One major challenge remains with renewable energy, as solar panels lose their ability to generate electricity after sunset. It’s how you store solar power for later use, even in cloudy weather or at night.
Researchers at the University of California, Santa Barbara think they may have found the answer to avoiding reliance on large-scale battery systems and the power grid. Writing in the journal Science, Associate Professor Grace Han and her research team describe a new material that can absorb sunlight, store its energy in chemical bonds, and release it as heat when needed. The material is based on a modified organic molecule called pyrimidone and represents a new advance in molecular solar thermal (MOST) energy storage technology.
“This concept is reusable and recyclable,” said Han Nguyen, a doctoral student at Han Group and lead author of the study.
“Think of photochromic sunglasses. When you’re indoors, they’re just clear lenses. When you go out in the sun, they naturally darken. When you come back inside, the lenses become clear again,” Nguyen continued. “That kind of reversible change is what we’re interested in. Instead of changing colors, we want to use the same idea to store energy, release it when we need it, and reuse the material over and over again.”
DNA-inspired solar energy storage
Scientists found inspiration from unexpected sources when designing molecules. DNA. The structure of pyrimidone is similar to components naturally found in DNA, which can reversibly change shape when exposed to ultraviolet light.
Using a synthetic version of that structure, the researchers designed a molecule that can repeatedly store and release energy. To better understand why this molecule is stable while retaining energy for long periods of time, the researchers partnered with Ken Houk, a distinguished research professor at UCLA. Computational modeling helped explain how materials can maintain stored energy for years without significant losses.
“We prioritized lightweight and compact molecular designs,” Nguyen said. “For this project, we cut out everything we didn’t need. We removed everything we didn’t need to make the molecule as compact as possible.”
“Solar cells” that can be used repeatedly
Unlike standard solar panels, which convert sunlight directly into electricity, this system stores energy chemically. Molecules behave like compressed springs. When it absorbs sunlight, it transforms into a tense, high-energy form and remains in that state until it is activated.
When exposed to a trigger, such as a small amount of heat or a catalyst, the molecule returns to its original shape and releases its stored energy as heat.
“We typically describe this as a rechargeable solar cell,” Nguyen said. “It can be charged by storing sunlight.”
This molecule also achieves amazing energy density. Researchers say they store more than 1.6 megajoules of energy per kilogram. By comparison, traditional lithium-ion batteries store approximately 0.9 MJ/kg. This new material demonstrated better performance than previous generation optical energy storage switches.
A new material that uses stored sunlight to boil water
Key milestones for the team included turning the molecule’s high energy storage capacity into a practical demonstration. The researchers showed in experiments that the material could release enough heat to boil water under ambient conditions, something that has been difficult to achieve in this field of research.
“Boiling water is an energy-intensive process,” Nguyen said. “The fact that you can boil water under ambient conditions is a huge accomplishment.”
This technology could eventually support a variety of real-world applications, including off-grid heating systems for camping and domestic hot water applications. Because the substance is soluble in water, it could be circulated through rooftop solar collectors during the day and stored in tanks that release heat at night, the researchers said.
“Solar panels require an additional battery system to store energy,” says co-author Benjamin Baker, a doctoral student in Han’s lab. “Molecular solar energy storage allows the material itself to store energy from sunlight.”
The project received support from a Moore Inventor Fellowship awarded to Han in 2025 to advance the development of these “rechargeable solar cells.”
