From light to hydrogen: the complete life cycle of free charges in organic photocatalytic nanoparticles

The blend of polymeric donor PM6 and non-fullerene acceptor Y6 is high performing as both an active layer in organic photovoltaics and as a nanoparticulate photocatalyst for the renewable production of hydrogen gas. Despite the high performance of PM6:Y6 blends, many aspects of the photophysics of this material remain unclear. Here we present a detailed spectroscopic analysis of bulk heterojunction PM6:Y6 nanoparticles for photocatalytic hydrogen evolution over 11 orders of magnitude in time, ranging from tens of femtoseconds to hundreds of microseconds. We find that the excitation of Y6 primarily results in the formation of charges first in Y6 domains, followed by diffusion of Y6 holes to PM6 domains. Upon excitation of PM6, charges are generated through two mechanisms: (1) energy transfer to Y6 followed by exciton dissociation and back hole transfer to PM6, and (2) electron transfer to Y6 facilitated by an interfacial charge-transfer state. We use kinetic modelling to confirm these mechanisms and determine the rates of formation and recombination of charges. We also investigate the PM6:Y6 nanoparticles under photocatalytic conditions, and show that the Pt co-catalyst can accept both Y6 electrons and Y6 holes on relatively fast (<100 ps) timescales, and that the sacrificial electron donor ascorbic acid scavenges holes from both components on picosecond and microsecond timescales. The results highlight the critical importance of rapid free polaron formation in Y6 domains, and point towards harnessing this property of the Y-series and other non-fullerene acceptors to develop industrially viable organic hydrogen-evolution photocatalysts.