This illustration shows the apparatus used to create a thin layer of a transparent, electrically conductive polymer for protecting 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 the top of the chamber. Illustration courtesy of the authors, edited by MIT News.Researchers at Massachusetts Institute of Technology (MIT) have improved on a transparent, conductive coating material, producing a 10-fold gain in its electrical conductivity. When incorporated into a type of high-efficiency solar cell, the material increased the cell's efficiency and stability. The researchers report their findings in a paper in Science Advances.
"The goal is to find a material that is electrically conductive as well as transparent," explains MIT professor Karen Gleason; such a material would be "useful in a range of applications, including touch screens and solar cells." The material most widely used today for such purposes is indium titanium oxide (ITO), but it is quite brittle and can crack after a period of use.
Gleason and her co-researchers developed a flexible version of a transparent, conductive material two years ago and published their findings, but this material still fell well short of matching ITO's combination of high optical transparency and electrical conductivity. The more ordered material reported in the new paper is more than 10 times better than the previous version.
The combined transparency and conductivity of a material is measured in units of Siemens per centimeter. For ITO, the values range from 6000 to 10,000, and though nobody expected a new material to match those numbers, the goal of the research was to find a material that could reach at least a value of 35. The earlier material exceeded that by demonstrating a value of 50, and the new material has leapfrogged that result, by clocking in at 3000; the team is still working on fine-tuning the fabrication process to raise that further.
The high-performing flexible material is an organic polymer known as PEDOT, which is deposited as an ultrathin layer just a few nanometers thick, using a process called oxidative chemical vapor deposition (oCVD). This process produces a layer where the structure of the tiny crystals that form the polymer are all perfectly aligned horizontally, giving the material its high conductivity. Additionally, the oCVD method can decrease the stacking distance between polymer chains within the crystallites, which also enhances electrical conductivity.
To demonstrate the material's potential usefulness, the team incorporated a layer of the highly aligned PEDOT into a perovskite-based solar cell. Such cells are considered a very promising alternative to silicon because of their high efficiency and ease of manufacture, but their lack of durability has been a major drawback. With the new oCVD-aligned PEDOT, the perovskite's efficiency improved and its stability doubled.
In the initial tests, the oCVD-aligned PEDOT layer was applied to substrates that were six inches in diameter, but the process could be applied directly to an industrial-scale, roll-to-roll manufacturing process. "It's now easy to adapt for industrial scale-up," says MIT postdoc Meysam Heydari Gharahcheshmeh. That's facilitated by the fact that the coating can be processed at 140°C – a much lower temperature than alternative materials require.
The oCVD method is a mild, single-step process, allowing direct deposition of PEDOT onto plastic substrates, as desired for flexible solar cells and displays. In contrast, the aggressive growth conditions of many other transparent conductive materials require an initial deposition on a different, more robust substrate, followed by complex processes to lift off the layer and transfer it to plastic.
Because the material is made by a dry vapor deposition process, the resulting thin layers can follow even the finest contours of a surface, coating them all evenly, which could be useful for certain applications. For example, it could be coated onto fabric to cover each fiber but still allow the fabric to breathe.
The team still needs to demonstrate the system at larger scales and prove its stability over longer periods and under different conditions, so the research 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," Gleason says.
This story is adapted from material from MIT, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.