Design and DFT study of nitrogen-rich donor systems for improved photovoltaic performance in dye-sensitized solar cells

Abstract

Eighteen electron-rich nitrogen incorporated donors with a butadiene π-spacer and a cyanoacrylic acid acceptor (A) as photosensitizers (D1–π–A to D18–π–A) for dye-sensitized solar cell (DSSC) applications have been designed for improving the photovoltaic performance. The significance of the nitrogen centres for revamping the donating strength (ΔV_mA) of D–π–A is scrutinized using molecular electrostatic potential (MESP) analysis at the B3LYP/cc-pVDZ level of density functional theory (DFT). During the transformation of a donor (D) to D–π–A, a certain delocalization of electron density from D to π–A has occurred, and the change in the MESP minimum (ΔV_mA) observed at the cyano region of D–π–A is related to the donating strength of D. Optical and photovoltaic properties are analyzed at the TD/CAM-B3LYP/cc-pVDZ/SMD//B3LYP/cc-pVDZ level. In D1–π–A to D18–π–A, ΔV_mA is in the range −7.0 to −19.0 kcal mol⁻¹ and the increase in donating strength is found to be proportional to the number of planar nitrogens in the donors. D12–π–A exhibited the most negative ΔV_mA (−19.0 kcal mol⁻¹), indicating the highest electron-donating strength of D12, whereas the least negative ΔV_mA (−7.0 kcal mol⁻¹) displayed by D7–π–A is correlated to the weak donating character of D7. By increasing the electron-donating strength of D in D–π–A, a red-shift in the absorption maximum (Δλ_max) by 162 to 294 nm is observed. Further, the open-circuit voltage (eV_oc) calculated for the D–π–A systems showed a strong linear relationship with ΔV_mA. The LUMO (lowest unoccupied molecular orbital) energy of all the D–π–A systems (−1.79 to −2.79 eV) is observed above the conduction band (CB) energy of TiO₂ (−4.0 eV), which ensured a desirable electron injection efficiency (ΔG_inject) for them. The analysis of the adsorption energy (E_ads) of the D–π–A systems on the TiO₂ semiconductor (D–π–A/TiO₂) showed that D12–π–A has the highest adsorption stability. Improving the adsorption stability is better for improving eV_oc and the power conversion efficiency (PCE). The maximum absorption wavelength (λ_max) of the D–π–A/TiO₂ systems ranges from 513 to 703 nm and all of them display a red-shift with respect to the bare D–π–A systems. The study suggests D12 as the most efficient photosensitizer for DSSC applications. Further, it deepens the understanding of the structure–performance relationship of D–π–A systems as photosensitizers.