Materials Today � Volume 16, Numbers 7/8 � July/August 2013 NEWS

Mapping the distribution of defect density in organic thin films. Source: Christian Westermeier.

and positively charged ‘holes’, with the

length of time it takes for the holes to be

extracted by the electrodes being depen-

dent on the structure of the active layer

of the cell. If there are defects in the arrange-

ment of the atoms, it behaves like a short-

term trap for the charge carriers, reducing

the size of the available current.

This study, reported in the journal

Advanced Materials [Westermeier et al.,

Adv. Mater. (2013) doi:10.1002/adma.

201300958], examined a thin pentacene

layer in which the majority of charge car-

riers are positively charged holes, showing

how changes in current flow by the loca-

lized excitation of defects from laser light

can be identified. When a voltage was

applied to a metallic back contact, the traps

in the material are either filled or emptied

in a controllable way, via the field effect. If

the frequency of the laser light is modu-

lated, it is possible to determine the tem-

poral dynamics of trap states.

This approach allowed them to image

trap states that can interact with the mobile

carriers. It is hoped that this will allow them

to identify the origin of the traps, or at least

help to confirm the effect of the different

preparation methods on the distribution of

traps. As study leader Bert Nickel points out,

‘It would be interesting to know what is

special about the surface layer at these

hot spots. What produces defects at these

sites? They could be due to chemical con-

taminants or to irregularities in the align-

ment of the molecules.’

The team now plans to investigate com-

plete solar cells, which consist of a hole-

conducting film that is in direct contact

with an electron-conducting layer, and

organic heterojunctions, the structural

backbone of organic solar cells.

Laurie Donaldson

Quantum processor solves optimization problems

A team of scientists from the University of

Southern California (USC) in the United

States has successfully validated the opera-

tional effectiveness of a prototype quantum

processor. Their research findings suggest

that the device can be used to solve opti-

mization problems, and has the potential

for scaling up quantum computing to offer

much faster processing power than cur-

rently achievable.

The study, which was published in the

journal Nature Communications [Boixo

et al., Nat. Commun. (2013) doi:10.1038/

ncomms3067], confirmed that quantum

mechanics plays a functional role in how

this first-of-its-kind process or works. The

quantum chip, developed by Canadian

manufacturer D-Wave, has 108 functional

qubits (quantum bits) and is the first com-

mercial quantum optimization processor.

Although a variety of such processors have

previously been made, they mostly operate

on a smaller scale and have fewer qubits.

Qubits, a unit of quantum information, are

able to encode the digits of one and zero

simultaneously, called a ‘‘superposition’’,

compared to the distinct coding of either

a one or zero carried out by traditional bits.

It is hoped that this superposition and the

ability of quantum states to effectively tun-

nel through energy barriers will be key to

the development of a new generation of

processors that can carry out faster optimi-

zation calculations. Sergio Boixo, first

author on the study, said ‘‘Our work seems

to show that, from a purely physical point

of view, quantum effects play a functional

role in information processing in the D-

Wave processor.’’

In addition, the device was found not to

be constrained by the problem of decoher-

ence, a mechanism which could have pre-

vented it from behaving in a quantum way.

Decoherence can be such an issue that the

Quantum Computing Center, where D-

Wave is housed, uses a magnetically

shielded box that is maintained at tempera-

tures near absolute zero to protect the pro-

cessor against its effects.

Also, as researcher Daniel Lidar points

out, they ‘‘have verified that the D-Wave

processor performs optimization calcula-

tions (that is, finds lowest energy solutions)

using a procedure that is consistent with

quantum annealing and is inconsistent

with the predictions of classical annealing.’’

Quantum annealing helps to solve optimi-

zation problems with quantum mechanics

on a large scale.

The team have undertaken a recent

upgrade to the original D-Wave processor

and, with a new 512-qubit chip now in

place, they are planning to test the new

chip to see if it can match up to its prede-

cessor.

Laurie Donaldson

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