Abstract
The successful development of multijunction photovoltaic devices with four or more subcells has placed additional importance on the design of high-quality broadband antireflection coatings. Antireflective nanostructures have shown promise for reducing reflection loss compared to the best thin-film interference coatings. However, material constraints make nanostructures difficult to integrate without introducing additional absorption or electrical losses. In this work, we compare the performance of various nanostructure configurations with that of an optimized multilayer antireflection coating. Transmission into a four-junction solar cell is computed for each antireflective design, and the corresponding cell efficiency is calculated. We find that the best performance is achieved with a hybrid configuration that combines nanostructures with a multilayer thin-film optical coating. This approach increases transmitted power into the top subcell by 1.3% over an optimal thin-film coating, corresponding to an increase of approximately 0.8% in the modeled cell efficiency.
Highlights
Multi-junction solar cells have achieved the highest efficiencies of all photovoltaic technologies
Since the underlying layers are typically composed of III-V materials with a refractive index similar to that of InGaP2, we find that there is not a significant difference in the design of an antireflection coating (ARC) that is optimized using a more complex optical model
We explore the design rules for the integration of broadband antireflective nanostructures and optical coatings with multijunction photovoltaics
Summary
Multi-junction solar cells have achieved the highest efficiencies of all photovoltaic technologies. World record efficiencies of 44.7% at 297-suns and 38.8% at 1sun have been reported using designs with four and five junctions respectively [3,8]. These four-junction (4-J) and five-junction (5-J) designs absorb light across a very broad wavelength range (~300-1800 nm) and have strict current matching requirements for each of the subcells [1,2,3,4]. The broadband performance of the antireflection coating (ARC) is more critical for these designs than for today’s best 3-J devices, which incorporate either a germanium bottom junction that is oversupplied with photons and can better tolerate high infrared reflectivity or a bottom junction with a bandgap around 1.0 eV and a narrower absorption range (~300-1250 nm) [9,10,11,12]
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