The Development History of Flexible Electronics Technology

The development of flexible electronics can be traced back to earliest... 20 Century 60 Over the years, it has been driven by applications, propelling the development of flexible substrates. / The development of technologies such as substrates, functional materials, conductive materials, and packaging materials, as well as techniques including roll-to-roll printing, transfer printing, spin coating, etching, and inkjet printing, 3D Innovation in technologies such as printing.

To date, flexible electronics technology can be categorized—based on different flexible and stretchable mechanisms and functional materials—into organic, inorganic, and organic. - Various flexible electronics, including inorganic composites and carbon-based materials.

Organic Flexible electronics Directly utilizing flexible organic materials, For example , Flexible functional devices and circuits are realized using organic semiconductors and conductive polymers, among other materials. Inorganic stretchable flexible electronics, on the other hand, achieve their lightweight, shape-variable, and reconfigurable characteristics through ingenious structural designs based on mechanical principles. , Radically transforms the rigid physical form of traditional inorganic solid-state devices.

1. The Development History of Organic Flexible Electronics

The concept of flexible electronics was initially developed as organic electronics. (organic electronics) proposed by its branch. People hope to replace silicon with organic semiconductors; organic polymers are highly flexible, and electronic devices fabricated from these materials naturally possess flexibility. The transformation brought about by organic polymer materials has had a profound impact on the rapid development of organic flexible electronics.

In 1986, Japanese researchers Koezuka Together with his colleagues, he reported the first organic field-effect transistor based on the semiconductor polymer polythiophene, thereby ushering in the era of research in organic electronics.

In 1987, two chemists from Eastman Kodak—Deng Qingyun and— Steven Van Slyke The first practical one was prepared. OLED The device employs a bilayer structure with separate hole-transport and electron-transport layers, enabling electrons and holes to recombine and emit light in the middle of the organic layer. This results in a reduced operating voltage and improved device efficiency.

In 1992, the University of California's Heeger Waiting for Nature First reported in polyethylene terephthalate (PET) Prepare flexible on top OLED 。

In 2017, Professor Zhenan Bao’s research group at Stanford University reported in the Proceedings of the National Academy of Sciences a flexible organic electronic device for transient electronic devices, achieved by incorporating a weak acid into biocompatible and fully degradable polymer semiconductors. ( such as acetic acid ) Moreover, it can be easily degraded, and such a device can achieve skin-like simulation as well as in-vivo sensing.

In 2018, at the Tokyo Institute of Technology in Japan, Michinobu and Wang The leader’s research team has designed a unipolar device with high electron mobility performance. n The organic transistor solves the challenge of low electron mobility in semiconductor polymers within the field of organic electronics.

In 2018, Professor Bao Zhenan’s research group at Nature The report details skin-like electronic devices fabricated using an expandable preparation process based on intrinsic stretchable transistor arrays, successfully achieving a transistor density of... 347cm ^-2 An array of intrinsic stretchable polymer transistors—this represents the highest density achieved to date among all reported flexible and stretchable transistor arrays.

In 2018, Someya The research group has developed a nanonetwork electronic system for recording the field potentials of cardiomyocytes, thereby addressing the issue of interference from natural cellular movements when using electrical probes to monitor cellular potentials over extended periods.

Organic flexible electronics have experienced decades of rapid development, including: OLED and TFT Many research findings, including those mentioned above, have already been brought to market. However, several new challenges are gradually emerging—for instance, the mobility of organic semiconductor materials and the operating frequencies of devices are several orders of magnitude lower than those of inorganic semiconductors. As technology continues to advance, the demands on device performance are becoming increasingly stringent, calling for multifunctionality, high performance, high-speed operation, high integration, and low power consumption.

However, organic semiconductor materials have limited performance and cannot meet the growing demands of devices. As a result, research efforts have shifted toward improving the performance of organic materials and conducting in-depth studies on inorganic flexible electronic devices.

2. The Development History of Inorganic Flexible Electronics

1) Expandable flexible structural design

Research shows that if large块 semiconductor materials are thinned down into nanometer-thick films, ( Thickness less than 100 nm) This allows the device to withstand significant deformation. Utilizing material thinning and mechanical structural design to achieve flexibility in traditionally rigid electronic devices is one effective approach for developing high-performance flexible electronics.

These stretchable, flexible electronic devices based on inorganic semiconductor materials, with their exceptional ability to adapt to deformation (including bending, twisting, and stretching), not only demonstrate... Yes base CMOS The advantages of complementary metal-oxide-semiconductor (CMOS) technology in conventional integrated circuits have also greatly expanded the application scope of flexible electronic devices.

In 2006, at the University of Illinois in the United States, Rogers Professor Huang Yonggang’s research group at Northwestern University in the U.S. was the first to propose the concept of stretchable flexible electronics by creating a wavy silicon film structure through pre-stretching and then releasing a flexible substrate, as shown in the figure. 1 As shown.

Figure 1: Wave Morphology

Based on this design concept, in the following years, several even more advanced structural designs emerged, further enhancing the mechanical performance of electronic devices and enabling them to withstand complex deformations such as stretching, torsion, and bending.

Currently, the design approaches for inorganic stretchable flexible structures can be broadly categorized into three main types: serpentine wire design, island-bridge structure design, and fractal structure design.

Snake-shaped wire design: The straight interconnect wires were replaced with serpentine interconnect wires; the serpentine wires are subjected to tensile stress. / Under compression, lateral buckling deformation is prone to occur, thus enabling it to withstand greater tensile forces. The introduction of serpentine wires has greatly enhanced the stretchability of electronic devices, allowing them to reach— 100%。

Island Bridge Structural Design: The wave-like morphology structure can only provide an elongation of up to 20% To break through this limit and achieve an ultra-high elongation rate ( >100% To protect the functional structure of rigid components while handling electronic devices, scientists have proposed an island-bridge structural design. In this island-bridge structural design, discrete islands... ( Rigid functional components ) Attached to a pre-stretched flexible substrate, the islands are connected by bridges. ( Interconnected wire ) Connection. In this structure, the island maintains strong adhesion to the substrate, whereas the bridge exhibits weak adhesion to the substrate. Pre-strain applied to the substrate induces out-of-plane buckling deformation in the bridge, thereby ensuring a low strain level in the functional device and imparting ductility to the device. Depending on the shape of the interconnect wires, the island-bridge structure can be classified into straight-interconnected island-bridge structures and serpentine-interconnected island-bridge structures. Figure: 2 For island bridge structure.

Figure 2: Island Bridge Structure

Fractal Structure Design: To further enhance stretchability while simultaneously boosting in-plane integration, researchers have introduced the concept of fractals. In flexible devices featuring a fractal design, when tensile strain is applied to thin-film devices integrated onto a substrate, the multi-level structure of fractal conductors—those not bonded to the substrate—unfolds sequentially. After unfolding, the strain within the conductors remains at a relatively low level. The unfolded configurations of the multi-level fractal conductors obtained from both experiments and finite-element simulations are largely consistent. By adopting this fractal conductor design approach, it becomes possible to achieve... 300% The elongation rate. Figure 3 Principles and development of fractal multi-level structures.

Figure 3: Design Principle and Unfolding of the Fractal Multi-level Structure

2) Flexible transfer technology

Flexible transfer technology is an advanced manufacturing technique used to transfer micro- and nanostructures, electronic devices, or functional materials from a primary substrate onto a flexible or heterogeneous substrate. This technology finds wide applications in fields such as flexible electronics, optoelectronics, biomedical devices, and wearable technologies. It has attracted considerable attention due to its ability to achieve high-precision, non-destructive transfers.

Thin-film transfer technology:

Another key issue in enabling rigid devices to become flexible is how to transfer single-crystal silicon thin films onto flexible substrates. In the early stages, Rogers The professor’s team has developed a brand-new thin-film transfer technology, the key processes of which include: SOI (silicon-on-insulator) Graphical preparation of the top-layer silicon ( Silicon nanofilm ) Peeling and transferring.

Transfer methods are divided into two categories:

① The direct flip-transfer method selectively applies an adhesive to the flexible substrate based on its specific characteristics, then directly inverts the flexible substrate onto... SOI On the substrate, and then peeled off. SOI Nanothin film on the surface ;

② Seal transfer method, will SOI The nanofilm on top is first peeled off onto polydimethylsiloxane. (PDMS) On the layer, then place PDMS Covered onto a flexible substrate and subjected to a secondary release process, the thin-film transfer technique represents an effective method for fabricating high-performance, large-scale flexible single-crystal silicon circuits and is also applicable to other inorganic semiconductor circuits. ( Such as gallium arsenide and gallium nitride )。

Following the development of thin-film transfer technology, transfer techniques for material assembly and micro- and nanoscale device fabrication have also received extensive research attention in recent years. This technology can be used to fabricate high-quality single-crystal silicon nanostrips or nanofilms via transfer processes.

Elastic stamp transfer:

Use elastic materials (such as PDMS As a stamp, it leverages its deformability to conform to curved surfaces.

Suitable for micrometers / Batch transfer of nano-scale patterned structures.

Laser-assisted transfer:

Local heating via laser pulses releases the target material and transfers it to a new substrate.

High precision, suitable for brittle materials (such as... Gallium nitride the transfer of silicon).

Sacrificial layer transfer:

A sacrificial layer (such as metal or polymer) is pre-deposited on the original substrate, and the device is released by dissolving or etching away the sacrificial layer.

Thin-film transistors commonly used in flexible displays ( TFT ) Manufacturing.

Roll-to-roll transfer:

Continuous processing is suitable for the large-scale production of large-area flexible electronics (such as flexible solar cells).

Flexible transfer technology is one of the core supporting technologies for the flexible electronics industry. With advancements in materials science and precision manufacturing, its application potential in fields such as smart wearables, healthcare, and green energy will continue to be fully realized.