Turning a solar cell into a catalyst: (Ag,Cu)(In,Ga)Se₂ p–n junction enabling ambient dry reforming of methane

Photocatalysis driven by solar energy offers a sustainable alternative to thermocatalysis for methane valorization, however large-scale deployment remains limited by catalyst efficiency and scalability. Meanwhile, photovoltaic technologies, though highly developed for electricity generation, still face challenges in costly energy storage and underutilized potential in direct solar-to-chemical energy conversion. In this context, CIGS thin-film solar cells emerge as promising candidates for photocatalytic applications due to their strong light absorption, tunable electronic properties, and industrial scalability. In the present work, we use a thin-film CIGS solar cell plates, re-designed as a monolithic photocatalyst, to drive DRM under ambient conditions. A 2 µm p-type (Ag,Cu)(In,Ga)Se₂ (ACIGS) absorber deposited on a soda-lime glass/Mo substrate is overcoated with an n-type CdS layer, forming a p–n junction that couples strong light absorption with built-in charge separation. Under irradiation, ACIGS/CdS plates produce > 2 mmol g_cat⁻¹ syngas with ≈ 85 % CO selectivity at ambient conditions, without any external electric power or thermal input. Mechanistic evidence indicates deep CH₄ dissociation to surface carbon and hydrogen, with subsequent CO₂ reduction by surface carbon to CO. The catalytic plates are air-regenerable under light and exhibit notable stability. Turning a solar-cell design into the catalytic junction tackles efficiency and manufacturing hurdles for CH₄/CO₂ conversion. Because CIGS and CdS processes already exist at industrial scale, this approach provides a practical route to deployable solar chemical hardware; further gains are expected from junction optimization and selective co-catalysts.