Position engineering of cyanoacrylic-acid anchoring group in a dye for DSSC applications

This study provides insights into the molecular tailoring of DSSC dyes via synthesis/investigation of three new organic dyes with the position engineering of the acceptor moiety. Differences in photophysical, electrochemical and photovoltaic properties are found and interpreted in detail. The excitation-spectrum study shows that the population of the intramolecular charge transfer (ICT) is incident-wavelength dependent; and the ICT excitons originate not only from direct ICT band transition but also from π-π* energy transfer, and the degree of the latter depends much on the position of the anchoring group. The transient photoluminescence discloses that the non-injection quenching of the ortho dye is the most severe possibly due to the instability of the excited state resulting from the hurdling of electron delocalization by the large steric hinderance. Large steric hinderance in the ortho dye also results in the least dye loading on TiO2 photoanode. Due to broader/stronger absorption, higher dye loading, high injection efficiency and low charge recombination rate, the para-substituted dye DSSC device achieves the best performance with power conversion efficiency of 6.63%; while the ortho dye based device performs the worst with significant lower of photocurrent and photovoltage compared to the para dye. The dye dependent VOC is explained by combination of dipole moment and dye loading. Particularly, the employing of excitation spectrum as tools to investigate the intrinsic photophysical complicity of dyes with D-π-A structure, which is scarcely reported, is proved to be an effective way.