TU Dresden Researchers Develop Flexible, Printable Organic Transistors

October 10, 2020 by Luke James

For the first time, researchers have successfully developed flexible, printable, and powerful vertical organic transistors.

Foldable smartphones and roll-up televisions may soon no longer be luxury goods, say researchers from Technische Universitat Dresden’s (TU Dresden) Institute of Applied Physics. They claim to have succeeded for the first time in developing flexible, printable, and powerful vertical organic transistors with two independent control electrodes.

In the research published in Nature Communications, this design, they say, overcomes the limits of conventional horizontal organic thin-film transistors (OTFTs) such as low on-current density and “hopping transport,” opening up a host of new potential applications that require high frequencies.


Vertical Organic Thin-Film Transistors

Although there is no shortage of works involving vertical OTFTs, they’ve been viewed as novel “lab curiosities” that are too difficult to integrate in electronic circuits—up until now, says Dr. Hans Kleemann of the TU Dresden research team.


Powerful vertical organic transistor

The team says this is the first time any researcher has developed a powerful vertical organic transistor. Image used courtesy of TU Dresden

When vertical OTFTs have two independent control electrodes, as is demonstrated in this study, they become “perfectly suited” for use in complex logic circuits while maintaining the benefits of vertical transistor devices, namely the high switching frequency that’s required for radio frequency identification (RFID) and low-power consumption. 

In this instance, the vertical OTFTs are characterized by a high switching frequency of just a few nanoseconds and an adjustable threshold voltage. This means that they’re able to independently represent different logical states while the adjustable threshold voltage bolsters signal integrity and low-power consumption.


Device Structure

In the study, the researchers report a device concept of vertical organic permeable dual-base transistors (OPDBTs). These combine two spatially separated base electrodes to enable change of the on-currents and tuning of the voltage threshold.

In its presence, a high on-current density of 1.54 A cm2, a large current gain of 9.2 × 105, and a high transmission value of 99.998% are demonstrated. 


A schematic and a cross-section TEM image of an OPDBT

A schematic and a cross-section TEM image of an OPDBT. Image used courtesy of Nature Communications


The above schematic shows the transistor, which consists of a sandwich-like architecture with four parallel electrodes (gray and green) separated by an organic semiconductor (orange).

Conductive buckminsterfullerene (C60) is used to realize an n-type OPDBT. All base contacts consist of a 15-nm-thick aluminum layer, and these are covered by a thin native oxide layer that is formed following exposure to air. In this transistor, a 20-nm-thick layer of C60 doped with an efficient n-dopant is placed underneath the emitter electrode to reduce contact resistance. 


The Future of Flexible Displays from Organic Components

The researchers reckon that vertical OTFTs, such as the type demonstrated in this study, could make it possible to realize advanced electronic functions like high-resolution flexible displays made entirely out of organic components.

By employing vertical-channel dual-base structures, the researchers were able to easily control the voltage threshold, which is a requirement for efficient logic circuits. With further research and engineering to optimize the performance of vertical OTFTs and OPDBTs, the researchers posit that it may be possible to completely eliminate the use of silicon in electronic devices.