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 CO reduction is presented. Central to this innovation is a thinner, Cu(In,Ga)S 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 CO reduction exceeding 10 mA/cm at 0.0 V vs. RHE under 1 Sun illumination, and ca. 16 mA/cm 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 CO reduction. It imposes minimal constraints on absorption properties, immobilization methods, and catalyst nature, setting the stage for high-performance, adaptable PEC devices.