Dicyano-Imidazole-Based Host Materials Possessing a Balanced Bipolar Nature To Realize Efficient OLEDs with Extremely High Luminance

Abstract

Most phosphorescent devices suffer from severe triplet−triplet annihilation (TTA), and the efficiency recording at high luminance is much lower than that at low luminance, making the practicality worse than expected. In this study, a series of donor−acceptor (D−A) molecules consisting of dicyano-imidazole and phenylcarbazole were synthesized and applied to the host materials. For electroluminescence applications, imM-m-Cz-based green-emitting organic light-emitting diodes show a maximum luminance of 1.68 × 10 5 cd m −2 at 11.2 V, which is 20% higher than that of the benchmark host 4,4′-bis(N-carbazolyl)-1,1′-biphenyl at 13.4 V; in addition, the turn-on voltage (V on ) is only 2.3 V. In terms of hole and electron mobility, imM-m-Cz shows one of the most balanced carrier mobilities in the reported literature (electron and hole mobilities of 3.64 × 10 −5 and 4.23 × 10 −5 cm 2 V −1 s −1 , respectively). The balanced carrier mobility can help expand the recombination region and thus reduce the formation of TTA. Furthermore, the high maximum luminance of about 2.80 × 10 5 cd m −2 obtained in an imM-m-Cz-based tandem device demonstrates a sufficiently high current density/luminance, and the peak efficiency achieves an even higher efficiency of 40.6% (1.51 × 10 5 cd A −1 and 98.6 lm W −1 ), which is among the highest reported properties based on imidazole-based host materials. Combining the balanced carrier mobility with the proper device design in this study, the efficiency roll-off under high luminance can be greatly reduced, and the practicality of the resulting phosphorescent device can be significantly improved.

Dicyano‐Imidazole: A Facile Generation of Pure Blue TADF Materials for OLEDs

“…The configuration was established in the form of D‐ π ‐A using the benzene ring as a conjugated link between chromophores. Commercially available 2‐amino‐1 H ‐imidazole‐4,5‐dicarbonitrile was methylated with sodium hydride (NaH) and iodomethane (CH 3 I) to form the key acceptor 2‐iodo‐1‐methyl‐1 H ‐imidazole‐4,5‐dicarbonitrile ( imM ) [14] . Four different electron donors (compound 1 – 4 ) were used in Suzuki coupling reaction to form C−C bonds in the presence of Pd catalyst, obtaining a series of targeted products with yields of 45–90 %.…”

“…Imidazole derivatives are proven to be promising materials with nonlinear optical, [10] fluorescent, [11] phosphorescent, [12] luminescent, and photochromic properties and are found to be suitable for the photonics, sensing, and optical switches, etc [13] . Our previous work proposed a series of imidazole‐based host molecules, which confirmed an extremely balanced carrier mobility strategy to produce maximum brightness and external quantum efficiency (EQE max ) (2.80×10 5 cd m −2 and 40.6 %) to achieve extraordinary tandem OLEDs [14] . On the other hand, in 2015, Zhu and co‐workers indicated a series of triphenylimidazole derivatives as blue fluorescent OLEDs with electroluminescent (EL) emission wavelengths in the range of 448 to 472 nm [15] .…”

“…Suzuki–Miyaura cross‐coupling reaction was applied to synthesize four targeted molecules. A mixture of corresponding starting materials 1 , [20] 2 , [21] 3 , [22] 4 [23] (0.10 g) and imM [14] (1.1 equivalence) was heated with K 2 CO 3 (7.5 equivalence), tetrakis(triphenylphosphine)palladium(0) (0.10 equivalence) in THF/H 2 O (4/1=v/v) for 16 h to produce imM‐ m ‐DMAC , imM‐DMAC , imM‐SPAC and imM‐DPAC , respectively. The organic layer was dried over anhydrous Na 2 SO 4 , and the crude solid was purified by silica gel column chromatography with n ‐hexane/ethyl acetate=4/1 as eluent.…”

“…Since these imidazole-based TADF emitters reveal emission colors ranging from deep-blue to sky-blue, the paired host materials should be carefully selected to fulfill the exothermic energy transfer. [14] Carrier balance was examined using two wide triplet-energy-bandgap host materials with different carrier transport capabilities, namely DPEPO and 9-(4tertbutylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi). [27] Two kinds of device architectures were designed for comparison.…”

“…[13] Our previous work proposed a series of imidazole-based host molecules, which confirmed an extremely balanced carrier mobility strategy to produce maximum brightness and external quantum efficiency (EQE max ) (2.80 × 10 5 cd m À 2 and 40.6 %) to achieve extraordinary tandem OLEDs. [14] On the other hand, in 2015, Zhu and co-workers indicated a series of triphenylimidazole derivatives as blue fluorescent OLEDs with electroluminescent (EL) emission wavelengths in the range of 448 to 472 nm. [15] The device with 1-NaCBI as the dopant turned out to be the bluest (448 nm), but the EQE max was only 3.50 %.…”