Selenorhodamine Dye-Sensitized Solar Cells: Influence of Structure and Surface-Anchoring Mode on Aggregation, Persistence, and Photoelectrochemical Performance

Abstract

A library of six selenorhodamine dyes (4-Se-9-Se) were synthesized, characterized, and evaluated as photosensitizers of TiO2 in dye-sensitized solar cells (DSSCs). The dyes were constructed around either a bis(julolidyl)- or bis(half-julolidyl)-modified selenoxanthylium core functionalized at the 9-position with a thienyl group bearing a carboxylic, hydroxamic, or phosphonic acid for attachment to TiO2. Absorption bands of solvated dyes 4-Se-9-Se were red-shifted relative to the dimethylamino analogues. The dyes adsorbed to TiO2 as mixtures of monomeric and H-aggregated dyes, which exhibited broadened absorption spectra and increased light-harvesting efficiencies relative to the solvated monomeric dyes. Carboxylic acid-bearing dyes 4-Se and 7-Se initially exhibited the highest incident photon-to-current efficiencies (IPCEs) of 65-80% under monochromatic illumination, but the dyes desorbed rapidly from TiO2 into solutions of HCl (0.1 M) in a CH3CN:H2O mixed solvent (120:1 v:v). The hydroxamic acid- and phosphonic acid-bearing dyes 5-Se, 6-Se, 8-Se, and 9-Se exhibited lower IPCEs (49-65%) immediately after preparation of DSSCs; however, the dyes were vastly more inert on TiO2, and IPCEs decreased only minimally with successive measurements under constant illumination. Power-conversion efficiencies (PCEs) of the selenorhodamine-derived DSSCs were less than 1%, probably due to inefficient regeneration of the dyes following electron injection. For a given anchoring group, the bis(half-julolidyl) dyes exhibited higher open-circuit photovoltages and PCEs than the corresponding bis(julolidyl) dyes. The hydroxamic acid- and phosphonic acid-bearing dyes are intriguing photosensitizers of TiO2 in light of their aggregation-induced spectral broadening, high monochromatic IPCEs, and relative inertness to desorption into acidic media.

Dye aggregation in dye-sensitized solar cells

“…89 Selenorhodamine dyes with carboxylic (46), hydroxamic (47), or phosphonic acid (48) anchoring groups exhibit different aggregation modes, which is reflected in different absorption bands in their UV/vis absorption spectra. 90 Considering the chemical nature of anchoring groups, first-principles 49…”

Excited-State Planarization in Donor–Bridge Dye Sensitizers: Phenylene versus Thiophene Bridges

“…The resurgence of interest in rhodamines stems in part from their recent application as effective sensitizers in the photogeneration of hydrogen − and dye-sensitized solar cells (DSSCs). − Because of their large molar absorptivities (ε ≈ 100 000 M –1 cm –1 ) and photostability, rhodamines have the potential to serve as robust and efficient light harvesters in solar energy applications. The rapidly increasing demand for clean-burning, renewable fuels requires that systems utilizing molecular sensitizers for solar light harvesting be optimized at a rapid pace to stay competitive with other contemporary alternative energy schemes.…”

Cause, Regulation and Utilization of Dye Aggregation in Dye-Sensitized Solar Cells

“…However, the effect of anchoring groups on dye aggregation haven’t been extensively investigated, and most of them focus on theoretical research. The anchoring group mainly effects the molecular space stretching ( Figure 4 ) and anchoring mode (adsorption site [ 112 ] and orientation [ 113 ], etc), and the intermolecular interaction between the anchoring groups have a direct influence aggregation [ 114 , 115 ].…”

“…Above all, the vast majority of studies focus on suppressing dye aggregation because the stronger intermolecular interactions within the compact dye layer leads to excited-state quenching, which will be detrimental to the electron injection efficiency. But the following points are worth noting: (1) The aggregation of dye molecules enables them to be arranged in order on the surface of TiO 2 , which facilitates efficient charge injection from aggregates [ 114 , 164 ] and causes higher absorption amount on TiO 2 [ 165 ]; (2) Dye aggregation can effectively reduce the bare surface of TiO 2 , which will help with blockading the redox mediator from infiltrating into the TiO 2 surface, and thus reduce electron recombination [ 166 ]; (3) Dye aggregation can broaden the absorption spectrum and enhance the ability of light-harvesting [ 167 , 168 , 169 ], especially J aggregation [ 64 ]. Although there is a competitive relationship between the advantages and disadvantages of dye aggregation, the prospect of using the advantages of dye aggregation to improve the overall performance of DSSC is considerable.…”