Investigation of the influence of π-conjugated bridge on photovoltaic parameters in A2-π2-D-π1-A1 dye sensitizers for efficient DSSCs

Abstract

Most notably, indole is the most abundantly found electron donor group. Eight $A_2$-${\pi }_2$-D-${\pi }_1$-$A_1$ dyes as sensitizers to DSSCs have been successfully designed on the basis of two categories (R1/dyes and R2/dyes). All of these eight dyes have the same donor fragment (indole), π-conjugated bridge ${\pi }_1$ (made of thiophene moiety/benzene) and acceptor fragment $A_1$ (made of cyanoacrylic/cyanure). Additionally, an acceptor $A_2$ group (cyanoacrylic acid) is incorporated with supplementary bridge ${\pi }_2$ units.

The photochemical properties, electronically excited states, and chemical reactivity influencing the produced dyes have been evaluated through DFT and TD-DFT calculations. They include bond lengths, dihedral angles between fragments, frontier molecular orbitals, density of states, isosurface molecular electrostatic potential, charge density differences, fragment transition density matrix, UV–Vis absorption spectra, quantum chemical parameters, photovoltaic characteristics. The investigation compares the dyes features in photophysics, electrically excited states and chemical reactivity.

Among all the donor dyes designed, R1-D1 and R2-D1 exhibit the most favorable properties. They possess the smallest energy gap values (1.187 eV, 1.165 eV), lowest vertical excitation energies (2.32 eV, 1.85 eV), longest absorption peaks (535.51 nm, 671.67 nm), and the most negative free energy change for electron injection (−1.74 eV, −1.33 eV). These characteristics make them the optimal designs within the two molecular groups. In accordance with the appropriate photophysical characteristics expressed by the remaining organic dyes, all afore mentioned dyes can be considered suitable for application in DSSCs.