Nanotechnology 17 (2006) 5387–5392 doi:10.1088/0957-4484/17/21/017
A large interconnecting network within
, Yun-Yue Lin1
, Hsi-Hsing Lo1
, Cheng-Hsuan Chen2
, Sz-Chian Liou2
and Wei-Fang Su1,2,3,4
1Department of Materials Science and Engineering, National Taiwan University, Taipei 106-17, Taiwan
2Center for Condensed Matter Sciences, National Taiwan University, Taipei 106, 106-17, Taiwan
3Graduate Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106-17, Taiwan
Received 24 June 2006, in final form 6 September 2006
Published 13 October 2006
This is a study of hybrid photovoltaic devices based on TiO2
-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV).
We use TiO2
nanorods as the electron acceptors and conduction pathways.
Here we describe how to develop a large interconnecting network within the
photovoltaic device fabricated by inserting a layer of TiO2
nanorod hybrid active layer and the aluminium
electrode. The formation of a large interconnecting network provides better
connectivity to the electrode, leading to a 2.5-fold improvement in external
quantum efficiency as compared to the reference device without the TiO2
nanorod layer. A power conversion efficiency of 2.2% under illumination at
565 nm and a maximum external quantum efficiency of 24% at 430 nm are
achieved. A power conversion efficiency of 0.49% is obtained under Air
Mass 1.5 illumination.
(Some figures in this article are in colour only in the electronic version)
Conjugated polymers have great utility for fabrication of large area, physically flexible and low cost solar cells [1, 2]. A basic requirement for making efficient photovoltaic devices is that the free charge carriers produced upon photo-excitation of the photoactive material must be transported through the device to the electrode without recombining with oppositely charged carriers. Photovoltaic devices merely composed of conjugated polymers as the only active material have extremely low electron mobility and, thus, limited performance. Recent developments have shown that the use of interpenetrating electron donor–
4 Address for correspondence: Department of Materials Science and Engineering, National Taiwan University, Taipei 106-17, Taiwan.
acceptor heterojunctions such as polymer:fullerene [2–4], polymer:polymer  and polymer:nanocrystal [6–8] can yield highly efficient photovoltaic conversions. Electron acceptors have been intermixed at the nanometre scale with an organic semiconducting polymer to obtain high charge separation yield. Following electron transfer, both electron and hole must be transported to the electrode before back recombination can occur. However, in some cases, electron transport is limited by inefficient hopping along poorly formed conduction paths. Thus, an enhanced charge transport route is desirable to achieve efficient electron conduction. One-dimensional semiconductor nanorods are preferable for offering direct pathways for electric conduction [1,9–11]. Huynh et al have produced high performance solar cells by combining CdSe nanorods with poly(3-hexylthiophene) (P3HT) .
Titanium dioxide (TiO2) nanocrystals are promising as
an electron accepting material in hybrid organic:inorganic photovoltaic device applications. Several different conju-gated polymers have been used for the polymer: TiO2
solar cells, such as poly[2-methoxy-5-(2 -ethyl-hexyloxy)-l,4-phenylene vinylene] (MEH-PPV) [12–15], MEH-PPV derivatives , poly[2-methoxy-5-(3,7 -dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) , P3HT [8,18,19] and water soluble polythiophene , etc. Many approaches produced devices by infiltrating polymers into sintered TiO2
nanoporous thin film. Polymer:TiO2solar cells made from spin
coating a blending of polymer–TiO2nanocrystals solution have
been presented less often. The photoinduced charge transfer and recombination of TiO2 nanorods and MEH-PPV hybrid
has been reported, which suggests that the MEH-PPV:TiO2
nanorod heterojunctions may be used as potential active ma-terial for photoconversion [21, 22]. In this study, we have fabricated an MEH-PPV:TiO2 nanorod heterojunction
photo-voltaic device. The device performance is further enhanced by inserting a thin layer of TiO2nanorods between the photoactive
material and the electrode for an efficient electron transport.
2. Experimental details
The controlled growth of high aspect ratio anatase titanium dioxide nanorods was accomplished by hydrolyzing titanium tetraisopropoxide according to the literature with some modifications . Typically, oleic acid (120 g, Aldrich, 90%) was stirred vigorously at 120◦C for 1 h in a three-neck flask under Ar flow, then allowed to cool to 90◦C and maintained at this temperature. Titanium isopropoxide (17 mmol, Aldrich, 99.999%) was then added into the flask. After stirring for 5 min, trimethylamine-N-oxide dihydrate (34 mmol, ACROS, 98%) in 17 ml water was rapidly injected. Trimethylamine-N-oxide dihydrate was used as a catalyst for polycondensation. This reaction was continued for 9 h to have complete hydrolysis and crystallization. Subsequently, the TiO2nanorod product was obtained (4 nm in diameter, 20–
40 nm in length). The nanorods were washed and precipitated by ethanol repeatedly to remove any residual surfactant. Finally, the TiO2 nanorods were collected by centrifugation
and then redispersed in chloroform or toluene.
The indium–tin-oxide (ITO)/poly (3,4-ethylenedioxythio phene)-poly(styrenesulfonate) (PEDOT:PSS)/MEH-PPV:TiO2
nanorods/Al device was fabricated in the following manner. An ITO glass substrate with a sheet resistance of 15/square (Merck) was ultrasonically cleaned in a series of organic solvents (ethanol, methanol and acetone). A 60 nm thick layer of PEDOT:PSS (Aldrich) was spin-cast onto the ITO substrate; this was followed by baking at 100◦C for 10 min. TiO2
nanorods in toluene and MEH-PPV (Aldrich, molecular weight 40 000–70 000 g mol−1) in chloroform/1,2-dichlorobenzene (1:1 to 100:1,vol/vol) were thoroughly mixed and spin-cast on the top of the PEDOT:PSS layer. The thickness of MEH-PPV:TiO2 nanorod film was 180 nm. Then, the 100 nm Al
electrode was vacuum deposited on the hybrid layer.
By inserting the TiO2nanorod thin film between the
MEH-PPV:TiO2 nanorod hybrid and Al electrode, an improved
device with a configuration of ITO/PEDOT:PSS/MEH-PPV:TiO2 nanorods/TiO2 nanorods/Al was made. The TiO2
Figure 1. Anatase TiO2nanorod structure images observed by TEM
and HRTEM (inset).
nanorods dissolved in chloroform:ethanol=4:1 solution were spin-cast on the top of the MEH-PPV:TiO2nanorods hybrid
to obtain a TiO2 nanorod thin film of 70 nm thickness. In
order to minimize the redissolving of MEH-PPV:TiO2 layer,
we have spin-coated concentrated nanorods solution (0.05 ml, 25 mg ml−1) on the MEH-PPV:TiO2nanorod hybrid at very
high speed (6000 rpm). An ITO/PEDOT:PSS/MEH-PPV:TiO2
nanorods/MEH-PPV/Al photovoltaic device was fabricated as a reference. A 130 nm MEH-PPV layer on an MEH-PPV:TiO2
nanorod hybrid was made by spin coating. The MEH-PPV in chloroform(20 mg ml−1)solution was also spin-cast at a very high speed of 6000 rpm.
The crystalline structure of the nanorods was studied using x-ray diffraction (XRD) (Philips PW3040 with filtered Cu Kα radiation (λ = 1.540 56 ˚A)). The analysis of TiO2
nanorods was performed using a JOEL JEM-1230 transmission electron microscope (TEM) operating at 120 keV or a 2000FX high resolution transmission electron microscope (HRTEM) at 200 keV. The film thickness was determined by anα-stepper (DEKTAK 6M 24383). The film morphology was observed by atomic force microscopy (AFM) (Digital Instruments Nanoscope III). The current–voltage (I–V) characterization (Keithley 2400 source meter) was performed under 10−3Torr vacuum, with monochromatic illumination at a defined beam size (Oriel Inc.). The Air Mass (AM) 1.5 condition was measured using a calibrated solar simulator (Oriel Inc.) with irradiation intensity of 100 mW cm−2. Once the power from the simulator was determined, a 400 nm cutoff filter was used to remove the UV light. The 80 nm PPV and MEH-PPV:TiO2 films were cast on quartz substrate to obtain UV–
Visible absorption (Jasco V-570) and photoluminescence (PL) (Perkin-Elmer FS-55) measurements.
3. Results and discussion
The TEM image of TiO2nanorods (figure1) reveals that the
TiO2nanorod dimension is 20–40 nm in length and 4–5 nm in
diameter. The HRTEM image indicates that the TiO2nanorods
2 300 400 500 600 700 800 0.00 0.25 0.50 0.75 1.00 0.00 0.25 0.50 0.75 1.00 MEH-PPV absorption MEH:PPV:TiO2 absorption MEH-PPV PL MEH:PPV:TiO 2 PL Wavelength(nm) Luminscence(arbitrary uni t) Abso rbance(arbitrary unit)
Figure 2. Absorption spectra of MEH-PPV films (solid line) and
MEH-PPV:TiO2nanorod (52 wt%) hybrid (dashed line) of thickness 80 nm, and photoluminescence spectra of MEH-PPV films (dotted line) and MEH-PPV:TiO2nanorod (52 wt%) hybrid (dash–dotted line), excited at 450 nm.
Figure 2 shows the absorption and PL spectra of pristine MEH-PPV and MEH-PPV:TiO2nanorod hybrid films
respectively. The optical density of the absorption spectrum in the hybrid increases with respect to the pristine polymer, and whose form is the result of contributions from each component. The absorption at wavelength less than 350 nm results mainly from the TiO2 nanorods. In contrast, the yield of the PL
emission decreases substantially, suggesting the occurrence of significant PL quenching in the hybrid . Decreases in PL yield are attributed to the quenching of the MEH-PPV PL emission by the TiO2nanorods, acting as an electron accepting
species, where significant charge separation takes place due to large interfacial areas for exciton dissociation.
As a starting point, we made a standard hybrid de-vice structure similar to those previous reported poly-mer:nanocrystal photovoltaic devices, resulting in devices with external quantum efficiencies of the order up to 10%. A schematic diagram of our standard device configura-tion is shown in figure 3(a), which consists of a transpar-ent indium–tin-oxide (ITO) conducting electrode, poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT: PSS), the MEH-PPV:TiO2 nanorod hybrid film, and an
alu-minium (Al) electrode. We have further modified the device configuration by including an additional electron conducting layer of TiO2 nanorods sandwiched between the active layer
and the aluminium electrode to improve device performance, as shown in figure3(b).
We used tapping-mode AFM to investigate the structures and film morphology of these devices. Figure4(a) shows the smooth topography of an MEH-PPV:TiO2nanorod hybrid with
roughness 2 nm. The TiO2nanorods were randomly distributed
in the polymer matrix for the interconnecting work formation. Figure 4(b) shows the phase image of an MEH-PPV: TiO2
nanorod hybrid. Tapping-mode AFM can also give information about the materials at the film surface via phase images. Because a hard material generally shows a positive phase shift with respect to a soft material due to the cantilever oscillation being related to the power dissipated in a nonelastic tip–sample interaction , the bright areas in figure4(b) are interpreted as the harder material of TiO2nanorods and the darker areas as the
Figure 3. (a) The schematic structure of standard configuration
MEH-PPV:TiO2nanorod hybrid photovoltaic devices. (b) Schematic structure of MEH-PPV:TiO2nanorod hybrid photovoltaic device included a TiO2nanorod layer.
soft material of polymer. A homogenous distribution of TiO2
nanorods in polymer is observed in figure4(b). Figure4(c) shows the surface topography of a spin-cast TiO2 nanorod
layer on an MEH-PPV:TiO2 nanorod hybrid. A feature of
aggregation of nanorod structure was found. The TiO2nanorod
thin film exhibits a porous structure of relatively high film roughness. Figure4(d), the phase image of a TiO2 nanorod
thin film, shows a single phase of bright areas consisting of TiO2nanorods. The dark region in figure4(d) could be seen as
deep pores of the TiO2nanorod thin film, which is consistent
with the surface topography observed in figure4(b). From the results above, we have constructed an interconnecting network in an MEH-PPV:TiO2nanorod photoactive hybrid and a thin
film composed of mere TiO2nanorods sandwiched between a
hybrid layer and Al electrode through our process conditions. An optimal composition between polymer and nanorods is required to achieve balanced exciton dissociation and charge transport. We investigated the effect of TiO2 nanorod
compositions on device performance. The best performance of this type of device was obtained at a concentration of MEH-PPV:TiO2 (52 wt%). Lower power conversion efficiencies
were obtained either at lower TiO2 concentration
(MEH-PPV:TiO2(40 wt%)) or higher concentration (MEH-PPV:TiO2
(64 wt%)). This implies that, under those conditions, MEH-PPV:TiO2 (40 wt%) or MEH-PPV:TiO2 (64 wt%),
polymer–TiO2 interfacial areas were not maximized for
exciton dissociation or that the donor–acceptor interpenetrating networks formed cannot meet the requirements for the most efficient charge transport. We have varied the compositions of TiO2in the hybrid, the film thicknesses of the active layer and
the types of solvent to achieve the optimal performance of the standard configuration device; however, the external quantum efficiency was limited to less than 10%.
Based upon considering the energy levels of the respective materials in the device, a TiO2nanorod layer inserted between
the active layer and the aluminium electrode is appropriate for offering a better connectivity of electron transport path to
Figure 4. AFM images showing the surface morphology of an MEH-PPV:TiO2nanorod hybrid film and TiO2nanorod film. (a) Height image of spin-cast film of MEH-PPV:TiO2(52 wt%) nanorod hybrid. The image size is 1.5 μm × 1.5 μm, and the vertical scale is 30 nm. (b) Phase image of MEH-PPV:TiO2(52 wt%) nanorod hybrid film. The image size is 1.5 μm × 1.5 μm, and the vertical scale is 30◦. (c) Height image of spin-cast film of TiO2nanorods on MEH-PPV:TiO2nanorod hybrid film. The image size is 1.5 μm × 1.5 μm, and the vertical scale is 30 nm. (d) Phase image of TiO2nanorod film on MEH-PPV:TiO2nanorod hybrid film. The image size is 1.5 μm × 1.5 μm, and the vertical scale is 30◦.
the electrode. The functions of the TiO2 nanorod layer can
be explained by the band diagram in figure5(a). The energy level diagram demonstrates that the TiO2 nanorod layer acts
as a hole-blocking electron-transporting layer in this device. As the electron–hole pairs are generated by incident light, an efficient charge separation occurs at the interface of the MEH-PPV:TiO2nanorod hybrid. Electrons move toward the
aluminium electrode and holes move toward the ITO electrode. The addition of the continuous TiO2nanorod thin film allows
for the current to be conducted effectively and also prevents electrons from back recombination with holes in the MEH-PPV. The TiO2 nanorod layer acts as a hole-blocking layer
because of lower valence band value. In contrast, on inserting a thin MEH-PPV layer instead, the device is energetically unfavorable for electron transport.
Figure 5(b) shows the current–voltage response of the devices with and without a TiO2 nanorod layer. The device
containing a TiO2 nanorod layer increases the short-circuit
current density by a factor of 2.5 with respect to a device without the layer. For a comparison, the thin TiO2 nanorod
layer was replaced with a thin MEH-PPV layer and a∼3 order of magnitude of decrease in the short-circuit current was found. Apart from the hole-blocking electron-transporting function of TiO2nanorod layer mentioned above, the interfaces introduced
(MEH-PPV:TiO2/TiO2 and TiO2/Al) seem more beneficial
to charge transport as compared to the MEH-PPV:TiO2/Al
contact. The TiO2nanorod layer can be connected to the TiO2
nanorods in the active hybrid. In addition, the rough surface of the TiO2 layer can lead to stronger contact and increased
contact area to the Al electrode. Besides, inserting this layer can create a second interfacial area for exciton dissociation that might increase the charge transfer rate. To introduce an additional titanium oxide thin film as a hole-blocking electron-transporting layer through various approaches has been presented in producing higher efficiency heterojunction organic solar cells [14,15,25,26]. Here we present a thin film of crystalline TiO2nanorods made via a fully solution process
that can lead to improvement in device performance. The TiO2nanorod thin film could be explored as a promising
hole-blocking electron-transporting layer in photovoltaic devices. An equivalent circuit has frequently been used to describe the electric behaviour of a photovoltaic device . We further analysed the characteristics of the devices based upon this equivalent circuit. The current density versus voltage characteristics can be described by the following equation:
I= I0× exp eU− I RS nkT −1 +U− I RS RSH − IPH (1)
where I0 is the saturation current, e is the magnitude of
2 0.00 0.25 0.50 0.75 1.00 -0.008 -0.006 -0.004 -0.002 0.000 400 450 500 550 600 650 0 5 10 15 20 25 External Quantum efficiency(%) Wavelength (nm) Cur re nt De nsi ty( m A /c m 2 ) Voltage(V) Dark Light -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 1x10-8 1x10-7 1x10-6 1x10-5 1x10-4 1x10-3 1x10-2 1x10-1 MEH-PPV:TiO 2 MEH-PPV:TiO 2/TiO2 MEH-PPV:TiO2/MEH-PPV Current density(m A/cm 2 ) Voltage(V) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 0.0 0.3 0.6 0.9 1.2 1.5 Cu rr e n t d e n s it y (m A /c m 2) Voltage (V) C u rr e n t d e n s ity (m A /c m 2 ) Voltage(V) (a) (c) (b) (d)
Figure 5. (a) Flat band energy-level diagram of ITO/PEDOT:PSS/MEH-PPV:TiO2nanorods/TiO2nanorods/Al devices. (b) Plots of current
density as the function of applied voltage for three different configuration devices under 0.09 mW cm−2illumination at 560 nm.
(MEH-PPV:TiO2nanorods (52 wt%) (dashed line); MEH-PPV:TiO2nanorods (52 wt%)/TiO2nanorods (solid line) and MEH-PPV: TiO2 nanorods (52 wt%)/MEH-PPV (dotted line)). (c) Plot of current density versus voltage in the dark (dashed line); and under 0.05 mW cm−2 illumination at 565 nm (solid line, Voc= 0.86 V, Jsc= −0.0035 mA cm−2, FF= 0.35 and η = 2.2%). The inset shows the external quantum efficiency versus wavelength of the device. The device structure is ITO/PEDOT:PSS/MEH-PPV:TiO2nanorods (52 wt%)/TiO2nanorods/Al. (d) The corresponding I –V curve of the ITO/PEDOT:PSS/MEH-PPV:TiO2nanorods (52 wt%)/TiO2nanorods/Al device at AM 1.5 illumination(100 mW cm−2). (Voc= 1.15 V, Jsc= −1.7 mA cm−2, FF= 0.25 and η = 0.49%). The logarithmic I –V characteristic of the device is shown in the inset.
ideality factor, k is Boltzmann’s constant, T is the absolute temperature, RS is the series resistance, RSH is the shunt
resistance and IPH is the photocurrent [2, 27]. The current–
voltage characteristics are largely dependent on the series and shunt resistance. A lower series resistance means that higher current will flow through the device. High shunt resistance corresponds to fewer shorts or leaks in the device. The ideal cell would have a series resistance approaching zero and shunt resistance approaching infinity. The series resistance can be estimated from the inverse slope at a positive voltage where the I–V curves become linear. The shunt resistance can be derived by taking the inverse slope of theI–V curves around 0 V. RS= lim V→∞ dV dI (2) RSH ≈ dV dI (V =0) RS RSH. (3)
The RS and RSH were analysed from the I–V curves of the
devices (figure 5(b)); it is found that a significant, nearly 60%, reduction inRSoccurred as the TiO2nanorod layer was
introduced into the device. A slight reduction of the shunt resistance was observed also. The series resistance can be
expressed as the sum of the bulk and interfacial resistance. It is likely that two interfaces that have been introduced (MEH-PPV:TiO2/TiO2and TiO2/Al) combined with the TiO2nanorod
layer offer a much lower magnitude of series resistance as compared to the MEH-PPV:TiO2/Al contact. The introducing
of the TiO2layer decreases the series resistance in the device
and thereby increases the current.
The performance of the device with a structure of ITO/PEDOT:PSS/MEH-PPV:TiO2 nanorods (52 wt%)/TiO2
nanorods/Al was evaluated. The I–V characteristic of the device exhibits a short-circuit current density (Jsc) of
−0.0035 mA cm−2, an open circuit voltage(Voc)of 0.86 V and a fill factor (FF) of 0.35. A power conversion efficiency(η)of 2.2% is achieved under 0.05 mW cm−2illumination at 565 nm (figure5(c)). The inset shows the external quantum efficiency (EQE) of the device under illumination. A maximum EQE of 24% under 0.07 mW cm−2at 430 nm is achieved. Figure5(d) presents the characteristics of the device tested under AM 1.5 illumination with an intensity of 100 mW cm−2. TheJsc, FF,
andVocare−1.7 mA cm−2, 0.25, and 1.15 V, respectively for
the device, yielding a power conversion efficiency of 0.49%. Work to optimize the device efficiency is still under way, to achieve better device efficiency.
In conclusion, we have used TiO2nanorods as efficient electron
acceptors and transport components in the active layer of our hybrid organic photovoltaic device. A TiO2 nanorod
layer between the active layer and the electron-collecting electrode provides an enlarged interconnecting network for electrical transport near the aluminium electrode, leading to a 2.5-fold increase in the short-circuit current under illumination. These results suggest that one-dimensional TiO2 nanorods are a promising material for hybrid organic
solar cell applications. Further improvements in the device performance could be accomplished by controlling the nanorod sizes and by improving the polymer:TiO2 nanorod
The authors thank the National Science Council of the Republic of China (NSC94-2120-M-002-012) and the US Air Force Office of Scientific Research (AFOSR-AOARD-04-23) for financial support of this research. Thanks are also due to Mr An-Jey Su of the University of Pittsburgh for editing our manuscript.
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