OLEDs are currently used extensively in wearable electronics, smartphones, flat displays, and solid-state lighting. This is largely driven by their numerous promising advantages compared to conventional liquid crystal displays (LCDs), such as lower cost, greater flexibility, better picture quality, and fast response time. To date, the mainstream commercial OLEDs are developed mainly based on expensive metal iridium (cost – US$5400/Oz, global production 3 tonnes per annum) and via vacuum deposition, which unavoidably increase the manufacturing cost. Although emitters that show thermally activated delayed fluorescence (TADF) have been designed as an alternative for realizing high-performance devices, sophisticated synthetic routes of the TADF emitters are needed to construct a steric donor–acceptor (D–A) motif or a multi-resonant (MR) rigid skeleton for a suitable S1–T1 energy gap (ES1T1 < 0.3 eV). Compared to TADF, organic RTP can be activated despite a large ES1T1 (>0.3 eV). Similar to TADF emitters, RTP counterparts can also enable 100% exciton utilization efficiency. Thus, organic RTP emitters can be promising candidates for OLEDs. Nevertheless, it remains a rather rarely documented occurrence for organic light-emitting diodes (OLEDs) with organic RTP emitters.
This project aims to achieve high-performance OLEDs without using noble metals and vacuum deposition technology. A feasible solution to this aim is to develop purely organic polymer emitters that show RTP in solution-processed OLEDs. To mitigate aggregation-caused quenching (ACQ), aggregation-induced emission (AIE) is expected to be introduced as a facilitator for improving the photoluminescence quantum yield (PLQY) of RTP. Thus, the major focus of this research proposal is to design high-efficiency AIE-RTP polymers for solution-processed OLEDs. Through thorough investigations, we have demonstrated that the triplet excited dynamics can be regulated on demand. Unfortunately, due to the time limitation, we cannot reach the final milestone to fabricate the AIE-RTP polymer OLEDs (PLEDs). Nevertheless, this project has demonstrated how to reduce the device efficiency roll-off via regulating higher-lying triplet excited state dynamics, building a roadmap for high-efficiency AIE-RTP PLEDs. The outcome of this project will strengthen the EU’s leading role as the material supplier for OLED displays.
We have demonstrated that RTP from a higher-lying triplet excited state (defined as T2 or T1H) can be achieved by regulating excitonic coupling between two triplet excited states. This breakthrough provides insight into designing high-performance TADF OLEDs utilizing the T2 state as a conduit for boosting reverse ISC (rISC). Thus, we fabricated TADF OLEDs with emitters showing divergent S1–T2 energy gaps (ES1T1) and confirmed that the T2 state plays a pivotal role in reducing the device efficiency roll-off. Next, we have developed an OLED with the emitter showing RTP from the T1H state. Due to degenerate S1 and T1H states, this kind of emitter possesses the potential to achieve high-performance RTP OLED without heavy atoms for improving ISC.
