Nanotechnology to harness infrared light

Plasmonics is an exciting and rapidly expanding research field. Of particular interest is the localised surface plasmon resonance (LSPR) observed in noble metal NCs that leads to strong light scattering and enhanced light–matter interaction. However, LSPR in such cases is restricted to visible wavelengths.

Recent studies showed that semiconductor NCs with reduced copper content exhibit strong LSPR in the near-infrared range of the electromagnetic spectrum. This exciting finding allows peak shifts to higher wavelengths compared to the visible ones, and thus semiconductor materials to be transparent near the LSPR wavelengths.

Based on this, the EU-funded project 'Near-infrared semiconductor plasmonic nanocrystals for enhanced photovoltaics' (NIRPLANA) focused on synthesising and incorporating plasmonic NCs into thin-film photovoltaic cells. Project advancements should pave the way for fabricating solar cells that capture infrared radiation that most cells ignore.

Scientists developed different plasmonic NC materials, with covellite being the most promising candidate for photovoltaics applications. Control over their thickness and diameter allowed tuning the LSPR amplitude that was high around wavelengths of 1 micrometre.

Three different materials for the absorber layer were also synthesised. The absorption edge of lead–sulphide NCs proved to be similar to the LSPR amplitude of covellite NCs. More suitable plasmonic NCs need to be developed for cadmium telluride.

Scientists replaced the long-chained organic ligands attached to the NC surface during synthesis by shorter ligands to achieve efficient charge transport throughout the solar cell. In addition, they established a suitable method to prepare quantum dot thin films with inorganic sulphur ligands for integration into quantum dot solar cells. A procedure was developed to prepare lead–sulphide NC solar cells in air, with conversion efficiencies of approximately 0.5 %.

Project findings can inspire new directions in NC synthesis, processing and applications in solar energy harvesting. Capturing a broader spectrum of light, NC solar cells have the potential to offer a viable low-cost alternative to current solar cell technologies.