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Organic light-emitting devices (OLEDs) already dominate the smartphone and tablet display market due to low power consumption, relatively cheap mass production technology and potential flexibility. In the past 20 years, research has successively led to fluorescence-based OLEDs as the first generation of these devices and phosphorescence-based OLEDs as the second. Today, phosphorescent OLEDs capable of converting 100% of created excitons into photons are mostly used in the display technology. Unfortunately, emitters in these devices contain heavy metal (typically iridium or platinum) atoms, which make them rather expensive. Recently, next-generation OLEDs based on thermally activated delayed fluorescence (TADF) were realized. This phenomenon also enables achieving 100% of internal quantum efficiency by reverse inter-system crossing (RISC) from exciton triplet state to the radiative singlet state in the materials with small singlet-triplet energy separation (of a few kBT). Although substantial progress has been made in research of blue-orange TADF OLEDs only a few attempts were made to realize efficient red TADF OLEDs, since their efficiency is lowered by not only concentration quenching but also by the enhanced non-radiative decay in low bandgap emitters due to Energy Gap Law. By fabricating an OLED with an emissive layer consisting of a host material doped with emissive (guest) molecules at a low concentration, emitter concentration quenching can be avoided. This is why finding an optimal host for TADF OLEDs is very important. The goal of this work is to fabricate, characterize and optimize the structure of the 3rd generation OLED with an emissive layer consisting of new bipolar host materials based on phenantroimidazole (PI) and a deep red TADF emitter TPA-DCPP (TPA = triphenylamine; DCPP = 2,3-dicyanopyrazino phenanthrene). Devices were fabricated by thermal evaporation in a high vacuum. Main OLED characteristics, i.e. external quantum efficiency (EQE), efficiency roll-off, I-V characteristics, turn on voltage and electroluminescence spectra were measured. OLED structure optimization was done considering EQE dependence on emitter concentration in the emissive layer as well as electron and hole current balance. By introducing PI hosts and optimizing device structure the efficiency of the red TADF OLED was increased 5-fold from 1.2% to 6%. The maximum EQE obtained is comparable to that reported in literature implying that PI hosts can be used as an alternative to commercial TPBI host. |
