Bio- & Eco-Friendly Organic Optoelectronics

High-efficiency stretchable TADF polymers and OLED devices. 

Stretchable light-emitting materials are the key components for realizing skin-like displays and optical bio-stimulation. All stretchable emitters reported to date have been based on “first-generation” electroluminescent (EL) polymers that can only harness the singlet excitons with a theoretical quantum yield of 25%. EL polymers based on TADF can break this limitation, However, due to the more complicated design requirements and chemical structures for efficient TADF processes, the achievement of the stretchable designs has been impeded by the lack of design principles for incorporating a suitable strain-dissipation mechanism without adversely influencing the EL behaviors, as well as suitable chemical synthesis approaches. To address this, we proposed a polymer design strategy of inserting flexible, linear units into the polymer backbone to increase mechanical stretchability without affecting the EL process. The resulting polymer achieved a high stretchability of 125% strain together with a record-high EQE of 10%. Demonstrated by the fully stretchable OLEDs, the stretchable TADF polymers provided a path toward achieving all the desired EL and mechanical characteristics, including high efficiency, brightness, switching speed, stretchability, and low driving voltage. (Nat. Mater. 2023, 22, 737-745.)

Depolymerizable and recyclable luminescent polymers.

Solution processable luminescent polymers are of great interest in a number of photonic technologies, including electroluminescence, bioimaging, medical diagnosis, bio-stimulation, and security signage. Ensuring the integration of depolymerizability and recyclability factors into the initial stages of the material design is of pivotal for promoting sustainability and mitigating environmental impacts throughout the entire product lifecycle. In this work, we unprecedently propose a general design concept utilizing cleavable moiety, herein tert-butyl ester, to create programmable depolymerizable and potentially recyclable thermally activated delayed fluorescence (TADF) polymers, without compromising their highly efficient luminescent properties. The depolymerizable and excellent light-emitting properties of these polymers open up the path for endowing end-life environmental friendliness and circular economy to current photonics technologies such as displays and bioimaging. New applications such as erasable emissive barcodes could also be enabled. (Nat. Sustain. 2024, 7, 1048-1056.)

Novel exciplex emitters with multiple up-conversion (Reverse intersystem crossing channels, RISC) channels.

Besides single-molecule TADF emitters, another class of TADF emitters is exciplex-based emitters, which form between electron-donating (D) and electron-accepting (A) molecules. Exciplex TADF emitters have some unique advantages over single-molecule emitters in terms of balancing bipolar charge transporting, simplifying OLED device structure, and decreasing device operating voltage. However, the reported exciplex-based TADF OLEDs showed much lower exciton utilization compared with the single-molecular TADF OLEDs. To address these, we proposed an effective strategy to improve the exciton utilization efficiency of exciplex-based emitters by introducing a single-molecule TADF emitter as one of the D-A pairs. Thus, the new type of exciplex TADF emitter will have two RISC routes on both the single-molecule TADF emitter and the exciplex emitter. (Adv. Funct. Mater. 2016, 26, 2002-2008.) Further, we constructed exciplex emitters with three components to realize multiple RISC channels in the exciplex systems. ( Adv. Sci. 2019, 6, 1801938.) Based on the concept, we pushed the EQE of exciplex-based emitters from the reported highest value of 17.8% to over 20%.