Celle solari organiche ad alta efficineza basate su nanostrutturazioni superficiali di materiali ibridi innovativi per il confinamento della luce

In the last years research in the field of photovoltaic conversion and materials for solar panel is enormously increased for many fundamental reasons such as: (1) reduction of pollution; (2) savings of energy costs; (3) reduction of oil warehouses.

Solar cells based upon the semiconductor technology are at present commercially available for small and medium scale applications and appear quite efficient. Their main disadvantage is the large energy amount required for the production of semiconductors and the rather high manufacturing cost. The main efforts in this field are therefore devoted to reduction of the costs and to an improvement of the efficiency of the solar cells, so to make this type of energy competitive with the most traditional energy sources. In addition to the well known silicon based solar cells, an increasing interest is addressed to organic solar cells which result to be more economic and easy to produce.

In this framework, an important class of photovoltaic cells termed DSSC (Dye-Sensitized Solar Cells) is extremely promising because it is made of low-cost materials.

In DSSC device, the anode is typically made of nanocrystalline mesoporous TiO2 sensitized by a dye. The dye anchored to TiO2 molecules is necessary for an efficient light absorption: the dye excitation leads to an electron injection into the TiO2 conduction band, the dye is reduced again to the original state by a redox couple which is regenerated in turn at the cathode.

The present project will be devoted to the study and optimization of DSSC developing and implementing physical and chemical methods to increase the solar cells efficiency. The activities will be carried out by three Research Units, having highly complementary skills in chemistry, physics, mechanical and electronical engineering): the Università degli Studi di Milano (UNIMI), the Mechanical Engineering Department of the Università di Padova (UNIPD), and the Università degli Studi di Roma “Tor Vergata” (UNIROMA2). Many efforts will be addressed to solve some limiting factors, working on different aspects of the DSSC. In particular, with the joint effort of all participating Research Units, the following issues will be faced:

- Design, synthesis, and purification of the photo-sensitizers

- Increase of dye concentration through a new sol-gel synthesis protocol for a highly porous TiO2 matrix, that allows the direct embedding of photo-sensitizer molecules

- Solar cells surface nanostructuration for an efficient sunlight-trapping system

Concerning the first point, much progress has been made in dye design; the most efficient photo sensitizers are reported to be ruthenium (II) bipyridinic complexes because of the long life-time excited states. Chemical synthesis and purification of dyes already described in the literature will be optimized so to reduce the synthesis times and improve yields. Besides, novel and stable ruthenium dyes with substituted phenylpyridines as ligands will also be investigated in order to obtain a more efficient harvesting of the solar light through a better overlap between the dye absorption and the solar spectrum. Studies of compatibility of different chromophores absorbing in complementary spectral regions will also be carried out. The main goal in this case will be the mixing of different dyes in DSSC to improve the amount of photon absorption extending the possible radiation range even in the near IR region.

Another limiting factor of the DSSC efficiency is the weak absorbance of the thin photoactive layer in the range of the solar spectrum. In fact, increasing the thickness of the active layer results in a higher series resistance of the device, that leads to a limited charge-carrier mobility of the crystalline TiO2 electrode. Two strategies are adopted to overcome this problem: a chemical and a physical approach.

In the first case, the conversion efficiency will be improved by the development of a low temperature sol-gel synthesis protocol for crystalline titania. In this way, the incorporation of dyes in the porous TiO2 layer can already occur during the synthesis, leading to an increment of dye amount available for photoconversion, without increasing the layer thickness and avoiding the impregnation step.

The second strategy will involve a morphological modification of the cell in order to improve the optical confinement of the solar light inside the device (light trapping).

Different geometries will be studied to fabricate Bragg gratings on innovative photosensitive hybrid sol-gel materials to be used as external or internal layers in the solar cells with the aim to diffract and/or couple light in the active region. Periodicity, as well as profile shape and depth of the gratings will be carefully studied and engineered in order to increase the number of photons entrapped into the device. Using this approach the photons travelling inside the cells will run for longer paths and the dye to photon interaction probability will be increased. Gratings will be fabricated in the inner and/or outer surfaces of the cell.

Photopolymerizable hybrid sol-gel materials will be exploited in order to fabricate the gratings by Laser Interference Lithography (LIL) or soft-lithographic methods.

The funding of this project could represent a unique opportunity to start a research activity in a fast growing field where important results for science applied to environmental protection may be expected. We consider this project as the first step to promote an interdisciplinary wider collaboration between researchers from Lombardy and other Italian Universities in the field of Photovoltaic.