DESIGN AND FABRICATION OF TOUGH, TRANSPARENT, AND SEMICONDUCTIVE POLYMER SUBSTRATES FOR FLEXIBLE OPTOELECTRONIC DEVICES

Abstract

Flexible electronics have come a long way in the last several decades, from the development of flexible solar cell arrays constructed of ultrathin single-crystal silicon to the development of flexible organic light-emitting diode displays on polymeric substrates. The recent speedy development of this field has been urged on the continuing evolution of large-area electronics with applications in disparate display devices, such as electrochromic devices, energy-efficient windows, organic lightemitting diodes, organic photovoltaic cells, and electronic paper. Flexible electronics are appealing for various reasons, including the fact that they are more flexible, lighter, portable, and inexpensive to manufacture than conventional rigid substrate counterparts. Currently, the glass substrate became an inevitable part of every display device because of its excellent transparency in the visible region, high corrosion resistance, and good weathering ability. However, along with those advantages, it has several drawbacks, e.g. (i) low impact strength, (ii) poor shatter resistance, (iii) heavyweight, and (iv) expensive mass production due to its high melting temperature (1500 ºC) and highly viscous melt. Therefore, the search for an alternate material to substitute glass in state-of-art electronic display devices has become one of the biggest challenges to the scientific community as light-weight, size reduction, and flexibility are the three most demanding criteria for modern electronics. On the other hand, indium tin oxide (ITO) is the most widely used transparent conducting electrode (TCE) due to its excellent optoelectrical properties with a sheet resistance of 10–25 Ωsq-1 at about 90% optical transparency in the visible region. Current optoelectronic devices are typically assembled on an ITO-coated glass substrate (ITO/Glass). However, flexible versions are becoming a developing trend and will open new applications and markets. This requires TCE to be flexible or even wearable and stretchable, cheap, and compatible with large-scale manufacturing methods, in addition to being conductive and transparent. Considering the advancement of optoelectronic devices, ITO has several limitations: (i) it is becoming increasingly expensive due to a predicted shortage of indium resources and its everrising consumption; (ii) the deposition of ITO requires vacuum procedures and often elevated temperatures; (iii) it lacks flexibility and cracks easily on flexural stress, which limits its use for flexible electronics; (iv) indium is known to diffuse into the active layers of OLEDs or OPV cells, which leads to a degradation of device performance. Therefore, in order to keep the pace of device development, new types of TCEs are urgently needed to replace the conventionally used electrode. This thesis is organized to provide a synthesis and systematic characterization of all three-set of copolymers, i.e., (a) Methyl methacrylate and Acrylonitrile, (b) Methyl methacrylate and Styrene, and (c) Methyl methacrylate and Vinyl neodecanoate materials. These copolymers are optimized based on mechanical, optical, and selective screening of UV portion of light properties to substitute the conventionally used glass substrate. Further, we have fabricated a series of nanocomposites of optimized copolymer with MWCNT (used as conductive filler) and optimized it based on optical and electrical properties. This optimized, flexible, tough, conductive, transparent, and selective UV screening substrate can be used to fabricate state-of-art optoelectronic devices, such as electrochromic devices, electronic skins, wearable electronics, electronic paper, smart windows, and solar cells. It is not the intent of this thesis to focus on optoelectronic technology but rather to provide alternative materials and applications; the chapters presented are also intended to augment existing literature in the field of flexible electronics.