Stretchable light-emitting materials are the key components for realizing skin-like displays and optical biostimulation. All the stretchable emitters reported to date, to the best of our knowledge, have been based on electroluminescent polymers that only harness singlet excitons, limiting their theoretical quantum yield to 25%. Here we present a design concept for imparting stretchability onto electroluminescent polymers that can harness all the excitons through thermally activated delayed fluorescence, thereby reaching a near-unity theoretical quantum yield. We show that our design strategy of inserting flexible, linear units into a polymer backbone can substantially increase the mechanical stretchability without affecting the underlying electroluminescent processes. As a result, our synthesized polymer achieves a stretchability of 125%, with an external quantum efficiency of 10%. Furthermore, we demonstrate a fully stretchable organic light-emitting diode, confirming that the proposed stretchable thermally activated delayed fluorescence polymers provide a path towards simultaneously achieving desirable electroluminescent and mechanical characteristics, including high efficiency, brightness, switching speed and stretchability as well as low driving voltage.
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The codes and force-field parameters used for the MD simulation are available via GitHub at https://github.com/czhangR/High-efficiency-stretchable-light-emitting-polymers-from-thermally-activated-delayed-fluorescence.git. The Gaussian (https://gaussian.com/) package is commercially available; the LAMMPS (https://www.lammps.org/#gsc.tab=0) and GROMACS (https://www.gromacs.org/) packages are open-source.
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This research is supported by the Start-Up Funds from the University of Chicago, National Science Foundation CAREER Award no. 2239618, and the University of Chicago Materials Research Science and Engineering Center, which is funded by the National Science Foundation under award no. DMR-2011854. We acknowledge the Research Computing Center of the University of Chicago for computational resources. We used the Center for Nanoscale Materials, an Office of Science user facility supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357. D.V.T. and J.J.d.P. acknowledge support from MICCoM, as part of the Computational Materials Sciences Program funded by the US Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division, under grant DOE/BES 5J-30161-0010A. D.V.T. acknowledges support from the National Science Foundation under grant no. DMR-2019444. R.A. is supported by the Dutch Research Council (NWO Rubicon 019.202EN.028). X.Z. acknowledges support from the National Natural Science Foundation of China under grant no. 51821002. We thank X. M. Lin for assisting us with the experiment at Argonne National Laboratory. We thank M. Zhang, X. K. Liu, X. D. Ma, P. J. Pei, C. Arneson and S. R. Forrest for helping with the experiments, and D. F. Yuan and L. P. Yu for discussions about polymer synthesis.
S.W., J.J.d.P., W.L. and C.Z. are inventors on a pending patent filed by the University of Chicago (no. UCHI 22-T-048). The other authors declare no competing interests.
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Extended Data Fig. 1 Optical microscopy images of PDKCM, PDKCP, DDKCH, and PDKCD films under stretching to the strains of 50%, 75%, and 100%.
The films were transferred from PDMS substrates to SEBS-coated Si substrates. The stretching direction was horizontal.
Extended Data Fig. 2 EL performance of the rigid OLED devices with PDKCM as the emitting layer under different strains.
(a) Representative EQE-current density plots. (b) Averaged maximum EQE (EQEmax) values as a function of different strains. The data of EQE are represented as mean values + /- s.d. from 3 devices. (c) Representative EL spectra at 8 V.
Extended Data Fig. 3 EL performance of the rigid OLED devices with PDKCD as the emitting layer under different strains.
(a) Representative EQE-current density plots. (b) Averaged EQEmax values as a function of different strains. The data of EQE are represented as mean values + /− s.d. from 3 devices. (c) Representative EL spectra at 8 V.
Extended Data Fig. 4 Snapshots taken from the MD simulations of PDKCM, PDKCP, and PDKCH at 0% and 100% strains.
One randomly selected chain is highlighted for each simulation and manually centred at the middle of the box, with the backbone rendered in grey, the electron-donor units in blue, and the electron-acceptor units in red.
The non-affine displacements of the alkyl main chain, rigid main chain, and side chain at 5% strain for polymers PDKCP (a) and PDKCH (b).
(a) Transmittance curve of the electrode. The inset is the photo of the electrode. (b) Stretching-induced resistance changes for the original electrode and the annealed electrode (140 °C for 1 h). The data of the resistance are represented as mean values + /− s.d. from 5 samples. (c) UPS test result of the electrode. The work function (EWF) of the electrode was calculated with the equation: EWF = hv – ESE = 21.22 eV – 16.48 eV = 4.74 eV, where hv is the incident photon energy, ESE is the secondary edge position. (d) Optical microscopy image of the electrode under different strains (60%, 80%, and 100%). The stretching direction is horizontal. The Scale bars are 10 μm.
(a) The chemical structures of PEIE and PFN-Br. (b) Optical microscopy images of PEIE_PFN-Br composite films at 50%, 60%, 70%, and 100% strains. The stretching direction is horizontal. Therefore, the composite film can be stretched to 60% without cracking.
(a) Chemical structures of PEDOT:PSS and PFI. (b and c) Optical microscopy images of PFI (b), and PEDOT:PSS_PFI composite film (c), under 100% strain. The stretching direction is horizontal, and the scale bar is 10 μm. (d) Raman spectra of PEDOT:PSS and PEDOT:PSS_PFI film under 632.8 nm laser excitation.
(a) Molecular depth (with the step size of ~5 nm) profiles of PEDOT:PSS_PFI composite film as inferred from the XPS results in Supplementary Figs. 36 and 37. Deconvoluted S 2p peaks for PEDOT (164.5 eV), sulfonic acid (S 2p peaks at 168.4 and 168.9 eV), C 1 s peak at 291.6 eV, and F 1 s peak for the PFI concentration were used. This shows that the PFI is rich at both top and bottom surfaces in the composite film, and the molecular concentration of PFI gradually decreased toward the centre in the thickness direction. (b) Proposed 3D illustration of the morphology of the PEDOT:PSS_PFI composite film.
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Liu, W., Zhang, C., Alessandri, R. et al. High-efficiency stretchable light-emitting polymers from thermally activated delayed fluorescence. Nat. Mater. 22, 737–745 (2023). https://doi.org/10.1038/s41563-023-01529-w