Insights into Backscattering Suppression in Solar Cells from the Helicity-Preservation Point of View

Abstract

We show that the anti-reflection performance of nano-particle arrays on top of solar cell stacks is related to two conditions: a high enough degree of discrete rotational symmetry of the array and the ability of the system to suppress cross-talk between the two handednesses (helicities) of the electromagnetic field upon light-matter interaction.For particle-lattice systems with high enough degree of discrete rotational symmetry 2π/n for n ≥ 3), our numerical studies link the suppression of backscattering to the ability of the system to avoid the mixing between the two helicity components of the incident field. In an exemplary design, we optimize an array of TiO 2 disks placed on top of a flat heterojunction solar cell stack and obtain a three-fold reduction of the current 1 arXiv:1902.07546v3 [physics.optics] 5 Jun 2019 loss due to reflection with respect to an optimized flat reference. We numerically analyze the helicity preservation properties of the system, and also show that a hexagonal array lattice, featuring a higher degree of discrete rotational symmetry, can improve over the anti-reflection performance of a square lattice. Importantly, the disks are introduced in an electrically decoupled manner such that the passivation and electric properties of the device are not disturbed. IntroductionMinimal reflection is an obvious design goal in solar cell technology which attracts much research attention. Different approaches to anti-reflection (AR) range from chemical texturing of the silicon waver 1,2 to sophisticated AR coatings 3-7 and plasmonic structures 8-10 .Recently, the use of arrays of dielectric nano-structures is being investigated as a possible avenue to improve AR properties of solar cells [11][12][13][14][15][16] . Due to the low profiles of the patterning nanostructures, this approach is suitable for ultra thin film solar cells. Clearly, understanding the underlying physical principles behind backscattering minimization is relevant for the AR aspect of solar cell design.Much of the nanophotonics research on backscattering minimization stems from the 1983 article of Kerker et al. 17 . This early work showed that a sphere whose relative electric permittivity and magnetic permeability are equal exhibits zero backscattering under plane wave illumination, i.e., there is no energy in the specular back reflection direction, independently of the polarization of the illuminating plane wave. Since then, the theoretical and experimental works on zero backscattering have been numerous, see e.g. Refs. 18-26. On the theoretical side, the relationship between electromagnetic duality symmetry and zero backscattering 18,20,27 has provided a new point of view on Kerker's result by connecting the backscattering suppression to a fundamental symmetry in electromagnetism. A system is symmetric under duality transformations 29 if and only if its electric and magnetic responses to incident radiation are equivalent. This equivalence connects directly to Kerker's = µ

Effects of symmetry-breaking on electromagnetic backscattering

“…Finally, we note that for systems made out of the same prisms, the hexagonal lattice results in smaller backscattering than the square lattice. This behavior has been reported for fully dielectric systems 18 .

Figure 5 ( a ) Backscattering ratio for a ellipsoid of permittivity for a varying axial ratio b / a (legend) as a function of the permeability.

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The annual energy yield of mono- and bifacial silicon heterojunction solar modules with high-index dielectric nanodisk arrays as anti-reflective and light trapping structures

“…The four different architectures studied in this contribution are schematically depicted in In the case of flat reference c-Si HJT solar cell stacks, the front hydrogenated amorphous silicon (a-Si:H) (passivation intrinsic and n + doped, the thickness can be found in 25 ) and conducting ITO (75 nm) layers were considered. For the rear side of the HJT solar cell stack, the a-Si:H (passivation intrinsic and p + doped) layer was slightly thicker than the a-Si:H layer on the front, while the ITO layer was thinner than its counterpart on the front side.…”

“…This design was based on the previous work on helicity preserving TiO 2 nanodisk array for the front interface of a c-Si HJT solar cell, where efficient and broadband backscattering suppression was achieved due to the ability of the system to suppress cross-talk between opposite handednesses of the electromagnetic field upon light-matter interaction. 25 The requirement for a system to be helicity preserving is to possess a high enough degree of rotational symmetry (n ≥ 3) along the illumination direction. For normal light incidence, for which the optimization of AR nanodisk array was performed, the illumination direction is along the symmetry axis of an individual nanodisk, which essentially means that n → ∞ in this case.…”

Antireflective Huygens’ Metasurface with Correlated Disorder Made from High-Index Disks Implemented into Silicon Heterojunction Solar Cells

“…19 As we will show, the latter can be achieved also in a disordered arrangement if spatial correlations are tuned appropriately. 26,27 The design of the metasurface proposed in this Article was inspired by our previous work 28 on a similar but periodic arrangement of disks, whose antireflective properties were designed and analyzed from the perspective of the helicity of the scattered light. Preserving the helicity of the incident light, plus a sufficiently high degree of rotational symmetry along the illumination axis, are sufficient conditions to ensure zero specular reflection from a general system.…”

“…While this approach nulls the specular reflection at a single or at most at a few discrete wavelengths, it turns out that the effect is all together rather broadband and suitable to the relevant spectral range for a solar cell. 28 In a disordered system, the concept of discrete rotational symmetry is not exactly applicable. However, if the pattern is tailored appropriately, one can speak of an effective continuous rotational symmetry (n → ∞) and, consequently, establish a one-to-one relationship in the long wavelength limit between the helicity preservation and the antireflective properties of the coated solar cell.…”