Highly efficient unsymmetrical squaraines for panchromatic dye-sensitized solar cells: A computational study

The geometries, electronic structures and absorption spectra of two unsymmetrical squaraine dyes (SQ1 and SQ2) were theoretically investigated by using density functional theory (DFT) and time-dependent DFT (TD-DFT). The dye-(TiO2)(9) nanocluster systems were also simulated to show the electronic coupling at the interface. The results reveal that compared to SQ1, SQ2 with an electron-rich thiophene spacer and a strongly pi-accepting carboxycyanovinyl group could cause a red shift of the absorption spectrum, increase the oscillator strength and improve the light harvesting efficiency, thus leading to the larger short-circuit photocurrent density (J(SC)) in dye-sensitized solar cells (DSSCs), in good agreement with experimental data. While the larger normal dipole moment pointing outward from the semiconductor surface of SQ2 is the major factor resulting in the higher open-circuit photovoltage (V-OC). Then, we designed another six dyes with different pi-spacers to screen dyes with improved performance. The results indicate that SQ6, SQ7 and SQ8 with benzathiadiazole, bithiophene and thienothiophene spacer, respectively, will be more efficient squaraine dyes used in DSSCs. This study is expected to assist the design and synthesis of novel squaraine dyes.