Light switch

By adding light-sensing capabilities researchers hope to expand the applications of OFETS.

Although inorganic ambipolar transistors have been around for more than two decades, their organic counterparts, available only in recent years, could be used to make integrated photodetectors, electro-optical circuits and full-colour and monochromatic image sensor arrays.

Traditionally, ambipolar organic field-effect transistors (OFETs) were developed as electronic switches. But recently, OFETs with additional functionalities have also made their debut — the most notable examples being the light-emitting (LE-OFETs) and light-sensing (LS-OFETs) transistors.

Now, researchers from Imperial College London and the London Centre of Nanotechnology aim to design, develop and study such OFETs for use in a variety of different optoelectronic applications. In particular, they aim to demonstrate LS-OFETs with high photoresponsivity and fast switching characteristics.

If the researchers’ work succeeds, they believe it has the potential to reshape the landscape of traditional organic electronics.

Their approach is based on generating a current in the ambipolar OFETs by illuminating the devices with light. The concentration of free carriers in the device generated due to this photoexcitation, modulate the current flow across the channel in the transistor, transforming it into to an electro-optical switch.

‘I don’t believe such devices could be faster than silicon but they do possess a number of advantages,’ said Dr Thomas Anthopoulos, principal investigator.

‘For example, we have a virtually endless library of organic materials with completely different characteristics — both optical and electrical — from which we can build the devices. So, unlike silicon, where you are stuck with a single material, we can target specific materials for specific applications.’

Using different organic materials, Anthopoulos believes he could create a range of transistors with wavelength-selective characteristics. An array of such OFET devices, all switching at different frequencies, could perhaps mimic the operation of the human eye.

Such devices could be exploited in low-cost, large-area and flexible image sensor array applications. Furthermore, such LS-OFETs might also pave the way towards electro-optical circuits, in which signal processing involves the use of both optical and electrical signals.

At present, speed is a limitation. ‘We have measured transistor operating speeds in the region of tens of kHz, which is quite suitable for imaging applications. Now, we are trying to improve that to build devices that can operate at higher frequencies above 1MHz,’ added Anthopoulos.

By adapting high-volume production techniques such as inkjet or radial printing, which are unsuitable for building silicon transistors, Anthopoulos believes the cost of manufacturing the transistors could be cut significantly.

‘If we can build an image sensor at a very low price, it could have a significant impact on the logistics, quality control and security industries,’ he said.

They could also be used in a number of different applications. ‘Organic devices are good for light sensing but they could also be used for gas and pressure sensing, too.

‘What’s more, all these functions could potentially be combined on a single array. Of course that is far in the future, but it could be an ultimate goal for this project,’ he added.

The researchers are also considering making novel types of devices such as a deformable image-sensing array, made possible by the flexibility of the organic material from which the devices are built. Applications could include optical scanners as well as more advanced systems such as medical X-ray imaging.

‘With OFETs you could have a flexible sensor array that could wrap around a body part, unlike silicon, which is rigid,’ said Anthopoulos.

The research is set to begin in April with the help of a £470,000 EPSRC grant and assistance from Merck Speciality Chemicals which will purchase materials, including polyhexylthiophene (PHT) and phenyl-butylic acid methyl ester, and Jacobs University, Bremen.