Abstract
Highly efficient organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters have been realized in recent years, but the device lifetime needs further improvement for practical display or lighting applications. In this work, we present a device design principle by tuning the optical cavity of single-layer undoped devices, to realize efficient and long-lived TADF OLEDs. Extending the cavity length to the second-order interference maximum by increasing the emissive layer thickness broadens the recombination zone, while the optical outcoupling efficiency remains close to that of the thinner first-order devices. Such a device design leads to efficient and stable single-layer undoped OLEDs with a maximum EQE of 16%, a LT90 of 452h and LT50 of 3693h at an initial luminance of 1000cd m-2 , which is doubled compared to the first-order counterparts. We further demonstrate that the widely-used empirical relation between OLED lifetime and light-intensity originates from triplet-polaron annihilation, resulting in an extrapolated LT50 at 100cd m-2 of close to 90,000h, approaching the demands for practical backlight applications. This article is protected by copyright. All rights reserved.
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