This study first demonstrates the potential of organic photoabsorbing blends in overcoming a critical limitation of metal oxide photoanodes in tandem modules: insufficient photogenerated current. Various organic blends, including PTB7-Th:FOIC, PTB7-Th:O6T-4F, PM6:Y6, and PM6:FM, were systematically tested. When coupled with electron transport layer (ETL) contacts, these blends exhibit exceptional charge separation and extraction, with PM6:Y6 achieving saturation photocurrents up to 16.8 mA cm⁻² at 1.23 V_RHE (oxygen evolution thermodynamic potential). For the first time, a tandem structure utilizing organic photoanodes has been computationally designed and fabricated and the implementation of a double PM6:Y6 photoanode/photovoltaic structure resulted in photogenerated currents exceeding 7 mA cm⁻² at 0 V_RHE (hydrogen evolution thermodynamic potential) and anodic current onset potentials as low as −0.5 V_RHE. The herein-presented organic-based approach paves the way for further exploration of different blend combinations to target specific oxidative reactions by selecting precise donor/acceptor candidates among the multiple existing ones.

The high voltage required to overcome the thermodynamic threshold and the complicated kinetics of the water splitting reaction limit the efficiency of single semiconductor-based photoelectrochemistry. A semiconductor/solar cell tandem structure has been theoretically demonstrated as a viable path to achieve an efficient direct transformation of sunlight into chemical energy. However, compact designs exhibiting the indispensable optimally balanced light absorption have not been demonstrated. In the current work, we design and implement a compact tandem providing the complementary absorption of a highly transparent BiVO₄ photoanode and a PM6:Y6 solar cell. Such bandgap combination approaches the optimal to reach the solar-to-hydrogen (STH) conversion upper limit for tandem photoelectrochemical cells (PECs). We demonstrate that, by using a photonic multilayer structure to adequately balance sunlight absorption among both tandem materials, a 25% increase in the bias-free STH conversion can be achieved, setting a clear path to take compact tandem PECs to the theoretical limit performance.