Light trapping using correlated disordered media for planarized solar cells

The history of solar cells is marked by a journey of innovation and discovery that has paved the way for a sustainable energy future. Since their inception, solar cells, also known as photovoltaic cells, have witnessed significant advancements in materials, technology, and efficiency. As the world faces critical challenges, including climate change, diminishing fossil fuel reserves, and the ever-increasing demand for clean energy, solar cells have emerged as a beacon of hope. The efficiency of solar cells plays a pivotal role in their significance. The pursuit of a high solar cell efficiency is of particular significance due to the increasingly limited availability of land for solar cell installations in the near future. For this purpose, integrating dielectric nanostructures into solar cells for the purpose of light management have recently emerged as a highly favoured approach for reducing the reflection loss from the front interface of the solar cell and increasing the absorption of the sunlight. This choice is more favourable when compared to the traditional approaches like direct texturing of the light-absorbing layer, as the dielectric nanostructures, when integrated into a standard planar solar cell, successfully overcome the detrimental impact on electrical performance due to texturing while also increasing the light absorption in comparison to a planar solar cell.
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In this thesis, we investigate the utilization of correlated disorder in the design of dielectric nanostructures. The resulting structures are used for planar crystalline silicon solar cells. The approach used in the thesis involves designing a spatially inhomogeneous distribution of material. It builds upon an existing notion of a smoothly varying refractive index coating being an ideal broadband anti-reflection structure for solar cells. However, due to light reciprocity, such planarized anti-reflection coatings also increase the portion of outcoupled light not absorbed in the solar cell. We discuss a graded-index based design strategy that includes lateral features for light-trapping in the solar cell. By introducing index variation along the lateral direction, the incoupled light can be scattered inside the absorbing medium of the solar cell into oblique angles, thereby trapping the light in the system while also serving the purpose of an anti-reflection coating. Our final structures contain lateral features tailored towards a correlated, hyperuniform disorder while individual scattering elements are designed seeking inspiration from transformation optics. The resulting metasurface designed this way, was experimentally realized on macroscopically large wafers using self-assembly nanofabrication and atomic layer deposition techniques by experimental partners from the Martin-Luther University Halle-Wittemberg. We numerically and experimentally demonstrate that our disordered graded-index metasurface can significantly improve the light-trapping and antireflective performance of a solar cell.