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A Peek Inside the World of Flexible Electronics

flexible electronics

A generic large-area electronic structure is composed of a substrate, a backplane and a frontplane system and an encapsulation. [1] To make the structure flexible, all components must comply with bending to some degree without losing their function and without delamination.

In other words, flexible electronics, also known as “flex circuits”, is a technology for assembling electronic circuits by mounting electronic devices on flexible plastic substrates. Commonly, the substrates are made of polyimide, PEEK or transparent conductive polyester film.[2]

Figure 1 : A flexible electronic alias flex circuits [3]
Figure 1 : A flexible electronic alias flex circuits [3]

What are flexible electronics?

Flexible electronics have a long history. The first flexible device was made in the 1960s by thinning crystalline silicon solar cells for use in extraterrestrial satellites. Today, smart credit cards carry bendable microchips which are made using stretchable Silicon. Financially, flexible electronics generate revenue worth USD 23.92 billion. [4] It is at an inflection point and set to boom in the next 5 to 10 years. The ruggedness, portability, light-weight and low cost of production of flexible electronics, when compared to rigid substrates, will revolutionize the electronics industry.

Materials are the heart of this technology. Flexibility can be attributed to a myriad of qualities ranging from how bendable a device is to whether it is manufactured using roll-to-roll processes. Simply, anything thin is flexible. To the commercial world, flexible electronics means flexible displays and X-ray sensor arrays. However, in my opinion, flexible means a laptop which can be rolled like a yoga mattress, textiles which have electronic circuits on them and an electronic skin.

Figure 2 : Flexible smartphone [5][6]

In this blog, I will focus on the role materials science has played to aid the growth of a segment in flexible electronics which is largely associated with active thin-film transistor (TFT) circuits. Thin Film Transistors (TFT’s) switch-on or switch-off each pixel on the display. High-performance TFT’s are extremely important.

Figure 3 : Display electronics on flexible plastic substrates [7]

Flexible electronics: the past

In my opinion, one of the biggest motivators for flexible electronics was the solar industry. The energy crisis in 1973 probed researchers to work on thin-film solar cells to reduce the cost of electricity. In the early 1980s, amorphous silicon solar cells were made on organic polymer films aka plastic substrates. This was the first time an official study on the flexibility of solar cells was carried out.

Around the same time, cadmium sulfide (CdS) was made by continuous deposition on a moving flexible substrate in a reel-to-reel vacuum coater. [8] It was a matter of a few years that the first amorphous silicon cell was coated using roll-to-roll processes. Today, it is an established manufacturing technique for solar cells.

Flexible TFTs were first researched by Dr. Peter Brody. His group made TFTs on flexible substrates like mylar, polyethylene, and anodized aluminum wrapping foil. [1] At 1/16’’ radius, the bendability of TFT was revolutionary for that generation but does not match the expectations of today’s scientists. The successful launch of the active matrix liquid-crystal display (AMLCD) industry in Japan which used the a-Si: H TFT as the backplane concomitant with the demonstration of a-Si: H solar cells on flexible substrates enthused excitement and stimulated the commencement of cutting-edge research on silicon-based thin-film circuits on novel substrates.

It is worth mentioning that the report in 1997 which stated the feasibility of coating polycrystalline silicon TFTs on plastic substrate paved the path for fast-processing flexible electronics.

Flexible electronics: the present

Silicon technology has been the main driving force behind miniaturizing devices to reduce costs while improving its performance. The material rigidity of silicon is an impasse of its ubiquitous use in soft electronics (flexible and stretchable electronics) applications. This resulted in an extensive search of prospective materials by the research community that has the potential to overcome the rigidity of conventional silicon technology.

Figure 4 : Flexible electronic materials classified from rigid to soft [9]

Nano-Carbon materials such as carbon nanotubes (CNTs) and graphene are promising due to outstanding elastic properties as well as an excellent combination of electronic, optoelectronic, and thermal properties compared to conventional silicon. [9]

The discovery of these nano-carbon materials has created a new wave of possibilities for soft electronics and have led to a technological trend in the market. Aside from carbon, other classes of graphene-like 2D materials such as transition-metal dichalcogenide (TMD) materials and boron nitride (BN), might also be promising in the field of soft electronics. The use of nano-carbons in biomedical applications are based on its biocompatibility and further research on this front is warranted.

Even though these materials can be used for innumerable applications there are a few limitations due to the lack of high yield assembly processes. CNTs must be oriented in a specific direction, with desired density and chirality. Large-area manufacture of graphene is feasible, but the damage introduced during the transfer to a substrate compromise the device performance. Finding robust techniques for assembling CNTs and transferring graphene are a hot topic in the research domain.

Flexible electronics: the future

With a strong proof of concept for flexible batteries and soft electronics the industry will witness a fusion of wearable technology with flexible electronics which I feel would be a critical advancement in the industry. Organic sensors will progress and features like gesture recognition, contactless control, and biometric sensor arrays would be made commercially ubiquitous.

“Stretchable silicon” will be a heavily researched field as nano-carbon materials will be unable to match the speed of silicon. Already commercialized LED, LCD technology may slow the growth of flexible electronics but the new era of electronics is real and flexible electronics are here to stay.

"Working with Matmatch was a delightful experience. It gave me the ability to share my thoughts of the past and my perception of the future of one of the fastest growing materials sector."

References:

[1] Cheng, I-Chun, and Sigurd Wagner. “Overview of Flexible Electronics Technology.” Flexible Electronics: Materials and Applications, Springer Science+Business Media, LLC, 2009, pp. 1–10.
[2] D. Shavit: The developments of LEDs and SMD Electronics on transparent conductive Polyester film, Vacuum International, 1/2007, S. 35 ff
[3] Francis, Matthew. “Bend Me, Shape Me: Flexible Electronics Perform under Punishing Conditions.” Ars Technica, Ars Technica, 5 June 2012, [Online] .
[4] “The Flexible Electronics Market Is Expected to Grow from USD 23.92 Billion in 2018 to Reach USD 40.37 Billion by 2023, at a CAGR of 11.0% between 2018 and 2023.” PR Newswire: News Distribution, Targeting and Monitoring, PRNewswire, 2018, [Online] .
[5] Chaaban, Omar. “Brand New BiCMOS Flexible Transistor.” Electronics-Lab, 22 Dec. 2017, [Online] .
[6] Arici, Alexandra. “Say Good Bye to Your Old Laptop and Hello to Rolltop Rollable Notebook/Tablet.” Softpedia, 25 Aug. 2014, [Online]
[7] Thompson, Michael. “ Laser Processing of Si-TFT’s on Plastic: Technology and Lessons from FlexICs .” Cornell University. 4 Apr. 2006.
[8] Russell TWF, Rocheleau RE, Lutz PJ, Brestovansky DF, Baron BN (1982) Properties of continuously-deposited photovoltaic-grade CdS. In: Conference record of the sixteenth IEEE photovoltaic specialists conference – 1982, San Diego, CA, USA, Sep. 27–30, San Diego, CA, USA, pp 743–747
[9] Chae, Sang Hoon, and Young Hee Lee. “Carbon nanotubes and graphene towards soft electronics.” Nano Convergence1.1 (2014): 15.

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