Osmium sensitizer with enhanced spin-orbit coupling for panchromatic dye-sensitized solar cells

Low-lying triplet metal-to-ligand charge transfer (3MLCT) transitions of osmium complexes induced by spin-orbit coupling (SOC) are promising for extending the photocurrent response in dye-sensitized solar cells. In this study, we present a new osmium complex (CYC-33O), incorporating a 2-thiohexyl-3,4-ethylenedioxythiophene functionalized bipyridyl ancillary ligand to red-shift the absorption and enhance the absorbance of singlet and triplet metal-to-ligand charge transfer (1MLCT and 3MLCT) transitions. Time-dependent density functional theory (TDDFT) calculations show that the reinforced 1MLCT and 3MLCT transitions of CYC-33O are mainly from osmium to the 4,4′,4′′-tricarboxy-2,2′:6′,2′′-terpyridine anchoring ligand, increasing the heterogeneous electron transfer between CYC-33O and TiO2. The device sensitized with CYC-33O exhibits panchromatic conversion beyond 1000 nm, yielding a photocurrent density of 19.38 mA cm-2, which is much higher than those of the cells based on the ruthenium analogue (CYC-33R) and model osmium complex (Os-3) sensitizers.