These cheap solar cells work better because they’re flawed

It raises an obvious question: how can a relatively simple, low-cost material compete with highly refined silicon technology developed over decades? Over the past 15 years, lead-halide perovskites have emerged as promising candidates for next-generation solar cells. Unlike silicon, which requires ultra-pure single-crystal wafers, these materials can be produced using inexpensive solution-based methods while delivering comparable performance.

Researchers Dmytro Rak and Zhanybek Alpichshev at ISTA have now identified the underlying mechanism behind these unusual properties. Their findings reveal a surprising contrast with traditional solar technology. Silicon depends on near-perfect purity to function efficiently, but perovskites benefit from their imperfections. According to the team, a naturally occurring network of structural defects allows electrical charges to travel long distances through the material, which is essential for efficient energy conversion. “Our work provides the first physical explanation of these materials while accounting for most-if not all-of their documented properties,” says Rak. This insight could help move perovskite solar cells closer to widespread real-world use.

From Overlooked Materials to Solar Breakthroughs

The term “lead-halide perovskites” refers to a group of compounds first identified in the 1970s. They were named for their structural resemblance to perovskites, a broader class of oxide materials widely studied in materials science. Aside from their ability to form stable hybrid organic-inorganic crystals, they initially attracted little attention and were largely set aside after basic characterization.

That changed in the early 2010s, when researchers discovered their impressive ability to convert light into electricity. Since then, perovskites have also shown promise in LEDs, as well as X-ray detection and imaging technologies. “In addition, these materials exhibit astounding quantum properties, such as quantum coherence at room temperature,” explains Alpichshev, whose research group studies complex phenomena in advanced materials.

How Solar Cells Generate and Transport Charge

For any solar cell to work efficiently, it must absorb sunlight and convert it into electrical charges. This process produces negatively charged electrons and positively charged “holes.” These charges then need to travel through the material and reach the electrodes to generate usable electricity.

This journey is not simple. Charges must move across distances of hundreds of microns, which would correspond to hundreds of kilometers on a human scale, without becoming trapped or lost along the way.

In silicon-based solar cells, this challenge is addressed by eliminating defects that could capture charges before they reach the electrodes. Perovskites, however, are created using solution-based methods and naturally contain many defects. This makes their strong performance even more surprising. How can charges move efficiently through such a flawed material, and why do they remain separated long enough to do so?

Discovering Hidden Forces Inside Perovskites

One known property of perovskites adds to the puzzle. When electrons and holes form a bound pair called an exciton, they tend to recombine quickly. Yet experiments show that these charges often remain separated for extended periods within the material.

To explain this contradiction, the ISTA team proposed that internal forces within perovskites actively pull electrons and holes apart, preventing recombination. To test this idea, they used nonlinear optical techniques to inject charges deep inside the material. Each time they introduced electrons and holes, they observed a consistent electrical current flowing in the same direction, even without applying any external voltage. “This observation clearly indicated that even deep inside single crystals of unmodified, as-grown perovskites, there are internal forces that separate opposite charges,” says Alpichshev.

Earlier studies had suggested that such behavior should not occur based on the material’s crystal structure. To resolve this discrepancy, the researchers proposed that charge separation is not uniform. Instead, it occurs at specific regions known as “domain walls,” where the structure of the material is slightly altered. These domain walls form interconnected networks throughout the material.

Visualizing Domain Walls With Silver Ions

Confirming the existence of these networks presented a major challenge. Most measurement techniques only probe the surface of a material, while the domain walls exist deep inside.

To overcome this limitation, Rak developed a new approach inspired by his background in chemistry. Since perovskites can conduct ions, he explored whether certain ions could act as markers to reveal internal structures. He introduced silver ions into the material, which naturally migrated and accumulated along the domain walls. These ions were then converted into metallic silver, making the network visible under a microscope.

“This qualitative technique, invented and implemented at ISTA, is much like angiography in living tissues — except that we are examining the micro-structure of a crystal,” says Alpichshev.

Charge “Highways” Enable Efficient Energy Flow

The discovery of a dense network of domain walls throughout perovskites proved to be a turning point. These structures act as pathways that guide electrical charges through the material.

As Rak explains, “If an electron-hole pair is created near a domain wall, the local electric field pulls the electron and the hole apart, placing them on opposite sides of the wall. Unable to recombine immediately, they can drift along the domain walls for what seems like eons on a charge carrier’s timescale and travel long distances.” In effect, these domain walls function as “highways for charge carriers,” allowing charges to move efficiently and contribute to electricity generation.

A Complete Explanation and a Path Forward

The researchers emphasize that their work provides a unified explanation for the behavior of perovskites. “With this comprehensive picture, we are finally able to reconcile many previously conflicting observations about lead-halide perovskites, resolving a long-standing debate about the source of their superior energy-harvesting efficiency,” says Rak.

Until now, most efforts to improve perovskite solar cells have focused on adjusting their chemical composition, with limited progress. This new understanding opens the door to engineering their internal structure instead, potentially increasing efficiency without sacrificing their low-cost production advantages. The findings could play a key role in bringing next-generation solar technology from the lab into widespread use.