Common myths about organic transistors

Paul Cain, Strategy Director

November 04, 2020

To the uninitiated, organic electronics might sound like something you would use to automatically water your vegetable patch. More commonly, it refers to devices and circuits that use carbon-based chemistry for the semiconductor in place of inorganic semiconductors, such as silicon. Different electronic components can be made with different organic semiconductors, with two common forms being switches (organic thin-film transistors or OTFTs), and light sources (organic light emitting diodes or OLEDs).

As well as having a different device design, each requires a different type of semiconductor material with very different attributes in terms of electrical properties and performance. For OTFT, today’s performance is now beyond the inorganic incumbent amorphous silicon (a:Si) in relation to performance and lifetime.

Why OTFT?

The market potential for flexible electronics is huge – from flexible displays to sensing applications all built onto low cost flexible substrates, allowing electronics on almost any surface. For example OTFT-based displays such as OLCD can meet the needs of existing and new applications in markets including consumer electronics, automotive, digital signage, TVs and monitors addressing what is today an estimated $80 billion display market.

Here we explain what is now possible with materials for mass production of solution-processed OTFTs. We also dispel a few myths along the way by looking at some of the key parameters for a TFT for large area electronics manufacture such as displays and sensor arrays.

OTFT mobility

In the 1990s the highest performing solution-processable organic semiconductors with reasonable stability were around 0.001 cm^2/Vs, a far cry from the most commonly used inorganic TFT at the time (and still today) – amorphous silicon, with a mobility of around 0.5cm²/Vs. Today OTFTs exceed amorphous silicon in mobility, meaning the same pixel density and resolution that’s possible for glass displays is achievable with OTFT. OTFTs built using FlexiOM™ organic semiconductor materials set have mobility 1.5cm²/Vs when manufactured using industry standard FPD equipment. 

OTFT stability / reliability

There are two parts to this – shelf life and operational stability. It’s often assumed that organic semiconductors are unstable in the presence of oxygen and water. This reputation comes in part from the fact that early materials (20 years ago) were unstable, and partly because it’s well-known that OLED devices today have greater sensitivity to oxygen and water, so it’s assumed that the same must be true of all organic semiconductors. Actually, for OTFT today that’s not the case. The materials themselves have long shelf life without any need to shield from oxygen or water, and the final OTFT devices are stable operationally to a degree that’s better than a:Si, meaning suitable for automotive as well as consumer electronics applications.

OTFT uniformity

Another key electrical parameter, which often comes under scrutiny, is uniformity. That’s because initial R&D on new materials is often done on small test chips – great for measuring mobility and stability, but not suitable for testing large area uniformity. Broadly speaking, solution-processed organic semiconductors can be divided into small molecule and polymer types. Usually the highest measured mobility is often seen in the small molecule types, but these typically have the lowest uniformity, because they tend to form polycrystalline films. Polymers on the other hand typically form amorphous films that have incredibly good large area uniformity – similar to or better than a:Si. All OTFT materials that have been used in large area electronics production to date have been polymers for this reason.

OTFT cost

It’s sometimes assumed that organic semiconductors are an expensive cost-adder for displays, but it’s not the case. Taking an OLCD for example, the costs of the OTFT materials is a very small proportion of the overall cost of manufacturing the display. A majority of the cost for manufacturing a display is the same as the components used in glass LCDs: backlight, polarizers, driver IC and other components, with the OTFT backplane only a small part. As a result, OLCD module costs are similar to glass LCDs, making OLCD by far the lowest cost flexible display technology for full colour, video rate displays.

Commercialising OTFT technology

With OTFT now meeting or exceeding a:Si in every important parameter, the focus in short term is to bring the technology into existing manufacturing lines using current flat panel display equipment in order to get flexible displays like OLCD into new product designs. FlexEnable is working with several leading manufacturing and supply chain partners in Asia to do just that.

If you want to find out more about FlexEnable’s OTFT technology or use it in your product, get in touch with us at info@flexenable.com. 

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