Dimers or Solid-State Solvation? Intermolecular Effects of Multiple Donor–Acceptor Thermally Activated Delayed Fluorescence Emitter Determining Organic Light-Emitting Diode Performance

Abstract

Organic light-emitting materials exhibiting thermally activated delayed fluorescence (TADF) show great promise for improving display applications. Recently, intermolecular effects between emitting molecules have been given more attention, revealing strong solid-state solvation or aggregation induced changes of sample performance. Implications of this on device performance are not yet fully covered. In this work, a thorough investigation of a novel TADF emitter, methyl 2,3,4,5,6-penta(carbazol-9-yl)benzoate (5CzCO2Me), is provided. Steady-state emission spectra reveal a luminescence redshift with increasing emitter concentration in a small molecule host. In all investigated concentrations, the emission profile remains the same; thus, the redshift is attributed to the solid-state solvation effect. The highest photoluminescence quantum yield (PLQY) is achieved in the 20 wt% sample, reaching 66%. The best organic light-emitting diode (OLED) in terms of current–voltage–luminance and external quantum efficiency (EQE) parameters is the device with 60 wt% emitter concentration, reaching maximal EQE values of 7.5%. It is shown that the emitter transports holes and that charge-carrier recombination does not take place on the bandgap of the host, but rather, a mixed host–guest concentration-dependent recombination is seen. The hole-transporting properties of 5CzCO2Me allow for a new dimension in tuning the device performance by controlling the emitter concentration.