Fully stretchable active-matrix organic light-emitting electrochemical cell array

The study addresses the challenge of realizing fully stretchable, skin-like displays capable of seamless integration with the human body. Prior work has demonstrated stretchable sensors and circuits, but not fully stretchable active-matrix displays. Achieving such displays requires two key components: intrinsically stretchable light-emitting device arrays and intrinsically stretchable thin-film transistor (TFT) active matrices, along with stretchable vertical interconnects to improve spatial resolution and contrast over passive-matrix devices. Existing stretchable displays are typically passive-matrix-driven or combine stretchable transistors with rigid LEDs; flexible AMOLEDs based on non-stretchable materials tolerate only about 5% strain, limiting stretchability. The purpose here is to develop materials and processes to integrate a fully stretchable TFT array with stretchable organic light-emitting electrochemical cells (OLECs) into a fully stretchable active-matrix OLEC (AMOLEC) display, demonstrating robust operation under mechanical deformation.

The paper situates the work within tissue-like electronics for consumer, wearable, and biomedical applications, citing advances in intrinsically stretchable pressure and temperature sensors and analog circuits. Prior displays have been passive-matrix stretchable light-emitting devices or hybrids using stretchable transistors connected to rigid LEDs. Attempts at stretchable AMOLEDs using flexible yet non-stretchable materials (e.g., poly-Si, commercial OLED) achieved only small strain tolerance (~5%), fundamentally limiting stretchability. Reported stretchable OTFTs often exhibited low drain currents (<10^-6 A) due to large channel lengths from low-resolution stretchable conductors and thick dielectric layers. Conventional elastomeric dielectrics (SEBS, PDMS) suffer solvent swelling and leakage; PVDF-HFP, while solvent-resistant with higher k, exhibits ionic effects that hinder fast switching required for displays.

Materials and dielectric development:

• Designed chemically orthogonal, intrinsically stretchable gate dielectrics based on photochemically crosslinked perfluoropolyether dimethacrylate (PFPE-DMA). PFPE-DMA offers strong solvent resistance to common organic semiconductor solvents and high breakdown (>100 V for <200 nm films). Dielectric constant measured 2.4 ± 0.1 at 20 Hz; co-polymerization with fluorinated acrylates can raise k to ~5.0 ± 0.2 at 20 Hz. Dielectric constant showed <10% change from 0–100% strain and remained stable after 100 cycles at 100% strain; negligible frequency dependence from 0.1–10^3 Hz.
• Addressed interfacial adhesion between perfluorinated dielectrics and organic semiconductors using an azide-functionalized PDMS crosslinker to covalently link semiconductor and dielectric, improving stretchability and preventing delamination under strain.

Transistor fabrication and characterization:

• Built top-gate bottom-contact (TGBC) OFETs using PFPE-DMA dielectrics to validate dielectric performance. For 1.5 µm dielectric thickness, p-type devices exhibited saturation mobility 0.43 ± 0.1 cm^2 V^-1 s^-1, on/off ~10^3. Reducing dielectric thickness increased mobility, consistent with field-dependent mobility in organic semiconductors and minimal ionic effects in PFPE-DMA.
• Developed an inkjet-printable intrinsically stretchable semiconductor ink: isoindigo-based PII2T mixed 1:1 (wt/wt) with PDMS-azide crosslinker in tetralin. Thermal annealing crosslinks the polymer, reducing modulus by ~2–3× and enhancing stretchability (vs. ~15% strain without crosslinker). Mobility decreased with crosslinker addition but was similar between 50–100 wt% crosslinker; 1:1 was chosen for optimum mechanical/electronic balance.
• Improved wetting on perfluorinated dielectric by modifying PFPE-DMA surface with a thin PDMS-azide layer, reducing water contact angle from ~120° to <30° and enabling uniform ink spreading (single-droplet diameter expansion from ~10 µm area to ~80 µm area). Enabled uniform inkjet-printed semiconductor patterns over millimeter scales and compatible spray-coating of CNT electrodes without swelling the dielectric.

Fully stretchable OTFT array fabrication:

• Process flow: spin-coat sacrificial dextran layer and PDMS-azide on Si/SiO2; spin-coat and UV-cure PFPE-DMA dielectric; spray-coat stretchable CNT/PEDOT:PSS gate electrodes; spin-coat PDMS substrate; release stack in water and flip; oxygen plasma to enhance wettability; inkjet print PII2T:crosslinker semiconductor patterns; thermal anneal; spray-coat PEDOT:PSS protective layer and CNT/PEDOT:PSS source/drain electrodes. Result: freestanding 5×5 stretchable OTFT array with interdigitated S/D (W/L large) and PFPE-DMA dielectric ~1.5 µm (capacitance ~1.4 nF/cm^2).
• Electrical/mechanical testing: Average saturation mobility 0.56 ± 0.17 cm^2/Vs. Devices operated under strains up to 100% and withstood up to 1000 stretch cycles at 100% strain with less than an order-of-magnitude reduction in on-current and mobility; leakage current changes were minimal.

Stretchable OLEC device:

• Structure: freestanding polyurethane acrylate (PUA) with Ag nanowires (AgNWs) electrodes; PEDOT:PSS hole injection/blocking on anode; light-emitting layer composed of Super Yellow (SY) emissive polymer, stretchable ion-conducting polymer (from prior work), ethoxylated trimethylolpropane triacrylate (ETT-15, heat-polymerizable ionically conductive component), and LiTf salt at 20:20:2:1 wt ratio. Active area: 2×3 mm^2. OLEC stretchability up to 30% strain without delamination/cracking.
• Optimization: Balancing stretchability and current density yielded an optimal SY:ionically conducting polymer ratio of ~1:1. Turn-on current density ~2 mA/cm^2; current density stable up to 30% strain.

Vertical integration to AMOLEC:

• Integrated each OLEC pixel with an underlying OTFT via Ag-filled vias connecting transistor drain to OLEC. Printed semiconductor area ~0.8×0.5 cm; interdigitated S/D W/L ≈ 140. Slight oxygen plasma treatment of PFPE-DMA doped the semiconductor to raise drive current while maintaining low off-current for proper pixel off-state. Photographs/videos demonstrated TFT-controlled pixel on/off under strain.
• Assembled a freestanding 5×5 AMOLEC array (3.9×3.7 cm^2). Average drain current 0.16 ± 0.04 mA. Device stack showed mechanical softness (~50 MPa). SEM cross-sections after 20 cycles at 30% strain showed negligible delamination; EDX mapping confirmed layer identities. Arrays could be bent, twisted, and stretched while lit.

• Developed perfluorinated elastomer PFPE-DMA as a chemically orthogonal, intrinsically stretchable gate dielectric with high solvent resistance, high breakdown (>100 V for <200 nm films), dielectric constant 2.4 ± 0.1 at 20 Hz (tunable to ~5.0 ± 0.2 via copolymerization), <10% change in k from 0–100% strain, stability after 100 cycles at 100% strain, and negligible frequency dependence from 0.1–10^3 Hz.
• TGBC OFETs on PFPE-DMA (1.5 µm) exhibited p-type mobility 0.43 ± 0.1 cm^2/Vs and on/off ~10^3; mobility increased with reduced dielectric thickness, indicating minimal ionic effects conducive to fast switching.
• Inkjet-printable stretchable semiconductor (PII2T + PDMS-azide 1:1) formed a crosslinked network with reduced modulus (~2–3× lower than pristine) and improved stretchability; surface modification of PFPE-DMA reduced water contact angle from ~120° to <30°, enabling uniform semiconductor and CNT electrode deposition.
• Fully stretchable 5×5 OTFT array achieved average saturation mobility 0.56 ± 0.17 cm^2/Vs, operated up to 100% strain, and maintained performance over 1000 cycles at 100% strain with <1 order decrease in on-current and mobility; leakage currents changed minimally.
• Stretchable OLEC devices with SY/ionic polymer/ETT-15/LiTf (20:20:2:1) exhibited turn-on current density ~2 mA/cm^2, stable current density under strains up to 30%, and optimal SY:ionic polymer ratio ≈ 1:1 for balancing stretchability and performance.
• Vertically integrated AMOLEC array (5×5, 3.9×3.7 cm^2) delivered average transistor drain current 0.16 ± 0.04 mA, operated under bending, twisting, and stretching; tolerated at least 20 cycles at 30% strain with negligible delamination seen by SEM; overall device modulus ~50 MPa. When mounted on skin, the array tolerated repeated cycles at 30% strain while functioning.

The work demonstrates, for the first time, a fully stretchable active-matrix-driven OLEC display by co-developing chemically orthogonal, intrinsically stretchable dielectrics (PFPE-DMA) and a crosslinkable, inkjet-printable stretchable semiconductor to realize high-current OTFT arrays capable of driving stretchable light-emitting pixels. The dielectric’s solvent resistance enabled direct patterning of semiconductors and electrodes without swelling or leakage, and its minimal ionic effects supported fast switching. The integrated AMOLEC array maintained electrical and optical performance under significant mechanical deformation, directly addressing the need for skin-like, conformable displays. The approach is compatible with solution processing and thus scalable. These results establish a foundation for future stretchable human–machine interfaces and enable vertical integration with other stretchable sensor arrays for on-body visual feedback and interaction.

This study introduces a fully stretchable active-matrix OLEC array (“skin display”) constructed entirely from intrinsically stretchable, solution-processable materials. Key advances include a perfluorinated stretchable dielectric (PFPE-DMA) with excellent solvent resistance and stable dielectric properties under strain, an inkjet-printable crosslinked stretchable semiconductor, robust stretchable OTFT arrays delivering sufficient current, and their vertical integration with stretchable OLEC pixels. The AMOLEC array functions during bending, twisting, and stretching, and withstands cyclic strain while maintaining performance, underscoring feasibility for skin-mounted displays. Future work should focus on increasing transistor mobility to raise pixel density, optimizing fluorinated dielectrics for higher permittivity to reduce operating voltages, and scaling array size and pixel counts for more complex displays.

• Operating voltages for the stretchable OTFTs are relatively high (e.g., |Vg| and |Vds| up to 100 V), suggesting the need for higher-k dielectrics to reduce drive voltage.
• Demonstrated strain endurance of the integrated AMOLEC array was tested to 30% strain and 20 cycles in the full stack (though individual components endured higher strain/cycles), indicating further long-term cyclic reliability studies are needed.
• Pixel density is limited by the required large W/L and device area to deliver sufficient current; higher-mobility semiconductors and finer patterning are needed to increase resolution.
• Light-emitting device characterization emphasizes current density and qualitative luminance; comprehensive optical metrics (e.g., luminance cd/m^2, efficiency, lifetime) under strain were not detailed.