Developing near-infrared persistent room-temperature phosphorescence for down-conversion OLEDs

Project Information

PNIRED

Grant agreement ID: 101068895

DOI

10.3030/101068895

Project closed

EC signature date 20 June 2022

Start date 1 December 2022

End date 31 January 2025

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 173 847,36

Coordinated by

TECHNISCHE UNIVERSITAET DRESDEN
Germany

Periodic Reporting for period 1 - PNIRED (Developing near-infrared persistent room-temperature phosphorescence for down-conversion OLEDs)

Reporting period: 2022-12-01 to 2025-01-31

Summary of the context and overall objectives of the project

The development of persistent room-temperature phosphorescence (p-RTP) materials represents a critical advancement in the fields of organic optoelectronics, bio-imaging, and sensing technologies. Traditional phosphorescent materials often require heavy-metal complexes or cryogenic conditions to achieve long-lived emission, limiting their applicability in practical devices. However, limited by the energy gap law, most of the reported p-RTP materials emitting in the visible region, near infrared (NIR) phosphors are rarely reported, not to mention their applications in optoelectronic devices, particularly affecting the interest in organic light-emitting diodes (OLEDs). Despite significant progress in p-RTP materials, developing general approaches to engineering NIR p-RTP emission together with constructing persistent NIR OLEDs is missing, which remains a major scientific challenge.
This project aims to overcome these limitations by developing heavy-atom-free organic NIR p-RTP materials and establishing their applications in NIR down-conversion OLED pixels and advanced optoelectronic devices with persistent emission. The exploration of NIR p-RTP materials is of great significance to increase the level of understanding molecular mechanisms as well as expanding these studies to wider sets of materials and functional systems. The project is structured around two main objectives: the development of high-performance NIR p-RTP material systems and the integration of NIR p-RTP systems into optoelectronic devices.
This project has the potential to develop approaches for organic NIR p-RTP material systems and significantly advance OLED technology, anti-counterfeiting, and sensing applications. The anticipated impact includes scientific advancements of getting a deeper understanding of the excited-state dynamics and molecular engineering strategies, technological innovations of improving NIR p-RTP systems for advanced applications, and societal and economic benefits of enabling cost-effective, environmentally friendly alternatives to existing phosphorescent materials, aligning with the European Green Deal and sustainable electronics initiatives.

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

The project was carried out along clear research directions addressing two objectives following the bottom-up approach. The development of NIR p-RTP emission has been achieved by trace doping and uniform dispersion of small molecules in highly packed and rigid host matrices. The main work performed includes:
Development of small molecule-based host/guest p-RTP material systems. After careful material screening, boronic ester-based molecules were selected as guest molecules, with TPO, BP, and PPT as small-molecule hosts. Photophysical characterization confirmed strong p-RTP properties in phenothiazine-based boronic ester systems, with RTP lifetimes increasing from 30 ms (in TPO) to over 300 ms (in PPT). The enhanced performance is attributed to improved spin-orbit coupling and suppressed non-radiative decay as well as the distribution of host-guest triplet excited states, emphasizing the role of molecular engineering in advancing organic p-RTP materials for optoelectronics.
Development of polymer-based host/guest NIR p-RTP material systems. 1) A phosphole molecule (pyrene-substituted azaphosphole) was presented, exhibiting RTP emission beyond 600 nm with a 300 ms lifetime but low photoluminescence quantum yield (PLQY <2%), likely due to phosphorus's heavy atom effect. We established design principles for future applications. 2) Expanding the previous cycloaddition method, a series of diazaphospholes were synthesized, achieving strong fluorescence with PLQYs up to 37%, making them promising for optoelectronics. 3) A supramolecular foldamer was designed, showing a face-to-face donor-acceptor stacking through non-covalent interactions. Confirmed by NMR and single-crystal analysis, it exhibits single-molecule exciplex thermally activated delayed fluorescence in solution and deep-blue RTP with a 236 ms afterglow in a PMMA matrix, setting a record lifetime for triplet charge transfer-character RTP. 4) By employing strong RTP emitters and fluorescence dyes as the donor and acceptor molecules, respectively, we successfully achieved efficient deep-red persistent emission via the triplet-singlet Fӧrster resonance energy transfer (TS-FRET) mechanism. The system reached a phosphorescence QY of 53.6% (10 wt% TA) and maintained 37.2% at 30 wt%. Total PLQY increased to 85.7% at 20 wt% TA.
Implement NIR p-RTP into applications. After trying and analyzing, a fully functional device is yet to be achieved. But the polymer host limitations with poor charge mobility were identified. Instead, I gained training in OLED simulation with SIMOJI tools and also came up with new ideas to fabricate OLEDs and expand the applications.
Based on the research, tunable NIR p-RTP in purely organic systems were demonstrated, establishing new molecular design principles for future materials. The results also contributed to the fundamental understanding of triplet exciton dynamics and generated new possibilities for applications. Three papers have been published: Chem, 2024, 10 (2), 644-659, Inorg. Chem. Front., 2025, https://doi.org/10.1039/D5QI00427F Adv. Optical Mater., 2024, 2400210; one paper submitted to J. Mater. Chem. C.

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)

In this project, we have exploited new approaches to engineering NIR p-RTP emission, made significant advancements to the current understanding of photonic and excitonic phenomena of organic p-RTP materials, and achieved beyond the fundamental research and have an immediate impact for applications. Key results include a record-long RTP lifetime (286 ms) in a non-covalent molecule with triplet charge transfer character; deep-red persistent emission with phosphorescence quantum yields up to 53.6% in a co-doping system based on TS-FRET; new phosphor systems tailored to host-guest interactions; OLED architecture simulated by SIMOJI tools; and alternative applications of NIR p-RTP systems in anti-counterfeiting and oxygen sensing, expanding their impact beyond OLEDs.
To fully translate these results into commercial and industrial applications, further steps are required: 1) continued optimization of material performance, stability, and compatibility with device architectures; 2) collaboration with industry partners for large-scale manufacturing and integration into commercial optoelectronic devices; 3) protection of key innovations through patents and alignment with industry standards to facilitate adoption; and 4) engagement with global research networks to enhance material development and application opportunities. These advancements mark a step forward in the field of organic p-RTP materials and open new possibilities for next-generation optoelectronic technologies.

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