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 (ΔVmA) between π–A and D–π–A. For unsubstituted D–π–A systems, ΔVmA is in the range −0.1 to −5.7 kcal mol−1, whereas the substitution enhances the negative character of ΔVmA in the range −0.8 to −8.0 kcal mol−1. In alkyls and Ph–CH3 substituted D–π–A systems, ΔVmA lies in the range −0.8 to −6.7 kcal mol−1, whereas the N(CH3)2 substituted systems exhibit more negative ΔVmA (more enhanced donating strength) in the range −5.1 to −8.0 kcal mol−1. The more negative value of ΔVmA implies the greater electron-donating ability of the D−π−A system. Optical and photovoltaic parameters (ΔGreg, ΔGinject, eVOC) 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 ΔVmA and the absorption maximum (λmax) showing that λmax 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 eVOC (1.38 eV). The strong linear correlation observed for the ground state property ΔVmA and open-circuit voltage eVOC provides guidelines for developing an effective strategy for designing dye sensitizers for desirable photovoltaic applications.