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

Donor-pi-acceptor (D-pi-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 pi-spacer butadiene and acceptor moiety cyanoacrylic acid. The substituent effect enhances electron donation from D to A through the pi-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 (Delta V-mA) between pi-A and D-pi-A. For unsubstituted D-pi-A systems, Delta V-mA is in the range -0.1 to -5.7 kcal mol(-1), whereas the substitution enhances the negative character of Delta V-mA in the range -0.8 to -8.0 kcal mol(-1). In alkyls and Ph-CH3 substituted D-pi-A systems, Delta V-mA lies in the range -0.8 to -6.7 kcal mol(-1), whereas the N(CH3)(2) substituted systems exhibit more negative Delta V-mA (more enhanced donating strength) in the range -5.1 to -8.0 kcal mol(-1). The more negative value of Delta V-mA implies the greater electron-donating ability of the D-pi-A system. Optical and photovoltaic parameters (Delta G(reg), Delta G(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 Delta V-mA and the absorption maximum (lambda(max)) showing that lambda(max) increases with enhanced donating strength. The higher absorption maximum obtained by N(CH3)(2) substituted D-pi-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-pi-A (D1-N(CH3)(2)) exhibits the highest eV(OC) (1.38 eV). The strong linear correlation observed for the ground state property Delta V-mA and open-circuit voltage eV(OC) provides guidelines for developing an effective strategy for designing dye sensitizers for desirable photovoltaic applications.