Soluble sulfoxide biphenyl polyimide film with transmittance exceeding 90%

Colorless polyimides (CPIs) have encountered new challenges for higher optical performance including transmittance (T% > 90%) in visible range and yellow index (YI < 2.0), to be utilized as substrates of cover window or touch panel in the foldable displays. With persistent efforts, the highest transmittance of conventional CPIs has barely reached about 88%, while still cannot satisfy the optical requirement. In this work, soluble sulfoxide biphenyl polyimide PI (TFSODA/BPDA) with high optical performance (T% = 88.5%, YI = 3.2) was successfully fabricated by a low-temperature (about 220 °C) process rather than high-temperature process (over 300 °C). Then, a finely designed inorganic anti-reflection coating was pioneering utilized on the sulfoxide polyimide film to further improve its transparency. The combination of chemical modification of molecular structure and physical regulation of incident light energy on the surfaces leads to a transmittance of 91% in the visible range, a YI of 2.9, and an effective enhancement of surface performance. In addition, the resultant sulfoxide biphenyl polyimide also exhibited excellent thermal durability (Tg = 327 °C) and dimension stability (CTE = 24.7 ppm/K in the range of 50–300 °C), which was attractive for applications in foldable displays.