Water-soluble pH neutral triazatruxene-based small molecules as hole injection materials for solution-processable organic light-emitting diodes

Abstract

Three novel water-soluble and pH neutral small molecules based on a triazatruxene (TAT) core containing sodium propanylsulfonate, sodium butanylsulfonate and sodium butanyl-2-sulfonate as the side chains, respectively, have been designed and used as hole injection materials in solution-processable organic light-emitting diodes. These molecules show a high transmittance of nearly 100% in the visible and near infrared region (400–1000 nm). Relative to PEDOT:PSS, the comparable HOMO levels and higher LUMO levels of TAT-based materials favor the efficient injection of holes to the emitting layer and the efficient blocking of electrons and excitons. Among the TAT-based materials, the device based on TAT-4C with the sodium butanylsulfonate side chains exhibits the best performance with a maximum EQE of 11.0%, a maximal current efficiency (LE) of 38.8 cd A⁻¹ and a power efficiency (PE) of 39.2 lm W⁻¹, which are also higher than those of the PEDOT:PSS-based device (an EQE of 10.6%, a LE of 37.6 cd A⁻¹ and a PE of 35.1 lm W⁻¹). In additon, all the devices based on the TAT-based molecules show better performance than PEDOT:PSS-based device at 10 000 cd m⁻², owing to their wider bandgaps and higher LUMO levels than PEDOT:PSS, indicating that the utilization of these TAT-based molecules as hole injection materials retards the efficiency decline.