RENDEMENT

The wide absorption and photovoltaic nature of metal complexes make them suitable for DSSC applications. Such types of complex sensitisers usually have anchoring ligands and ancillary ligands. Anchoring ligands are responsible for the complexes adsorption onto the semiconductor surface and are also chromophoric groups.

Ancillary ligands are not directly attached onto the semiconductor surface and can be used for tuning the overall properties of the complexes. Polypyridinic complexes of d⁶ metal ions show intense metal to ligand charge transfer (MLCT) bands in the visible region, with potential interest for promoting charge injection processes to the conduction band of wide band gap semiconductors, such as TiO2, SnO2 and ZnO. The energies of the (MLCT) states can be altered systematically by modifying the anchoring ligands as well as by changing the ancillary ligands or their substituents.

The wide possibilities to tune the MLCT energy resulted in the preparation of many different compounds that have been investigated for semiconductor sensitization.

Among them, the best light-to-electricity conversion efficiency has been achieved by using ruthenium(II) polypyridyl complexes as TiO2 sensitisers in dye-sensitized solar cells. In fact, a ruthenium bipyridine complex was used in the first efficient DSSC cell example reported in the breakthrough article by Grätzel and O’Regan in 1991 [11]. A metal complex consists of a central metal ion with an ancillary binding molecule bearing at least one anchoring group. The absorption of light in the discernible element of the solar spectrum is caused by a charge transfer process from

38 a metal to a ligand (MLCT). Thus, the central metal ion is critical for the complex in which ancillary binding molecules, typically bipyridines or terpyridines, can be used with different substituents (alkyl, aryl, heterocycle, etc.). The purpose of this is to improve the photovoltaic performance by altering the original photophysical and electrochemical properties. In order to enable the injection of the excited electron into the conduction band (CB) of the semiconductor anchoring groups can be used to attach the dye. The adjustment of energy levels of the MLCT to enhance electron injection and dye regeneration kinetics can be done on any part of the new structure.

3.2.1 Ruthenium complex dyes

Ruthenium DSCs were first reported in 1991 by O’Regan and Grätzel [11]. These first ruthenium dye DSCs achieved 7.1% conversion efficiency (Figure 3.1). However, the structure of the ruthenium dye was complicated and contained three ruthenium metal centres. In 1993, Nazeeruzzin et al. published DSCs with 10.3% conversion [12], using a ruthenium dye sensitiser N3 (Figure 3.1). N3 contained one ruthenium center and was thus simpler than the ruthenium dye reported in 1991. The family of complexes [{(4,4’-CO2H)2(bipy)}2RuX2] (bipy=2,2’-bipyridyl; X=Cl, Br, I, CN, NCS) all perform well [12]. For example, the dye [{(4,4’-CO2H)2bipy}2Ru(NCS)2] (N3) and the doubly deprotonated analogue N719 shown in Figure 3.2 give a solar-to-electrical energy conversion efficiency of over 10%. Use of a terpyridyl ligand led to the so-called “black dye” shown in Figure 3.2, which gives a very high IPCE (incident photon to current conversion efficiency) across the wavelength range 400–

700 nm and a cell efficiency of over 10% [13, 14].

39 Figure 3.1: A ruthenium dye reported in Nature (1991) by Dr. O’Regan and Prof. Grätzel [11].

The most efficient DSSCs are based on ruthenium dyes. These were originally produced by the Grätzel group, and are known as N3, N719 and ‘black’ dyes [13, 14]

(Figure 3.2). The metal-ligand charge transfer (MLCT) transition that mediates how the photoelectric charge is injected into TiO2 makes these dyes more durable and superior at harvesting light. In the ruthenium complexes the charge transfer is faster and more effective. This allows electrons to merge with the oxidised dye instead of entering into the circuit to yield energy [1].

Considering that the efficiency of conversion in absorbed atoms is particularly high, most research to date has been focused on enhancing the absorption of incident light by altering the molecular structure of the dye. The main objective of this modification is to increase the extent to which photons are absorbed within a functional wavelength (as measured by the molar extinction coefficient, ε) and/or to increase the functional wavelength in itself near infrared (N-IR) [1, 12, 15].

40 Figure 3.2: The chemical structures of the ruthenium-based dyes N3, N719 and

‘black dye’ developed by the Grätzel group [16, 17].

Among these complexes, polypyridyl ruthenium sensitisers have outstanding redox properties and good responses to natural visible sunlight. Using a semiconductor film electrode with either a carboxylate or a phosphonate group, sensitisers are attached to the surface allowing the injection of electrons into the band of the semiconductor. In the case of the polypyridyl ruthenium dye, the number of metal centres determines the extent of light adsorption of the sensitisers. Whereas carboxylate polypyridyl ruthenium dye and phosphonate ruthenium dye bear similar metal centres, hence the polynuclear bipyridyl ruthenium dye is clearly different. The carboxylate polypyridyl ruthenium dye shows an in level constitution and that allows quantitative electron injection into the conduction band of the semiconductor.

N3 and its tetrabutylammonium salt N719 (also known as black dye) are the most efficient amongst this type of sensitisers. N719 is considered the standard for dye sensitised solar cells, for instance. Recently, other ruthenium-based dyes such as amphiphilic ruthenium (Z907), and high-molar-extinction-coefficient ruthenium sensitisers (K19) have been the focus of extensive research for carboxylate polypyridyl ruthenium sensitisers. Both the molecular configuration and photovoltaic performance of solar cells are shown in Figure 3.3 and Table 3.1 [18 - 31].

41 Figure 3.3: The chemical structures of the several ruthenium dye complexes [14, 18 - 31].

42 Table 3.1: Absorption spectra and photoelectric performance of different polybipyridyl ruthenium (II) complexes.

Dye Abs/nm