Nanowires as future transparent electrical

A team led by Clivia
Sotomayor Torres, ICREA Research Professor and Leader of the
Phononic and Photonic Nanostructures Group at the Centro de
Investigación en Nanociencia y Nanotecnología, CIN2 (ICN-CSIC) has
shown that Nanowires provide the solution to overcome the trade-off
between electrical conductivity and optical transparency for organic light
emitting diodes and photovoltaics.
CIN2 is a mixed centre joined by Institut Català de Nanotecnologia (ICN),
whom the mentioned group belongs to, and the Consejo Superior de
Investigaciones Científicas (CSIC).
From this research, the article “Bottom-up growth of fully transparent
contact layers of indium tin oxide nanowires for LEDs” has been
published on line on 1st February 2009, March Issue of Nature
Nanotechnology, DOI: 10.1038/NANO.2008.418.
Transparent conducting oxides (TCO) play a crucial role in a range of
passive and active optical and energy devices. They are used for
transparent anti-reflecting coatings, electrodes in solar cells, Li-ion
batteries and flat-panel displays. However, they are far from ideal. For
example, as electrical contacts for organic light emitting devices (OLEDs)
and prospective organic photovoltaics (OPV), they are required to
conduct electric current and at the same time be transparent to the
frequencies of the emitted light, in the case of OLEDs, or absorbed sun
light, in the case of OPV. These two needs are usually conflicting due to
losses by light absorption in a sufficiently thick and uniform layer needed
for conduction and a thin one needed for transparency. This usually
results in a trade off.
Clivia Sotomayor Torres together with Colm O’Dwyer of the University of
Limerick (Ireland), Vladimir Lavayen of Universidad Técnica Federico
Santa Maria (Chile)/UNIFRA (Brazil), Marta Szachowic and Giuseppe
Visimberga of University College Cork, Tyndall National Institute (Ireland)
and Simon Newcombe of Glebe Scientific Ltd (Ireland), have shown that
this compromise is no longer necessary. By growing crystalline branched
nanowires of Indium Tin Oxide (ITO) with sub- 60 nm diameter in the form
of a layer, they demonstrated that both electrical conductivity and
transparency can be optimized in the visible and the near-infrared part of
the spectrum.
At the centre of this breakthrough is the nanowire structure of ITO. Single
phase ITO nanowires evolved from seed, through nucleation, growth with
progressive branch seeding and further growth. The team studied the
growth phase diagram to determine optimum conditions going from a
porous, rough but compact layer of ITO to dendritic nanowires.
The team measured optical properties and determined that the refractive
index of the electrical contact layer composed of nanowires is close to
unity, i.e., close to the refractive index of air. This means that the Fresnel
reflection, which affects thicker and more compact ITO films, is
dramatically reduced in branched nanowire structures. Important practical
implications are that light emitted from a LED with such a top TCO
contact is isotropic and suffers minimum losses thereby enhancing the
extraction efficiency. The room temperature electrical conductivity of
branched nanowires is just above 1 S/cm, which is comparable to that of
individual wires and two orders of magnitude larger than that of individual
In2O3 or SnO2 nanowires. Optimising further the growth conditions, it is
expected that the sheet resistance can be four orders of magnitude larger
than that of commercially available TCO. Charge transport mechanisms
are undergoing further studies.
This nanotechnology research goes to show that nanowires have much to
offer with their enhanced electrical and optical properties in areas of fast
technological development, such as those driven by the pressing energy
challenges. Understanding the nanometre-scale science behind will be
crucial to find alternatives to ITO nanowires as TCO, since the Indium
reserves are rather limited and the demand for organic optoelectronic
devices is soaring. This work is a solid step in this direction.