Graphene and Organic Flexible Electrodes for Highly Efficient Flexible Polymer Light-Emitting Diodes

Abstract

In this Ph.D. dissertation, I investigated transparent and flexible electrodes as the alternative indium tin oxide (ITO) electrode for the highly efficient flexible display applications. Recently, flexible displays were successfully demonstrated using several flexible electrodes including graphene, silver nanowire, conductive polymeric anode, carbon nanotubes, metal grids, and thin metal films. However, the graphene electrodes have not been utilized for polymer light emitting diodes (PLEDs) yet, although their simple device structure and the solution-based fabrication process are expected to be more advantageous in terms of time and cost. I demonstrated high performance PLEDs which composed with simple two-layer structures using interface-engineered single-layer graphene films as anodes. The single-layer graphene synthesized by chemical vapor deposition methods was transferred onto a glass substrate utilizing an elastic stamp, and its work function was engineered by varying the duration and the power of ultraviolet ozone (UVO3) treatment. I was able to optimize the contact between silver electrodes and the graphene anodes, leading to the considerable enhancement of light-emitting performance. In case of the experiments about the conductive polymeric electrodes, highly efficient flexible PLEDs which composed with highly conductive and transparent foldable polymer electrodes were fabricated. New doping material, n-methyl-2-pyrrolidone (nMP) and n-methylformamide (nMF), have much improved the conductivity of poly(3,4-ethylenedioxythiophene) : poly(styrenesulfonate) (PEDOT:PSS). The selectively modulated inkjet-printing method facilitated the PEDOT:PSSs application to both transparent anodes and highly conductive bus line electrodes. Multiple-time printed electrodes showed similar performance to Ag bus lines while one-time printed anode showed much better figure of merit than ITO on the plastic substrates. Due to flexible property, high transparency and high work function of the polymeric anode, ITO-free PLEDs showed high performance with foldable characteristics. In order to increase the output efficiency of our device, high efficient flexible PLEDs which composed with the two dimensional nano hole arrays and highly conductive transparent polymer anodes were fabricated. The polymeric anodes were selectively deposited by an inkjet printing method. The photonic crystal structures embedded PLEDs showed high performance in the rigid and flexible substrates. The PC structures and polymeric anodes embedded flexible PLEDs showed an enhancement efficiencies by a factor of 1.31 and 1.28 in comparison with the normal PLEDs for the current and power efficiency at 979 cd/m2, respectively. Moreover, ITO-free PLEDs showed the outstanding flexible property due to the mechanical property of the polymeric anode. With the impedance spectroscopy, PLEDs with conductive polymeric anodes deposited by the inkjet printing process were investigated with impedance spectroscopy (IS). I analyzed the operational status of the PLEDs. In this work, the five devices were fabricated to study the effect of HIL and bus line electrode with two different anode materials, ITO and PEDOT:PSS. The equivalent circuit of our devices was established from IS measurements and analysis, including Cole-Cole plots, M-plots, Bode plots, and normalized resistance versus frequency plots. IS analysis showed that HIL should be included in the series resistance, and EML should be divided to the interface region and the bulk region. Moreover, to investigate the relationship between the IS parameter and the efficiency of the PLEDs, I fabricated PLEDs with PEDOT:PSS anodes on the various plastic substrates such as PA substrate, PS/PA substrate, and SiNx/PA substrate. I showed the relationship between the IS parameters and the efficiency of the PLEDs, which the slope of the mobility extracted from the on-set frequency is related to the current and power efficiency of the PLEDs.