Planar D–D−π-A Organic Sensitizers for Thin-Film Photoanodes

The planarity of an organic sensitizer is one of the most crucial factors for determining molar absorptivity and intramolecular charge transfer (ICT). The photovoltaic performance of dye-sensitized solar cells dramatically changed depending on the planarity of the donor, although all dyes exhibited similar extinction coefficients, electrochemical characteristics, as well as the amount of loaded dye. The power conversion efficiency (PCE) of planar donor dyes was 3 times greater than that of twisted donor dyes because of more rapid ICT. In addition, RK-3 dye, characterized by additional donor groups on an indoline unit, exhibited broad light-harvesting ability with higher performance as compared to those observed for planar dyes with a single donor attached to the thin-film photoanode (1.8 μm transparent + 2.5 μm scattering film). The champion cell of RK-3 exhibited a PCE of 10.3% when the thickness of the active film increased to 3.5 μm, as well as when an antireflection layer was applied with an iodine-based electrolyte.