Insights into charge dynamics in Y6-based heterojunction organic nanoparticles for hydrogen evolution Keren A i 1 , Martina Rimmele 1 , Suzzy Su 1 , Yasmine Baghdadi 2 , Charles Jeffreys 3 , Bhatti, Humza 2 , Salvador Eslava 2 , Soranyel Gonzalez-carrero 4 , Martin Heeney 1 and James Durrant 1 1 Department of Chemistry, Imperial College London, London, UK, 2 Department of Chemical Engineering, Imperial College London, London, UK, 3 Applied Physics Program, Physical Science and Engineering Division (PSE), King Abdullah University of Science and Technology (KAUST), Saudi Arabia, 4 Department of Organic Chemistry/Institute of Molecular Science (ICMol), University of Valencia, Spain Organic semiconductors, known for their tunable energy levels and strong light absorption, have emerged as promising materials for various optoelectronic applications. Over the past two decades, significant advancements in organic photovoltaics (OPVs) have been driven by the bulk-heterojunction (BHJ) architecture, which facilitates efficient charge separation through donor-acceptor interfaces that enable effective exciton dissociation upon photoexcitation. Recently, the BHJ architecture has been adapted for photocatalytic hydrogen evolution reactions (HER) via water splitting. BHJ-based nanoparticles (NPs), comprising both donor and acceptor components, have demonstrated superior performance in exciton dissociation and HER efficiency 1,2 . In this study, we investigated the photocatalytic performance of the widely successful non-fullerene acceptor (NFA) Y6 and its derivative L8-BO in BHJ nanoparticles prepared via the mini-emulsion method 2 . When blended with the low-cost polymer donor FO6-T, the L8-BO BHJ NPs exhibited nearly twice the HER rate of Y6 at an optimized donor/acceptor (D/A) ratio of 1:4. Cryo-transmission electron microscopy (cryo-TEM) revealed a highly crystalline packing of L8-BO inside the BHJ NPs, in contrast to the amorphous morphology observed in Y6 BHJ NPs. Interestingly, this morphology trend differs from film morphologies, where Y6 exhibits higher crystallinity 3 . These morphological differences were further reflected in the absorption spectra: Y6 showed a significant change in absorption between films and NPs, whereas L8-BO maintained consistent spectral features. To correlate the observed morphology with HER performance, transient absorption spectroscopy (TAS) was used to investigate charge generation. Due to its high crystallinity, the L8-BO BHJ NPs exhibited low yield charge generation, likely caused by poor intermixing between donor and acceptor phases. In contrast, the Y6 BHJ NPs showed efficient charge generation due to its amorphous and intermixed morphology. Despite this, photoluminescence (PL) quenching measurements also supported the charge generation difference. To understand why good charge generation in Y6 BHJ NPs doesn’t correlate with the HER performance, the photo- induced absorption (PIA) technique is utilised to probe the charge accumulation in the timescale of seconds. The PIA revealed that charge accumulation in the L8-BO BHJ NPs was approximately twice as high as in the Y6 BHJ NPs, aligning with the observed HER performance. This highlights the role of crystalline morphology in stabilizing charge accumulation, which prolongs charge lifetimes and enhances interactions with co-catalysts. These findings reveal an inverse morphology-performance relationship between films and NPs for Y6 and L8-BO, offering new insights into charge dynamics within BHJ NPs. This work underscores the importance of morphology control in optimizing photocatalytic performance and opens up new avenues for BHJ-based photocatalysis. References 1. Kosco, J., Gonzalez-Carrero, S., Howells, C.T. et al. Generation of long-lived charges in organic semiconductor heterojunction nanoparticles for efficient photocatalytic hydrogen evolution. Nat Energy 7 , 340–351 (2022). 2. Kosco, J., Bidwell, M., Cha, H. et al. Enhanced photocatalytic hydrogen evolution from organic semiconductor heterojunction nanoparticles. Mater. 19 , 559–565 (2020). 3. An, K., Zhong, W., Peng, F. et al. Mastering morphology of non-fullerene acceptors towards long-term stable organic solar cells. Nat Commun 14 , 2688 (2023).
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