Realizing fully stretchable electronic materials is central to advancing new types of mechanically agile and skin-integrable optoelectronic device technologies. Here we demonstrate a materials design concept combining an organic semiconductor film with a honeycomb porous structure with biaxially prestretched platform that enables high-performance organic electrochemical transistors with a charge transport stability over 30–140% tensional strain, limited only by metal contact fatigue. The prestretched honeycomb semiconductor channel of donor–acceptor polymer poly(2,5-bis(2-octyldodecyl)-3,6-di(thiophen-2-yl)-2,5-diketo-pyrrolopyrrole-alt-2,5-bis(3-triethyleneglycoloxy-thiophen-2-yl) exhibits high ion uptake and completely stable electrochemical and mechanical properties over 1,500 redox cycles with 104 stretching cycles under 30% strain. Invariant electrocardiogram recording cycles and synapse responses under varying strains, along with mechanical finite element analysis, underscore that the present stretchable organic electrochemical transistor design strategy is suitable for diverse applications requiring stable signal output under deformation with low power dissipation and mechanical robustness.
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We thank Air Force Office of Scientific Research (AFOSR) (FA9550-18-1-0320), the Northwestern University Materials Research Science and Engineering Center (NU-MRSEC) (NSF DMR-1720139;) and Flexterra Corporation for support of this research. This work made use of the J. B. Cohen X-Ray Diffraction Facility; the Electron Probe Instrumentation Center (EPIC) facility, Keck-II facility, the Scanned Probe Imaging and Development (SPID) facility and Northwestern University Micro/Nano Fabrication Facility (NUFAB) of the Northwestern University’s Atomic and Nanoscale Characterization Experimental Center (NUANCE) at Northwestern University, which received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the MRSEC programme (NSF DMR-1121262); the International Institute for Nanotechnology; the Keck Foundation; and the State of Illinois, through the International Institute for Nanotechnology. Use of the Advanced Photon Source (beamline 8-ID-E), an Office of Science User Facility operated for the US Department of Energy Office of Science by Argonne National Laboratory, was supported by the US Department of Energy under contract DE-AC02-06CH11357. We also acknowledge the support from the National Natural Science Foundation of China (grant nos 61804073, 12072057, U1830207), Dalian Outstanding Young Talents in Science and Technology (grant no. 2021RJ06), LiaoNing Revitalization Talents Program (grant no. XLYC2007196), the Fundamental Research Funds for the Central Universities (grant no. DUT20RC(3)032), City University of Hong Kong (grants nos 9610423, 9667199), the Research Grants Council of the Hong Kong Special Administrative Region (grant no. 21210820), the Intelligence Community Postdoctoral Research Fellowship Program and Guangdong Provincial Key Laboratory Program (2021B1212040001) from the Department of Science and Technology of Guangdong Province.
A.F. is the CTO of Flexterra Corporation. The remaining authors declare no competing interests.
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Cyclic voltammetry of (a) DPP-g2T and (b) DPP-2T drop-cast films on Pt electrodes. The oxidation onsets are +0.34 and +0.75 V for DPP-g2T and DPP-2T, respectively, indicating that DPP-g2T is more readily oxidized. The spectro-electrochemical properties of (c,e) DPP-g2T and control (d,f) DPP-2T films on ITO/glass were characterized under biases ranging from 0.0 V to +0.8 V (in 0.1 M aqueous KPF6) to investigate the electrochemical doping efficiency. For DPP-g2T, upon application of a low bias the intensities of the π–π* absorption (420 nm), intramolecular charge transfer (ICT, 783 nm), and π–π aggregate (848 nm) peaks decrease and polaronic absorption at longer wavelengths (> 1000 nm) increases, indicating full and efficient doping of DPP-g2T. In contrast, the DPP-2T films show no obvious absorption changes at biases ≤ +0.5 V and even at biases ≥ +0.8 V, while the absorption features of the neutral polymer persist, indicating poor electrochemical doping efficiency. In addition, the absorption spectrum of DPP-g2T returns to the original line shape when applying a bias of –0.2 V, while that of DPP-2T only partially recovers, indicating inferior redox stability, a result confirmed by CV measurements on the same platform.
(a) Atomic force microscopy (AFM) images and (b) corresponding line cuts of the indicated DPP-2T and DPP-g2T d- and h-films. (c)The scheme of calculated thickness of h-films. The average film thickness and wall loading of h-films were calculated from the AFM software. Transferred d-films are relatively smooth with RMS roughness (σRMS) = 0.75 nm (d-DPP-2T) and 0.89 nm (d-DPP-g2T). For the transferred h-films, h-DPP-g2T exhibits a uniform honeycomb structure with an average pore diameter of 1.33±0.15 μm and wall height of 183±34 nm. In contrast, the h-DPP-2T films exhibit very non-uniform craters of diameters ranging from 1.3~3.6 μm (average = 2.14±0.83 μm) and wall height of 322±86 nm.
(a)2D-GIWAXS images and (b)corresponding one-dimensional line-cuts for d- and h-DPP-2T and DPP-g2T films. Solid line: out-of-plane line cuts; dashed line: in-plane line cuts. (c) The Lorentz corrected partial pole figure and (d) calculated relative degree of crystallinity for dense DPP-2T and DPP-g2T films. All films are textured and exhibit similar reflections, demonstrating that porosity does not alter the overall film microstructure and polymer backbone orientation. Thus, distinct (100) and (200) reflections associated with a lamellar d-spacing of 18.9 Å, as well as a (010) reflection due to the backbone π-π stacking measuring 3.7 Å are observed in both out-of-plane and in-plane directions for the h- and d-DPP-2T films. In contrast, only a stronger (100) reflection (d-spacing=19.6 Å) in the in-plane direction and a (010) reflection (π-π stacking distance = 3.9 Å) in the out-of-plane direction are detected for the h- and d-DPP-g2T films. The relative degree of crystallinity for d-DPP-g2T was calculated to be 0.766 ± 0.007 that of d-DPP-2T. These data demonstrate that the DPP-g2T crystallinity is notablely lower than that of DPP-2T, it is less tightly packed, and DPP-g2T backbone is preferentially oriented π-face-on.
(a) Scanning electron microscopy (SEM) images of h-DPP-g2T film after KPF6 electrolyte evaporation. (b) SEM/energy-dispersive X-ray spectroscopy (SEM/EDS) mapping of (b) carbon, (c) fluorine, (d) phosphorus, and (e) potassium. (f) SEM image, along with carbon (red) and fluorine (blue) element mapping show excellent matching. The results show that the electrolyte solution penetrates into the h-DPP-g2T pores since cubic KPF6 crystals are detected in the pore areas and but are not located on the honeycomb top walls.
SEM images of d-DPP-2T films with (a) 0% and (b) 30% strain; h-DPP-2T films with (c) 0% and (d) 60% strain; d-DPP-g2T films with (e) 0% and (f) 60% strain; h-DPP-g2T films with (g) 0% and (h) 60% strain (The red square stands out the cracks).
Extended Data Fig. 6 Schematic illustrations for OECT fabrication using stretched semiconductor films and film morphology.
(a) Schematic illustrations for OECT fabrication using stretched semiconductor films: Transfer of h- or d-DPP-g2T films on SEBS (Step 1), stretching at different strains (Step 2); Upon stretching, the h- or d-DPP-g2T/SEBS film were adhered to a metal sheet by using a 3MTM VHBTM double side tape (Step 3); The patterned Au source/drain electrodes (W/L = 600/25 μm) on OTS-modified Si/SiO2 wafer was adhered to the h- or d-DPP-g2T/SEBS/metal sheet (Step 4); Lamination of Au S/D electrodes onto the surface of DPP-g2T film (Step 5); Application of aqueous KPF6 electrolyte and Ag/AgCl gate electrode to complete the OECT architecture(Step 6). SEM images of OECTs with (b) 60% and (c) 90% stretched d-DPP-g2T and h-DPP-g2T, respectively.
(a) Transfer and transconductance curves and (b) gm /Ion of OECTs based on d-DPP-g2T under different strains with transferred Au as S/D electrodes.
(a) Transfer and transconductance curves and (b) gm /Ion of OECTs based on h-DPP-g2T under different strains with transferred Au as S/D electrodes.
Extended Data Fig. 9 SEM images for a psh-DPP-2gT (εps = 100%) films and gm /Ion for psh-DPP-2gT (εps = 100%) OECTs based on Ecoflex/SEBS bilayer substrates.
(a) SEM images for a psh-DPP-2gT (εps = 100%) under 0%, 40%, and 80% strains. Note obvious Au line rupture when the h-film remains connected. (b) gm /Ion for psh-DPP-2gT (εps = 100%) OECTs based on Ecoflex/SEBS bilayer substrates under various strains (from 0% to 200%) in ε┴ or ε// direction. The insert is the optical image of the stretched OECT device.
Synaptic characteristics of psh-DPP-2gT (εps = 100%) OECT (electrolyte: aqueous KClO4) under 0%, and 60% elongation strains in ε┴ or ε// direction. (a) PPF (A2/A1) based on different interspike intervals (from 100 ms to 1900 ms). (b) EPSC versus spike number under different spike frequencies. (c,d,e) EPSC gain (from A10/A1 to A70/A1) with spike frequency from 0.5 to 5 Hz.
Supplementary Figs. 1–53, Tables 1–3 and details of the experimental procedures, synthesis and characterization of polymers, optical images, OECT characterizations, AFM images and SEM images.
UV, cyclic voltammetry and OECT properties.
FEA simulation and tensile results and electrical properties of stretched DPP-g2T films.
Electrical properties of psh-DPP-2gT (εps = 100%) OECTs.
ECG recording data.
Cyclic voltammetry and UV−visible−near-infrared data.
AFM line-cut data.
Electrical properties of OECTs based on d-DPP-2gT under different strains.
Electrical properties of OECTs based on h-DPP-2gT under different strains.
Electrical properties of psh-DPP-2gT OECTs based on Ecoflex/SEBS bilayer substrates.
Synaptic characteristics of psh-DPP-2gT (εps = 100%) OECT.
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Chen, J., Huang, W., Zheng, D. et al. Highly stretchable organic electrochemical transistors with strain-resistant performance. Nat. Mater. 21, 564–571 (2022). https://doi.org/10.1038/s41563-022-01239-9
Nature Materials (2022)