Efficiency Roll‐Off in Organic Light‐Emitting Diodes

Abstract

Organic light-emitting diodes (OLEDs) have attracted much attention in research and industry thanks to their capability to emit light with high efficiency and to deliver high-quality white light that provides good color rendering. OLEDs feature homogeneous large area emission and can be produced on flexible substrates. In terms of efficiency, OLEDs can compete with highly efficient conventional light sources but their efficiency typically decreases at high brightness levels, an effect known as efficiency roll-off. In recent years, much effort has been undertaken to understand the underlying processes and to develop methods that improve the high-brightness performance of OLEDs. In this review, we summarize the current knowledge and provide a detailed description of the relevant principles, both for phosphorescent and fluorescent emitter molecules. In particular, we focus on exciton-quenching mechanisms, such as triplet-triplet annihilation, quenching by polarons, or field-induced quenching, but also discuss mechanisms such as changes in charge carrier balance. We further review methods that may reduce the roll-off and thus enable OLEDs to be used in high-brightness applications.

Bis‐Tridentate Iridium(III) Phosphors with Very High Photostability and Fabrication of Blue‐Emitting OLEDs

“…However, it should be noted that the chemical stability is not the solely factor in determining the OLED stability and efficiency roll‐off. Other device parameters such as the exciton interaction, exciton–polaron interaction, field‐induced quenching, and charge carriers imbalance also need to be considered 31. In an effort to unveil this efficiency roll‐off, a 2 nm thick, red‐emitting [Ir(piq) 2 (acac)] dopant was co‐deposited to the emitting layer, at a conc.…”

Monte Carlo study of efficiency roll-off of phosphorescent organic light-emitting diodes: Evidence for dominant role of triplet-polaron quenching

“…Using this model, we can reproduce the measured efficiency roll-off for prototypical phosphorescent OLED stacks based on the green dye tris[2-phenylpyridine]iridium (Ir(ppy) 3 ) and the red dye octaethylporphine platinum (PtOEP) and study the cause of the roll-off as function of the current density. Both the voltage versus current density characteristics and roll-off agree well with experimental data.…”

“…We investigate the shape of the roll-off for two prototypical OLED stacks, studied intensively in the literature, 2 based on the green phosphorescent emitter tris[2-phenylpyridine]iridium (Ir(ppy) 3 ) and the red phosphorescent emitter octaethylporphine platinum (PtOEP). As the triplet radiative lifetime of Ir(ppy) 3 is much shorter than that of PtOEP ($1 ls vs $100 ls), a comparison between both devices will reveal to what extent the relative contributions of TPQ and TTA to the roll-off are sensitive to the emissive lifetime.…”

“…These bimolecular processes present a considerable challenge to the development of electrically pumped organic lasers, 1 and cause in phosphorescent OLEDs a decrease of the IQE with increasing current density. 2,3 Elucidating the origin of this roll-off and disentangling the roles of both types of loss processes has been a subject of intensive study. [2][3][4][5][6][7] However, so far, no widely accepted picture has emerged.…”

“…2,3 Elucidating the origin of this roll-off and disentangling the roles of both types of loss processes has been a subject of intensive study. [2][3][4][5][6][7] However, so far, no widely accepted picture has emerged. Obtaining such a picture is hampered by the need to carefully consider the strong spatial non-uniformity of the charge and exciton densities in actual devices.…”