D–A−π–A System: Light Harvesting, Charge Transfer, and Molecular Designing

Abstract

Three organic dyes (XY1, XY2, and XY3), synthesized by inserting a thiophene heterocycle and shifting the position of benzothiadiazole (BTZ) auxiliary acceptor on the cyclopentadithiophene(CPDT) π-bridge, are theoretically investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) for their potential applications in solar cells. The photoelectric paremeters (short-circuit current, open circuit voltage, conversion efficiency) are evaluated by the following factors: energy levels and the orbital compositions, the absorption spectra, NBO analysis from S 0 to S 1, the qualitative descriptions of D CT, Δr, S, and ΔQ for intramolecular charge transfer (ICT) processing, the adsorption energy ( E ads ), and the excited lifetime ( t ), the shift of CB and vertical dipole moment (μ normal ), the electron injection lifetime (τ), the Marcus rate of inner-path recombination, the simulation of outer-path recombination dye···I 2 and dye regeneration [dye +• I – ], etc. The E ads, the excited lifetime ( t ) and the electron injection lifetime (τ) explain the order of experimental J SC, and the trend of V oc is interpreted by the calcuations on the Δ E CB, μ normal, the Marcus rate and the dye···I 2 mode. Among three molecules, XY2 possesses the significance advantage in ICT mechanism and dye regeneration ability based on analysis of qualitative descriptions and two-step regeneration mechanism. Moreover, 12 dyes are designed by the substitution of π-bridge of XY2, which provide a strategy to improve the abilities of absorption and electron injection as well as photoelectric properties on the comparison of XY2.