Efficient near IR porphyrins containing a triphenylamine-substituted anthryl donating group for dye sensitized solar cells

Three porphyrin dyes comprising 4-(10-ethynylanthracen-9-yl)-N,N-diphenylaniline as an extended pi-conjugated donor integrated with systematically varied acceptor groups, i.e., 2-cyano-3-(5-methylthiophen-2-yl)acrylic acid (LG19), 2-cyano-3-(5-(7-methylbenzo[c][1,2,5] thiadiazol-4-yl) thiophen-2-yl)acrylic acid (LG20), and 2-cyano-3-(4-(7-methyl benzo[c][1,2,5] thiadiazol-4-yl) phenyl)acrylic acid (LG21), have been synthesized and implemented in dye sensitized solar cells (DSSCs). All these sensitizers were characterized by various spectroscopic and electrochemical techniques. The absorption maxima of LG20 significantly red-shifted to 707 nm, when compared to those of the LG19 and LG21 sensitizers. Optical, electrochemical and theoretical studies suggest that the lowest unoccupied molecular orbitals (LUMOs) of LG20 and LG21 are very close to the TiO2 conduction band, when compared to that of LG19. The IPCE values of LG20 and LG21 were found to be 81% at 486 nm and 83% at 482 nm, respectively, and the onset wavelengths were shifted up to 900-980 nm. With favourable red shifted absorption of the LG20 sensitizer, a high power conversion efficiency (PCE) of 10.0% has been observed for the fabricated DSSC, which was higher than those of the DSSCs fabricated from LG19 (PCE = 8.51%) and LG21 (PCE = 9.04%). Therefore, the effect of the extended pi-conjugated donor, anchoring group, benzothiadiazole auxiliary acceptor and presence of dodecyloxy groups significantly contributed to enhancing the efficiency of the device using a liquid redox electrolyte. Finally, we have used intensity modulated photovoltage spectroscopy and nanosecond transient studies to explain the design and efficiency relationship of these sensitizers.