Ultrathin transistor promises transparent flexible OLED displays

April 25, 2018 // By Julien Happich
Ultrathin transistor promises transparent flexible OLED displays
In a paper titled "Flexible active-matrix organic light-emitting diode display enabled by MoS2 thin-film transistor" published in Science Advances, a team of South Korean researchers from Yonsei University, and Chung-Ang University propose a novel ultrathin transistor architecture with drastically increased charge mobility, which could be used for transparent flexible OLED displays.

In their paper, the researchers highlight the drawbacks of 2D MoS2 used as an active channel material in thin film transistors, lacking charge mobility due to a large contact barrier between the source/drain (S/D) metal electrode and the MoS2 channel. In such architectures, electron transport is hindered by Coulomb scattering and trap charges at the interface between the gate dielectric and MoS2, lowering charge mobility to under 1 cm2 V−1 s−1 on par with that of a-H Si.

They propose an Al2O3-capped TFT device, whereby a bilayer of Molybdenum disulphide is sandwiched between two layers of high-k dielectric aluminium oxide.

According to the paper, the high-k Al2O3 layer reduces the contact resistance at the metal/MoS2 interface and facilitates considerable n-type doping of the MoS2 layer owing to its oxygen-deficient surface. This in turn reduces the scattering charge impurities. What's more, the bottom Al2O3 layer reduces surface roughness, further improving the device's performance as it effectively decreases the interface-trapped charge density.

Fig. 1: (top) the high-mobility MoS2 TFT using an Al2O3 passivation layer capping the MoS2 channel region. An ultra-thin MoS2-based backplane array drives an AMOLED display (middle), capable of operation when affixed to human skin (bottom).

All these effects combined yield a 28-fold increase in mobility value compared to conventional back-gate structure, report the authors, with a positive threshold voltage of about 5V, a high ON/OFF ratio at circa 108.

As an extra benefit, the positive turn-on voltage of the top-gated TFT can maintain the OFF state of the pixel without the supply of an additional gate bias voltage, reducing unnecessary power consumption during selective pixel operation.

Fig. 2: The AMOLED stack using the novel MoS2 TFT
architecture (left), and the ultra-flexible display system (right),
less than 7μm thin.

To prove their novel MoS2 TFT architecture, the researchers then fabricated a backplane circuit for a flexible OLED display, consisting of a 6x6 array of transistors switching OLED pixels ON and OFF on a 6μm-thick ultrathin polyethylene terephthalate (PET) substrate.

After it was peeled from the carrier glass substrate, the ultra-thin active-matrix OLED display, only seven micrometers thick, exhibited stable performance during continuous operation even when affixed to the skin of a human wrist. Switching between the alphanumeric characters “M,” “O,” “S,” and “2”, the display was also driven under flexure at a repeated bending radius of 0.7mm, only suffering minor current variations (within 10%), which recovered when display was laid flat again.

Fig. 3: Sequential photos of the display in dynamic operation on a human wrist (top) and a close-up of the peel-off process from a carrier glass substrate.

The OLED was driven at approximately 8V while 9V were applied to the gate and drain terminals of the driving TFT. At VGate of 9V, the OLED pixels reached a maximum luminance of 408 cd m−2 and could be switched ON and OFF in milliseconds.

Next, the researchers will investigate if their novel transistor can be manufactured economically at scale for use in a high-resolution devices.

Yonsei University - www.yonsei.ac.kr
Chung-Ang University - https://neweng.cau.ac.kr

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