Photodoping and Fast Charge Extraction in Ionic Carbon Nitride Photoanodes

Ionic carbon nitrides based on poly(heptazine imides) (PHI) represent one of the most vigorously studied class of low-cost, tunable and stable polymeric materials with possible applications in photocatalysis and energy storage. However, the fundamental photophysical properties and processes that govern the performance of these materials are still poorly understood and have been studied mostly in form of particle suspensions. We study, for the first time, the photogenerated charge dynamics in highly stable and binder-free PHI photoanodes with excellent performance in photoelectrocatalytic alcohol conversions using in operando transient photocurrents (TPC) and spectroelectrochemical photoinduced absorption (PIA) measurements. Interestingly, we discover that light-induced accumulation of long-lived trapped electrons within the PHI film leads to effective photodoping of the PHI film, resulting in a significant improvement of photocurrent response due to more efficient electron transport. While photodoping has been previously reported for various inorganic and organic semiconductors, the here reported beneficial photodoping effect has never been shown before for carbon nitride materials. Furthermore, we find that the extraction kinetics of untrapped electrons are remarkably fast in these PHI photoanodes, with electron extraction times (ms) comparable to those measured for commonly employed metal oxide semiconductors (e.g., TiO₂, WO₃, BiVO₄). These results shed light on the excellent performance of PHI photoanodes in alcohol photoreforming, including very negative photocurrent onset, outstanding fill factor, and the possibility to carry out photoreforming under zero-bias conditions. More generally, the here reported photodoping effect and fast electron extraction in PHI photoanodes represent thus yet another intriguing property of ionic (PHI-based) carbon nitride materials, and establish a strong rationale for the use of PHI films in various applications, such as bias-free photoelectrochemistry or photobatteries.