Multi-resonant light trapping in ultra-thin solar cells (Conference Presentation)

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Efficiency of state-of-the-art single-junction solar cells is approaching the Shockley-Queisser limit (c-Si, GaAs). In contrast, the thickness of state-of-the-art solar cells is far from its theoretical limits and could be reduced by more than one order of magnitude with efficient light-trapping. In this talk, I first present a benchmark of recent advances of ultra-thin solar cells (c-Si, CIGS, GaAs), using short-circuit current as a function of absorber thickness. I show that current state-of-the-art solar cells operate close to single-pass absorption and I highlight different light-trapping strategies proposed in the literature to approach the Lambertian limit. I then introduce our strategy for efficient light-trapping based on multi-resonant absorption. This approach overcomes the 4n2 limit, making use of coherent scattering of a discrete number of resonant modes. In the second part, I apply these concepts to CIGS and GaAs solar cells. The goal is to reduce the thickness of the semiconductor absorber by one order of magnitude while preserving the short-circuit current. This study is not only pertinent from an academic point of view, but is of practical relevance to CIGS manufacturers for reducing material consumption and time deposition and for space power applications, where ultra-thin solar cells based on III-V semiconductors outperform long-term efficiency and power production of thicker cells due to their intrinsically higher radiation tolerance. We propose a simple and scalable light-trapping architecture based on nanostructured TiO2/silver back-mirror fabricated by direct nanoimprint of sol-gel derived films over large surface areas. Electromagnetic simulations predict a short-circuit current of 36.3 mA/cm2 for a 150 nm thick CIGS solar cell, and I present our roadmap for implementing these concepts in an industrial CIGS solar cell fabrication process. Finally, I detail the design and fabrication of a 205 nm thick GaAs solar cell featuring certified efficiency of 19.9%, and I discuss possible improvements to achieve 25% efficiency.