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