Metallic nanomesh electrodes with controllable optical properties for

APPLIED PHYSICS LETTERS 100, 143109 (2012)
Metallic nanomesh electrodes with controllable optical properties
for organic solar cells
Jinfeng Zhu,1,2,a) Xiaodan Zhu,3 Ryan Hoekstra,2 Lu Li,3 Faxian Xiu,4,b) Mei Xue,2
Baoqing Zeng,1 and Kang L. Wang2
1
School of Physical Electronics, University of Electronic Science and Technology of China,
Chengdu 610054, China
2
Department of Electrical Engineering, University of California, Los Angeles, California 90095, USA
3
Department of Materials Science and Engineering, University of California, Los Angeles,
California 90095, USA
4
Department of Electrical and Computer Engineering, Iowa State University, Ames, Iowa 50011, USA
(Received 4 February 2012; accepted 12 March 2012; published online 6 April 2012)
We have fabricated a metallic nanomesh using nanosphere lithography and metal evaporation. The
metallic nanomesh has a precisely controlled nanostructure showing excellent uniformity with
hexagonally arrayed periodic circular holes. A P3HT:PCBM organic solar cell, with the gold
nanomesh electrode, demonstrates a high fill factor of 61% and a considerable power conversion
efficiency of 3.12%. Electromagnetic simulation indicates that the optical properties of the metallic
nanomesh can be optimized for organic photovoltaic devices by tuning the film thickness, hole
diameter, and periodicity. These results show the promising potential of using a metallic
C 2012 American Institute of Physics.
nanomesh as the transparent electrode in organic solar cells. V
[http://dx.doi.org/10.1063/1.3701582]
Organic solar cells (OSCs) are a promising alternative
to conventional photovoltaic devices due to their flexibility,
light weight, low cost, and convenient fabrication.1 Indium
tin oxide (ITO) is the most widely used material for transparent electrodes in OSCs because of its electrical conductivity
and optical transparency. However, due to the scarcity of indium, it is a relatively expensive material, which directly
raises the manufacturing costs of OSCs.2 The brittleness of
ITO also further obstructs its application as a transparent
electrode in flexible optoelectronic devices. In the past few
years, researchers have been seeking alternatives to ITO,
such as: carbon nanotubes,3 graphene,4 and metal thin films.5
Very recently, transparent electrodes based on metallic nanostructures have attracted a great deal of attention due to their
flexibility, high transparency, and high conductivity.6,7 In
contrast with carbon nanotubes and graphene, the metallic
nanostructures can also have plasmonic effects which could
improve the optoelectronic performance of devices.8,9
Since the momentous research by Ebbesen et al.,10 the
extraordinary optical transmission of metal films with subwavelength holes has attracted a surge of interest.11 Reilly
et al. found that a metal film with nanoholes demonstrated a
promising route to replacing the ITO transparent electrode.12
Despite the advantages of nanohole metal films, there are still
numerous technical difficulties when integrating them with
OSCs as the transparent electrodes. The difficulties arise from
the changes of surface roughness and material work function.
In this research, we investigate the use of a metallic nanomesh
electrode in OSCs by employing a polymer bulk heterojunction
blend. The nanomesh is made out of gold to match the work
function of the anode to the active layer.13 The gold nanomesh
is fabricated by using nanosphere lithography. This fabrication
method is versatile and can be applied to other common metals
a)
E-mail: [email protected].
E-mail: [email protected].
b)
0003-6951/2012/100(14)/143109/4/$30.00
including aluminum, copper, silver, and their alloys. The active
layer of the OSC is a popular polymer bulk heterojunction of
poly(3-hexylthiophene) and [6,6]-phenyl-C61 butyric acid
methyl ester (P3HT:PCBM, 1:1 in weight). The device with
the gold nanomesh shows a high fill factor, short circuit
current, and power conversion efficiency (PCE). We also
demonstrate, using finite-difference time-domain (FDTD)
simulations, that the optical properties of the nanomesh can
be manipulated by controlling the geometric size of the
nanostructure.
The gold nanomesh with a hexagonal array of periodic
circular holes is fabricated using a polystyrene (PS) nanosphere template.14 First, a commercial suspension of PS nanospheres, with a weight concentration of 10%, is diluted with
an equal volume of ethanol. The PS nanospheres have a diameter of 430 nm with a size deviation of less than 3%. The PS
FIG. 1. (a) SEM image of close-packed PS nanospheres without oxygen
RIE. SEM images using a RIE power of 50 W and an oxygen flow rate of 50
sccm, (b) PS nanospheres etched for 60 s and (c) PS nanospheres etched for
120 s. (d) Diameter of PS nanosphere as a function of RIE time T.
100, 143109-1
C 2012 American Institute of Physics
V
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
218.193.57.47 On: Thu, 02 Apr 2015 00:58:12
143109-2
Zhu et al.
Appl. Phys. Lett. 100, 143109 (2012)
FIG. 2. (a) SEM image of the gold nanomesh on a glass substrate. The inset
is a photograph of the 1.5 1.5 cm2 glass substrate with gold nanomesh on
the left half. (b) SEM image of the gold nanomesh on a PET substrate. The
inset shows the photograph of the PET substrate with gold nanomesh.
nanospheres are self-assembled onto a surface of deionized
water in a vessel to form a monolayer of nanospheres.14 The
monolayer is then transferred onto a clean glass substrate and
the substrate is dried in air. The substrate is then baked on a
hot plate at 100 C for 25 min to remove any water residue.
The PS nanospheres form a close-packed periodic hexagonal
array on the substrate, as characterized by scanning electron
microscope (SEM) in Fig. 1(a). Oxygen reactive-ion etching
(RIE) is then used to etch the edge of the PS nanospheres and
reduce their size, as shown in Figs. 1(b) and 1(c). The size of
the etched PS nanospheres can be tuned by changing the oxygen flow rate, RIE power, and time, as shown in Fig. 1(d).
Electron beam evaporation is used to deposit metals on the
substrate. In this work, a 2 nm chromium film followed by an
18 nm gold film is vertically deposited on the substrate. Chromium is used to improve adhesion between the gold layer and
the glass substrate. Finally, the etched PS spheres are removed
by ultrasonication in chloroform. The gold remains on the
substrate forming the nanomesh and can be seen in Fig. 2(a).
The metallic nanomesh can also be fabricated on a flexible
transparent substrate of polyethylene terephthalate (PET)
instead of glass, as shown in Fig. 2(b). The hole periodicity is
determined by the original diameter of the PS nanospheres.
The OSCs are prepared on the glass/Au nanomesh substrates. The schematic structure of the solar cell using gold
nanomesh as the anodic electrode is illustrated in Fig. 3(a).
The nanomesh substrates are thoroughly cleaned in an ultrasonic bath with deionized water, acetone, and isopropyl alcohol successively for 10 min each and dried in a vacuum oven
for 20 min at 60 C. Subsequently, the substrates are processed
by oxygen plasma at a power of 200 W for 1 min to remove
any organic residue and smooth the surface. An electron
FIG. 4. Photovoltaic characteristics of OSCs using ITO and the gold nanomesh. The diameter of hole on the nanomesh is about 357 nm.
blocking layer of aqueous poly(3,4-ethylenedioxythiophene)
and poly(styrenesulfonate) (PEDOT:PSS) is spun on top of
the nanomesh at 4500 rpm for 1 min and baked in air at
130 C for 35 min. The surface is uniform with hexagonally
periodic circular pits less than 8 nm deep, as determined by
atomic force microscope (AFM) seen in Fig. 3(b). The
PEDOT:PSS layer acts as a buffer layer and allows for proper
wetting between the gold nanomesh and the active layer. The
substrates are transferred into a nitrogen glove box where the
active layer is deposited by spin coating as described in our
previous research.15 The samples are solvent annealed in
covered Petri dishes for about 16 min and then baked at
140 C for 5 min. Finally, a Ca/Al (20 nm/100nm) layer is deposited as the back electrode.
The photovoltaic performance of OSCs under AM 1.5 G
illumination is investigated and shown in Fig. 4. The open
circuit voltage is minimally changed when the ITO electrode
is replaced by gold nanomesh, since the work functions of
gold and ITO are so similar.16 The device with the gold
nanomesh demonstrates lower photocurrent density than the
ITO reference device. The short-circuit current density (Jsc)
of the nanomesh device is 9.6 mA/cm2 while the Jsc of the
ITO device is 10.8 mA/cm2. It has been suggested that the
incorporation of gold nanomesh could induce surface plasmon enhanced optical absorption of the OSC.12 However,
the near-field enhancement effect should be mostly limited
within the PEDOT:PSS layer in the gold nanomesh device
FIG. 3. (a) Schematic structure of the OSC with gold
nanomesh as the anode. (b) AFM image of the gold
nanomesh coated by PEDOT:PSS.
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
218.193.57.47 On: Thu, 02 Apr 2015 00:58:12
143109-3
Zhu et al.
Appl. Phys. Lett. 100, 143109 (2012)
FIG. 5. (a) Validation of the optical
simulation for t ¼ 20 nm, D ¼ 316 nm,
and P ¼ 430 nm, the transmission spectrum of the gold nanomesh is measured
using an advanced electronics spectrometer. Simulated transmission of gold
nanomesh, (b) with different thicknesses: t ¼ 10, 15, 20, and 25 nm for
D ¼ 400 nm and P ¼ 430 nm, (c) with
different hole diameters: D ¼ 240, 320,
360, and 400 nm for t ¼ 20 nm and
P ¼ 430 nm, and (d) with different periodicities: P ¼ 370, 430, and 490 nm, for
t ¼ 20 nm and D ¼ 320 nm.
that we fabricated. The reduction of Jsc mainly corresponds
to the decrease of optical transparency of gold nanomesh
versus ITO. The optical transmission of ITO is about 85%,
which is higher than the optical transmission of gold nanomesh fabricated in this work, seen in Fig. 5. Even though the
nanomesh device has a lower Jsc than ITO, it maintains a
high fill factor of 61% and a considerable PCE of 3.12%,
versus the result of 1.22% PCE when using a silver nanohole
film electrode as reported by Reilly et al. in Ref. 12.
The photocurrent density depends on the optical absorption
in the active layer and is partially determined by the optical
transmission of the front transparent electrodes. We perform an
electromagnetic simulation using the 3-dimensional FDTD
method to illuminate the optical properties of the metallic nanomesh. This modeling method is validated by experimental measurement as shown in Fig. 5(a). The optical transmission of the
metallic nanomesh can be manipulated by tuning the film thickness (t), hole diameter (D) and periodicity (P), as shown in
Figs. 5(b)–5(d). The suppression of optical transmission in a
certain wavelength range is attributed to the excitation of a
short range surface plasmon.17 The near-field enhancement
induced by the short range surface plasmon is confined within
the PEDOT:PSS layer and likely cannot effectively enhance
the optical absorption in P3HT:PCBM. The transmission suppression due to the plasmon resonance should be tuned below
the energy band gap of the OSC to allow more useful light to
enter the active layer. The simulation indicates that the optical
transmission of the nanomesh could approach nearly 80% with
a weak suppression around the plasmon resonance frequency.
Further work could be done to decrease the plasmon resonance
energy below the band gap of P3HT and raise the performance
of the cell. The electrical resistance of the nanomesh as a function of film thickness, hole diameter and periodicity can also
influence the device performance.18 Future research could be
carried out to quantify the trade-off between the optical properties and the conductivity of the metallic nanomesh.
In summary, a gold nanomesh was fabricated using a
versatile and convenient method. The design and fabrication
of polymer bulk heterojunction solar cells using gold nanomesh was demonstrated. OSCs using the gold nanomesh
showed a high fill factor and considerable PCE. The optical
and electrical properties of the metallic nanomesh could be
further optimized by tuning the film thickness, hole diameter
and periodicity. These results demonstrate that the metallic
nanomesh should have great potential to replace ITO electrodes in the organic photovoltaic industry.
This work is partially supported by the KACST under
the KACST-CAL Green Technology Center. J. Zhu thanks
China Scholarship Council for the fellowship support. B.
Zeng acknowledges the support from National Key Laboratory for Vacuum Electronics.
1
G. Li, V. Shrotriya, J. S. Huang, Y. Yao, T. Moriarty, K. Emery, and
Y. Yang, Nature Mater. 4, 864 (2005).
2
J.-F. Tremblay, Chem. Eng. News 84(26), 13 (2006), http://pubs.acs.org/
cen/coverstory/84/8426cover1.html.
3
C. D. Williams, R. O. Robles, M. Zhang, S. Li, R. H. Baughman, and
A. A. Zakhidov, Appl. Phys. Lett. 93, 183506 (2008).
4
K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, K. S. Kim, J.-H. Ahn,
P. Kim, J.-Y. Choi, and B. H. Hong, Nature 457, 706 (2009).
5
B. O’Connor, C. Haughn, K. H. An, K. P. Pipe, and M. Shtein, Appl.
Phys. Lett. 93, 223304 (2008).
6
H. Wu, L. Hu, M. W. Rowell, D. Kong, J. J. Cha, J. R. McDonough,
J. Zhu, Y. Yang, M. D. McGehee, and Y. Cui, Nano Lett. 10, 4242 (2010).
7
Z. Yu, L. Li, Q. Zhang, W. Hu, and Q. Pei, Adv. Mater. 23, 4453 (2011).
8
H. A. Atwater and A. Polman, Nature Mater. 9, 205 (2010).
9
J. Zhu, M. Xue, H. Shen, Z. Wu, S. Kim, J. Ho, A. Hassani-Afshar,
B. Zeng, and K. L. Wang, Appl. Phys. Lett. 98, 151110 (2011).
10
T. W. Ebbesen, H. J. Lezec, H. F. Ghaemi, T. Thio, and P. A. Wolff, Nature 391, 667 (1998).
11
F. J. Garcia-Vidal, L. Martin-Moreno, T. W. Ebbesen, and L. Kuipers,
Rev. Mod. Phys. 82, 729 (2010).
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
218.193.57.47 On: Thu, 02 Apr 2015 00:58:12
143109-4
12
Zhu et al.
T. H. Reilly III, J. van de Lagemaat, R. C. Tenent, A. J. Morfa, and K. L.
Rowlen, Appl. Phys. Lett. 92, 243304 (2008).
13
S. Gunes, H. Neugebauer, and N. S. Sariciftc, Chem. Rev. 107, 1324 (2007).
14
J. Rybczynski, U. Ebels, and M. Giersig, Colloids Surf., A 219, 1 (2003).
15
M. Xue, L. Li, B. J. Tremolet de Villers, H. Shen, J. Zhu, Z. Yu, A. Z. Stieg,
Q. Pei, B. J. Schwartz, and K. L. Wang, Appl. Phys. Lett. 98, 253302 (2011).
Appl. Phys. Lett. 100, 143109 (2012)
16
D. Cheyns, J. Poortmans, P. Heremans, C. Deibel, S. Verlaak, B. P. Rand,
and J. Genoe, Phys. Rev. B 77, 165332 (2008).
J. Braun, B. Gompf, G. Kobiela, and M. Dressel, Phys. Rev. Lett. 103,
203901 (2009).
18
Y.-H. Ho, K.-Y. Chen, S.-W. Liu, Y.-T. Chang, D.-W. Huang, and P.-K.
Wei, Org. Electron. 12, 961 (2011).
17
This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
218.193.57.47 On: Thu, 02 Apr 2015 00:58:12