Molecular photovoltaic devices with extended absorption in the near-IR region of the spectrum are currently a hot topic in the area of solar cells. Porphyrins and Phthalocyanines are widely investigated as promising materials for their integration in such PV systems. These intensely colored macrocycles exhibit interesting physical properties having, precisely, high extinction coefficients in the visible and near infrared, where there is the maximum flow of sunlight photons. In addition, these materials have a rich redox reactivity, ability to transfer photo-induced electrons and interesting conduction properties.
This research unit have been working on phthalocyanine chemistry for many years addressing the research to different targets as chemical sensor, electrochromic materials, non linear optical properties etc. This intensive activity has allowed to acquired a great experience in the chemical handling of this class of materials which results particularly useful in designing new phthalocyanines/porphyrins-based dyes capable of performing the expected functions when inserted in a photovoltaic device either type DSSC or as components in a hetero-junction organic cell.
The experimental work will deal with the synthesis of new molecular systems for both DSSC (Dye Sensitized Solar Cells) [A] and organic hetero-junction solar cells (HJS) [B].
1. As it regards the studies to carry on in the field of the solar cells DSSC, the new dyes have to contain groups able to form a chemical bond with the semiconductor, to this aim the new molecules (dyes) will be functionalised with a anchoring group (-COOH) able to form a strong bond with the semiconductor. Particular attention will be devoted to the length and the conjugation of such groups to the light of the role that these characteristics can have in the transfer of the photo-induced electrons from the dye to the semiconductor.
2. The solubility of the new systems (especially in the case of the phthalocyanines) will be increased with appropriate synthetic strategies that variously consist in reactions of substitution on the external benzene rings of the macrocycle with groups (as -C(CH3)3 ) that, without changing their reactivity, increases the solubility.
3. In the meanwhile the molecules have to display a favorable trend to separate the electron/hole couple induced by the light and quickly transfer the electrons to the conduction band of the semiconductor. When the construction of the photo-anode is realized with these type of coloured molecules, the molecular aggregation is a recognized drawback that can be reduced by insertion of suitable groups on the external benzene rings of the macrocycle.
4. Theoretical studies can offer support in driving the choice of the molecules and can address the synthesis toward products with appropriate positioning of the frontier orbitals (HOMO-LUMO).
5. Phthalocyanine/porphyrin derivatives, containing peripheral substitutions with different inductive character and positioned with a such geometry to get a natural electronic polarization of the molecule, are to be synthesized. Recent studies pointed out that electron-pulling anchoring groups positioned to the opposite one of electron pushing groups polarize the orbital one of frontier LUMO. We consider it a starting point to improve the strategy.
6. The possibility to use analogous macrocycles (benzo-porphyrins), with electronic structure intermediate between porphyrins and phthalocyanines, has been also sounded in order to understand the role of the aromatic conjugation in the electron transfer process.
1. In the case of the construction of organic hetero-junction solar cells, both phthalocyanines and porphyrins are fluently used for their donor properties and among the donor/acceptor systems porphyirins / phthalocyanine-fullerene couples are currently investigated.
2. Besides these systems supra-molecular structures can be built using carbon nanotubes that result to be photo-active and able to transfer electrons.
3. The strategy of synthesis that will be realized consists of getting systems donor-acceptor self assembled departing from fullerene functionalized with ligand as pyridine or imidazole. Such ligands can coordinate to the central ion of the macrocycle (zinc or magnesium) and form so a supra-molecular aggregation.
4. The choice of the metal will depend on the strength of metal-nitrogen bond, on the redox behavior of the macrocycle and on its photo-physic properties.
5. These compounds will be tested with solution deposition methodologies, in order to obtain the formation of a sensitive layer from solution; for example a solution in which a mixture of donor and acceptor are dissolved can be spin-coated on a transparent conductor substrate(ITO).
Contatto: Gianna Pennesi