-Developments of backplane technologies, which are one of the challenging topics, toward the realization of flexible active matrix organic light-emitting diodes (AMOLEDs) are discussed in this paper. Plastic substrates including polyimide are considered as a good candidate for substrates of flexible AMOLEDs. The fabrication process flows based on plastic substrates are explained. Limited by the temperature that plastic substrates can sustain, TFT technologies with maximum processing temperature below 400 C must be developed. Considering the stringent requirements of AMOLEDs, both oxide thin-film transistors (TFTs) and ultra-low-temperature poly-silicon TFTs (U-LTPS TFTs) are investigated. First, oxide TFTs with representative indium gallium zinc oxide channel layer are fabricated on polyimide substrates. The threshold voltage shifts under bias stress and under bending test are small. Thus, a 4.0-in. flexible AMOLED is demonstrated with indium gallium zinc oxide TFTs, showing good panel performance and flexibility. Further, the oxide TFTs based on indium tin zinc oxide channel layer with high mobility and good stability are discussed. The mobility can be higher than 20 cm 2 /Vs, and threshold voltage shifts under both voltage stress and current stress are almost negligible, proving the potential of oxide TFT technology. On the other hand, the U-LTPS TFTs are also developed. It is confirmed that dehydrogenation and dopant activation can be effectively performed at a temperature within 400 C. The performance of U-LTPS TFTs on polyimide is compatible to those of TFTs on glass. Also, the performance of devices on polyimide can be kept intact after devices de-bonded from glass carrier. Finally, a 4.3-in. flexible AMOLED is also demonstrated with U-LTPS TFTs.

A 9.4-inch 228-ppi full-color micro-LED display using the flexible low temperature polysilicon thin-film transistor (LTPS-TFT) backplane has been successfully developed. In order to solve the interconnection of tiny micro-LEDs, those size are less than 30 μm, we introduced the flip-chip soldering technology for high-resolution flexible micro-LED displays. Contrast ratio of >1,000,000:1 with a brightness of 700 nits was realized by the full-color micro-LED display with uniform brightness and color shift free at any off-axis viewing angles. These supreme micro-LED visual performances will provide a more comfortable user experience for high-resolution flexible automotive applications.

A highly reliable ultra-high gas barrier (UGB) was developed and applied on the fabrication of 4.3-inch flexible AMOLED. The water vapor transmission rate of the UGB on flexible substrate could achieve ~ 10 -6 g/m 2 -day under a calcium test (60 C and 90%RH). In addition, a delamination method to remove the flexible AMOLED from the glass carrier was proposed where the mechanical strain during the delamination is controlled within 0.2%. With the incorporation of a metallic interlayer, the damage of TFT due to the electrostatic discharges from the delaminated surfaces can be prevented.

A 4.1‐inch flexible QVGA AMOLED display with microcrystalline silicon (μc‐Si:H) TFTs backplane on colorless polyamide (PI) substrate is demonstrated. The PI substrate has the features of high Tg (∼350°C) and high light transmittance (∼90%). The bottom‐gate μc‐Si:H TFTs backplane is fabricated at 200°C by a conventional (13.56 MHz) plasma‐enhanced chemical vapor deposition (PECVD) system. The flexible μc‐Si:H TFTs backplane shows better electrical stability, flexibility, and uniformity.

Influences of silicon nitride (SiNx) films on the electrical performances of hydrogenated amorphous silicon thin film transistors (a-Si : H TFTs) are studied. Relatively low temperature (200 °C) SiNx films are prepared by plasma enhanced chemical vapour deposition at different radio-frequency powers. Results indicate that the SiNx films at a radio-frequency power of 340 W (Power density = 1.96 × 10−1 W cm−2) are near-stoichiometric and have better interface quality. Therefore, a-Si : H TFTs with this SiNx gate dielectric layer have a high field effect mobility and sustain the bias stress. The field effect mobility is 0.59 cm2 V−1 s−1 and the threshold voltage shift after a constant voltage stress (CVS) for 2.8 h is 3.18 V. The electrical degradation mechanism of a-Si : H TFTs is studied from the capacitance–voltage measurement. The degradation of the a-Si : H TFT after CVS is due to the defect generation in the SiNx gate dielectric and a-Si : H active layers. However, when the surface roughness of the SiNx film is poor, the degradation from the a-Si : H/SiNx interface is predominated. Therefore, if the SiNx film is used as a gate dielectric layer to fabricate a-Si : H TFTs, the surface roughness and chemical composition of the SiNx film should be considered simultaneously.