Synthesis, characterization and introduction of a new ion-coordinating ruthenium sensitizer dye in quasi-solid state TiO2 solar cells

https://doi.org/10.1016/j.jphotochem.2011.05.020Get rights and content

Abstract

A new ion-coordinating heteroleptic ruthenium(II) dye was synthesized by attaching two crown ether moieties in the 4,4′ positions of one of the bipyridine ligands. This new dye (named as RC730) was characterized by UV-Vis spectroscopy, CHN elemental analysis, NMR and electrospray ionization Fourier transform-ion cyclotron resonance mass spectrometry (ESI FT-ICR MS). In order to investigate the properties of these ion-coordinating species, dye-sensitized solar cells were assembled with a gel polymer electrolyte based on two different cations: lithium and sodium. The devices were characterized by JV curves under 100 mW cm−2, incident photon to current conversion efficiency spectra (IPCE) and photovoltage decay transients under open-circuit conditions. The solar cells based on the new heteroleptic dye provided higher photocurrent and photovoltage when lithium was used in the electrolyte instead of sodium cations, reaching overall conversion efficiencies up to 2%. This behavior might be related to the ability of the ion-coordinating RC730 dye to trap Li ions, minimizing the conduction band edge shift. When the polymer electrolyte based on lithium is used, the IPCE spectra show a maximum efficiency of 31% at the maximum absorption peak of the RC730 dye (ca. 530 nm).

Highlights

► A new ion-coordinating heteroleptic ruthenium(II) dye was synthesized by attaching two crown ether moieties in the 4,4′ positions of one of the bipyridine ligands. ► In order to investigate the influence of these ion-coordinating species, dye-sensitized solar cells were assembled with a gel polymer electrolyte based on two different cations: lithium or sodium. ► The solar cells based on the new heteroleptic dye provided higher photocurrent and photovoltage when lithium was used in the electrolyte instead of sodium cations, reaching overall conversion efficiencies up to 2%.

Introduction

Since their inception 19 years ago, dye-sensitized solar cells (DSCs) have emerged as a viable alternative to inorganic silicon-based solar cells [1]. Their low cost of production and high efficiency of energy conversion, recently reaching more than 11% [2], make such devices promising alternatives for the generation of new, inexpensive solar cells [3].

DSCs are made up of two major chemical components: a sensitizer dye (responsible for light absorption and generation of charge carriers) and an electrolyte that serves as a shuttle between the two electrodes and is responsible for dye regeneration. To date, highly absorptive, relatively stable and well-tested dyes consist of ruthenium polypyridyl complexes anchored to a porous metal oxide electrode such as TiO2 nanoparticulated films [4]. The most efficient electrolytes are liquid substances that allow for the rapid diffusion of ions from the counter-electrode to the oxidized dye complex [5], [6]. Whereas liquids are the optimum medium to facilitate ion flow, there are some inherent drawbacks when incorporating them into DSCs. For instance, electrolyte leakage, evaporation, corrosion and difficulty of large scale production all cause substantial problems when bringing DSCs to market [5]. Many efforts have been made to overcome these challenges, for example, by replacing the liquid electrolyte by room temperature ionic liquids [7], [8], hole transport materials [9], [10], [11], [12] or polymer and gel electrolytes [13], [14], [15], [16], [17], [18], [19], [20].

Over the past 14 years, our laboratory has focused on novel types of electrolytes for DSCs in the form of solid-state polymer electrolytes based on copolymers of poly(ethylene oxide) (PEO) [19], [20], [21] and two recent reviews on these efforts have appeared [19], [21]. Although the quasi-solid nature of this polymeric material is a great advantage because it reduces the problems mentioned above, it also has the effect of slowing down ionic diffusion, thereby reducing the efficiency of the solar cell. With the intent of improving ionic mobility, we have recently incorporated lithium coordinating 12-crown-4 ether into our gel electrolyte formulation [22]. The results showed an overall increase in solar cell efficiency due to two factors: (a) an increased open-circuit voltage (Voc) due to the coordination of Li+ ions by the crown ether [23] and (b) effective shielding from the porous metal oxide layer. The other benefit was increased short-circuit current (Isc) as a consequence of decreased Li+ mobility, whereas increased iodide mobility was observed [22], [24]. To further analyze the effect of the crown-ether moiety within the solar cell, we have designed a novel sensitizer complex with two crown-ethers covalently linked to the ruthenium complex itself.

The modification of the ruthenium complex discussed in this work was inspired by Grätzel and coworkers [25], who to improve solar cell efficiencies, have covalently modified ruthenium sensitizer dyes with different organic groups. One particular dye of note is the K51 dye in which the triethylene oxide methyl ether (TEOME) group was introduced on the 4,4′ position of a 2,2′-bipyridine ligand with ion-coordinating properties to capture the Li+ ions [25]. Devices tested with K51 and a solid-state electrolyte showed a significant increase in the open-circuit voltage, when Li+ was added into the electrolyte. Unfortunately, ethylene oxide chain ion-coordinating arm of the K51 dye increased its instability during accelerated aging tests due to the desorption of the bipyridine ligands. This effect was related to the hydrophilic nature of the ligand. To improve the stability of the DSCs using an ion-coordinating sensitizer, Grätzel and coworkers have also developed a more hydrophobic K68 dye [26]. This dye exhibited ion-coordinating properties, due to the presence of TEOME substituents, and also high hydrophobicity due to presence of heptyl chains at the end of the alkoxy chains. Therefore, the K68 dye presented more stability and less aggregation than K51. Fig. 1 shows the molecular structure of the heteroleptic RC730 dye. The molecular structures of the sensitizers K51 and N719 are also shown for comparison.

As may be seen in Fig. 1, RC730 dye has a cyclic ethylene oxide chain opposed to the open-chain ethylene oxide moieties in the K51 dye. Therefore, in this work we present the synthesis, characterization and introduction of a new ion-coordinating ruthenium sensitizer (named as RC730, Fig. 1) in quasi-solid state solar cells assembled with gel polymer electrolyte based on poly(ethylene oxide-co-2-(2-methoxyethoxy) ethyl glycidylether) (P(EO/EM), γ-butyrolactone (GBL), iodine and two different salts (LiI or NaI).

Section snippets

RC730 synthesis

Synthesis of the 4,4′-bis(12-crown-4 methylether)-2,2′ bipyridine ligand (1).

A THF solution (3 mL) of 2-hydroxymethyl 12-crown-4 ether (0.160 g, 0.78 mmol) was combined with NaH (20.5 mg, 0.85 mmol) and stirred under argon for 1 h. A THF solution (2 mL) of 4,4′-bis(chloromethyl)-2,2′-bipyridine, Cl2Bpy, prepared as described previously [27] (98.2 mg, 0.38 mmol) was then added and the mixture was stirred for 4 h at room temperature, until thin layer chromatography (TLC) showed total consumption of the

ESI FT-ICR MS

Fig. 2 shows the ESI(−) FT-ICR mass spectrum for a methanolic solution of the RC730 ruthenium dye. Note the detection of the dye as the [M+TBA] anion of m/z 1294.4 (whereas M2− is the doubly charged Ru-anion and TBA+ is the counter cation) with the characteristic cluster of isotopologue ions. The experimental isotopologue pattern of the dye is quite diverse mainly due to the Ru, and closely matches the theoretical pattern as calculated by the Xcalibur 2.0 software (insets in Fig. 2). This

Conclusion

A new heteroleptic Ru dye (RC730) containing crown-ether moities on 4,4′ positions of the bipyridine ligand was successfully synthesized, as confirmed by ESI(−) FT-ICR MS analysis. DSCs were assembled using two different types of gel electrolytes (with NaI or LiI) and the N719 dye was used for comparison. The higher photocurrent density values observed for DSCs based on the N719, dye compared to the new heteroleptic Ru dye prepared in this work, can be explained by a different dye structure

Acknowledgements

The authors acknowledge FAPESP (fellowships 06/58998-3 and 08/51001-9), Renami, CNPq and National Science Foundation (USA) for financial support, Daiso Co. Ltd., Osaka (Japan) for kindly providing the copolymer and Prof. Carol H. Collins for English revision. We also acknowledge the LMF/LNLS (National Synchrotron Light Laboratory, Campinas, SP, Brazil) for providing the platinized counter-electrodes

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