Flexible silicon solar cells that can roll up

Highly efficient silicon solar cells that are as flexible as a sheet of paper could offer a lightweight power source for applications such as uncrewed aerial vehicles while cutting the cost of solar panels on the ground (Nature 2024, DOI: 10.1038/s41586-023-06948-y).

Conventional silicon photovoltaic (PV) cells, which supply more than 95% of the world’s solar electricity, contain brittle crystalline silicon wafers that are typically 150–200 μm thick. The best silicon cells can convert light into electricity with an energy efficiency of just over 27%. Although bendable cells can be made from thinner silicon wafers, they have lower efficiencies.

Meanwhile, some thin-film solar cells—based on materials such as copper indium gallium selenide—are much more flexiblebecause they contain light-absorbing layers just 1 μm thick. That means they can wrap around the curves of vehicles or fit around parts of buildings that are out of bounds for stiff panels. This flexibility would help squeeze more solar power out of every available surface.

Yet these thin-film solar cells are less efficient, less durable, or much more expensive than their rigid silicon counterparts. Consequently, “significant efforts are now being made to develop high-efficiency, flexible silicon solar cells that leverage both the physical and chemical stability of crystalline silicon,” says Han-Don Um, a PV researcher at Kangwon National University who peer-reviewed the new work.

The consortium behind the new cells includes researchers from Chinese company LONGi, one of the world’s leading solar manufacturers, and universities in China and Australia. The team made a series of 274.4 cm² cells—a standard commercial size—with silicon wafer thicknesses ranging from 57 to 125 μm. Each cell set a new efficiency record for a wafer of its particular thickness: even the 57 μm cell managed to convert 26.06% of light energy into electricity, a result described as “very impressive” by PV researcher Wenzhu Liu at the Shanghai Institute of Microsystem and Information Technology who was not involved in the work.

Crucially, the researchers say their 57 μm cell also offers the highest power-to-weight ratio of any silicon cell, a key consideration for applications in which mass must be minimized. The cells performed well in standard stability tests, and the team estimates they would have a 20-year operational lifetime. “The LONGi team’s achievement shows that it’s possible to maintain high efficiency in flexible solar cell formats, a crucial step forward for the industry,” Um says.

These high efficiencies were largely achieved by improving the flow of charge through other key layers within the cell. “The most important thing is to separate charge carriers effectively,” says team member Yang Li of Jiangsu University of Science and Technology and the LONGi Central R&D Institute.

When light hits the cell’s silicon layer, it frees electrons and positive “holes” that travel to opposite faces of the wafer. Here, they meet passivation layers that help prevent electrons and holes from recombining before they can deliver their energy. The researchers used a chemical vapor deposition (CVD) technique to gradually adjust the composition of these passivation layers as they grew on top of silicon; these adjustments improved the layers’ performance. They used a related CVD method to form nanocrystals that eased charge flow through the next layer. Finally, the team tweaked the composition of the cell’s outermost layer to improve its conductivity and used a laser-printing technique to lay extremely thin silver wires that blocked less incoming light on the cell’s surface.

All these techniques are compatible with LONGi’s existing production lines, Li says. But commercial cells would also need to be encapsulated in a robust flexible covering, and the team is still working on a way to achieve that in mass production. “The problem with flexible solar panels is finding a flexible encapsulant that doesn’t fail,” says Jenny Chase, a solar analyst at the data provider BloombergNEF.

Flexibility aside, the thinner cells might also be cheaper than conventional cells. Li’s team points out that every 10 μm of thinning could save enough silicon to reduce manufacturing costs by 3%. That ultimately means cheaper power, and Li suggests that using the thinnest of the researchers’ cells could potentially shave 20% off the cost of solar electricity.

Last year, Liu’s team in Shanghai made flexible cells with a 60 μm silicon wafer that offered efficiencies of more than 24% (Nature 2023, DOI: 10.1038/s41586-023-05921-z). The researchers improved their cell’s flexibility by texturing the silicon’s surface and edges to prevent cracks from forming and growing. “One thing we want to do is to apply the technology from Shanghai to make our highly efficient solar cells even more flexible,” Li says.