MIT researchers have improved on their initial version of a transparent, conductive coating material for solar cells and touch screens, producing a tenfold gain in its electrical conductivity. The high-performing, flexible material, an organic polymer known as PEDOT, is deposited in an ultrathin layer just a few nm thick, using an oxidative chemical vapor deposition (oCVD) process. This process results in a layer where the structure of the tiny crystals that form the polymer are perfectly aligned horizontally, enabling the material to be highly conductive. Additionally, the oCVD method can decrease the stacking distance between polymer chains within the crystallites, which also enhances electrical conductivity. This 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 the top of the chamber. Courtesy of Meysam Heydari Gharahcheshmeh et al. To demonstrate the material’s potential usefulness, the research team incorporated a layer of PEDOT into a perovskite-based solar cell. With the new oCVD-aligned PEDOT, the perovskite’s efficiency improved and its stability doubled. In the initial tests, the oCVD layer was applied to substrates that were 6 inches in diameter. However, the process could be applied directly to a large-scale, roll-to-roll industrial scale manufacturing process, according to researcher Heydari Gharahcheshmeh. “It’s now easy to adapt for industrial scale-up,” he said. The coating can be processed at 140 °C — a much lower temperature than alternative materials require. The oCVD PEDOT is a mild, single-step process, enabling direct deposition onto plastic substrates, which is desirable 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 thin layers produced can follow even the finest contours of a surface, coating them evenly, which could be useful in some applications. For example, if PEDOT were coated onto fabric, it could cover each fiber but still allow the fabric to breathe. The team still needs to demonstrate the material at larger scales and prove its stability over longer periods and under different conditions. The research is ongoing. “There’s no technical barrier to moving this forward,” professor Karen Gleason said. “It’s really just a matter of who will invest to take it to market. “The goal is to find a material that is electrically conductive as well as transparent, [which would be] useful in a range of applications, including touch screens and solar cells,” Gleason said. The material most widely used today for such purposes is indium titanium oxide (ITO), but ITO is brittle and can crack after a period of use. The combined transparency and conductivity of both ITO and PEDOT are measured in units of Siemens per cm. ITO ranges from 6000 to 10,000, and although no one 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 MIT team’s earlier version of the material demonstrated a value of 50. The new material demonstrates a value of 3000. The team continues to work on fine-tuning the process to raise that number even further. The research was published in Science Advances (www.doi.org/10.1126/sciadv.aay0414).
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