Flexible Solar Cell Achieves 7.6% Efficiency

A thin-film technology called CZTS has been applied to photovoltaics to achieve a world-leading 7.6 percent efficiency in a 1-cm² solar cell. The achievement marks a milestone for thin-film photovoltaic (PV) technology — which is being explored for zero-energy buildings, among other applications — on its path toward commercially competitive 20 percent efficiency.

High-efficiency, low-toxicity solar cells developed by UNSW's Australian Centre for Advanced Photovoltaics. Courtesy of Robert Largent/UNSW.

A team of researchers from the University of New South Wales (UNSW) led by professor Xiaojing Hao fabricated the cell; efficiency results were confirmed by the U.S.’s National Renewable Energy Laboratory.

"In addition to its elements being more commonplace and environmentally benign, we're interested in these higher bandgap CZTS cells for two reasons," said professor Martin Green, a mentor of Hao and a pioneer of photovoltaic research. “They can be deposited directly onto materials as thin layers that are 50 times thinner than a human hair, so there's no need to manufacture silicon 'wafer' cells and interconnect them separately. They also respond better than silicon to blue wavelengths of light, and can be stacked as a thin film on top of silicon cells to ultimately improve the overall performance."

Because they can deposit CZTS solar cells on various surfaces, Hao's team believes they can create thin-film PV cells that are either rigid or flexible, and durable and cheap enough to be widely integrated into buildings to generate electricity from the sunlight that strikes structures such as glazing, façades, roof tiles and windows.

Hao said UNSW is collaborating with a number of large companies to develop applications well before it reaches 20 percent efficiency, enabling competition with commercially available crystalline silicon PV systems, for example.

Professor Xiaojing Hao of UNSW's Australian Centre for Advanced Photovoltaics holding the CZTS solar cells. Courtesy of Quentin Jones/UNSW.

CZTS has none of the toxicity problems of its two thin-film rivals, CdTe (cadmium-telluride) and CIGS (copper-indium-gallium-selenide). Cadmium and selenium are toxic at even tiny doses, while tellurium and indium are extremely rare. Currently, thin-film photovoltaic cells like CdTe are used mainly in large solar power farms, as the cadmium toxicity makes them unsuitable for residential systems, while CIGS cells is more commonly used in Japan on rooftops.

Thin-film solar cell technologies are attractive because they are physically flexible, increasing the number of potential applications, such as curved surfaces, roofing membranes, or transparent and translucent structures like windows and skylights.

Hao said CZTS's cheapness, benign environmental profile and abundant elements may be the trigger that brings architects and builders onboard to using thin-film solar panels more widely in buildings.

Solar cell efficiency is defined as the ratio of energy output from the solar cell to input energy from the sun.