Beyond ray optics absorption of light in CsPbBr₃ perovskite nanowire arrays studied experimentally and with wave optics modelling

We study experimentally and with wave optics modelling the absorption of light in CsPbBr ₃ perovskite nanowire arrays fabricated into periodic pores of an anodized aluminum oxide matrix, for nanowire diameters from 30 to 360 nm. First, we find that all the light that couples into the array can be absorbed by the nanowires at sufficient nanowire length. This behavior is in strong contrast to the expectation from a ray-optics description of light where, for normally incident light, only the rays that hit the cross-section of the nanowires can be absorbed. In that case, the absorption in the sample would be limited to the area fill factor of nanowires in the hexagonal array, which ranges from 13% to 58% for the samples that we study. Second, we find that the absorption saturates already at a nanowire length of 1000-2000 nm, making these perovskite nanowires promising for absorption-based applications such as solar cells and photodetectors. The absorption shows a strong diameter dependence, but for all diameters the transmission is less than 24% already at a nanowire length of 500 nm. For some diameters, the absorption exceeds that of a calculated thin film with 100% coverage. Our analysis indicates that the strong absorption in these nanowires originates from light-trapping induced by the out-of-plane disorder due to random axial position of each nanowire within its pore in the matrix.