“The cover shows a perspective view of an array of inverted pyramids engraved in silicon and covered with a thin gold layer, as seen in scanning electron microscopy (SEM). The magnifying glass zooms into a single pyramid, revealing the existence of a "hot spot" at the tip of the pyramid (red region). Hot spots are characterized by a strong enhancement of the electromagnetic field intensity driven by excitation of plasmons by the incident sunlight” explains ICMAB researcher Alejandro Goñi from the Nanostructured Materials for Optoelectronics and Energy Harvesting (NANOPTO) group and corresponding author of the article that illustrates the cover of the journal Materials Advances (Royal Society of Chemistry).
These pyramid arrays form a plasmonic material that is used as infrared sunlight absorber. A sunlight absorber is a material or device that absorbs solar radiation and converts it either into heat or electricity, depending on the particular application. In this case, the sunlight absorber is intentionally targeted to absorb the near infrared (NIR) part of the solar spectrum, instead of the UV or visible regions.
“Our plasmonic devices are particularly suitable for working in tandem with conventional photovoltaic devices” explains Goñi, since the NIR part of the solar spectrum is typically wasted in conventional solar cells “due to the limitation of the exploitable spectral range by the band gap of the active photovoltaic material”.
According to Goñi, this NIR light harvesters based on gold-covered silicon inverted pyramids presented in this article are an interesting alternative to current ones, and can be used not only in NIR harvesting deviced, but also in applications of photodetection, photocatalysis, or cell photostimulation in the biological IR window. “The strong absorption and the existence of hot spots, which are a prerequisite for enhanced photocurrents generated by internal photoemission of hot electrons, let foresee the great potential of this type of plasmonic nanostructures for their implementation in many devices”.
This article, titled "Efficient infrared sunlight absorbers based on gold-covered, inverted silicon pyramid arrays" (Materials Advances) is a result of the MSCA-IF project PLASMIONICO by IP Alejandro Goñi and fellow Luís A. Pérez. The project aims to advance in the sustainable production of electricity by harnessing near infrared (NIR) solar light, which is typically wasted in conventional solar cells.
A science outreach article of this project was published at the Project Repository Journal "Using plasmons to harness infrared solar light", which we recommend you to read, for more insights on this project.
The transparency of silicon in the infrared region enables the design of nano/microstructures for implementation in devices to harvest the infrared (IR) part of the solar spectrum. Herein we report a strategy that uses arrays of inverted silicon pyramids covered with a thin gold film, which exhibit substantial light absorption in the infrared spectral range (below the gap of Si). The absorption stems from the resonant excitation at infrared wavelengths of surface-plasmon polaritons at the metal/ dielectric interface mainly by tuning size and separation of the inverted pyramids.
The array-parameter optimization proceeded by iteration of the calculation and measurement of the infrared response using finite difference time-domain simulations and Fourier-transform IR spectroscopy, respectively. We analyse the calculated near-field distributions specifically looking for the presence of hot spots, i.e. nano-sized regions of very high concentration of the electronic charge and strong electromagnetic field enhancement, and discuss their potential for hot-electron generation. We show two fabrication routes for this kind of metal/silicon metamaterial either by photolithography or scalable nanoimprint techniques for a seamless integration in optoelectronic fabrication processes.
Efficient infrared sunlight absorbers based on gold-covered, inverted silicon pyramid arrays
Jinhui Hu, Luis A. Pérez, Juan Luis Garcia-Pomar, Agustín Mihi, Miquel Garriga, M. Isabel Alonso and Alejandro R. Goñi
Materials Advances, 2022, 3, 2364