Donor-pi-Acceptor Based Stable Porphyrin Sensitizers for Dye-Sensitized Solar Cells: Effect of pi-Conjugated Spacers

摘要

Porphyrins are major sensitizers in dye-sensitized solar cells (DSSCs) and result in very high power conversion efficiency; however, aggregation tendency and visible range absorption prevent realistic applications. Thus, designing of novel porphyrins based sensitizers is essential to resolve the current existing issues. In this context, seven D-pi-A porphyrin dyes (LG1-LG7) engineered with 3-ethynyl phenothiazine tethered at the meso-position and pi-spacers, such as 4-ethynyl phenyl (LG1), 5-ethynylthiophene (LG2), 5-ethynyl furan (LG3), 2,1,3-benzothiadiazole (BTD)-phenyl (LG6), and BTD thiophene (LG7), were incorporated between porphyrin macrocycle and anchoring carboxylic acid. Similarly, pi-spacers 4-ethynyl phenyl (LG4) and 4-ethynylthiophene (LG5) were functionalized between porphyrin and anchoring cyanoacrylic acid. LG5 and LG6 showed significant near-infrared absorption resulting in the highest efficiency of 10.20% and 9.64% among other derivatives. UV-vis-NIR absorption, cyclic voltammetry, and density functional theory calculations of LG1 LG7 suggested that LG5 exhibits strong absorption, and optimized lowest unoccupied molecular orbitals aid to inject electrons very effectively from the excited state of dye into the TiO2 conduction band. Current density-voltage (J-V) of LG1-LG7 revealed that LG5 exhibits the highest short-circuit current density of 21.01 mA cm(-2), resulting in a power conversion efficiency of 10.20% in-a liquid I-/I-3(-) redox couple electrolyte. Panchromatic incident photon-to-current conversion efficiency response of LG5 was observed between 400 and 900 nm, when compared to other derivatives. Thus, these results suggest that LG5 attained the highest efficiency in liquid electrolyte based DSSCs. Subsequently, durability studies of LG5 performed by continuous light exposure have shown that this sensitizer retained 80% initial efficiency after 1000 h. Therefore, the effect of spacer length and anchoring significantly contributed to improve the efficiency in liquid electrolyte, which is very useful to make efficient future generated DSSCs.