Synthetic Efforts to Investigate the Effect of Planarizing the Triarylamine Geometry in Dyes for Dye-Sensitized Solar Cells

Abstract

The geometry of a dye for dye-sensitized solar cells (DSSCs) has a major impact on its optical and electronic properties. The dye structure also dictates the packing properties and how well the dye insulates the metal–oxide surface from oxidants in the electrolyte. The aim of this work is to investigate the effect of planarizing the geometry of the common triarylamine donor, frequently used in dyes for DSSC. Five novel dyes were designed and prepared; two employ conventional triarylamine donors with thiophene and furan π-spacers, two dyes have had their donors planarized through one sulfur bridge (making two distinct phenothiazine motifs), and the final dye has been planarized by forming a double phenoxazine. The synthesis of these model dyes proved to be quite challenging, and each required specially designed total syntheses. We demonstrate that the planarization of the triarylamine donor can have different effects. When planarization was achieved by a 3,7-phenothiazine and double phenoxazine structures, improved absorption properties were noted, and a panchromatic absorption was achieved by the latter. However, an incorrect linking of donor and acceptor moieties has the opposite effect. Further, electrochemical impedance spectroscopy revealed clear differences in charge recombination depending on the structure of the dye. A drawback of planarized dyes in relation to DSSC is their low oxidation potentials. The best photovoltaic performance was achieved by 3,7-phenothazine with furan as a π-spacer, which produces a power conversion efficiency of 5.2% ( J sc = 8.8 mA cm –2 , V oc = 838 mV, FF = 0.70).

Enhancement of photovoltaic performance in ferrocenyl π-extended multi donor–π–acceptor (D–D′–π–A) dyes using chenodeoxycholic acid as a dye co-adsorbent for dye sensitized solar cells

“…The ferrocenyl multi-donor coupled with strong acceptors like cyanoacrylic acid and 4-(cyanomethyl) benzoic acid, which can be used as sensitizers in DSSCs and their electron withdrawing effects have been utilized to develop charge-transfer process. [33][34][35] In this expedition, we have synthesized new D-p-A type of ferrocenyl methoxyphenylsubstituted acids and tested their ability for sensitizers in DSSCs. In addition, we have done the theoretical calculations using density functional theory (DFT/TD-DFT) using B3LYP/6-31+G(d,p)/LanL2TZf level of theory and these results were correlated with experimental calculations.…”

Aniline and Indoline Donors Based Far‐Red Active Unsymmetrical Squaraine Dyes for Dye Sensitized Solar Cells

“…SQA1 , SQA2 and SQA3 dyes showed the absorption maxima at 629 nm, 641 nm and 654 nm, respectively. The λ max for SQA2 dye red‐shifted with 12 nm compared to SQA1 dye, due to restricted bond rotation and structural rigidification [27,51–53] . Further, introduction of alkyl groups for SQA3 dye showed 13 nm red‐shifts compared to methylated SQA2 dye, which may be due to the solvent‐solute interaction [54–55] .…”

“…The λ max for SQA2 dye red-shifted with 12 nm compared to SQA1 dye, due to restricted bond rotation and structural rigidification. [27,[51][52][53] Further, introduction of alkyl groups for SQA3 dye showed 13 nm red-shifts compared to methylated SQA2 dye, which may be due to the solvent-solute interaction. [54][55] The molar extinction coefficient of SQA dyes showed within the range of 1.49-0.94×10 5 M À 1 cm À 1 (Table 1 and Figure S17).…”

“…[49] As most of the organic sensitizers possess πconjugated framework, design principles that reduce the number of single bonds in the sensitizer for reducing the nonradiative process upon photoexcitation, [50] and including out-ofplane alkyl groups for controlling the aggregation of dyes on TiO 2 are required. In the present work, a series of D-A-D based unsymmetrical dyes have been designed and synthesized, where semisquaric acid condensed with N,N-dimethyl aniline and simple and branched-indoline (i) to compare and study the effect of cyclization, which rigidify the dye structure, [51][52][53] on the photophysical, electrochemical, and photovoltaic properties (ii) to study the effect of introducing branched alkyl groups on the photovoltaic properties (Figure 1). As the effect of number carbon atoms in the branched chains of the D-A-D based squaraine dyes, [27] as well as the solvent utilized for the sensitization process modulates the DSSC device performance, [37] the dye designed SQA3 possess an octyl and hexyl groups (moderate number of carbon atoms) which helps the dye solubility in MeCN.…”

Steric and Electronic Effect in Unsymmetrical Squaraine Dyes for Dye-Sensitized Solar Cells

“…To get insights into molecular structures and distribution of electron/electron density in the HOMO, HOMO – 1, LUMO, and LUMO + 1 electronic energy level of SQ-X dyes, DFT calculations were carried out at the B3LYP/6-311G++ level (Figure ). Structural orientation, planarity, and electronic distribution within the molecular orbitals are very important parameters for the DSSC applications. , The electronic distributions within the HOMO and LUMO molecular orbitals were observed to be uniform for SQ-X donor, acceptor, and donor moieties whether the donor was connected with electron-donating or electron-withdrawing groups. In the HOMO energy level of SQ-X dyes, the electron density was located on the donor group (indoline) side, whereas the density of electrons was observed mostly toward the anchoring group side, which favors electron injection processes to the TiO 2 conduction band from the LUMO of the dyes upon photoexcitation.…”