Operando ESR observation in thermally activated delayed fluorescent organic light-emitting diodes

Abstract

Organic light-emitting diodes (OLEDs) using thermally activated delayed fluorescence (TADF) materials have advantages over OLEDs using conventional fluorescent materials or high-cost phosphorescent materials, including higher efficiency and lower cost. To attain further high device performance, clarifying internal charge states in OLEDs at a microscopic viewpoint is crucial; however, only a few such studies have been performed. Here, we report a microscopic investigation into internal charge states in OLEDs with a TADF material by electron spin resonance (ESR) at a molecular level. We observed operando ESR signals of the OLEDs and identified their origins due to a hole-transport material PEDOT:PSS, gap states at an electron-injection layer, and a host material CBP in the light-emitting layer by performing density functional theory calculation and studying thin films used in the OLEDs. The ESR intensity varied with increasing applied bias before and after the light emission. We find leakage electrons in the OLED at a molecular level, which is suppressed by a further electron-blocking layer MoO3 between the PEDOT:PSS and light-emitting layer, resulting in the enhancement of luminance with a low-voltage drive. Such microscopic information and applying our method to other OLEDs will further improve the OLED performance from the microscopic viewpoint.

Avoiding Photocyclized Degradation of Electrochromic Materials Based on Quantum Chemical Molecular Design

“…A set of acceptor substitutions in the p -position of TPA has been applied to avoid the photocyclization process by enhancing the S 0 –S 1 transition dipole moment (Scheme : 2nd generation). , However, this approach alters the π conjugation among TPA and the acceptor groups, resulting in non-expected changes in the color and the oxidation potential. As this example shows, achieving photodegradation resistance while maintaining the critical properties of EC materials is still a significant challenge. − Therefore, this study proposes a quantum chemical approach to reduce the T 1 spin localization on the TPA unit by introducing a non-reactive antenna with a lower T 1 state than the TPA unit itself (Scheme : 3rd generation).…”