Organic semiconductors make cheap LEDs

The future could be filled with T-shirts that light up or beach umbrellas that collect solar energy to run portable televisions, if new organic semiconductor technology comes into play.

Cornell University researchers have developed a new type of organic semiconductor device that shows electroluminescence and acts as a photovoltaic cell.

The device uses, what is called, an ‘ionic junction,’ which the scientists say could lead to improvements in LEDs – as organic semiconductors can be made in thin, flexible sheets, they could potentially create displays on cloth or paper.

Flexible means low-cost fabrication and that means another result of the research could be mass-produced, inexpensive solar cells.

The Cornell researchers made a diode out of organic semiconductors that also contain free ions (molecules with an electrical charge) – a diode is a device, as a two-element electron tube or a semiconductor, through which current can pass freely in only one direction.

They laminated together two organic layers, one that contained free positive ions and the other negative ions.

They then added thin conducting films on the top and bottom; the top conductor is transparent to allow light in and out.

Where the two films meet, negative ions migrate across the junction to the positive side and vice versa, until an equilibrium is reached.

This is analogous, to what happens in a silicon diode, where electrons and holes migrate across the junction.

Taking the organic semiconductor further

When a voltage is applied across the top and bottom electrodes, the current flows through an ‘electron junction’ that moves in one way while the holes move the other way.

The migration of ionic charge across the junction causes a higher potential (voltage difference) than normal, which affects the way electrons combine with holes.

This raises the energy of the molecules, which quickly release the energy as photons of light.

The junction shows “intense light emission”.

On the other hand, when a bright light is applied, photons are absorbed by the molecules, causing them to expel the electrons - the ionic charges create a ‘preferential direction’ for the electrons to move, making a current flow.

Since the device is created by laminating together materials that are flexible, large quantities could be manufactured very cheaply by feeding two films together from rolls.

The next step is to try modifying the metal content of the semiconductors to make more efficient materials.
Oh SNAP! Photovoltaic OLEDs

Organic light emitting diodes or OLEDs are heralded as the future of LCD technology, for its ability to turn electricity into light with very little loss of energy, making them appear brighter and more efficient.

Scientists decided to think backwards and realized if you reversed the process, (light turning into electricity), you get a photovoltaic cell. They put that theory to the test and found OLEDs can not only generate light but also recover some of the energy lost by turning light back into electricity.

Imagine your cell phone screen bright as it wants to be, when it’s in idle mode, the screen captures ambient light and recharges your battery. The same tech can apply to everything from TV’s to windows. Soon, OLEDs will offer both low-cost lighting and low-cost energy production.