Anti-Reflective Dielectric Nanostructures for Solar Cells Analyzed from a Helicity Preservation Perspective
Creators: name: Slivina, Evgeniia
Publisher: name: Karlsruher Institut für Technologie (KIT)
Year: 2021

description: 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.