Advancing conductivity in coating for touchscreens

Materials World magazine
6 Jan 2020

A new conductive coating has been incorporated into advanced solar cells, increasing their efficiency and stability. Shardell Joseph finds out more.

A transparent coating material with increased electrical conductivity has been incorporated into high-efficiency perovskite solar cells that are suitable for touchscreen technology. Researchers from Massachusetts Institute of Technology, USA, claimed the new material has potential for relatively easy production at industrial scale.

Improving on an existing conductive material they developed two years ago, the researchers stated that the new coating outperformed the previous one with 10 times the conductivity. By utilising poly(3,4-ethylene dioxythiopene) known as PEDOT – a polymer mixture made of two ionomers – the team used an oxidative chemical vapour deposition (oCVD) method, enhancing intercrystallite mobility. The integration of oCVD EDOT also improved both power conversion efficiency and shelf-life stability of inverted perovskite solar cells.

A transparent solution

The aim of the research, according to MIT Materials Science and Engineering Professor, Karen Gleason, is to find a material that exhibits both electric conductivity and transparency. ‘This would be useful in a range of applications, including touchscreens and solar cells. The material most widely used today for such purposes is indium titanium oxide (ITO), but that material is quite brittle and can crack after a period of use,’ she said.

The material used today for such purposes is ITO, but this is brittle and prone to cracking. The conductivity and transparency are measured in units of siemens per centimetre (s/cm). ITO will usually range from 6,000-10,000s/cm – a figure that is not expected to be reached by any newly developed materials. The team developed a material two years ago that reached 50s/cm, whereas the researchers’ newest material reached 3,000s/cm. The team expects there is potential to raise this further.

For applications such as organic light-emitting diodes, batteries, supercapacitors and biosensors, there is demand for lightweight, breathable and mechanically flexible devices developed with highly electrically conducting thin layers.

However, flexibility and breathability within electrodes have many limitations, including lack of compatibility, cost-ineffectiveness and complex integration conformal coverage complications.

As described in the paper, Tuning optimisation, and perovskite solar cell device integration of ultrathin poly(3,4-ethylene dioxythiophene) films via a single-step all-dry process, published in Science Advances, the team used three key methods. This included polymerisation, doping and thin film formations to develop the conductive, transparent polymer by permitting direct deposition of conducting layers onto thermally sensitive substrates.

Once polymerisation has reached completion, the oCVD process is used to deposit the organic polymer PEDOT into a layer that is only a few nanometres thick. The purpose of this process is to give the material its high conductivity, resulting in a horizontally aligned structure consisting of the tiny crystals that form the polymer. The oCVD method can also further enhance electrical conductivity by decreasing the stacking distance between polymer chains with crystallites.

Once the material was developed, the team demonstrated its potential by incorporating a layer of the PEDOT into a perovskite-based solar cell. According to the study, the integration of PEDOT improved the efficiency of the perovskite and increased its stability two-fold.

In the preliminary tests, the oCVD layer was applied to substrates that were approximately 15cm in diameter, but the process could be applied directly to a large-scale, roll-to-roll industrial-scale manufacturing process, said MIT Materials Science and Engineering Researcher, Heydari Gharahcheshmeh.

‘It’s now easy to adapt for industrial scale-up. That’s facilitated by the fact that the coating can be processed at 140°C — a much lower temperature than alternative materials require,’ said Gharahcheshmeh.

The team still needs to demonstrate the system at larger scales and prove its stability over longer periods and under different conditions, so research is ongoing. But Gleason stated that ‘there is no technical barrier to moving this forward. It’s really just a matter of who will invest to take it to market.’