Semiconductor Thin Film Based Metasurfaces and Metamaterials for Photovoltaic and Photoelectrochemical Water Splitting Applications

Abstract

throughput and large-scale compatibility. The photoactive material is generally composed of single or multiple semiconductor layers responsible for harvesting solar energy. This harvested solar irradiation can be used to generate electricity using photovoltaic (PV) devices, or it can be converted to chemical energy in the form of hydrogen which is the case for photoelectrochemical water splitting (PEC-WS). In both PV and PEC-WS applications, the ultimate goal is to establish an efficient photon capturing and electron collecting scheme to generate a huge number of photocarriers (including electron-hole pairs) with rational carrier dynamics (efficient charge generation, separation, and collection). This means that the device should be optically thick enough to generate high carrier's density and electrically thin enough to collect photogenerated carrier before they recombine. When light impinges a semiconductor surface, a part of the light is reflected back due to the mismatch between the air and the semiconductor layer refractive index. The rest will propagate within the bulk until it is fully absorbed by the semiconductor slab. The optical penetration depth (LPD) is defined as the depth at which the incident power falls to 1/e (≈37%) of its input value. This parameter differs from one semiconductor to another and it depends on the extinction coefficient (κ) of the 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 bulkysemiconductor-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.

Highly Efficient Semiconductor-Based Metasurface for Photoelectrochemical Water Splitting: Broadband Light Perfect Absorption with Dimensions Smaller than the Diffusion Length

“…The concept of metamaterials and metasurfaces has emerged as a promising route to engineer the light-matter interaction in nanostructure geometries [1][2][3][4][5][6][7]. Light confinement in subwavelength geometries has turned into one of the most intensively explored areas in recent years.…”

Hot‐carrier solar cells and improved types using wide‐bandgap energy‐selective contacts

“…As is clear from Equations 4, 12, and 14, d is required to be as thin as possible for reduction in N SRH , U SRH , and U th . 19,58 For this purpose, light-trapping structures have been investigated and demonstrated to secure high absorptivity with a thin d. 59,60 However, it would be difficult to achieve notable light-trapping effects over the whole of the absorption range. Considering these present statuses, d was set to be 1 μm.…”

Strong Light–Matter Interactions in Au Plasmonic Nanoantennas Coupled with Prussian Blue Catalyst on BiVO4 for Photoelectrochemical Water Splitting

“…Unlike semiconductors, which only harvest photon energies above their band gap, nanometals exhibit resonant light absorption in the whole electromagnetic spectrum, through the excitation of localized surface plasmon resonances (LSPRs) and inter‐band transitions. Thus, plasmonic photoelectrochemical water splitting (PEC‐WS) offers a promising approach to convert sunlight into chemical energy, which has recently received intense research . Plasmonic nanometals can contribute to the semiconductor activity enhancement through two main pathways; i) radiative (scattering, optical near field coupling) and ii) non‐radiative energy transfer (hot electron injection, plasmon resonant energy transfer) .…”

“…Although one of the most successful semiconductors—for plasmonic PEC‐WS—is titanium dioxide (TiO 2 ), mainly owing to its chemical stability, earth abundance, and cost effectiveness; however, it suffers from a poor absorption response that only covers the ultraviolet (UV) portion of the solar spectrum. Therefore, in recent years, extensive attempts have been made for the design and realization of plasmonic coupled low band gap metal oxides for driving water oxidation and reduction reactions . By decorating plasmonic deep sub‐wavelength nanoparticles on a semiconductor, near field effects and hot electron injection can simultaneously contribute to the overall activity of the cell.…”