Solvent-Acidity-Driven Change in Photophysics and Significant Efficiency Improvement in Dye-Sensitized Solar Cells of a Benzothiazole-Derived Organic Sensitizer

Abstract

The photophysics and solar cells efficiency of organic sensitizers comprising a cyanoacrylic acid group are greatly influenced by an equilibrium between the neutral ("non-deprotonated", COOH) and anionic ("deprotonated", COO -) forms, whose ratio depends on the solvent polarity and its H-bonding properties, dye concentration and temperature used. Herein, we report a detailed investigation on the relationship between the portions of COOH and COOdye forms and the photophysical and solar cell properties of an organic dipolar sensitizer, BTZA-II, bearing triphenylamine electron-donating and benzothiazole electron-withdrawing moieties. The photophysics has been studied by stationary and time-resolved fluorescence spectroscopy in apolar and polar solvents with a dye concentration ranging from 5×10 -7 M to 5×10 -5 M, also upon addition of small amounts of an external acid or base in order to change the solvent acidity, allowing us to distinguish the contribution and lifetime of the neutral and anionic form. The fluorescence of BTZA-II in apolar toluene originates from the neutral form, which has a lifetime of 1.9 ns. Addition of a strong base (1,8-diazabicyclo[5.4.0]undec-7-ene, DBU) shifts the equilibrium towards the less fluorescent anionic COOform with a lifetime of 1.1 ns. The situation is different in the polar acetonitrile, where the fluorescence of the anionic form dominates (with a lifetime of 2.0 ns). Adding small amounts of acetic acid (AcOH) protonates the COOform of the BTZA-II dye and reveals significant quenching of the fluorescence because of the increased contribution of the neutral species with a lifetime of 0.4-0.5 ns. This quenching of the neutral species in acetonitrile has been also observed in concentrated solutions and is due to excited state proton transfer. By contrast, the photophysics of a dye Btz-NPh2, similar to BTZA-II but without the cyanoacrylic acid group, is not affected upon adding acetic acid (H-donor) or DBU base (H-acceptor), that rules out the Brønsted-base role of the benzothiazole scaffold in the aforementioned observations. Finally, dye-sensitized solar cells (DSSCs) with a solid-state electrolyte were prepared from toluene and toluene+acid solutions. A significant increase of the solar cell efficiency, η, by 58% (η reaching a value of 4.9%) has been achieved after addition of a small, appropriate amount of acetic acid into the initial BTZA-II dye solution.3

Exploring Solvent and Substituent Effects on the Excited State Dynamics and Symmetry Breaking of Quadrupolar Triarylamine End-Capped Benzothiazole Chromophores by Femtosecond Spectroscopy

“…In the present work, we apply two fs spectroscopic techniques, namely FU and fs-TA spectroscopy, to unravel the dynamics and symmetry breaking of three quadrupolar molecules of the type D-π-A-π-D comprising triarylamine end-donors and a functionalized benzothiazole acceptor core (Figure , left). The latter S , N -heteroarene unit is, due to its inherent stability and electron-withdrawing nature, recognized as an efficient building block in chromophores attractive for nonlinear optics, dye-sensitized solar cells, or light emitting materials with tunable fluorescence upon metal-ion complexation. − A systematic change of both the electron accepting core and electron donating edge substituents allowed us to examine the effect of molecular and electronic structure on SB and its dynamics. Specifically, the molecule Qbtz-H bears a benzothiazole electron accepting core and triphenylamine edge donors.…”

Solvent-Dependent Functional Aggregates of Unsymmetrical Squaraine Dyes on TiO2 Surface for Dye-Sensitized Solar Cells

“…Drastic changes in J SC , V OC , and device efficiency have been observed when varying the solvent from dichloromethane and ethanol to N , N -dimethylformamide for triphenylamine-based D-π-A dyes . The BTZA-II dye showed significant changes in J SC and device efficiencies with an increasing concentration of acetic acid in toluene . Furthermore, different sensitizing solvents provide different electronic couplings and interactions between the dye and TiO 2 and change the dye adsorption geometry or dye aggregation on the TiO 2 surface, which influences the device performance of the DSSC. , Aggregated structures have been utilized to enhance emission (aggregation-induced emission), photovoltaic action, and J-type aggregate formation by hydrogen bonding for organic electronics and in cellular membrane imaging. − Further, solvent additives act as morphology-directing agents in organic solar cells .…”

Donor‐Acceptor Based 1,8‐Naphthalimide Substituted Phenothiazines: Tuning of HOMO‐LUMO Gap

“…in donor-acceptor (D-A) chromophores have received substantial attention in the field of nonlinear optics (NLO), organic light-emitting diodes (OLEDs), organic photovoltaics (OPVs), etc. [6][7][8][9] In order to fine tune the HOMO/ LUMO energy levels, D-A based chromophores can be utilized by; (a) changing the donor or acceptor strength or (b) incorporation of different π-linkers. [10][11][12][13][14][15][16][17] Therefore, our group is interested to design and synthesize push-pull based chromophores by modulating the donor and acceptor moieties for the application in optoelectronics.…”

“…The incorporation of heteroatom‐based moieties such as phenothiazine, thiazole, benzothiazole, benzothiadiazole etc . in donor‐acceptor (D‐A) chromophores have received substantial attention in the field of nonlinear optics (NLO), organic light‐emitting diodes (OLEDs), organic photovoltaics (OPVs), etc [6–9] . In order to fine tune the HOMO/LUMO energy levels, D‐A based chromophores can be utilized by; (a) changing the donor or acceptor strength or (b) incorporation of different π‐linkers [10–17] .…”