A general method for developing high-efficiency organic light-emitting diodes based on AIE-active room-temperature phosphorescent polymers

Project Information

AIE-RTP-PLED

Grant agreement ID: 897098

DOI

10.3030/897098

Project closed

EC signature date 24 March 2020

Start date 12 October 2020

End date 11 October 2022

Funded under

• EXCELLENT SCIENCE - Marie Skłodowska-Curie Actions

Total cost

€ 212 933,76

EU contribution

€ 212 933,76

Coordinated by

THE UNIVERSITY COURT OF THE UNIVERSITY OF ST ANDREWS
United Kingdom

Periodic Reporting for period 1 - AIE-RTP-PLED (A general method for developing high-efficiency organic light-emitting diodes based on AIE-active room-temperature phosphorescent polymers)

Reporting period: 2020-10-12 to 2022-10-11

Summary of the context and overall objectives of the project

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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Each WP consists of designing materials, computational screening, designing synthetic routes, successful synthesis with high purity, optoelectronic characterization, and exploration of applications.
Work Package 1
Thermally Activated and Aggregation-Regulated Excitonic Coupling Enable Emissive High-Lying Triplet Excitons: we have developed two emitters that show dual phosphorescence. Our study reveals that the dual phosphorescence originates from the different accessible triplet conformers, where the excitonic coupling between two triplet excited states (T1H and T1L) can be regulated by temperature and host-guest interactions. This work has been published in Angew. Chem. Int. Ed. (doi: 10.1002/anie.202206681).
Work Package 2
Conjugation-Modulated Excitonic Coupling Brightens Multiple Triplet Excited States. we have reported a general multiple RTP design principle, which is informed by our study of four compounds where there is modulation of the linker hybridization between donor (D) and acceptor (A) groups. We demonstrate that room-temperature dual phosphorescence (T1H and T1L) can be realized from the well-separated donor and acceptor subunits due to the suppressed D-A excitonic coupling. These findings provide insight into a fundamental design principle for designing compounds that show multiple RTP. This work has been published in J. Am. Chem. Soc. (doi: 10.1021/jacs.2c12320).
Work Package 3
Room-Temperature Multiple Phosphorescence from Functionalized Corannulenes: Temperature Sensing and Afterglow Organic Light-Emitting Diode. Based on strategy WP1, we demonstrate that room-temperature dual phosphorescence (T1H and T1L) can be regulated by judiciously decorating the corannulene core with different donors. Our study reveals that T1H-dominated RTP can be realized via constructing degenerate S1 and T1H states. This design provides another strategy for achieving 100% exciton utilization in solution-processed phosphorescent OLEDs. This work is going to be submitted to a peer-reviewed journal.
Work Package 4
Improving Efficiency Roll-off in Multi-Resonant Thermally Activated Delayed Fluorescent OLEDs Through Modulation of the Energy of the T2 State. We designed and synthesized two emitters that show multi-resonant thermally activated delayed fluorescence (MR-TADF) based on WP2. By modulating the conjugation between the MR-TADF DiKTa emissive center and donor substituent, emission directly from the T2 state was for the first time observed in MR-TADF emitters. Our study demonstrates that the T2 state can be utilized as a conduit for boosting kRISC and mitigating the OLED efficiency roll-off. This work has been accepted by Adv. Optical Mater. (doi: 10.1002/adom.202300114).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

We have developed two design strategies related to how to regulate the excitonic coupling between higher-lying and lower-lying triplet excited states for enabling multiphosphorescence. One strategy is devised based on the multiple accessible triplet conformers, and the other is proposed via regulating the conjugation between donor and acceptor. These outcomes provide us with methods to modulate the energy gap between the S1 and high-lying triplet excited states for boosting (reverse) ISC between singlet and triplet excited states. We have demonstrated that the T2 state can be utilized as a receiver state for boosting kRISC and thus mitigating the OLED efficiency roll-off. Besides, we demonstrate that solution-processed afterglow OLED can be developed by constructing degenerate S1 and T1H states. The outcome of this project related to multiple room-temperature phosphorescence design strategies provides a route to high-performance TADF OLEDs and a clue in developing solution-processed RTP OLEDs from the T1H state.
All the work carried out during this project is aimed at developing cost-effective, easy-to-access, and sustainable solution-processable emitters for display applications. The work carried out in this project provides mechanistic insight into how to regulate excitonic coupling between triplet excited states for efficient (reverse) ISC and demonstrates the feasibility of developing RTP OLEDs.

Summary for publication