Is shading no longer a problem? This new solar cell continues to function

Shading represents one of the main degradation factors for thin-film solar cells. Trees, buildings, clouds, or even the passage of a bird can cover part of the module, inducing a reverse bias that reduces energy output and, in the worst cases, causes permanent damage to the active materials. A research team from the Hong Kong Polytechnic University (PolyU) has developed a new generation of perovskite-organic tandem solar cells (POTSC, Perovskite-Organic Tandem Solar Cells) designed specifically to address this issue.

The results, published in the journal Nature Materials, show devices capable of maintaining over 90% of their initial efficiency even after exposure to an extreme reverse bias of -40 volts, a value significantly higher than that normally tolerated by current thin-film photovoltaic technologies. Thin-film technologies, including cadmium telluride (CdTe), copper-indium-gallium-selenide (CIGS), perovskite cells, and organic cells, are considered particularly interesting for their lightweight, flexibility, and lower production costs compared to traditional silicon panels. However, they all share a critical issue: when part of the module is shaded, the affected cells can develop negative voltages that, due to the hybrid nature of electron and ion transport, gradually compromise performance and reliability.

To understand the origin of the phenomenon, the group led by Professor Li Gang focused on organic solar cells, whose behavior in reverse bias conditions has been little studied despite significant progress in efficiency in recent years. The analysis identified the main cause of degradation in the so-called "deep trap states", defects present within the bulk heterojunction, the active layer where charge generation occurs. These defects trap electrons and holes, reducing the cell's yield and promoting irreversible damage during operation under reverse voltage.

To limit the phenomenon, researchers modified the microstructure of the donor-acceptor blend, eliminating isolated clusters of the acceptor material. This strategy significantly reduced the formation of "deep trap states", allowing the creation of organic cells with an irreversible breakdown voltage exceeding -35 V. In practice, below this threshold, the cell does not suffer permanent damage.

The approach also reflects on the performance of the tandem cells, where the organic layer plays a protective role with respect to the perovskite. The reduction of the reverse tunneling phenomenon, that is, the reverse current flow generated during shading, indeed prevents damage to the perovskite layer. In addition to withstanding exposures at -40 V while retaining more than 90% of the initial efficiency, the devices have shown remarkable stability over time. After 12 hours of continuous operation at -20 V, they still maintain 90% of their initial performance, while after 2,000 hours of operation at -4.5 V, they retain 97% of their original efficiency, values that the authors indicate as superior to those achieved by current thin-film photovoltaic technologies.

This work also represents an evolution of research published by the same group in 2025 in Nature Energy, when a perovskite-organic tandem cell with a conversion efficiency of 25.9%, independently certified at 25.1%, was presented, achieved through a technique of lower contact modulation. In the new study, in addition to a significant increase in robustness against reverse voltage, the conversion efficiency exceeds 26%, further bringing this technology closer to potential use in commercial photovoltaic modules.

According to Professor Li, the stability demonstrated by the mini-tandem modules under shading conditions represents an important step toward the practical adoption of perovskite-organic solar cells, providing useful insights for the development of more reliable and durable devices for future renewable energy production systems.