Studies of nanostructured materials for dye sensitized solar cells (dssc) electrodes

Abstract

Exploitation of solar irradiation, in particular by development of photovoltaic technologies, is a widely recognized target for renewable energy production. Present R&D trends span from highly efficient expensive devices for aerospatial industry to less efficient cheap devices for building integration. In last contest, where the coverage of buildings or greenhouses requires versatile shapes and colours and low production costs, the so-called Dye Sensitized Solar Cells (DSSC) have attracted a particular interest, starting from the publication of the seminal paper of Gratzel and O’Reagan in 1991. A DSSC is a photoelectrochemical system, as the ones studied since 1960s, based on a dye sensitized semiconductor, where a nanostructured high specific surface area porous titania electrode was introduced as a turning point to reach energy conversion up to 11%. The development of the photoanode functionality, depending on both the porous semiconductor and the dye, is one of the keys to improve the efficiency of DSSC. The research is directed on one side to the synthesis of highly absorbing dyes over a wide spectral range; on the other side to the development of novel semiconductor nanostructures providing better optical and electronic transport properties. Electrodes with the high specific surface necessary for efficient dye anchorage are usually produced by sinterization of titania nanobeads. However, several different shapes and morphologies are under investigations, in order to overcome the poor electron conduction characteristics of this kind of anodes. In particular, well-ordered vertically aligned titania nanotubes have been proposed, thanks to the long electron diffusion length predicted for this geometry. In parallel, research is in progress with the aim of producing sustainable devices based on widely available and cheap materials, focussing on the cathode. The last is a conductive surface functionalized with Pt nanoclusters, enabling both electron conduction and catalysis of the reduction reaction occurring in the iodine-based electrolyte that provides electrochemical carrier transport in the device. Since carbon is known to be a catalyst for the iodine reduction, several efforts are devoted to exploit carbon based catalysts for Pt substitution at the DSSC cathode. This thesis deals with the study of DSSC photoanodes based on different nanostructured titania morphologies, and novel carbon-based cathode nanomaterials. To this purpose, the procedure to fabricate small lab-scale devices based on both commercial and novel materials was set-up, and the devices were routinely characterized by current-voltage measurements under AM1.5 standard illumination. Moreover, a great effort was devoted to the standardization of the protocols to prepare photoanodes and cathodes, and to assemble the complete device, in order to allow meaningful comparison of performances. As novel material for the cathode, carbon nanostructures were made on purpose by CVD on different substrates, characterized, and then tested in complete devices. The efficiency of the cells with carbon-based cathodes is still low compared to those obtained with standard materials, and work is in progress in order to synthesize carbon nanostructures with improved catalytic performance. For the study of photoanodes, a simple model, based on geometrical considerations, was developed in order to calculate the specific surface of both conventional spherical nanoparticles -based electrodes, and new photoanodes based on tubular nanostructures (titania nanotubes, TNT). The knowledge of the specific surface enables to estimate the dye (N719) load on the electrode and then, neglecting light diffusion phenomena as a first approximation, the photoanode optical absorption. From the absorption of the electrode is finally possible to estimate the short circuit current density under standard solar illumination. To take into account the light diffusion in the mesoporous film, an absorption enhancement factor was considered. In order to calculate the maximum achievable short circuit current density, a statistical ray approach was used to simulate the case of limit light trapping. In parallel, the experimental measure of the diffuse and collinear optical trasmittivity and reflectivity of a NP-based electrode enabled quantification of the real enhancement factor of the absorption coefficient as a function of the wavelength. The experimental values of the short circuit current density, obtained by measuring the characteristics of a complete cell, were discussed and compared to the values predicted by applying the optical model. Novel photoanodes based on titania TNT grown on transparent electrodes (fluorine doped tin oxide coated glasses, FTO) were prepared and tested in complete cells. A large bibliography shows the possibility to grow by electrochemical process titania TNT with controlled variable morphologies, up to tens of microns of length from titanium foils. Nevertheless, in order to fully exploit the solar irradiation, growth of TNT on FTO is mandatory. For this purpose, different electrochemical conditions were applied to synthesize TNT from Ti films sputtered on FTO. Tubular structures were easily produced, but the interface Ti-FTO is severely damaged by the anodization process, so that film peeling remains a challenge for most of the conditions. TNT films up to 3.5 m thick were obtained in carefully chosen anodization conditions and tested as photoanodes in real cells, showing, however, conversion efficiency lower than that attainable from standard NP-based devices. In conclusion, small lab-scale DSSC, based both on conventional and novel materials, have been fabricated and characterized. The performances of devices using cathodes based on carbon nanostructures were measured, pointing out the necessity of improving the catalytic properties, in order to reach the standard Pt nanoparticles characteristics. The optical functionality of photoanodes was assessed. The light scattering properties of practical photoanodes were experimentally quantified and related to illuminated IV characteristics of real cells. Several photoanodes were realized by electrochemical processing of Ti films on FTO, and used to assembly complete devices. Complete characterizations of the cells, jointly with the optical model, pointed out the necessity of overcoming the technological bottlenecks in the anodization process, in order to produce high specific surface morphologies.