Ultrathin coating pushes solar cell efficiency past 33%, UNIST says

UNIST uses vacuum deposition to eliminate surface defects in perovskite solar cells

The UNIST research team behind the study. From left: professors Yang Chang-duk and Choi Kyung-jin, and researchers Park Ji-won, Kim Ji-eun and Noh Young-im. [UNIST]

South Korean researchers have developed a breakthrough technology that simultaneously boosts the efficiency and durability of next-generation perovskite solar cells.

The Ulsan National Institute of Science and Technology (UNIST) announced Sunday that a joint research team led by Yang Chang-duk, a professor in the Department of Energy and Chemical Engineering, and Choi Kyung-jin, a professor in the Department of Materials Science and Engineering, had succeeded in improving both the power generation efficiency and durability of perovskite solar cells using vacuum deposition technology.

In perovskite solar cells, microscopic defects on the surface of the light-absorbing material cause electrons to be lost, reducing power generation efficiency. Those defects also serve as starting points for material degradation, making them a key factor in shortening the lifespan of the cells.

A technique known as "passivation" — coating the surface with a special substance — addresses this problem. Conventional solution-based processes, however, have a drawback: the coating material tends to clump or spread unevenly, causing performance variations from cell to cell.

The research team heated the passivation material in a vacuum, converted it into a gas and deposited it directly onto the solar cell surface. The key is precisely controlling the supply rate and deposition speed to form a uniform ultrathin film.

After comparing the thermal stability and volatilization characteristics of six passivation materials, the team selected piperazinium diiodide (PDI) as the optimal substance. Depositing it at a rate of 0.01 nanometers per second, the molecules bonded more closely and uniformly to the surface than with conventional solution-based treatment, reducing energy loss.

The peak power generation efficiency of perovskite solar cells treated with this technology reached 26.05 percent, surpassing the 25.43 percent achieved with conventional solution processing. In a tandem solar cell combining perovskite and silicon, the team achieved a certified efficiency of 33.09 percent.

When 12 cells were fabricated under identical conditions and compared, efficiency variation between cells also decreased.

The process of uniformly depositing PDI gas onto a perovskite surface to form a passivation layer, and the structure of a perovskite-silicon tandem solar cell. [UNIST]

Durability also improved significantly. In a test in which cells were operated under continuous light at 65 degrees Celsius, the cells retained 90 percent of their initial efficiency for 1,005 hours — about 55 percent longer than cells treated with conventional solution processing.

The technology proved effective not only in single perovskite solar cells but also in 3-square-centimeter mini-modules and silicon tandem solar cells. Because it relies on vacuum deposition — a process already widely used in semiconductor and display manufacturing — it also points to the potential for scaling up to large-area, high-volume production.

"This technology offers a way to improve both the efficiency and durability of perovskite solar cells while also reducing performance variation between cells," Yang said. "We expect it to contribute to the large-area, mass production of high-efficiency solar cells, having secured a certified efficiency of more than 33 percent in tandem solar cells as well."

The findings were published Sept. 1 in Energy & Environmental Science, an international journal covering energy and environmental research.

nbgkoo@heraldcorp.com