High-efficiency stretchable light-emitting polymers from thermally activated delayed fluorescence

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High-efficiency stretchable light-emitting polymers from thermally activated delayed fluorescence
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The development of stretchable electroluminescent (EL) materials is crucial for creating wearable and implantable EL devices that can be integrated with the human body. Such devices offer unique functionality for information visualization, wireless signal/power transmission, and medical therapies. However, the reported EL materials are all based on the conventional fluorescence (FL) emitters, which can only harvest up to 25% of the excitons (known as singlet exciton) since the rest excitons (known as triplet exciton) are spin forbidden for florescence emission, therefore with a maximum external quantum efficiency (EQE) of 5%.

To break such fundamental efficiency limitation, our recent work published in Nature Materials focuses on the new generation of OLED emitters, known as thermally activated delayed fluorescence (TADF) emitter. TADF emitters, with the fine molecular design, can realize 100% of exciton harvest by allowing the up conversion of the triplet excitons to the singlet excitons. But the realization of stretchable properties for this class of emitters has remained an open challenge, due to the significantly more complicated photophysics process compared with the conventional FL emitters.

Drawing on the potential of this class of high-efficiency emitters, we have demonstrated an effective molecular design strategy to achieve TADF polymers that combine high efficiency and high stretchability. Specifically, we proposed to incorporate the soft chains, i.e., linear alkyl chains, into the polymer backbone by copolymerization with TADF units (Fig. 1). Our findings indicate that incorporating a 10-carbon alkyl chain into the polymer backbone results in a TADF polymer that can withstand an impressive crack-on-set strain of up to 125%, which is twice the maximum strain that can be generated by human tissue (i.e., elbow). At the same time, the TADF polymer exhibited an EQE of 10%, which is twice the theorical limitation of the conventional FL emitters.

Figure 1: a, Design principle for realizing stretchable TADF polymers via the incorporation of soft alkyl chains into the polymer backbone. The soft alkyl chains can effectively dissipate the strain energy on stretching, keeping the TADF units unaffected. b, Mechanism of TADF light-emitting process in stretchable polymers. S1, T1 and S0 stand for the lowest singlet state, lowest triplet state and ground-state energy levels, respectively. ISC, RISC, Fp and Fd stand for intersystem crossing, reverse intersystem crossing, prompt fluorescence and delayed fluorescence, respectively. c, Chemical structures of stretchable TADF polymers having different alkyl chains in the polymer backbone. d, Glass transition temperature (Tg) and crack-onset strains of stretchable TADF polymers.

In order to provide a comprehensive understanding of our newly developed stretchable TADF polymers, we conducted a thorough investigation of their characteristics and properties. This included analysis of their basic photophysical properties, performance in EL devices, density functional theory calculations, and molecular dynamics simulations (Fig.2). Counter-intuitively, our findings show that the incorporation of alkyl chains into the polymer has minimal impact on the photophysical and EL properties of the TADF materials, while realizing high levels of stretchability. The details can be found in our published paper.

Fig. 2 | a, Optical microscopy images of PDKCM and PDKCD thin films under 100% strain, with the AFM height image as the nanoscale view of the PDKCD film. b, Schematic of the OLED device structure for characterizing the EL performance of the stretchable TADF polymers under strain. c, EQEmax from PDKCD under different strains. d, Snapshots of one representative chain taken from an MD simulation of PDKCD at 0% and 100% strain. e, Schematic representation of non-affine atomic displacements (left). Non-affine displacement distributions for the alkyl main chains, the rigid main chains, and the side chains at 5% strain for polymers PDKCM and PDKCD (right).

We also made a further step towards stretchable OLED and display technologies, by demonstrating a fully stretchable OLED based on the most stretchable TADF polymer – PDKCD (Fig. 3). Benefited from the TADF mechanism and careful material selection, our device structure exhibited a record-high EQE of 3.3%, a current-efficiency of 10.2 cd/A-1, a low turn-on voltage of 4.75 V with little precedent in the literature, and a skin-like stretchability of 60%. Additionally, the device can be easily patterned to various light-emitting shapes and arrays. Those results suggest that the stretchable TADF polymers proposed here provide a highly promising path toward delivering highly desirable EL and mechanical characteristics, including high efficiency, brightness, switching speed, stretchability, and low driving voltage.

Figure 3: a, Schematic of the device structure. PEIE and PFI are used in hole and electron injection layers, respectively, to both tune the work function of the AgNW electrodes and enable layer stretchability. b, Energy-level diagram of the device. c, Photograph showing the stretchable OLED lit up by a battery. d, Photographs of a representative OLED device stretched from 0% to 20%, 40% and 60% strains. e, Pictures of a fully stretchable OLED and an array built with the stretchable TADF polymer PDKCD.

We believe the feasible strategy to develop highly efficient stretchable light-emitting polymers based on the TADF mechanism would open up a much wider platform for further improvements of stretchable OLED performance. In academia, we anticipate that this will create numerous research opportunities to achieve high efficiency in stretchable TADF devices with emitters in various colors, longer lifetimes, and higher brightness. In industry, it would offer considerable potential for a wide range of applications, particularly in the field of stretchable optoelectronic devices used in human-interactive applications, including but not limited to on-skin display, wearable sensors for blood oxygenation/heart rate, and implantable light source for optogenetics.

This work titled “High-efficiency stretchable light-emitting polymers from thermally activated delayed fluorescence” was published in the Nature Materials.

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