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Putting A Spin On Organic Semiconductors

Currently receiving substantial attention, organic semiconductors are increasing commercial momentum, here SciTech Europa Quarterly delve further into the world of the evolving material.

As a dynamic material with unusually high conductivity, organic semiconductors are often enhanced by the presence of certain gases, and other properties commonly associated with semiconductors. With applications ranging from organic light-emitting diodes (OLEDs), field-effect transistors (OFETs) to solar cells and potentially even power energy-efficient computers, the prospects of organic semiconductor applications are now greater than ever.

The field of semiconductors intrigues materials for the next generation of lightweight and flexible electronic applications, however researchers have discovered that specific organic semiconducting materials can transport spin faster than they conduct charge, a phenomenon which could eventually power faster, more energy-efficient computers.

The world of organic semiconductors

As of yet, organic semiconductors have not been realistic candidates for spintronics as it was impossible to move spins far enough without losing the original information. Spintronics is essentially the field of electronics concerned with the detection and manipulation of electron spin in solid-state physics. This differs from fundamental electronics, whereby in addition to electron charge, the electron spin is taken into account and exploited as a further degree of freedom with possible effects toward increasing the efficiency of data storage and transfer. Spintronics is a fundamental application in quantum computing.

‘Spin’ is the term for the intrinsic angular motion of electrons, which is referred to as up or down. Using the up/down states of electrons instead of the 0 and 1 in conventional computer logic could transform the way in which computers process information.

Here, an international team from the UK, Germany and the Czech Republic have uncovered that the material of organic semiconductors could be used for ‘spintronic’ applications, which could create cheap organic semiconductors competitive with silicon for future computing applications.

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Published in Nature Electronics, the research was coordinated by Professor Henning Sirringhaus at the Cavendish Laboratory, Department of Physics, University of Cambridge, England and funded through a European Research Council (ERC) Synergy Grant jointly held by the University of Cambridge, Imperial College London, Hitachi Cambridge Laboratory, England, University of Mainz, Germany, and Czech Academy of Sciences, Czechia.

Enabling the existence of faster, energy-efficient computers

Instead of moving packets of charge around, a device constructed on spintronics would transmit information using the relative spin of a series of electrons, otherwise known as a pure spin current. By eliminating the movement of charge, any such device would need less power and be less prone to overheating – removing some of the most significant obstacles to further improving computer efficiency.

Spintronics could therefore provide us with faster, energy-efficient computers, capable of performing more complex operations than at present.

As organic semiconductors are widely used in applications such as OLEDs, they are cheaper and easier to produce than silicon, and it had been thought that spintronic devices based on organic semiconductors could power a future computer revolution. However, as of yet, it has not worked out that way.

Dr Shu-Jen Wang, of the University of Cambridge’s Cavendish Laboratory, and the paper’s co-first author explains: “To actually transfer information through spin, the electron’s spin needs to travel reasonable distances and live for a long enough time before the information encoded on it is randomised.

“Organic semiconductors have not been realistic candidates for spintronics so far because it was impossible to move spins around a polymer circuit far enough without losing the original information,” added Dr Deepak Venkateshvaran, also from the Cavendish Laboratory. “As a result, the field of organic spintronics has been pretty quiet for the past decade.”

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Studying spin in organic semiconductors

The internal structure of organic semiconductors is inclined to be highly disordered. As such, packets of charge do not move nearly as fast as they do in semiconductors like silicon or gallium arsenide, both of which have a highly ordered crystalline structure.

Most experiments studying spin in organic semiconductors have found that electron spins and their charges move together, and since the charges move more slowly, the spin information does not go far: typically, only a few tens of nanometres.

Now, the research team claim they have found the conditions that could enable electron spins to travel far enough for a working organic spintronic device.

Artificially increasing the number of electrons in the materials, the researchers were able to inject a pure spin current into them using a technique called spin pumping. Highly conductive organic semiconductors, the researchers found, are governed by a new mechanism for spin transport that transforms them into excellent conductors of spin.

This mechanism essentially decouples the spin information from the charge, so that the spins are transported quickly over distances of up to a micrometre, a method that is far enough for a lab-based spintronic device.

According to the researchers, organic semiconductors that have both long spin transport lengths and long spin lifetimes are promising candidates for applications in future spin-based, low energy computing, control and communications devices, a field that has been predominantly dominated by inorganic semiconductors to date.

As a next step, the researchers intend to investigate the role that chemical composition plays in an organic semiconductor’s ability to efficiently transport spin information within prototype devices.

Source: scitecheuropa

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