Giant bulk photovoltaic effect driven by interfacial symmetry breaking in MoS2/Ta2NiSe5 heterostructures

Sunlight hitting a solar cell usually needs a push to get electricity flowing. In most solar panels, the light knocks electrons loose, but they wander aimlessly until a built-in electric field, created by joining two different types of materials, forces them to move in one direction. This is how standard solar cells work, and it requires a specific internal architecture to function. However, nature offers a different, stranger way to generate power from light that does not rely on this usual separation of materials. In certain crystals that lack a center of symmetry, light can directly push electrons in a specific direction without any external help or internal junction. This phenomenon, known as the bulk photovoltaic effect, has long been a theoretical curiosity and a potential path to more efficient energy harvesting, but it has been difficult to harness because it typically only appears in a very small set of rare, complex materials.

The challenge for scientists has been finding a way to create this effect in materials that are easy to work with and to understand exactly how it happens. When researchers try to build these effects by stacking thin layers of different materials on top of each other, the process often becomes a tangled mess. The layers interact in so many ways—shifting electrons between them, changing their internal structure, and creating their own electric fields—that it becomes impossible to tell which part of the system is actually generating the power and which part is just getting in the way. To solve this puzzle, a team of researchers has developed a new way to stack two specific materials, molybdenum disulfide and tantalum nickel selenide, in a cross-shaped pattern. By doing this, they have created a device that produces a massive surge of electricity from light with no battery or external voltage needed, and they have managed to separate the different physical forces at play to understand exactly why it works.

The researchers started with two very different materials. One, molybdenum disulfide, is a thin, hexagonal sheet that is symmetric and stable. The other, tantalum nickel selenide, is a material with a chain-like structure that conducts electricity well and has a strong directionality, meaning its properties change depending on which way you look at it. When they placed a sheet of the first material directly on top of the second, the perfect symmetry of the bottom layer was broken by the presence of the top layer. Imagine placing a square tile on top of a round table; the combination creates a new shape that is no longer perfectly round. In this case, the stacking broke the internal balance of the bottom material, creating a new, uneven electrical landscape right at the interface where the two touch. This broken symmetry is the key that unlocks the ability to generate a direct current from light without any external push.

To see if this new arrangement actually worked, the team built a device shaped like a cross. They laid a strip of the bottom material down, then placed a strip of the top material across it, forming a small square where they overlapped. This specific shape was crucial because it allowed them to measure the electricity flowing along the top strip separately from the electricity flowing through the bottom strip. When they shined a red laser beam onto the center of the overlap, a strong electric current immediately appeared, flowing along the top strip even though no battery was connected. The voltage they measured was small, but the current density was enormous, reaching 247 amperes per square centimeter. This performance is competitive with established van der Waals counterparts, representing a significant achievement for this type of effect.

The team then had to prove that this electricity was coming from the unique symmetry breaking at the interface and not from some other common cause, like the metal wires touching the materials or simple heating effects. They scanned the device with a tiny, focused laser spot, moving it across the surface to map exactly where the electricity was being generated. The results showed that the power was produced only in the small square where the two materials overlapped. The areas where the materials were alone, or where the metal wires touched the edges, produced no such signal. This confirmed that the effect was happening strictly at the boundary between the two layers, driven by the way their atomic structures interacted. Furthermore, they tested the device with different colors of light and found that it only worked with light that the bottom material could absorb, ruling out other potential mechanisms.

A major part of the discovery was understanding that two different forces were working together to make the effect so strong. First, the breaking of symmetry created a direct push for the electrons. Second, because the two materials have different natural electrical properties, electrons spontaneously jumped from the bottom layer to the top layer when they were stacked. This created a built-in electric field that acted like a hidden battery, helping to separate and guide the electrons generated by the light. The researchers showed that they could control both of these forces. By applying a voltage to the silicon base underneath the device, they could change how many electrons were available in the bottom layer. By applying a voltage across the two layers, they could strengthen or weaken the built-in electric field. When they adjusted both of these controls at the same time, the amount of electricity generated increased by more than ten times compared to when no extra voltage was applied.

This work demonstrates that by carefully choosing materials and designing the shape of the device, scientists can turn complex interactions into a clear, powerful source of energy. The researchers showed that the giant surge of electricity they observed was not a fluke or a result of a single factor, but the result of a precise combination of symmetry breaking and charge transfer. They ruled out the idea that the effect came from the edges of the device or from the metal contacts, proving instead that the heart of the phenomenon lies in the microscopic interface between the two layers. The findings suggest that this approach could be used to design a new generation of self-powered electronic devices that can harvest energy from light without needing the bulky junctions found in traditional solar cells. By proving that these effects can be tuned and amplified, the study opens a path toward creating flexible, efficient, and highly sensitive light detectors and energy harvesters that could be integrated into a wide variety of future technologies.