Periodic Reporting for period 1 - PLASMIONICO (Plasmon-resonance driven thermionic emitters for improved solar energy harvesting)
Período documentado: 2019-09-01 hasta 2021-08-31
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.
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.
In the framework of PLASMIONICO, great effort was invested in controlling the nanostructuring of crystalline Si wafers in the form of perfectly ordered arrays of inverted pyramids covered with a thin gold film. Silicon has many advantages over other semiconductors: the ready integration of our NIR harvesters into conventional electronic devices and the transparency in the infrared spectral range, which enables the optimum illumination of the inverted pyramids arrays from the substrate side. In addition, low-cost and scalable manufacturing methods were employed. Fabrication protocols were developed to obtain nanostructures by combining chemical and physical etching processes, together with optical lithography (for big pyramids) and nano-imprint lithography, when the lattice parameter was smaller than 1 micron. A wide set nanostructures were fabricated, changing geometric parameters such as the size of the pyramids (from 400 nm to 7 microns), the lattice parameter used in the arrangement, the thickness of the deposited metal layer, etc. The obtained nanostructures show high light absorption in the NIR region (in excess of 80%) that can be tuned by modifying their geometric parameters. The spectral features are in excellent agreement with computational calculations.
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.
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.
Looking ahead, we would like to point out that the mechanism of plasmon-assisted hot-electron generation studied and developed in the framework of PLASMIONICO will have direct impact not only in solar-energy conversion technologies. On the contrary, other research and technological areas can profit from its implementation, such as high-sensitivity detection, photo-catalysis, biological electro-stimulation, the synthesis of added-value chemicals, water splitting and hydrogen technology, etc. All these areas would very much profit from the knowledge gained in the framework of PLASMIONICO regarding the mechanisms of plasmon-resonance driven generation and injection of hot electrons for current generation or triggering specific chemical reactions; the latter being an aspect which is often underestimated in its complexity.
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.
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.