The Hidden Chemistry Inside Tomorrow’s Solar Cells
Researchers uncover why perovskite solar cells degrade under reverse bias, opening new pathways to longer-lasting solar technologies.
Researchers just figured out why perovskite solar cells break down surprisingly easily under partial shading, and how to stop it. Perovskite solar cells are among the most promising candidates for the next generation of solar technology. They can convert sunlight into electricity with remarkable efficiency while offering the potential for lower manufacturing costs than today’s silicon-based panels. Before they can be scaled and deployed, there are some critical challenges to be solved, including ensuring the cells remain stable for decades under real-world conditions. A central question has been what happens when a perovskite solar panel is pushed into an unusual electrical state known as reverse bias.
Like all solar panels, perovskite solar cells occasionally experience reverse bias, an unusual electrical condition that can occur when part of the solar panel is shaded while the rest is in full sunlight. It has long been known that reverse bias quickly damages perovskite devices, but the pieces were not all fitting together. Current flowing through the cell under reverse bias is at the heart of the problem, and there are two ways this can happen. The first is apparent if the materials have big defects, electrons bolt through those defects like lightning and fry the cell, an acute failure mechanism. The second mechanism happens even in cells where those large defects are avoided or covered up, current can flow by a process known as quantum tunneling; much like taking a short-cut through interfaces in the cell. This current flows everywhere and while it is less abrupt than electrons flowing through defects, it will cause degradation in the cell over time. This gradual problem is much harder to understand because seeing these subtler processes in the cell is not easy. Existing theories explained parts of the problem, but they couldn’t fully account for how quickly some cells degraded or why certain device architectures proved far more resilient than others. Something important was happening inside the device that researchers couldn’t yet see.
Late last year, in October of 2025, a report led by RASEI Fellow Michael McGehee, a Professor at the University of Colorado Boulder, in collaboration with researchers at the National Laboratory of the Rockies (NLR; then named NREL), that explored the underlying mechanisms of the abrupt failure mechanisms of perovskite films under reverse-bias was published in . The team identified that the defects were the sites of failure and demonstrated how to create films without defects that were much more robust to exposure to reverse bias.
To explore and better understand the gradual mechanism for degradation Michael McGehee assembled a new team of collaborators with the required expertise, including the groups of from Northwestern University, at the University of Arizona, at the University of Washington, and Joey Luther another RASEI Fellow at NLR. Together they combined advanced time-resolved electrical measurements, device modeling and materials characterization to watch how perovskite solar cells responded under reverse bias conditions, just published in . Rather than just observing the damage, they set out to answer the key question of what hidden process was driving it?
The layers inside a solar cell create an energy landscape that electrons must navigate to get through the cell. By design, the interfaces act as energy “mountains” separating two valleys where electrons can be. Under reverse bias, ideally electrons cannot get over (or through) the mountain; the mountain is wide enough to keep electrons where they belong, and no current flows. Using these advanced measurement techniques, the team, led by (a Graduate Student in the McGehee group) and (a Research Associate with the McGehee group) was able to investigate the current-flow and degradation mechanism in remarkable detail. They were able to uncover a hidden process that had largely escaped attention: Under reverse bias, electrochemical reactions inside the solar cell weren’t just moving charged particles around as previously thought, they were actually creating new mobile ions. As these newly created ions redistribute and accumulate near interfaces, they gradually reshape the energy mountain, making it thinner until quantum tunneling becomes easy, and making it much easier for damaging current to pass through. Essentially the shifting chemistry was changing the device physics, providing important insights into why some device architectures show electrical breakdown and degrade much faster than existing models could explain.
These observations underlined an important perspective about these devices. Rather than simply behaving as electronic devices that move electrical charge, the cells also function as tiny chemical systems, with electrochemical reactions reshaping their internal structure under certain conditions. The initial device physics governs the ion creation, and the ion creation changes the device physics; this is an important feedback loop that can accelerate degradation. Identifying the hidden chemistry gives the researchers a much clearer picture of why the degradation occurs, and possible ways to prevent it.
Based on these findings, the team was able to identify where to intervene. The answer isn’t to try and repair the damage after it has begun, but to stop the chemical reactions from starting in the first place. By improving on the ultra-thin transport layers inside the device, the so-called hole transport layer (or HTL), the researchers showed that they could block the reactions responsible for creating new mobile ions. This solution is one that doesn’t require new equipment or exotic new materials, the team used standard materials and procedures but ensured that the layer placed in the device was very smooth, improving the coverage of the layer. With the electrical landscape preserved, the pathway to damaging currents remains closed, helping the cells maintain their performance for longer.
This new work, in combination with the study published last year, provides a holistic view of the two pathways that lead to perovskite decomposition under reverse bias. Importantly, both studies go beyond just observing the problem, they offer proven approaches to solve the abrupt and gradual degradation pathways. Preventing reverse bias degradation is a crucial component for the commercialization of perovskite solar cells.
For devices that may one day be expected to operate for decades on rooftops and in solar farms around the world, understanding the failure points is as important as improving efficiency. Efficiency will often get the headlines, but durability and resilience will decide whether perovskites make it onto a rooftop. This work doesn’t just observe and explain a failure mode, it provides a solution for one of the major challenges to scaling this new technology.