Organic emitters have been widely developed since the demonstration of high efficiency in pure organic emitters represented by thermally activated delayed fluorescent (TADF) emitters. Pure organic‐based TADF emitters are found to harvest all singlet and triplet excitons for radiative transition processes, like phosphorescent emitters that utilize all generated excitons for light emission by strong spin–orbit coupling. Red, green, and blue organic emitters are investigated, but major interest is focused on green and blue organic emitters. However, recent studies are being directed to red and even near‐infrared emitters because of their wide applications in bioimaging, sensors, telecommunications, and night vision. Although the external quantum efficiency of the red and near‐infrared TADF organic light‐emitting diodes (OLEDs) is low, the external quantum efficiency of red OLEDs has been advanced to close to 30% and that of near‐infrared OLEDs to the 10% level. Therefore, more attention is being paid to red and near‐infrared TADF emitters. In this work, recent progress of the red and near‐infrared TADF emitters is reviewed to understand the molecular design principle and the molecular structure–photophysical characteristics and molecular structure–device performance relationships. In addition, future prospect of the red and near‐infrared TADF emitters is proposed.

Blue organic light‐emitting diodes (OLEDs) have been a bottleneck for OLEDs lighting and flexible displays. Improving the operational lifetimes and simultaneously decreasing efficiency roll‐off while maintaining high quantum efficiency is currently a challenge in the scientific community and industry. Optimizing the fabrication process of devices and developing stable and efficient luminescence and transporting materials and developing host materials with high triplet energy is an effective way to overcome this obstacle. On the one hand, the host material can disperse the blue emitters to reduce the possibility of exciton annihilation. On the other hand, it can adjust the carrier transport, exciton formation, and energy transfer in the device. In recent years, many efforts have been undertaken for the design, synthesis, and applications of the novel host. A systematic summary and comments on the recent advances of high triplet energy hosts for blue OLEDs are provided here, which specifically include bipolar transport hosts, single thermally activated delayed fluorescence (TADF) hosts, TADF assistant hosts, exciplex hosts, exciplex free type mixed hosts, and electroplex hosts. Moreover, future prospects for host for high performance blue OLEDs are also proposed.

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.

No abstract

Stable host materials based on benzocarbazole and quinazoline were synthesized to establish a design strategy of bipolar hosts for elongated lifetime in orange phosphorescent organic light-emitting diodes (PHOLEDs).