Organic Light-Emitting Diodes Based on Imidazole Semiconductors


Student thesis: Doctoral Thesis

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Award date16 Apr 2018


  Organic light-emitting diodes (OLEDs) are the most successful product of organic electronics in the market and continually receiving attention from scientific and industrial communities. Imidazole derivatives are widely used for OLED applications, and demonstrated their versatility in molecular design for different functions. This thesis focuses on novel imidazole semiconductors for OLEDs, including two main applications: deep-blue fluorescent emitters and host materials for phosphorescence OLEDs (PhOLEDs), employing phenanthro[9,10-d]imidazole (PI) and benzo[d]imidazole (BI) as building blocks, respectively.

  Firstly, a new member of polycyclic aromatic hydrocarbons named 1,2,3,4-tetraphenylnaphthalene (TNa) is designed and synthesized as a building block for deep-blue emitters. By incorporating TNa with PI, three blue emitters named TNa-PI, TNa-BPI and TNa-DPI featuring different length of phenyl linkers are designed. The new fluorophores show intramolecular charge transfer (ICT) properties in excited state with the TNa group serving as an electron acceptor. The use of TNa also improves electron conductivities of the emitters. A non-doped OLED based on TNa-DPI exhibits excellent electroluminescence (EL) performance with a maximum external quantum efficiency (EQE) of 5.78% and a deep-blue emission with Commission Internationale de l'Èclarage (CIE) coordinates of (0.152, 0.085).

  Secondly, I establish a new approach for developing efficient violet-blue-emitting materials featuring hybrid local and charge transfer (HLCT) excited state through incorporation of naphthyl group(s) as a weak n-type π spacer in a donor-π-acceptor system. The incorporation of a naphthyl/binaphthyl spacer between donor and acceptor moieties can alleviate a common dilemma that enhancing device performance by increasing charge transfer excited properties often leads to red-shifted emissions. A device using TPINCz as a dopant emitter shows a violet-blue emission with CIE coordinates of (0.153, 0.059) and a record high EQE of 6.56% at a luminance of 1000 cd m-2
  Thirdly, a series of aromatically substituted PI fluorophores at C6 and C9 are synthesized and systematically characterized by theoretical, thermal, photophysical, electrochemical and EL studies. Theoretical calculation suggests that the C6 and the C9 positions of PI are electronically different because of the two different nitrogen atoms of imidazole. Photophysical properties of the fluorophores are greatly influenced by size and conjugation extent of the substituents as well as the linking steric hindrance. Most of the devices show significantly improved efficiencies comparing to the OLED using the non-modified emitter, with the pyrene-based emitter showing the highest performances.

  Fourthly, a novel spiro-configured BI host material (SPBI-TPA) with a high triplet energy (ET) is designed. The C2 atom of BI is jointed to the spiro carbon, while a benzene ring is simultaneously attached to the pyrrole-like nitrogen of BI and spiro carbon with an sp3 linkage, forming a rigid non-conjugated triangular configuration. This special donor-spiro-acceptor design endows the resulting material SPBI-TPA with a bipolar nature and a high ET of 3.07 eV. SPBI-TPA is effective as host for FIrpic-, FIr6- and FK306-based blue PhOLEDs with maximum EQEs over 20%, which outperform its fluorene-based counterpart. 

  Lastly, two novel donor-σ-acceptor host materials BII-BCz and BII-TPA are synthesized by using the sp3 carbon attached in a 11H-benzo[4,5]imidazo[1,2-a]indole building block to link two electron-donating groups. The resulting materials feature with high ETs, good thermal properties and suitable photophysical properties as universal hosts for full-color PhOLEDs. Both BII-BCz and BII-TPA reveal excellent performances in blue, green, yellow, orange, red and white PhOLEDs with a simple device structure.

    Research areas

  • Organic light-emitting diode, Electroluminescence, Imidazole derivative