Invisible
conductors
A new high-technology coating
has the potential to significantly
reduce the manufacturing costs of
new-generation solar photovoltaic
cells and other optoelectronic
devices. Melanie Rutherford talks to
researchers at the University of Oxford
and their colleagues at its technology
company, Isis Innovation Ltd.
I
t’s hard to remember life before smart phones,
tablets and high-definition LCD televisions. While
the development of modern display applications
has experienced a relatively recent surge, the
organic LEDs (OLEDs) used by many mobile phones
and, more recently, large-screen displays, are actually
based on advances in small molecule dyes pioneered
by Kodak in the USA back in the 1960s and 1970s.
While technology has come a long way since then, the
theory remains the same.
Photovoltaic (PV) cells, also known as solar
cells, convert light into electrical current via a
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M AT E R I A L S WO R L D N OV EM B ER 2 012
semiconducting material. When a photon is absorbed by this material, an electron
is knocked loose from the atom that ‘excites’ an electron in the crystal lattice
that would normally be tightly bound by covalent bonds between surrounding
atoms. This allows it to move freely within the semiconductor, leaving its previous
covalent bond short of one electron – known as a hole. This hole allows space for
neighbouring electrons to free themselves of their respective atoms, which in turn
creates holes into which other electrons can move. As a result, the hole can move
through the lattice. Intrinsic to this is the semiconducting material, which creates
these mobile electron-hole pairs. Different semiconductor materials have different
properties, and improving existing materials and finding new ones is of great
interest in materials science.
Professor Peter Edwards of the University of Oxford, UK, says, ‘An intrinsic and
significant issue for any organic-based materials is the
very low mobilities of both electrons and holes. On
the positive side, this means that the probability of
light-emitting recombination is high, so they exhibit
good properties for light emission. The principle
problem appears to be the lifetime of such devices
and the complexity of circuitry, as the efficiency of
different colours decreases at different rates over time.’
An example is blue OLEDs that, when used for
flat-panel displays, have a lifetime of around 14,000
hours until brightness is reduced by half. Furthermore,
the material used to produce blue light degrades at
a faster rate than that used for other colours, the
resulting change in colour balance more noticeable
than an overall decrease in brightness.
‘One has to recall, of course, that electron and hole
production in organic-based materials in terms of solidstate physics corresponds in chemistry terms to radical
anion–cation production,’ says Dr Vladmir Kuznetsov
of the University of Oxford. ‘Radical anions and
cations are known for their high chemical reactivity
and, therefore, their high chemical instability.’
Dope testing
Zinc oxide (ZnO) is a semiconductor noted for its
transparency, high electron mobility, wide bandgap
and strong luminescence at room temperature. As
such it is a common transparent conducting oxide
component of thin films used for PV applications.
Doping a semiconductor with impurities to modify
its electronic properties is a technique widely used to
enhance the electronic properties of these materials.
‘We have found that silicon dioxide (SiO2) acts as an
effective n-type dopant to ZnO thin films prepared
by both vacuum deposition and solution-phase
deposition techniques,’ explains Edwards. ‘The doping
effect arises due to a substitution of silicon for zinc
in the ZnO host crystal structure, which leads to a
high carrier concentration and electrical conductivity
of thin films. As a result, the electrical properties of
silicon-doped ZnO (ZnO:Si) significantly exceed those
typical for aluminium or indium-doped ZnO films
prepared by solution routes.’
A research team at the University of Oxford is
developing two approaches for depositing the PV
coating. The first is a vacuum-phase deposition
process (also known as sputtering), involving a pressed
target of ZnO:Si for use in magnetron sputtering
equipment. ‘Vacuum-deposited ZnO:Si offers a direct
replacement for sputtered indium tin oxide (ITO, or
tin-doped indium oxide) at a lower materials cost,’
says Ferguson. ‘This should result in manufacturers
being more competitive and a reduction in the price
of consumer products that have a large transparent
conducting oxide (TCO) component.’
The second approach is an ambient pressure
approach using liquid precursors and spray pyrolysis.
‘Precursors may be deposited as an aerosol over a
large area substrate, which is heated to decompose
the precursors into the desired ZnO:Si thin films.
The spray pyrolysis technique allows large areas
F EAT U R E
to be covered – a scalable and relatively low-cost process compared to vacuum
techniques. ‘The solution-phase atmospheric pressure approach would lend itself to
wide-area use, as the spraying process is readily scalable,’ Kuznetsov adds. ‘Solution
phase processes would also work on web production systems. We regard these
developments as being important to industries such as third-generation solar cells
and plastic electronics, with the drive to fabricate high-performance devices on
flexible substrates and decrease manufacturing costs.’
Solution phase TCO also allows the use of non-vacuum processing equipment
for liquid phase coating – not only enabling coverage of large areas at lower cost,
but also offering potential for web-based processing, which is essential for low-cost
plastic electronics.
Glass substrate
Si:ZnO
(spray pyrolysis)
Transmittance (%)
MATER I A L S WORL D
Si:ZnO
(PLD)
ITO
Wavelength (nm)
Results from a 200nm thin film.
Visible results
‘In terms of transmittance, our silicon doped zinc oxide (ZnO:SiO2) materials
compare very favourably to indium tin oxide (In2O3:SnO2, ITO),’ says Edwards
(see graph above). ‘Note the optical cut off, which signifies comparable optical
transparency of our SiO2 doped ZnO thin film with ITO, and indeed conventional,
non-conducting glass itself.’
Optical transmittance of ZnO:Si films is around 85% over the 400nm–800nm
range, a transmittance value that also includes the absorption by a glass substrate.
Other transparent coatings have recently been developed by scientists at the
University of California, USA, derived from combinations of ultra-fine silver wires,
ZnO:Si 200nm thin film covering half
of the Armourers and Brasiers’ crest.
The glass substrate is on the right
hand side, while the yellowish TCO film
covers the middle part of the substrate.
The electrical conductivity of the doped
slide is 1018 times higher than that of
the glass substrate slide.
The University of Oxford / Isis
Innovations team was awarded the
£25,000 Materials Science Venture
Prize by the Worshipful Company of
Armourers and Brasiers in 2012.
N OVEM B ER 2012 MAT E R I AL S WO R LD
34
MATER I A L S WORL D
carbon nanotubes and graphene within the transparent conducting composite.
While electrons can move faster through graphene than they can through silicon,
Edwards highlights the relative expense and availability of these materials, ‘which
also demand complete control of quality and reproducibility,’ he says.
‘Silver, also expensive and a rare metal, has some similarities with indium as a
component in a TCO device, hence our initial drive away from indium-based TCO
materials. Silver will also degrade in the presence of atmospheric sulphur to form
silver sulphide (Ag2S) as a black semiconducting material. These cost and stability
issues are some of the reasons why silver is not used in conventional solar cells,
which favour gold or steel alloys. It is for the same reasons that silver is never used
for electronic wiring, despite having higher molar conductivity than copper.’
Transparent conductor
Comparative advantages
Limitations
ZnO:Si
•
•
•
•
Earth Abundant
Low cost
Good performance
Can be deposited from
solution
May not be suitable as
an ITO replacement for
the most demanding (ie
high end) applications
In2O3:SnO2 (ITO)
•
•
Excellent performance.
Use expected
where the highest
performance is
required
•
•
•
Expensive
Scarce
Most suitable for
vacuum processing
Antimony tin oxide
•
•
Good performance
Can be deposited from
solution
•
Toxic
Price versus performance for the various classes of TCO materials.
On the circuit
‘The use of organic devices as electronic circuit elements also has the intrinsic
problem that carrier mobility is extremely low – often much less than 1cm2V-1/sec-1,’
says Edwards. ‘Compare these with ZnO:Si (see table above), which have the similar
advantage of solution processing and large area deposition but with significantly
higher carrier mobilities and carrier densities – typically by a factor of 10, even for
the solution-based routes. This is adequate for a number of applications where the
highest conductivity performance is not required.’ The Oxford researchers say these
materials will allow better device and circuit characteristics including power use,
frequency response, and significantly higher environmental stability.
While the team is now fabricating PV devices using ZnO:Si coatings, the next
step will be the further development of low-temperature solution-phase deposition
of thin film coatings from carefully designed precursor molecules. They are about
to test its performance with several photovoltaic technologies, with a view to
manufacturing and testing operating device demonstrators across a variety of
photovoltaics, OLED lighting and LCD displays.
In as little a year’s time, a ZnO:Si-based target for vacuum deposition of TCO
coatings could be introduced to market, although Edwards stresses that further
Potential
application
Customer benefits
CIGS PVs
•
CIGS is a leading intermediatecost PV material.
Amorphous
Silicon PVs
•
Much investment has been made
in amorphous silicon technology.
TCO is major part of the cost.
Solution phase coating would fit
well with low cost manufacture
and the desire to work on flexible
polymer substrates.
A specific target will be the
lowest cost high-performance PV
•
•
•
Conductive
panels for LCD
displays
•
•
•
OLED pixel
•
•
•
NEWS
LCD displays are a large market &
are price driven.
Requires the TCO to carry charge
(& not current).
Solution phase lends itself well to
large area coverage.
New area. Highly cost sensitive.
Large area processing is urgently
needed.
Solution or atmospheric pressure
phase processing lends itself well
to variable topography.
Advantages of ZnO:Si coatings for a range of potential
applications.
careful optimisation is needed to ensure maximum
performance. ‘We also have plans to scale up
the solution phase work, where there is clearly a
variety of parameters to optimise – particularly the
role of precursors and post-deposition treatment
technologies.’
For further information contact Dr Jamie Ferguson
of Isis Innovation Ltd, [email protected].
uk, or Professor Peter Edwards, peter.edwards@
chem.ox.ac.uk or Dr Vladimir Kuznetsov, vladimir.
[email protected], both of Department of
Chemistry at the University of Oxford, UK.
The Armourers and Brasiers Livery Company
presents an annual £25,000 Venture Prize to
help scientists and researchers exploit exciting
ideas in materials science. For more information,
contact [email protected] or visit
www.armourershall.co.uk
Sample
Deposition technology
Mobility cm2/Vs Carrier conc. cm-3
Electrical cond. Ω-1 cm-1
Sheet resistance Ω/*
ZnO:Si
Vac.
27
6.0´10+20
2550
18
ZnO:Si
Soln.
10
1.6´10+20
270
65
ITO
Vac.
38
6.5´10+20
3900
10
Vacuum-produced samples show electrical conductivity to be around two-thirds that of ITO, while solution processing produces carrier
concentration comparable to ITO. Although carrier mobility for both processes is less than that for ITO, note that organic-based materials
typically rise to mobilities no greater than 1cm2/Vs.
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N OVEM B ER 2012 MAT E R I AL S WO R LD
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