Significant differences in photophysical and photovoltaic properties of flexible chain terminated homoleptic tris-Ir(iii) complexes

Cyclometallated heavy metal complexes as photoactive materials have not received as much attention as pure organic/polymer counterparts in organic solar cells (OSCs). In this work, a novel homoleptic Ir(iii) complex, tris(2-ethylhexyl 5′′-(benzo[d]thiazol-2-yl)-[2,2′:5′,2′′-terthiophene]-5-carboxylate) Ir(iii) (ATBz3Ir), is presented for potential use in OSCs. Compared to hexyl-terminated TBz3Ir with the same main ligand backbone, ethylhexyl carboxylic ester-terminated ATBz3Ir exhibits significantly attenuated absorption at longer wavelengths of 400-600 nm. ATBz3Ir only shows phosphorescence peaking at 706 nm with a triplet lifetime of 280 ns in solution, which is dramatically different from the double modal profile for TBz3Ir with high-intensity fluorescence (564 nm/0.59 ns) and low-intensity phosphorescence (790 nm/174 ns). When employed as the third component for PM6:Y6 blended OSCs, ATBz3Ir and TBz3Ir exhibit opposite effects. The maximum power conversion efficiency (PCE) is improved from 15.14% for binary PM6:Y6 to 16.50% for PM6:ATBz3Ir:Y6 based ternary devices, while the PCE decreases to 14.49% for the PM6:TBz3Ir: Y6 device. The main reason could be attributed to the better miscibility between ATBz3Ir and the Y6 acceptor, which induces enhanced Y6-aggregation, and thus increased Y6-absorption, more efficient exciton dissociation, improved electron transport and reduced charge recombination. Similarly, an enhanced PCE was also achieved from 16.11% to 17.04% for other batches of PM6′:Y6′ and from 17.25% to 18.37% for PM6:L8-BO based ternary devices, respectively, which further support the efficiency of the third component of ATBz3Ir.