Common Materials and Application Fields of Flexible Electronics!

Flexible electronic displays are new products developed on the platform of flexible electronic technology. They are variable and bendable display devices made of flexible materials. Currently, flexible display technologies (electronic paper, LCD, OLED, etc.) can be implemented on flexible substrates, enabling the development of devices such as foldable phones, writable e-books, and USB capacity displays.

Flexible electronics is a technology that connects inorganic/organic devices to flexible substrates to form circuits. Compared to traditional silicon electronics, flexible electronics refers to thin-film electronic devices that can be bent, folded, twisted, compressed, stretched, or deformed into any shape while maintaining efficient optoelectronic performance, reliability, and integration.

Countries like the United States, Japan, and South Korea have strategically positioned flexible electronics projects, which will maintain a high-speed growth trend in high-precision fields. This is also a historical opportunity that our country should strive to seize.

Common Materials for Flexible Electronics

1. Flexible Substrates

To meet the requirements of flexible electronic devices, flexible substrates need to be thin, transparent, flexible, and stretchable. Insulation and corrosion resistance have become key indicators for flexible substrates.

Common flexible materials include polyvinyl alcohol (PVA), polyester (PET), polyimide (PI), polyethylene naphthalate (PEN), paper, textile materials, etc.

Polyimide materials have advantages such as high temperature resistance, low temperature resistance, chemical resistance, and good electrical properties. They are the most promising materials for flexible electronic products. Apart from high-temperature resistance, when selecting a flexible substrate, factors such as light transmittance, surface roughness, and material cost must be considered.

Polydimethylsiloxane (PDMS) is also a widely recognized flexible material. Its advantages include ease of use, chemical stability, transparency, and good thermal stability. Particularly under ultraviolet light, the difference between adhesion and non-adhesion characteristics makes it easy to adhere electronic materials to the surface.

Although PET has a low conversion temperature, between 70-80 °C, it is inexpensive and has good light transmittance. It is a cost-effective transparent conductive film material.

2. Metal Materials

Metal materials such as gold, silver, copper, etc., are generally used as conductive materials for electrodes and wires. For modern printing processes, conductive nano inks are mainly used as conductive materials, including nano particles and nano wires. In addition to good conductivity, metal nanoparticles can be sintered into thin films or wires.

3. Organic Materials

Large-scale pressure sensor arrays are crucial for the development of future wearable sensors. Pressure sensors based on the piezoresistive capacitive signal mechanism have problems with signal crosstalk, leading to inaccurate measurements. This issue has become a development problem and one of the biggest challenges in wearing sensors.

Due to the perfect signal conversion and amplification performance of transistors, the use of transistors makes it possible to reduce signal crosstalk. Therefore, a lot of research in the field of wearable sensors and artificial intelligence focuses on obtaining large-scale flexible pressure-sensitive transistors.

In transistor research, the p-type polymer materials traditionally used are mainly thiophene polymers, with the most successful example being poly(3-hexylthiophene) (P3HT) systems. Naphthalene tetracarboxylic diimide and perylene tetracarboxylic diimide show good n-type field-effect performance and are the most widely studied n-type semiconductor materials, widely used in small molecule n-type field-effect transistors.

4. Inorganic Semiconductor Materials

Inorganic semiconductor materials represented by ZnO and ZnS demonstrate excellent piezoelectric performance in the field of wearable flexible electronic sensors. The application prospects are vast.

For example, a flexible pressure sensor has been developed that directly converts mechanical energy into light signals. This matrix utilizes the photoluminescent properties of ZnS:Mn particles.

The core of the lychee luminescence is photon emission caused by the piezoelectric effect.

Under pressure, the electron energy band of piezoelectric ZnS tilts, which can promote the excitation of manganese ions, and the subsequent de-excitation process emits yellow light.

5. Carbon Materials

Common carbon materials used in flexible wearable electronic sensors include carbon nanotubes and graphene. Carbon nanotubes have high crystallinity, good conductivity, large specific surface area, and the size of micropores can be controlled through the synthesis process, achieving a utilization rate of up to 100%.

Graphene is lightweight, thin, transparent, and has good electrical and thermal conductivity. It has extremely important and broad application prospects in sensing technology, mobile communication, information technology, and electric vehicles.

In the application of carbon nanotubes, multi-walled carbon nanotubes and silver composites are used, and conductive polymer sensors obtained through printing retain a high conductivity of 20S/cm even under 140% stretching.

When carbon nanotubes and graphene are combined, highly stretchable transparent field-effect transistors can be fabricated. It combines graphene/single-walled carbon nanotube electrodes, single-walled carbon nanotube grid channels, and a corrugated inorganic dielectric layer. Due to the corrugated alumina oxide dielectric layer, the leakage current under more than a thousand 20% amplitude stretching and relaxation cycles remains unchanged, demonstrating good sustainability.

Application Fields of Flexible Electronics

Flexible Electronics Applications

1

Flexible Electronic Displays

Flexible electronic displays are new products developed on the platform of flexible electronic technology. Made from flexible materials, they are variable and bendable display devices. Currently, flexible display technologies (e-paper, LCD, OLED, etc.) can be implemented on flexible substrates, enabling the production of devices such as foldable phones, writable e-books, and USB capacity displays.

2

Flexible Energy Storage

Flexible energy storage refers to the flexible and efficient low-cost manufacturing technology of organic/inorganic material electronic devices on flexible/stretchable plastic or thin metal substrates. It is an emerging energy storage technology with broad application prospects in fields such as information, energy, healthcare, and defense. It has already been successfully applied in flexible electronic displays and organic light-emitting diodes (OLEDs). Examples include OLED, printed RFID, thin-film solar panels, and adhesive surfaces for electronic products.

For instance, Samsung manufactures a foldable 210 mAh/hour battery for wearable devices. The battery itself can reach a thickness of 0.3 millimeters and can be worn on the human body. It can withstand bending on the wrist for 50,000 times without any failures.

3

Flexible Medical Electronics

The fundamental characteristic of flexible medical electronics is the integration of various electronic components on flexible substrates, forming a skin-like flexible circuit board. Similar to skin, it possesses high flexibility and elasticity.

Flexible medical electronics can seamlessly integrate with human tissue over long periods. They can accurately measure medical indicators such as body temperature, respiration, blood pressure, electrocardiogram, etc., and provide real-time fundamental medical data for big data healthcare analysis.

4

Flexible Printed Circuit Boards

Flexible printed circuit boards (FPCs) are highly reliable and excellent flexible printed circuit boards made of polyimide or polyester film as the substrate. They have high wiring density, light weight, thin thickness, good flexibility, and perfectly align with the development trend of lightweight, thin, and miniaturized devices.

The FPC industry is primarily dominated by Japan, the United States, and South Korea. In recent years, the increase in production costs has prompted a shift in the focus of the FPC industry to China. FPCs are located in the middle and upper reaches of the electronics industry chain, with flexible copper-clad laminate (FCCL) as the direct upstream raw material and downstream terminal consumer electronic products.

Currently, Japanese companies hold a dominant position in the upstream of the industry chain and have a first-mover advantage. They started relatively late in China and have relatively weak strength.

In recent years, the flexible electronics market has developed rapidly and has become a pillar industry in some countries. It has broad application prospects in fields such as information, energy, healthcare, and defense.