Numerical predictions of the improvement of solar energy harvesting in inverse opal photonic crystals by tuning the slow-light effect to higher photonic bands

Improvement of solar light harvesting for energy or environmental applications such as photocatalysis can be achieved through the manipulation of light. Inverse opal (IO) photonic crystals have emerged as promising material structures, due to their ability to slow down light and enhance light-matter interactions at specific wavelengths. While previous studies have primarily focused on slow light modes at the lower (i.e., red) and upper (i.e., blue) edges of the first partial photonic band gap (PBG), the potential of higher order, almost flat photonic bands for light conversion has recently been highlighted. Nonetheless, understanding and optimization of this effect require further investigation. In this context, we present a numerical study of anatase TiO2 IOs featuring interpenetrating pores. To this end, we developed a three-dimensional model based on the finite element method (FEM) and benchmarked it against rigorous coupled-wave analysis (RCWA). Excellent agreement was found between the two methods. Light harvesting enhancement was quantified via the ratio of photon fluxes absorbed by the IO structure and a reference homogeneous TiO2 slab of equivalent material volume. By tuning the IO lattice constant (pore diameter), the spectral positions of slow light modes associated with nearly flat bands were controlled. Adjustment of the slow light mode wavelength using a pore diameter of 170 nm enabled tuning to the blue edge of TiO2 electronic band gap, resulting in an enhancement factor equal to 1.62. More notably, we demonstrated that tuning higher photonic flat bands provided even better enhancement with a factor reaching 2.39 for a larger pore diameter of 280 nm. We believe that our FEM-based numerical model could help further understand and optimize slow light effect in IO, opening new perspectives for the exploitation of higher photonic bands in solar light harvesting applications.