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Lawrence Berkeley National Laboratory
Oligo and Poly-thiophene/Zno Hybrid Nanowire Solar Cells
Briseno, Alejandro L.
Lawrence Berkeley National Laboratory
Oligo, Polythiphene, Zno, Hybrid Nanowire
LBNL Paper LBNL-3396E
Nano Letters , DOI 10.1021, nl9036752, pg 334-340, 12/15/2009
We demonstrate the basic operation of an organic/inorganic hybrid single nanowire solar cell.
End-functionalized oligo- and polythiophenes were grafted onto ZnO nanowires to produce p-
n heterojunction nanowires. The hybrid nanostructures were characterized via absorption and
electron microscopy to determine the optoelectronic properties and to probe the morphology
at the organic/inorganic interface. Individual nanowire solar cell devices exhibited well-resolved
characteristics with efficiencies as high as 0.036percent, Jsc = 0.32 mA/cm2, Voc = 0.4 V, and
a FF = 0.28 under AM 1.5 illumination with 100 mW/cm2 light intensity. These individual test
structures will enable detailed analysis to be carried out in areas that have been difficult to study
in bulk heterojunction devices.
Oligo- and Poly-thiophene/ZnO Hybrid Nanowire Solar Cells
Alejandro L. Briseno, Thomas W. Holcombe, Akram I. Boukai, Erik C. Garnett, Steve
W. Shelton, Jean J. M. Fréchet*, Peidong Yang*
Department of Chemistry, University of California, Berkeley, California 94720, and
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley,
Email: firstname.lastname@example.org; email@example.com
We demonstrate the basic operation of an organic/inorganic hybrid single nanowire solar
cell. End-functionalized oligo- and poly-thiophenes were grafted onto ZnO nanowires to
produce p-n heterojunction nanowires. The hybrid nanostructures were characterized via
absorption and electron microscopy to determine the optoelectronic properties and to
probe the morphology at the organic/inorganic interface. Individual nanowire solar cell
devices exhibited well-resolved characteristics with efficiencies as high as 0.036%, Jsc=
0.32 mA/cm2, Voc= 0.4 V, and a FF= 0.28 under AM 1.5 illumination with 100 mW/cm2
light intensity. These individual test structures will enable detailed analysis to be carried
out in areas that have been difficult to study in bulk heterojunction devices.
Hybrid solar cells composed of organic semiconductors
1 and inorganic
nanostructures 2 are an area of immense study as they are alternatives to organic bilayer 3
and bulk heterojunction device structures.4,5 The organic/inorganic hybrid system 6-9 has
opened new opportunities for the development of future generation solar cells, new
device technologies, and a platform to study three-dimensional morphology.10 A
multitude of concepts have been demonstrated by combining p-type donor polymers with
n-type acceptor inorganic nanostructures such as CdSe,6,7,11 TiO2,8-10,12-15 and ZnO.8-10,14
One-dimensional (1-D) inorganic semiconductor nanostructures are among some of the
most attractive nanomaterials for solar cell devices because they provide a direct path for
charge transport.2 Other advantages include high carrier mobilities, solution
processability, thermal and ambient stability, and a high electron affinity necessary for
charge injection from the complementary organic donor material. ZnO nanowires are an
example of this class of materials that have been used for hybrid solar cells.8-10,14,16
Poly(3-hexylthiophene) (P3HT)/ZnO nanowire composite solar cells are benchmark
systems that have attained power conversion efficiencies ranging from 0.02 to 2%.9,16,17
In spite of the vast efforts in this area of research, solar cells based on hybrid composites
have yielded efficiencies only close to those of organic bilayer devices and significantly
less than organic bulk heterojunction solar cells. Knowledge regarding interfacial charge
separation and/or transport in hybrid nanowire devices is only partly understood.10,17 If
this class of materials is to play a part in the future of next generation solar cells, then
there must be an improved fundamental understanding of the organic/inorganic interface
in order to improve power conversion efficiencies. While nanowire array and bulk
inorganic/organic blend devices are technologically relevant, their electrical properties
depend on nanostructure size, uniformity, crystallinity, phase segregation, interfacial
interactions, mobility, trap density and many other factors. For macroscopic devices,
these parameters can vary significantly over the active area, making it difficult to
attribute any change in performance to a particular phenomenon. Single nanowire
devices allow for more precise control over and characterization of the properties listed
above, greatly reducing the uncertainty in data interpretation.
In this study, we utilize end-functionalized p-type oligo- and poly-thiophene to
chemically graft the organic component to an n-type ZnO nanowire, producing a p-n
core-shell nanowire from which we subsequently fabricated a single nanowire solar cell.
We end-functionalized P3HT and quaterthiophene with a phosphonic ester and acid,
respectively, and self-assembled the semiconductors onto the ZnO surface in the solution
phase to yield organic shells with thicknesses of about 5-20 nm. We present results on the
synthesis and characterization of the organic/ZnO composites, high-resolution
transmission electron microscopy (TEM) of the organic/ZnO interface, as well as results
on the photovoltaic characteristics of individual nanowire devices. The nanowire devices
yield low efficiencies of about 0.03%, but provide an effective platform for isolating and
studying the many phenomena that affect bulk hybrid solar cell performance.
ZnO nanowires were prepared via solution and vapor-phase synthesis as previously
reported.18-20 Both methods can produce high-quality, single crystalline nanowires with
lengths of several microns and diameters ranging from 30-100 nm. Regioregular P3HT
was prepared from 2-bromo-3-hexyl-5-iodo thiophene through the Grignard metathesis
(GRIM) reaction21 to afford a bromine-terminated polymer with a molecular weight of
~7000 Da as determined via MALDI-TOF. End-functionalization was carried out by
reacting P3HT-Br with butyllithium and then diethylchlorophosphate to yield a
phosphonic ester. Didodecylquaterthiophene (QT) was end-functionalized via a similar
pathway, however, the ester was subsequently hydrolyzed to afford a phosphonic acid.
We note that the P3HT-phosphonic ester was not hydrolyzed to the phosphonic acid since
the reaction conditions using trimethylsilyl bromide degrade the properties of P3HT.
Figure 1 shows the synthetic steps towards functionalization of the two organic materials.
Details of the synthesis and characterization are included in the supporting information.
We obtained core-shell nanowires by stirring a suspension of ZnO nanowires in a 2
mg/ml chlorobenzene solution of the respective functionalized organic components
overnight. The composite materials were purified by centrifugation followed by removal
of the supernatant containing excess organic component. THF was added to the
precipitated nanowires and the purification step was repeated an additional three times.
No efforts were made to vary the concentration of the oligothiophene in organic solvent.
However, we did substitute THF and chloroform for chlorobenzene, and similar results
were obtained. Large-scale quantities (~20 mg) of functionalized nanowires were
prepared from both organic components. Dry powders were stored in a nitrogen box to
prevent oxidation. ZnO/P3HT composites are light purple in color while the ZnO/QT
composites are yellow (Figure 2). The hybrid nanowires can be easily re-dispersed by
sonicating in methanol for 5-10 seconds.
We verified the grafting of oligothiophenes onto ZnO nanowires via UV absorption
spectroscopy. Arrays of ZnO nanowires on quartz substrates were employed in this
experiment because light scattering prevented solutions of the hybrid nanowires from
yielding quality spectra. Grafting of the organic component onto ZnO arrays were carried
out under similar conditions as described for solution-phase nanowires. Figure 2 shows
the schematic structure of the hybrid ZnO nanowire arrays and the solid state absorption
spectra of ZnO/P3HT and ZnO/QT. Figure 2C shows an overlay of the pristine end-
functionalized P3HT spin-coated on a quartz substrate and the P3HT-modified ZnO
nanowires. The peak centered at ~370 nm corresponds to the ZnO absorption, while the
three low energy bands are attributed to the vibronic transitions of P3HT. Figure 2D
shows the spectra of the pristine phosphonic acid-terminated quaterthiophene and an
overlay of the ZnO/QT nanowire array. The ZnO absorption is again observed at ~370
nm while a broad shoulder centered at ~400 nm correlates well with the absorption band
of the free QT oligomer.
High-resolution TEM was used to determine the thickness of the grafted organic
components onto ZnO nanowires. Figures 3A-3C show TEM micrographs of ZnO/P3HT
hybrid structures with P3HT thicknesses ranging from about 7 - 20 nm. The grafting of
P3HT onto ZnO results in a somewhat uneven morphology, yet, complete coverage
throughout the surface of the nanowires is reproducibly observed. Figures 3D-3F show
TEM micrographs of ZnO/QT hybrid nanowires with shell thicknesses ranging from 6 –
13 nm. It is worth pointing out that small-molecule grafting shows significantly smoother
shells compared to P3HT grafts. The reason is unclear, but it may be possible that
nanoscale disorder predominates in self-assembled polymer thin films compared to small-
molecule thin films. This does not imply that P3HT is unable to self-assemble into highly
ordered domains. In fact, we repeatedly observed highly organized domains when
ZnO/P3HT nanowires were imaged with HR-TEM as shown in Figure 3C. This
organization is known as lamellar chain packing and is documented in literature for thin
films of P3HT.22 Brinkman and coworkers observed lamellar organization with P3HT
thin films using HR-TEM bearing similar molecular weights to that of our P3HT.23 From
the known unit cell parameters of P3HT,22,23 the averaged molecular weight, and the
chemical repeat unit of P3HT, one could estimate the length of a P3HT chain and explain
the shell thickness based on the lamellar repeat folding (along c-axis of chain). For
instance, the estimated unit cell parameters for P3HT from literature are, a = 16.2 Å, b =
3.8 Å, and c = 7.8 Å, 21-23 and from an ~7 kDa polymer determined from MALDI-TOF, a
shell thickness of about 6 - 11 nm is estimated. This is assuming the lamellar fold length
is on the order of 5 - 10 nm based on our TEM images (Figure 3C) and from
measurements in literature reports.22,23 A diagrammatic illustration of the chain packing
at the ZnO interface as shown in Figure 4A. We note that this illustration is not to scale
and the intent is to show a plausible scheme of lamellar chain packing on the surface of
The molecular packing of QT on ZnO is highly uniform, as evidenced by the smooth
shell on ZnO from Figure 3E. The length of a QT molecule is about 1.9 nm, however,
shell thicknesses of 6-13 nm are measured by TEM. This can be explained by the QT
bilayers which are strongly interacting via three molecular forces. A schematic
illustration of the proposed solid-state structure is shown in Figure 4B. The planar
backbone of QT forms - interactions with next-nearest neighbor molecules along the
plane of the ZnO nanowire. H-bonding between end-functionalized phosphonic acid
groups from QT bilayers also interact strongly with one another to stabilize the molecular
framework. Phosphonic acids are capable of forming strong P–O–H···O=P hydrogen
interactions with a bond strength of approximately 10-20 kcal/mol.24 Jen and coworkers
previously reported similar interactions with pyrene phosphonic acid molecules that form
polycrystalline films through -interactions and H-bonding.25 In addition to the two
forces that we discussed, QT frameworks are also assisted by van der Waals interactions
between dodecyl side-chains. Therefore, if three bilayers of QT stack vertically, a
thickness of about 9 nm can be estimated. Well-ordered architectures composed of
alternating oligothiophene/ZnO lamellar nanostructures were recently reported by Stupp
and coworkers.26 In their study, the hybrid nanocomposite was employed as a
photoconductor device that generated extremely large spectral responsivities. It has also
been shown that end-functionalized oligothiophenes can self-assemble onto ZnO
nanorods as highly crystalline monolayers.27 The molecules were found to pack in a
herringbone pattern and the packing density correlated well with a core-shell thickness of
~3 nm. The hybrid nanostructures were subsequently fabricated into transistors and
ambipolar behavior was observed.27
The p-n heterojunction nanowires were further characterized by measuring their
photovoltaic characteristics. The organic/ZnO nanowires were dispersed in methanol and
drop cast onto oxidized silicon substrates. The nanowire devices were fabricated by a top
contact approach using electron beam lithography (EBL). Part of the organic shells were
etched away from ZnO nanowires using oxygen plasma through defined EBL patterns.
Next we deposited aluminum electrodes (100 nm) to make ohmic contacts directly onto
ZnO. EBL was utilized again to define and deposit gold electrodes (100 nm) directly onto
oligothiophene shells. Figure 5A shows a schematic configuration of a completed
nanowire device. All photovoltaic measurements were carried out in a vacuum probe
station (10-6 Torr) equipped with a solar simulator (Oriel). Figure 5B and 5C show
current-voltage (I-V) characteristics for the two types of photovoltaic cells investigated
for this work. The plot of a typical ZnO/P3HT nanowire device yielded a JSC of ~0.32
mA/cm2, VOC of ~0.40 V, FF of ~0.28, and an efficiency of ~0.036% under AM 1.5
illumination with 100 mW/cm2 light intensity (Figure 5B). These results are consistent
with literature values of ZnO/P3HT bulk nanowire solar cells where the average
characteristics yielded a Jsc of 0.74 mA/cm2, VOC of 0.17 V, FF of 0.34, and an
efficiency of 0.04%. 9 This suggests that increasing the P3HT thickness will not
significantly improve the performance of the nanowire device (assuming the exciton
diffusion length of P3HT is similar or less than the core-shell thickness of 10 nm). Since
P3HT defines the optical absorption in our nanowire device, we calculated the maximum
current density by integrating the absorption coefficient of end-functionalized P3HT over
the solar spectrum (AM 1.5) 28 and estimated a current density of ~0.75 mA/cm2. This
difference in experimental versus theoretical current density is similar to that in bulk
heterojunction solar cells where a maximum current is calculated at ~19 mA/cm2 and
experimental results yield about 10-12 mA/cm2. 28
Similar photovoltaic results were found for a ZnO/QT nanowire device (Figure 5C)
yielding a Jsc of 0.29 mA/cm2, VOC of 0.35 V, FF of 0.32, and an efficiency of 0.033%.
The important finding in this work is that the open circuit voltage in our devices are
larger than those reported in literature 9,29 from ZnO/P3HT bulk nanowire array devices.
This may suggest that the ZnO/P3HT interface for the grafted polymer is superior to the
bulk spin-coated method. It is also possible that single-nanowire devices increase the
shunt resistance by eliminating shorting paths present in the bulk ZnO array devices.
There remains significant room for improvement of these devices by modifying the
device fabrication procedure and electrode deposition technique. Nevertheless, ideal
characteristics and reproducible measurements were observed in these devices. Although
one could argue that device performance will be limited by the thickness of the grafted
oligothiophene (since more material will absorb more sunlight), the reality is that only the
excitons that are photogenerated within a diffusion length of the respective material will
be effectively dissociated at the interface, transported, and collected into the external
circuit. Smaller band gap semiconductor nanowires and organic semiconductors with
larger absorption coefficients, longer diffusion lengths, and higher mobilities will be best
suited for improving the performance in single nanowire devices. These devices will play
a critical role in understanding fundamental device physics at interfaces and in the
development of new device concepts and technologies.
In summary, we demonstrated basic operation of individual organic/inorganic hybrid
nanowire solar cells. End-functionalized oligo- and poly-thiophenes were grafted onto
ZnO nanowires to produce p-n heterojunction nanowires. The hybrid nanostructures were
characterized via absorption and electron microscopy to determine the optoelectronic
properties and to probe the morphology at the organic/inorganic interface. Hybrid p-n
heterojunction nanowire solar cell devices exhibited ideal characteristics. These
individual test structures will enable detailed analysis to be carried out in areas that have
been difficult to study in nanostructured array and heterojunction devices.
Acknowledgments. This work was supported by the Director, Office of Science, Office
of Basic Energy Sciences, Materials Sciences and Engineering Division, of the U.S.
Department of Energy under Contract No. DE-AC02-05CH11231. TWH thanks the NSF
for a graduate research fellowship. The authors thank Yunjeong Hwang for assistance
with SEM measurements, Michael Moore and Michelle N. Comte for the preparation of
ZnO nanowires, and Ruoxue Yan for the schematic drawings in Figure 2 & 5.
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on ZnO NWs
1.) nBuLi, Toluene
1.) nBuLi, -78 °C
on ZnO NWs
Figure 1. Synthetic pathway for end-functionalizing (A) poly(3-hexylthiophene) (P3HT)
and (B) didodecylquaterthiophene (QT) with a phosphonic ester and acid, respectively.
The oligothiophenes are self-assembled onto ZnO nanowires overnight in 2mM
700 300400 500600 700
ZnO/P3HT ZnO/QT ZnO NWs
Figure 2. (A) Schematic structure of the oligothiophene-modified ZnO nanowire
substrate utilized for UV-vis absorption measurements. (B) Digital photograph of the as-
prepared solution-phase ZnO nanowires, ZnO/QT, and ZnO/P3HT in their dry powder
form. (C) A UV-vis absorption spectra of the P3HT-modified ZnO nanowires, and (D)
QT-modified ZnO nanowires. An overlay of the pristine polymer and oligomer is also
included in the respective spectra.
Figure 3. Transmission electron microscopy (TEM) images of (A-C) ZnO/P3HT core- Download full-text
shell nanowires, and (D-F) ZnO/QT core-shell nanowires.