Dye‐sensitized solar cells

Among the different types of second and third generation solar cells, Dye‐Sensitized Solar Cells (DSSC) or Grätzel cells have reached global AM 1.5 power conversion efficiencies of up to 12% using cheap, non toxic and widely available components like TiO₂, representing a promising alternative to  silicon based photovoltaic devices. However the learning curve of these devices has not improved significantly in the last years. The DSSC contains several different components: a conducting glass substrate, a mesoporous semiconductor film, a sensitizer, a hole conducting layer or an electrolyte with a redox couple and a counter electrode. The overall efficiency of DSSC is a delicate balance of several concurrent and consecutive processes (sensitizer relaxation, electron injection in the VB of TiO₂, electron percolation in the TiO2 NPs network, back electron transfer to electrolyte among others). Despite the great research efforts of the last decade, the thermodynamic and kinetic aspects of these electron transfer processes are not yet fully understood. To obtain a breakthrough in the comprehension of these processes  and, consequently, a significant improvement in the efficiency and durability of the DSSC, a key aspect is the use of TiO₂ NPs with a precise control of not only specific surface area, composition, size, crystallinity and porosity, but also morphology.

In the SETNanoMetro project, the high homogeneity in shape and size of prepared TiO2 NPs will be exploited to disclose the mechanisms of the relevant electron transfer  and percolation processes and how them depends on the morphology and facets exposed of TiO₂ NPs. Moreover, the disclosure of methods and procedure to obtain assemblies as much ordered as possible, with controlled amount, shape and relative extent of particles/voids ratio will be pursued, in order to attain an optimized balance between interparticle aggregation/junction (required for an effective transport of charge carriers) and space for the diffusion of reactant/functional molecules. The final goal  will be the control of materials macroscopic properties.

Further processing benefits are clearly seen from controlling TiO₂ NPs morphology which may lead to better device reproducibility and efficiency.

In detail the following activities will be undertaken:

• Characterization of the TiO₂ films loaded with the dye sensitizer  as a function of NPs morphology and size

• Stability characterization (conversion efficiency over time) of the laboratory cells
• Correlation of the macroscopic properties of the DSSC to the properties of NPs/films down to the nanometer (NP assembly and inter-particle contacts) and molecular level (localization of the sensitizer and dependence on it of the electron injection processes)