Perfect light absorption at subwavelength scale is promising for various applications in terms of cost, flexibility, weight and electrical performance. Especially, ultrathin form of them have a number of applications that is related to photon-to-carrier conversion processes. They can provide near-unity internal quantum efficiency since they absorbs most of incident light and the diffusion length of generated charge carriers exceeds the film thickness, minimizing the recombination loss. However, achieving incident angle-/polarization-independent perfect absorption within desired wavelength range without any lithographic procedures remains a challenge. Recently, it is reported that lossy semiconductors on metal substrates show omnidirectional absorption at the resonance wavelength. By in depth analysis, we reveal that radiative and non-radiative modes coexist, and a radiative mode can be coupled only when the semiconductor layer is ultrathin, while non-radiative mode exist independent on the semiconductor layer thickness. However, the resonance wavelength range is narrow due to the high refractive index contrast (impedance mismatch) between the lossy semiconductor and air.
In this work, we experimentally demonstrate highly omnidirectional, broadband and lithography-free superabsorber by introducing impedance matching layers on top of the lossy semiconductor/metal configurations. This layer not only improve absorption, but also act as a passivation layer to increase electrical performances. Unlike conventional anti-reflection (AR) coatings with quarter-wave-thick, our superabsorbers are much thinner (~$\lambda$/7.4). The superabsorbers show day-integrated solar energy absorption up to 96%, which is 32% enhanced when compared to the lossy semiconductor/metal absorbers. Moreover, this method can be generally applied to other highly lossy semiconductors including emerging 2D materials.