See-through solar cells offer the exciting prospect of turning gleaming skyscrapers into photovoltaic powerhouses. But it has been difficult to make solar cells that are both transparent and efficient. Now researchers have found a simple, low-cost trick to make opaque silicon solar cells transparent: punch tiny, carefully patterned holes in them (Joule 2019, DOI: 10.1016/j.joule.2019.11.008).
Some researchers and companies have already developed ways to make solar windows. These include thin-film solar cells made with amorphous silicon, quantum dots, and, more recently, perovskites. However, all of these devices are semi-transparent and have a reddish or brownish tint. “Colored windows look pretty, but nobody wants to look outside through a colored window always,” says Kwanyong Seo, a chemist at the Ulsan National Institute of Science and Technology. To make the devices clearer, researchers would have to use thinner materials, which would further decrease their efficiency.
Seo, Seungwoo Lee of Korea University, and their colleagues wanted to make neutral-colored transparent cells with crystalline silicon, which Seo calls “the best material for solar cells with high efficiency and long-term stability.” Using conventional silicon wafers could also make commercialization a cinch because they are a mainstay of the electronics and photovoltaics industries.
To make their transparent devices, the team etched a grid of 100-µm-wide holes into a conventional industrial silicon wafer. Visible light passes through the holes without diffracting to create color, while the silicon around the holes absorbs light for conversion into electricity. By controlling the number and spacing of the holes, the silicon becomes anywhere from 20–50% transparent. Solar cells made with these wafers range in efficiency from 12.2–7.4%, respectively.
Those efficiencies are less than half those of the best conventional opaque silicon cells but still beat some other transparent devices made so far. Amorphous silicon and perovskite-based transparent solar cells, for instance, can have efficiencies up to 14% and are 50% transparent but have a strong reddish color. Organic photovoltaic materials give colorless, 50% transparent devices, but the best devices have reached just 5% efficiency.
The etching process also wastes silicon, but the researchers plan to try another approach that involves depositing crystalline silicon in a pattern instead of etching a wafer.
The devices are also small with a size of 1 cm2. “The next step for my team is to scale up the device to 25 cm2 and increase the efficiency to 15%,” Seo says.
Nam-Gyu Park, a chemical engineer at Sungkyunkwan University, calls this transparent silicon solar cell technology “unique and a breakthrough.” The devices can’t be made flexible or colorful like with other transparent solar cell technologies, he says. But he thinks they could find a large market in buildings and vehicle sunroofs, where they would have the added benefit of increasing building energy efficiency by regulating temperature because silicon blocks heat-producing infrared rays. Seo also imagines the see-through solar panels leading to power-generating phone and tablet screens.