Selenium-dependent Structural and Optoelectronic Properties of Mo-Free Cu 2 ZnSn(S,Se) 2 Solar Cells Prepared from Elemental Precursors

The world runs on energy, but the sources we rely on today are finite and damaging to the planet. Fossil fuels like coal and oil will eventually run out, and burning them warms the atmosphere. This reality has pushed scientists to look for cleaner alternatives, with sunlight offering the most abundant promise. The challenge lies not in capturing the sun's light, but in turning it into electricity efficiently and affordably. For decades, silicon has been the standard material for solar panels, but it is expensive to refine and requires complex manufacturing. A newer generation of solar cells uses thin films of special crystals called kesterites. These materials are made from elements that are common and non-toxic, such as copper, zinc, tin, and sulfur. They hold the potential to be cheaper and more environmentally friendly than traditional silicon, provided researchers can figure out how to make them work as well as the best existing technologies.

A team of researchers in Indonesia recently explored a new way to build these solar cells, focusing on a specific type of kesterite crystal that includes selenium, a chemical element related to sulfur. Their goal was to create a complete solar cell using only locally available, basic materials and a simple liquid-based process, avoiding the expensive vacuum chambers usually required for high-tech manufacturing. They constructed a device with layers of transparent glass, a special oxide coating, and a thin film of their custom-made crystal, sandwiched between other conductive layers. By swapping out some of the sulfur in their mixture for selenium, they aimed to tune the material's ability to absorb light and conduct electricity. The team compared cells made from standard laboratory chemicals against those made from raw, locally sourced powders to see if the cheaper route could yield similar results.

The researchers found that they could successfully synthesize the desired crystal structure using their liquid method, creating a material that is rich in zinc and slightly poor in copper, a specific balance known to improve performance. When they examined the crystals under powerful microscopes and analyzed their internal structure, they discovered that adding selenium changed the physical shape of the material significantly. The selenium atoms caused the tiny grains of the crystal to shrink dramatically. In the samples made from standard chemicals, the grains dropped from a size of about 32.8 nanometers down to just 10.7 nanometers. Similarly, the grains in the locally sourced samples shrank from roughly 195.9 nanometers to 98.1 nanometers. This reduction in size created a more porous surface, which the researchers believe helps the material trap more light and reduces the amount of light that bounces off the surface.

Optical tests revealed that this change in size and composition had a direct effect on how the material interacts with light. The addition of selenium shifted the material's absorption toward longer wavelengths, a phenomenon known as a redshift, which effectively lowered the energy gap the material needs to overcome to generate electricity. The best-performing sample, made from standard chemicals and containing selenium, achieved a bandgap of 1.16 electron volts, the lowest among all the tested samples. This lower energy gap allowed the cell to harvest a broader spectrum of sunlight, resulting in a light-harvesting efficiency of 88.12 percent. In contrast, the samples without selenium had higher energy gaps and harvested less light. The study also showed that the selenium-containing samples had fewer unwanted impurities, known as secondary phases, which can block the flow of electricity. The best sample had a secondary phase content of only 16.2 percent, the lowest of all the groups tested.

When the team tested the actual electricity generated by these cells, the results confirmed the benefits of the selenium addition. The solar cell made from the standard selenium mixture produced the highest overall efficiency of 0.166 percent. While this number is still far below the efficiency of commercial silicon panels, it represents a meaningful step forward for this specific type of low-cost, non-vacuum manufacturing. The cell also achieved the highest voltage and the best fill factor, a measure of how effectively the cell delivers power. Interestingly, the cell made from the locally sourced materials performed nearly as well as the one made from standard chemicals, reaching 92 percent of the efficiency of the best sample. This suggests that the expensive, imported chemicals are not strictly necessary to achieve good results, provided the synthesis process is carefully controlled.

The researchers noted that the performance was limited by the high amount of amorphous, or non-crystalline, material in their films, which occurred because they did not use a specific gas treatment during the heating process. This lack of a perfect crystal structure meant that the cells could not yet reach their full potential. However, the study successfully demonstrated a new solar cell design that does not use a molybdenum backing layer, a material that is difficult to source and expensive. Instead, they used a transparent conductive layer that is much cheaper and easier to obtain. The work proves that it is possible to build a functional solar cell using a simple solution-based method and locally available raw materials. By showing that selenium can reduce grain size, lower the energy gap, and minimize impurities, the study provides a clear path for improving the efficiency of these earth-abundant solar cells in the future.