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.
Highlights
Multilayer films without any micro/nanoscale patterns have been widely used due to their translational invariance and the resultant resonance excited for a particular incident angle and wavelength
We experimentally and theoretically characterized an 18 nm Ge/Ag absorber and a 22 nm hematite/Ag absorber, to demonstrate that the high refractive index is the physical origin of the angle insensitivity for absorbers consisting of an ultrathin planar semiconductor layer and an opaque metallic substrate
To determine whether these angular optical properties are caused by the ultrathin absorber thickness, we studied the absorber configuration with the Ag substrate replaced by a perfect electric conductor (PC) as shown in the inset of Fig. 2e
Summary
Two challenging problems still remain for optical absorbers consisting of an ultrathin planar semiconductor film on top of an opaque metallic substrate. We experimentally and theoretically characterized an 18 nm Ge/Ag absorber (representative of small band gap semiconductors for photovoltaic applications) and a 22 nm hematite/Ag absorber (representative of large band gap semiconductors for photoelectrochemical applications), to demonstrate that the high refractive index is the physical origin of the angle insensitivity for absorbers consisting of an ultrathin planar semiconductor layer and an opaque metallic substrate. These absorbers show omnidirectional resonance for TE polarization due to the high refractive index difference between the semiconductor and air. The photovoltaic and photoelectrochemical cells work as spectrum splitters to utilize a broad range of the solar energy without an external spectrum splitting element
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