Degradation Mechanisms in Blue Organic Light-Emitting Diodes

Abstract

An organic light-emitting diode (OLED) is required to exhibit long-time operation without degradation as an inorganic LED. Sufficiently long operation time has been demonstrated for green-and red-emitting OLEDs. However, a blue device that is important for full-color display and lighting exhibits a much shorter operational lifetime than the other color devices. The short lifetime is mainly attributed to the molecular dissociation and the defects and radical species formation through various unimolecular and bimolecular processes, including direct photolysis, exciton-exciton interaction, and exciton-polaron interaction, and so on. Different novel techniques of chemistry and physics have been employed for blue devices to suppress the degradation process induced by high-energy excitons. A deep understanding of the degradation mechanism is still needed for all three kinds of blue OLEDs employing fluorescence, phosphorescence, and thermally active delayed fluorescence. In this brief review, we introduce and discuss several degradation mechanisms for these three kinds of OLEDs.

Hybridized Local and Charge-Transfer Excited-State Fluorophores through the Regulation of the Donor–Acceptor Torsional Angle for Highly Efficient Organic Light-Emitting Diodes

“…It is speculated that o-2CzBT and m-2CzBT have potential as host materials for orange and red organic emitters, considering their good charge-carrier transporting abilities with greenish-blue emission. In this regard, 4-(7-(4-(diphenylamino)phenyl)benzo[c] [1,2,5] thiadiazol-4-yl)benzaldehyde (TPABCHO) (inset of Figure 5a), which is an orange HLCT emitter we previously reported, 16 was chosen as a dopant. There is considerable overlap between the UV-vis absorption spectrum of TPABCHO and the PL spectra of o-2CzBT and m-2CzBT (Supporting Information Figure S25), facilitating efficient Förster resonance energy transfer (FRET) from o-2CzBT/ m-2CzBT to TPABCHO.…”

“…Although second-generation phosphorescent emitters can achieve an IQE of 100%, they suffer from high cost due to the incorporation of noble metal, as well as shortage of efficient blue phosphorescent emitters. 1,2 In this regard, third-generation pure organic emitters including thermally activated delayed fluorescence (TADF) [3][4][5][6][7] and hybridized local and charge-transfer (HLCT) [8][9][10][11] excited-state emitters, which render a 100% exciton utilization efficiency while avoiding noble metal, have been considered as potential candidates. Among these, TADF emitters have been widely explored, as their intrinsic long exciton lifetimes tend to cause serious efficiency roll-off when the devices are driven under high currents.…”

TADF sensitization targets deep-blue

“…Such devices are desired for next-generation displays with improved colour gamut and brightness. It is a challenging goal, however, as many blue organic materials suffer molecular bond dissociations, which in turn leads to device degradation under electrical operation 1 .…”

Thioxanthone-based thermally activated delayed fluorescence emitters showing fast reverse intersystem crossing for efficient organic light-emitting diodes with small efficiency roll-off

“…However, TADF OLEDs usually suffer from e ciency roll-off at high brightness due to the long-lived T 1 excitons, as observed from long-delayed uorescence with the lifetime ranging from several microseconds to milliseconds. [13][14][15] The delayed uorescence of TADF materials occurs via a RISC (T 1 →S 1 ) process followed by a radiative decay (S 1 →the ground state(S 0 )) pathway. The relatively long-delayed lifetime is closely related to the small rate constant of RISC (k RISC ), as reported that RISC process is the rate-limiting process of the TADF.…”