Degradation Mechanism and Lifetime Improvement Strategy for Blue Phosphorescent Organic Light‐Emitting Diodes

Abstract

caused by the low efficiency of traditional fluorescent OLEDs. [15][16][17] The invention of PhOLEDs quadrupled the quantum efficiency (QE) of OLEDs and resulted in power savings in the OLED display panel. Presently, commercialized OLED products employ red, yellow and green phosphorescent emitters, but cannot use a blue phosphorescent emitter because of the poor lifetime of blue PhOLEDs. [18,19] Therefore, the power consumption cannot be further reduced due to the inherently low QE of blue fluorescent OLEDs.Currently, the QE of the blue PhOLEDs is sufficiently high, but the lifetime of blue PhOLEDs is less than one tenth that of blue fluorescent OLEDs. [20,21] Furthermore, there are no papers that report the lifetime of deep blue PhOLEDs that have a CIE color coordinate below 0.20. Therefore, a breakthrough that improves the lifetime of blue PhOLEDs is badly needed. Before we can effectively extend the lifetimes of blue PhOLEDs, it is important to understand the origin of the poor lifetimes of blue PhOLEDs. In this work, we classified and described the reasons for poor lifetimes and reviewed research results that focused on improving the lifetime of blue PhOLEDs from two perspectives, materials and devices. This was based on recent work dealing with lifetime issues in blue PhOLEDs, and we proposed a solution to resolve the key challenge of poor blue PhOLED lifetime. Basic Mechanism of DegradationDevice degradation, which is quantified by the lifetimes of the devices, is an efficiency loss process that happens in the OLEDs over time during the electrical driving process, which originated from the inherently poor stability of organic materials inserted in the carrier transport or emitting layers of OLEDs. [22][23][24][25][26] It is true that the intrinsic instability of organic materials is unavoidable. Therefore, many degradation processes of OLEDs have their origin in improper management of device structures, which aggravate material degradation. Therefore, both material and device related degradation routes should be managed at the same time.There are several material related degradation mechanisms in OLEDs. [14,25] One of the most important material related degradation pathways is the inherent degradation of organic Providing adequate lifetimes for organic light-emitting diodes (OLEDs) has been a challenging issue for a long time because of the naturally weak chemical bonds of organic materials that can be damaged during electrical processes that drive light emission. The lifetime of OLEDs has been dramatically extended to the point where commercialization is feasible due to the development of stable materials and device structures that lessen the damage of the organic materials. However, the lifetime of high efficiency OLEDs represented by phosphorescent OLEDs is still short, and this prevents full utilization in red, green and blue colors. The lifetime of the blue phosphorescent OLEDs is particularly short, i.e., less than one tenth of the lifetime of conventional blue fluorescent OLEDs. Therefore, a large in...

Mer‐Ir(ppy)3 to Fac‐Ir(ppy)3 Photoisomerization

“…Daniel Escudero* [a] In this contribution a detailed mechanistic investigation of merto-fac photoisomerization occurring in Ir(ppy) 3 (ppy = 2-phenylpyridine) is presented. Facial/meridional stereoisomerism in trisbidentate Ir(III) complexes strongly determines their photophysical properties and thereby their efficiency and stability within a phosphorescence-based organic light-emitting diode (PhOLED) device.…”

“…In the case of phosphors, ligand dissociation reactions are the most common degradation routes. 3 MC states are usually highly distorted with respect to the ground state ( 1 GS) and lowest triplet (T 1 ) excited state geometries, [17] the latter typically being of predominant metal-to-ligand charge transfer character, i. e., 3 MLCT. Thus, these final products are ultimately responsible for the significant luminance loss in the aged devices.…”

“…3 MC states are usually highly distorted with respect to the ground state ( 1 GS) and lowest triplet (T 1 ) excited state geometries, [17] the latter typically being of predominant metal-to-ligand charge transfer character, i. e., 3 MLCT. [19] Besides being relevant for PhOLEDs' degradation, 3 MC states also act as very efficient funnels for the radiationless deactivation to the 1 GS, due to the presence of minimum energy crossing points (MECP) between the 1 GS and 3 MC PES. [17,18] Therefore, ligand dissociation is more likely to occur from a 3 MC state than from either 1 GS or T 1 .…”

“…For instance, in cyclometalated Ir(III) complexes, both 1 GS and T 1 exhibit a pseudo-octahedral geometrical disposition while the 3 MC state often displays a pentacoordinate trigonal bipyramid arrangement bearing one broken IrÀ N bond. [21][22][23] Thus, 3 MC states are also responsible for the diminished photoluminescence quantum yields of phosphors at room temperature. As recently disclosed, the excited state potential energy surfaces (PES) of these complexes, but also in the case of e. g., [Ru(bpy) 3 ] 2 + , [19] might be more complex than originally believed.…”

“…

In this contribution a detailed mechanistic investigation of merto-fac photoisomerization occurring in Ir(ppy) 3 (ppy = 2-phenylpyridine) is presented. [3][4][5][6][7][8] Degradation of PhOLEDs' materials originates from parasitic exciton-polaron and exciton-exciton annihilation processes, [4,7] such as e. g., triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA), which also compromise PhOLEDs efficiency at high brightness levels (i. e., roll-off effect). Double-hybrid density functional theory (DFT) calculations are performed herein to identify the photoactive species involved in the photoisomerization reaction and to assess its kinetic and thermodynamic feasibility at room temperature.

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