Tuning Optical Absorption and Photovoltaic Performance for Novel Near-Infrared Squaraine Molecules in Dye-Sensitized Solar Cells.

Asymmetric squaraine dyes featuring a D1-A-D2-COOH architecture have emerged as promising candidates for DSSCs due to their intense near-infrared absorption, tunable electronic structures, and superior charge transfer capabilities. However, theoretical investigations focused on the rational design of such dyes, particularly through density functional theory (DFT) and time-dependent DFT (TD-DFT), remain limited. In this study, two experimentally synthesized squaraine dyes, SQ-NPh2 and SQ-CN, were systematically analyzed with respect to their electronic structures, optical absorption properties, and photovoltaic performance. The results reveal that SQ-NPh2, incorporating an electron-donating D1 unit, exhibits a lower energy gap, red-shifted absorption, improved intramolecular charge transfer (ICT), and superior photovoltaic parameters, including enhanced power conversion efficiency (PCE), compared to the electron-withdrawing SQ-CN. Building on a D1-substitution strategy, two new dyes, SQ-MPTA and SQ-PPP, were rationally designed by introducing stronger electron-donating groups at the D1 position. Comprehensive theoretical assessments, including analyses of frontier molecular orbitals, ICT behavior, light-harvesting capability, open-circuit voltage (), short-circuit current density (), and recombination, demonstrate that SQ-PPP exhibits the most favorable optoelectronic characteristics and significantly improved photovoltaic performance. These results revealed the effectiveness of D1-donor engineering in tuning key properties of squaraine dyes and offer valuable insights for designing high-efficiency squaraine dyes.