Development of Flexible and Transparent Ultrathin Metal Electrodes for Printable Optoelectronic Devices

Abstract

Flexible electronics based on organic materials such as organic solar cells (OSCs), organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs) have shown great potential for next generation platform techniques because of their portability, wearability and lightweight. Especially, with the high throughput roll-to-roll printing systems known as “printable electronics,” flexible electronics have been considered as one of the most promising research fields for ubiquitous electronic dㅁrview of flexible electronics based on organic electronics. First, the general features of flexible organic electronics are presented. Second, various alternative FTEs for bottom electrodes are discussed. Third, the fundamental operating mechanism of representative organic electronic devices such as OSCs and OLEDs is described. Fourth, various scalable printing technologies for flexible electronics are introduced. Chapter 2 deals with the development of large-area, extremely flexible, and highly transparent electrodes based on ultrathin Ag films. Various polymeric metal nucleation inducers (MNIs) with different surface energies and functional groups were used to fabricate the Ag electrodes, taking into account their properties such as coordination bond with deposited metal atoms and ionic self-assembly with plastic substrates. High performance and flexible white light-emitting diodes are developed, which outperform those based on widely used indium tin oxide (ITO) electrodes. Chapter 3 illustrates novel strategies to develop device structures for printable and flexible electronics that are fabricated with ultrathin Ag FTEs. First, a new and facile microcavity white OLED structure using an ultrathin Ag FTE is introduced that functions both as a partially reflective mirror and a transparent electrode. Second, an OPV color filter, which not only performs as an efficient color filter, but also generates electrical energy during the day, is developed by fabricating OLED-OPV tandem devices. Chapter 4 presents a simple and efficient printing method, which completely eliminates temperature-dependent performance issues commonly associated with OSCs based on printed photovoltaic films. By controlling the concentration of the photoactive solutions, favorable film morphologies and nano-structures are formed, thereby removing temperature-dependent performance issues arising from intrinsic differences in the coating dynamics of spin-coating and printing techniques including anisotropic movement, external force (centrifugal force and shear force), and extra control factors. Chapter 5 provides an overall summary of the work and the conclusions drawn in this thesis.