Commercialization of solar hydrogen production technology, which extracts hydrogen from water using sunlight, is expected to accelerate following the development of a thin-film material that enhances the performance of photoelectrodes essential for the process.
A research team led by Professor Cho Han-hee at the Department of Materials Science and Engineering at Ulsan National Institute of Science and Technology (UNIST) announced Wednesday that they have developed a naphthalimide-based self-assembled monolayer (SAM) thin film that can improve solar hydrogen production performance.
Solar hydrogen production is a technology that splits water into hydrogen and oxygen by exposing photoelectrodes submerged in water to sunlight. When the semiconductor inside the photoanode absorbs light, electrons are generated. These electrons move to the substrate and trigger a chemical reaction that splits water into hydrogen and oxygen.
The self-assembled monolayer developed by the research team serves to transfer electrons between the organic semiconductor and the substrate. Previously, this role was performed by metal oxide layers, which are thick and have poor charge transfer performance.
When applied to photoelectrodes, the material recorded a current density of 7.97 mA/cm². This represents the highest current density performance among photoelectrodes based on bulk heterojunction (BHJ) organic semiconductors. Higher current density performance in photoelectrodes means faster hydrogen production at the counter electrode. Additionally, unlike metal oxide layers, this material forms thin films through self-assembly of molecules, reducing manufacturing process costs.
The research team was able to develop this material by designing the molecules forming the thin film with a "push-pull structure." Push-pull refers to a structure where electron-donating and electron-withdrawing parts coexist within a single molecule. Molecules with this structure can combine forces to create a strong electric field. The generated electric field lowers the energy barrier, activating the "electron tunneling" phenomenon where electrons pass through the self-assembled monolayer.
"Organic semiconductor-based photoelectrodes have advantages of low cost and large-area manufacturing capability," Professor Cho said. "The self-assembled monolayer developed in this study is a material that significantly increases the commercialization potential of organic photoelectrode-based solar hydrogen production technology."
Professor Cho added, "The 'push-pull' molecular structure, which is the molecular design strategy of this research, can be widely applied not only to solar hydrogen production technology but also to solar cells, light-emitting diodes, and optical sensor devices where electron extraction is important."
The research was published online on November 11 in ACS Energy Letters, an internationally prestigious journal in the energy field. The study was supported by the Ministry of Science and ICT's National Research Foundation of Korea (NRF) through the Basic Research Laboratory Program, Excellent Young Researcher Program, and Innocore Program (Intelligent Hydrogen Technology Innovation Center, AI-Space Solar Cell Project), as well as the ETH Leading House Asia in Switzerland.
