Current trend of the average laser power increase follows Moore's law. The average power of ultra-short lasers in 2000 was 1 W and now is 1 kW following trend of doubling per year: 2^(20years/2) = 1024. This trend can be harnessed for large area patterning. Here we show application of direct laser writing for processing of surface of solar cells.
Light trapping photonic crystal (PhC) patterns on the surface of Si solar cells provides a novel opportunity to approach the theoretical efficiency limit of 32.3% for light-to-electrical power conversion with a single junction cell. This is beyond the efficiency limit implied by the Lambertian limit of ray trapping ~29%. The interference and slow light effects are harnessed for collecting light even at the long wavelengths near the Si band-gap. We compare two different methods for surface patterning, that can be extended to large area surface patterning: 1) laser direct write and 2) step-&-repeat 5-times reduction projection lithography.
Photonic crystal patterns for light trapping in Si solar cells are developed via different process flows: 1) step-and-repeat projection lithography with lift-off or 2) laser ablation and subsequent etching dry or/and wet. Both methods are amenable for large area (2×2 cm2) fabrication and can be used to break the ray-optics light trapping limit. This is required to surpass the record high efficiency ∼ 26% of solar-to-electrical power conversion of Si solar cells and approach the theoretical limit of ∼ 30%. Also, standard electron beam lithography (EBL) was used to define Si3N4 or Cr masks for wet KOH etching on silicon-on-insulator (SOI) and Si wafers. Direct laser writing of the etch mask by ablation (10 nJ, 515 nm, 230 fs pulses) has an advantage due to its scalability. The large area patterning is important for industrial application of direct laser writing of light trapping patterns in solar cells and absorbers/emitters for the IR spectral range.
Light harvesting using photonic crystal (PhC) surface patterns provides an opportunity to surpass the ray-optics defined light trapping and to approach thermodynamic ShockleyQueisser limit of solar cell efficiency, which for a single junction Si solar cell is ~ 32%. For an industry amenable nano-patterning of Si solar cells, we used laser direct write and stepper lithography based approaches for defining a large area (1 cm2) light trapping PhC patterns on silicon. Nanoholes of ~ 500 nm in diameter were fabricated by direct laser writing in a thin layer of chromium to act as a mask for subsequent reactive plasma etching to fabricate the nanostructures forming a PhC surface over a square centimeter. Surface area fabrication throughput was improved by more than order of magnitude as compared with electron beam lithography required to achieve sub-1 μm resolution.
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