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Growing "green" electronics

29 January 2013

The transparent conductor film on glass panel (left) beside a blank glass panel for comparison. The grown transparent conductor was optically smooth and transparent with the NUS logo background.

Transparent conductor that has been architectured with lithography process to form 3D multilayer trenches and pits in a checker-board configuration.

Copyright: Energy & Environmental Science

Dr Ho (left) with PhD student Mr Kevin Moe, holding a transparent conductor grown on a glass panel

The search for cleaner energy is seeing rising interest in "transparent" conductors (TCs), a key component of solar cells and displays for mobile devices. A breakthrough fabrication approach by a group of engineers at NUS, a first for TCs used in solar cells, could pave the way towards achieving cheaper and more energy-efficient electronics.

Thanks to its optical property, a TC conducts electric current while at the same time lets light go through to the active material below. It is thus ideal for use in solar cells to absorb sunlight, or in screens for clear viewing. Such a conductor is currently made from doped indium-based metal oxides.

Indium, the main element used for TCs, is extremely rare, with less than 6,000 metric tonnes available in the world. Other alternatives like aluminium-doped zinc oxide are being used to reduce the quantity of expensive indium, but they are not as effective and depend on the situation being applied.

Led by Dr Ho Ghim Wei from the NUS Engineering Science Programme and NUS Department of Electrical and Computer Engineering, the team managed to overcome the problem of growing conductors on glass to produce their own compound - comprising zinc oxide (ZnO) and a small amount of gallium - to make a new kind of TCs. Moreover, the process allows them to create microscopic three-dimensional (3D) structures that raise the efficiency of solar cells by as much as 50 per cent.

Besides showing comparable performance as commercial ones, the new conductors boast several advantages. One key benefit is the "green" water-based chemistry process known as "low temperature aqueous synthesis" that addresses environmental concern.

Explained Dr Ho: "This process, as its name suggests, does not require high heat so this helps to lower the cost of the product. We use ZnO which is abundantly found compared to other oxides currently used in the production of TCs to further reduce the cost. The process also allows great scalability as we can produce numerous TC films simultaneously. Another added novelty is the ability to grow the TC in a 3D fashion for enhanced performance."

The environmentally friendly approach means that fabrication on an industrial scale will be feasible, which can lead to much cheaper electronics. Think mass production for low-cost solar cells and screens for portable devices.

The team has filed a US patent for their invention and published their research results in leading journal Energy & Environmental Science. They plan to do a proof of concept and commercialisation when they get funding. They are also exploring new applications such as flat panel display, "smart" or electro-chromic windows and thin film transistor based on their method.