How to stretch a display and maintain pixel resolution
We dream of a single device with a size-variable screen display that can function as a phone, a pad, and a tablet as required. We want a small screen device for voice communications but a large screen when reading text or watching movies on the same device. Several engineering prototypes of so-called rollable or foldable, sometimes multi-axis foldable, screen displays have been demonstrated. The size of the viewing area—thus the device planar area as well—is small when the displays are stored in a rolled or folded form, but can be enlarged when unrolled or unfolded. However, such devices are generally bulky due to the form factor of the rolled or folded screen displays. We considered how to change the size of the screen display and its form factor.
There have been two previous approaches to developing stretchable display technology, characterized by a very thin form factor based on an elastomeric substrate and light-emitting elements such as organic light-emitting diodes (OLEDs) or conventional LEDs. White and Liang made the OLED element itself stretchable.1, 2 The light-emitting area is expanded when tensile stress is applied, but device performance is degraded when stretched. Kim reported a stretchable LED array where the LED elements are integrated with stretchable electrodes, so that the array area is expanded but the LEDs remain intact under the tensile stress.3 Although the light-emitting performance of the array is maintained, the pixel resolution is reduced as the distance between the light-emitting pixels increases. We have been working on a ‘hidden pixel structure’ to vary the screen size while maintaining the pixel resolution (see Figure 1).
Typically, the electrical resistance of stretchable electrodes increases with tensile stress, and researchers have used various approaches to minimize such change to this ‘positively strain-dependent electrical resistance.’4–8 In our device, hidden pixels are located between conventional pixels (on pixels in Figure 1) and connected via novel electrodes whose electrical resistance decreases with tensile stress. (We call this negatively strain-dependent resistance.) In our array structure, the conventional LEDs are initially on. The hidden LEDs are turned on as the tensile stress is applied, maintaining the original pixel resolution.
To realize this concept, we developed two types of stretchable electrodes. The first is a simple method of magnetically arranging and patterning ferromagnetic conductive nickel fillers embedded in poly(dimethylsiloxane) (PDMS), which is elastomeric. Unlike typical conductive composites, our magnetically arranged nickel composite shows negatively strain-dependent resistance. Very low initial conductivity of the nickel composite electrode significantly increases, up to a maximum of 115S/cm at 100% tensile strain. For the second type, we applied inkjet-printed silver thin films on the nickel composite electrodes to improve the initial conductivity. We obtained a highly stretchable electrode with less than 20Ω resistance (1cm length and 1mm width) for zero to 100% tensile strain.9
We used both types of stretchable electrodes to fabricate a 1D LED array with conventional pixels (first, third, and fifth LEDs in a short movie10) and hidden pixels (the second and fourth LEDs), which were connected to power supply lines via silver-covered nickel composite and nickel composite-only electrodes, respectively. The conventional pixels are initially turned on and are continuously on under the tensile stress as the silver-covered electrode contains highly stretchable property. The nickel composite-only electrodes provide power and turn on the hidden pixels as the electrodes' resistance reduces with the tensile stress. Finally, all five LEDs are turned on when the array is stretched, sustaining its original pixel resolution.
To enable resolution-sustainable stretchable display technology in real devices, the use of the hidden pixel structure and negatively strain-dependent nickel composite electrodes seems highly promising. Our results have shown that we can easily make appropriate electrode patterns for LED arrays by using a patterned magnetic field. Our next step will be to show that this method can provide a platform technology for 2D and high-resolution pixel arrays. Our approach and concept could be one of the key enabling technologies for future stretchable high-performance electronic display devices. To make a practical display using this concept, we now need to improve the operational stability of the composite electrodes and develop a high-resolution patterning method for the composite based on the patterned magnetic field.
This work was supported by a grant (2013M3A6A5073181) from the Center for Advanced Soft Electronics under the Global Frontier Research Program of the Ministry of Science, ICT and Future Planning (MSIP), Korea, and the Converging Research Center Program through the MSIP (2013K000371). It was also supported by the IT R&D program of MSIP/Korea Evaluation Institute of Industrial Technology (10041416, The Core Technology Development of Light and Space Adaptable New Mode Display for Energy Saving on 7inch and 2W).
Department of Electrical and Computer Engineering (ECE)
Seoul National University
Yongtaek Hong received his BS and MS in electronic engineering from Seoul National University in 1994 and 1996, respectively, and a PhD in electrical engineering from the University of Michigan, Ann Arbor, in 2003. After working for the Eastman Kodak Company, he joined ECE and is now an associate professor.
Sangwoo Kim received his BS and PhD in electrical engineering and computer science from Seoul National University in 2008 and 2014, respectively. His research interests include stretchable sensors and electronics, electronic skin development, and conductive polymer composite materials.