alternative is a transparent material known as aluminium-doped zinc oxide, researchers said.

"The elements that make up this material are more abundant than indium, making AZO a commercially sensible option," said Professor Husam Alshareef from the King Abdullah University of Science and Technology (KAUST) in Saudi Arabia.

"However, electronic devices made using AZO have traditionally shown inferior performance to devices made using ITO," said Alshareef, who led the research.

To overcome this limitation, researchers used a high-precision technology called atomic layer deposition, a process in which the circuitry is built up a single layer of atoms at a time.

Volatile vapours of aluminium and zinc in the form of trimethyl aluminium and diethyl zinc were alternately introduced onto the transparent substrate, where they adhere to the surface in a single layer before reacting in situ to form AZO.

"Using atomic layer deposition to grow all active layers simplifies the circuit fabrication process and significantly improves circuit performance by controlling layer growth at the atomic scale," Alshareef said.

For many electronic devices, the key component is the thin film transistor.

When combined in great numbers, these devices allow computers to do calculations, drive displays and act as active sensors.

Alshareef used a transparent material called hafnium oxide that was sandwiched between layers of zinc oxide to form the highly-stable transistors used to fabricate the transparent circuits.

"Our transistor properties are the best reported so far for fully transparent transistors using AZO contacts," said Zhenwei Wang, PhD student at KAUST.

Another advantage of Alshareef's approach is that atomic layer deposition only requires a temperature of 160 degrees Celsius to form each layer, which is low enough for the transparent circuitry to be formed on flexible plastic substrates as well as on rigid glass.