Influence of the Nature of the Absorption Band on the Potential Performance of High Molar Extinction Coefficient Ruthenium(II) Polypyridinic Complexes As Dyes for Sensitized Solar Cells

Abstract

When tested in solar cells, ruthenium polypyridinic dyes with extended π systems show an enhanced light-harvesting capacity that is not necessarily reflected by a high (collected electrons)/(absorbed photons) ratio. Provided that metal-to-ligand charge transfer bands, MLCT, are more effective, due to their directionality, than intraligand (IL) π-π* bands for the electron injection process in the solar cell, it seems important to explore and clarify the nature of the absorption bands present in these types of dyes. This article aims to elucidate if all the absorbed photons of these dyes are potentially useful in the generation of electric current. In other words, their potentiality as dyes must also be analyzed from the point of view of their contribution to the generation of excited states potentially useful for direct injection. Focusing on the assignment of the absorption bands and the nature of the emitting state, a systematic study for a series of ruthenium complexes with 4,4'-distyryl-2,2'-dipyridine (LH) and 4,4'-bis[p-(dimethylamino)-α-styryl]-2,2'-bipyridine (LNMe(2)) "chromophoric" ligands was undertaken. The observed experimental results were complemented with TDDFT calculations to elucidate the nature of the absorption bands, and a theoretical model was proposed to predict the available energy that could be injected from a singlet or a triplet excited state. For the series studied, the results indicate that the percentage of MLCT character to the anchored ligand for the lower energy absorption band follows the order [Ru(deebpy)(2)(LNMe(2))](PF(6))(2) > [Ru(deebpy)(2)(LH)](PF(6))(2) > [Ru(deebpy)(LH)(2)](PF(6))(2), where deebpy is 4,4'-bis(ethoxycarbonyl)-2,2'-bipyridine, predicting that, at least from this point of view, their efficiency as dyes should follow the same trend.

Effect of free rotation in polypyridinic ligands of Ru(ii) complexes applied in light-emitting electrochemical cells

“…32 As has already been described for these types of complexes, the HOMO is expected to be mainly concentrated on the metal (t 2g orbitals), while the LUMO is expected to reside on the ligands. 33 Compared to the oxidation potential measured for the [Ru( phen) 3 ] 2+ and [Ru( phen) 2 Lhydro] 2+ complexes, which present similar values (+1.30 and +1.26 V, respectively), the [Ru( phen) 2 LH] 2+ complex shows a less positive oxidation potential (+1.17 V). This finding suggests a destabilization of the HOMO for the [Ru( phen) 2 LH] 2+ complex due to the higher conjugation present in the LH ligand that affects the charge density of the metal center and facilitates the oxidation.…”

Interaction of LD14 and TiO2 in dye-sensitized solar-cells (DSSC): A density functional theory study

“…ruthenium and zinc) on nanocrystalline titanium dioxide films [4,[7][8][9][10]. Among the studied compounds, ruthenium(II)bipyridyl (N3, N719 and N749) complexes have proved to be the more efficient TiO 2 sensitizers [7][8][9][10][11][12][13][14]. This efficiency is measured by the power conversion index (η) and Grätzel cells based on the sensitization of TiO 2 by molecular dyes have a η among 7-11%.…”

“…In several studies it has been mentioned that the use of supramolecular interactions in solar cells is scarce [25][26][27][28]. Solar energy conversion efficiency (η) in photoelectrochemical cells is a function of several factors such as: the collection of electrons by the external circuit, the electron injection quantum yield (i.e., the ratio of injected electrons to absorbed photons), and the absorbance (the ratio of absorbed photons to incident photons) [10,11]. Dyes that support efficient intramolecular energy transfer (i.e., the antenna effect) can be designed to display broader absorption bands and/or absorption bands with high extinction molar coefficients [15][16][17][18][19][20][21].…”

Interaction of YD2 and TiO2 in dye-sensitized solar cells (DSSCs): a density functional theory study

“…Grätzel's DSSCs are based on the adsorption of metallic complexes (e.g., ruthenium and zinc), which are usually termed "dyes" in this context, on nanocrystalline titanium dioxide films [9,[12][13][14][15][16][17][18][19]. Among the dye compounds studied, ruthenium(II) bipyridyl complexes have proven to be the most efficient TiO 2 sensitizers [13][14][15][16][17].…”

“…Among the dye compounds studied, ruthenium(II) bipyridyl complexes have proven to be the most efficient TiO 2 sensitizers [13][14][15][16][17]. The efficiency of a solar cell is measured by its power conversion index (η).…”