Organic light emitting (OLED) technology keeps advancing significantly in areas like solid-state lighting and display innovations [1, 2]. In the past decade, the development of unconventional display forms has been a key challenge in materials science, electrical engineering, and mechanical engineering, driven by their potential for diverse applications with outstanding features like ultra-lightweight design, unbreakable durability, and flexibility [3, 4].
Third generation, purely organic TADF emitters get a lot of attention as a potential replacement of the expensive phosphorescent emitters because of their ability to use 100% excitons for light emission without the need of noble metal emitters [5, 6]. Here, we demonstrate synthesis and properties of the new electroactive compounds, which have promising properties to be used as host materials for TADF based emitters.
The synthesis and full characterization of newly developed bipolar phenanthro[9,10-d]imidazole-based compounds are reported. The new derivatives deminstrated very high thermal stability and they formed amorphous layers with glass transition temperatures of 73-167 ºC. The electron photoemission spectra of the layers showed ionization potentials in the range of 5.39-5.59 eV. Hole drift mobility in thin layers of some derivatives exceed 2.8 × 10\(^{-5}\) cm\(^{2}\)V\(^{-1}\)s\(^{-1}\) at high electric fields at room temperature. The new derivatives were tested as hosts in OLEDs for commercial red TADF based emitter: 7,10-bis(4-(diphenylamino)phenyl)-2,3-dicyanopyrazino-phenanthrene (TPA-DCPP). The OLED device with 9-(2-ethylhexyl)-3-(1-phenylphenanthro[9,10-d]imidazol-2-yl)carbazole host exhibited superior performance with rather low turn on voltage of 3.5 V, maximum brightness of about 1000 cd/m\(^{2}\) and peak efficiency exceeding 5.6%. The characteristics are better than those of a similar device using the well-known commercial host material: 4,4′-bis(9-carbazolyl)-1,1′-biphenyl (CBP). We gratefully acknowledge the funding support from the Research Council of Lithuania (grant No. S-MIP-22-84).