Transparent films are cheaper to make, more environmentally friendly

Conventional TCO films consist of a mixture of tin oxide and the increasingly expensive indium. However, a less expensive option uses zinc oxide mixed with aluminium, which is usually applied to the substrate in a high vacuum by means of plasma sputtering. The manufacturing process is complex, making it expensive, and energy intensive. Looking to address these issues, Empa researchers have developed a water based method of applying a TCO film made of aluminium and zinc salts onto a substrate without a vacuum. The film can also be cured at 90°C, rather than up to 600°. "This means that our method is not only cheaper and more environmentally friendly, but also requires less energy," said researcher Harald Hagendorfer. "It is even possible to use more heat sensitive substrates, such as flexible plastics." In the Empa method, the film grows 'by itself' through a type of molecular self organisation. A short irradiation process with a UV lamp is said to be sufficient to produce excellent conductivity. However, aluminium zinc oxide (AZO) prefers to grow tapering upwards, but for optimum conductivity, , there must be no gaps between the crystal 'pillars'. The solution devised by Empa was to use a 'molecular lid', allowing the material to only grow to a limited height and to then grow laterally, resulting in a compact transparent film with optimum conductivity.