Molecular twisting, donor engineering, and BF₂ coordination in chalcone-coumarin sensitizers for DSSCs

Efficient organic sensitizers for dye-sensitized solar cells (DSSCs) need to absorb light effectively while maintaining good charge separation and dye regeneration. In this work, twelve chalcone-coumarin-based D-π-A sensitizers were designed to study the effects of donor structure and BF₂ coordination on their electronic, excited-state, and photovoltaic properties. Benchmark calculations using experimentally reported parent dyes showed that CAM-B3LYP gives a suitable description of the charge-transfer excitations in these systems. Increasing the strength of the D unit raises the HOMO energy and reduces the HOMO-LUMO gap. BF₂ coordination mainly lowers the LUMO energy and increases polarization on the acceptor side. The energy gap decreases from 1.843 to 1.327 eV for the AX1 dyes and from 1.632 to 1.136 eV for the AX2 dyes. These changes are accompanied by red shifts of the absorption bands of up to 186 nm. Hole-electron and interfragment charge-transfer analyses show that stronger donors promote charge transfer within the molecule, while BF₂ coordination increases the contribution of the acceptor region. All of the sensitizers have favorable electron-injection driving forces. The strength of the D unit also affects the thermodynamics of dye regeneration. Based on the calculated electronic, optical, and photovoltaic properties, 2-AX2, 3-AX2, and 2-BX2 show the most favorable overall properties.