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Experimenting with OLED: a technology for next generation electronics

01 April 2010

In this article, Stephen Clemmet explores organic light emitting diode (OLED) technology. He describes how to make a proof-of-principle ‘kitchen chemistry’ OLED using a commercially available science kit, and suggests some experiments that can be carried out to further investigate the technology

Organic light emitting diodes are an example of the next generation of electronics that comes under the heading of ‘printed electronics’. Printed electronics are devices that can be fabricated on to flexible materials, such as plastic and thin metal foils.

OLED displays have a resolution that is unsurpassed by technologies such as liquid crystal displays (LCDs), or thin-film transistor (TFT) displays that are found in laptops and mobile phones. The resolution is so good that razor-sharp film quality images can be played on mobile phone sized screens. This is more than a technological advancement, it’s a revolutionary technology change. OLEDs create new and exciting product types that were once the preserve of science fiction.

The term ‘organic electronics’ means that the device’s molecules are bound together using carbon atoms. Conventional electronics are based on materials such as copper or silicon.

Light emission from organic materials was first demonstrated by Professor A. Bernanose in 1953, at the Nancy-Universite in France. The first OLED device was created by Dr C Tang and Dr S Slyke of Eastman Kodak in 1985. Since that time, there has been considerable research and developments in OLED material science and applications. The market value for the technology is projected to be worth $16.7billion by 2017. Of that figure OLED TVs are estimated to be worth $7.8billion. The entire field of printed electronics has a 2017 market-value of $30billion.

There are a number of factors associated with OLED fabrication. The first is cost. The vapour disposition chambers used in their production cost upwards of $200,000 and sputter coaters upwards of $20,000. Secondly, 95% of the materials used are wasted during OLED fabrication. Materials coat the internal walls of the deposition chamber and in the case of spin-coating, the material is flung off the substrate altogether. For the 5% of material that is where you want it, the product yield of properly working devices is no more than 60%. Moreover, the larger the OLED area is, the greater is the probability that the device will be faulty. For an A6 sized OLED, the yield is approximately 0%.
Making an OLED
The world of OLED manufacturing is focused on novel materials that use high-vacuum thin-film fabrication technologies. This does suggest that gaining a mere toe-hold on this market is impossible without having at least $1,000,000 to invest. This is very true for any in-depth material science research. Though it’s not true for those that wish to step on the bottom few rungs of the OLED ladder, or develop applications. It’s quite possible to make a working OLED for less than £50 and to experiment with the chemistry. Here’s how to do it using an OLED science kit that is available from the author’s company. There are eight steps, as follows:

1. Using double-sided sticky tape, stick an indium tin oxide (ITO) coated glass slide (ITO facing up) to a de-vaned fan (de-vaned to prevent a draft over the device).
2. Clean the ITO of grease and dirt using an alcohol swab, then use a hair dryer set to approximately 60℃ to drive off any water vapour.
3. Spread an amount of PEDOT:PSS (a semiconductor polymer) over the entire surface of the ITO glass. Switch the fan on so that the centrifugal force creates a thin, even coat across the slide. Bake the slide in an oven at 60℃ for a couple of hours to drive off the water content. For very simple OLED devices, this layer can be omitted - it will still work.
4. Switch the fan on and place a drop of emissive material on the centre of the slide. The centrifugal force will instantly spread it out across the slide.
5. Switch the fan off, then add an amount of the alloy gallium-indium (Ga:In) that is the area of OLED that you want to light up. This should be no more than 5mm in diameter. Remember, small devices are more reliable.
6. Attached wires to the ITO anode and Ga:In cathode.
7. Encapsulate the entire device using a resin.
8. Apply a voltage to light the device.

Experiment with steps 1-4 until you are satisfied with the result. The ITO glass can be cleaned with acetone (nail varnish remover, for example), but do not use an abrasive to clean it. The metal alloy used in step 5 is expensive, so practice with the previous steps to ensure a good working OLED device.
A first device will emit an orange light for a few minutes. With a little practice it is possible to make a device light for several hours and be comparable with the light intensity that is found on many electronic items with conventional LEDs.

Applying 3-10Vdc is adequate for testing a device’s light emission. Applying a dc voltage for a long time will result in the device heating, which will degrade it. If the applied voltage is pulsed with either a square wave, or pulse width modulation, the operational lifetime of the device can be extended for up to 100 hours. Pulsing the light at 75Hz is fast enough for the human eye not to notice the flicker. Further performance improvements can be made by using an electronic capacitive charge pump as the voltage source.

To increase the reliability of the OLED, the substrate must be clean, free of water molecules and free of any dust particles. The organic portion is no more than 200nm thick, so it doesn’t take very much to cause a short circuit between the anode and the cathode. Water vapour is a known killer of OLEDs, so care must be taken to ensure none is present during device manufacture.

So, how does an OLED work?
When a dc voltage is applied across the anode and cathode, there is a flow of electrons from the cathode to the anode. As an electron leaves the anode for the positive terminal of the power source, it leaves an absence of electrons in the anode. This absence is termed a ‘hole’, which moves through the anode and conductive layers to the conductive-emissive layer interface. Here, the holes are filled by electrons via the cathode. This process is called recombination and it occurs at the boundary between the conductive layer and the emissive layer. Light is emitted on recombination. The process repeats itself for as long as a voltage is applied.

The choice of materials used when making an OLED is very important. For the device to emit light, the OLED’s molecules in the organic portion (conductive and emissive layers) require a minimum amount of electrical energy called the ‘work-function’. To prevent heat being dissipated, the organic portion’s work-function must be small but sufficient to make the material fluoresce. The difference in work-function between the anode and the cathode must be at least that of the work-function of the organic portion.

For the anode, the choice is simple; ITO is the only sensible candidate. It has a very low resistivity (less than 100ohm/cm2), it is transparent and can be deposited (by sputter coating) on to glass, or on to polyethylene terephthalate (PET) plastic. PET is flexible, so is a common substrate for flexible OLED devices. For convenience, ITO can be bought already coated on substrates.

The organic conductive layer improves the electronic efficiency and reliability of the device. A commonly used material is the semiconductor polymer PEDOT:PSS - poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate). It is possible to fabricate OLEDs without the conductive layer. If so, the emissive layer is deposited directly on the ITO glass. The emissive layer when on ITO can result in tiny holes in the layer. These are called ‘pin holes’ and become localised hot-spots when the device is on. A device can still work, even with pin holes.

The organic emissive layer material depends on the colour of light that you want to emit. Derivates of MEH-PPV - poly[2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylene vinylene] - are the usual choice. For making proof-of-principle OLEDs, ruthenium compounds work very well. It is very easy to spin-coat ruthenium compounds on to ITO and PEDOT:PSS. Ruthenium is available as crystals from chemical suppliers and can be processed under laboratory conditions. Alternatively it can be bought as a ‘ready-to-go’ fluid from the author’s company.

The cathode material has to be a metal to reflect the light out through the ITO glass. The choice of metal is predominately dictated by the required work-function of the cathode. The cathode’s work-function must be at least the sum of the anode’s work-function plus the organic portion’s work-function. This limits the choice of materials. Aluminium is often used, having a suitable work-function. Surprisingly, gold does not work, having a too high a work-function. For experimentation purposes, the alloy Ga:In is extremely good as it has a low work-function and is a liquid above 16oC.

For device longevity, encapsulation is important. This keeps water vapour out and protects the device from physical damage. Encapsulation can be a glass layer, or it can be a resin. Resins and plastic substrates are permeable to oxygen molecules over many months. Oxygen and water ingress limits the shelf-life and operational-life of OLED’s, even for commercial devices.

Experimenting with OLEDs
There are a range of experiments that can be conducted having made a working OLED. You can vary the spin coater speed to observe the effect on the device or make different sized OLEDs. It is also possible to measure the power per unit area of an OLED. Comparisons between OLED with conventional LED technologies can include studies of power consumption, light emissive waveband, light intensity and viewing angle. And you may wish to develop and compare OLED electronic drivers, including dc sources, charge pumps and pulse width modulation drivers.

OLEDs and plastic electronics offer an exciting new technology that has applications beyond high-resolution mobile phone screens. This article has scratched the surface of printable electronics. While it is improbable that plastic electronics will replace conventional electronics, they do nonetheless facilitate new technology product opportunities.

Stephen Clemmet is chief executive officer at E2M Technology
 


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