Plasmonic electrophosphorescence: "Feeling" blue

Surface plasmon polaritons (SPPs), excited at the interface between the metallic electrode and dielectric layer, are commonly considered detrimental to the performance of organic light-emitting diodes (OLEDs).1,2,3 Approximately 20%–40% of photons are confined within the SPP mode, resulting in exciton annihilation and energy dissipation. Compared with the low internal quantum efficiency (IQE) of fluorescent materials and the unpleasant efficiency roll-off of thermally activated delayed fluorescence (TADF)-based devices, phosphorescent materials have received increasing attention for blue OLEDs, owing to their near-unity IQE and relatively better stability. Owing to the prolonged blue triplet lifetimes and slow radiative decay rates, the phosphorescent molecules are prone to dissociation under electrical excitation. This results in the excessively short operational lifetimes for blue phosphorescent OLEDs (PHOLEDs). Inspirationally, the stability of blue phosphorescent devices with plasmonic structures has been demonstrated as a big breakthrough by utilizing the polariton-enhanced Purcell (PEP) effect to accelerate radiative decay, using metallic nanoparticles (MNPs) to extract energy from the SPP mode.

Blue phosphorescence

First-generation OLEDs employ fluorescent emitters with a low IQE of only 25%, thereby facing the dilemma of inherent performance limitations. Although the state-of-the-art TADF emitters can achieve up to 100% IQE, TADF devices face significant external quantum efficiency (EQE) roll-off and inferior stability because of the accumulation and annihilation of excitons caused by inefficient reverse intersystem crossing. In contrast, heavy-metal-based phosphorescent materials, capable of reaching an IQE of 100% as well, exhibit faster radiative decay of triplet excitons due to the heavy-atom effect. While high-performance green and red PHOLEDs have been employed in commercial displays, blue PHOLEDs still remain constrained by their short operational lifetimes and high cost. Serious nonradiative recombination processes, such as triplet-triplet annihilation (TTA) and triplet-polaron annihilation (TPA), frequently rule blue PHOLEDs.

To tackle this issue, researchers have exploited lots of strategies for luminescent materials and emission layers (EMLs). On the materials front, some useful strategies include developing more rigid molecular structures to suppress vibrational relaxation and enhancing chemical bond stability through methods such as deuteration. For the EML, some unique approaches, such as gradient doping of host and guest, mixed hosts, and hot excited state management, have been employed to reduce triplet exciton density and accelerate exciton decay. In addition, EML systems, such as TTA upconversion, hyperfluorescence, inverted singlet-triplet system, and phosphor-assisted TADF-sensitized fluorescence, have been developed to enhance device stability. However, the effectiveness of these methods diminishes rapidly in deep-blue devices due to the increased exciton energy, which triggers molecular degradation processes more easily.

Recently, Forrest's group demonstrated a significant improvement in the operational lifetimes of blue plasmonic PHOLEDs with different planar-cavity architectures, such as single-junction devices and tandem devices, manipulating the PEP effect. They achieved a long lifetime of LT90 = 830 ± 30 h with an initial luminance of L0 = 500 cd m−2 in blue planar tandem plasmonic PHOLEDs.5 The lifetime enhancement was nearly 250 times that of conventional single-junction devices. To be specific, such a strategy generally employs optical tuning of a microcavity to actively induce resonance between the SPP mode and excitons within the adjacent organic layer, such as the electron transport layer and hole transport layer, respectively, thereby forming strongly coupled plasmon-exciton polaritons. Then, it accelerates the radiative decay rate (kr) of excitons and reduces the triplet radiative lifetime, fundamentally reducing the probability of TTA and TPA.