Effect of Carrier Balance on Device Degradation of Organic Light‐Emitting Diodes Based on Thermally Activated Delayed Fluorescence Emitters

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Abstract

The relatively short device lifetime of blue organic light‐emitting diodes (OLEDs) when compared with the lifetimes of green and red OLEDs is one of the crucial problems that must be overcome to enable practical application of these devices to full‐color OLED displays. This work focuses on the degradation phenomena of OLEDs that are based on sky‐blue thermally activated delayed fluorescence emitters and clarifies the degradation mechanisms based on spectral change of the electroluminescence, which indicates the formation of electromer emission from an electron transport layer. Additionally, it is determined that the change in the carrier balance that occurs during this degradation process can be ascribed to the formation of electron traps.

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Highly efficient organic light-emitting diodes from delayed fluorescence.

Hiroki Uoyama, Kenichi Goushi, Katsuyuki Shizu … (2012)

The inherent flexibility afforded by molecular design has accelerated the development of a wide variety of organic semiconductors over the past two decades. In particular, great advances have been made in the development of materials for organic light-emitting diodes (OLEDs), from early devices based on fluorescent molecules to those using phosphorescent molecules. In OLEDs, electrically injected charge carriers recombine to form singlet and triplet excitons in a 1:3 ratio; the use of phosphorescent metal-organic complexes exploits the normally non-radiative triplet excitons and so enhances the overall electroluminescence efficiency. Here we report a class of metal-free organic electroluminescent molecules in which the energy gap between the singlet and triplet excited states is minimized by design, thereby promoting highly efficient spin up-conversion from non-radiative triplet states to radiative singlet states while maintaining high radiative decay rates, of more than 10(6) decays per second. In other words, these molecules harness both singlet and triplet excitons for light emission through fluorescence decay channels, leading to an intrinsic fluorescence efficiency in excess of 90 per cent and a very high external electroluminescence efficiency, of more than 19 per cent, which is comparable to that achieved in high-efficiency phosphorescence-based OLEDs.

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Organic electroluminescent diodes

S. A. VanSlyke, C. W. Tang (1987)

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Highly efficient blue electroluminescence based on thermally activated delayed fluorescence.

Shuzo Hirata, Yumi Sakai, Kensuke Masui … (2015)

Organic compounds that exhibit highly efficient, stable blue emission are required to realize inexpensive organic light-emitting diodes for future displays and lighting applications. Here, we define the design rules for increasing the electroluminescence efficiency of blue-emitting organic molecules that exhibit thermally activated delayed fluorescence. We show that a large delocalization of the highest occupied molecular orbital and lowest unoccupied molecular orbital in these charge-transfer compounds enhances the rate of radiative decay considerably by inducing a large oscillator strength even when there is a small overlap between the two wavefunctions. A compound based on our design principles exhibited a high rate of fluorescence decay and efficient up-conversion of triplet excitons into singlet excited states, leading to both photoluminescence and internal electroluminescence quantum yields of nearly 100%.