Highly Efficient Nondoped Near‐Ultraviolet Electroluminescence with an External Quantum Efficiency Greater Than 6.5% Based on a Carbazole–Triazole Hybrid Molecule with High and Balanced Charge Mobility

Abstract

Organic light-emitting diodes (OLEDs) with conjugated organic molecules as fluorescent emitters continue to attract significant interest because they have flexible molecular structures and are easy to prepare and abundant resources of organic fluorophores. [1] To date, while red, green, and even ordinary blue emitters with desirable electroluminescence (EL) efficiencies have been achieved, highly efficient organic emitters exhibiting the near-ultraviolet (NUV) EL (such as with emission peak at Low charge injection and unbalanced carrier transport are unfavorable factors for highly efficient near-ultraviolet (NUV) electroluminescence (EL) emitters. The construction of weak donor-and acceptor-containing fluorophores has been utilized as a common solution to these issues. Few fluorescent emitters simultaneously exhibiting NUV emission and high/balanced charge-transport properties have been reported. Herein, a new 1D donor-π-acceptor molecule 9-(4′-(4,5-diphenyl-4H-1,2,4-triazol-3-yl)-[1,1′-biphenyl]-4-yl)-9H-carbazole named PCZTZ consisting of 9-phenylcarbazole as a donor and 2,3,4-triphenyl-1,2,4-triazole as an acceptor with a biphenyl π-bridge is constructed. The backbone-twisted PCZTZ exhibits not only strong solid-state NUV fluorescence but also high and balanced hole and electron mobilities (both up to 10 −4 cm 2 V −1 s −1 ), benefiting from the 3D supramolecular H-bonding network containing strong CH⋯N interactions. Remarkably, a fabricated nondoped light-emitting diode (OLED) emits 408 nm bright NUV light with a maximal external quantum efficiency of 6.57% and Commission Internationale de L'Eclairage coordinates of (0.17, 0.07). Thus, this OLED is the most efficient NUV OLED reported thus far. These results provide a valuable strategy for the design of highly efficient NUV EL emitters by modulating the linking positions of carbazole and triazole on a biphenyl bridge.

Giant Enhancement of External Quantum Efficiency in Near‐UV Organic Light‐Emitting Diodes via Device Aging and Impedance Spectroscopy Analysis

“…We posit that this result is caused by a hybridized excited state consisting of locally excited and charge-transfer states in the PCZTZ molecules. [18] Furthermore, the 3D supramolecular H-bonding network of PCZTZ facilitates carrier migration, which promotes charge exchange between neighboring molecules through tripletpolaron-interaction-induced upconversion. [18,22] These factors contribute to highly efficient light emission of the PCZTZ molecule.…”

“…[18] Furthermore, the 3D supramolecular H-bonding network of PCZTZ facilitates carrier migration, which promotes charge exchange between neighboring molecules through tripletpolaron-interaction-induced upconversion. [18,22] These factors contribute to highly efficient light emission of the PCZTZ molecule. The EL spectra of the NUV OLED have short-wavelength emission with an emission peak of 404 nm, which remains almost unchanged after aging (Figure 2d).…”

“…Indium-tin-oxide (ITO)-coated glass was sequentially cleaned with acetone, isopropyl alcohol, and deionized water, followed by a 1a). CBP, PCZTZ, [18] and BPhen were used as the hole transport layer, NUV fluorescent molecule, and electron transport layer, respectively. The current density-voltage-radiance ( J-V-R) characteristics and EL spectra were simultaneously measured using a Keithley 2400 Source Meter, Ocean Optics Maya2000Pro Spectra Scan, and data acquisition software.…”

19‐2: Invited Paper: Design of Multi‐Resonance Thermally Activated Delayed Fluorescence Materials for Organic Light‐Emitting Diodes

“…A stable and efficient high energy emitting OLED can significantly simplify the fabrication process and thus reduce the cost of full color displays by generating other emission colors without direct electrical excitation of blue, green, and red pixels [7]. Most of the examples of violet OLEDs (lEL < 400 nm) use fluorescent emitters [7][8][9][10][11][12].…”

The origin of the unusual red-shifted aggregation-state emission of triphenylamine-imidazole molecules: excimers or a photochemical reaction?

“…As a classical molecular structural unit of functional materials, 1-3 imidazole possesses a typical five-member ring containing two entirely different nitrogen atoms, where one is electron-rich and the other is electron-deficient, which allows great control of the photoelectrical properties of imidazole derivatives. This is why imidazole derivatives can be extensively applied to OLEDs, [4][5][6][7][8][9][10][11][12] fluorescent switches, [13][14][15] optical sensors, [16][17][18] electron transfer mediators, 19 memory devices 20 and other fields of interest. [21][22][23][24][25][26] With the requirement for improved material performance, it is even more necessary to further explore imidazole molecules with new structures and investigate their photophysical and photochemical properties.…”