A flexible transparent conductor consisting of multiple oxide layers doped onto an ultrathin silver film. [Image: ICFO]
Researchers at a Spanish laboratory have invented a multilayer transparent conductor with high mechanical flexibility (Nat. Commun., doi: 10.1038/ncomms13771). Not only does the composite of semiconductor oxides and silver transmit more than 98 percent of visible light, but it also bends easily without breaking. The researchers believe that the conductor could lead to more flexible optoelectronic devices or to lower-cost production of existing devices.
Transparent conductors, such as indium tin oxide (ITO), already exist and find uses in touch screens, flat-panel displays, thin-film photovoltaic cells and optical coatings. ITO, however, doesn't bend and must be processed at high temperatures, and is comparatively expensive to produce.
A team at the Institut de Ciències Fotòniques (ICFO) in Barcelona, Spain, built up the flexible semiconductor from layers of silver, aluminum-doped zinc oxide and titanium dioxide, all over a silica substrate, and all deposited in a room-temperature vacuum chamber. Alone, the silver has very low electrical sheet resistance, but it doesn't transmit light unless it's sandwiched in between anti-reflective coatings, which produce destructive interference between light rays reflected at the interfaces of the stack.
Transfer-matrix calculations, followed by experimental measurements of light transmission and electrical resistance, found optimal thicknesses for the oxide overcoat and undercoat layers touching the 12-nm-thick silver. The titanium dioxide layer, in particular, keeps the thin silver layer continuous and smooth—necessary qualities for good conductivity and transparency. The silver-oxide multilayer demonstrated electrical sheet resistance of 5.75 Ω/square, which the researchers said is about half that of commercially available ITO.
Next, the researchers substituted flexible polyethylene terephthalate (PET) for the silica substrate and tested their own transparent conductor against ITO-coated PET. Thanks to the ductility of silver, the multilayer conductor continued to work over a bending radius of as little as 3 mm, while the more brittle ITO cracked. On repeated bending, the silver-and-oxide sandwich gained much less electrical resistance than did ITO under similar mechanical tests.