Design and Synthesis of Novel NIR Dyes with Phosphonic Acid Derivative Anchoring Groups Aiming Towards Stable Dye-Sensitized Solar Cells

Abstract

Introduction: The solar energy is a promising solution to the sustainable energy supply and the relevant environmental concerns. Production of the traditional solar cells is expensive and this poses difficulty for the mass-level implementation of this kind of solar cells. So, there has been a lot of effort and interest to propose cost-effective and efficient solar cells, where dye-sensitized solar cells (DSSCs) are one of the potential candidates. DSSCs have emerged due to their good photo conversion efficiency (PCE) beyond 12 % in spite of photon harvesting window mainly in the visible region (400-700 nm). Dyes are the “Heart” of the solar cells and anchoring group present in it not only controls the overall PCE but also stability of photo anode, which ultimately controls the stability of DSSCs. Amongst various anchoring groups, phosphonic acid has been widely reported to give improved stability to the DSSCs as compared to their most commonly used carboxylic acid anchoring group counterparts. Owing to huge possibilities of molecular structures, quantum chemical (QC) calculations have emerged as one of the potential tools for the prediction of their energetics and spectral behavior. The goal of this research is to propose and develop of novel near infrared (NIR) dyes bearing phosphonic acid groups having the capability of improved stability as well as PCE focusing on Donor-Acceptor-Donor framework bearing squaraine dyes. Experimental: In this work, a series of unsymmetrical squaraine (SQ) dyes such as SQ-143, SQ-148, SQ-151, SQ-157, SQ-159 bearing different phosphonic acid anchoring groups with the molecular structure as shown in the Fig. 1 were logically designed and subjected to QC calculations using 6-311G/DFT/B3PW91 and PCM model using ethanol solvent to judge their suitability as sensitizer before the synthesis. Some of the suitable dyes having favorable energetics have been synthesized by multistep synthetic route. Structural characterization and confirmation have been done by 1H NMR and Fab Mass techniques followed by their Photophysical characterizations in 5 µM ethanolic solution for electronic absorption spectral behavior. Adsorption behavior of these dyes on the mesoporous TiO2 has been investigated by adsorbing the respective dyes from their 0.1 mM solution in ethanol. Photovoltaic characterization was also performed after fabricating DSSCs based on 10-12 µm thick mesoporous TiO2 with adsorbed dyes from their 0.2 mM solution as photo anode, catalytic amount of Pt coated conductive glass as counter electrode and I-/I3 - as redox electrolyte. Results and Discussion: After the successful synthesis and structural characterizations, SQ dyes under investigation were subjected to their photophysical characterizations. It can be clearly seen from the solution as well as solid-state absorption spectra shown in the Fig. 1 that due to extended p-conjugation in SQ-148 and SQ-157, exhibit bathochromically shifted absorption maxima as compared to their SQ-143 and SQ-151 dye counterparts. At the same time, in spite of having two free –OH groups in the dyes SQ-148 and SQ-143, former exhibited less extent of the dye aggregation. A swift dye adsorption in combination with reduced aggregated species are highly desired for commercial applications. Investigation pertaining to the dye adsorption behavior on the TiO2 reveals the trend of SQ-143>SQ-148>SQ-157>SQ-151 suggesting that less aggregation tends to the relatively higher rate of dye adsorption. This can be attributed to the swift diffusion of the dye molecules in the nanopores of the mesoporous TiO2 during the dye adsorption process. Stability of the adsorbed dyes on the mesoporous TiO2 is another important parameter responsible to impart the stability to the DSSCs. This was conducted by investigating the dye binding strength based on dye desorption studies. Apart from their synthesis and photophysical characterizations, dyes were also used as sensitizer to fabricate the DSSCs and implication of the nature anchoring groups on photon harvesting and overall PCE was also investigated. Results on the photovoltaic performance revealed that newly designed dye SQ-159 makes a good compromise imparting improved stability as well as PCE amongst the dyes under investigation. Acknowledgement: SSP would like to express sincere thanks to the Japan Society for Promotion of Science for the financial support by Grant-in-Aid for scientific research (C) [Grant No.-18K05300] to carry out the research Figure 1