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The shadow-effect energy generator developed by researchers from the National University of Singapore uses the contrast in illumination between the lit and shadowed areas to generate electricity. [Image: Royal Society of Chemistry]

Shadows are often taken for granted or even regarded as undesirable, except perhaps on a hot day as respite from the sun or as a way to create moodiness or contrast in a photograph. In particular, developers of photovoltaic and optoelectronic technologies view shadows as a problem that can limit the performance of certain devices.

A team of researchers from the National University of Singapore looked at shadows in a different light to create a device that harvests useful energy from illumination contrast (Energy Environ. Sci., doi: 10.1039/d0ee00825g). The shadow-effect energy generator converts partial shadow castings into electricity—enough to power a digital watch or other wearable electronics.

A new application for shadows

Swee Ching Tan, the study’s senior author, takes inspiration from nature for much of his research. He studies organisms that use photosynthesis to convert sunlight into chemical energy to design better photovoltaic devices with higher conversion efficiency.

In his field, shadows were regarded as a nuisance that lowers the power output of solar cells. But his 2018 study on metallic thin films showed that selective illumination can create a work function contrast, enabling in-plane charge transport. The work function refers to the minimum energy needed to eject an electron from a solid into vacuum.

“Our group had earlier demonstrated that the work function of metallic thin films coated on silicon wafers can be manipulated by the intensity of light falling on them,” said Tan, Assistant Professor of Materials Science and Engineering at the National University of Singapore. “By using this effect, we developed the idea of harvesting the illumination contrast that arise when shadows are cast.”

Ideal for indoor use

The shadow-effect energy generator consists of a series of cells made of gold thin film deposited on an n-type silicon wafer. The cells are arranged side-by-side on a flexible and transparent plastic strip. The work function difference in the film on the light-exposed side and the shadowed side induces a voltage difference, which causes the flow of electrons to produce energy.

During testing, Tan and his colleagues found that their device worked best when half-illuminated, half-in-shadow, producing the highest short-circuit current and open-circuit voltage in this condition. It even outperformed a commercial silicon solar cell in an indoor, low-light environment. Even with low light intensity, the shadow-effect energy generator could produce enough electricity to stably power an electronic watch.

“The shadow-effect energy generators are particularly designed for indoor applications where shadows are persistent and in places where solar cells are deemed inefficient,” Tan said. “Since the array is light and flexible, it could be used as a replacement to bulky batteries for wearable applications.”

A next-generator motion sensor

The device also has promise as a self-powered sensor for monitoring passing objects such as vehicles, humans or animals. Unlike many conventional motion sensors, which use cameras or RFID systems, the shadow-effect energy generator is small in size, cost-effective and doesn’t need an external power source.

“With densely built urban spaces and a large amount of people movement, there is ample scope for shadows to be cast, and this gives numerous opportunities to deploy the device for energy harvesting,” said Tan. “Simple processing steps and inexpensive materials enable easy scalability and commercialization.”