Pyridine‐Containing Electron‐Transport Materials for Highly Efficient Blue Phosphorescent OLEDs with Ultralow Operating Voltage and Reduced Efficiency Roll‐Off

Abstract

3268www.MaterialsViews.com wileyonlinelibrary.com reached 90 lm W −1 at 1000 cd m −2 , [ 2 ] which is readily above the effi ciency of a general fl uorescent tube. However, there is still a big room for improvement compared to the theoretical limit of 248 lm W −1 . [ 3 ] Among the three primary colors, effi cient blue light emission is the bottleneck for effi cient full-color OLED display and white OLEDs. [ 4 ] In the past few years, many studies have focused on improving external quantum effi ciency ( h ext ) of blue OLEDs by utilizing a common blue phosphorescent emitter of iridium(III) bis [(4,6-h ext values over 20% have been achieved by developing new materials, such as bipolar host materials and electron-transport materials (ETMs) with high triplet energy level, due to effi cient harvesting of both electro-generated singlet and triplet excitons for phosphorescent emission from OLEDs. [ 1c , 5 ] Most recently, h ext values were further improved to ≈30% by Lee et al. by using a pyrido [2,3-b ] indole derivative as the host material and a 50-nm-ITO-coated glass as the substrate due to the improved carrier balance and light outcoupling. [ 6 ] Aside from the external quantum effi ciency, reducing operating voltage is also crucially important for high power conversion effi ciency. As the most recent report, although h ext value of ≈30% has been achieved, the power effi ciency was compromised by the relatively high driving voltage, giving the maximum power effi ciency ( h P ) of only 50.6 lm W −1 . [ 6b ] Nevertheless, unlike the most intensively investigated EQE, reducing operating voltage without compromising external quantum effi ciency seems to be ignored or underestimated. The limit of operating voltage is generally believed to be equal to the energy band gap ( E g ) of the emitter molecule or the photon energy ( hv ) of the emission light divided by the electron charge ( e ), which is corresponding to the energy difference between the highest occupied molecular orbital (HOMO) and the unoccupied molecular orbital (LUMO). Although a few exceptions were reported, where the devices could even be operated at half of the voltage of E g / e , [ 7 ] the electroluminescence was achieved with an Augerassisted mechanism, and multiple electrons should be used for one photon emission to give a very low h ext of 0.4%.

Solution-Processed Blue/Deep Blue and White Phosphorescent Organic Light-Emitting Diodes (PhOLEDs) Hosted by a Polysiloxane Derivative with Pendant mCP (1,3-bis(9-carbazolyl)benzene)

“…48 that an additional electron-transport layer enhances efficiency in FIrpic devices. 59 The E T values of PmCPSi and Tm3PyPB (2.75 eV) 15,60 are sufficiently high to confine triplet excitons in the emitting layer and to prevent luminescence quenching. The electroluminescence spectra of the blue PhOLEDs are shown in Figure 4d.…”

Synthesis and optoelectronic properties of novel fluorene-bridged dinuclear cyclometalated iridium (III) complex with A–D–A framework in the single-emissive-layer WOLEDs

“…Over the past two decades, considerable progresses have been achieved for red, green and blue OLEDs via utilizing phosphorescent materials of iridium complexes and platinum complexes, which could reach 100% internal quantum efficiency (IQE) in theory [6][7][8][9][10][11]. However, high-efficiency stable white organic light-emitting diodes (WOLEDs) remain a challenge [12,13].…”

P‐170: Organic Light‐Emitting Diodes Incorporating a Novel Exciplex‐Forming Host: A Synergistic Strategy to Design Highly Simplified OLEDs for Application

“…[1] An additional hole transport layer (HTL) is also typically used and introduce additional interface, [6] with the most common examples being thin films of 1,1-bis [4-[N,N-di(ptolyl)-amino]phenyl] cyclohexane) (TAPC), which was reported and significant decrease in the drive voltage was shown. [7] However, each additional layer that is required to balance the carrier transport and lower the charge transport barrier, largely increases the manufacturing costs. Hence, it shows greatly demanded to make the device structure and fabrication process more simplify.…”