Anti-Reflective Dielectric Nanostructures for Solar Cells Analyzed from a Helicity Preservation Perspective

Continuing increase of carbon dioxide (CO$_2$) emissions and subsequent growth of the global average temperature pushes us towards a faster transition from fossil fuels to renewable energy sources. In this respect, photovoltaics (PV) may play a decisive role in achieving net zero CO$_2$ emissions within the desired time frame. While new PV technologies have been actively researched over recent years, silicon (Si) PV continues to dominate the world market. Despite the maturity of Si PV, there is still room for improvement. In particular, to keep up with the estimates for the global installed PV capacity for upcoming decades, one has to consider how much energy is actually used for manufacturing Si wafers. Thus, it is feasible to consider a transition to thinner Si absorbers. However, such transition requires adjustments of the industrially accepted processes used to negate optical losses since the standard approach employing random pyramidal textures is no longer feasible for rather thin wafers. Thus, alternative strategies have to be established. For this purpose, nanophotonic structures are of interest. In particular, dielectric scatterers supporting Mie resonances attracted attention from the research community over the last few years. In this thesis, we perform a holistic study of periodic and disordered anti-reflective (AR) dielectric nanostructures applied to crystalline silicon (c-Si) heterojunction (HJT) solar cells. We optimize the optical performance of these systems and show that the AR properties of the nanostructure arrays on top of solar cell stacks are related to two requirements: a sufficiently high degree of discrete rotational symmetry of an array and the ability to preserve helicity of the incident illumination. For a periodic system, the first condition can be readily met. The second condition generally requires the system to be made from materials with an equal electric permittivity and magnetic permeability. Since this is unfeasible with naturally available materials, this condition has to be relaxed. Indeed, similar effects can be achieved if only the electric and magnetic response from the photonic nanostructure is balanced. This balance is accomplished by tuning the geometrical parameters of scatterers made from high index materials. For a disordered system, the helicity preservation condition can be reduced similarly to a periodic system. However, in such a system, the first condition is not exactly applicable. Luckily, the disorder can be tailored such that it becomes stealthy hyperuniform, and large-scale density fluctuations are suppressed. Such tailored disordered patterns are fully isotropic, thus possessing effective continuous rotational symmetry. Therefore, the AR properties of these systems are also related to the requirements stated above. Furthermore, we fabricate solar cells coated with periodic and tailored disordered nanodisks based on the optimal designs. We characterize the optical and electrical properties of the samples and observe the improvement of the AR properties and subsequent positive influence on the short-circuit current density. A complementary analysis of the annual energy yield of the solar modules employing solar cells with nanodisk coatings shows that our designs can potentially be integrated into the module with their positive effect preserved

Effects of symmetry-breaking on electromagnetic backscattering

International audienceAbstract Systems with a discrete rotational symmetry $$2\pi /n$$ 2 π / n where $$n\ge 3$$ n ≥ 3 that also have electromagnetic duality symmetry exhibit zero backscattering. The impact of breaking one of the two symmetries on the emerging backscattering has not yet been systematically studied. Here, we investigate the effect that perturbatively breaking each of the two symmetries has on the backscattering off individual objects and 2D arrays. We find that the backscattering off electromagnetically-small prisms increases with the parameters that determine the symmetry breaking, and that the increase of the backscattering due to the progressive breaking of one of the symmetries can be related to the other symmetry. Further exploration of the interplay between the two symmetries reveals that, in systems lacking enough rotational symmetry, the backscattering can be almost-entirely suppressed for a given linear polarization by deliberately breaking the duality symmetry. This duality breaking can be interpreted as an effective increase of the electromagnetic degree of rotational symmetry for that linear polarization

Tailored Light Scattering through Hyperuniform Disorder in Self-Organized Arrays of High-Index Nanodisks

Arrays of nanoparticles exploited in light scattering applications commonly only feature either a periodic or a rather random arrangement of its constituents. For the periodic case, light scattering is mostly governed by the strong spatial correlations of the arrangement, expressed by the structure factor. For the random case, structural correlations cancel each other out and light scattering is mostly governed by the scattering properties of the individual scatterer, expressed by the form factor. In contrast to these extreme cases, it is shown here that hyperuniform disorder in self-organized large-area arrays of high refractive index nanodisks enables both structure and form factor to impact the resulting scattering pattern, offering novel means to tailor light scattering. The scattering response from the authors’ nearly hyperuniform interfaces can be exploited in a large variety of applications and constitutes a novel class of advanced optical materials

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

While various nanophotonic structures applicable to relatively thin crystalline silicon-based solar cells were proposed to ensure effective light in-coupling and light trapping in the absorber, it is of great importance to evaluate their performance on the solar module level under realistic irradiation conditions. Here, we analyze the annual energy yield of relatively thin heterojunction (HJT) solar module architectures when optimized anti-reflective and light trapping titanium dioxide (TiO$_2$) nanodisk square arrays are applied on the front and rear cell interfaces. Our numerical study shows that upon reducing crystalline silicon (c-Si) wafer thickness, the relative increase of the annual energy yield can go up to 11.0 %$_\text{rel}$ and 43.0 %$_\text{rel}$ for mono- and bifacial solar modules, respectively, when compared to the reference modules with flat optimized anti-reflective coatings of HJT solar cells.Comment: 24 pages, 7 figure

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

A large variety of different strategies has been proposed as alternatives to random textures to improve light coupling into solar cells. While the understanding of dedicated nanophotonic systems deepens continuously, only a few of the proposed designs are industrially accepted due to a lack of scalability. In this Article, a tailored disordered arrangement of high-index dielectric submicron-sized titanium dioxide (TiO$_{2}$) disks is experimentally exploited as an antireflective Huygens’ metasurface for standard heterojunction silicon solar cells. The disordered array is fabricated using a scalable bottom-up technique based on colloidal self-assembly that is applicable virtually irrespective of material or surface morphology of the device. We observe a broadband reduction of reflectance resulting in a relative improvement of a short-circuit current by 5.1% compared to a reference cell with an optimized flat antireflective indium tin oxide (ITO) layer. A theoretical model based on Born’s first approximation is proposed that links the current increase in the arrangement of disks expressed in terms of the structure factor S(q) of the disk array. Additionally, we discuss the optical performance of the metasurface within the framework of helicity preservation, which can be achieved at specific wavelengths for an isolated disk for illumination along the symmetry axis by tuning its dimensions. By comparison to a simulated periodic metasurface, we show that this framework is applicable in the case of the structure factor approaching zero and the disks’ arrangement becoming stealthy hyperuniform

Effects of symmetry-breaking on electromagnetic backscattering

International audienceAbstract Systems with a discrete rotational symmetry $$2\pi /n$$ 2 π / n where $$n\ge 3$$ n ≥ 3 that also have electromagnetic duality symmetry exhibit zero backscattering. The impact of breaking one of the two symmetries on the emerging backscattering has not yet been systematically studied. Here, we investigate the effect that perturbatively breaking each of the two symmetries has on the backscattering off individual objects and 2D arrays. We find that the backscattering off electromagnetically-small prisms increases with the parameters that determine the symmetry breaking, and that the increase of the backscattering due to the progressive breaking of one of the symmetries can be related to the other symmetry. Further exploration of the interplay between the two symmetries reveals that, in systems lacking enough rotational symmetry, the backscattering can be almost-entirely suppressed for a given linear polarization by deliberately breaking the duality symmetry. This duality breaking can be interpreted as an effective increase of the electromagnetic degree of rotational symmetry for that linear polarization