New Direct Approach for Determining the Reverse Intersystem Crossing Rate in Organic Thermally Activated Delayed...

We developed a new optical method to determine the rate of reverse intersystem crossing (k_rISC) in thermally activated delayed fluorescent (TADF) organic chromophores using time-resolved transient absorption spectroscopy. We successfully correlated the k_rISC of the TADF-chromophores with device performance. Specifically, we focused on the external quantum efficiency (η_EQE) and the stability of the device at high brightness levels. It is believed that by obtaining a large k_rISC one may reduce the possibility of triplet-triplet annihilation (TTA) and increase the long-term stability of organic light emitting diodes (OLEDs) devices at high brightness levels (η_EQE roll-off). In this contribution, we investigate the photophysical mechanism in a series of TADF-chromophores based on carbazole or acridine derivatives as donor moieties, and triazine or benzonitrile derivatives as the acceptor moieties. We found a relationship between large k_rISC values and high η_EQE values at low operating voltages for the TADF-chromophores investigated. In addition, those chromophores with a larger k_rISC illustrated a smaller η_EQE roll-off (higher stability) at high operating voltages. These features are beneficial for superior OLEDs performing devices. Contrarily, we found that if a chromophore has a k_rISC ≤ 10⁵s^-1 its η_EQE is ≤5%. Such a small k_rISC suggests that there is no TADF effect operating in these organic systems and the molecule is not efficient in harvesting triplet excitons. Emission lifetime-based methodologies for determining the k_rISC were included for comparison but failed to predict the devices performance of the investigated TADF-chromophores to the same extent of our proposed methodology.