3 OVERVIEW OF THE THESIS
SYNTHESIS, SPECTRAL, ELECTROCHEMICAL AND PHOTOVOLTAIC PROPERTIES OF NOVEL HETEROLEPTIC POLYPYRIDYL RUTHENIUM(II) DONOR‐ANTENNA DYES
The focus of this investigation was the design, synthesis and characterisation of a series of novel heteroleptic Ru(II) (4,4´‐dicarboxylic acid‐2,2´‐bipyridine)(bipyridyl donor‐antenna ligand)(NCS)2 complexes. The objective was to elucidate structure‐property relationships between the donor‐
antenna ligands and the photovoltaic performance of the associated Ru(II) donor‐antenna complexes in SDSCs. The molecular structures of the synthesised complexes are depicted in Figure 2.
Ru-NMe2-NCS
Ru-TPA-NCS
After the successful synthesis of the bipyridyl donor‐antenna ligands, the Ru(II) donor‐antenna complexes were obtained in one‐pot reactions under conventional or microwave assisted conditions. The electrochemical behaviour of the donor‐antenna ligands and the Ru(II) complexes was investigated thoroughly by cyclic voltammetry in solution to ascertain the energy levels and to examine the role of the donor‐antenna groups on electron transfer processes. The HOMO levels of the Ru(II) complexes were determined to be ‐5.13 ± 0.05 eV. On the basis of cyclic voltammetry experiments on the donor‐antenna ligands and the commercially available reference dye N719 it could be stated that the HOMO levels of Ru‐tS‐NCS, Ru‐TPA‐NCS and Ru‐
TPA‐EO‐NCS are mainly delocalized over the ruthenium core and certain groups which are equal in all complexes like the NCS groups. In these cases, the second oxidation involves only the donor‐ligands. For Ru‐DTBT‐NCS and Ru‐NMe2‐NCS the HOMO level is assumed to be delocalized
Overview of the Thesis
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over the ruthenium core and the bipyridyl donor antenna moiety. The values of the LUMO energy levels of all donor‐antenna complexes and the reference dye are ‐3.13 ± 0.03 eV. The LUMO is mainly delocalized over the anchoring ligand. In conclusion, the values of the energy level determined for the complexes are ideal for electron injection into the n‐type semiconductor TiO2 and regeneration of the oxidized dye by the solid hole transport material spiro‐OMeTAD.
To elucidate the influence of the donor‐antenna groups on the optical properties of the complexes, steady‐state UV/vis spectra were measured for all complexes. Figure 3 shows these spectra in comparison to the reference dye N719 which is lacking any donor groups.
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0
extinction coefficient [104 M-1 cm-1]
wavelength [nm]
Figure 3. Comparison of the optical properties of a series of Ru(II) donor‐antenna complexes and the complex N719 without donor‐antenna groups. The calculated extinction coefficients of Ru‐DTBT‐NCS (in DMF, blue), Ru‐NMe2‐NCS (in dioxane/H2O/DMF 1:1:1 + 1 wt% KOH, purple), Ru‐tS‐NCS (in DMF, cyan), Ru‐TPA‐NCS (in MeOH + 1 wt% KOH, green), Ru‐TPA‐EO‐NCS (in MeOH + 1 wt% KOH, red) and N719 (in MeOH + 1 wt% KOH, back) are shown as function of the wavelength.
The Ru(II) donor‐antenna complexes and N719 exhibit three absorption bands leading to a broad absorption almost throughout the whole visible region. The maxima arise from ligand‐centred (LC) and MLCT transitions. The high energy transition bands at 305 ± 5 nm were attributed to LC transitions in the anchoring and the donor‐antenna ligand. The second absorption band is determined by two influences, LC π‐π* transitions and MLCT d‐π* transitions. The third low energy band with maxima between 515 and 550 nm is a MTLC transition associated with the introduction of NCS ligands. The most important aspect, however, is that all donor‐antenna complexes provide higher extinction coefficients than the reference dye. This is a direct benefit from the extended delocalized π‐systems of the donor‐antenna ligands.
These superior optical properties make Ru(II) donor‐antenna complexes interesting candidates as sensitizers in SDSCs. The current‐voltage characteristics of the Ru(II) donor‐antenna complexes are depicted in the following figure:
Figure 4. Current‐voltage characteristics of Ru(II) donor‐antenna dyes used as sensitizers in SDSCs in comparison to the performance of the reference dye N719.
It was clearly shown that especially the photocurrent density of the solar cells is strongly dependent on the respective donor group. As a general trend, a significant increase in the photocurrent density was observed with extension of the delocalized system (with exception of Ru‐DTBT‐NCS). The photocurrent density of solar cells sensitized with Ru‐NMe2‐NCS, which is carrying the smallest donor groups, is rather the same as of N719‐sensitized devices (2.15 and 2.21 mA cm‐2 for Ru NMe2‐NCS and N719, respectively). The optimum photocurrent density was achieved with Ru‐TPA‐NCS (4.30 mA cm‐2). This dye features a large delocalized π–system and the highest optical density as adsorbed on mesoporous TiO2. Although Ru‐TPA‐EO‐NCS features the same conjugated system as Ru‐TPA‐NCS, its bulky side chains reduce the optical density as adsorbed on mesoporous TiO2 and thus the current density is lower compared to Ru‐TPA‐NCS.
However, the side chains were introduced with the aim to provide an ion‐coordinating functionality. This causes an increase in the open‐circuit voltage by preventing Li+‐ions from reaching the TiO2 surface, where they are supposed to lower the Fermi‐level of TiO2. For Ru‐TPA‐
EO‐NCS, the gain in voltage surpasses the loss in current density. Therefore, the highest efficiency of 1.37 % was achieved with Ru‐TPA‐EO‐NCS in non‐optimized devices.
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-4 -3 -2 -1 0 1
Ru-DTBT-NCS Ru-NMe2-NCS Ru-tS-NCS Ru-TPA-NCS Ru-TPA-EO-NCS N719 photocurrent density J [mA cm-2 ] photovoltage V [mV]
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