- UNIST creates 'LY-m' by shifting molecular linkage position in self-assembled monolayer

UNIST Professor Yang Chang-duk holds up a perovskite solar cell. [UNIST]
UNIST Professor Yang Chang-duk holds up a perovskite solar cell. [UNIST]

Researchers have developed a new solar cell material that operates normally despite temperature swings ranging from minus 100 degrees Celsius to plus 100 degrees Celsius.

The National Research Foundation of Korea announced Thursday that a joint research team — led by UNIST professors Yang Chang-duk and Shin Seung-jae of the Department of Energy and Chemical Engineering, together with Professor Song Myung-hoon of the Department of Materials Science and Engineering — had developed an interfacial material called "LY-m" by controlling the linkage position of molecules in a self-assembled monolayer (SAM), enabling a dense and stable arrangement on the electrode surface.

Among perovskite solar cells, which have attracted intense research interest as a next-generation photovoltaic technology, the inverted-structure design — in which the layers responsible for charge transport are flipped upside down — has drawn particular attention for its potential to maximize both efficiency and stability.

In an inverted-structure device, the interface between the transparent electrode (ITO) and the perovskite light-absorbing layer is the critical zone that determines overall solar cell performance.

SAMs are widely used as hole-transport layers at this interface, but uneven molecular alignment or heat exposure can cause significant energy losses and reduce long-term stability.

Previous SAM research aimed at addressing these issues often changed both the chemical structure and the electrical properties simultaneously, making it difficult to isolate the effect of the linkage position itself on interfacial performance.

The research team synthesized three isomers that preserved the basic molecular structure of the SAM while varying only the position of the central linking group.

When applied to the perovskite interface, the "LY-m" molecule — connected at the meta position — demonstrated the best overall characteristics.

The LY-m molecule forms a dense, uniform film in a tilted orientation on the transparent electrode substrate, exhibiting the highest binding energy among the three isomers while also minimizing defects.

The structure of the self-assembled monolayer used in the solar cell capable of withstanding extreme temperature swings. [UNIST]
The structure of the self-assembled monolayer used in the solar cell capable of withstanding extreme temperature swings. [UNIST]

When applied to an actual inverted-structure perovskite solar cell, LY-m achieved a peak power conversion efficiency of 26.39 percent — an exceptionally high figure.

Particularly notable was the performance in an extreme thermal cycling test, in which devices were repeatedly exposed to minus 100 degrees Celsius and plus 100 degrees Celsius for 30 minutes each. Devices using conventional commercial materials stopped functioning, while the LY-m device maintained normal operation with an efficiency loss of only about 9 percent.

"This design principle can be applied to improve the hole-selective interface in high-efficiency inverted perovskite single-junction and tandem solar cells," Yang said. "However, while it may also serve as a reference for environments subject to large temperature fluctuations — such as in space — further verification is needed, including more thermal cycling, combined conditions such as vacuum and radiation exposure, large-area uniform coating, and long-term encapsulation reliability."

The research was supported by the Ministry of Science and ICT and the National Research Foundation of Korea through the Nano and Materials Technology Development project and published in the energy journal Joule.


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