Semiconductor thin film based metasurfaces and metamaterials for photovoltaic and photoelectrochemical water splitting applications

In both photovoltaic (PV) and photoelectrochemical water splitting (PEC‐WS) solar conversion devices, the ultimate aim is to design highly efficient, low cost, and large‐scale compatible cells. To achieve this goal, the main step is the efficient coupling of light into active layer. This can be obtained in bulky semiconductor‐based designs where the active layer thickness is larger than light penetration depth. However, most low‐bandgap semiconductors have a carrier diffusion length much smaller than the light penetration depth. Thus, photogenerated electron–hole pairs will recombine within the semiconductor bulk. Therefore, an efficient design should fully harvest light in dimensions in the order of the carriers' diffusion length to maximize their collection probability. For this aim, in recent years, many studies based on metasurfaces and metamaterials were conducted to obtain broadband and near‐unity light absorption in subwavelength ultrathin semiconductor thicknesses. This review summarizes these strategies in five main categories: light trapping based on i) strong interference in planar multilayer cavities, ii) metal nanounits, iii) dielectric units, iv) designed semiconductor units, and v) trapping scaffolds. The review highlights recent studies in which an ultrathin active layer has been coupled to the above‐mentioned trapping schemes to maximize the cell optical performance.