Hybrid CIGS-Cobalt Quaterpyridine Photocathode with Backside Illumination: A New Paradigm for Solar Fuel Production.

Chalcogenide-based thin-film solar cell optimized for rear illumination and used for CO2 reduction is presented. Central to this innovation is a thinner, Cu(In,Ga)S2 chalcopyrite absorber coated with a robust metallic top layer, which potentially surpasses the performance of conventional front-illuminated designs. Using cobalt quaterpyridine molecular catalyst, photocurrent densities for CO2 reduction exceeding 10 mA/cm2 at 0.0 V vs. RHE under 1 Sun illumination, and ca. 16 mA/cm2 at -0.25 V vs. RHE were achieved in voltammetry experiments. Controlled potential electrolysis showed catalytic activity over 20 h with selectivity for CO ranging from >92 % (first 4 hours) to 86 % at the end of the experiment. This approach opens limitless possibilities for employing various reduction catalysts, extending far beyond CO2 reduction. It imposes minimal constraints on absorption properties, immobilization methods, and catalyst nature, setting the stage for high-performance, adaptable PEC devices.