due to its abundance and low-cost, is an attractive compound for photoelectrochemical splitting of water to produce hydrogen. However, one major obstacle preventing hematite from achieving the target efficiencies comprises its significantly smaller minority carrier transport distance relative to its optical absorption depth in the visible part of the optical spectrum. Here, we combine host-guest and Mie resonance concepts to achieve significant optical absorption in extremely thin layers of hematite. We propose and theoretically evaluate transparent particles coated with an extremely thin hematite shell as building blocks for hematite photoanodes. By full-field optical simulations we found out that maximal optical absorption is achieved when the particle supports two to three Mie resonance modes above the hematite optical absorption edge. Optical absorption efficiencies integrated over the air mass 1.5 global (AM1.5G) irradiance spectrum, hh abs i AM1.5 , reach more than 2 mA cm À2 within a 10 nm thick hematite shell of a particle with optimal dimensions and hh abs i AM1.5 comes close to 5 mA cm À2 in a 25 nm thick hematite shell. Furthermore, we evaluate the performance when the particles are part of an array or stacked atop each other. The concept introduced here could be useful for improving optical absorption in semiconductors with extremely short carrier transport distances.

Metallic nanoparticles (NPs) have not been effective in improving the overall performance of the cells with micrometer-thick absorbing layers mainly due to the parasitic optical dissipation in the metal. Here, using both experiment and theory, we demonstrate that aggregates of metallic NPs enhance the light absorption of dye-sensitized solar cells of a few micrometer-thick light absorbing layers. The composite electrode containing the optimal concentration of 5 wt% Au@SiO2 aggregates shows the enhancement of 80% and 52% in external quantum efficiency and photocurrent density, respectively. The superior performance of the aggregates relative to NP is attributed to their larger scattering efficiency using full-wave optical simulations. This is further confirmed by optical spectroscopic measurements showing that a large fraction of the incident light couples into the diffused components because of the presence of these metallic aggregates. The optical absorption enhancement is broadband and it is particularly strong at wavelengths larger than 680 nm where the optical absorption of dye molecules is weak.

A large class of photoelectrodes for water splitting are processed by assembling nanoparticles onto a silicon solar cell substrate. A fundamental question is the optimal size of constituent nanoparticles that maximizes optical absorption in the photoanode. We use electromagnetic optical calculations to study the impact of particle size on optical absorption of Ta 3 N 5 /Si tandem photoanodes. We found that optical absorption efficiency dramatically increases when particle support Mie resonances, independent of particle shape. Subsequently, monolayers of resonant-size particulate and thin films of Ta 3 N 5 on silicon substrate are studied. Ta 3 N 5 limits overall performance of Ta 3 N 5 /Si tandem device in both devices with the particulate one showing better performance. We take into account material charge transport properties and conditions of imperfect surface coverage of Si solar cell with Ta 3 N 5 nanoparticle. Furthermore, we found that an intermediate reflective layer placed between Ta 3 N 5 and Si solar cell improves photocurrent density of Ta 3 N 5 and the tandem device.