Periodic Reporting for period 1 - PLASMIONICO (Plasmon-resonance driven thermionic emitters for improved solar energy harvesting)
Periodo di rendicontazione: 2019-09-01 al 2021-08-31
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.
Selected nanostructures were integrated into a cold-cathode thermionic emitter (Au/insulator/Si or Au/Si). The Si doping level was selected so as to have a good electrical conductivity and simultaneously sufficient transmission for light in the NIR spectral range. Special attention was devoted to optimization of the front (Al or Au) and back (the same gold of the pyramids) contacts in order to obtain diode-like I-V curves, despite of not using an insulating layer as Schottky barrier other than a 400 nm thick undoped Si layer. The characterization of the photocurrent-generation performance of the ready devices was carried out using a home-made external quantum efficiency (EQE) setup. The n-Si/Au inverted pyramid arrays exhibited very high photocurrents for wavelengths longer but close to the Si gap due to the so-called PIRET effect (plasmon-induced resonance energy transfer). On the other hand, we developed a proof of concept architecture were silicon is replaced by a solution-processable conductive polymer. For this purpose, we adapted soft nanoimprint fabrication methods to integrate the soft material to our plasmonic nanostructure. The results are promising with hot-electron photocurrents more than ten times larger than the state of the art.
The dissemination of the project results was carried out in different ways: At the educational outreach level, we participated in talks for secondary school students within the framework of the European Researchers Night, and a broad audience article has been published in the Project Repository Journal (https://www.europeandissemination.eu/project-repository-journal-volume-10-july-2021/14505) and with a posters at the European Corner 2021, Nit Europea de la recerca (https://lanitdelarecerca.cat/plasmonico/) . On the other hand, some of the results obtained during the project have been presented in conferences and symposium, have been submitted for publication, or are currently under preparation.
From the socio-economical point of view, PLASMIONICO has contributed, on the one hand, to enhance the efficiency of the sustainable conversion of solar energy into electricity and, on the other hand, to set the grounds for the development of a scalable technology which can be directly implemented for the capture of CO2 to tackle the acute problem of the climate change due to greenhouse-gases emissions. In both cases, we believe to have contributed to decarbonizing the footprint of human activities, in consonance with the European Green Deal.