Journal of the American Chemical Scoiety (2020), DOI: 10.1021/jacs.0c01225

Author(s):

Vazquez, R. J. (Vazquez, Ricardo); Yun, J. H. (Yun, Ju); Muthike, A. K. (Muthike, Angelar); Howell, M. (Howell, Madeline); Kim, H. (Kim, Hyungjun); Madu, I. K. (Madu, Ifeanyi Kizito); Kim, T. (Kim, Taesu); Zimmerman, P. M. (Zimmerman, Paul); Lee, J. Y. (Lee, Jun); Goodson III, T. (Goodson, Theodore, III)

Abstract:

We developed a new optical method to determine the rate of reverse intersystem crossing (krISC) in thermally activated delayed fluorescent (TADF) organic chromophores using time-resolved transient absorption spectroscopy. We successfully correlated the krISC 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 krISC 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 krISC values and high ηEQE values at low operating voltages for the TADF-chromophores investigated. In addition, those chromophores with a larger krISC 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 krISC ≤ 105s-1 its ηEQE is ≤5%. Such a small krISC 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 krISC were included for comparison but failed to predict the devices performance of the investigated TADF-chromophores to the same extent of our proposed methodology.