Carbazole–phenothiazine-based organic sensitizers via π-bridge functionalization with different electronegative/steric substituents: photophysical properties and DSSC performance†

Abstract

As pivotal components in D–π–A dye sensitizers for dye-sensitized solar cells (DSSCs), π-bridge engineering strategies have demonstrated that structural and electronic modifications critically govern charge transfer dynamics and photovoltaic performance. This study systematically investigates four D–D–π–A sensitizers (CP-Ph, CP-Ph-F, CP-Ph-OMe, and CP-Ph-Cl) with tailored substituents (–H, –F, –OMe, and –Cl) on the phenyl π-bridge, enabling direct comparison of different electronegative/steric substituents to the performance of DSSCs. The interplay between electronic and steric effects dictates intramolecular charge transfer (ICT) absorption shifts in solution: strongly electronegative substituents (e.g., –F) induce a redshift through LUMO stabilization via dominant electronic effects, while sterically bulky groups (e.g., –Cl and –OMe) cause a blueshift by increasing dihedral angles and disrupting conjugation. Upon TiO₂ adsorption, all dyes exhibit pronounced bathochromic shifts in absorption spectra, enhancing light-harvesting efficiency. Electrochemical impedance spectroscopy and open-circuit voltage decay analyses reveal that both electronegative and steric substituents promote interfacial charge recombination, significantly shortening electron lifetimes. Among DSSC devices fabricated with CP-series sensitizers, CP-Ph demonstrates optimal performance with a J_sc of 14.25 mA cm⁻², a V_oc of 0.83 V, a FF of 63.5%, and a PCE of 7.54%, highlighting the balance between electronic optimization and minimal steric compromise.