Stable AgBiS2 near-infrared absorbers for thin film tandem photovoltaics

Sunlight is a vast, uneven river of energy, containing colors that range from the violet we can see to the invisible infrared that warms our skin. For decades, the most efficient way to capture this energy has been to use a single material, like silicon, to turn light into electricity. But a single material acts like a sieve with holes of a fixed size; it catches some colors well but lets others pass right through, wasting their energy as heat. To catch more of the river, scientists have turned to stacking different materials on top of each other, creating "tandem" solar cells. Each layer is tuned to catch a different slice of the sunlight spectrum, working together to harvest far more energy than a single layer ever could. The challenge has always been finding the right materials for the bottom layer, which must be thin, flexible, and able to catch the long, invisible waves of infrared light without degrading.

A team of researchers at the University of Oxford and their collaborators has now identified a new material that could solve this problem, offering a path to solar cells that are not only more efficient but also cheaper and more stable to make. They focused on a compound called silver bismuth sulfide, a material that had shown promise in single-layer solar cells but had never been successfully used as the bottom layer in a stacked, tandem device. The researchers discovered that this material has a natural advantage: it can absorb infrared light more effectively than the current leading candidate, a mixture of lead and tin, and it does not break down when exposed to air. By developing a new way to lay down thick films of this material, they demonstrated the first working tandem solar cells using silver bismuth sulfide, proving that it can be paired with other layers to create a highly efficient, stable, and scalable power source.

The journey to this discovery began with a simple question of physics. In a tandem solar cell, the bottom layer needs to have a specific "bandgap," a term that describes the minimum amount of energy a material needs to turn light into electricity. If the bandgap is too high, the material ignores the low-energy infrared light; if it is too low, it wastes energy from the high-energy light. The ideal bandgap for the bottom layer of a two-layer solar cell is just under one electronvolt, and for a three-layer cell, it is even lower. The current standard for the bottom layer in advanced solar cells is a mix of lead and tin. While this mix can be tuned to absorb infrared light, its bandgap is still slightly too high to be perfect, and the tin component is chemically unstable, prone to reacting with oxygen and ruining the cell unless it is made in a perfectly sealed, oxygen-free environment.

The researchers turned their attention to silver bismuth sulfide. This material naturally has a bandgap of about one electronvolt, which is much closer to the ideal value for a bottom layer than the lead-tin mix. It is also made from elements that are abundant and non-toxic, and unlike the lead-tin mix, it is stable in normal air. However, there was a significant hurdle. In previous experiments, silver bismuth sulfide worked best only when made into extremely thin films, just a few dozen nanometers thick. This was because the material had a short "diffusion length," meaning that once a particle of light created an electric charge, that charge could not travel very far before getting lost. For a solar cell to work well, especially one designed to catch infrared light, the film needs to be much thicker, around a hundred nanometers or more, to absorb enough of the light. If the film is too thick, the charges get trapped and the electricity stops flowing.

To overcome this, the researchers developed a new method to prepare the material. Instead of building the film layer by layer in a slow, tedious process, they created a liquid ink containing tiny crystals of silver bismuth sulfide. They then treated these crystals with a mixture of chemical agents, specifically bromide and iodide, to clean and protect their surfaces. This treatment, known as solution-phase ligand exchange, allowed the crystals to stick together tightly and conduct electricity much better. The result was a film that could be deposited in a single step, just like spreading paint, and could be made as thick as 340 nanometers without cracking or losing its ability to carry electricity. This was a crucial breakthrough, as it meant the material could finally be thick enough to catch the infrared light it was designed for.

With this new, thick film in hand, the team built the first tandem solar cells using silver bismuth sulfide. They created two types of devices. The first was a "four-terminal" tandem, where two separate solar cells were placed on top of each other but operated independently, like two separate batteries connected in a circuit. They paired a silver bismuth sulfide bottom cell with a top cell made of a different material called a perovskite. The combination produced an efficiency of nearly 24 percent, a strong result that proved the two materials could work together effectively. The second type was a "monolithic" or "two-terminal" tandem, where the layers were fused into a single, solid unit that acted as one battery. This was a much harder engineering challenge, requiring a special transparent layer between the two cells to allow electricity to flow between them without short-circuiting. Despite the complexity, the team successfully built these devices, showing that the voltages of the two layers added up correctly, a key requirement for a working tandem cell.

The simulations the researchers ran before building the devices suggested that silver bismuth sulfide could push the theoretical efficiency of three-layer solar cells to nearly 48 percent, far higher than the 36 percent limit of the current lead-tin technology. While the devices they built were not yet at that theoretical peak, the experimental results confirmed that the material works and that the new method of making it is viable. The team also tested the stability of their new cells. They left unsealed devices sitting on a shelf in dry air for three months. While a similar device made with the lead-tin mix would have lost most of its power in just a day and a half, the silver bismuth sulfide cells retained more than 90 percent of their original efficiency. This durability suggests that future solar panels made with this material could be manufactured in regular factories without the need for expensive, oxygen-free rooms, potentially lowering the cost of solar energy.

The researchers also looked at how easy it would be to make these films on a large scale. They compared their new single-step method to the old, multi-step method and found that the new approach was significantly faster and cheaper. They successfully scaled up their process from tiny laboratory samples to larger panels, showing that the film remained uniform and free of defects even when made on a surface the size of a small tile. This scalability is essential for moving from a laboratory curiosity to a product that could be used on rooftops or integrated into buildings.

The work does not mean the problem is completely solved. The researchers noted that even with their improvements, the charges in the silver bismuth sulfide still get trapped relatively quickly, which limits how thick the film can be and how much current it can generate. They identified that the next step is to find ways to make the material even more conductive, perhaps by changing how the crystals are arranged, so that the charges can travel even further. However, the study has established a clear new path forward. By proving that silver bismuth sulfide can be made into thick, stable, and efficient films, and by demonstrating its success in the first tandem solar cells, the team has opened the door to a new generation of solar technology that is more efficient, more stable, and potentially much cheaper to produce than what exists today.