Structure-efficiency relationship of newly synthesized 4-substituted donor–π–acceptor coumarins for dye-sensitized solar cells

Abstract

Four (MC1–MC4) 4-substituted coumarin dyes having N,N-diethyl aniline as donor and rhodanine-3-acetic acid as acceptor have been synthesized for use in nano-crystalline TiO₂-based dye-sensitized solar cells (DSSCs). The alteration at the 4-position of coumarin was carried out by substituting H atom by chlorine (–Cl), piperidine (–NC₅H₁₀), and cyano group (–CN) to study the effect of substitution on the efficiency of the DSSCs. All the dyes were characterised by ¹H-NMR and ¹³C-NMR spectroscopy and CHN analysis. The absorption and emission spectra of these coumarin dyes were recorded in eight different solvents and show positive solvatochromism. The TiO₂-based DSSCs were fabricated using the four dyes and their photovoltaic properties were estimated. A maximum efficiency of 4.60% was obtained for the MC4 device, which has an electron-withdrawing cyano group at the 4-position of coumarin, while 2.64% efficiency was obtained for the MC1 device, which has only hydrogen at the 4-position. The efficiencies of the MC2 and MC3 devices lie between those of MC1 and MC4, suggesting the significance of substitution at the 4-position. The introduction of a cyano group, which acts as an electron trap, increases the electron-withdrawing capacity of the dye; thus, more electron density is withdrawn from the donor, resulting in enhanced efficiency. These dyes were studied by using density functional theory (DFT) and time dependent density functional theory (TD-DFT) to obtain the vertical excitation, HOMO–LUMO energy levels, band gap and electron distribution in the ground and excited state. The calculated values showed good correlation with the experimental values. Our results suggest that substitution at the 4-position is essential for enhancing the efficiency of coumarin-based DSSCs.