“Random network” nanotube films could improve the long-term stability of solar cells made of perovskite. Researchers from Aalto University in Finland, Uppsala University in Sweden, and École polytechnique fédérale de Lausanne (EPFL) in Switzerland are using these films, composed of single-walled carbon nanotubes, to increase the lifetime of solar cells. Illustration of a perovskite solar cell. Courtesy of Aalto University / University of Uppsala / EPFL. Uppsala University researcher Kerttu Aitola said the conductor layer in traditional perovskite solar cells consists of organic material and, on top of it, a thin layer of gold that easily starts to disintegrate and diffuse through the solar cell structure. “We replaced the gold and also part of the organic material with films made of carbon nanotubes and achieved good cell stability in 60 degrees and full one sun illumination conditions,” said Aitola. In the joint study, thick black films with high conductivity were used in the back contact of the solar cell where light does not need to get through. According to Aitola, nanotube films can also be made transparent and thin, which would make it possible to use them as the front contact of the cell as well. Cross-section of the solar cell in an electron microscope image. The fluff seen in the front of the image is composed of bundles of nanotubes that have become half-loose when the samples have been prepared for imaging. Courtesy of Aalto University / University of Uppsala / EPFL. The lifetime of solar cells made of silicon is 20 to 30 years and their industrial production is very efficient. Still, alternatives are needed as reducing the silicon dioxide in sand to silicon consumes a huge amount of energy. It is estimated that a silicon solar cell needs two or three years to produce the energy that was used to manufacture it, whereas a perovskite solar cell would only need two or three months to do it. Flexible solar cells were previously manufactured on conductive plastic. Compared with the conductive layer of plastic, the flexibility of nanotube films is superior and the raw materials are cheaper. They are also more flexible, which could mean production using the roll-to-roll processing method known from the paper industry. “Light and flexible solar cells would be easy to integrate in buildings and you could also hang them in windows by yourself,” said Aitola. The study has been published in the journal Advanced Materials (doi: 10.1002/adma.201606398).