Electronics all rolled up

C&I Issue 13, 2009

‘We can throw away all the toolkit from the world of silicon.’ So claims Sir Richard Friend, professor of physics at the University of Cambridge’s Cavendish Laboratory and a pioneer in organic electronics.

At the moment, practical applications of organic electronics are mainly restricted to organic light emitting diodes (OLEDs) in display screens. They can be found in some mobile phones and MP3 players, and in 2007 Sony launched the first OLED TV, known as XEL-1. As the prices of OLED displays come down over the next few years, US market analyst firm DisplaySearch predicts that the global market will take off, growing from $600m in 2008 to $5.5bn in 2015.

OLEDs have a number of advantages over more conventional liquid crystal displays (LCDs). As OLEDs generate their own light, displays don’t need a backlight, which means they can be much thinner than LCD screens and more energy efficient. They also generate crisper images that can be viewed from any angle.

But the potential of organic electronics goes far beyond displays. This is because organic molecules like polymers that can act as semiconductors offer the opportunity to take electronics to places where silicon is unable to go. They promise a world in which computer circuits can be printed onto almost any material, including flexible plastics, fabrics and paper.

Some of the first of these non-display applications were recently presented at a Polymer Electronics seminar in London, jointly organised by the Institution of Engineering and Technology and the Institute of Nanotechnology. They may represent the first steps on the path to organic electronics taking over the world.

In addition to conducting electricity (see Box on page 22), some semiconducting polymers, such as PPV, also naturally emit light via electroluminescence. These light-emitting polymers form what is known as the emissive layer in OLEDs. Deposited on top of this layer are an n-type semiconductor and a cathode; deposited under this layer are a p-type semiconductor and an anode.

When a voltage is applied such that the anode is positive and the cathode negative, the negatively charged electrons in the n-type semiconductor are attracted towards the anode. As a result of electrons moving towards the anode, the positively charged holes in the p-type semiconductor appear to be attracted towards the cathode.

As they travel in opposite directions, the electrons and holes meet at the emissive layer where they can combine to form a particle called an exciton, with the release of light energy.

In this sense, OLEDs work in a similar way to conventional light emitting diodes (LEDs). But unlike conventional LEDs, the different layers of an OLED can be processed in solution. This means that an OLED can be printed onto a wide range of materials using the same kind of inkjet technology used in conventional printers.

This opens up the possibility of cheaply printing OLEDs on a wide range of materials. Two UK companies are now taking advantage of this ability to develop light plasters for treating skin cancer.

These light plasters are designed to work as part of photodynamic therapy, which involves rubbing a cream containing a porphyrin-based drug onto a patch of skin cancer.

The drug naturally soaks into the skin and congregates around cancer cells, remaining harmless until activated by red light. This promotes the formation of highly reactive oxygen radicals that proceed to destroy the cancer cells, but without harming any of the surrounding healthy cells.

At the moment, the red light is supplied by expensive lamps or lasers, and so has to be performed within a hospital. The heat produced by the lamps and lasers can also be quite painful. Inexpensive OLED-containing plasters offer a potentially painless way for skin cancer patients to treat themselves at home.

This is what Oxfordshire-based Polymertronics and Lumicure, a spin-off from the University of St Andrews, are now both developing. Lumicure is embedding OLEDs into thick plastic pads, while Polymertronics has developed a way to print an array of tiny OLEDs onto flexible plastic strips.

The idea is for patients to rub the cream into the area of skin cancer and then attach the pad or strip for several hours. This allows the OLEDs to emit sufficient light to activate the drug, but at levels that don’t generate too much heat.

Polymertronics has shown that its light plaster kills cancer cells in the laboratory and plans to launch a product within two years. Lumicure has conducted a pilot clinical trial of its pad, which it plans to launch before of the end of 2009.

Molecular Vision, a spin-off from Imperial College, London, is taking advantage of the ease of fabrication and low cost of organic electronics to produce novel diagnostic devices. The most advanced of these is a device for detecting four heart disease biomarkers in samples of blood and urine, which the company hopes to bring to market in around 18 months.

The device, which combines organic electronics and microfluidics, works in the same way as a normal immunoassay. Fluorescently-tagged antibodies capture the four biomarkers, forming complexes that are then captured by antibodies attached to a solid substrate. The concentrations of the four biomarkers in the samples can then be determined based on the amount of light given off; more light means more captured complexes, which means higher concentrations.

This fluorescence is stimulated by the light emitted by an OLED and is detected by a photodetector that also comprises organic molecules. This photodetector works in almost the opposite way to the OLED. A photon hitting the light-responsive layer generates an exciton, which splits into an electron and a hole that are each pulled toward the opposite electrodes, generating a detectable electric current.

According to John de Mello, a nanomaterials researcher at Imperial College and a co-founder of Molecular Vision, the advantage of using organic electronics is that it simplifies the design of the diagnostic device, greatly reducing the cost.

‘What we really want to do is to lay down multiple light sources and photodetectors directly onto the chip, and to minimise extraneous components,’ explains de Mello.

Some companies are now taking the ability to print organic electronics to its logical conclusion, by producing computer circuits using the same roll-to-roll process used to print newspapers. THe German firm PolyIC, a joint venture between Siemens and the printing company Leonhard Kurz, has established the first roll-to-roll process for printing simple integrated circuits composed of semiconducting polymers. It is able to produce 15 circuits in parallel at speeds of around 20m/minute, with each circuit costing just a few pennies to make.

It envisages that the first application of these chips will be as advanced barcodes, able to act as radiofrequency identification (RFID) tags for tracking products. But it eventually sees them forming the basis for fully-interactive tickets, labels and credit cards. It also plans to develop printed versions of more advanced electronic devices, including batteries and computer memory.

At that point, organic electronics will start to appear everywhere you look: from packaging to clothes to windows.

‘So you thought the silicon chip was everywhere? Well it actually isn’t,’ says Peter Harrop, chairman of IDTechEx, a UK printed electronics consultancy. ‘But there will be an enormous amount of printed electronics.’

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