DFT/TD-DFT studies on carbazole-diphenylamine-based dyes using different π-bridges for DSSCs

Abstract

Molecular engineering of metal-free organic sensitizers with a Donor-π-Acceptor (D-π-A) structure is fundamental for the development of next-generation dye-sensitized solar cells (DSSCs). In this work, we present a systematic study based on Density Functional Theory (DFT) and its Time-Dependent extension (TD-DFT) on a series of eight novel dyes integrating a potent hybrid carbazole-diphenylamine donor unit and a cyanoacrylic acid anchor. The research elucidates how the chemical constitution of various conjugated (π) bridges modulates a comprehensive set of structural, electronic, optical, and charge-transport properties. Our findings reveal that molecular planarity, quantitatively assessed using dihedral angles and planarity parameters (MPP/SDP), is a critical performance factor. Specifically, the incorporation of thiophene-based π-bridge promotes superior planarity, leading to a significant reduction in chemical hardness and promoting efficient intramolecular charge transfer (ICT), a conclusion visually corroborated by Transition Density Matrix (TDM) analysis. Furthermore, the calculation of reorganization energies indicates that the designed dyes possess favorable intrinsic kinetics for ambipolar charge transport. Among the evaluated candidates, the sensitizer DC5A, incorporating a benzodithiophene bridge, is identified as exceptionally promising, exhibiting a red-shifted absorption maximum of 645 nm, a HOMO–LUMO energy gap of 2.60 eV, and free energy of electron injection of 0.80 eV. This study not only identifies a series of synthesis-feasible dyes with high potential for photovoltaic applications but also establishes a key design principle, the use of planar bridges to enhance ICT in D-π-A systems, offering a clear and rational route for the development of future high-efficiency organic photosensitizers.