Harnessing hierarchical architectures to trap light for efficient photoelectrochemical cells

Photoelectrochemical (PEC) cells including dye-sensitized solar cells and photoelectrochemical water splitting devices are a promising approach to effectively harvesting and storing solar energy, but the contradictory requirements on the device design with sustainable materials between charge collection and light absorption hinder their development. An ideal material possessing simultaneously wide and strong photo-absorption and high electrical conductivity is hard to come by, but these features can be built into hierarchical structures by rationally combining materials with multiple dimensions and multiple functions, thereby solving the contradiction between charge collection and light absorption. In this article, we review recent progress in the design and utilization of hierarchical architectures here, specifically referring to structures with a combination of 0–3D entities, for PEC cells. Inspired by the natural photosynthesis system, which divides the overall task among chloroplasts, foliages, trunks and branches, we split the hierarchical structure into three functional parts, a surface coating layer for anti-reflection and charge transfer, a light absorbing block and a support structure good in anti-transmittance and charge transport. In this way, the above-mentioned contradiction in solar energy conversion can be solved by decoupling the charge collection from optical propagation. The design ideas and the underlying mechanisms of each part of the hierarchical structure are discussed in detail. In general, appropriately designing each individual unit to control light absorption and charge transport is the main strategy to trap light and to direct charge carriers. Finally, the prospects and future directions are presented.