Transcript No Slide Title

Gold-black as an IR absorber and solar cell enhancer
J. W. Cleary1, R. E. Peale1, C. W. Smith1, M. Ishigami1, K. Baillie1, J. E. Colwell1, K. M. Beck2, Alan G. Joly2, O. Edwards3, C. J. Fredricksen4
1Department
of Physics, University of Central Florida, Orlando FL 32816
2Pacific Northwest National Laboratory, William R. Wiley Environmental Molecular Science Laboratory, Richland, WA
3Zyberwear, Inc., 2114 New Victor Road, Ocoee FL
4LRC Engineering, Inc., 9345 Chandon Dr., Orlando FL 32825
ABSTRACT
Fig. 5 (below) presents PEEM images of samples C01 (left), and C13 (right) with field of
view = 150 mm. The bright areas are “hot spots”, where plasmon resonances facilitate
photoemission. C01 appears to have a higher concentration of hotspots. The lower left part
of C13 has been modified by a focused ion-beam mill (FIB) to make a marker for alignment.
PEEM shows that it has less plasmon activity in the FIB milled region.
EXPERIMENTAL
• IR absorbance and VIS/NIR excited plasmon resonances are investigated in Aublack.
• IR Transmittance and Reflectance measured with a Bomem Fourier transform
spectrometer.
• 2 level full factorial optimization
• evaporation-chamber pressure
• boat current
• substrate temperature
• degree of polymer infusion (for hardening)
• The figure of merit (FOM) was defined as 1-T-R, a measure of absorbance. It is not exactly
the same as absorbance, however, since the reflectance was not at normal incidence.
• Polymer infusion was found generally to reduce absorbance in the LWIR but has
little effect in the THz region.
Samples with the highest Figure of Merit (FOM) for absorbance show a
slight improvement in the absorbance in both wavelength regimes with higher
polymer infusion.
• A discrete wavelet analysis on each image using a Morlet wavelet was performed on each
SEM image.
• Scanning electron microscope (SEM) images were collected on various instruments at UCF
and EMSL.
INTRODUCTION
• Metal-black : nano-structured conducting films that have been exploited as
broad-band surface absorbers for bolometers .
RESULTS
• IR FOM was determined at 150 and 650
cm-1
(67 and 15 mm respectively ).
• Polymer infusion tended to reduce the FOM at 600 cm-1. This is less significant at 150
cm-1.
• For the samples with highest FOM , there is a tendency for polymer infusion to increase
the FOM slightly.
• Mid-point measurements show that FOM depends on each variable, and as well as
interactions between all variables.
-1
• Created by evaporating metal in an inert gas at tenths to tens of Torr.
0.6
0.4
C13
0.2
0.0
1
10
Wavelength (mm)
150 cm
1000
100
0.2
0.0
0.0
3.0x10
6
6.0x10
6
9.0x10
6
1.2x10
7
1.5x10
7
Intensity
-0.2
-0.4
-1
600 cm
-0.6
0
5
10
15
20
0
5
10
10000
15
20
Sample
SAMPLE PREPARATION
• Thermal evaporation in a chamber that has been back-filled with 1-2 Torr of inert
gas (helium).
• Parameters: He pressure ( P ), evaporation current ( I ), and substrate temperature
( T ) with two levels chosen for each. In addition, after characterizing these
samples, they were infused with polymer to two different levels of saturation. 20
samples (16 + 4 mid points)
Fig. 1. (above) Figures of merit at THz and Long-wave IR frequencies for gold-black samples of Table
I. Solid (open) symbols indicate values after (before) polymer infusion.
Fig. 2 (below) present SEM images of two of the gold black samples C04 (left) and C09 (right). The
metallic particles are arranged in interconnected groups with a broad-range of characteristic length
scales or spatial wavelengths. These two images were chosen for presentation because of the large
difference in their characteristic length scales.
# of pixels
• Presented are initial investigations of the plasmon resonance characteristics of
Au-black, which has potential to increase the efficiency of thin film solar cells via
resonant scattering and field enhancement.
100
Fig. 7 (left) present histograms
of the PEEM images from Fig.
5 for samples C01 (top) and
C13 (bottom). The higher
concentration of bright hot
spots for C01 is revealed as a
bump at the highest intensity
level. In principle, the ratio of
this bump to the area under the
lower intensity part of the curve
might be used as a measure of
hot-spot density in properly
controlled experiment.
0.4
FOM
•Au-black films are powder-like (broad particle-size distribution). Hardening of
Au-black films is achieved here via polymer infusion.
C01
0.8
10000
0.8
0.6
Fig. 6 (right) shows the Wavelet
analysis of the PEEM spectra of Fig.
5. Symbols indicate the minimum
length scale observed in the SEM
image. This result suggests a certain
minimum for the excitation of
plasmon resonances, and that this
minimum which may be larger (as in
in the case of C01) than the
minimum size in the sample.
Amplitude
• Initial investigations of Au-black by photoelectron emission microscopy (PEEM)
reveal plasmon resonances, which have potential to enhance the efficiency of thin
film solar cells. For films with different characteristic length scales, the plasmon
resonances appear in portions of the film with similar length scales.
1.0
# of pixels
• Characteristic length scales of the structured films vary considerably as a
function of deposition parameters, but the IR FOM is found to be only weakly
correlated with these distributions.
• PhotoElectron Emission Microscopy (PEEM) was used to characterize the plasmon
resonances.
1000
100
0.0
3.0x10
6
6.0x10
6
9.0x10
6
1.2x10
7
1.5x10
7
Intensity
Fig. 8 (below) shows PEEM images of a gold black film (C13) with 50 mm field-of-view.
The bright areas correspond to photoelectron emission for laser illumination at 370 nm
(left) and 420 nm (right) wavelengths. We see higher plasmon activity in the longer
wavelength image when we are closer to the bulk surface plasmon resonance wavelength
of gold (~500 nm).
• Experiments were randomized to avoid systematic errors.
Table I: Matrix of experiments in two-level full factorial optimization of Au-black.
The variables are He pressure (P), substrate temperature (T), boat current (I), and
level of polymer infusion. High (+), low (-), and intermediate (*) levels are
indicated. The final column shows which samples were also characterized by
PEEM. Underlined samples are those with exceptional FOM.
1.0
Sample
P
T
I
Polymer
PEEM
0.8
C01
+
+
-
+
C03
C04
+
+
-
-
+
C05
C06
*
-
*
-
*
-
*
-
C07
C08
+
+
+
-
+
-
C09
C10
*
+
*
-
*
+
*
+
C11
C12
+
-
+
+
+
+
+
-
C13
C14
+
+
-
+
+
+
-
X
X
Amplitude
===================================================
0.6
C04
X
X
+
+
+
C17
C18
-
+
+
-
-
0.2
C19
+
+
+
*
C21
*
*
*
*
150 cm -1
0.4
FOM
ACKNOWLEDGEMENTS
.
0.0
-0.2
-0.4
600 cm -1
-0.6
2
3
4
5
• The observation of photoelectron emission from gold-black samples for
energies well below the work function of gold (~ 5.1 eV) suggests that the
emission occurs due to the field enhancement at the plasmon resonances. Thus,
there is potential for similar useful effects in to enhance the efficiency of solar
cells.
• A broad range of length scales contribute significantly to the plasmon-assisted
photoemission which supports our suggestion that Au-black may be a suitable
material for plasmon-resonance enhancement of solar cell efficiency over the
broad solar spectrum.
0.8
-
*
• Experiments do not clearly show that the IR absorbance is correlated with the
characteristic length scale of the gold-black films.
Fig. 4 (below) plots FOM values at THz (left) and LWIR (right) wavelengths as a function of the
minimum length scale from the wavelet analysis of SEM images. There may be a tendency for
samples with smaller length scales to give higher absorbance.
-
*
• Hardening of Au-black tends to lower the LWIR FOM somewhat, while this
characteristic is less effected in the THz spectral range.
1
10
Characteristic length (mm)
X
C15
C16
*
C09
0.2
0.6
C20
Fig. 3 (left) presents the wavelet
analysis of SEM images for C04 and
C09. We define the minimum
characteristic length scale as the point
where the power is half the maximum
value.
0.4
0.0
0.1
SUMMARY
6
7
8
2
3
4
Minimum length scale ( mm)
5
6
7
8
JWC and are supported by a grant from AFOSR. Pacific Northwest National Laboratory is
operated for the U.S. Department of Energy by Battelle. PEEM and some of the SEM
experiments were performed in the Environmental Molecular Sciences Laboratory, a U.S.
Department of Energy user facility operated by the office of Biological and Environmental
Research. JEC and KB were supported by NASA's Cassini Data Analysis Program Grant
NNX08AJ68G. The contributions of LRC Engineering were supported by an AFOSR Phase
I STTR.
University of Central Florida Physics Department:
http://physics.cos.ucf.edu/content/index.html