Low Fluorine Colorless Polyimide Substrate for Flexible OLED Display

Flexible display has become the main stream for the next innovative display. To build the flexible electronics, polyimide varnish is usually coated on the carrier glass with the thickness of a few micrometer and various devices such as flexible solar cell, flexible display, flexible printed circuit board, flexible terahertz sensor, and smart window [1]. Currently available colorless polyimide varnish is usually made of fluorine contained 6-FDA, which has good characteristics such as high transmittance, low yellow index, excellent solubility, but also has bad characteristics such as low decomposition temperature, high coefficient of thermal expansion, high cost, and environmentally unfriendliness due to fluorine. In this study, we make low fluorine contained polyimide varnish by substituting 6-FDA to BPDA + ODPA.

Fig. 1 shows the molecular structure of 6-FDA, BPDA, and ODPA. Low fluorine contained polyimide varnish is composed of 40% 6-FDA, 40% BPDA, 20% ODPA and it showed the excellent characteristics such as the glass transition temperature of 300 deg, the decomposition temperature of 366 deg, the yellow index of 2.2, the coefficient of thermal expansion of 20 ppm/deg, and the transmittance at 550 nm wavelength of 98.1%.

Because polyimide substrate made from coating polyimide varnish on carrier glass have to be protected using inorganic barrier material such as SiO₂ and Si₃N₄. Also, transparent electrode is also required to apply voltage to the OLED display. Because the color of the polyimide substrate can be changed with the variation of barrier material thickness and transparent electrode thickness, we used optical simulator, Macleod, to access the transmittance change with the thickness change of SiO₂ and Si₃N₄ as shown in fig. 2. Optimal thickness is selected at SiO₂ of 400 nm and Si₃N₄ of 50 nm. The experimental transmittance is well matched with the simulation transmittance as shown in fig. 3.

Fabricated device will be detached using innovative laser lift-off (LLO) that utilized excimer laser which scan the interface between the polyimide substrate and the carrier glass from the backside [2]. Fig, 4 shows the laser irradiation and the detached wrinkle free polyimide substrate. Before the delamination, the polyimide substrate is clear, but shows yellowish color during delamination process. Optimum laser energy for the delamination is compared with the color index as shown in fig. 5.

We will make the OLED display on the polyimide substrate.

1. Yi et al., Solar Energy 195, 340 (2020),

2. Bian et al., App. Sur. Sci. 499, 143910 (2020).

Figure 1