ULSAN — The Ulsan National Institute of Science and Technology (UNIST) said on the 26th that a team of professors Cho Han-hee, Choi Mun-ki and Song Myoung-hoon in its Department of Materials Science and Engineering has developed a high-performance organic semiconductor photoanode that sharply improves the efficiency and durability of solar water splitting, in collaboration with researchers at the Swiss Federal Institute of Technology in Lausanne (EPFL).
Solar water splitting is a technology in which a photoelectrode absorbs sunlight to break down water and produce clean green hydrogen. Conventional organic photoelectrodes mainly used a "bulk heterojunction" structure, in which materials are mixed at random. In this approach, the pathways of electrons and holes become entangled, causing heavy recombination loss as they merge again during transport, while the electrode surface is easily damaged, limiting long-term stability.
The team solved the problem by stacking the acceptor layer first and then adding the donor layer on top, a sequential deposition method that creates a "quasi-bilayer" structure. During the solution-coating process, the researchers induced a mixed layer to form naturally between the two layers. Charges generated in the intermediate mixed layer moved downward to the acceptor layer, while holes moved upward to the donor layer in contact with the water — traveling in opposite directions to minimize recombination loss.
In performance tests, the new photoanode recorded an average photocurrent density of 1.75 mA/cm², about 22% higher than the conventional level. The external driving voltage needed for the water oxidation reaction fell from 0.35 V to 0.15 V, less than half, allowing smooth operation with only a small amount of energy. Continuous operating stability also improved by about 300% compared with the conventional approach, a result of suppressing chemical damage to the internal mixed layer where charges separate.
"By controlling the arrangement of the two materials within the photoanode thin film, we were able to raise both performance and operating stability," Cho said. "Because it works well even at low voltage, it should help in developing devices that produce hydrogen using only sunlight, without an external voltage."
The findings were published online on July 16 in the Chemical Engineering Journal, a leading international journal in the field of chemical engineering. The research was supported by the National Research Foundation of Korea (NRF) under the Ministry of Science and ICT and by the InnoCORE program.
