New Direct Approach for Determining the Reverse Intersystem Crossing Rate in Organic Thermally Activated Delayed Fluorescent (TADF) Emitters

• Published in: Journal of the American Chemical Society Vol. 142; no. 18; pp. 8074 - 8079
• Main Authors: Vázquez, Ricardo Javier, Yun, Ju Hui, Muthike, Angelar K, Howell, Madeleine, Kim, Hyungjun, Madu, Ifeanyi K, Kim, Taesu, Zimmerman, Paul, Lee, Jun Yeob, III, Theodore Goodson
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 06.05.2020
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.0c01225

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 (ηEQE) 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 (ηEQE 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 ηEQE values at low operating voltages for the TADF-chromophores investigated. In addition, those chromophores with a larger k rISC illustrated a smaller ηEQE 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 ηEQE 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.