Influence of the non-conjugated 5-position substituent of 1,3,5-triaryl-2-pyrazoline-based photosensitizers on the photophysical properties and performance of a dye-sensitized solar cell

We report the effect of the non-conjugated 5-position substituent of pyrazoline-based photosensitizers on the photophysical characteristics and performance of dye-sensitized solar cells (DSSCs). Four photosensitizers which contained different para-substituted phenyl groups (dimethylamine (6a), hexyloxy (6b), ethyl ester (6c), and no substituent (6d)) at the 5-position were synthesized. Although these substituents were not part of the conjugated pi-system of the pyrazoline, their absorption maxima showed a red shift from 496 to 510 nm with increasing electron-donating ability. The same trend was observed for their electrochemical characteristics: the oxidation potentials decreased from 1.14 to 0.97 V. In terms of DSSC performance, 6a, which had a strongly electron donating substituent at the 5-position, delivered the highest short-circuit current (J(sc)) of 12.3 mA cm(-2), an incident photon-to-current conversion efficiency (IPCE) as high as 75%, and the best power conversion efficiency (PCE, eta) of 5.7% under AM 1.5 G conditions. Further investigation with the stepped light-induced photocurrent and voltage transients (SLIM-PCV) method revealed that the steric differences between the groups at the 5-position clearly influenced the recombination reaction, resulting in the highest V-oc of 636 mV in the DSSC employing 6a with the bulky dimethylamine substituent.