Solar hydrogen generation from water represents a compelling component of a future sustainable energy portfolio. Recently, chemically robust heptazine-based polymers known as graphitic carbon nitrides (g-C3N4) have emerged as promising photocatalysts for hydrogen evolution using visible light while withstanding harsh chemical environments. However, since g-C3N4 electron-transfer dynamics are poorly understood, rational design rules for improving activity remain unclear. Here, we use visible and near-infrared femtosecond transient absorption (TA) spectroscopy to reveal an electron-transfer cascade that correlates with a near-doubling in photocatalytic activity from 2050 to 3810 μmol h–1 g–1 when we infuse a suspension of bulk g-C3N4 with 10% mass loading of chemically exfoliated carbon nitride. TA spectroscopy indicates that exfoliated carbon nitride quenches photogenerated electrons on g-C3N4 at rates approaching the molecular diffusion limit. The TA signal for photogenerated electrons on g-C3N4 decays with a time constant of 1/ke′ = 660 ps in the mixture versus 1/ke = 4.1 ns in g-C3N4 alone. Our TA measurements suggest that the charge generation efficiency in g-C3N4 is greater than 65%. Exfoliated carbon nitride, which liberates only trace hydrogen levels when photoexcited directly, does not appear to independently sustain appreciable long-lived charge generation. Thus, the activity enhancement in the two-component infusion evidently results from a cooperative effect in which charge is generated on g-C3N4, followed by electron transfer to exfoliated carbon nitride containing photocatalytic chain terminations. This correlation between electron transfer and photocatalytic activity provides new insight into structural modifications for controlling charge separation dynamics and activity of carbon-based photocatalysts.

Using transient absorption, time-resolved photoluminescence, and device measurements, we show that fullerene aggregation in small-molecule organic photovoltaic blends correlates with photocurrent enhancement due to kinetically avoided recombination to thermodynamically favored triplet states. We evaluate the electron donor chloroboron subphthalocyanine (SubPc) blended with a C60 fullerene electron acceptor. We show that photocurrent generation nearly doubles for SubPc:C60 blends with a higher C60 ratio (1:2 versus 1:1) and enhanced fullerene aggregation. Our spectroscopic results suggest that aggregation at the higher C60 loading ratio aids in sustaining the free charge population by inhibiting recombination to form SubPc triplets. By also examining blended SubPc:C60 films in which aggregation and charge transfer are disrupted by an inert matrix, we further highlight an additional energy transfer pathway for SubPc triplet formation facilitated by intersystem crossing centered on C60. This energy transfer pathway is kinetically outcompeted by charge transfer in the condensed films employed in devices. Our results provide new insight into the role that aggregation plays in promoting charge separation and photocurrent collection in small-molecule organic photovoltaics. Our findings suggest new avenues for improving device performance by kinetically avoiding recombination to triplet states, despite the presence of multiple thermodynamically accessible pathways for triplet formation in these blended films.