Silicon, an earth-abundant material with mature technology, low-cost manufacturing, and high stability, holds promise to enable the sustainable exploitation of solar energy resources currently under utilized at the world-scale. Apart from traditional interest in the photovoltaic industry, recent years have seen increasingly large activity in the study of Si-based photo-electro-chemical (PEC) cells for water splitting and CO2 reduction. This research established an exciting area with the potential to address the present environmental crisis originating from unregulated CO2 emission levels. In this review, the recent work on Si-based PEC devices for large scale production of hydrogen via water splitting, and carbon-negative technologies for the solar-driven reduction of CO2 into chemical fuels of industrial interest are summarized. Bias-assisted and bias-free PEC architectures are discussed, highlighting the motivations, challenges, functional mechanisms, and commenting on the perspectives related to this field of research both as a science and engineering.