New Insight into the Angle Insensitivity of Ultrathin Planar Optical Absorbers for Broadband Solar Energy Harvesting

Abstract

Two challenging problems still remain for optical absorbers consisting of an ultrathin planar semiconductor film on top of an opaque metallic substrate. One is the angle-insensitive mechanism and the other is the system design needed for broadband solar energy harvesting. Here, first we theoretically demonstrates that the high refractive index, instead of the ultrathin feature as reported in previous studies, is the physical origin of the angle insensitivity for ultrathin planar optical absorbers. They exhibit omnidirectional resonance for TE polarization due to the high complex refractive index difference between the semiconductor and the air, while for TM polarization the angle insensitivity persists up to an incident angle related to the semiconductor refractive index. These findings were validated by fabricating and characterizing an 18 nm Ge/Ag absorber sample (representative of small band gap semiconductors for photovoltaic applications) and a 22 nm hematite/Ag sample (representative of large band gap semiconductors for photoelectrochemical applications). Then, we took advantage of angle insensitivity and designed a spectrum splitting configuration for broadband solar energy harvesting. The cascaded solar cell and unassisted solar water splitting systems have photovoltaic and photoelectrochemical cells that are also spectrum splitters, so an external spectrum splitting element is not needed.

Highly Efficient Semiconductor-Based Metasurface for Photoelectrochemical Water Splitting: Broadband Light Perfect Absorption with Dimensions Smaller than the Diffusion Length

“…As shown in Fig. 1c, this matching is obtained in a range of 600-700 nm for Ge (in agreement with previous studies [85][86][87]), while the matching is satisfied in wavelength values below 500 nm for PVSK and Fe 2 O 3 [82]. Increasing D S to 25 nm, Ge loses its matching, while Fe 2 O 3 absorption upper edge experiences a red shift (see Fig.…”

“…The strong interface effects, induced by a bottom mirror, can provide light nearly perfect absorption in this design. In this configuration, the right choice of semiconductor geometries can significantly enhance the light-matter interaction [82][83][84][85][86][87]. However, this configuration can enhance light absorption to some extent.…”

Semiconductor Thin Film Based Metasurfaces and Metamaterials for Photovoltaic and Photoelectrochemical Water Splitting Applications

“…Later, a proof‐of‐concept device has been made to reveal the potential of ultrathin transition metal dichalcogenides (TMDs) for energy conversion applications . A 22 nm thick Fe 2 O 3 on Ag mirror has also been shown to have strong light absorption at λ < 450 nm . Similar optical responses have been realized for another type of metals and semiconductors .…”

Strong Interference in Planar, Multilayer Perfect Absorbers: Achieving High-Operational Performances in Visible and Near-Infrared Regimes

“…This strong, light-matter interaction can lead to exceeding the Yablonovitch limit [48]. Later studies demonstrated many different MS configurations that achieve stronger and broader light absorption [49]- [51]. This is of great interest in photoelectronic applications, which confine light in dimensions much smaller than the carrier's diffusion length.…”