The Behavior of TADF Materials with Fast Inter-convertible Spin-flip Process

Abstract

Herein, we propose a series of thermally activated delayed fluorescent (TADF) materials with a fast spin-flip system. In general, TADF emitters take a relatively long time-scale for delayed fluorescence (DF) enabled by spin-conversion from the triplet state manifolds, although TADF is the effective route to harvest triplet excitons, leading to the internal quantum efficiency of OLEDs as high as unity. From this intractable trade-off relation, one further needs to suppress or at least mitigate the OLED device efficiency roll-off at a high luminance level. To cope with this issue, we need to concentrate on the system showing a very fast inter-convertible spin-flip process between intersystem crossing (ISC) and reverse ISC (rISC) and, moreover, its implication on OLEDs device performance. In this context, we discuss the behavior of characterized TADF materials showing the rate constant of RISC over 108 s-1 by the introduction of internal heavy atom effect. Furthermore, the spectroscopic and theoretical results in this study unravels which factors should be addressed with a high priority to provide an effective means to mitigate the efficiency roll-off behavior.