Transparent thin films with high conductivity

US researchers have discovered a nanoscale-thin film material with what is said to be the highest ever conductivity in its class, in a breakthrough which could lead to smaller, faster and more powerful electronics. Their study has been published in the journal Nature Communications.

Currently, most of the transparent conductors in electronics use a chemical element called indium. The price of indium has generally gone up over the last two decades, which has added to the cost of current display technology.

As a result, there have been efforts to find alternative materials that work as well as, or even better than, indium-based transparent conductors. Now, researchers led by the University of Minnesota have found a solution.

The team developed a transparent conducting thin film using a novel synthesis method, in which they grew a BaSnO₃ thin film (a combination of barium, tin and oxygen called barium stannate) but replaced the elemental tin source with a chemical precursor of tin. This chemical precursor has radical properties that enhanced the chemical reactivity and greatly improved the metal oxide formation process. Both barium and tin are significantly cheaper than indium and are abundantly available.

“We were quite surprised at how well this unconventional approach worked the very first time we used the tin chemical precursor,” said Abhinav Prakash from the University of Minnesota, the first author of the paper. “It was a big risk, but it was quite a big breakthrough for us.”

The researchers noted that the material not only has a high conductivity, which helps electronics conduct more electricity and become more powerful, but it also has a wide bandgap, which means light can easily pass through it. In most cases, materials with wide bandgap usually have either low conductivity or poor transparency.

“The high conductivity and wide bandgap make this an ideal material for making optically transparent conducting films which could be used in a wide variety of electronic devices, including high-power electronics, electronic displays, touch screens and even solar cells in which light needs to pass through the device,” said lead researcher Bharat Jalan, also from the University of Minnesota.

According to the researchers, the process enabled the creation of material with unprecedented control over thickness, composition and defect concentration. In fact, they said it was the structurally superior quality with improved defect concentration that allowed them to discover high conductivity in the material in the first place.

The process is also reproducible and scalable and should be suitable for a number of other material systems where the element is hard to oxidise, according to the researchers. Their next step is to continue to reduce the defects at the atomic scale.