Bulk versus surface charge recombination kinetics in polymer heterojunction nanoparticle photocatalyst dispersions

Organic donor-acceptor heterojunction nanoparticles have emerged as promising photocatalysts for solar hydrogen production, combining efficient light harvesting, donor-acceptor charge separation, and the generation of long-lived charge carriers. However, the identity of the charge populations responsible for photocatalytic turnover remains unresolved. Here, we study photogenerated polarons in aqueous oligo(ethylene glycol)-functionalized IDTBT:IDTBR-based heterojunction nanoparticles using transient absorption spectroscopy (TAS) and photoinduced absorption spectroscopy (PIAS) to probe, respectively, the transient polarons generated under pulsed excitation and long-lived polarons that accumulate under continuous illumination. By directly correlating spectroscopic signatures with photocatalytic hydrogen evolution, we show that the steady-state PIAS response tracks photocatalytic activity, establishing long-lived accumulated charges as the reactive population governing photocatalytic turnover. Excitation-fluence studies further demonstrate that the contrasting TAS and PIAS responses originate from distinct trapped-charge populations, with decay times at matched charge densities being five orders of magnitude faster for TAS compared to PIAS. We attribute the exceptionally long lifetimes measured in PIAS to the synergistic effects of donor-acceptor charge separation, progressive trap filling under continuous illumination, and dielectric stabilisation provided by the aqueous glycol-functionalized environment, which together suppress charge recombination and enable long-lived charge accumulation under operating conditions. These findings identify operando PIAS as a powerful approach for probing catalytically relevant charge populations and provide mechanistic design principles for engineering organic semiconductor photocatalysts with enhanced solar fuel performance.