Theoretical Design of Near-Infrared-Absorbing D-A-π-A Dyes with Modified Hagfeldt Donors: A DFT/TDDFT Study

Dye-sensitized solar cells based on Hagfeldt donor sensitizers achieve high open-circuit voltages through effective suppression of interfacial charge recombination. However, their absorption remains largely confined to the visible region, which limits further gains in the photocurrent and overall efficiency. This study addresses the issue of spectral limitation by designing six D-A-π-A organic dyes, HJ101~HJ106. These dyes are derived from the reference sensitizer XY1 through systematic modification of the donor unit with anthracene and squaraine moieties combined with two benzothiadiazole-type auxiliary acceptors. Geometric structures, frontier molecular orbitals, absorption spectra, and excited-state charge transfer characteristics were investigated using density functional theory and time-dependent density functional theory. The results demonstrate that donor and acceptor modifications act synergistically to control the spectral direction and magnitude, with several dyes achieving pronounced redshifts extending into the near-infrared region while retaining thermodynamically favorable electron injection and regeneration driving forces. Among the designed structures, HJ106, which combines a squaraine-modified donor with a redshifting acceptor, exhibits the largest bathochromic shift and an extended excited-state lifetime. HJ105 delivers the highest molar extinction coefficient and light-harvesting efficiency. Collectively, these findings identify squaraine-based donor engineering as the most promising strategy for near-infrared-responsive Hagfeldt-type sensitizers. These findings offer practical structural guidance for the development of next-generation dye-sensitized solar cell sensitizers with broadened spectral coverage.