Advancing Silicon-Based Photoelectrodes toward Practical Artificial Photosynthesis

ConspectusArtificial photosynthesis is a sustainable technology to convert solar energy into storable chemicals or fuels, which potentially paves the way for coping with the greenhouse gas emission and growing energy demand. Semiconductor photoelectrodes are vital constituents in artificial photosynthesis systems. Among them, silicon (Si) is extensively employed due to its earth abundance, suitable band gap, and low cost. However, Si-based photoelectrodes suffer from insufficient photovoltage, serious interfacial charge recombination, sluggish reaction kinetics, and low stability in electrolyte.Numerous strategies have been proposed to address these challenges. Si-based photoelectrodes comprising buried junctions, passivation/protective layers, and electrocatalysts as the main components have thus been developed. Unfortunately, several issues persist: (1) the additional defect state at the newly formed interfaces; (2) the contradiction between adequate passivation/protection and sufficient charge transfer; (3) the parasitic light absorption of electrocatalyst; and (4) the lack of compatibility with different application scenarios. Therefore, it is paramount to meticulously balance the functionalities and potential detrimental impacts brought about by different components to govern the photoelectrode design.This Account describes recent advances in Si-based photoelectrode design toward high efficiency, excellent stability, and practical applications in artificial photosynthesis. The charge utilization efficiency is determined by charge generation, separation, and transport processes, which could be simultaneously improved by the careful designs of homo/heterojunctions, bifacial passivation, and illumination-reaction decoupling. The above methods have been proven to be efficient, as through them we have obtained benchmark Si-based photocathodes. Apart from efficiency, stability is another key factor. We explored the charge transport and deactivation mechanisms of the traditional metal oxide protective layers, varied the coverage and surface energy of organic protective layers to control the flux of photogenerated charge, and discussed the merits of metal protective layers in illumination-reaction decoupled photoelectrodes. Moreover, the versatility of our Si-based photoelectrodes in unbiased water splitting with separated and integrated configurations, as well as in CO2 reduction with H-cell and flow cell, is proposed. Finally, the key challenges and future perspectives toward practical artificial synthesis are suggested. Our collective work outlines strategies for efficient and stable Si-based photoelectrodes. These general methods may be applicable to other semiconductor photoelectrodes for artificial synthesis.