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Keywords:

  • ambipolar transport;
  • electroluminescence;
  • emission color;
  • singlet–triplet splitting;
  • organic light-emitting diodes

Abstract

Two coordination complex emitters as well as host materials Be(PPI)2 and Zn(PPI)2 (PPI = 2-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenol) are designed, synthesized, and characterized. The incorporation of the metal atom leads to a twisted conformation and rigid molecular structure, which improve the thermal stability of Be(PPI)2 and Zn(PPI)2 with high Td and Tg at around 475 and 217 °C, respectively. The introduction of the electron-donating phenol group results in the emission color shifting to the deep-blue region and the emission maximum appears at around 429 nm. This molecular design strategy ensures that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) HOMO and LUMO of Be(PPI)2 and Zn(PPI)2 localize on the different moieties of the molecules. Therefore, the two complexes have an ambipolar transport property and a small singlet–triplet splitting of 0.35 eV for Be(PPI)2 and 0.21 eV for Zn(PPI)2. An undoped deep-blue fluorescent organic light-emitting device (OLED) that uses Be(PPI)2 as emitter exhibits a maximum power efficiency of 2.5 lm W−1 with the CIE coordinates of (0.15, 0.09), which are very close to the National Television Standards Committee (NTSC) blue standard (CIE: 0.14, 0.08). Green and red phosphorescent OLEDs (PhOLEDs) that use Be(PPI)2 and Zn(PPI)2 as host materials show high performance. Highest power efficiencies of 67.5 lm W−1 for green PhOLEDs and 21.7 lm W−1 for red PhOLEDs are achieved. In addition, the Be(PPI)2-based devices show low-efficiency roll-off behavior, which is attributed to the more balanced carrier-transport property of Be(PPI)2.