Extreme light absorption in a necking-free monolayer of resonant-size nanoparticles for photoelectrochemical cells
Semiconductor photoelectrodes for water oxidation that absorb visible light usually have poor electronic transport properties and small optical absorption coefficients near their absorption edge. Therefore, innovative designs that lead to significant optical absorption in relatively thin layers of these compounds are highly desirable. Here, using full-field electromagnetic optical simulations, we demonstrate that a monolayer of resonant-size BiVO4 spheres can provide enhancement up to a factor of two in solar light absorption relative to dense planar layers. In this monolayer, BiVO4 spheres do not need to be interconnected; therefore, such monolayers are flexible and their fabrication process does not require the complicated necking steps to establish electrical contact among the nanoparticles. These resonant-size spheres support Mie resonance modes that efficiently trap light and hence significantly increase effective optical path length. Under air mass 1.5 global (AM1.5G) irradiation, the maximum achievable photocurrent density (MAPD) in a monolayer of 250 nm diameter BiVO4 spheres reaches 4.9 mA. cm(-2). This is about twofold improvement over the MAPD for a 250 nm thick dense planar layer and well above the 3.8 mA. cm(-2) MAPD for a 1 mu m thick dense planar layer. In addition, it is shown that lower-order resonance modes of the spheres are superior to higher-order modes for broadband optical absorption. The insight provided in this work can also be applied to nitride and oxynitride photoanode materials.