Tuning the donating strength of dye sensitizers using molecular electrostatic potential analysis

Abstract

Donor–π–acceptor (D–π–A) systems typically used in dye-sensitized solar cells (DSSC) have been studied for assessing the donating strength of six donors (D1–D6) under the influence of substituents such as CH₃, C₅H₁₁, isopropyl, t-butyl, OH, OCH₃, OC₂H₅, NH₂, N(CH₃)₂, PhCH₃, and PhNH₂ along with π-spacer butadiene and acceptor moiety cyanoacrylic acid. The substituent effect enhances electron donation from D to A through the π-spacer. The enhancement in electron density at A has been quantified in terms of the difference in the molecular electrostatic potential (MESP) minimum at the cyano nitrogen (ΔV_mA) between π–A and D–π–A. For unsubstituted D–π–A systems, ΔV_mA is in the range −0.1 to −5.7 kcal mol⁻¹, whereas the substitution enhances the negative character of ΔV_mA in the range −0.8 to −8.0 kcal mol⁻¹. In alkyls and Ph–CH₃ substituted D–π–A systems, ΔV_mA lies in the range −0.8 to −6.7 kcal mol⁻¹, whereas the N(CH₃)₂ substituted systems exhibit more negative ΔV_mA (more enhanced donating strength) in the range −5.1 to −8.0 kcal mol⁻¹. The more negative value of ΔV_mA implies the greater electron-donating ability of the D−π−A system. Optical and photovoltaic parameters (ΔG_reg, ΔG_inject, eV_OC) are analyzed at the TD-CAM-B3LYP/SMD/cc-pVDZ//B3LYP/cc-pVDZ level of DFT. An excellent linear correlation is observed in all six sets between ΔV_mA and the absorption maximum (λ_max) showing that λ_max increases with enhanced donating strength. The higher absorption maximum obtained by N(CH₃)₂ substituted D–π–A systems lies in the range 430 nm to 490 nm, explaining the outstanding donating ability of N(CH₃)₂ compared to other substituents. The reduced highest occupied molecular orbital (HOMO) – lowest unoccupied molecular orbital (LUMO) gap (from 3.14 to 2.17 eV) with enhanced donating strength confirms the influence of substituent effects in broadening the absorption maximum. Furthermore, in photovoltaic parameters, a strong influence of the substituent effect is observed. The N(CH₃)₂ substituted D1–π–A (D1–N(CH₃)₂) exhibits the highest eV_OC (1.38 eV). The strong linear correlation observed for the ground state property ΔV_mA and open-circuit voltage eV_OC provides guidelines for developing an effective strategy for designing dye sensitizers for desirable photovoltaic applications.