André S. Polo

A high efficiency ruthenium(II) tris-heteroleptic dye containing 4,7-dicarbazole-1,10-phenanthroline for nanocrystalline solar cells

The tris-heteroleptic polypyridyl ruthenium(II) dye, cis-[Ru(cbz2-phen)(dcbH2)(NCS)2], cbz2-phen = 4,7-dicarbazole-1,10-phenanthroline and dcbH2 = 4,4′-dicarboxylic acid 2,2′-bipyridine, was designed, synthesized, purified via liquid chromatography and characterized using 1H NMR, FTIR, cyclic voltammetry, absorption and emission spectroscopy. The compound exhibits broad and intense MLCT bands that overlap the visible spectrum and it is capable of sensitizing TiO2 films. The energy levels of cis-[Ru(cbz2-phen)(dcbH2)(NCS)2] are adequate for its use in DSSCs. The solar cells prepared using this dye achieved a performance that surpasses N3 dye under the same conditions. The introduction of the extended π-conjugated carbazole substituent at the 4 and 7 positions of 1,10-phenanthroline increases the IPCE and the overall solar cell efficiency. The high performance is ascribed to the HOMO stabilization and to the increase in electron delocalization of the triplet excited state, which favors the electron injection via singlet and triplet pathways.

Nanomaterials for Solar Energy Conversion: Dye-Sensitized Solar Cells Based on Ruthenium (II) Tris-Heteroleptic Compounds or Natural Dyes

The worldwide energy demand is growing and the development of sustainable power generation is a critical issue. Among several possibilities, dye-sensitized solar cells, DSSCs, have emerged as a promising device to meet the energy needs as an environmentally friendly alternative and investigation for academic and technological improvement of DSSCs are being carried out. One of the most important components of this device is the dye-sensitizer, since it is responsible for the sunlight harvesting and electron injection, the first steps of energy conversion. Herein, we review the developments on tris-heteroleptic ruthenium dye-sensitizers, which have been gaining much attention on the last years due to the possibility of modulating their photochemical and photophysical properties of the complex by using different ligands. Besides synthetic compounds, natural dyes have also been employed as semiconductor sensitizers for these devices and are also reviewed. These dyes can lower the device production costs since they can be promptly obtained from fruits or flowers in a very simple way. Among numerous classes of natural dyes, anthocyanins have been the most investigated ones and gained special attention in this work. 1 Aims and Scope Dye-sensitized solar cells, DSSCs, gained much attention since it is a simple and cheap device capable of converting the sunlight into electricity through a regenerative photoelectrochemical process. DSSCs overall efficiency attained 11 % and it is estimated to last around 20 years. Besides the economic advantages, these J. d. S. de Souza (&) L. O. M. de Andrade A. S. Polo Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Rua Santa Adélia 166, Santo André, SP 09210-170, Brazil e-mail: andre.polo@ufabc.edu.br F. L. de Souza and E. R. Leite (eds.), Nanoenergy, DOI: 10.1007/978-3-642-31736-1_2, Springer-Verlag Berlin Heidelberg 2013 49 devices can be transparent and allows their use for distinct architectonic purposes, such as facades of buildings. DSSCs are based on a nanocrystalline mesoporous semiconductor films sensitized by dyes, which are responsible for light harvesting and electron transfer, these processes, start the energy conversion and are directly responsible for its overall efficiency. This chapter aims to review a specific class of synthetic dye, the tris-heteroleptic ruthenium sensitizers, which have been attracting much attention on the last years due to the possibility of tune their spectroscopic and electrochemical properties as well as to improve the stability of the device. The recent advances on the use of natural dyes as semiconductor sensitizers, from 2003 to 2010, are also reviewed. 2 Introduction The use of fossil fuel based technologies is the major responsible for the continuous increase in the pollution and in the concentration of greenhouse gases. Renewable sources must have higher contribution on the energetic matrix in providing more energy available for the humanity in a short period, having low environmental impact [1, 2]. The interest on the conversion of environmentally friendly energy sources led to the development of several devices that took the advantage of the continuous evolution on several fields of research, which can result in new materials for already developed devices. For instance, the performance of direct methanol fuel cells, a well known technology [3, 4] was improved due to the development of nanomaterials especially designed for the energy conversion process [5, 6] and their evolution allows the use of light to boost the process through a synergic arrangement [7–10]. The use of sunlight has been gaining much attention due to its abundance. For instance, it is possible to supply human energy needs up to 2050 covering only 0.16 % of the earth surface with 10 % efficiency solar devices [1, 11]. There are several investigations on the conversion of sunlight in substances with more chemical energy than the reactants in a process that mimics the photosynthesis; this approach is known as artificial photosynthesis [12]. Most recently, the investigation on this research field is being called solar fuels and several papers were published describing photochemical approaches to produce high energy content substances, or fuels, from simple reactants such as water or CO2 [13–19]. Great interest is dedicated to an especially attractive, the Dye-sensitized solar cells, DSSC, since they are capable of converting the sunlight into electricity based on photoelectrochemical principles. The materials employed for the construction of these new solar cells are common and cheap and the procedures do not require controlled environment, thus clean rooms or any other sophisticated control can be avoided, consequently a very low production cost is estimated (less than 1 € per Wp) [20]. The use of new nanomaterials allows interesting features of these devices, such as transparency, possibility to have distinct colors, among others. These characteristics are very interesting for new applications of solar cells, since 50 J. d. S. de Souza et al. it can substitute glass windows and promote the co-generation of energy, or for any other architecture design. Albeit the possible use of sensitization effect for solar energy conversion is known for a long time [21], the breakthrough of these solar cells was in 1991 when B. O’Reagan and M. Grätzel published the use of nanocrystalline and mesoporous TiO2 film [22]. This film enhanced the light absorption due to its sponge-like characteristic which increases the surface area. The nanocrystallinity plays an important role on the electron injection and transport in these devices [23]. Since the paper of 1991, this field has been growing very fast and all the aspects of these solar cells are investigated [24–27]. In this review, the focus is on the development of tris-heteroleptic ruthenium (II) dyes as well as the use of natural extracts as a source of sensitizers. The absorption spectra and photoelectrochemical parameters published for these compounds since 2003 will be reviewed and discussed. 2.1 Dye-Sensitized Solar Cells: Principles and Operation Dye-sensitized solar cells are prepared in a sandwich arrangement and are comprised by two electrodes, the photoanode and the counter-electrode, Fig. 1. The photoanode is a conducting glass covered by a mesoporous and nanocrystalline TiO2 film, sensitized by the dye-sensitizers. The counter electrode is a conducting glass covered by a thin film of catalyst, such as platinum or graphite. Between these electrodes is placed a mediator layer, usually a solution of I3 /I in nitriles. In order to promote the energy conversion, the sunlight is harvested by the dye-sensitizers leading to an excited-state capable of inject an electron into the semiconductor conducting band. The oxidized dye is immediately regenerated by the mediator and the injected electron percolates through the semiconductor film, reaches the conducting glass and flows by the external circuit to the counterelectrode. The counter electrode is responsible for regenerating the oxidized specie of the mediator, reducing it by a catalyzed reaction using electrons from the Fig. 1 Schematic arrangement of a dyesensitized solar cell Nanomaterials for Solar Energy Conversion 51

A importância do estado excitado 3MLCT de compostos de Ru(II), Re(I) e Ir(III) no desenvolvimento de fotossensores, OLEDS e fotorredução de CO2

The photochemistry and photophysics of coordination compounds have been extensively investigated not only because their structure, stability, reactivity dependence on the metal center oxidation state and the coordinated ligand; but also for their electronic transitions in a wide range of visible radiation. The knowledge of light absorption, excited state deactivation, sensitization and quenching processes are crucial to their manipulation aiming the development of systems capable of execute useful functions such as photosensors and/or probes, luminescent devices and molecular systems to convert sunlight into other types of energy. In this review, the progresses and challenges of biomolecules photosensors, organic light emitting diodes and CO2 photoreduction catalysts based on ruthenium(II), rhenium(I) or iridium(III) coordination compounds are discussed based on their photochemical and photophysical processes.