Doping of the Metal Oxide Nanostructure and its Influence in ...

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Doping of the Metal Oxide Nanostructure and its
Influence in Organic Electronics
By Mi-Hyae Park, Juo-Hao Li, Ankit Kumar, Gang Li,* and Yang Yang*
FULL PAPER
Synthesizing metal oxides through the sol–gel process provides a convenient
way for forming a nanostructured layer in wide band gap semiconductors. In
this paper, a unique method of introducing dopants into the metal oxide
semiconductor is presented. The doped TiO 2 is prepared by adding a Cs 2 CO 3
solution to a nanocrystalline TiO 2 solution that is synthesized via a nonhydrolytic
sol–gel process. The properties of the TiO 2 :Cs layer are investigated
and the results show stable nanostructure morphology. In addition to
providing morphological stability, Cs in TiO 2 also gives rise to a more
desirable work function for charge transport in organic electronics. Polymer
solar cells based on the poly(3-hexylthiophene) (P3HT): methanofullerene
(PC 70 BM) system with the addition of a TiO 2 :Cs interfacial layer exhibit
excellent characteristics with a power conversion efficiency of up to 4.2%. The
improved device performance is attributed to an improved polymer/metal
contact, more efficient electron extraction, and better hole blocking
properties. The effectiveness of this unique functionality also extends to
polymer light emitting devices, where a lower driving voltage, improved
efficiency, and extended lifetime are demonstrated.
1. Introduction
Electronic devices based on organic materials (small molecules and
polymers), such as organic light emitting devices (OLEDs), [1]
organic photovoltaic cells (OPVs), [2] transistors, [3] bistable devices,
and memory devices, [4] have attracted considerable attention. The
most salient attribute of polymer electronics is the potential to be
low-cost and versatile while having low-energy consumption and
high-throughput processing. [5] For polymer solar cells, the polymer/
fullerene based bulk-heterojunction (BHJ) system is the most
commonly used device architecture [6–8] for which a certified
efficiency of 5.4% for a single cell configuration was achieved. [9]
In the field of organic electronics, the metal/organic interface
plays a critical role in influencing device performance. The
[*] Dr. G. Li
Solarmer Energy, Inc.
El Monte, CA 91731 (USA)
E-mail: gangl@solarmer.com
Prof. Y. Yang, M.-H. Park, J.-H. Li, A. Kumar
Department of Materials Science and Engineering
University of California Los Angeles
Los Angeles, CA 90095 (USA)
E-mail: yangy@ucla.edu
DOI: 10.1002/adfm.200801639
interface can often be modified by an
insertion of a functional interfacial layer to
improve the device performance. Depending
on the characteristics of the material,
the functional interfacial layer can be
employed in different configurations. Early
prominent examples of functional interfacial
layers used in the development of
OLEDs and OPVs include: i) introduction
of LiF, CsF, AlO x , etc. as an electron buffer
layer in OLEDs, [10–12] ii) application of
polyaniline (PANI) [13] and poly(3,4-ethylenedioxythiophene):poly(styrene
sulfonate)
(PEDOT:PSS) as a hole transport/buffer
layer, [14] iii) insertion of a TiO x layer as an
optical spacer/hole blocking layer, [15,16] and
iv) combination of an n- and p-type
transport layer for tandem OLEDs (e.g.,
LiF–V 2 O 5 ). [17] The role of cesium as an
effective interfacial material has been
proven. Recently, it has been shown that
the use of salts, such as Cs 2 CO 3 or CsF, as a
source of Cs component for an n-type
interfacial layer, can improve solar cell efficiency. [18] In addition,
Cs 2 CO 3 , which can be deposited either by thermal evaporation or
solution processing, can serve as an effective electron injection/
buffer layer, leading to record high white and red PLEDs
efficiencies with significantly reduced driving voltages and
enhanced lifetimes. [19] Furthermore, combined with novel
p-type interfacial layer materials, such as transition metal oxides
(V 2 O 5 , MoO 3 ,WO 3 , etc.), we have successfully demonstrated
efficient inverted polymer solar cells. [20,21]
Semiconducting TiO 2 has been extensively studied as a
promising material in a variety of applications including dyesensitized
solar cells, photocatalysts, and organic photovoltaics.
[22,23] Sol–gel chemistry is widely accepted as a valuable
process used for preparing materials with well-controlled
morphological and structural properties. However, it is widely
accepted that the application of nanocrystalline TiO 2 through the
typical sol–gel method in organic photovoltaics remains limited
due to the hydrothermal processing or calcinations required to
induce crystallization. [24] On the other hand, a non-hydrolytic
sol–gel process can provide several important advantages:
i) the elimination of additional agents allows for less particle
agglomeration; ii) no exposure to air is needed; and iii) the
elimination of water enables the formation of homogeneous
films. [24] The use of nanocrystalline anatase phase TiO 2 produced
by a non-hydrolytic sol–gel process eliminates the need for a high
temperature sintering process (400–500 8C), which would
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otherwise inhibit the application of crystalline TiO 2 in regular
OPV structures. In addition, the introduction of dopants in sol–
gel chemistry offers a convenient method for producing
functional materials. It has been demonstrated that the properties
of metal oxides can be enhanced and tuned through the addition
of various dopants and processing methods. [25,26] The design and
synthesis of functional materials formed by the sol–gel method
via doping are being widely investigated and possess a great
potential for the development of nanoscale technology.
In this study, we present a method for doping metal oxides via a
non-hydrolitic sol–gel method and demonstrate an approach to
make efficient organic electronic devices. A functional interfacial
layer prepared by mixing solution processable semiconducting
metal oxides and salts is inserted into the polymer electronic
device. It is found that introducing a nanoscale Cs doped TiO 2
layer can enhance the solar cell performance. The effectiveness of
this unique approach was also expanded to polymer LEDs, where
a lower driving voltage, improved efficiency and extended lifetime
are demonstrated once again. The properties of the TiO 2 :Cs layer
were investigated and a discussion of the source of improvement
in device performance is presented.
2. Results and Discussion
2.1. Synthesis and Characterization
Synthesis of crystalline TiO 2 nanoparticles follows a previously
published method. [27] A Cs doped TiO 2 was obtained by mixing
the individual solutions of Cs 2 CO 3 and TiO 2 together. Transmission
electron microscopy (TEM) images of TiO 2 and the TiO 2 :Cs
are shown in Figure 1a and b, respectively. An overview image of
Figure 1. a) TEM images of TiO 2 and b) a Cs doped TiO 2 (TiO 2 :Cs).
c) X-Ray powder diffraction patterns for TiO 2 (bottom), and TiO 2 :Cs
overview (middle) and zoomed in (top). d) XPS profiles of TiO 2 (dot line)
and TiO 2 :Cs (solid line) samples for Ti peak.
the TiO 2 nanoparticles illustrates that the material is entirely
composed of nanosized particles that are homogeneously
distributed throughout the material. As we continue to blend
with Cs 2 CO 3 , the TEM images show that the product consists of
markedly more monodispersed shapes. A comparison of the TEM
images (not shown) of the TiO 2 and TiO 2 :Cs (taken 1 week after
being exposed to air) shows that the mixture is stable and that the
product has not agglomerated upon addition of Cs 2 CO 3 . This may
be explained by Cs 2 CO 3 having a stabilizing effect on the solution,
which prevents the three-dimensional titania network from
shrinking.
The crystalline phase evolution of these two samples was
monitored with an X-ray powder diffractometer (XRD data shown
in Fig. 1c). The X-ray powder diffraction pattern for TiO 2 ,
obtained by the sol–gel method, confirms the existence of
nanocrystalline TiO 2 in the anatase phase, which agrees with the
literature. [24,27] All the peaks are ascribed to the anatase crystal
structure without any secondary reaction impurities. The indexed
broad peaks indicate the nanocrystalline nature of TiO 2 with sizes
between 7 and 8 nm. The XRD spectrum of TiO 2 :Cs, along with
an enlarged spectrum, are shown. When Cs 2 CO 3 is added to
TiO 2 , the peak patterns for both the anatase phase of TiO 2 as well
as the CsCl cubic structure can be assigned to the XRD spectrum.
The CsCl can be formed through the reactions of the residual
benzyl chloride, which is a by-product from the stock TiO 2
solution with cesium in Cs 2 CO 3 . The narrow peak width of CsCl
shows highly ordered crystalline characteristics of CsCl as
compared to TiO 2 . On the other hand, the existence of
nanocrystalline anatase TiO 2 in the TiO 2 :Cs sample is evident
from the similar peak width and intensity from the enlarged XRD
data.
X-Ray photoemission spectroscopy (XPS) was performed to
further investigate the surface characteristics of TiO 2 and the
TiO 2 :Cs interfacial layer. The data is shown in Figure 1d. The
samples were prepared by spin casting the films on an Ag-coated
Si wafer, where the instrument was calibrated using an internal
Ag standard. The atomic ratio of oxygen to titanium was
estimated to be 1.99 based on the integrated area under the
element peak and the sensitivity factor, with commercially
available crystalline TiO 2 powder used as a reference (Sigma–
Aldrich, used as received). The data imply that titanium dioxide
prepared from the non-hydrolytic sol–gel method is chemically
stoichiometric, which is also in good agreement with previously
reported Ti 2p 3/2 peak position for TiO 2 . [28] We observe that the Ti
2p 3/2 spectra for TiO 2 :Cs shifts toward a lower binding energy by
0.78 eV in comparison to the value for TiO 2 . We suspect that this
change is attributed to the creation of partially reduced Ti ions,
which is consistent with previous reports that the Ti (2p) peaks
shift considerably to a lower binding energy upon Cs or K
adsorption. [29,30] The Cs 3d 5/2 peak position of TiO 2 :Cs shifted
toward a lower binding energy, compared to that of Cs 2 CO 3 ,
which also supports the idea of charge transfer between TiO 2 and
Cs. Metal ions in an organic/inorganic matrix can act as a doping
component. This discrepancy in the XPS survey spectra may be
explained by the possible formation of Cs-doped TiO 2 materials.
The energy levels of both TiO 2 and TiO 2 :Cs samples were
determined through electrochemical cyclic voltammetry (C-V)
and the energy offset wavelength from the UV–Vis absorption
spectra. The energy level diagram is shown in Figure 2.
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Figure 2. Corresponding energy level diagram of a device based on a
TiO 2 :Cs interfacial layer.
2.2. Photovoltaic Device Performance
The photovoltaic devices were fabricated using a blend of poly(3-
hexylthiophene) (P3HT) and [6,6]-phenyl-C 71 -butyric acid methyl
ester (PC 70 BM) with Al as the cathode. Cs 2 CO 3 ,TiO 2 , and the
TiO 2 :Cs were inserted individually as an interfacial layer between
the active layer and the cathode. The current density–voltage (J–V)
characteristics under AM 1.5G one-sun illumination condition is
shown in Figure 3a. Table 1 summarizes the characteristics of the
device performance. A comparison of devices with an Al electrode
to those with a Cs 2 CO 3 /Al electrode shows a decreasing open
circuit voltage (V oc ) and short circuit current density ( J sc ) upon
insertion of the Cs 2 CO 3 spin-casted film. This implies that a
Cs 2 CO 3 -only interfacial layer does not provide the appropriate
function in terms of charge extraction and charge transport to the
electrode. The insertion of a TiO 2 layer between the active layer
and the evaporated Al cathode layer leads to an increase in V oc up
to 0.46 V. It is known that the open circuit voltage is generally
determined by the difference between the highest occupied
molecular orbital (HOMO) of the donor and the lowest
unoccupied molecular orbital (LUMO) of the acceptor in the
case of an Ohmic contact between the active layer and the
cathode. [31] Thus, the increase in V oc may arise from the work
function of TiO 2 . The conduction band level of TiO 2 is 4.3 eV, as
determined from the C-V experiments, which is slightly higher
than the work function of 4.2 eV of the Al electrode. This results in
unfavorable electron charge extraction from the active layer to the
electrode and an S-shape J–V curve is observed. On the other
hand, we clearly see an improvement in V oc , J sc , and fill factor (FF)
for the devices fabricated with a functional TiO 2 :Cs layer,
resulting in efficient device performances. The V oc increases
from 0.42 V (for the device with no interlayer) to 0.58 V and the FF
improves dramatically up to 67%. This yields the average power
conversion efficiency (PCE) of 4.0% and the highest PCE
Figure 3. a) J–V characteristics of a P3HT:PC 70 BM based photovoltaic cell
with an evaporated Al cathode and different interfacial layers (none;
Cs 2 CO 3 ; TiO 2 ; TiO 2 :Cs), and b) external quantum efficiencies (EQE) of
the device with and without the TiO 2 :Cs interfacial layer.
achieved is 4.2%, which is comparable to a device with a Ca/Al
electrode. As a result, TiO 2 :Cs is a promising candidate for
replacing Ca, as it has been shown that inorganic oxides are quite
stable to oxygen and moisture. [32] One possible reason for
the increased performance of the devices with TiO 2 :Cs is the
formation of a better Ohmic contact that is created by the
decreased conduction band level of the TiO 2 :Cs layer (3.93 eV)
such that the interfacial layer facilitates electron transport from
the active layer to the cathode. Under dark conditions, the
rectification ratio is on the order of 10 6 , the serial resistance is
Table 1. Summarized photovoltaic performance characterisitcs of corresponding
regular configuration devices with different interfacial layers.
Device V oc [V] J sc [mA/cm 2 ] PCE [%] FF [%]
None 0.42 9.64 2.0 48
Cs 2 CO 3 0.36 5.34 0.7 37
TiO 2 0.46 10.48 2.4 50
TiO 2 :Cs 0.58 10.76 4.2 67
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considerably decreased to 1–2 V cm 2 , while the shunt resistance
remains as high as 10 7 V cm 2 , making it ideal for photovoltaics.
It is believed that the TiO 2 :Cs layer can keep the hot Al
electrode from diffusing into the active layer during evaporation
and can offer good contact morphology between the active layer
and the electrode. This is also supported by the dark current
characteristics of the device with no interlayer having a similar
shunt resistance as that of the TiO 2 :Cs layer but a higher serial
resistance of several tens of V cm 2 . In addition, the highly
negative valence band level of the interfacial layer serves as an
efficient hole blocking layer, which is confirmed by the small
leakage current for the TiO 2 :Cs based device. The external
quantum efficiency (EQE) of both the reference device using no
interlayer and the device with a TiO 2 :Cs interfacial layer is shown
in Figure 3b; the EQE is consistent with J–V characteristics. We
note that the device utilizing only a CsCl interfacial layer does not
display any of the improved device characteristics, including high
V oc , high FF, and small serial resistance. This indicates that
although the CsCl seems to be a major component in the XRD
data, the CsCl layer does not play a direct role in improving the
efficiency. Instead, the nanocrystalline anatase phases derived
from TiO 2 , such as doped TiO 2 , is a possible contributor to the
enhancement in efficiency. The related improvements in V oc and
FF were observed with a TiO 2 :CsF interlayer, where CsF acts as
another source of Cs component, and also with several other
polymer systems. However, further investigation is required to
clarify the mechanism.
We used the charge extraction by linearly increasing voltage
(CELIV) method to investigate the charge carrier transport
characteristics of the TiO 2 :Cs layer for some of the representative
regular configuration devices. In CELIV, the initial rise speed
provides information on the bulk conductivity of the sample and
the time of extraction current maximum, t max , is used for
estimating the drift mobility of equilibrium charge carriers. [33]
Under a ramping speed of 10 5 V cm 1 , CELIV extraction
peaks were obtained as shown in Figure 4a. The change in t max
is negligible, indicating the fairly consistent mobility values
obtained from the different devices, all of which have mobilities
on the order of 10 4 cm 2 V 1 s 1 . Impedance spectroscopy was
used to measure the bulk conductivity of the samples. All of our
devices fitted well to the R p –C p (resistor–capacitor in parallel)
model wherein the conductance (G ¼ 1/R p ) should be independent
of the frequency and the susceptance ½B ¼ jð2pfC p ÞŠ should
vary linearly with frequency. The conductance data derived with
this method is shown in Figure 4b. The conductance for the
device with a TiO 2 :Cs layer was at least three orders of magnitude
higher than the corresponding Cs 2 CO 3 /Al or TiO 2 /Al devices.
Since the number of charges extracted is directly proportional to
the ratio of the conductivity divided by the mobility, we concluded
that the devices with TiO 2 :Cs improve charge extraction from the
polymer active layer. Additional support for this argument can be
observed from the CELIV data. The area under the current
density–time curve is the sum of the capacitive charges and the
equilibrium charges extracted from the device. Subtracting the
capacitive charges [initial current rise j(0)], we see that the area for
the TiO 2 :Cs/Al device is larger than that for the TiO 2 /Al device.
Hence, more equilibrium charge carriers are extracted under no
illumination for the TiO 2 :Cs/Al devices than for the TiO 2 /Al
devices.
Figure 4. a) CELIV extraction peaks and b) conductance data for the ITO/
PEDOT/P3HT:PC 70 BM/interfacial layer/Al device with Cs 2 CO 3 ; TiO 2 ;
TiO 2 :Cs interfacial layers.
An inverted structure was investigated for polymer solar cells
using Cs 2 CO 3 to modify the ITO electrode as a cathode and using
a transition metal oxide V 2 O 5 as a hole buffer layer. We have
reported inverted solar cells with an efficiency of 2.25% due to
non-optimized interfacial layer and active layer processes. [20] A
thick buffer layer on top of the active materials can be applied in
inverted cells, so that the structure is more robust to transparent
electrode deposition, e.g., ITO sputtering. A lamination fabrication
process of semitransparent and flexible solar cells based on
the same interface modification approach was recently shown. [34]
Here, we apply a TiO 2 :Cs to replace the Cs 2 CO 3 layer and
demonstrated fabrication of highly efficient inverted polymer
solar cell based on the P3HT and PCBM system. The structure of
the inverted device is as follows: ITO/TiO 2 :Cs/P3HT:PC 70 BM/
V 2 O 5 /Al. The dark and photo (AM1.5G, 100 mA cm 2 ) J–V curves
of the device with a TiO 2 :Cs interfacial layer are shown in
Figure 5. For the device with a TiO 2 :Cs layer, optimization of the
device fabrication process again leads to improvements in V oc and
FF. Subsequently, this results in a device efficiency of 3.9%, with
the V oc , J sc , and FF being 0.60 V, 11.5 mA cm 2 , and 57%,
respectively. The high rectification ratio is also attributed to the
improved injection current under forward bias as shown from
the dark current.
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Figure 5. J–V curve of an inverted solar cell with TiO 2 :Cs in the dark and
under illumination of AM 1.5.
2.3. Polymer Light Emitting Devices (PLEDs) Performance
In an effort to explore the effectiveness of the TiO 2 :Cs layer,
green-polyfluorene based PLEDs were constructed with the
structure ITO/PEDOT:PSS(40 nm)/light emitting polymer (LEP)
(80 nm)/interfacial layer/Al, where the interfacial layer is i) TiO 2 ,
ii) Cs 2 CO 3 , and iii) TiO 2 :Cs, all in 2-ethoxyethanol. To exclude
the solvent effects on the device performance, the solvent itself
was spun-cast between the LEP and Al to make a reference
diode. Figure 6a shows a comparison of the current-density–
voltage–brightness (J–V–L) characteristics of the devices with
different interfacial layers and its effect on device performance.
The Cs 2 CO 3 interfacial layer has been shown to be an effective
electron-injection layer, which leads to white and red emission
PLEDs reaching record highs in power efficiencies. [35,36] The
significant improvements in device performances have been
attributed to the formation of a low work-function complex and
surface dipole, which can facilitate electron injection from the
cathode. [19] Surprisingly, with the use of the TiO 2 :Cs interfacial
layer, further improvements in both the current density and
brightness were observed in comparison to devices with only a
Cs 2 CO 3 or TiO 2 interfacial layer. As shown in Figure 6b, the
device with a TiO 2 :Cs interfacial layer has a current efficiency of
11.5 cd A 1 or power efficiency of 14 lm w 1 at a bias of 2.8 V. The
turn-on voltage (around 2.3 V) does not change, which implies
that the PLEDs with the TiO 2 :Cs interfacial layer may not further
lower the electron injection barrier as compared to the reference
devices. However, the increase in current density and brightness
suggests that the better charge balance should be responsible for
the efficiency enhancement. As discussed above, a nearly Ohmic
contact is observed with the interfacial layer of TiO 2 :Cs, supported
by the conductivity and energy level alignment between the
organic materials and the metal cathode. Moreover, the valence
band level (7.6 eV) of the interface layer is lower than the HOMO
(5.4 eV) level of the organic active layer, providing a hole-blocking
effect in our device structure. By combining the Ohmic contact
and the hole-blocking effects, a better charge balance and
enhanced device performance can be achieved. Therefore,
compared to PLEDs containing only a Cs 2 CO 3 or TiO 2 interfacial
layer, the TiO 2 :Cs layer exhibits the advantageous characteristics
of both the lower work function and the hole-blocking effect from
Cs 2 CO 3 and TiO 2 , respectively.
FULL PAPER
3. Conclusions
In summary, we have demonstrated a novel approach for
fabricating efficient organic electronic devices by introducing
dopants into solution processable metal oxides as an interfacial
layer. The nanocrystalline TiO 2 was synthesized using a nonhydrolytic
sol–gel approach and was mixed with a Cs 2 CO 3
solution. Polymer solar cells based on the P3HT:PC 70 BM system
with a TiO 2 :Cs interfacial layer reached a PCE of 4.2% in regular
configurations. Significant improvements in PLED performances
have also been obtained. We anticipate that this study will
stimulate further research on metal oxides and salts as materials
for combined functional layers to achieve efficient charge
transport properties.
Figure 6. a) Current-density–voltage–brightness characteristics and
b) current efficiency for the ITO/PEDOT/LEP/EIL/Al device with different
interfacial layers (none; Cs 2 CO 3 ; TiO 2 ; TiO 2 :Cs).
4. Experimental
Material: All chemicals were purchased from Sigma–Aldrich and used
as received. TiO 2 was synthesized from a non-hydrolytic sol–gel approach
described as follows: After stirring a solution of TiCl 4 , ethanol, and benzyl
alcohol for 9 h at 80 8C, it was washed with diethyl ether. The white TiO 2
precipitate was obtained by centrifuging the crude product. The final TiO 2
solution was prepared by dispersing it in ethanol. A solution of TiO 2 :Cs was
obtained by blending 0.2 wt % of Cs 2 CO 3 in 2-ethoxyethanol solution with
the TiO 2 solution (0.2 wt %) at a 1:1 volume ratio.
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Characterization: The nanotextures of TiO 2 and TiO 2 :Cs were characterized
by TEM (JEOL JEM-2000FX). XRD analysis was performed with a
PANalytical X’Pert Pro Powder Diffractometer on finely powdered samples
using CuKa radiation. The powder samples containing TiO 2 and TiO 2 :Cs
for XRD analysis were prepared by evaporating off the solvent at 110 8C in
an oven. The XPS experiment was performed in an Omicron Nanotechnology
system with a base pressure of 2 10 10 Torr. A MgKa radiation
source was used for the XPS measurements. For CELIV analysis, the data
were taken with a Tektronix TDS-430A digital oscilloscope and a Wavetek
Datron 195 waveform generator. The impedance measurements were
carried out using a HP4284A Precision LCR meter.
Device Fabrication: The polymer blend of P3HT:PC 70 BM at a 1:1 weight
ratio was spin-casted at 800 rpm on top of a layer of PEDOT:PSS deposited
on ITO-coated glass. This was followed by thermal annealing at 110 8C. The
interfacial layer was spin-casted from each solution and that film was
annealed at 80 8C. The device fabrication was completed by thermal
evaporation of 100 nm of Al as the cathode. For an inverted configuration
device, the TiO 2 :Cs layer was spin-casted on ITO-coated glass and thermal
annealing was performed at 150 8C for 30 min. After spin-casting a polymer
blend solution of P3HT:PC 70 BM, another thermal annealing step was
performed at 110 8C for 10 min. The device fabrication was completed by
thermal evaporation of 5 nm of V 2 O 5 and 80 nm of Al as the anode.
For OLED device fabrication, 1% green polyfluorene in p-xylene (as a
LEP) was spin-casted on a layer of PEDOT:PSS deposited on ITO-coated
glass. The cathode was formed by spin coating from each interfacial layer
solution, followed by thermal deposition of Al.
Acknowledgements
This work was financially supported by Solarmer Energy, Inc. (grant no.
20061880) and UC-Discovery Grant (no. GCP05-10208). The authors thank
Mr. Hyun Cheol Lee for recording TEM images.
Received: November 7, 2008
Published online: February 25, 2009
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