High-performance organic light-emitting diodes (OLEDs) with tetradentate Pt(II) complexes and device structure improvement

Abstract

Cyclometallated platinum(II) complexes linked by tetradentate (O^N^C^N) or (N^C^C^N) ligands were characterized and employed as emitting dopants to fabricate organic light-emitting diodes (OLEDs) in the work presented in this thesis. High electroluminescent efficiency and improved efficiency roll-off at the practical luminance of 1000 cd/m2 were achieved in the optimized Pt-OLEDs. These desirable outcomes were attributed to the suppressed triplet-triplet annihilation (TTA) and concentration quenching effect originating from intermolecular interactions in our modified device structure. By utilizing emissions originating from both monomers and excimers of a tetradentate Pt(II) complex, voltage-dependent color-tunable OLEDs with a single emitter were realized. To the best of our best knowledge, such unique simple-structured devices demonstrated the greatest performance when compared with the current literature-reported, color-tunable OLEDs. The voltage-dependent color-tunable mechanism of this kind of single-emitter, color-tunable OLEDs was experimentally investigated and theoretically simulated; the resultant model, based on the competition between charge trapping and energy transfer, fits the electroluminescent behavior. In addition, three novel device structures for white OLEDs with tetradentate Pt(II) complexes were designed: 1) one with a combination of a sky-blue, Ir(III) complex emitter and a yellow-emitting Pt(II) complex, 2) one with an excimer-forming Pt(II) complex, and 3) one with an exciplex-formation co-host system as the blue emitter. Chapter 3 presents the fabrication of full-color phosphorescent OLEDs with tetradentate Pt(II) complexes. With the use of a double-host or double-emissive-layer (DEML) strategy, the efficiency roll-off of these OLEDs was largely improved within a practical luminance of 1000-10000 cd/m2, since the exciton recombination zone could be broadened via the double-host or DEML structure, which in turn suppressed the TTA. Meanwhile, the improved balance of the electron/hole after the optimization of each layer, the high photoluminescent quantum yield (PLQY), and the short excited-state lifetime of the tetradentate Pt(II) emitter also contribute to the improvement of the device efficiency roll-off. As a result, maximum external quantum efficiencies (EQEmaxs) of 23.2%, 24.6%, 16.1%, and 16.1% were respectively achieved in yellow, green, red, and blue OLEDs. The efficiency roll-offs were 1.8%, 0.6%, 13%, and 4.3% at a luminance level of 1000 cd/m2 for these devices. Because of the low driving voltage of our device, impressive power efficiencies (PE) of up to 102.8 and 102.0 lm/W were demonstrated in yellow and green Pt-devices, respectively, which are comparable to the best reported OLEDs. Chapter 4 presents the application of the strategies described in the previous Chapter, DEML and double hosts structures, to design voltage-dependent, color-tunable OLEDs with a single emitter. High EQEmax of up to 21.6% and high luminance of 88600 cd/m2 were achieved in a device with tetra-Pt-2. With the improved efficiency roll-off of this device, high EQEs of 20.6% and 17.6% were maintained at high luminance of 1000 and 10000 cd/m2, respectively. At the same time, the “Commission Internationale De l’Éclairage 1931” (C.I.E.) coordinates could be tuned from (0.54, 0.43) to (0.40, 0.58) by increasing the voltage from 3 to 12 V. To the best of our knowledge, the efficiency at high luminance of our device is the best described in the literature, with regards to color-tunable OLEDs. Based on a similar device structure, voltage-dependent color-tunable OLEDs with tetra-Pt-1 or Et-Pt-F2 were also realized with slight efficiency roll-offs of 0.9% or 9.3%, respectively, at 1000 cd/m2, suggesting that this device design is universal for single-emitting, color-tunable OLEDs. The voltage-dependent, color-tunable phenomenon in this kind of device can be explained by a “trapping–energy transfer” model that describes the competition between the direct trapping of carriers in the low-energy Pt(II) excimers and the energy transfer from the host to Pt(II) monomers. Since the aggregation of tetradentate Pt(II) complexes discussed in this chapter does not decrease the PLQY value, and even shortens the emissive lifetime of these complexes, the efficiency remained at a high value at high luminance in our device. The combined DEML/double-host device structure further suppresses the TTA, leading to the lower efficiency roll-off of our color-tunable OLEDs. Chapter 5 presents the design and characterization of three types of white OLEDs (WOLEDs) with tetradentate Pt(II) complexes. In the first design, a white OLED was composed of a sky-blue, Ir(III) complex emitter and a low-energy, Pt(II) complex emitter. EQE of up to 16.0%, a color rendering index (CRI) of 86, and C.I.E coordinates of (0.45, 0.43) at 1000 cd/m2 were achieved in this device. In the second design, warm-white OLEDs were realized using an excimer-forming Pt(II) complex as a red-emitter, sandwiched between two blue emitters. EQE of up to 16.7%, a CRI of 85, and C.I.E. coordinates of (0.48, 043) at 1000 cd/m2 were observed in this device. In the third design, the exciplex formed by CDBP: PO-T2T was employed as a blue emitter and tetra-Pt-2 was employed as a low-energy emitter to fabricate WOLEDs, since the exciplex can theoretically utilize 100% excitons via reverse intersystem crossing (RISC) from triplet states to singlet states, usually possessing satisfactory bipolar charge-transport properties. As a result, EQEmax of 12.2% and a CRI of 74 were achieved, with the C.I.E. coordinates of (044, 0.49) at 1000 cd/m2.