Maximal Visible Light Energy Transfer to Ultrathin Semiconductor Films Enabled by Dispersion Control

Abstract

dielectric and semiconductor [2] nanostructures. In addition, to extend absorption over a broader range of the solar spectrum, multiple semiconducting layers with different absorption bands have been used to create multijunction solar cells. [11,12] Despite such efforts, the high efficiency thin-film semiconductor absorber is still a topic of active research. The classical Yablonovitch limit [13] of maximum achievable absorption, which is based on geometrical optics, does not apply to ultrathin semiconductor absorbers. Attempts have been made to overcome the Yablonovitch limit using wave optics rather than geometrical optics. [14,15] For example, the enhancement of local density of states (LDOS) using optical resonators can exceed the Yablonovitch limit with the upper bound determined by the LDOS sum rule. [16] However, the performance of ultrathin semiconductor absorbers based on the LDOS enhancement can be limited by Ohmic losses and narrow working frequency band in optical resonators.Here, we propose a new dispersion control scheme for maximal light absorption by ultrathin semiconducting absorbers. We analyze the typical configuration of a thin-film absorber deposited on a metal substrate coated with a dielectric spacer layer and find the condition for maximal light energy transfer to the thin absorber over the entire visible spectrum. Specifically, we show that the near perfect absorption of visible light by an ultrathin film is possible when the permittivity of the absorber satisfies the ideal dispersion relation. We explain how the ideal dispersion relation can be achieved with semiconductor bandgap materials by controlling resonances. The inevitable loss, arising only from the absorption by the reflector possessing finite conductivity, can be minimized by controlling the thicknesses of dielectric spacer and absorber, resulting in typically less than 5% loss from the aluminum reflector and less than 1% from the silver reflector. As experimental verification, we fabricated ultrathin lead selenide (PbSe) absorbers on an aluminum reflector with SiO 2 spacers and controlled the dispersion of PbSe permittivity by changing the sputtering condition. We find that dispersion-controlled 9 nm thick PbSe film can reduce the reflectance averaged over the entire visible range to less than 4% and the aluminum absorption to 8% so that the absorption of visible light by an ultrathin PbSe film reaches 88% which is close to the theoretical maximum value of 95%.Increasing light absorption in an ultrathin semiconductor is critical for developing thin-film photovoltaic devices. Here, it is shown that a maximal absorption of visible light is possible through controlling the dispersion of thin-film materials. The ideal dispersion relation is determined for the permittivity of a thin film placed on a reflector with a dielectric spacer, and it is explained how the ideal dispersion relation can be realized for semiconductor materials possessing bandgaps. To experimentally verify dispersion control and maximal absorption, the permitti...

Ultrahigh omnidirectional, broadband, and polarization-independent optical absorption over the visible wavelengths by effective dispersion engineering

“…Moreover, because the Brewster mode mostly exists in transvers magnetic (TM) polarization only, the absorption of transverse electric (TE) polarization is much lower than that of TM polarization. Some research groups have addressed these problems by using a spacer below the highly absorbing layer 23–25 , metal-semiconductor-metal cavity structure 26 , metamaterials 27–29 , lossy magnetic mirrors 17 , and cavity tuning for aesthetic purpose 30,31 . However, so far, it remains a challenge to make incident-angle and polarization-independent optical absorbers that cover a wide range of wavelengths (over the whole visible range) without using complex fabrication procedures.…”

Angle-Insensitive Ultrathin Broadband Visible Absorber Based on Dielectric–Semiconductor–Lossy Metal Film Stacks

“…For example, the maximum absorptivity of ~82.57% appears at 550 nm, and the average absorption remains above around 65% over the visible regime for a typical design of 10 nm Ge/100 nm silver (Ag), as presented in Figure 1b(i). To further enhance the optical absorption, one widely used design is the semiconductor-dielectric-metallic mirror structure, in which the top semiconductor film is required to exhibit an unusual type of material dispersion [2,32,34]. Although an effective medium design using two absorbing films [34] and controlling the semiconductor's polycrystallinity [32] was proposed to match the ideal optical complex refractive index, the near-unity absorption band still cannot cover the entire visible range, and the fabrication costs were also increased.…”

“…Recently, it was found that strong interference effects could be excited in an ultrathin absorbing semiconductor film coated on a metallic substrate, leading to strong absorption with a broad bandwidth at the resonant wavelength [30]. Although many broadband absorber designs have been proposed by utilizing ultrathin semiconductor films, the near-unity absorption characteristic cannot cover the whole visible range [2,31,32]. Therefore, how to achieve more efficient absorption over a wider spectral range with simpler planar thin-film structures still remains a significant challenge.…”

Optical properties of bismuth nanostructures towards the ultrathin film regime

“…Tailoring the film nanostructure [5,22] enables tuning its effective optical dispersion in a broad range. This is useful to meet the requirements for an optimal light harvesting [23] in Bi nanostructured materials, in particular for photocatalytic systems based on near-percolation Bi nanostructures [24][25].…”