Nanomaterials and nanofabrication for next generation photovoltaics

On December 2015, almost 200 countries were represented in Paris, discussing on the topics of climate change and Kyoto Protocol. According to the organizing committee at the outset of the talks, the expected key result was an agreement to set a goal of limiting global warming to less than 2°C compared to pre-industrial levels [1]. In order to reach this goal, huge cut on polluted emissions has to be done, finding new resources to produce energy. Photovoltaics is a sustainable and environmentally clean energy source that has the potential to become one of the main ones, in the near future. In order for this to happen, photovoltaics needs to be economically competitive with other conventional energy sources: despite the price of conventional Si solar cells dropped from ~4 USD/WP to ~0.54 USD/WP in the last 15 years [2], the use of new materials to get even more low-cost solar cells created the field of the so-called “new generation photovoltaics”. Organic (polymeric and small molecules) solar cells have been extensively investigated, in the last years, not only for the potential low fabrication costs, but also for the possibility of building integration and for the solution process realization that allows spray, Roll-toroll and printing techniques, over rigid or flexible substrates. Moreover, perovskite solar cells, that are also compatible with the above mentioned production methods, recently outperformed multicrystalline Si, showing an impressive certified efficiency of 22.1 % [3]. This “new gen” solar cells strictly rely on research performed in the nanometric world: the typical thickness of an active layer for organic solar cell is hundreds of nanometers; phase separation between different components of polymeric solutions are investigated by the mean of Atomic Force Microscopy, which allows to see features in the order of some nanometers; perovskite is likely deposited on top of scaffold with nanometric pores. So, integrate nanomaterials and nanofabrication steps in the realization of such solar cells is of great interest. In this work, the possible interactions of nanomaterials as Carbon Nanotubes (CNTs) and Nanoporous Alumina, as well as the integration of nanotechnology process such as 4 Nanoimprint Lithography, have been investigated in the field of new gen solar cells. A thorough research on Carbon nanotubes (CNTs) has been carried out to realize semitransparent electrodes to be used as contacts in an organic solar cell. A “two temperature zone” Chemical Vapour Deposition (CVD) set-up is assembled to synthesize CNTs directly on top of Fluorine Doped Tin Oxide and standard glass. The transparency of the asprepared contacts is increased by the mean of a laser patterning technique. An additional technique used to deposit a very thin layer of Nickel, to enhance the conductivity of such semitransparent electrodes, is also presented. The necessity of decreasing the roughness of the CNTs based electrodes, in order to prevent short circuits in organic solar cells, leads to the development of a spray deposition technique of nanotubes solutions on top of glass substrates. The use of different solutions, with and without the addition of surfactants, as well as the upgrade of the spray set-ups and the design of appropriate metal contacts for inverted organic P3HT:PCBM solar cells are shown. The inverted architecture of bulk heterojunction solar cell is used also in combination with the Nanoimprint Lithography Technique (NILT): the creation of a pattern of nanometers size on the polymer surface, in order to realize a Photonic Crystal effect designed by optical simulations, is carried out through several technology obstacles, for each of which is given a proper solution. Lastly, scaffolds of ordered Nanoporous Alumina, realized through the anodization of pure and evaporated Aluminium on glass, are used for the growth of perovskite active layers that are characterized by absorbance measurements.