Physicists invent ‘Spintronic’ LED which promises to be brighter and cheaper

Physicists at the University of Utah have invented a new ‘spintronic’ organic light-emitting diode or OLED that promises to be brighter, cheaper and more environmentally friendly than the kinds of LEDs currently used in television and computer displays, lighting and traffic lights.

The Utah physicists made a prototype of the new kind of LED - known as a spin-polarised organic LED or spin LED - that produces an orange colour light. It is expected that within two years the new technology will be able to produce red and blue and eventually white spin OLEDs.

However, it could be five years before the new LEDs hit the market because, right now, they operate at temperatures less than -33°C. The research was funded by the US National Science Foundation, the US Department of Energy, the Israel Science Foundation and US-Israel Bi-national Science Foundation.

Evolution of LEDs and OLEDs

The original LEDs introduced in the early 1960s used a conventional semiconductor to generate coloured light. Newer organic LEDs or OLEDs with an organic polymer or ‘plastic’ semiconductors have become increasingly common in the last decade, particularly for displays in MP3 music players, cellular phones and digital cameras. OLEDs also are expected to be used increasingly for room lighting. Big-screen TVs with existing OLEDs will hit the market later this year.

The new kind of OLED invented by the Utah physicists has the ability to store information based not only on the electrical charges of electrons, but using the ‘spins’ of the electrons.

The invention of the new spin OLED was made possible by another device - the organic spin valve - which could regulate electrical current flow and so researchers expected they could eventually modify it to also emit light, making the organic spin valve a spin OLED.

Spin valves are electrical switches used in computers, TVs, mobile phones and many other electrical devices. They are so named because they use a property of electrons called ‘spin’ to transmit information. Spin is defined as the intrinsic angular momentum of a particle. Electron spins can have one of two possible directions: up or down. By changing the spin direction as up or down it correlates to the zeroes and ones in binary code.

Organic spin valves comprise three layers: an organic layer that acts as a semiconductor and is sandwiched between two metal electrodes that are ferromagnets. In the spin OLED, one of the ferromagnet metal electrodes is made of cobalt and the other one is made of a compound called lanthanum strontium manganese oxide. The organic layer in the OLED is a polymer known as deuterated-DOO-PPV, which is a semiconductor that emits orange-coloured light.

The whole device is 300 microns wide and long, which is only the width of three to six human hairs. It is 40 nanometres thick, which is 1000 to 2000 times thinner than a human hair.

A low voltage is used to inject negatively charged electrons and positively charged ‘electron holes’ through the organic semiconductor. When a magnetic field is applied to the electrodes, the spins of the electrons and electron holes in the organic semiconductor can be manipulated to align either parallel or anti-parallel.

Advances made to produce spin OLEDs

In the new study, the physicists reported two crucial advances in the materials used to create ‘bipolar’ organic spin valves.

The first big advance was to use deuterium instead of normal hydrogen in the organic layer of the spin valve. Deuterium is ‘heavy hydrogen’ or a hydrogen atom with a neutron added to regular hydrogen’s proton and electron.

The second advance was the use of an extremely thin layer of lithium fluoride deposited on the cobalt electrode. This layer allows negatively charged electrons to be injected through one side of the spin valve at the same time as positively charged electron holes are injected through the opposite side. That makes the spin valve ‘bipolar’, unlike older spin valves, into which only holes could be injected.

This ability to inject electrons and holes at the same time allows light to be generated. When an electron combines with a hole, the two cancel each other out and energy is released in the form of light. The light intensity of the new spintronic OLEDs can be controlled with a magnetic field, while older kinds were controlled by electrical current.

Existing OLEDs produce a particular colour of light based on the semiconductor used. It is expected that the new spin OLEDs may produce different colours by only controlling the magnetic field.