Development of active-matrix full-color flexible OLED based on two-dimensional semiconductors

A giant step toward the commercialization of future displays including wearable electronic devices

The results of the joint research by Professor Kim Soo-young’s team of Korea University and Professor Ahn Jong-hyun’s team of Yonsei University were published in Science Advances, an internationally renowned journal.

▲ (From left) Professor Kim Soo-young of the Department of Materials Science and Engineering (corresponding author) 

and Bae Sa-rang, a doctoral student (first author)

Professor Kim Soo-young’s team of the Department of Materials Science and Engineering, Korea University, and Professor Ahn Jong-hyun’s team of the Department of Electrical & Electronic Engineering, Yonsei University, conducted a joint study, which represents a significant step toward the commercialization of future-type displays attachable to human skin and clothes.

The joint research team successfully fabricated an active-matrix full-color flexible OLED that can be easily attached to human skin and clothes by using a two-dimensional semiconductor device to incorporate thin-film transistors and logic circuits on ultrathin plastic substrates, wherein the ultrathin plastic substrates show relatively high performance.

The joint research team has successfully fabricated an active-matrix full-color flexible OLED by using a two-dimensional semiconductor device. This is the very first flexible ‘full-color’ display in the world that operates in an active-matrix mode.

The study was supported by the Basic Research in Science & Engineering Program (Basic Research Laboratory and Research Leader Program) funded by the Ministry of Science and ICT and the National Research Foundation of Korea. The results were published in the online edition of Science Advances, an internationally renowned journal, on the dawn of July 9 (afternoon on July 8, US EST).

*Article Title: Full color active-matrix organic-light emitting diode display on human skin based on a large area MoS2 backplane

*Authors: Dr. Choi Min-woo (first author, graduated from Yonsei University), Bae Sa-rang (first author, doctoral student of Korea University), Luhing Hu (co-author, doctoral student of Yonsei University), Anh Tuan Hoang (co-author, doctoral student of Yonsei University), Professor Kim Soo-young (corresponding author, Department of Materials Science and Engineering, Korea University), and Professor Ahn Jong-hyun (corresponding author, Department of Electrical & Electronic Engineering, Yonsei University).

▲ A TFT array was formed on a thin MoS2 film, and RGB OLEDs were deposited on the TFT electrodes. 

The display was detached from the carrier substrate and placed on the back of a human hand.

Two-dimensional semiconductor materials, including MoS2, WS2, MoSe2 and WSe2, have unique electrical and mechanical properties suitable for application to backplane circuits of wearable electronic devices. Among them, MoS2 has been employed to fabricate a switching device for wearable applications.

A MoS2 film and transistor arrays were fabricated directly on a SiO2/Si wafer by metal-organic chemical vapor deposition (MOCVD), and a polyethylene terephthalate (PET) substrate was coated with Al2O3 using atomic layer deposition (ALD).

The structure of the transistor device was encapsulated with ALD-formed Al2O3 to improve metal contact and carrier mobility due to the n-doping effect in the contact and channel regions.

A high carrier mobility (>18 cm2V-1s-1) and high ON/OFF ratio (>107) were observed, and the light emission of the RGB OLED pixels was controlled by applying a gate voltage of between 4 and 9 V. The proposed direct fabrication method using a plastic substrate combined with 2D semiconducting materials and OLED resulted in excellent electrical, optical, and mechanical performance.

In addition, the RGB OLED pixels integrated with the MoS2 transistors were connected to a data line and a scanning line, and the entire display circuitry operated in an active-matrix manner. The pixel current was precisely controlled according to the drain and gate signals of the transistor, thereby changing the brightness of the OLED. The mechanical properties of the ultrathin display allowed for stable transfer of the detached device from the carrier glass substrate without a decrease in the performance of the device.

Professor Kim Soo-young of the Department of Materials Science and Engineering, the corresponding author of this article, stated, “This study is significant because we have improved the ability of the device to tolerate a curved surfaces, such as clothes, while maintaining the color expression performance of existing full-color displays.” He added, “We may be able to bring forward the commercialization of future-type wearable displays.”

*Description of Terms

▶MoS2: MoS2, molybdenum disulfide, consists of molybdenum and sulfur, and is a transition metal dichalcogenide. All types of MoS2 have a laminated structure in which the plane of the molybdenum atom is sandwiched in between planes of sulfide ions. The three layers constitute a MoS2 monolayer. Bulk MoS2 consists of stacked monolayers held together by a vulnerable van der Waals interaction.

▶ Thin Film Transistor (TFT): TFT is an element of an electric circuit comprising a semiconductor, and it acts as a valve that controls the flow of electric current. TFT, a transistor in the form of a thin film, controls the brightness of individual pixels on a display screen.

▶ Active matrix: Active matrix refers to the manner of operating pixels in which each pixel has a TFT, a semiconductor device, and electrodes such that the pixels can be turned on and off individually. Most LCD and OLED displays follow the active matrix approach.