Theoretical study of novel D-A-π-A-π-A conjugated organic dye sensitizers: impact of alkyl chain and electron-withdrawing functional groups substitution

The influence of alkyl chain on the physical and photochemical properties of D-A-π-A-π-A sensitizers (GZ-121 and GZ-124) for dye-sensitized solar cells (DSSCs) was investigated through density functional theory (DFT). Key aspects examined include geometry, optical characteristic, electron injection, intramolecular charge transfer, and dye regeneration. Adsorption models of dye@TiO₂, short-circuit current density (J_sc), open circuit voltage (V_oc), and photoelectric conversion efficiency (PCE) were also analyzed. The presence of the alkyl chain in GZ-121 inhibits molecular distortion, promotes the separation of holes and electrons, and extends the excited state lifetime, facilitating electron injection. GZ-124 exhibits a faster charge transfer rate due to lower reorganization energy, with electrochemical parameters indicating superior electron transport capacity and stability. The narrower energy gap results in a more red-shifted absorption spectrum and wider spectral response range, thereby enhancing the light harvesting efficiency. Consequently, GZ-124 exhibited a higher J_sc of 19.77 mA cm⁻² and a PCE of 12.92 %. Additionally, molecular design involved substituting furan and thiophene at different π-bridge positions to investigate the impact of electron-rich functional group substitution on the photoelectric properties of DSSCs. Compared to the original molecule, the modified designs exhibit higher J_sc and PCE values. Theoretical simulation results demonstrate the feasibility of substituting electron-withdrawing functional groups to enhance photovoltaic performance, thereby providing valuable guidance for the design of photosensitizers with dual auxiliary acceptors.