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Nanostructured antireflective coating for gallium arsenide solar cells

Source：pv magazine

Researchers led by a team at Spain's Technical University of Madrid (IES-UPM) have demonstrated a broadband antireflective coating (ARC) for gallium arsenide-based solar cells. The nanostructured coating was based on thermally oxidized gallium nanoparticles.

“Unlike conventional plasmonic metallic nanoparticles that can cause parasitic absorption, the fully oxidized gallium oxide nanoparticles (GaxOy-NPs) preserve the nanoparticle morphology and act as non-resonant, all-dielectric ARCs,” Sergio Catalán Gómez, corresponding author, told pv magazine.

“These GaxOy-NPs reduce reflectance by about 30% across the solar spectrum and improve solar cell external quantum efficiency and short-circuit current density by approximately 10%,” he stated.

The research, which is detailed in “Thermally Oxidized Gallium Nanoparticles as Broadband Antireflective Coatings for GaAs Solar Cells,” published in Optical Materials, builds on earlier work with Ga-NPs but with a focus on oxidation and dielectric nanoparticle layer technology for antireflective functionality. It offers “new insights into “scalable oxide nanostructured coatings for photovoltaics,” according to Catalán Gómez.

“While plasmonic Ga-NPs have attractive light-scattering properties, their practical use in GaAs solar cells is limited because plasmonic resonances can overlap with the cell absorption spectrum, causing losses,” explained Catalán Gómez.

The search for an alternative with the “morphological advantages of these NPs but without plasmonic losses” resulted in the choice of gallium oxide. Gallium oxide emerged as an “excellent” candidate due to its refractive index and wide bandgap.

The group fabricated the GaAs solar cells in-house using standard photolithography and metallization procedures to ensure the study reflected performance on devices it controlled throughout the fabrication and coating processes.

“We then directly deposited Ga-NPs onto the front surface of these custom-fabricated cells and performed the oxidation treatment to form the GaxOy-NPs,” said Catalán Gómez. The process enabled precise optical property tuning of both size and surface coverage.

The results indicated that when the “initial NP radius remains below ∼30 nm”, then there was complete oxidation without compromising structural integrity. It was confirmed by microscopy analysis.

Simulations based on atomic force microscopy (AFM) measurements accurately reproduced the experimental spectra, validating the optical model. “The resulting GaxOᵧ-NPs exhibit a uniform, smooth morphology essential for predictable optical behavior and robust antireflective performance,” said the researchers.

“When implemented on GaAs solar cells, these coatings yield reproducible enhancements in external quantum efficiency and short-circuit current density, with average improvements of around 10 %,” they noted, adding that control experiments confirmed that the gains are” solely due to the presence of these oxidized NPs.”

Further discussing the results, the group stressed how the plasmon-free technology is compatible with standard device processing and that the graded-index optical coating shows “great promise” for III-V photovoltaic applications.

Researchers from Universidad de Cádiz participated in the study.

Of interest for future research is optimizing antireflective performance and making larger stable GaxOy-NPs. Additionally, integrating these coatings into other III-V multijunction solar cells and long-term stability studies under operational conditions are priorities for future work, according to Catalán Gómez.