Combined Theoretical & Experimental Approaches for Molecular Design of NIR Dyes for Dye-Sensitized Solar Cells Aiming Towards Improved Efficiency and Stability

Abstract

Introduction: Since its inception and first report in 1991, dye sensitized solar cells (DSSCs) have got great attentions towards the realization of lost cost solar energy harvesting owing to easy fabrication, lower raw materials and aesthetic beauty. In the recent past, DSSCs came in to the lime light owing to their potential applications for efficient room light harvesting due to very good performance under low light intensity and highly suitable for building integrated photovoltaics due to color tunability in combination of the controllable transparency. State-of-art DSSCs exhibiting power conversion efficiency (PCE) >13% in spite efficient photon harvesting only in the visible wavelength region give a good hope for the further enhancement in the PCE by development and implementation of novel potential dyes having sensitivity near-infrared (NIR) wavelength region utilizing the existing state-of-art DSSC fabrication technologies developed in last two decades. Apart from enhancing the PCE, increasing the stability is inevitable for the commercialization of DSSCs and nature of the anchoring group of the sensitizers are expected to play a vital role by controlling efficiency of charge injection and binding strength on the mesoporous wide band gap semiconductor like TiO2. A series squaraine dyes (SQ) having same p-conjugated mother core but varying anchoring group has been designed and subjected to QC calculations in both of the ground and excited states using Gaussian program. In parallel, some of the potential sensitizers have also been synthesized and subjected to photo-physical investigation aiming towards the verification of the theoretically predicted results. Experimental: In this work, a series unsymmetrical squaraine dyes (SQ) bearing different anchoring groups (SQ-138-SQ-156) as shown in the Fig. 1 were logically designed and subjected to QC calculations. In order to construct theoretical energy band diagram, HOMO energy level was calculated after structural optimization using 6-311G/DFT/B3PW91 and PCM model using Ethanol solvent. Optimized structures were then used for TD/DFT excited state absorption spectral calculations and wavelength associated with the full width at half maximum of the spectrum was considered to be Eg followed by calculation of energy of LUMO by the relation LUMO = HOMO + Eg. Some of the designed dyes bearing different anchoring groups were synthesized, characterized and subjected to photo-physical investigation including the DSSC fabrication and characterizations. Results and Discussion: A perusal of the Fig. 1, clearly corroborates that except two dyes that is SQ-141 and SQ-146 bearing hydroxyl and catechol anchoring groups, all of the designed dyes seems to suitable as sensitizers for the fabrication DSSCs using TiO2 as mesoporous electron accepting scaffold and I-/I3 – redox electrolyte. In parallel to theoretical QC calculations, some of the potential dyes were also synthesized and subjected to their photo-physical characterization before their utilization for DSSC fabrication. Photo-physical results for some of the experimentally synthesized dyes indicate that there was only a difference of about 60-70 nm (0.06 eV) in λmax and (±0.06 eV) in Eg between theoretical prediction and experiment results. At the same time, experimental energy band diagram for four of the synthesized dyes as shown in the inset of Fig. 1, exhibits excellent match with the theoretical results. Rate of dye adsorption and their binding strength on the mesoporous TiO2 for some of the synthesized dyes was also investigated. It has been found that dye with Phosphonic acid anchoring group although exhibit nearly rate of dye adsorption as compared to that of one bearing carboxylic acid anchoring group but depicted >60 times higher binding strength. On contrary, it exhibited drastically hampered PCE than that of dye bearing –COOH, which has been explained by poor electron injection from the excited dye molecule to the conduction band of TiO2 owing to highly diminished intramolecular charge transfer and electronic coupling with d-orbitals of the TiO2 as indicated by QC calculations. One of the newly synthesized dye SQ-140 bearing cyanoacrylic acid anchoring, exhibited not only very good photo-voltaic performance with PCE of about 6 % after simulated solar irradiation but also NIR photosensitization having photon harvesting mainly between the 500 nm-800 nm with peak of the incident photon to current conversion efficiency of nearly 80 % at 700 nm. Acknowledgement: SSP would like to express sincere thank JSPS to 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