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

Our reported intrinsically stretchable organic transistor active-matrix driven light-emitting array exhibits high stability and ‘skin-like’ mechanical properties, making it possible for wearable human-interactive systems.

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Soft skin-like electronics are particularly attractive for realizing human-interactive functionalities. However, skin-like displays based on stretchable transistor-driven light emitting devices are challenging to fabricate and has yet been demonstrated. In this work we demonstrated an intrinsically stretchable organic transistor active-matrix driven light-emitting array, which can serve as an essential ‘output’, making it a step closer to ‘skin-like’ wearable interactive systems.

Our work is enabled by three new developments: an intrinsically stretchable organic light-emitting electrochemical cell (OLEC), an intrinsically stretchable organic thin-film transistor (TFT)-driven active-matrix, and the integration between these two components. In order to achieve sufficient light emission from an OLEC device, powered by a stretchable TFT, the main challenge is to develop TFTs with matching power. Based on performance of our OLECs, the minimum current density to generate human eye-distinguishable luminescence is around 2-2.5 mA/cm2. Thus to drive a such a stretchable OLEC pixel for visualization (e.g. 2 mm x 2 mm for one pixel), the individual stretchable TFT needs to provide around 10-4 A of current at moderate voltages1; while for the reported stretchable TFT, the current were mostly <10-6 A even at high gate voltages2.

In order to overcome this nearly two order of magnitude shortfall in current, we developed a stretchable gate dielectric layer with reduced thickness and increased the active channel area by employing an interdigitated channel design. Our stretchable dielectric is compatible with solution processed stretchable semiconductor over a large active area with little leakage current. 

This dielectric material is based on perfluorinated elastomers. We previously reported a photo-patterning method of related material for encapsulation layer for bioelectronics3. This class of elastomer have both high stretchability and chemically inertness which are desirable features for subsequent deposition of solution-processed polymer semiconductors and conductors. However, it was challenging to inkjet printing large-area semiconductor ink on a fluorinated dielectric surface due to it lyophobicity. We gradually improved the ink property, developed a surface modification method and ink-jet printing conditions to eventually obtain uniform semiconductor patterning. finally. We were able to obtain TFT arrays with 3.5 mm x 5 mm active area for each single pixel, providing a W/L ratio greater than 140. This TFT array can provide required drain current (average >10-4 A) and on-off ratio to drive the stretchable OLECs with high air stability, low leakage current and high stretchability. Finally, we achieved the successful vertical integration of these two components, using active-matrix as a controlling part to drive a light emitting panel. 

Figure 1. Left: Schematics of the vertical integrated active matrix and LEC array. Right: A SEM cross-sectional image of an AMOLEC device stack after subjected to 30% strain and released. The false colour highlights the active layer of OTFT (red) and LEC (yellow), respectively.

The final demonstration of this work is a freestanding 2 x 3-pixel array with 30% stretchability shown in the figure. 

Figure 2. Top: Photographic images of the AMOLEC skin display pixels with different columns being selectively turned on. Bottom: Photographic images of AMOLEC skin display pixels subjected to both bending and stretching. 

The most important finding in our work is show the possibility to realize the world’s first intrinsically stretchable active-matrix driven light-emitting array composed of intrinsically stretchable materials and fabricated by all solution-processing techniques. We expect the future work of higher density or multi-color display, or with further integration of other fully stretchable sensor arrays to provide human-interactive systems for sensing on-body information display and detection through visual interaction. For more details, please see our recent work published in Nature Communications:

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

Reference

1       Liang, J., Li, L., Niu, X., Yu, Z. & Pei, Q. Elastomeric polymer light-emitting devices and displays. Nature Photonics 7, 817 (2013).

2       Oh, J. Y. et al. Intrinsically stretchable and healable semiconducting polymer for organic transistors. Nature 539, 411-415 (2016).

3       Liu, Jia, et al. Intrinsically stretchable electrode array enabled in vivo electrophysiological mapping of atrial fibrillation at cellular resolution. Proceedings of the National Academy of Sciences (2020).

Jiechen Wang

PhD Candidate, Stanford University

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