Synthesis of New Metal Complexes as Chromophores for Dyesensitised Solar Cells

Nowadays the production of energy originating from renewable sources is a burning issue, in particular a lot of efforts are made for a reduction of worldwide energy consumption for a sustainable world. The efficient and low-cost direct conversion of solar photons into electricity is one of the most important scientific and technological challenges of this century. Up to now, commercially available photovoltaic technologies are based on inorganic materials, which require high costs and highly energy consuming preparation methods. Organic photovoltaic can avoid those problems, but the best efficiencies of organic-based photovoltaic cells are at the moment around 7%. Dye-sensitised solar cells (DSSCs) represent a concrete solution for harnessing solar energy and converting it into electrical energy and 11% efficiencies have been reached with the most performing Ru(II) sensitizers, such as N3 (cis-di(thiocyanato)bis(2,2- bipyridyl-4,4′-dicarboxylate) ruthenium(II)) and N719 (bis(tetrabutylammonium)-cisdi(thiocyanato)-N,N′-bis(4-carboxylato-4′- arboxylic acid-2,2′-bipyridine) ruthenium(II)).
The photosensitiser dye plays a strategic role in DSSCs, absorbing the solar light and promoting the formation of an electron-hole pair which is eparated, transported, and then collected at the electrodes. Other organometallic complexes have also been used as dyes in DSSCs, for examples complexes of Pt(II), Fe(II), Os(I), Cu(I), Re(I) and Ir(III). Iridium complexes are potentially good candidates for application in DSSCs. The absorbed photon to current yield in iridium based DSSC devices is comparable to the ruthenium dyes. Moreover, the ruthenium dyes produce current only by injection from metal to-ligand charge transfer (MLCT) states whereas, for iridium dyes, it would be possible to combine injection from both MLCT and ligand-to-ligand charge transfer (LLCT). However, up to now, low molar extinction coefficient and a narrow absorption spectrum at relatively high energy (380 nm) are critical factors that limit the efficiency of Ir(III) dyes. For this reason, the reported DSSCs solar cells based on Ir(III) complexes are characterized by low efficiencies.
In this thesis, I present my results on the design and synthesis of new dyes for DSSC application.
Described in Chapter 2 is the synthesis of iridium (III) complexes where aryl-1,2,3- triazole ligands act as cyclometalating ligand and 4,4‟-dicarboxy-bipyridine as N^N ancillary/anchoring ligand. The photophysical effects of these complexes were investigated. It was found that by using different substituents on the phenyl ring, or a different aryl system, it is possible to tune the absorption and the emission of these complexes. Computational studies showed HOMO-LUMO directionality of these complexes is ideal for the electron injection once they are applied on DSSC devices. Preliminary efficiency test have been carried out with promising results.
Described in Chapter 3 is the synthesis of iridium (III) complexes designed as dyes for p-type DSSC. For these complexes a phenylpyridine containing a carboxylic group has been used as cyclometalating/anchoring ligand whereas different diimine ligands act as ancillary ligands. The photophysical effects of these complexes were investigated. It was found that using different π-systems on the ancillary ligand is possible to tune the absorption of these complexes and to enhance the spatial separation between HOMO and LUMO. Computational studies confirm the potential use of these complexes on DSSC devices. Preliminary tests on NiO devices have been carried out.
Described in Chapter 4 is the synthesis of a novel bipyridine-1,2,3-triazole based anchoring group. The novel triazole ligand might act as spacer between the metal oxide and the metal center insulating the electronic coupling and avoid the recombination. This N^N ligand, obtained through click of azido-bpy and acetylenedicarboxylate, was used as an anchoring ligand in ruthenium, iridium and rhenium complexes. The set of three metal complexes was compared with their dicarboxybipyridine analogues. Looking at the photophysical and electronic properties of these new complexes they seem comparable with their dicarboxybipyridine analogues. Preliminary anchoring tests on TiO2 have been carried out.
Described in Chapter 5 is the design and synthesis of two cyclometalated Ir(III)– coumarin molecular arrays, which show intense absorption of visible light, one belonging to p-type dyes family and another belonging to n-type one. The two complexes have been characterised and tested on both TiO2 and NiO cells.