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

Abstract

Functionalizing the light harvesting sensitizers with additional electron-donating or -withdrawing groups is a potential approach to modulate the photophysical and electrochemical properties which in-turn optimizes the driving force associated with the charge injection and dye-regeneration processes at the dye-TiO 2 / electrolyte interface and the photovoltaic device performance in dyesensitized solar cells (DSSCs). Furthermore, modulated electronic levels of the dyes provide an opportunity to reduce the overpotential associated with the dye-regeneration process and make the dye-TiO 2 interface compatible with various electrolytes. Furthermore, an in-built steric feature by means of introducing linear/branched alkyl groups in the sensitizer is important in controlling the aggregation of dyes on the TiO 2 surface. Hence, to integrate both steric and electronic properties, a series of alkyl group-wrapped unsymmetrical squaraine dyes (SQ-X) with electron-donating and -withdrawing groups have been designed, synthesized, and utilized for DSSC device fabrication. These dyes are functionalized with alkyl groups at both sp 3 -C and N-atoms of the indoline donor moiety at the nonanchoring side to have a similar steric feature. Photophysical and electrochemical studies revealed that the HOMO and LUMO energy levels of the SQ-X series of dyes have been modulated systematically with sufficient driving forces for both charge injection and dye-regeneration processes with iodolyte (I − /I 3 − ) electrolyte. In the presence of electron-donating groups in SQ-X (where X = −NPh 2 and −OMe), the HOMO energy levels are less positive than SQ-H, whereas the presence of electron-withdrawing groups such as −CO 2 Me, − CN, and −NO 2 pushed the HOMO energy levels toward more positive potentials. Enhanced photovoltaic performances have been obtained for the dyes containing electron-donating groups, where the dye with the −NPh 2 group showed a maximum of η 7.03% (V OC 708 mV, J SC 13.16 mA cm −2 , and ff 78%). The dye with the strong electron-withdrawing group −NO 2 showed an efficiency of 1.49% (V OC = 634 mV, J SC = 3.13 mA cm −2 , and ff 75%). As the dyes with the electron-withdrawing group possess deep positive HOMO energy levels, the DSSC device characterization has been investigated with the Cu +/2+ redox shuttle. The reduced device performance of electron-withdrawing-group-containing dyes is due to the unfavored charge distribution in the LUMO compared to the presence of electron-donating-group-containing dyes, and it was supported by the difference in the charge injection efficiency.

Extracts from Capsicum annuum as Natural sensitizer in Dye Sensitized Solar Cell : Theoretical analysis using DFT and TD-DFT

“…Thus, we propose to these molecules as promise natural dye and thus a computational study about photoexcitation and electron injection will be fundamental to understand the role of carotenoids as sensitizing on TiO 2 . [42,43]…”

Visible-Light-Active Unsymmetrical Squaraine Dyes with Pyridyl Anchoring Groups for Dye-Sensitized Solar Cells

“…However, to control the aggregation of dyes by anchoring group engineering, a set of selenorhodamine dyes have been developed, and they showed that the H-aggregated dyes can participate in photocurrent generation . Furthermore, including alkyl groups and electron-donating groups in squaraine dye structures helps minimize the aggregation of sensitizers on TiO 2 and modulates the electronic properties of the dyes for enhanced device performance. , We designed and synthesized two novel pyridine-based dyes SD1 and SD2 and a carboxylic acid-based visible-light-active dye SD3 , in which pyridine binds mostly to the Lewis acid sites of TiO 2 while the carboxylic acid binds to the Brønsted acid sites of TiO 2 . SD1 and SD2 dyes having pyridyl anchoring groups were used to study their cosensitization using a blue dye, that is SD4 , having a carboxylic acid anchoring group (Figure ).…”

“…Dyes SD1 or SD2 were cosensitized with SD4 as the absorption spectra complemented each other ( SD1 and SD2 absorb in the visible region, whereas SD4 absorbs in the far-red region) (Figure ). The dye SD4 possesses indoline donor groups with long alkyl chains, which have been proven to be useful in iodine-based electrolyte systems, particularly in reducing the charge recombination process in a DSSC device …”