(496j) Near-Infrared-Absorbing Open-Shell Donor-?-Acceptor Metal-Free Conjugated Dyes for Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSCs) have drawn a significant interest due to their low production cost, design flexibility, and the easy tunability of the sensitizer. However, the power conversion efficiency (PCE) of the metal-free organic dyes is limited due to their inability to absorb light in the near-infrared (NIR) region, leaving a large amount of energy unused. Herein, we have designed new DSC dyes with open-shell character, which significantly red-shifts the absorption spectra from its counterpart closed-shell structure. A small radical character (y < 0.10) is found to be beneficial in red-shifting the absorption maxima into the NIR region and broadening up to 2500 nm. Also, the open-shell dyes significantly reduce the singlet-triplet energy gaps (ΔE_ST), increase the total amount of charge transfer to the semiconductor surface, reduce the exciton binding energy, and significantly increase the excited-state lifetimes than the closed-shell dyes. However, the closed-shell dyes have large injection efficiency with increased intramolecular charge transfer (ICT) character. Furthermore, the open-shell dyes found more closely adsorbed into TiO₂surface than the closed-shell dyes.