Vertical Structure Strategy Could Improve Next-Gen Wearable Displays

The skin-like displays under development for future wearable electronics can be based on passive- or active-matrix designs. Active-matrix designs, however, provide better spatial resolution and contrast and a faster response. All-organic active-matrix OLEDs (AMOLEDs) make it easier to use flexible or elastic materials to build the displays because AMOLEDs are flexible and can be processed at low temperatures. A vertical structure is conducive to making AMOLEDs with a high aperture ratio, which is a key performance indicator for display technology.

A research team at the Changchun Institute of Optics, Fine Mechanics and Physics (CIOMP) developed an integration strategy for building vertically structured AMOLEDs that addresses the challenges of working with a vertically-stacked design. The strategy, called discrete preparation-multilayer lamination, could provide a platform for building AMOLEDs with high aperture ratio for next-generation skin-like displays.

The strategy calls for each functional layer of the AMOLED to be prepared separately. The layers are then laminated together vertically. The functional layers include a top-emitting OLED array and an organic thin-film transistor (OTFT) array. To ensure mechanical compatibility and interconnection between layers, a single interconnect layer is placed between each functional layer. The OLED array is vertically stacked on the all-photolithographic OTFT array through a link between the interconnect layer and the functional layers.

(a) Schematics of skin-like AMOLED device and main fabrication scheme (bottom inset). (b) 3D optical microscope image. Scale bar: 5 mm. Illustrations are digital photographs and microscope images of individual pixels. (c) Photograph of a skin-like AMOLED array adhered to a transparent plastic hemisphere. Scale bar: 5 mm. (d) Photographs of the array adhered onto a human palm. Scale bar: 5 mm. (e) Photographs and 3D optical microscope image of the array adhered to a dragonfly wing. Scale bar: 2 mm. Courtesy of J. Li, Y. Ni, X. Zhao, B. Wang, C. Xue, Z. Bi, C. Zhang, Y. Dong, Y. Tong, Q. Tang, and Y. Liu.

The functional layers are free from the mechanical limitations imposed by the rigid substrates frequently used in layer-by-layer preparation methods. Thus, the AMOLED displays created with the discrete preparation/multilayer lamination method exhibit good mechanical flexibility and good conformability.

By preparing each layer on a different substrate, the researchers eliminate the potential for damage to organic materials caused by chemical solvents or by etching and heating processes during fabrication. The ability to protect fragile organic materials is essential for fabricating reproducible, high-quality vertically stacked displays.

No solvents or water are used during the peeling process and no additional adhesives are used during lamination. The all-dry fabrication approach prevents mutual dissolution and interface pollution, resulting in a clean, complete contact interface for fabricating the vertically stacked displays.

The researchers showed that the key metrics resulting from the use of the discrete preparation/multilayer lamination method are significantly better than those reported previously for all-organic AMOLEDs.

(a) Schematics of vertically stacked AMOLED. (b) Schematic diagram of the AMOLED array control circuit. (c) AMOLED array optical microscope images. Scale bar: 10 μm. (d) Device performance comparison of our device with reported all-organic flexible AMOLED displays. (e) Vertically stacked AMOLED arrays conformally fit on the back of an artificial hand. Digital photos with OTFT control of single and multiple pixels off and on. Courtesy of J. Li, Y. Ni, X. Zhao, B. Wang, C. Xue, Z. Bi, C. Zhang, Y. Dong, Y. Tong, Q. Tang, and Y. Liu.

Most existing AMOLED displays are parallel structures consisting of OTFTs and OLEDs placed side-by-side. In a parallel structure, part of the pixel area is covered by nonluminous OTFTs and wiring, resulting in a low active area, low luminance, low resolution, and a low aperture ratio. The aperture ratio of AMOLED displays with parallel structures has typically been less than 37%, with a maximum aperture ratio of only 52%.

Vertically stacking OLEDs and OTFTs eliminates obstruction from nonluminous OTFTs and wiring parts, maximizing the active area. In addition, a vertically structured pixel minimizes the occupied area, improving the display’s resolution and quality.

The discrete preparation-multilayer lamination strategy for vertically structured AMOLED displays allows multiple pixels to collectively display a pattern of “1” with an aperture ratio of 83%. The OLED device in these displays was found to maintain 80% of its initial current density even when the bending radius was as small as 2.5 mm. The all-photolithographic OTFTs were found to maintain 92% of their initial mobility value after 10,000 folding cycles. The mechanical flexibility and conformability of all-organic AMOLEDs created with the discrete preparation-multilayer lamination method allows the AMOLEDs to operate normally even on 3D curved objects or skin.

Modern microelectronic techniques can be integrated into the discrete preparation-multilayer lamination approach. The researchers believe that their strategy could form a basis for the construction of vertically stacked active-matrix displays that would enable the commercial development and mass production of integrated, high-resolution skin-like displays for next-generation wearable electronics and optoelectronics.

The research was published in Light: Science & Applications (www.doi.org/10.1038/s41377-024-01524-z).