Spin-Coating-Speed-Dependent Structural and Optical Properties of Open-Air-Processed MAPbI 3 Thin Films: Experimentally Informed SCAPS-1D Device Simulation

Solar energy has long been the domain of heavy silicon panels, but a newer, lighter contender has emerged from the laboratory: perovskite solar cells. These devices use a special class of crystals that can be painted onto surfaces like ink, offering a potential path to cheap, flexible electricity. The most promising version of this material is a compound called methylammonium lead iodide. To make it work, scientists must grow a thin, flawless layer of these crystals on a glass slide. The quality of this layer determines how well the solar cell captures sunlight and turns it into power. However, growing these crystals is a delicate balancing act. If the process is too slow, the material dries unevenly; if it is too fast, the crystals do not have time to form properly. The challenge lies in finding the exact speed that allows the crystals to grow large and orderly, creating a smooth film without cracks or gaps.

A team of researchers at Obafemi Awolowo University and the Federal University of Technology Owerri in Nigeria set out to solve this specific puzzle. They focused on a single variable: how fast the glass slide should spin while the liquid solution is applied. This spinning process, known as spin coating, spreads the liquid into a thin film. The team tested four different speeds, ranging from 2,000 to 5,000 rotations per minute, while keeping every other condition exactly the same. They worked in a controlled room with specific temperature and humidity levels to ensure that the air itself did not interfere with the results. By using a solvent that evaporates quickly and a specific heating process, they guided the liquid to transform into solid crystals on the glass.

The results revealed that the relationship between speed and quality is not a straight line. The researchers found that spinning the glass too slowly or too fast both led to messy, imperfect crystals. The slowest speed left the liquid too thick, causing it to dry unevenly. The fastest speed stripped the liquid away so quickly that the crystals could not grow large enough. The sweet spot was found at 3,000 rotations per minute. At this specific speed, the crystals grew to their largest size, measuring about 22.73 nanometers across, and the internal strain within the material was at its lowest. This created the most orderly and high-quality film of all the samples tested. The team confirmed this by examining the film with X-rays, which showed a perfect crystal structure with no leftover raw materials.

Once they identified the best film, the researchers wanted to know how this physical quality would translate into electrical power. They used a computer simulation to build a virtual solar cell using the exact properties of the films they had made. The simulation showed that the film spun at 3,000 rpm, which had a slightly lower energy gap for light absorption, performed slightly better than the others. In this virtual test, the best prototype achieved an efficiency of 16.67 percent. However, the researchers did not stop there. They used the simulation to ask what would happen if they could fix other problems in the device, such as making the layers thicker or reducing tiny defects that trap electricity. When they optimized these factors in the computer model, the predicted efficiency jumped to 25.38 percent.

This study highlights that while the speed of the spinning process is critical for making good crystals, it is only the first step. The researchers demonstrated that the best physical film does not automatically guarantee the best solar cell, but it provides the essential foundation. The computer models showed that to reach the highest possible power levels, one must also carefully manage the thickness of the layers and minimize the tiny imperfections inside the material. The work confirms that making solar cells in normal air is possible if the process is tightly controlled, and it provides a clear roadmap for how to move from a basic lab sample to a highly efficient device. The findings suggest that the future of these solar cells lies not just in finding new materials, but in mastering the precise, everyday details of how they are built.