Fiber-based polymer light emitting diodes for wearable displays

Abstract

Advances in material science and electronics technologies have promised to enable the form-factor of electronic devices to be miniaturized into nearly imperceptible configurations, and wearable electronics have recently been spotlighted for advanced capabilities exceeding the spatial and temporal limitations of hard electronics. Among various strategies, e-textiles integrating electronic functions into human-friendly clothes, have been studied at both the fiber and fabric levels. While most earlier work has focused on transistors, sensors, photovoltaic devices, and electric connectors, there are few studies of wearable display devices. Among the more promising applications, wearable displays have emerged as a technological collaboration between the textile and fashion industries, and state-of-the-art electronics. Fiber-based wearable displays are considered highly desirable because they allow display functions to be incorporated without losing the inherent properties of hierarchically-woven clothes. These include important characteristics such as flexibility, comfort, durability, and washability, all at the same time. However, in spite of its great potential, few workers have attempted to realize fiber-based wearable displays utilizing organic/polymer light-emitting diodes and polymer light-emitting electrochemical cells. Through the work described in this dissertation, we have broken from the existing ways in which display devices are produced on flat substrates such as glass and polymer films. Instead, we produced cylindrical fiber-based OLEDs by the dip coating method, the most appropriate solution process for cylindrical fibers. We also analyzed the peculiar electrical and optical properties of these light-emitting devices due to the effect of the cylindrical geometry. However, they, like other previously reported fiber-shaped light-emitting devices, are considerably behind the performance and stability of well-established planar-shaped display devices. Based on the results from our analysis, we proposed a tailored device structure suitable for cylindrical fibers to achieve fiber-based display devices with performance at the level of planar displays. We developed and demonstrated fiber inverted OLEDs composed of a low-temperature solution-processable ZnO/PEI bilayer to compensate for a deficient electron-injection barrier over the entire surface of a fiber. The fiber OLEDs showed an unprecedentedly high luminance and efficiency in comparison with their conventional counterparts. Notably, the efficiency and lifetime achieved with the fiber OLEDs were on a par with glass-based devices. These results are significant because these state-of-the-art, highly efficient solution-processed flat OLEDs can be applied to fibers without any degradation in performance. Furthermore, we verified that the resulting fiber OLEDs are the most desirable for the application of wearable displays by analyzing properties such as outcoupling efficiency, flexibility, and weavability, with the assistance of simulation. Regarding the fabrication process, we also proved that the proposed approach, using a low-temperature process, holds the promise of scalability and versatility of application. Last, we tried to develop all dip-coated fiber OLEDs, a near ideal form of inexpensive manufacturing. Given the findings in this dissertation, we believe that this work represents an important milestone on the way toward low-cost commercially feasible, wearable displays. This, in turn, could reveal the easiest way to achieve low-cost mass-produced fiber-based light emitting devices using a roll-to-roll process.