Thickness dependence of the MoO
3
blocking layers on ZnO
nanorod-inverted organic photovoltaic devices
Mingjun Wang,
1,2
Yuan Li,
1
Huihui Huang,
1,2
Eric D. Peterson,
1
Wanyi Nie,
1
Wei Zhou,
1
Wei Zeng,
2
Wenxiao Huang,
1
Guojia Fang,
2,a͒
Nanhai Sun,
2
Xingzhong Zhao,
2
and David L. Carroll
1,a͒
1
Department of Physics, Center for Nanotechnology and Molecular Materials, Wake Forest University,
Winston-Salem, North Carolina 27109, USA
2
Key Laboratory of Acoustic and Photonic Materials and Devices of Ministry of Education and Department
of Electronic Science and Technology, School of Physical Science and Technology, Wuhan University,
Wuhan 430072, People’s Republic of China
͑Received 6 August 2010; accepted 20 January 2011; published online 10 March 2011͒
Organic solar cells based on vertically aligned zinc oxide nanorod arrays ͑ZNR͒ in an inverted
structure of indium tin oxide ͑ITO͒/ZNR/poly͑3-hexylthiophene͒: ͑6,6͒-phenyl C61 butyric acid
methyl ester͑P3HT:PCBM͒ /MoO
3
/aluminum͑Al͒ were studied. We found that the optimum MoO
3
layer thickness condition of 20 nm, the MoO
3
can effectively decrease the probability of
bimolecular recombination either at the Al interface or within the active layer itself. For this
optimum condition we get a power conversion efficiency of 2.15%, a short-circuit current density of
9.02 mA /cm
2
, an open-circuit voltage of 0.55V, and a fill factor of 0.44 under 100 mW /cm
2
irradiation. Our investigations also show that the highly crystallized ZNR can create short and
continuous pathways for electron transport and increase the contact area between the ZNR and the
organic materials. © 2011 American Institute of Physics. ͓doi:10.1063/1.3554381͔
Photovoltaic ͑PV͒ devices based on conjugated polymers
such as poly͑3-hexylthiophene͒͑P3HT͒ and ͑6,6͒-phenyl
C61 butyric acid methyl ester ͑PCBM͒ have attracted a lot of
attention in recent years because they are easily fabricated.
This offers the possibility of solar cells that are cost-
effective, flexible, and of large area. However, the strong
acidic nature of the poly͑3,4-ethylene dioxythiophene͒:͑poly-
styrene sulfonic acid͒͑PEDOT:PSS͒ modified layer, can lead
to corrosion of the indium tin oxide ͑ITO͒ electrode
1
and
degradation of the conjugated polymer in the active layer.
Several groups have constructed inverted structure PV de-
vices with highly transparent metal oxides, i.e., TiO
x
͑Refs. 2
and 3͒ or ZnO ͑Refs. 4 and 5͒ on the transparent electrode as
the cathode buffer layer. ZnO is a good candidate for this
application because it is a cheap and environmentally
friendly material that can be synthesized with high purity and
crystallinity at low temperature. Its high electron mobility
and high transparency in the visible-wavelength range allow
it to be both an effective electron transporter and excellent
wave guide. Olson et al. have reported efficient ZnO/
polymer devices with different ZnO morphologies.
6,7
Verti-
cally oriented ZnO nanorods arrays ͑ZNR͒ can provide con-
tinuous electron transport pathways. Moreover, the high
surface-to-volume of the vertically aligned ZNR can ensure
three-dimensional contact with the polymer and provide
many direct electrical pathways to transport electrons to the
cathode directly. Vishal Shrotriya et al. have verified that
MoO
3
can replace PEDOT:PSS as the hole selective layer,
which can be conveniently deposited by evaporation before
depositing the anode metal.
8
The thickness of the MoO
3
hole
selective layer is a vital parameter in the inverted solar cell
structure. To further understand the effect of MoO
3
thickness
on the ZNR-based invert structures, layers of various thick-
nesses were deposited onto a series of inverted ZNR solar
cells by thermal evaporation. We found the thickness of the
MoO
3
layers had a significant effect on the performance of
the inverted ZNR solar cells.
The ZNR-based inverted solar cell was fabricated by the
normative process as illustrated in Fig. 1. Devices were fab-
ricated on precleaned and etched conducting ITO ͓Delta
Technologies, R
s
=10 ⍀ sq
−1
͑Delta Technologies Ltd, Still-
water, Minnesota͔͒ coated glass substrates. A 130 nm
P3HT:PCBM ͑1:0.8 Weight ratio, 15 mg/ml in chloroben-
zene͒ layer was spin-cast on top of pregrown ZNR
9
substrates. Different thicknesses of MoO
3
were deposited
and capped with a 100 nm Al by thermal evaporation
͑5 ϫ 10
−6
torr͒ with a shadow mask. The devices were then
annealed under N
2
ambient at 150 °C for 7 min. The PV
characteristics were measured with a Keithley 236 source-
measurement unit under 100 mW / cm
2
using an AM 1.5G
standard Newport #96000 Solar Simulator. The external
quantum efficiency ͑EQE͒ was measured using a Newport
Cornerstone 260 Monochromator in conjunction with a New-
port 300W Xenon light source, and a Merlin Lock-In ampli-
fier. The morphology analysis of the ZNRs and solar cell
a͒
Electronic mail: carroldl@wfu.edu. Tel: ϩ1-͑336͒ 727-1806.
FIG. 1. ͑Color online͒ Schematic illustration of the fabrication process of
ZNR based bulk heterojunction ͑BHJ͒ solar cells.
APPLIED PHYSICS LETTERS 98, 103305 ͑2011͒
0003-6951/2011/98͑10͒/103305/3/$30.00 © 2011 American Institute of Physics98, 103305-1