Fluoro substitution effect on 2,1,3-benzothiadiazole-based hybridized local and charge transfer state fluorophores for high-performance organic light-emitting diodes

Abstract

Fluoro substitution of organic semiconductors is an effective way to improve some related performance of the materials, such as the carrier mobility in organic photovoltaics (OPVs) and organic field-effect transistors (OFETs) applications. However, the influence of fluoro substitution for fluorophores in organic light-emitting diodes (OLEDs) applications is rarely reported. Here, with fluorinated electron-deficient 2,1,3-benzothiadiazole (BT) unit as the acceptor and triphenylamine (TPA) as the donor, we construct two new D-A-type fluorophores DTPA-fBT and DTPA-ffBT. Theoretical calculations and experimental results reveal that the lowest excited states (S1) of the materials are characterized by hybridized local and charge-transfer (HLCT) properties, in which the fluorine substitution will increase the proportion of the charge-transfer (CT) component. Due to the introduction of fluoro substitution, multiple fluorine hydrogen bonds are formed, which endow the molecules with more rigid conformation, leading to lower non-radiative rates and higher photoluminescence quantum yield (PLQY) in solution and doped film, as well as improved thermal stability in the solid state. As a result, the two fluorinated DTPA-ffBT demonstrate significantly improved performance in OLEDs with a maximum external quantum efficiency (EQE) of 7.25% (5.90% for fluorine-free counterpart) and very low-efficiency roll-off (roll-off <7% at 10,000 cd m−2).