Displays and other electronics are rapidly evolving as they become lighter, cheaper, and more ergonomic. The next stage in this evolution is to add flexibility to their properties to substantially reduce their size and increase their usability.
A key component in the function of electronic devices, including displays and photovoltaics, is the optically transparent and electrically conductive electrode. Presently, transparent conductive electrodes are made from Indium Tin Oxide (ITO) deposited onto glass or polymer film substrates. This is carried out by physical vapor deposition or chemical vapor deposition; both are expensive and batch processes. Regardless of the substrate it is deposited on, ITO is an inorganic ceramic material which lacks the ability to flex. At a very small strain, 2.5%, the thin ITO layer cracks and loses its ability to conduct electricity and therefore is not suited as a component for flexible electronic devices.
To produce flexible transparent conductive films, a hybrid electrospinning and solution casting process was developed. With this process conductive nanofibers (wires) are generated by electrospinning and partially embedded into transparent films. Poly(methyl methacrylate) (PMMA) conductive films were produced with a surface resistivity of 1.8 kohm/sq and a normalized transmittance of 91% at the wavelength of 550 nm. In addition to PMMA films, conductive polycarbonate (PC) films were also produced with a resistivity of 700 ohm/sq and a normalized transmittance of 75% at 550 nm. Highly flexible polyimide (PI) films with a resistivity of 2.8 kohm/sq and a normalized transmittance of 93% at 550nm were also produced. These films were subjected to flex testing using a specially developed device and were found to maintain their conductivity even after repeated flexing (300 +) around rod with a radius of 1 mm and a curvature up to 1 mm-1. Using these films a flexible single pixel Polymer Dispersed Liquid Crystal (PDLC) display was prepared. The PDLC cell exhibited fast switching response and functioned normally even when it was bent almost 180 degrees.
Beyond flexible electronics, the need will come in the future for deformable electronics to create devices with single and double curvatures such as goggles with displays. To demonstrate that the hybrid films we prepared could also be used for thermoforming to achieve such structures, they were heated and stretched in the rubbery state using a highly instrumented stretching machine that measures true stress, true strain, birefringence and electrical conductivity simultaneously. Upon stretching up to 2X, these films were found to maintain conductivity with minimal loss (about one decade) with deformations .This makes them very attractive for applications where thermoforming can be applied to create devices with single and double curvatures such as visors.
The flexible transparent conductive films were also tested as flexible transparent conductive heaters for their potential application in car windshields and airplane canopies. The tests showed that all the flexible transparent conductive films had very short response times below 20 seconds; and they can be heated rapidly to 70 °C while maintaining transparency.