Fabrication of practical deformable displays: advances and challenges

Abstract

Display form factors such as size and shape have been conventionally determined in consideration of usability and portability. The recent trends requiring wearability and convergence of various smart devices demand innovations in display form factors to realize deformability and large screens. Expandable displays that are foldable, multi-foldable, slidable, or rollable have been commercialized or on the edge of product launches. Beyond such two-dimensional (2D) expansion of displays, efforts have been made to develop three dimensional (3D) free-form displays that can be stretched and crumpled for use in realistic tactile sensation, artificial skin for robots, and on-skin or implantable displays. This review article analyzes the current state of the 2D and 3D deformable displays and discusses the technological challenges to be achieved for industrial commercialization.

Fig. 1. Technological roadmap of deformable displays.

It shows the form factor and functional innovation for deformable displays

Fig. 2. Patent registration for deformable displays.

Changes in the number of patents related to deformable displays registered in four countries (United States, China, South Korea, and Japan) every 5 years

Fig. 3. Technological classfication of deformable displays.

Basic technologies and technological challenges in the 2D expandable and 3D free-form deformable displays

Fig. 4. Display structure of the display and hinge adopted in foldable displays.

a Unit device layers in the foldable displays. Optimization of each layer and design of neutral planes determine the stress distribution applied to cover glass, polarizer, touch screen, and display panel layers. Reproduced with permission Copyright 2019 Wiley-VCH. b Thickness change effect of the optical clear adhesive (OCA) layers on the formation of the neutral planes and the prevention of display delamination. Reproduced with permission Copyright 2021 ASME. c A hinge system adopted in foldable displays to reduce the stress applied to the panel during repeated folding and un-folding cycles. Reproduced with permission Copyright 2021 ASME. d Hideaway dual cam mechanism applied in Samsung Galaxy Z Flip. During folding and unfolding, two cam pieces slide up and down against each other, and the friction between them allows the device to stand alone at different angles Reproduced with permission Copyright 2020 Samsung Electronics Co., Ltd

Fig. 5. Stress minimization in the 2D expandable displays.

a Mechanical failure that can occur during repeated folding cycles. Reproduced with permission Copyright 2021 ASME. b, c Patterned supporting structure (b) and protecting cover layer (c) to absorb stress in the folded region. Reproduced with permission Copyright 2021 ASME. d Partial laser etching in the folded region of the top panel layer to realize multi-axis foldable displays. Reproduced with permission Copyright 2021 Springer Nature. e, f Folding/bending characteristic of a patterned QLED (e) and 3D multi-axis folding of QLED (f). Reproduced with permission Copyright 2021 Springer Nature

Fig. 6. Basic technologies in the 3D free-form stretchable displays.

a Serpentine-shaped interconnections and integrated µLEDs driven by Si-based TFTs. Reproduced with permission from ref. Copyright 2017 Wiley-VCH. b Stretchable OLED display deposited on a patterned substrate with a stress-relief elastic pillar array and interconnection bridges. Reproduced with permission from ref. Copyright 2020 American Chemical Society. c 14.1-inch stretchable active-matrix OLED display with wave-shaped interconnecting hinges. Reproduced with permission from ref. Copyright 2019 Wiley-VCH. d Intrinsically stretchable transistor array integrated on a patterned elastomer layer with tunable stiffness. Reproduced with permission from ref. Copyright 2021 Springer Nature. e Stretchable OLED composed of intrinsically stretchable (is-) layers. Reproduced with permission from ref. Copyright 2021 AAAS. f Structure and light-emitting polymers of the on-skin stretchable all-polymer LED (APLED) displaying pulse signals in real-time. Reproduced with permission from ref. Copyright 2022 Springer Nature

Fig. 7. Crumpable and feelable displays.

a Deformable smart textile display system with multifunctional devices. The right photograph shows the fiber photodetector that displays environmental UV light intensity. Reproduced with permission from ref. Copyright 2022 Springer Nature. b Large-area ACEL textile display made by weaving conductive and luminescent fibers. Reproduced with permission from ref. Copyright 2021 Springer Nature. c 3D shape-morphing system with interactive interfaces composed of mechanical actuators and visual-tracking systems. Reproduced with permission from ref. Copyright 2015 IEEE. d Electric stimulation of the human skin mechanoreceptors. Reproduced with permission from ref. Copyright 2013 IEEE. e AC-driven organic light-emitting board (OLEB) visualizing the finger touches on the OLEB. Reproduced with permission from ref. Copyright 2017 Springer Nature

Fig. 8. On-skin displays and implantable optoelectronic devices.

a Stand-alone health monitoring patch containing a stretchable OLED array powered by an external battery. Reproduced with permission from ref. Copyright 2021 AAAS. b On-skin electrocardiogram (ECG) sensor composed of permeable Au nanofiber mat with both waterproofing and hydro-wetting properties. Reproduced with permission from ref. Copyright 2022 Wiley-VCH. c Cardiac optogenetics therapy composed of strain sensors for heart rate monitoring and self-adaptive LEDs for optical stimulation. Reproduced with permission from ref. Copyright 2021 AAAS. d, e Use of implantable optoelectronic devices for wireless optical stimulation (d) and neuromodulation (e). Reproduced with permission from ref. Copyright 2020 Wiley-VCH. Reproduced with permission from ref. Copyright 2015 Springer Nature