Significance of anchoring group design on light harvesting efficiency of dye-sensitized solar cells and non-linear optical response

Abstract

Dye-sensitizers designed from coumarin backbone are widely utilized in solar light harvesting due to their versatile performance in dye-sensitized solar cells (DSSC). They are well-known as non-linear optical (NLO) materials. In this case study, we have designed coumarin-based sensitizers containing thiophene as a common spacer with the different acceptor-anchoring groups, ethyl-2-cyanoacrylate (SPDSSC 1) and 2-cyanoacrylic acid (SPDSSC 2). Density functional theory (DFT) and time-dependent- DFT (TD-DFT) computational calculations of these molecules predict superior light harvesting capability and NLO properties. The calculated HOMO–LUMO energy gap of these dyes showed an inverse relation with chemical potential and electrophilicity index and was directly related to hardness and hyperhardness. It was also seen β0 and γ and PCE follow a similar trend. Further, computations of sensitizers@TiO2 indicated sensitizers with ethyl-2-cyanoacrylate bonded with TiO2 cluster via unidentate linkage, whereas 2-cyanoacrylic acid containing sensitizer involved bidentate bonding with TiO2. Apart from this, sensitizers with 2-cyanoacrylic acid anchors showed much planer geometry. Also, the quantitative analysis of the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) and MEP plots showed that more electron clouds shifted to the TiO2 cluster in the sensitizer with 2-cyanoacrylic acid (SPDSSC 2) than ethyl-2-cyanoacrylate (SPDSSC 1). Further, both molecules were taken as sensitizers in the DSSC device. SPDSSC 2 showed higher power conversion efficiency than SPDSSC 1. Overall, results disclosed that sensitizers with ethyl-2-cyanoacrylate having two oxygen atoms in their anchoring part could bind with only carbonyl oxygen. And this monodentate binding of ethyl-2-cyanoacrylate with TiO2 leads to poor charge transfer in the semiconductor's conduction band, resulting in poor DSSC performance.