New high-performance flexible electronics could make foldable phones more affordable - Vertically layered high-performance organic transistor -

 

 

# Transistors are the most frequently used semiconductor electronics. Their function is often compared to that of a water tap. A good tap allows water to flow smoothly when open and does not let a single drop leak when closed. The most important factors in the role of transistors, which regulate electrical current, are ‘speed’ and ‘switching’. When a transistor is turned on, the current should flow rapidly and promptly; when the transistor is turned off, the current should stop.

 

A novel vertical organic transistor has been developed at the Korea Research Institute of Standards and Science. The greatest advantage of this technology is that it will allow flexible devices, the key to wearable and flexible technology, to be made more affordably and to perform better.

 

The research team led by Dr. Kyung-Geun Lim, senior research scientist at the KRISS Center for Nanocharacterization, and Prof. Karl Leo at Technische Universität Dresden, developed a vertical organic transistor using an electrochemical oxidation process. This innovation is expected to greatly accelerate the commercialization of flexible devices by solving issues in the conventional manufacturing process and switching performance.

 

Organic transistors have been gaining attention as next generation semiconductor devices that not only enable displays, sensors, and memory to be made lightweight and flexible, but can also be used to mass produce devices affordably. With recent breakthroughs having solved issues of low charge transfer and stability in organic transistors, it is looking more likely that they these devices will eventually replace conventional inorganic transistors.

 

However, many obstacles still have to be overcome before organic transistors can reach the commercial market. The performance of organic transistors is affected by the horizontal array of electrodes, which increases the device surface area, raising the operating voltage and response time. There is also the structural issue of high parasitic capacitance on the electrodes.

 

In order to overcome these issues, the international research team led by Dr. Lim and Prof. Leo stacked electrodes and organic semiconductor vertically. When the electron flow is made vertical, the distance becomes several hundred times shorter, enhancing the speed dramatically. In such a vertical structure, the key factor is the performance of the permeable base* within the semiconductor layer. The team used an anodizing* method in which the permeable base is placed in an aqueous solution of citric acid, commonly found it citrus fruits, and potential is applied on it. This resulted in a permeable base with nanometer scale pinholes and a fine-controlled oxide layer. The leakage current from this permeable base is only 1/10,000 that from conventional transistors.

 

? Permeable base

The permeable base in a vertical organic transistor is located between organic semiconductor layers that are stacked like a sandwich. When the transistor is on, a large number of electrons are allowed to pass through rapidly. When it is off, charge transfer is blocked, without leakage current. The permeable base has a complex microstructure, with a nanometer scale dielectric material covering a metal thin film containing tiny pinholes.

 

? Anodizing

Electrochemical oxidation process used to create an oxide layer on the surface of metals. Anodizing is used in various industrial applications because the oxide layer increases the durability of metallic parts and can be used to create desired colors on a metal surface. It is also widely used in physics, chemistry, and biology research to generate nanoscale metallic oxides using a very simple electrochemical reaction.

 

The permeable base has to allow a large number of electrons to pass through quickly when the device is on, and block charge transfer effectively without leakage current when off. However, permeable bases produced using conventional methods such as thermal growth performed less efficiently due to the lack of control over nanostructures.

 

This technology not only offers effective control of electron flow through the permeable base, but also a stable production method that can be controlled by the anodizing voltage, without any costly or complicated processes. Needless to say, it has huge potential for industrial application.

 

“With the enhancement of performance through this technology, organic transistors will be applied in many areas,” says Dr. Lim at KRISS, adding, “vertical organic transistors made using the environmentally friendly anodizing process are not only affordable, but also easy to produce. This technology will eventually lower the production cost of devices such as foldable phones and wearable computers.”

 

This study was funded by the National Research Foundation of Korea's Young Researcher Program and published in Advanced Materials (IF 21.950), the top journal in the field of materials science.

 

○ Explanation of vertical organic transistor technology

 

▲ Difference in electron transfer distance between conventional horizontal (top) and new vertical (bottom) transistor due to structural difference (white arrows)

- Electrons move between the two electrodes of source (S) and drain (D), delivering electric signals in the direction of the arrows.

- The conventional horizontal structure takes up more surface area, which means a longer electron transfer distance, which increases the operating voltage and response time. The electron transfer distance is several hundred times shorter in the vertical structure, resulting in a lower driving voltage and the fastest possible performance among organic transistors.

 

 

▲ Diagram of vertical organic transistor

- Transistor uses a permeable base electrode with fine-controlled pinholes and AlOx nanostructures produced by anodizing method.

 

 

 

▲ Comparison between conventional permeable base and permeable base made using anodizing method

- The existing permeable base had an uneven oxide layer and damaged nanostructure. The anodizing process results in an even pinhole-oxide structure.

 

 

○ What are its applications?

 

1) Enhancing performance and reducing cost in flexible/wearable gadgets

Flexible/wearable gadgets are made using flexible organic semiconductors. Until now, organic transistors had been more affordable but less efficient than inorganic semiconductors. The newly developed vertical organic transistor overcomes the issue of performance with lowered driving voltage and response time. Using the affordable and high performance vertical organic transistor should lead to enhanced performance and lowered costs in flexible/wearable technology.

 

2) Development of optical-data hybrid devices

Optical devices such as LEDs and solar cells are made in vertical structures that allow light to pass through them. With this new technology changing the paradigm for transistors to vertical structures, transistors can now be integrated into optical devices. This opens the door to the development of new optical-data hybrid devices such as ultra-sensitive light sensors that amplify signals 10,000 times, light emitting transistors that control light, and ultra-high speed memory devices.

 

▲ Dr. Kyung-Geun Lim, senior research scientist at the KRISS Center for Nanocharacterization is holding the high performance vertical organic transistor developed by his team.

 

 

 

▲ Dr. Kyung-Geun Lim, senior research scientist at the KRISS Center for Nanocharacterization is working on the vertical organic transistor.