Abstract

Organic photoelectrochemical (PEC) cells for solar water splitting typically utilize bulk heterojunction (BHJ) structures to circumvent the intrinsically short exciton diffusion lengths of organic semiconductors. However, stochastic donor/acceptor networks in BHJs often result in disordered interfacial contacts, leading to severe charge recombination. Herein, we report a pseudo-bilayer (PS-Bi) organic photoanode featuring a vertically ordered donor–acceptor configuration established via sequential deposition. Our systematic investigation reveals that this PS-Bi structure promotes selective charge transport and suppresses recombination due to its high domain purity, thereby facilitating efficient charge transfer at the interface. Consequently, the optimized PS-Bi photoanodes deliver an enhanced average photocurrent density (Jph) of 1.75 ± 0.03 mA cm−2 at 1.23 V versus the reversible hydrogen electrode for solar water oxidation, while simultaneously exhibiting a significant cathodic onset shift and extended operational stability compared to their BHJ counterparts (Jph = 1.44 ± 0.07 mA cm−2). This study demonstrates that employing the PS-Bi structure is a promising strategy for achieving high-performance and stable organic PEC systems for sustainable solar fuel production.

Producing hydrogen from sunlight depends in part on how efficiently a photoelectrode can separate and move electrical charges. In organic photoelectrodes, however, the conventional practice of mixing semiconductor materials can create disordered pathways that allow those charges to recombine, reducing both performance and stability.

A research team, led by Professors Han Hee Cho, Moon Kee Choi, and Myung Hoon Song of the Department of Materials Science and Engineering at UNIST, working with researchers at École Polytechnique Fédérale de Lausanne (EPFL), has developed an organic photoanode with a more ordered internal structure. The new design improved photocurrent by about 22% while extending operational stability.

The team created what is known as a pseudo-bilayer (PS-Bi) structure by depositing two organic semiconductor materials in sequence. As the second layer is applied, it partially penetrates the first, forming a mixed region at the interface while preserving more distinct layers above and below.

Conventional organic photoelectrodes typically use a bulk heterojunction (BHJ), in which donor and acceptor materials are mixed throughout the film. This arrangement helps generate and separate charges, but its irregular internal pathways can also bring electrons and holes back together before they contribute to the chemical reaction.

The PS-Bi combines the advantages of both approaches. Charges are generated and separated in the mixed region, while the more ordered layers guide electrons and holes in different directions. This reduces recombination and allows more of the generated charge to drive water oxidation.

In tests, the PS-Bi photoanodes achieved an average photocurrent density of 1.75 mA cm⁻², compared with 1.44 mA cm⁻² for conventional BHJ photoanodes, an increase of about 22%. The voltage required to initiate water oxidation also fell from 0.35 V to 0.15 V, allowing the reaction to begin with less external electrical input.

The structural change also improved durability. Based on the time required for photocurrent density to fall to 0.5 mA cm⁻², the pseudo-bilayer photoanodes remained operational substantially longer than their BHJ counterparts.

The researchers also found that the two structures degrade differently. In BHJ photoanodes, highly reactive holes can damage the organic semiconductor within the intermixed structure. The more ordered pseudo-bilayer reduces damage within the donor–acceptor region, helping the photoanode maintain its performance for longer.

“Conventional organic photoelectrodes mix donor and acceptor materials throughout the film, which can lead to charge losses and faster degradation,” said Professor Cho. “By controlling how the two materials are arranged, we were able to improve both charge transport and operational stability.” He further added, “The lower operating voltage could also help in the development of systems that ultimately produce hydrogen using sunlight without an external electrical bias.”

The study was published online in  Chemical Engineering Journal on July 16, 2026. The research was supported by the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (MSIT), and the InnoCORE program.

Journal Reference

Gyu Won Chae, Seong Rae Kang, Jin Su Park,   et al. , “A pseudo-bilayer organic photoanode for solar water oxidation,”   Chem. Eng. J., (2026).