Differently substituted benzonitriles for non-doped OLEDs

Abstract

Towards highly efficient doping-free organic light-emitting diodes, five benzonitrile-based emitters with different substitution pattern were designed exploiting symmetrical donor-acceptor-donor and asymmetrical donor-acceptor-donor* structures. As it was predicted by theoretical calculations, different thermally activated delayed fluorescence of different energy with relatively high absolute quantum yields (11–42%) was detected for non-doped films of the studied compounds. The smallest singlet-triplet energy splitting of 0.05 eV thus the most efficient TADF was estimated for the film of compound with the asymmetrical donor-acceptor-donor* structure containing carbazole and acridan donor moieties. Thermogravimetry revealed sublimation of the materials with the onset temperatures in the range of 350–383 °C. Glass transition temperatures of the molecular materials were in the range of 82–94 °C. Tuning of hole injection properties of these compounds in solid-state was demonstrated. Their ionization potential was in range from 5.8 to 6.0 eV. Strong effect of different substitutions was observed on hole mobilities of the layers of compounds. They were found to be in the wide range from 3 × 10−7 cm2V−1s−1 to 1 × 10−4 cm2V−1s−1. Electron mobility values of the compounds were found to be comparable and ranged from 1.5 × 10−4 cm2V−1s−1 to 3 × 10−4 cm2V−1s−1 at electric field of 6.9 × 105 Vcm−1. Reflecting effect of substitution pattern of benzonitrile on electroluminescent properties of OLEDs, maximum external quantum efficiencies in the range from 1.6 to 5% as well as maximum brightness in the wide range from 1200 to 22600 cd/m2 were observed for the devices based on the doping-free light-emitting layer.