Novel materials and New design for Dye Sensitized Solar Cells Technology

One of the main differences between classical solar energy systems and photoelectrochemical cells, as said in the introduction chapter, is the way in which electrons are carried untill external circuits in order to generate a potential energy.

In fact in typical silica-based solar cells photons are harvested by the semiconductor, then, electrons generated are carried outside the cell by the semiconductor itself. Instead, in photoelectrochemical cells, the process is splitted into different parts.

Dyes, organic or metal-based, harvest photons, while carry mechanism is done by the semiconductor. In every semiconductor-based systems there is, instead, a huge difference with the photosynthesis natural process in particular in the part concerns the transport.

In fact, in nature, carriers move through the system by a molecular hopping mechanism, while in SC systems carriers are higly delocalized and move following a simple diffusion process; consequentely can quickly move for large distances avoiding surface recombination and other "dark current" mechanism. Well-understanding of these mechanisms is possible only with a more deep point of view.

Electronic properties of semiconductors
Crystal structures
First deep look necessary to understand SC properties concerns lattice structures of these inorganic solids. There are many different available structures for SCs. It's possible to subdivide the major part of SCs into two big classes: Adamantine solids, sharing common feature of having an average per atom of 4 valence electrons; Non adamantine solids.

The most basic structure is the so-called face-centered cubic (fcc) lattice. Many covalent semiconductors, belonging to the adamantine class, are based on fcc lattice with some variants. For example, Si and Ge, attempt to achieve maximum bonding between nearest neighbors; this implies that tetrahedral structure is maintained but atoms follow the same principles of NaCl crystals adopting a cubic diamond lattice.

Another typical structure is the zinc blende , taken by covalent, binary SCs, with atoms of similar electronegativities, as GaAs, InAs, InP and GaP. Otherwise in SCs with atoms with different radii and electronegativities, in order to maintain a lattice with a 1:1 cation-anion stoichiometry, the lattice adopted is hexagonally close-packed with only one-half of the possible sites are occupied.

This configuration,called wurtzite structure, adopted by ZnO and CdSe is more open than the zinc blend and the bonding is more ionic. Non adamantine solids include the general classes of metal oxides like Fe2O3 , TiO2 and SrTiO3 . In particular titanium dioxide occurs in two different structures:the rutile lattice, derived from a body-centered cubic (bcc) configuration and the anatase lattice, derived from a fcc. In both cases to maintain a 1:2 stoichoimetry only on-half of sites are occupied by Ti atoms.