Nov 25 2019
Researchers at the Massachusetts Institute of Technology (MIT) have further enhanced a transparent and conductive coating material by increasing its electrical conductivity by 10 times.
Illustration shows the apparatus used to create a thin layer of a transparent, electrically conductive material, to protect solar cells or other devices. The chemicals used to produce the layer, shown in tubes at left, are introduced into a vacuum chamber where they deposit a layer on a substrate material at top of the chamber. Image Credit: MIT.
When this coating material was integrated into a certain high-efficiency solar cell, it boosted the stability and efficiency of the solar cell.
This latest discovery has been recently reported in the Science Advances journal, in a paper written by Meysam Heydari Gharahcheshmeh, a postdoc at MIT; professors Karen Gleason and Jing Kong; and also three others.
“The goal is to find a material that is electrically conductive as well as transparent,” explained Gleason, which would be ‘“useful in a range of applications, including touch screens and solar cells.”
For such purposes, a material called indium titanium oxide (ITO) is extensively utilized today. However, that material is rather fragile and can break following a period of use, added Gleason.
A couple of years ago, Gleason along with her co-researchers enhanced a flexible type of transparent and conductive material and published the findings. However, this material still did not match the combination of high electrical conductivity and optical transparency of ITO. According to Gleason, the latest, more ordered material is over 10 times better when compared to the earlier version.
Conductivity and transparency together is quantified in units of Siemens per centimeter. ITO spans from 6,000 to 10,000, and although no one anticipated a novel material to align with those numbers, the aim of this study was to identify a material that can reach a minimum value of 35.
That value was exceeded by the previous publication by demonstrating a value of 50, and the latest material has surpassed that result, by currently clocking in at 3000; the researchers are still exploring ways to fine-tune the process to further raise that value.
An organic polymer called PEDOT is a flexible and high-performing material. It is deposited in an ultrathin layer that has a thickness of only a few nanometers, through a process known as oxidative chemical vapor deposition, or oCVD.
The oCVD process produces a layer where the structures of the small crystals forming the polymer are all seamlessly aligned in a horizontal fashion, providing high conductivity to the material. The oCVD technique can also reduce the stacking distance between the polymer chains inside the crystallites, and this approach also improves electrical conductivity.
In order to reveal the potential usefulness of the material, the researchers integrated a highly aligned PEDOT layer into a perovskite-based solar cell. These cells are believed to be a highly potential substitute to silicon because they can be easily produced and have excellent efficiency.
However, a major disadvantage of these solar cells is that they are not durable. Now, with the novel oCVD aligned PEDOT, the efficiency of the perovskite not only improved but its stability also increased two-fold.
In the preliminary tests, substrates measuring 6″ in diameter were applied with the oCVD layer, but the process can be directly applied to a roll-to-roll industrial-scale manufacturing process at the commercial level.
It’s now easy to adapt for industrial scale-up.
Meysam Heydari Gharahcheshmeh, Postdoc, MIT
That is simplified by the fact that it is possible to process the coating at 140 °C—a considerably lower temperature when compared to the one required by alternative materials.
Being a mild, single-step process, the oCVD PEDOT allows direct deposition onto plastic substrates, as required for flexible displays and solar cells. On the other hand, a majority of other transparent conductive materials have adverse growth conditions and therefore need a different and stronger substrate that is initially deposited followed by intricate processes to remove the layer and eventually transfer it to plastic.
Since a dry vapor deposition process is used to make the material, the thin layers can follow even the finest shapes of a surface and coat them uniformly; this may prove useful in certain applications. For instance, the material can be coated onto a piece of fabric and each fiber can be covered and still enable the fabric to breathe.
The researchers still have to demonstrate that the system can work at larger scales and establish its stability over extended periods and under a variety of conditions, hence the study is ongoing. But “there’s no technical barrier to moving this forward. It’s really just a matter of who will invest to take it to market,” stated Gleason.
The researchers included Mohammad Mahdi Tavakoli and Maxwell Robinson, both MIT postdocs; and research affiliate Edward Gleason. The study was supported by Eni S.p.A. under the Eni-MIT Alliance Solar Frontiers Program.