Mater Renew Sustain Energy (2015) 4:12
DOI 10.1007/s40243-015-0055-8
ORIGINAL PAPER
Limitations of ultra-thin transparent conducting oxides
for integration into plasmonic-enhanced thin-film solar
photovoltaic devices
Jephias Gwamuri1 • Ankit Vora2 • Rajendra R. Khanal3 • Adam B. Phillips3 •
Michael J. Heben3 • Durdu O. Guney2 • Paul Bergstrom2 • Anand Kulkarni2 •
Joshua M. Pearce1,2
Received: 20 April 2015 / Accepted: 3 July 2015 / Published online: 16 July 2015
Ó The Author(s) 2015. This article is published with open access at Springerlink.com
Abstract This study investigates ultra-thin transparent
conducting oxides (TCO) of indium tin oxide (ITO), aluminum-doped zinc oxide (AZO) and zinc oxide (ZnO) to
determine their viability as candidate materials for use in
plasmonic-enhanced thin-film amorphous silicon solar
photovoltaic (PV) devices. First a sensitivity analysis of the
optical absorption for the intrinsic layer of a nano-disk
patterned thin-film amorphous silicon-based solar cell as a
function of TCO thickness (10–50 nm) was performed by
simulation. These simulation results were then used to
guide the design of the experimental work which investigated both optical and electrical properties of ultra-thin
(10 nm on average) films simultaneously deposited on both
glass and silicon substrates using conventional rf sputtering. The effects of deposition and post-processing parameters on material properties of ITO, AZO and ZnO ultrathin TCOs were probed and the suitability of TCOs for
integration into plasmonic-enhanced thin-film solar PV
devices was assessed. The results show that ultra-thin
TCOs present a number of challenges for use as thin top
contacts on plasmonic-enhanced PV devices: (1) optical
and electrical parameters differ greatly from those of
thicker (bulk) films deposited under the same conditions,
(2) the films are delicate due to their thickness, requiring
very long annealing times to prevent cracking, and (3)
reactive gases require careful monitoring to maintain stoichiometry. The results presented here found a trade-off
between conductivity and transparency of the deposited
films. Although the sub 50 nm TCO films investigated
exhibited desirable optical properties (transmittance greater
than 80 %), their resistivity was too high to be considered
as materials for the top contact of conventional PV devices.
Future work is necessary to improve thin TCO properties,
or alternative materials, and geometries are needed in
plasmonic-based amorphous silicon solar cells. The stability of ultra-thin TCO films also needs to be experimentally investigated under normal device operating
conditions.
Keywords Transparent conducting oxide (TCOs)
Plasmonics Solar photovoltaics Indium tin oxide Zinc
oxide Aluminum-doped zinc oxide
Introduction
& Joshua M. Pearce
pearce@mtu.edu
1
Department of Materials Science and Engineering, Michigan
Technological University, 1400 Townsend Dr., Houghton,
MI 49931-1295, USA
2
Department of Electrical and Computer Engineering,
Michigan Technological University, 1400 Townsend Dr.,
Houghton, MI 49931-1295, USA
3
Wright Center for Photovoltaic Innovation and
Commercialization, Department of Physics and Astronomy,
School of Solar and Advanced Renewable Energy, University
of Toledo, Toledo, OH 43606, USA
Despite the material, sustainability, economic and technical
benefits of thin-film solar photovoltaic (PV) devices [1–3],
conventional crystalline silicon (c-Si) modules dominate
the market [4]. The cost of c-Si PV has fallen to the point
that the balance of systems (BOS) and thus the efficiency
of the modules plays a major role in the levelized cost of
electricity for solar [5]. There is thus a clear need to
improve the efficiency of thin-film devices further [6].
Recent developments in plasmonics theory promise new
methods with great potential to enhance light trapping in
thin-film PV devices [7–14]. To fully exploit these
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potential benefits offered by plasmonic-based devices,
TCOs with high transmittance (low loss) and low enough
resistivity are to be used as device top contacts. However,
for current transparent conducting oxides (TCOs) to be
successfully integrated into the novel proposed plasmonicenhanced PV devices, ultra-thin TCOs films are required
[14]. For example, simulations by Vora et al. showed a
19.65 % increase in short circuit current (JSC) for nanocylinder patterned solar cell (NCPSC) in which the ITO
layer thickness was kept at 10 nm to minimize the parasitic
Ohmic losses and simultaneously act as a buffer layer
while helping to tune the resonance for maximum
absorption [14]. TCOs such as the most established indium
tin oxide (ITO), aluminum-doped zinc oxide (AZO) and
zinc oxide (ZnO) are standard integral materials in current
thin-film solar PV devices [15–18]. Bulk material properties for common TCOs including ITO have been well
researched and documented for different processing conditions and substrates [15, 16, 19–23]; however, this is not
the case for ultra-thin TCOs. The few exceptions include
Sychkova et al. [24], who reported both optical and electrical properties of 9–80 nm ITO films deposited by pulsed
DC sputtering varied with thickness and showed a general
increase in resistivity with decrease in film thickness [24].
Other notable studies on ultra-thin ITO films using various
deposition techniques include the following: Chen et al.
who used filtered cathodic vacuum arc (FCVA) to deposit
30–50 nm on heated quartz and Si substrates [25]; Tseng
and Lo, who used DC magnetron sputter for
34.71–71.64 nm ITO film on PET (polyethylene terephthalate) [26]; Kim et al. who used RF magnetron sputter for
films between 40 and 280 nm deposited on PMMA substrate heated at 70 °C [27]; Alam and Cameron, who used
sol–gel process for 50–250 nm film deposited on titanium
dioxide film [20]; and Betz et al. who used planar DC
magnetron sputtering for 50, 100 and 300 nm films on
glass substrates [28]. The results from these few thin TCO
studies reveal a pattern in which resistivity increases
rapidly as film thickness decreases from 50 to 10 nm.
The electrical properties of ITO thin films depend on the
preparation method, the deposition parameters used for a
given deposition technique and the subsequent heat treatments. Key factors for the low resistivity have not been
clearly documented because of the complex structure of the
unit cell of crystalline In2O3 formed by 80 atoms and the
complex nature of the conducting mechanisms in polycrystalline films [29]. The issue is further complicated by
the large number of processing parameters, even for a
single technique.
To probe these challenges and to determine if ITO, AZO
and ZnO are viable candidate materials for use in plasmonic-enhanced thin-film PV devices, sensitivity analysis
on TCO thickness (10–50 nm) versus absorption was
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Mater Renew Sustain Energy (2015) 4:12
performed using COMSOL Multiphysics RF module v4.3b
on the optical absorption in the i-a-Si:H layer of nano-disk
patterned thin-film a-Si:H solar cells (NDPSC) shown in
Fig. 1a [15]. These simulation results are used to guide the
experimental work which investigated both optical and
electrical properties of ultra-thin (10 nm on average) films
simultaneously deposited on both glass and silicon substrates (with a thermally grown oxide layer. The effects of
deposition and post-processing parameters on material
properties of ITO, AZO and ZnO ultra-thin TCOs were
probed and the suitability of TCOs for integration into
plasmonic-enhanced thin-film solar PV devices was
assessed. From these results some of the limitations of thin
TCOs for plasmonic optical enhancement of thin-film PV
were identified.
The optical effects of TCO thickness
Sensitivity analysis for the proposed silver nano-disk patterned solar cell (NDPSC) was performed in the
300–750 nm spectral range to determine the optimum ITO
layer thickness which would promote maximum enhancement and minimize Ohmic losses. Having a TCO spacer
layer with as low as possible Ohmic losses is desirable for
efficient coupling of light from the silver nano-discs into
the active layers of the device. The results are shown in
Fig. 1b and theoretically show 10 nm films offer the best
absorption and hence the greatest potential to improve
efficiency in plasmonic-based PV devices. From these
results, AZO and ITO offer the best potential due to lower
Ohmic losses and ZnO, despite having the greatest Ohmic
losses among the three TCOs, is still promising particularly
for the sub 20 nm films since its absorption ([250 W/m2)
is still higher than that expected of a standard PV device.
Experimental details
The focus of the study was to investigate ways of
improving material properties of ultra-thin TCOs for integration into plasmonic-enhanced thin-film solar PV devices
by studying the effects of different process parameters on
both optical and electrical properties of sub 50 nm films. A
comparative study of the three most commonly used TCOs
in thin-film commercial solar cells is undertaken, and a
more in-depth study of ITO is performed.
Sample preparation and fabrication
Samples of ITO with thickness ranging from 10 to 50 nm
were deposited on both glass and n-doped silicon (with a
32-nm thermally grown oxide layer) substrates using rf
sputter deposition techniques previously described in refs
Mater Renew Sustain Energy (2015) 4:12
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12
Fig. 1 a Structure of the NDPSC with an enlarged unit cell,
b absorption as a function of ITO, ZnO and AZO thickness. The
results simulated using COMSOL show how the useful optical
absorption in the active regions of plasmonic PV devices varies with
TCO type and thickness. Theoretically, at small film thicknesses
Ohmic losses decrease and useful optical absorption increases [15]
[30–32]. A 99.99 % 4-inch pressed ITO (Sn2O:In2O3
10:90 % wt) target was used, and an average base pressure
of 7 9 10-8 torr was achieved before deposition. Both the
glass and silicon substrates were ultrasonically cleaned in
isopropanol for 5 min. All other process parameters such as
target bias [900 V (ITO and ZnO) and -500 V (AZO)] and
substrate distance (75 mm) were kept constant through the
experiment. Substrates and target were sputter pre-cleaned
in an argon environment for 5 min before each run. The
protocol for pre-cleaning is described in Ref. [29]. To
investigate substrate dependency, ITO was deposited on a
pair of substrates for 1 min with 0 % oxygen ratio and
100 W rf power. ZnO samples were processed at rf power
of 100 W on glass and silicon substrates in an argon
environment and 0 % oxygen in the same system as ITO
using a stoichiometric 99.99 % 4-inch pressed ZnO target.
The process pressure was maintained at 7.1 9 10-3 torr
and the deposition rate was calculated to be 8 nm/min.
AZO was processed using a Perkin–Elmer Model
2400-8 J rf sputter deposition system using an 8-inch
(203.2 mm) target. The rf power was kept at 500 W, argon
flow rate at 18.0 sccm, oxygen rate of 2.0 sccm and process
pressure at 7.3 9 10-3 torr. The system was initially
pumped to a base pressure of 6.0 9 10-8 torr. The process
parameters are summarized in Table 1.
To investigate the effects of post-processing treatment
on both optical and electrical effects, additional samples of
ITO films on sodalime glass (SLG) substrates were processed using a different instrument [33] to obtain a pair of
film samples with varying thicknesses from 10 to 50 nm in
steps of 10 nm. The system is a four-gun sputtering system
with a target to substrate spacing of approximately 400 . An
ITO (90 % In2O3/10 % SnO2 from Lesker) target was used.
The material was sputtered using 100 W rf under 4 mTorr
of Ar. Deposition time was varied for film thickness with
36 s resulting in 10 nm (*3 A/sec). This deposition rate
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Mater Renew Sustain Energy (2015) 4:12
Table 1 Summary of process parameters for the TCOs
Sample name
0A
TCO
ITO
Substrate type
Glass
RF power (W)
100
Target bias (V)
900
Process gases flow rates (sccm)
Ar
O2
10
0
Film thickness (nm)
9.55
0B
Si/SiO2
10.02
1A1
Glass
19.92
1A2
Si
19.75
1B
Glass
10.23
1C
20.01
1D
1E
30.79
39.70
1F
2A1
50.03
ZnO
2A2
Glass
9.51
Si/SiO2
10.05
2B
20.01
2C
29.72
2D
38.98
2E
3A
3B
48.31
AZO
Glass
500
500
Si/SiO2
18.0
2.0
12.16
11.93
3C
20.39
3D
30.04
3E
40.63
was determined by depositing for a set amount of time and
measuring the resulting film thickness using stylus profilometry (Veeco Dektak 150).
One sample for each as-deposited pair was divided into
three samples using a diamond scriber. The three pieces
were then annealed separately at 400 °C for 10, 20 and
30 min, respectively, using UHP forming gas (FG) (95 %
N2/5 % H2 from Air Gas) in a sealed (by vacuum coupling
components) quartz tube inside a tube furnace. The furnace
was equilibrated at the heating temperature prior to sample
introduction. The samples were placed in the quartz tube;
then the tube was purged with FG at 5 scfm for 5 min—this
was approximately four exchanges of tube volume. After
purging, the samples were introduced into the hot zone
with a vacuum-sealed push rod, and the flow rate was
reduced to approximately 150 sccm for the duration of
heating. After heating, the samples were removed from the
hot zone and cooled by increasing the gas flow. After
characterization, the sample previously annealed at 400 °C
for 30 min was further annealed at 500 °C for 10 min.
Optical and electrical characterization process
The film thickness measurements and optical characterization were carried out using spectroscopic ellipsometry
(J.A Woollam Co UV–VIS V-VASE with control module
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VB-400). In each case, a standard scan was performed
ranging from 300 to 1000 nm in increments of 10 nm for
the 65°, 70° and 75° incident angles. Random detailed
scans were performed for the quality check purposes
although they are normally not necessary for isotropic
samples. Ellipsometry analysis was performed following
the process by Synchkova [24]. Intensity measurements
were carried out using the VASE for normal transmission
incidence (0° reflection angle) for the three TCOs on glass
substrates for the same wavelength range as above. A
baseline scan was obtained for the clean SLG substrate first
followed by the main data scans using baseline data. Both
the baseline and the data scans were acquired in close
successions to minimize errors due to light source intensity
fluctuations.
Electrical characterization was performed using a fourpoint probe system consisting of ITO optimized tips consisting of 500 micron tip radii set to 60 g pressure and an
RM3000 test unit from Jandel Engineering Limited, UK.
The sheet resistance of the 10 and 20 nm TCOs on glass
and on silicon substrates with a spacer oxide layer was
determined by direct measurement for both forward and
reverse currents. For each TCO on glass sample, a mean
sheet resistance value from three random points was used
in the final results whilst a mean of only two points was
used for the TCO on Si samples since they were smaller.
Mater Renew Sustain Energy (2015) 4:12
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All samples were imaged for film quality and a compositional analysis was done using a Hitachi S4700 field
emission scanning electron microscope (FE-SEM). Atomic
force microscopy (AFM) was performed using a Veeco
Dimension 3000 equipment with cantilever tips
(Tap300Al-G) on a 1:1 acquisition aspect ratio. The field of
view was 2 lm at 512 pixel width and scans performed at a
speed of 0.5 Hz. Three randomly selected fields of view
were acquired per sample and the analyzed areas were
limited near to the center of the sample. Roughness analysis was then performed on a defect-free region.
Theory and calculations
The theoretical derivations of both the resistivity and
attenuation coefficient of the ITO films are highlighted in
Sects. 3.1 and 3.2 below to explain the underlying processes contributing to the results reported in this paper.
Resistivity measurements
Sheet resistance measurement was used to obtain the
resistivity:
R¼
qL
L
¼ Rs ;
tW
W
ð1Þ
where R is the resistance, Rs is the sheet resistance, and L,
W and t are the sheet length, width and thickness,
respectively.
As the film thickness is measured, the bulk resistivity q
(in ohm cm) can be calculated by multiplying the sheet
resistance by the film thickness in cm:
q ¼ Rs t
ð2Þ
Transmittance
To determine the true transmittance of the TCOs, it was
necessary to perform a correction on the experimental data
to compensate for losses due to both surface reflection and
absorption due to the glass substrate. It is assumed light
passing through the glass substrate undergoes attenuation
according to Beer-Lambert’s law:
Ig ¼ Io e
ag tg
12
Results and discussion
TCOs characterization
The transmittance and resistivity measurement results for
the TCOs are discussed below.
Transmittance
Figure 2 below shows how transmittance of the TCOs
studied varied within the 300–1000 nm wavelength range.
Transmittance results support the sensitivity analysis
results. For the 20-nm films, AZO has greater than 90 %
transmittance for the 300–1000 wavelength range, whilst
ITO and ZnO show an average transmittance greater than
80 and 70 %, respectively, in the same spectral range.
Sheet resistance
The resistivity of the 20 nm as-deposited TCO films on
SLG substrates are shown in Table 2. ZnO, despite having
the worst transmittance (Fig. 2), has the lowest resistivity
among the three TCOs being compared here and AZO has
the highest resistivity value. ITO has transmittance comparable to that of AZO and its resistivity is slightly higher
than that of ZnO, making it the most promising candidate
material for plasmonic-based devices.
Table 3 shows the dependence of ITO sheet resistance
with substrate type and thickness. There was a marked
difference between the readings on the 10- and 20-nm Si
samples; however, there was no discernible difference
between the readings on the 10 and 20 nm on glass. There
were very small amounts of fluctuation which can be
expected on high resistance samples, and it was more
prominent on the Si samples. The readings reversed well,
indicating that the film was uniform, with the worst correlation on the 10-nm Si sample. This is the limit of fourpoint probe capability. The 10-nm ITO on glass showed the
ð3Þ
;
where I0 and Ig represent the initial incident intensity and
intensity through the glass substrate, ag and tg are the
attenuation coefficient of the glass and glass thickness,
respectively.
The total normalized transmittance, T is given by
T¼1
A
R
ð4Þ
where A and R represent the total absorbance and reflectance, respectively.
Fig. 2 Transmittance results for 20 nm thick ITO, ZnO and AZO
films
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Mater Renew Sustain Energy (2015) 4:12
Table 2 Resistivity of 20 nm as-deposited ITO, AZO and ZnO films on SLG substrates
Sample
Substrate
Thickness (nm)
Sheet resistance, Rs (X/h) 9 103
Resisitivity, q (X cm)
ITO
Glass
20
623
1.3 9 10-3
AZO
Glass
20
876
1.7 9 10-3
ZnO
Glass
20
390
7.8 9 10-4
Table 3 Sheet resistance of various as-deposited TCO samples
Sample
Substrate
Thickness (nm)
Input current
Sheet resistance, Rs (X/h) 9 103
Resisitivity, q (X cm)
ITO
Glass
10
100 nA
830
8.3 9 10-4
623
1.3 9 10-3
422
4.2 9 10-4
20
Si
10
1 lA
20
highest resistivity whilst the lowest resistivity value was
recorded for the 20-nm Si substrate sample. The results are
further confirmed by the nature of the microstructure
observed by SEM (vide infra) for these samples.
ITO characterization
Transmittance measurements for ITO
Transmittance measurements for ITO samples deposited on
SLG substrates are shown in Fig. 3. All transmittance
values were normalized as given in Eq. (4). It can be noted
that there is no discernible difference between the as-deposited and the heat-treated samples particularly for the
30-, 40- and 50-nm films. However, it is also interesting to
note that for the 10- and 20-nm films, the as-deposited films
have the highest transmittance with the 10-nm film being
almost 100 % transmitting throughout the visible spectra.
For the 40-nm film, annealing at 400 °C for 20 min gives
the best transmittance. Generally it is observed that heattreated ITO films in FG environment improve transmittance in the UV region of the spectra.
As-deposited thinner ITO samples (10 and 20 nm) have
the transmittance greater than 95 %. It is interesting to note
that the 40-nm film sample does not seem to follow this
general trend, particularly the sample annealed for 20 min.
This sample film shows the greatest increase in mean
roughness (vida infra) when all other films’ roughness is
decreasing and it also has the best transmission for all the
40-nm film samples. The general trend is that the overall
transmittance curve for the as-deposited ITO shifts down
with increasing film thickness (i.e., the as-deposited film
becomes less transparent with increasing thickness as
expected). Around visible spectrum and at higher wavelengths, the transmittance for the as-deposited ITO
approaches that of the annealed ITO (i.e., annealing is not
much effective here in improving transparency). However,
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83.9
1.7 9 10-4
around UV wavelengths the transmittance for the as-deposited ITO shifts down below that of the annealed samples (i.e., at small wavelengths annealing is more effective
as the annealed samples are more transparent). This is a
well-known phenomenon (Burstein–Mess shift) which is a
result of ITO optical band gap shifting towards higher
energies when annealed in FG or H2 gas. This is attributed
to increase in carrier concentration and is well documented
[25]. In addition, it appears that among the annealed
samples, 20 min gives the optimum transmittance for
thicknesses below 50 nm, especially at large wavelengths.
There observed trend means that the use of thinner
(10 nm), more transmitting and low loss (Ohmic losses)
films will result in more light being coupled into the
underlying i-a-Si:H layer rather than being absorbed in the
TCO layer as is the case with thicker film ([20 nm).
Electrical characterization
Figure 4 shows the dependence of sheet resistance on film
thickness, annealing temperature and time. Films annealed
for 20 min give the lowest resistivity and show the same
trend as those annealed for 30 min whilst the as-deposited
resistivity versus thickness trend is similar to films
annealed for 10 min. Results here show that annealing in
FG lowers the resistivity. The lowest resistivity of
approximately 4 9 10-4 Xcm is for the 40-nm film
annealed for 20 min. The highest resistivity value for the
annealed samples is for the 20-nm film annealed for
10 min.
Film morphology and roughness
Effect of substrate on ultra-thin ITO films
Figure 5 shows results from SEM scans showing the surface morphology for both 10 and 20 nm as-deposited ITO
Mater Renew Sustain Energy (2015) 4:12
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12
Fig. 3 Transmittance spectra for ITO as-deposited and annealed films on sodalime glass for a 10 nm, b 20 nm, c 30 nm, d 40 nm and e 50 nm
ITO thickness
films. Figure 5a–c shows that the film surface is relatively
smooth and predominantly amorphous in nature. Figure 5d
shows signs of grains development. The AFM analysis
results are shown in Fig. 6.
The results in Fig. 6 show how mean roughness values
of ITO vary with substrate type and are in agreement with
the results shown in Fig. 5. It can be observed in these
images that ITO tends to form uniform features on silicon
with no evidence of defects. This is not the case with ITO
on glass substrate which, despite having finer features
(10 nm film) exhibits some larger defects. These defects
seem to increase with the increase in film mean roughness
and thickness. Despite the presence of a few dust particles
on the sample surface, results confirmed that the sputtered
films were of good quality. The AFM roughness results are
summarized in Table 4.
Effect of annealing time on ultra-thin ITO films
When ultra-thin ITO films were subjected to post-processing treatment at 400 °C in a FG environment, different
treatment times produced different effects.
The as-deposited films mean roughness for this second
batch of ITO samples were observed to vary between 0.67
and 0.85 nm. The 10-nm film had the smallest mean
roughness value whilst the 20-nm film had the largest
value. This may be due to the presence of surface defect
features which seem to be more pronounced on the 20-nm
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Fig. 4 Variation of resistivity with ITO film thickness for (a) asdeposited/room temperature (RT) (b) annealed films for 10, 20 and
30 min
film compared to all the other samples. Generally, all
samples show a varying degree of dust particles’ presence
and potential artifacts. Sections of the film samples which
exhibited heavy dust particles’ (and any other contaminants) presence, striations and potential artifacts that were
not consistent with other areas on the sample were excluded from the analysis.
The images show a sharp increase in the mean roughness for generally all films after 10 min of heat treatment.
Whilst the film roughness is small for both the 10- and
20-nm films, it is observed to increase by a factor of two
for the 30- to 50-nm film thickness samples. There is a
trend for all films showing a decrease in mean roughness
after 20 min of post-processing treatment with the 30-nm
film showing the greatest decrease from approximately 1.9
to 1 nm. Evidently, annealing for 30 min results in a slight
improvement in film roughness for the 30- to 50-nm range
of film. However, the thinner films (10 and 20 nm) show
great deterioration in film mean roughness when annealed
for longer periods of time (30 min or greater). This can be
explained by the onset of islands on both of these films.
Island formation is more pronounced on the quasi 2D
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Mater Renew Sustain Energy (2015) 4:12
10-nm film resulting in the mean roughness increasing
from the initial value of approximately 0.7–1.9 nm.
The effects of annealing different ITO films in forming
gas at 400 °C for 10, 20 and 30 min on films surface
roughness are compared and summarized in Fig. 7.
The detailed study on ITO showed some dependency of
electrical properties and surface roughness with substrate
type which is consistent with results from previous studies
on slightly thicker films. Also ITO films on glass show a
high degree of surface defects and finer amorphous-like
features which may explain the high and oscillating values
of sheet resistance on these films. Films grown on Si
substrate have uniform, but large features. However, the
same films have higher resistivity values. All samples, asdeposited and annealed, have a transmittance value greater
than 80 % with the as-deposited films being superior
except for the 40-nm films for which the annealing for
20 min gives the best transmittance. Further analysis of
samples shows films annealed for 20 min generally have
the lowest resistivity and lower roughness values.
Future work is needed to improve other TCOs such as
AZO and ZnO and to engineer new high-conductivity lowloss materials for integration into plasmonic devices. AZO
exhibited a transmittance superior to that of ITO while ZnO
had the best sheet resistance among the three TCOs being
compared. Further investigative work is needed to find the
balance between films with useful resistivity and acceptable Ohmic losses in plasmonic-based PV devices. Future
work should focus on different processing techniques such
as DC sputtering as well as exploring other post-processing
environments.
Conclusions
Ultra-thin TCOs and in particular ITO present a number of
challenges for use as thin top contacts on plasmonic-enhanced PV devices. First, both ultra-thin TCO optical and
electrical parameters differ greatly from those of thicker
(bulk) films deposited under the same conditions. Second,
they are delicate due to their thickness, requiring very long
annealing times to prevent film cracking. The reactive
gases (usually oxygen or hydrogen) require careful monitoring to avoid over-oxidizing or over-reducing the film as
it impacts their stoichiometry. There is a trade-off between
conductivity and transparency of the deposited films. The
sub 50 nm thick TCO films investigated exhibited desirable optical properties (transmittance greater than 80 %),
which makes them viable for plasmonic PV devices
Mater Renew Sustain Energy (2015) 4:12
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Fig. 5 FESEM images for (a) 10 nm ITO on glass, (b) 10 nm ITO on silicon (with oxide spacer), (c) 20 nm ITO on glass and (d) 20 nm ITO on
silicon (with oxide spacer)
Fig. 6 AFM images for asdeposited (a) 10 nm ITO on
glass, (b) 10 nm ITO on silicon
(with oxide spacer), (c) 20 nm
ITO on glass and (d) 20 nm ITO
on silicon (with oxide spacer).
Image scale is 10 nm
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Mater Renew Sustain Energy (2015) 4:12
Table 4 Summary of AFM results for as-deposited ITO films on glass and Si substrates
Film
Roughness (nm)
Observations
RMS roughness (Rq)
Mean roughness (Ra)
ITO on Si wafer, 10 nm
0.44
0.35
Uniform, small features
ITO on Si wafer, 20 nm
0.58
0.45
Uniform features larger than 10 nm
ITO on glass, 10 nm
0.37
0.29
Very fine features with some larger defects
ITO on glass, 20 nm
0.83
0.52
Fine features with many large defects
Fig. 7 RMS roughness of annealed ITO films. The figure shows a
time series with an overlap of error bars
applications. However, all films evaluated here had resistivity values too high to be considered as materials for the
top contact of conventional PV devices.
Acknowledgments Authors would like to acknowledge the support
from faculty start-up funds from the University of Toledo, Fulbright
(Science and Technology) funds, and the National Science Foundation (CBET-1235750). Furthermore, authors would like to acknowledge the helpful discussion with Dr. J. Mayandi.
Open Access This article is distributed under the terms of the
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