Nearly non-roll-off high efficiency fluorescent yellow organic light-emitting diodes

Abstract

This paper demonstrates the employment of a multi-emissive-layer device architecture with smooth cascading energy levels to improve markedly the efficiency, roll-off, and color-stability of a fluorescent yellow organic light-emitting diode. As a tri-emissive layer structure was used in lieu of neat film, for example, the resultant external quantum efficiency was increased from 2.2 and 1.8% to 8.2 and 8.6% between 100 and 1000 cd m−2, while current efficiency increased from 5.8 and 4.6 cd A−1 to 26.5 and 27.2 cd A−1 and efficacy from 4 and 2.4 lm W−1 to 20.6 and 18.3 lm W−1. Moreover, the color stability of the tri-emissive layer structure has become of even higher quality than the neat film counterpart has. The reason why the multi-emissive-layer device exhibits both high efficacy and high color-stability may be attributed to the stepwise energy levels that enable a significant reduction in injection barriers, a wider recombination zone, and a more effective carrier confinement. Additionally, the paired host and guest energy-levels in two of the emissive layers allow excitons to generate on the host to facilitate the occurrence of host-to-guest energy transfer and, thus, high device efficiency. The little roll-off may be due to the different paired host and guest energy-levels in the third emissive layer allowing excitons to generate predominantly on the guest at low voltage, but with increasing excitons generating on the host as the voltage increases, fully utilizing all the possible recombination sites.