Tuning photophysical properties via alkoxyl groups in charge-separated triphenylamine sensitizers for dye-sensitized solar cells

Abstract

Two charge-separated (CS) sensitizers (MoTPA-Pyc and OcTPA-Pyc) with methoxyl or octyloxyl groups attached to the para position of the triphenylamine donor moiety were synthesized to investigate the effect of the alkoxyl groups on the photophysical properties of the CS sensitizers and photovoltaic performances of the corresponding dye-sensitized solar cells (DSSCs). The introduction of alkoxyl groups can increase the energy gap between LUMO+1 and LUMO, leading to an increased intramolecular photo-induced electron transfer (PET) efficiency. Both MoTPA-Pyc and OcTPA-Pyc show elongated lifetimes for their CS states, resulting in an enhanced internal quantum efficiency (IQE) of DSSCs. It has been demonstrated that intramolecular PET is the key step for efficient DSSCs due to the presence of long-lived CS states allowing efficient and fast interfacial electron transfer from the CS sensitizers to the TiO2 nanoparticles. The best efficiency of 5.76% was achieved by the MoTPA-Pyc based DSSC through a combined contribution of high IQE and efficient light harvesting. The results suggest that enhancing the intramolecular PET efficiency and the light harvesting ability of the dye-loaded TiO2 film are effective strategies for achieving higher photovoltaic performance for CS sensitizer-based DSSCs.