Internal Electric Fields in Polar TADF Emitters: Insights from Electroabsorption Studies of TPA-DCPP

Thermally activated delayed fluorescence (TADF) materials are promising candidates for organic light-emitting diodes (OLEDs). In this work, we investigated the internal electric field in devices based on triphenylamine–2,3-dicyanopyrazinophenanthrene (TPA-DCPP), a red-emitting TADF material. The internal electric field plays a key role in determining carrier injection, transport, recombination, and dissociation processes in organic optoelectronic devices. Using electroabsorption (EA) spectroscopy, we obtained the electric field profiles inside TPA-DCPP devices of different thicknesses under varying external biases. The study revealed the presence of an auxiliary field within the devices, in addition to the applied external field. The contributions from both the bulk and the interfaces to this auxiliary field were examined by introducing MoO3 buffer layers into the device structure. The interfacial contribution was found to arise from charge accumulation at the metal–organic interface, while the bulk contribution was attributed to spontaneous orientation polarization (SOP), which was further verified by spectroscopic ellipsometry. Additionally, current–voltage and capacitance–voltage measurements indicated the presence of charge traps in the active layer, which modified the internal field distribution of the device.