Breaking the 800 mV open-circuit voltage barrier in antimony sulfide photovoltaics

Solar cells are the workhorses of the renewable energy transition, capturing sunlight and turning it into electricity. For decades, the most efficient panels have relied on silicon, but scientists are constantly searching for new materials that are cheaper to make, less toxic, and capable of being printed onto flexible surfaces. Among the most promising candidates are compounds made from antimony and sulfur, known as antimony sulfide. These materials are abundant, non-toxic, and absorb light exceptionally well. However, despite their potential, they have hit a frustrating wall. For years, the best solar cells made from this material could not generate an electrical pressure, known as voltage, higher than about 800 millivolts. This ceiling has held back their efficiency, leaving them far below their theoretical potential and making them less competitive with established technologies.

The scientific community has long debated why this voltage limit exists. One prevailing theory suggested that the material itself was fundamentally flawed. It was proposed that when light hits the antimony sulfide, the energy gets trapped inside the crystal structure in a way that wastes it before it can be turned into electricity. This idea, called self-trapping, implied that no matter how perfectly the material was made, the voltage could never exceed that 800-millivolt barrier because the loss was built into the physics of the atoms. If this were true, the path forward would be to abandon the material or accept its limitations. However, a competing view suggested that the problem was not the material itself, but rather the imperfections in how the films were manufactured. If the voltage loss was caused by defects—tiny errors in the crystal structure—then careful engineering might be able to fix it and push the performance higher.

A team of researchers has now settled this debate by breaking the barrier. They successfully created antimony sulfide solar cells that generate a voltage of 824 millivolts, surpassing the long-standing limit and setting a new record for this type of technology. Their work demonstrates that the material is not fundamentally broken; rather, its performance was held back by the way it was grown. By changing the chemical recipe used to deposit the thin films, the scientists reduced the number of defects and grain boundaries—essentially the seams where tiny crystals meet—within the material. This reduction in imperfections allowed the solar cells to operate much closer to their theoretical maximum, proving that the voltage ceiling was an engineering challenge, not a physical law.

The breakthrough came from a technique called chemical bath deposition, a method where a substrate is dipped into a solution to grow a thin film of the desired material. The researchers added a specific ingredient, a compound known as citrate, to the solution. This additive acted as a regulator, slowing down the reaction and changing how the crystals formed. Without this additive, the crystals grew in a chaotic manner, forming many small grains with numerous boundaries between them. These boundaries are problematic because they act as traps where electrons and holes—the charge carriers that create electricity—can get stuck and recombine, losing their energy as heat. By introducing the citrate, the researchers encouraged the crystals to grow larger and more orderly, effectively smoothing out the internal landscape of the film.

The results of this change were dramatic. The density of these grain boundaries was cut nearly in half, dropping from over 1,100 per square millimeter in the standard films to about 585 in the improved ones. This structural improvement had a direct impact on the electrical performance. The researchers used a sensitive technique called deep-level transient spectroscopy to look inside the devices and identify the specific types of defects that were causing the voltage loss. They found two main types of deep traps, which are energy states that capture charge carriers and prevent them from contributing to the current. Through detailed computer simulations, they determined these traps were likely caused by missing sulfur atoms or antimony atoms sitting in the wrong spots within the crystal lattice.

By reducing the number of these defects, the new solar cells showed a significant improvement in how they handled charge. The voltage increased steadily as the concentration of the citrate additive was optimized, peaking at 824 millivolts. This value is not just a small increment; it is a definitive breach of the 800-millivolt barrier that had seemed unbreakable. The researchers also confirmed that the material itself was not suffering from the self-trapping effect that some had feared. If self-trapping were the primary culprit, the voltage would have remained capped regardless of how perfect the film was made. The fact that they could push the voltage higher by simply cleaning up the defects proves that the material is capable of much higher performance.

The implications of this finding extend beyond just a single number. The study provides a clear roadmap for improving antimony sulfide solar cells. It shows that the path to higher efficiency lies in defect engineering—carefully controlling the growth process to minimize imperfections. The new cells also maintained excellent stability, retaining most of their efficiency after a month of storage in normal conditions, which is a crucial requirement for practical applications. Furthermore, the researchers tested the cells under indoor lighting conditions, where they achieved an efficiency of nearly 19 percent, making them a strong candidate for powering the growing number of low-power electronic devices that run on indoor light.

This work resolves a long-standing uncertainty in the field of photovoltaics. It moves the conversation away from the idea that antimony sulfide is a material with an intrinsic, unfixable flaw. Instead, it confirms that the limitations were due to the manufacturing process. By demonstrating that the voltage can be raised by reducing defects, the researchers have opened the door for further optimization. The path forward is now clear: continue to refine the growth methods to further reduce defects, and the efficiency of these solar cells will continue to climb toward their theoretical limits. The barrier was not a wall of physics, but a hurdle of chemistry, and it has been cleared.