Dye Aggregation, Photostructural Reorganization and Multiple Concurrent Dye···TiO₂ Binding Modes in Dye-Sensitized Solar Cell Working Electrodes Containing Benzothiadiazole-Based Dye RK-1

Dye-sensitized solar cells (DSSCs) are widely considered to be promising contenders for the next generation of photovoltaic devices. The light-harvesting component in the DSSCs, that is, the sensitizers, are adsorbed onto the semiconductor layer to form the photoelectrode that absorbs solar energy and converts it into electrical power. The choice of dye has a profound impact on the DSSC performance. Thus, substantial research efforts have been devoted to the development and testing of a large variety of sensitizers during the past decades. We, herein, focus our attention on the metal-free benzothiadiazole-based dye, 2-cyano-3-(4-(7-(5-(4-(diphenylamino)phenyl)-4-octylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)phenyl) acrylic acid (RK-1), which exhibits excellent power conversion efficiency and high stability. Simulations and experiments were combined to conduct a systematic study of its optical and structural properties. RK-1 shows a broad absorption band over the visible region and the formation of H aggregates in solution. Deprotonation of the cyanoacrylic acid group in RK-1 occurs upon dye adsorption onto TiO2. This induces a hypsochromic shift of the absorption band. The emission spectrum of RK-1 shows a broad band and a large Stokes shift (6949 cm–1), indicating the presence of several different structural reorganization processes during excitation. Time-dependent density functional theory (TD-DFT) calculations reveal that intramolecular charge transfer in RK-1 molecules should be predominantly attributed to the HOMO–LUMO transition. DFT modeling also reveals the involvement of the cyano group of RK-1 in anchoring to TiO2, which makes the dye···TiO2 binding structure more stable. Fourier transform infrared spectroscopy data prove that bridging bidentate (BB) and COO/CN (A2) binding modes coexist in the RK-1···TiO2 interfacial structure.