MOLECULAR DESIGN AND SYNTHESIS OF DYES FOR DYE-SENSITIZED SOLAR CELLS (DSSCS)

ABSTRACT Background Solar energy plays a critical role in meeting the global energy challenge and represents one of the most promising energy sources for the future of the planet. Solar or photovoltaic cells are currently a hot topic on the market, these are devices that convert the energy of sunlight directly into electricity trough the photovoltaic effect. Strong and competitive research is currently devoted to lower the material costs of solar cells, and to increase their energy conversion efficiency. Up to now, commercially available photovoltaic technologies are based on inorganic materials, mainly crystalline silicon (first generation) and other semiconductors, such as gallium arsenide, indium phosphide and cadmium telluride (second generation In addition to high costs, also in terms of energy consuming, in fabrication processes, several of those materials are toxic and have low natural abundance. Therefore, in the last two decades the research focused on the development of a third generation of solar cells based on hybrid or organic materials, that offers a number of advantages, such as: high molar extinction coefficients, versatility of the chemical design for modulating the electronic properties, easy processability as well as low manufacturing costs. Although the efficiencies of organic-based photovoltaic cells ( 8%) are still at the moment a long way behind those obtained with purely inorganic based photovoltaic technologies ( 20%), the power conversion efficiency of organic solar cells have been significantly improved and there are expectations for more important results. Among the third generation of solar cells, the Dye-Sensitized Solar Cells (DSSC), also called Grätzel cells, have emerged as very promising candidates for low-cost alternative to conventional semiconductor photovoltaic devices. A DSSC cell scheme is shown in Figure 1. The cell components are: a mesoporous film of TiO2 (anode), a dye-sensitizer, an electrolyte, an electrochemical mediator and a cathode. The photovoltaic process in this cells can be resumed as follows: the dye-sensitizer (S), linked to semiconducting TiO2 surface (usually through a carboxylic group), absorbs a photon passing to the excited state S*, which transfers an electron to the conduction band of TiO2. The oxidized S+ thus obtained, is reduced by a redox mediator, generally I- from the couple I-/I3- dissolved in the electrolyte. The electron injected in TiO2 through the external circuit arrives to the cathode, where the reduction of I3- regenerates the iodide, closing the circuit. (Figure 1). Figure 1 The DSSC technology separates two requirements as: i) the charge generation, done at the semiconductor-dye interface and ii) the charge transport, done by the semiconductor and the electrolyte. Consequently, carrier transport properties can be improved by optimizing the semiconductor and electrolyte composition, while the spectral properties and thus charge generation can be improved by modifying the dye structure, that can be tailored in many ways by organic chemistry contribution. Many kinds of dyes have been studied for DSSCs application and in principle they could be divided in two classes (Figure 2): 1. metal complexes (N719, Zn-porphyrine e.g YD2-o-C8) , , 2. metal-free system Donor-Spacer-Acceptor (TA-St-CA) Figure 2 Up to now the best efficiencies (~11%) have been reached using ruthenium complexes, thanks to their large absorption range from visible to near infrared (NIR), and their capability to easily inject electrons in the conducting band of the semiconductor. The metal based chromophores still have several disadvantages such as not very high molar extinction coefficient and the presence of the expensive metal, such as ruthenium, which involves complicated synthesis and hard purification steps. On the contrary, metal-free dyes are simple and cheap to prepare and it is possible to easily modulate their photo- and electrochemical properties varying the functionalization, but very high efficiencies have not been achieved yet. The obtainment of new and more efficient dyes is therefore object of competitive international researches. Within this context, the present Ph.D. research project has focused on the synthesis of new metal-complexes and metal-free organic dyes characterized by a Donor-Spacer-Acceptor (D-π-A) structure, (Figure 3) in which the novelty is represented by the presence of benzo-condensed thiophene units as π bridge spacer. Figure 3 Aim of the work In such chromophores the π spacer plays a fundamental role, as it is responsible for the electronic communication between acceptor and donor moiety and for the extension of the conjugation that lead to wider and red-shifted absorption spectra. To date a number of new π-conjugated aromatic and heteroaromatic systems have been investigated and among these, thiophene or thienothiophene π-bridges have been reported to give remarkable efficiency. Benzodithiophenes systems BDT and BDT1 (Figure 4) attracted our attention because their rigid, π-conjugated, condensed-polycyclic structure , leads to unique electronic properties such as conductivity, high field effect mobility and tunable stacking in the solid state; rigid structures hamper the roto- vibrational modes responsible for the deactivation of the excited states in functional materials. Figure 4 In this Ph. D. work we investigated synthesis of suitably functionalized BDT and BDT1 derivatives as well as their use for the construction of two classes of dyes: 1) Zn-porphyrin based dyes (in collaboration with the research group of Prof. Pizzotti and Prof. Ugo) and 2) metal-free dyes and, Zn-porphyrin based dyes In addition the design of the new dyes have been oriented by preliminary theoretical calculations, done in collaboration with Dr. Filippo De Angelis of CNR-ISTM in Perugia, that allowed to gain insight into the molecular, ground and excited state electronic structure of the new chromophores. 1. Synthesis of new benzodithiophene containing Zn-porphyrins Metal porphyrins, characterised by very strong absorption bands around 450 nm (Soret band) and 600-700 nm (Q band) are potentially interesting as dyes for DSSC. For example, some push-pull type porphyrins bearing a carboxylic acid moiety as an anchoring group, have disclosed a remarkably high power conversion efficiency (6-7%), therefore in the recent years some research efforts have been devoted to the design, synthesis and application of new porphyrin-based chromophores for DSSC. , , The unique feature of these sensitizers is that the porphyrin chromophore itself constitutes the π-bridge of the D-π-A structure and with the aim of increasing the conjugation of the system, some new Zn porphyrins, containing the BDT1 unit (Figure 5), have been designed in our group. These porphyrin molecules are differently functionalized in 5,15 and 10,20 meso positions. In positions 5 and 15, aromatic rings bearing bulky groups are needed to avoid aggregation on the semiconductor surface, that drastically Figure 5 reduce the dye light-harvesting by a filtering effect. In 10,20 meso positions the structure presents two π-delocalized aromatic systems with opposite (electron-withdrawing or electron-donating) properties, in order to realize a push-pull system in which is possible to modulate the position and the intensity of the Q band and to favor the electron flow. The most promising structures were selected on the basis of preliminary theoretical calculations done by Dr. De Angelis and synthesized in collaboration with Prof. Ugo and Prof. Pizzotti’s research group. The novel Zn-porphyrin system 1 (Figure 6) was first synthesized, whose structure is characterized by the presence of BDT1 system in the acceptor part of the molecule. The suitable 2,6 di-functionalized BDT1 derivative was prepared and then linked to the porphyrin core. Figure 6 The resulting new Zn-porphyrin 1 was completely characterized from the analytical and photophysical point of view and used in preliminary tests as dye in Grätzel solar cells, giving an efficiency of 0.6%. Slightly optimization of the cell structure and in the composition of the electrolyte led to an increased efficiency of 2,54%. This result, although unsatisfactory, served as a starting point for the set-up of a number of synthetic protocols and for designing more targeted substitution and variation in the molecule structure. This part of the work is currently under progress. 2. Synthesis of benzodithiophene containing metal-free dye As already mentioned, the general structure of a metal-free dye, reported in Figure 3, presents a donor and an acceptor unit linked by a π-conjugate system. The most efficient structures reported in the literature contain triarylamines as donor unit, because of the prominent electron-donating ability and hole-transport of such molecules. Within this topic we designed novel metal-free triarylamine-containing organic dyes endowed with the innovative spacers BDT1 and its isomer BDT. Also in this case the design of the new compounds was oriented by preliminary TD-DFT calculations made by Dr. De Angelis, on two parent BDT1-containing structures 15 and 16, which differ from each other by the presence of a triple or a double bond. (Figure 7) With the aim to investigate the structure-performance relationship of the dyes in the cell, we designed a small library of structures, changing the BDT-bridge (17), the acceptor group (18) or the donor (19, 20) with respect of the model compound 16. (Figure 7). This allowed us to investigate the potentiality of BDT and BDT1 in the dyes in combination with double or triple bond in order to elongate the conjugation, and to obtain band gap reduction and enlarge the absorption spectra. In particular, the presence of the triple bond should ensures more planarity and therefore conjugation and avoids energy losses due to photoisomerization. The series of synthesized dyes are reported in figure 7. Figure 7 Almost all the dyes synthesized have also been characterized from a photophysical as well as electrochemical point of view, with the aim of identifying, among them, the most interesting and promising compounds for application in solar cells and try to clarify the relationship between the chemical structure and photovoltaic performances. Preliminary test in DSCs have been carried out for some of the dyes and among these dye 16 has emerged as the most promising one leading to an efficiency in liquid state cell of 5.11% and confirming the potential of BDT1 π-spacer for application in DSSCs. The cell efficiency found for 16, which is however still under optimization, allows us to say that this dye ranks among the promising dyes to date reported in literature. In addition, it must be pointed out that dye 16 seems to possess most of the essential chromophore characteristics required for obtaining high-performance DSSCs. The systematic study developed during the present Ph.D. thesis will be very useful for future improvement of the synthesized structures and their photovoltaic performances in DSSCs.