Scientists in Ningbo anchored a fragile one-molecule-thick contact layer with an indium chloride interlayer, producing 20.4%-efficient small cells and devices that kept more than 85% of their performance after 2,000 hours.

Organic solar cells promise something silicon panels cannot easily offer. Built from carbon-based molecules, they can be lightweight, flexible, semi-transparent and potentially printed like ink on large rolls of plastic. In the laboratory, they have also reached impressive efficiencies.

The trouble comes when researchers try to scale them up and keep them running. Now a team at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences reports a way to tackle one of the weak points in these devices, a delicate layer that sits between the solar material and the electrode. Their results appear in Joule.

Using what they describe as a molecular locking strategy, the researchers built small cells with a power conversion efficiency of 20.4% and large modules with a record active-area efficiency of 18.8%. The devices also proved durable under continuous operation.

A one-molecule-thick weak link

An organic solar cell is a stack of thin layers. Light is absorbed in the active layer, generating electrical charges. Those charges must then be pulled out through selective contact layers to the electrodes, one of which is usually indium tin oxide, or ITO, a transparent conductor coated on glass or plastic.

In recent years, researchers have increasingly used self-assembled monolayers, or SAMs, as contact layers. These are films just one molecule thick, made from molecules that attach themselves to the ITO surface by one end, like bristles on a brush. SAMs can extract charges very efficiently while wasting little light, and they have helped push efficiencies of both organic and perovskite solar cells to new highs.

But forming a dense, uniform and stable SAM on ITO is difficult. Gaps in the layer create pathways for losses, and the molecules can detach, or desorb, when the device is exposed to light and heat. Over a large area, small inconsistencies multiply, which is one reason why efficiencies tend to fall as solar cells grow from postage-stamp size to practical modules.

Locking the molecules in place

The NIMTE team, led by Professor Ge Ziyi, addressed the problem by adding a vacuum-deposited interlayer of indium chloride, written as InCl_x, between the ITO and the SAM.

The chloride triggers what chemists call a ligand-exchange reaction, in which the SAM molecules bind to indium through multiple connections at once, forming a network of indium-oxygen-phosphorus bonds. Because each molecule is held at several points, rather than one, the result is a denser, more uniform and more firmly anchored layer, the molecular lock of the title.

Multidentate binding, in which a molecule grips a surface or metal atom with several attachment points, is a familiar principle in chemistry. It tends to make bonds much harder to break than single attachments, in the same way that a shelf held up by several screws is less likely to come loose than one held by a single nail.

From small cells to modules

With the locked contact layer, small-area devices achieved a power conversion efficiency of 20.4%, among the highest values reported for organic solar cells.

The more important test was scaling up. “We scaled the technique to large-area modules with an aperture area of 15.6 cm2, reaching a record active-area PCE of 18.8%,” said Professor Liu Quan, the study’s corresponding author.

Modules are built by connecting a series of individual cells on a single substrate. The aperture area refers to the overall area that collects light, while the active area efficiency counts only the portions of the module that actually generate power, excluding the narrow lines where cells are joined together. Maintaining performance over a larger area requires that every layer be uniform across the whole device, which is exactly where the denser SAM appears to help.

Holding up over time

Durability has long been one of the biggest obstacles for organic photovoltaics. Carbon-based materials can degrade under sunlight, heat, oxygen and moisture, and interfaces between layers are frequent points of failure.

The NIMTE devices were tested by maximum power point tracking, in which a cell is kept operating continuously at the voltage that delivers the most power, mimicking real-world use. After 2,000 hours of such operation at 25 °C, the devices retained more than 85% of their initial efficiency.

Two thousand hours is roughly 83 days of continuous operation. That is still far short of the decades-long lifetimes expected of commercial silicon panels, and tests at room temperature are less demanding than the heat a rooftop panel experiences in summer. But it represents meaningful progress for a technology in which stability is often the limiting factor.

Researchers often use a benchmark called T80, the time it takes a device to fall to 80% of its starting efficiency, to compare stability across studies. Retaining more than 85% after 2,000 hours means the NIMTE devices had not yet reached that threshold by the end of the test.

A platform for other devices

The researchers describe the approach as a universal interface platform that could be applied to efficient, stable and scalable organic photovoltaic modules. Because the indium chloride layer is deposited under vacuum, a family of techniques already widely used in the electronics and display industries, it could in principle be integrated into existing manufacturing processes.

Organic solar cells are unlikely to replace silicon on power-plant scales any time soon. Their most promising niches are applications where light weight, flexibility or transparency matter, such as building windows and facades, portable electronics, vehicles and indoor devices powered by ambient light. In those markets, closing the gap between laboratory efficiency and real-world modules, while extending operating lifetimes, is the main challenge.

By shoring up a molecular layer only a single molecule thick, the NIMTE team has shown that small changes at an interface can make a significant difference as devices grow from laboratory samples toward products.

The research was supported by the National Natural Science Foundation of China and the Key Program of the Ningbo Public Welfare Research Plan.

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Notes

1. “Chloride-mediated SAM locking via ligand-exchange reaction enables 18.8% active-area efficiency and stable organic solar modules.” Joule, 2026. DOI: 10.1016/j.joule.2026.102709.
2. Chinese Academy of Sciences. “Molecular locking strategy enables efficient and scalable organic solar cells.” EurekAlert!, 22 October 2026. https://www.eurekalert.org/news-releases/1148864.

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