Towards stable solar hydrogen generation by tandem organic bulk heterojunction photoanodes Matyas Daboczi 1,2 , Noof Al Lawati 1,3 , Flurin Eisner 3,4 , Maoqing Zhi 1,3 , Joel Luke 3 , Katherine Stewart 3 , Junyi Cui 1 , Shi Wei Yuan 3 , Jolanda Simone Müller 3 , Ji-Seon Kim 3 , Levente Tapaszto 2 , Jenny Nelson 3 , Salvador Eslava 1 1 Department of Chemical Engineering and Centre for Processable Electronics, Imperial College London, London SW7 2AZ, UK, 2 Centre for Energy Research, Institute of Technical Physics and Materials Science, Budapest, 1121, Hungary, 3 Department of Physics and Centre for Processable Electronics, Imperial College London, London SW7 2AZ, UK, 4 School of Engineering and Materials Science, Queen Mary University of London, E1 4NS, UK Photoelectrochemical water splitting offers a promising pathway for sustainable hydrogen production using solar energy. While organic photoactive materials have demonstrated record-high efficiencies in single-junction solar cells, their broader application in PEC devices remains limited due to poor operational stability in aqueous environments. In this talk, I present a cost-effective strategy to protect a high-performing PM6:D18:L8-BO bulk heterojunction photoanode using a multifunctional, catalyst-functionalised graphite sheet. This architecture enables photocurrent densities exceeding 25mAcm ‑2 at 1.23 V RHE for solar water oxidation. Furthermore, by integrating two photoactive layers with complementary absorption into a tandem photoanode, we demonstrate unassisted ( i.e., bias-free) hydrogen generation with a solar-to-hydrogen efficiency of 5%. 1 I will discuss in detail the degradation pathways of these photoanodes, with a focus on the morphological instability of the organic photoactive layer as the primary limitation to long-term stability.Next, I will present another organic bulk heterojunction blend (PTQ10:L8BO) that addresses the challenge of morphological instability. The introduction of this new photoactive layer leads to single-junction organic photoanodes with significantly enhanced operational lifetime under full solar illumination ( i.e. , without using a UV-filter) and to increased solar-to-hydrogen efficiency (up to 6.2%). Finally, I will provide guidelines for further improving the stability and efficiency of integrated organic tandem photoelectrodes for unassisted solar hydrogen production. References 1. Daboczi, M. et al. Enhanced solar water oxidation and unassisted water splitting using graphite-protected bulk heterojunction organic photoactive layers. Nature Energy 1–11 (2025) doi:10.1038/s41560-025-01736-6.
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