Active-matrix organic light-emitting diode display on human skin

The development of electronic applications can take on many new forms to include foldable and wearable displays to monitor human health and act as medical robots. Such devices rely on organic-light emitting diodes (OLEDs) for optimization. However, it is still challenging to develop semiconducting materials with high mechanical flexibility due to their restricted use in conventional electronic formats. In a new report on Science Advances, Minwoo Choi and a team of scientists in Electronic Engineering and Materials Science in the Republic of Korea, developed a wearable, full-colour OLED display using a two-dimensional (2-D) material-based backplane transistor. They engineered an 18-by-18 thin-film transistor array on a thin molybdenum disulfide (MoS₂) film and transferred it to an aluminium oxide (Al₂O₃)/polyethylene terephthalate (PET) surface. Choi et al. then deposited red, green and blue OLED pixels on the device surface and observed excellent mechanical and electrical properties of the 2-D material. The surface could drive circuits to control the OLED pixels to form an ultrathin, wearable device.

Scientists and engineers must conduct extensive research in the field of wearable electronics to develop smart electronic systems focused on flexible devices and ultrathin substrates. Inherent limits of such materials have motivated the use of alternative semiconductor materials such as MoS₂ for inclusion in thin-film transistors (TFTs) and logic circuits with relatively high performance. These materials are known as transition metal dichalcogenides and they provide unique electrical, optical, and mechanical properties for backplane circuitry of wearable electronics. Researchers had recently developed MoS₂ transistors with sophisticated red, green and blue (RGB) colors as a fundamental and essential requirement for practical displays. In this work, Choi et al. developed a large-area MoS₂ TFT array to operate 324 pixels in a 2-inch RGB OLED, in which the full-color display demonstrated an active-matrix configuration. The RGB OLEDs were made of different optoelectronic characteristics, therefore the team designed the backplane TFTs to control each color pixel. The experimental setup was promising as a wearable display and functioned steadily on human skin without adverse effects. The team used heterogenous material designs to form optoelectronics in the present work.

Large-area active-matrix OLED (AMOLED) display

The team designed a large-area active-matrix OLED (AMOLED) display with an MoS₂ backplane via a sequence of processes. They first formed a thin-film-transistor (TFT) array on a thin MoS₂ film, then deposited an RGB OLED on the drain electrode of the TFTs and peeled the display from the carrier to transfer it to the human hand (the target). During the process, they synthesized a bilayer MoS₂ film on a 4-inch SiO₂/Si wafer via metal organic chemical vapor deposition (MOCVD). Then they coated a polyethylene terephthalate (PET) substrate with aluminium oxide using atomic layer deposition and transferred the MoS₂ film from the SiO₂/Si wafer to this PET substrate to produce an MoS₂ transistor array with a driving backplane configuration. The resulting structure was unique and encapsulated with aluminium oxide for improved metal contacts and carrier mobility. The full-color AMOLED display uniformly controlled the RGB OLED pixels, where each pixel connected to a data and a scanning line and the entire display circuit functioned in an active-matrix design. Choi et al. controlled the pixel current based on the drain and gate signals of the transistor to change the brightness of the OLED. They could then transform the ultrathin display from the carrier glass substrate to a curved surface without device degradation.

Stable display applications

The team examined the current-voltage output curves to determine the drain characteristics of the TFTs to illustrate the relationship between the drain current (I_DS) and the bias voltages (V_DS and V_GS). The homogeneity of the MOCVD-grown MoS₂ film allowed high uniformity for stable display applications. The device properties were consistent across all the samples, allowing the single pixel to operate in the full-color AMOLED, while the efficiency did not decrease. The team measured the highest luminescence at 460, 530, and 650 nm for the blue, green and red OLEDs.

At a repeated gate pulse bias of +10 volts, the OLED exhibited a rapid transition between on and off states, although the response time was limited by the measurement system, the delay time was short. Gate modulation did not occur during the off state and the pixel state remained stable, providing efficient leak-proof operation of the TFT. The pixel current also dramatically increased with increasing gate bias (V_G) during the on state to reach a threshold voltage of 5 volts across the RGB OLEDs.

Proof-of-concept—wearable electronic device

The team confirmed the performance of the individual RGB pixels using the transistors and integrated an 18 x 18 array (324 pixels) to the data and gate lines of the transistor backplane circuit to form a full-color AMOLED display. They controlled each pixel via the matrix line and maintained consistent light luminescence in each individual pixel in the OLED displays. The RGB OLED pixels showed consistent and uniform brightness due to the stable control of the gate and data signals. Choi et al. drove the RGB pixel arrays sequentially via an external drive circuit configured in a commercial strip pixel structure representing the characters ‘R’, ‘G’, and ‘B’.

The low stiffness of the ultrathin device prevented the deterioration of optical and electrical properties during substantial mechanical deformation reflexes—after its transfer to a human hand. Based on the current-voltage characteristics (I-V), the current level did not change during skin shrinkage or skin stretching exercises and the on-state also did not fluctuate during active-matrix display operation. While the device stability is still in development, the team aim to conduct further engineering to improve the MoS₂ film for practical applications as a wearable, full-color AMOLED display.

In this way, Minwoo Choi and colleagues developed a thin (2-inch), wearable and full-color AMOLED display with 18 x 18 arrays using MoS₂-based backplane TFTs. They built the transistor array directly on a bilayer MoS₂ film grown using MOCVD and observed a high carrier mobility and on/off ratio. The team controlled the light emission of the RGB OLED pixels by applying a gate-voltage between 4 and 9 volts. They used an ultrathin plastic substrate (PET) combined with 2-D semiconducting materials to directly fabricate OLEDs for excellent electrical, optical, and mechanical performance. This experimental system can be improved for integration in wearable and electronic devices beyond the existing conventional and rigid organic materials.