UNIST develops non-destructive inspection technology to detect minute distortions in solar cells

Researchers have developed a non-destructive inspection technology capable of mapping crystal alignment and minute distortions in perovskite solar cells without damaging the material.

A research team led by professors Kim Dong-seok, Shin Tae-joo and Kim Jin-young at the Ulsan National Institute of Science and Technology, working with Shin Yun-seop of the Korea Research Institute of Chemical Technology, announced Wednesday that they had developed two-dimensional X-ray diffraction mapping (2DXDM) technology to analyze the structure of large-area perovskite thin films without causing damage.

Perovskite has drawn attention as a next-generation solar cell material that could replace silicon, thanks to its high photoelectric conversion efficiency and low processing costs. However, while small-scale cells achieve high efficiency, scaling up to large areas causes efficiency to drop sharply.

The problem stems from uneven growth of perovskite thin films during the scale-up process, which generates internal stress and impedes charge transport. This leads not only to reduced solar cell efficiency but also to long-term degradation.

The technology can diagnose crystal non-uniformity and lattice strain without physically cutting into the solar cell. It exploits the principle that when X-rays strike a thin-film crystal, the intensity, direction and position of the resulting diffraction patterns vary according to the crystal's orientation and the degree to which its lattice is compressed or stretched. Much like a hospital CT scanner reconstructing internal images by compiling X-ray data from multiple points, the technique repeats measurements across the entire thin film to produce a map showing where structural non-uniformity occurs — and to what degree — from the center to the edges.

Applying the technology, the research team identified which of two solution-based fabrication processes produces a more uniform thin film and used that process to manufacture a high-efficiency large-area solar cell. Comparing static spin coating (SSC) — in which the source solution is deposited all at once and spread by spinning the substrate — with dynamic spin casting (DSC), in which the solution is continuously supplied in droplets while the substrate rotates, the team found that DSC produced superior-quality perovskite thin films.

A 100-square-centimeter solar module fabricated using DSC recorded a photoelectric conversion efficiency of 23.0 percent over an active generation area of 86.4 square centimeters, with a certified efficiency of 22.75 percent. That figure approaches the current certified world record of 23.55 percent for perovskite modules of comparable size. The sealed module retained more than 80 percent of its initial efficiency after 1,000 hours of continuous light exposure.

"For large-area perovskite thin films, it is important to understand structural uniformity across the entire surface, not just in localized regions," Kim Dong-seok said. "This research allows us to determine under which process conditions thin films become more uniform and module efficiency improves, and it can be used to refine the manufacturing process for high-efficiency large-area solar cells."

The findings were published in the energy journal Joule on July 23.