Effect of Molecular Structure Perturbations on the Performance of the D-A-π-A Dye Sensitized Solar Cells

Three D-A-pi-A organic dyes based on 5,6-difluoro-2,1,3-benzothiacliazole (DFBTD) were synthesized by sequential direct arylation and characterized by spectroscopic and electrochemical techniques. Compared to 2,1,3-benzothiadiazole (BTD) analogue, the presence of two fluorine atoms on DFBTD results not only in a significant increase in the molar absorptivity but also in a blue shift of the onset of the absorption spectra. In the system of DFBTD-based sensitizers, replacing the thienyl unit bridge with disubstituted cydopenta[1,2-b:5,4-b']dithiophene (CPDT) further increases molar absorptivity and decreases the oxidation potential of the excited state E(s+/s*). Similarly, changing the indoline donor to 4-butoxy-N-(4-butoxyphenyl)-N-phenylaniline increases the optical band gap and decreases the oxidation potential of the excited state E(s+/s*) of the sensitizer. The correlation between the molecular structure of these sensitizers and the photovoltaic performance of the dye cells were examined using current-voltage scan, incident photon to electron conversion efficiency (IPCE), femtosecond transient absorption spectroscopy (TAS), and electrochemical impedance spectroscopy (EIS). The observed change of the photovoltage and photocurrent upon changing the molecular structure of the sensitizers are discussed in terms of the charge injection from the excited state to the interband gap states of TiO2 and the charge recombination between injected electrons into TiO2 film and electrolyte. In all, it is found that the most efficient D-A-pi-A dye achieved an open circuit voltage of 0.717 V, short circuit current density of 18.8 mA/cm(2), corresponding to an overall power conversion efficiency of 9.1%.