10.1117/2.1200912.1848
Thin metal films as simple
transparent conductors
Max Shtein
Because of optical microcavity effects, using thin nonpatterned metal
films instead of indium tin oxide in organic solar cells can result in
similar efficiencies.
Large-area transparent conductors are essential in many
important applications, such as thin-film solar cells, traditional
LCDs, and organic LEDs (OLEDs). The widely used transparent conducting oxides (TCOs), such as indium tin oxide
(ITO), are typically deposited using plasma sputtering or sol-gel
methods. There is a natural tradeoff between transparency and
conductivity, with the best films exceeding 90% transparency
in the visible part of the spectrum at sheet resistances below
15˝/square. This level of performance is suitable for thin-film
solar-cell applications, where a sparse metal grid can be added
to the TCO film as an auxiliary conductor to minimize ohmic
losses during charge collection.
However, TCOs typically exhibit a combination of shortcomings (e.g., brittleness, expensive source materials, processing problems, or availability of suitable flexible substrates).1
They are particularly problematic in reel-to-reel processing of
thin-film, flexible devices because they are susceptible to cracking, which raises the film’s electrical resistance and makes it
permeable to oxygen and moisture that accelerate device degradation. An acute need exists for transparent conductors that
are fundamentally different from TCOs in their mechanical,
processing, and cost characteristics.2
The search for TCO replacements for organic photovoltaic
(OPV) devices has focused on carbon nanotubes,3 graphene,4
highly conductive polymers,5 and metallic microgrids combined
with conducting polymers.6 But few of these approaches have
yielded devices that perform as well as those using ITO, and
fewer can be scaled up cost-effectively.
Instead, we considered using a very thin, unpatterned metal
film. Metals are malleable and can be deposited relatively
cheaply and rapidly onto continuously spooled substrate. In organic optoelectronics, thin metal films have been investigated
as stand-alone transparent electrodes7–11 and in conjunction
Figure 1. Metal-organic-metal photovoltaic (PV) cells with thin nonpatterned metal films achieve the same power-conversion efficiency as
those with conventional indium tin oxide (ITO) electrodes. h: Light
energy.
with conducting oxides.12–14 Generally, the transparency of a
metal film drops exponentially with increasing thickness, while
the sheet resistance rises rapidly. This tradeoff between transparency and electrical conductivity limits the range of feasible
metal thicknesses to 10–20nm. At the low extreme, it can be
difficult to maintain film continuity because the metal tends to
aggregate into droplets on glass and plastic, while at the high
end transparency suffers. As a result, OPV cells using continuous metal films as transparent electrodes have not achieved
parity with ITO-based cells.
We recently examined15 how the sheet resistance varies with
thickness and found that 9–10nm-thick silver films exhibit
15˝/square sheet resistance. This is comparable to device-grade
ITO films. We mitigated silver aggregation by co-evaporating it
with magnesium, producing films with roughness below 4nm
(compared to 7nm roughness for ITO). Using this film as the
anode, we vacuum deposited an archetypal copper phthalocyanine (CuPc)/fullerene (C60 ) planar heterojunction solar cell,
Continued on next page
10.1117/2.1200912.1848 Page 2/2
Table 1. Summary of OPV device characteristics.15 Ag: Silver. ITO:
Indium tin oxide. FF: Fill factor. jsc : Short-circuit photocurrent. Voc :
Open-circuit voltage. : Efficiency.
PV type
ITO
Ag
FF
0.60
0.61
jsc , mA/cm2
6.97
6.00
V oc , V
0.48
0.55
, %
1.86
1.88
completing it with a silver cathode. The metal-organic-metal
device exhibited a power-conversion efficiency of 1.88% at
AM1.5 (typical daylight) illumination, on par with the 1.86%
efficiency of an ITO-based control device (see Table 1 for device
characteristics.)
The reason that the device achieved this efficiency is twofold.
First, the open-circuit voltage was higher than in the ITO control
device (0.55 versus 0.48V). Second, despite the 20% lower overall transmittance of the metal anode than its ITO counterpart,
the short-circuit photocurrent suffered only by 13%. This was
attributable to optical microcavity effects in thin-film OPV
stacks, which we explained using optical and transport
modeling.15 The modeling results were also extended to bulkheterojunction polymer solar cells, suggesting that metalorganic-metal architectures could slightly exceed the efficiency
of ITO-based cells.
The optical behavior of thin metal electrodes is reciprocal,
suggesting that these results obtained in OPV cells should translate well to OLEDs, benefiting in particular large-area solid-state
lighting applications. They could be used in conjunction with
dielectric capping layers to further boost device efficiency.16
These nonpatterned films could be superior to almost any
alternative transparent conductor from the standpoint of manufacturing scalability and cost-effectiveness. We plan to verify
the long-term reliability of thin metal electrodes and their compatibility with a wider range of substrates. We also plan to
apply such electrodes more effectively in the various nonplanar energy-conversion devices (such as solar cells and OLEDs
on fibers or on atomic-force-microscope probe tips) we demonstrated previously,17–20 in addition to the conventional planar
devices.
Max Shtein is an assistant professor and the recipient of several
awards, including the 2007 Presidential Early Career Award for
Scientists and Engineers. His research interests are in novel semiconductor device modeling, processing, and characterization.
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Author Information
Max Shtein
Materials Science and Engineering
University of Michigan
Ann Arbor, MI
c 2009 SPIE