Ag Fiber Electrodes significantly improve OLED efficiency

Team led by Prof. Byeong-Kwon Ju develops flexible OLEDs
Results published in the international journal Small
 

▲ Prof. Byeong-Kwon Ju, School of Electrical Engineering, College of Engineering

Professor Byeong-Kwon Ju of the School of Electrical Engineering in the College of Engineering and Professor Young Wook Park of Sun Moon University succeeded in improving OLED efficiency by developing flexible transparent electrodes with Ag fibers that are 100 times longer than Ag nanowires.

Ag nanowires are comprised of thin wires of Ag with diameters in the order of a nanometer. Ag nanowires offer the flexibility that is lacking in existing transparent electrodes (ITO), and at the same time, match ITO electrodes in terms of electrical conductivity and transparency. They are recognized as a promising material for transparent electrodes in display panels and lighting equipment. However, Ag nanowires are difficult to utilize in OLED displays, as the length of nanowires are limited to the order of a micrometer (㎛), which places constraints on improving conductivity and transparency. The increase in surface roughness due to nanowire junctions also results in electrical instability.

* Organic Light Emitting Diode (OLED): A light-emitting diode made from organic compounds and used in display panels. Widely used in mobile displays due to advantages such as high flexibility, thinness of film, and lightweight properties.
* Surface roughness: The extent of irregularity on the surface of a material.

The team employed electrospinning in order to develop junction-free Ag fiber electrodes of a few centimeters in scale, and thus significantly improved conductivity, transparency, and electrical stability. Electrospinning is a facile method that produces ultralong fibers using polymer solutions. The process is simple and can produce fibers of large area, which can be easily applied to large OLEDs. Electrical conductivity and transparency were maximized by adjusting the thickness and density of Ag fibers, and the EQE was improved by 19% compared to that of ITO electrodes.

Professor Ju said, “We are the first to develop and apply Ag fiber electrodes to OLEDs, thereby overcoming the limitations of nanowires. This is expected to contribute to the development of key technologies for wearable displays and lighting markets.”

The study, supported by the National Research Foundation of Korea grant funded by the Ministry of Science and ICT, was published in the international journal Small on December 28.

 - Title of the Article: Junction-Free Electrospun Ag Fiber Electrodes for Flexible Organic Light-Emitting Diodes
 - Authors: Prof. Byeong-Kwon Ju (corresponding author, Korea University), Prof. Young Wook Park (corresponding author, Sun Moon University, Junhee Choi (first author, Korea University), Yong Sub Shim (co-author, Korea University), Cheol Hwee Park (co-author, Korea University), Ha Hwang (co-author, Korea University), Jin Ho Kwak (co-author, Samsung Display), Dong Jun Lee (co-author, Korea University)

Terminology:
1. Small
  ○ An international journal in the field of nanomaterials with an impact factor of 8.643

2. Organic Light Emitting Diode (OLED)
  ○ A light-emitting diode made from organic compounds and used in display panels. Electrons and holes injected through the cathode and anode recombine to form an exciton, and this energy produces light of a certain frequency.

3. Surface roughness
  ○ The extent of irregularity on the surface of a material.

4. Transparent Conductive Electrode (TCE)
  ○ A material with high light transmittance and conductivity. Used in optical devices. Indium tin oxide (ITO) is a commonly used material, and Ag nanowires are used to create flexible transparent electrodes.

5. Nanowire
  ○ A nanostructure with diameter in the order of a nanometer. Size ranges from a diameter less than 10 nm to hundreds of nm. Length ranges from a few ㎛ to tens of ㎛.

6. Electrospinning
  ○ A fiber production method that applies electrical force on a polymer solution to produce fibers having a thickness of a few hundred nanometers. 

7. Sheet resistance
  ○ Used as a method to measure the resistance of thin films. Expressed as specific resistance per unit thickness.

Figure Description:

Figure 1. Schematic of fabrication of Ag fiber electrodes

Ag is deposited on flexible substrates, and electrospun polymer fibers are used as an etch mask. When polymer fibers are removed after solution etching, long Ag fibers with adjusted surface roughness are produced.

Figure 2. SEM images and photographs of Ag fiber electrodes

(a-d) Images showing the change in polymer fiber density with varying spinning times (e) image of electrospun polymer fiber (f) cross-linked fibers forming a complete etch mask after heat treatment (g) SEM images of Ag fibers (h) photographs of flexible PEN substrate with and without Ag fibers

The density of polymer fibers can be adjusted using spinning time, which can also be used to adjust electrode resistance and transparency. The resulting Ag fibers, as shown in SEM images, are junction-free and continuous. The photographs of Ag fiber electrodes show that the fiber electrodes are highly transparent.