Improving the flexibility of large-area transparent conductive oxide electrodes on polymer substrates for flexible organic light emitting diodes by introducing surface roughness

In this study, transparent conductive oxide (TCO) electrodes with highly enhanced flexibility were developed on polymer substrates for application in flexible organic emitting diodes (OLEDs). TCOs, particularly indium tin oxide (ITO), have superior functional properties as electrodes compared to other materials but are inherently brittle, which significantly limits the bendability of the flexible devices. To improve the fracture strength of ITO on a polymer substrate under bending, we investigated the effect of expanding the film surface areas on the reduction of the stresses induced by an external bending force. Regularly spaced channels were imprinted at an elevated temperature onto polymer substrate surfaces using Teflon^®. Then, both amorphous (a-ITO) and crystalline ITOs (c-ITO) were dc magnetron sputter deposited. As the channel patterns on the substrate surfaces were reflected into the growing film surfaces, the ITO surfaces became unidirectionally wavy, which increased the surface area by approximately 500%. The electrical and optical properties of the wavy ITOs were measured using a four-point probe and a UV-visible spectrophotometer, respectively, and the flexibility was evaluated with cyclic bending tests. For comparison, flexible OLEDs were also fabricated on both wavy ITO and conventional ITO. Our results revealed that the functional properties of ITOs with expanded surfaces are equivalent to those of conventional ITOs on the polymer substrates. However, their cyclic bending stability was significantly improved. After 10,000 cycles at a bending radius of 10 mm, the electrical resistivity change was less than half of the conventional ITO. The current density-voltage (J-V) characteristics of the flexible OLEDs on the wavy ITOs were also nearly equal to those on conventional ITOs.