The worldwide exponential growth in industrial activity and the increasing dependence on energy of our society have boosted anthropogenic emissions that unbalanced the natural carbon cycle. As a consequence, global warming is one of the greatest challenges nowadays and it requires immediate actions like the European Green Deal, to massively reduce greenhouse gas emissions. One way to mitigate it, is the replacement of fossil fuels with green sources. This prompted us to seek for clean energy solutions, either by developing new concepts or by improving existing ones. This is where this Marie Skłodowska Curie Action (MSCA) project, entitled “Plasmon-resonance driven thermionic emitters for improved solar energy harvesting (PLASMIONICO)” comes in. The project has had a clear perspective regarding this scenario, searching for alternatives to photovoltaics to harness near-infrared (NIR) solar light, the part of the solar spectrum which is normally wasted in conventional solar cells. The key concept is the use of purposely designed interfaces between a nanostructured metal and a semiconductor to efficiently absorb NIR solar light. The absorbed photons would then excite surface plasmons which end up injecting electrons from the metal into the semiconductor, generating a photocurrent.
The underlying physical principle involves the utilization of the plasmonic modes supported by metallic nanostructures. Metals contain high densities of free electrons, moving through the solid like a compressible fluid but transporting electricity. Plasmons correspond to compressive waves of the electronic charge density, in an analogy of sound waves in liquids. At well-defined frequencies (wavelengths), so-called resonances, an electromagnetic field (light) readily couples to the charge carriers, transferring energy to the metal by launching plasmons. In nanostructured metals, plasmons are restricted to travel along the metal/dielectric interfaces. These (surface) plasmons have extremely large electromagnetic field enhancement close to places where the electronic charge density is higher, named hot spots. We aim to utilise these hot spots for the internal injection of electrons to produce a photocurrent.
The project roadmap included the design, fabrication and characterization of photonic/plasmonic nanostructured NIR absorbers, the optimization of the material interfaces (i.e. metal/semiconductor interface) and the development a thermionic emitter demonstrator. The obtained results have shown that the fabricated nanostructures are compatible not only with materials typically used in photovoltaics such as silicon, but also with soft materials like conductive polymers.
