Exploring photophysical properties of metal-free coumarin sensitizers: an efficient strategy to improve the performance of dye-sensitized solar cells

An efficient strategy was provided by adopting different numbers of electron-deficient units (pyrimidyl and quinolyl) into parent coumarin sensitizers to obtain excellent absorption in the short-wavelength region (B2 band), which eventually improves the performance of DSSCs. Density functional theory calculations were performed on both free dyes and dye-TiO2 complexes. As expected, introducing a single electron-deficient unit results in a positive influence on the power conversion efficiency (eta) of DSSCs because of the larger short-circuit current density (J(sc) is proportional to optical absorption (phi(LHE)), charge separation, dye regeneration (phi(reg)) and electron injection (phi(inject))) and the higher open circuit voltage (V-oc). The introduction of more pyrimidine facilitates charge separation and favors effective electron injection, whereas the second quinoline displays the opposite effect. The results give guidance to design promising candidates for future DSSCs applications.