Rational Molecular Engineering of Indoline-Based D‑A-π‑A Organic Sensitizers for Long-Wavelength-Responsive Dye-Sensitized Solar Cells

Indoline-based D-A-π-A organic sensitizers are promising candidates for highly efficient and long-term stable dye-sensitized solar cells (DSSCs). In order to further broaden the spectral response of the known indoline dye <b>WS-2</b>, we rationally engineer the molecular structure through enhancing the electron donor and extending the π-bridge, resulting in two novel indoline-based D-A-π-A organic sensitizers <b>WS-92</b> and <b>WS-95</b>. By replacing the 4-methylphenyl group on the indoline donor of <b>WS-2</b> with a more electron-rich carbazole unit, the intramolecular charge transfer (ICT) absorption band of dye <b>WS-92</b> is slightly red-shifted from 550 nm (<b>WS-2</b>) to 554 nm (<b>WS-92</b>). In comparison, the incorporation of a larger π-bridge of cyclopentadithiophene (CPDT) unit in dye <b>WS-95</b> not only greatly bathochromatically tunes the absorption band to 574 nm but also largely enhances the molar extinction coefficients (ε), thus dramatically improving the light-harvesting capability. Under the standard global AM 1.5 solar light condition, the photovoltaic performances of both organic dyes have been evaluated in DSSCs on the basis of the iodide/triiodide electrolyte without any coadsorbent or cosensitizer. The DSSCs based on <b>WS-95</b> display better device performance with power conversion efficiency (η) of 7.69%. The additional coadsorbent in the dye bath of <b>WS-95</b> does not improve the photovoltaic performance, indicative of its negligible dye aggregation, which can be rationalized by the grafted dioctyl chains on the CPDT unit. The cosensitization of <b>WS-95</b> with a short absorption wavelength dye <b>S2</b> enhances the IPCE and improves the η to 9.18%. Our results indicate that extending the π-spacer is more rational than enhancing the electron donor in terms of broadening the spectral response of indoline-based D-A-π-A organic sensitizers