The Ultrafast Photophysics of Y-Series-Based Organic Photocatalysts for Hydrogen Evolution
Date
2025
Authors
de la Perrelle, Jessica Michelle
Editors
Advisors
Kee, Tak
Huang, David
Huang, David
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Thesis
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Abstract
Organic semiconductors are a fascinating and diverse class of materials: carbon-based small molecules and polymers whose extensive conjugation yields strong visible and near-infrared (NIR) light absorption. Devices based on organic semiconductors are lightweight and flexible, and their properties are broadly tunable via synthetic design. Organic semiconductors have found applications in technologies such as light-emitting diodes (LEDs), photovoltaic cells, and more recently photocatalysts for hydrogen (H2) evolution and other chemical transformations. However, for applications requiring the generation of free charges, the efficiency of organic semiconductors lags behind their inorganic counterparts. To improve the charge-generation efficiency, the active component of devices is typically a blend of two materials, an electron-rich donor and an electron-poor acceptor. Traditionally, excitation of the donor results in donation of an electron to the acceptor, yielding spatially separated charges that can be collected and used. In the organic photovoltaic (OPV) field, a new class of acceptors has boosted performance dramatically. The so-called non-fullerene acceptors (NFAs) are typically small molecules with fused-ring backbones consisting of alternating donor and acceptor moieties. The Y-series NFAs (Y6 and its derivatives), which have a characteristic A– D–A’–D–A structure and curved backbone, are consistently found in high performing OPVs. In this thesis, the Y-series acceptors are leveraged for the photocatalytic evolution of H2 from water. Photocatalytic H2 evolution using organic semiconductor nanoparticles (NPs) has the potential to produce low-cost green H2 from water and sunlight, providing renewable combustible fuel for sectors that cannot be easily electrified. Chapter 1 provides the theoretical background and motivation for this work, while essential experimental methods are summarized in Chapter 2. In Chapter 3, the photophysics and H2 evolution of NPs of the high-performing blend of donor PM6 and acceptor Y6 are investigated. We find that the 1:1 PM6:Y6 blend NPs stabilized with surfactant 2-(3-thienyl)ethyloxybutylsulfonate sodium salt (TEBS) are highly active photocatalysts under solar-like illumination, with a 2% by mass loading of Pt co-catalyst and using ascorbic acid (AA) as a sacrificial hole scavenger. Much of this performance is retained under red and NIR illumination, where only Y6 is excited. We show that the external quantum efficiency (EQE) of this blend remains consistent at 405nm, 565nm, and 780nm. We use ultrafast transient absorption (TA) spectroscopy to probe the photophysics of this blend, and attribute the retained performance under excitation of Y6 to charge generation by transfer of holes to PM6. This Chapter establishes the critical role of excitation of the acceptor Y6 in PM6:Y6 blends and is one of the first studies of H2 production under exclusively red and NIR illumination. In Chapter 3, we note that while PM6 NPs are rather inactive photocatalysts, Y6 NPs retained a significant proportion of the activity of PM6:Y6 blend NPs, despite significantly lower absorption in the yellow to blue wavelengths due to the absence of PM6. This behavior is unusual, as free charge yields in single-component NPs are typically low due to high exciton binding energies. In Chapter 4, we present a more detailed study of neat Y6 NPs. We also take the opportunity to investigate the impact of two common surfactants, sodium dodecyl sulfate (SDS) and TEBS, on H2 evolution rates, as the use of single-component NPs eliminates the surfactant-induced morphological changes often observed in blend NPs. We find that Y6 NPs prepared with SDS and TEBS have similar photophysics, with evidence of free charge formation in both cases, which may explain the impressive H2 evolution rates of TEBS-stabilized Y6 NPs. However, the H2 evolution rate of Y6 NPs stabilized with SDS is 21 times lower than the rate for Y6 NPs stabilized with TEBS. This difference is attributed to poor deposition of the Pt co-catalyst on NPs prepared with SDS, most likely due to the formation of a dense, insulating layer of SDS on the NP surface. This work highlights the potential of neat Y-series NPs as simple but high-performing photocatalysts for H2 evolution, as well as the importance of surfactant selection in promoting favorable surface conditions for co-catalyst deposition. In Chapter 5, we return to PM6:Y6 NPs as well as neat PM6 NPs and neat Y6 NPs with a detailed photophysical study using ultrafast TA with an expanded time axis ranging from tens of femtoseconds to hundreds of microseconds. This large time range allows tracking of charges across their entire lifetime, from generation under PM6 or Y6 excitation to eventual recombination. By applying kinetic modeling to the spectroscopic data, we confirm that Y6 excitons are weakly bound, and rapidly dissociate to form Y6 electrons and Y6 holes in neat Y6 NPs and in Y6 domains in PM6:Y6 NPs. We also show that upon excitation of PM6, both electron transfer and Förster resonance energy transfer (FRET) to Y6 occurs, but FRET is not a loss pathway as it is followed by back hole transfer from Y6 domains. This thorough photophysical study demonstrates the importance of exciton dissociation in Y6 domains and recommends optimization of exciton dissociation in Y-series acceptors to improve photocatalytic performance. In Chapter 6, we broaden our study to include other Y-series acceptors (Y6, L8- BO, PC6, and PY-IT) and focus on ultrafast exciton dissociation, which occurs largely within the instrument response time even for ultrafast TA. PC6 and L8-BO are Y6 derivatives with modified side chains to influence their solid-state packing, while PY-IT is a polymerized Y-series acceptor. We find that in NP suspensions, there is a clear range of morphologies obtained by the side-chain modifications and polymerization. Y6 NPs show two distinct aggregation types, both with mixed H/J character, as well as significant population of non-interacting (disordered) chromophores. Progressing along the series, from Y6 to PC6, PY-IT, and L8-BO, we observe increasing J-aggregation and decreasing energetic disorder as one aggregate type dominates over the other. To resolve the ultrafast kinetics of charge generation in the Y-series NPs, we use broadband pump–probe (BBPP) spectroscopy with broadband 10 fs laser pulses. Spectral deconvolution confirms that exciton dissociation occurs in all the Y-series NPs, with time constants of 28±3 fs for Y6 NPs, 45±5 fs for PC6 NPs, 89±10 fs for PY-IT NPs, and 149 ± 16 fs for L8-BO NPs. The trend in these rate constants correlates with the trend in morphology; chromophores arranged with mixed H/J-character aggregation and increased energetic disorder show faster exciton dissociation. Finally, we characterize the coherence induced by the broadband pump, finding it primarily vibrational in nature. We find preliminary evidence that a low-frequency vibration around 460 cm−1 may be involved in exciton dissociation. Overall, this study highlights the universality of ultrafast exciton dissociation within a range of Y-series acceptors, and quantifies the dependence of exciton dissociation rate on morphology. Together, these four projects represent a comprehensive description of the photophysics and photocatalytic performance of Y-series acceptors is neat NPs and when blended with a donor.
School/Discipline
School of Physics, Chemistry and Earth Sciences : Chemistry
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Physics, Chemistry and Earth Sciences : Chemistry, 2025
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