Effects of methoxy group(s) on D-π-A porphyrin based DSSCs: efficiency enhanced by co-sensitization

Abstract

We have engineered, synthesized, and characterized a series of porphyrin sensitizers that contain either one or three methoxy groups on the phenyl ring of the triphenyl imidazole donor moiety and either 3-(5-(benzo[c][1,2,5]thiadiazole-4-yl)thiophene-2-yl)-2-cyanoacrylic acid (LG24 and LG26) or 2-(5-(benzo[c] [1,2,5]thiadiazole-4-yl)thiophene-2-yl)methelene-4-oxo-2-thiathiazolidin-3-yl-acetic acid (LG25 and LG27) acceptor groups, adopting a donor-π-acceptor structure. All four sensitizers are characterized using various spectroscopic techniques and electrochemical methods. The Q bands of all four sensitizers are redshifted when compared to the previously detailed LG18 sensitizer, and the absorption onset is extended to 800 nm. Density functional theory calculations suggest that the highest occupied molecular orbital (HOMO) levels are delocalized on the triphenylimidazole donor and porphyrin macrocycle, while the lowest unoccupied molecular orbital (LUMO) levels reside on auxiliary acceptors and anchoring groups, facilitating intramolecular interactions. Finally, we have evaluated the photovoltaic performances of these sensitizers in dye sensitized solar cells (DSSCs) using a liquid I⁻/I₃⁻ redox electrolyte, and the performances were compared with the previously reported LG18 sensitizer. We observed lower efficiencies from the porphyrin dyes having rhodanine-3-acetic acid anchoring groups (LG25 and LG27) than from the porphyrin dyes having cyanoacrylic acid anchoring groups (LG24 and LG26). Both the LG24 and LG26 dyes with conjugated electron acceptors can not only maintain a broad absorption spectrum but they can also enhance the electronic structure, considerably improving electron injection and lowering charge recombination compared to our previous reports. On the other hand, the LG18 dye showed better photovoltaic performance than the LG24 and LG26 dyes, probably due to its more negative LUMO, and as a result its V_oc was enhanced. The power conversion efficiency (PCE) order was LG26 ≥ LG24 > LG27 ≥ LG25. Furthermore, we fabricated a co-sensitized DSSC using the UV-absorbing dye Y1. The LG26 + Y1-based DSSC showed the highest PCE of 10.45%. To understand the reasons for the photovoltaic performance trends, we have carried out intensity-modulated photovoltage spectroscopy and nanosecond transient absorption studies on the present sensitizers and compared them with the previously reported sensitizer LG18.