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Simultaneous Enhancement of Open-Circuit Voltage,
Short-Circuit Current Density, and Fill Factor in
Polymer Solar Cells
Zhicai He, Chengmei Zhong, Xun Huang, Wai-Yeung Wong, Hongbin Wu,*
Liwei Chen,* Shijian Su, and Yong Cao
The growing interests in polymer solar cells (PSCs)[1,2] are
related to their unique advantages such as low-cost manufacturing and easy processability over large-area size via printing
and roll-to-roll coating technologies,[3] and compatibility with
flexible substrates and materials availability, which enable them
for potential applications in low-cost photovoltaic systems.
Although the power conversion efficiency (PCE) of state-of-theart PSCs has already exceeded 7% in scientific literature,[4,5]
further improvements are needed for mass production and
practical applications, especially simultaneous enhancement
of short-circuit current density (JSC),[6] open-circuit voltage
(VOC),[7,8] and fill factor (FF)[9] are in urgent need. Among all
of the approaches to improve the PCE, the most common and
successful strategy is developing new low bandgap donor materials[5,10–12] to maintain a broad overlap with the solar spectrum
and ensure effective harvesting of solar photons, which leads
to higher JSC. However, since there is a trade-off between light
harvesting (JSC) and open-circuit voltage (VOC),[13] a gain in PCE
can be expected only if a decrease of VOC can be avoided. Apart
from this, other approaches based on device physics have also
been studied and proved to be effective, but these methods
normally can only improve one or two key parameters of PSCs
substantially at a time. For example, incorporation of an additive,[14] controlling the growth rate of film,[15] or optimizing film
thickness in combination with proper annealing[16] alter the
active layer morphology and enhance JSC and FF; inserting an
Z. C. He, C. M. Zhong, Prof. H. B. Wu, Prof. S. J. Su, Prof. Y. Cao
Institute of Polymer Optoelectronic Materials and Devices
State Key Laboratory of Luminescent Materials and Devices
South China University of Technology
Guangzhou 510640, P. R. China
E-mail: hbwu@scut.edu.cn
X. Huang, Prof. L. W. Chen
Suzhou Institute of Nano-Tech and Nano-Bionics
Chinese Academy of Sciences
398 Ruoshui Road
Suzhou Industrial Park
Suzhou 215123, P. R. China
E-mail: lwchen2008@sinano.ac.cn
Prof. W.-Y. Wong
Institute of Molecular Functional Materials (Areas of Excellence Scheme
University Grants Committee, Hong Kong)
and Department of Chemistry and Centre
for Advanced Luminescence Materials
Hong Kong Baptist University
Waterloo Road, Hong Kong, P.R. China
DOI: 10.1002/adma.201103006
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optical spacer layer solely increases the JSC;[17] and the tandem
cell approach greatly enhances either the JSC or VOC depending
on the device structure.[18–19] Besides the methods described
above, it was reported by our group that alcohol/water-soluble
polymer cathode interlayers can enhance VOC of PSCs prepared
from polyfluorene and polycarbazole derivatives.[20–22] In parallel, other polyelectrolytes have also been reported to have similar functions,[23,24] and in particular the Bazan group improved
the efficiency of PSCs based on poly[N-9´´-hepta-decanyl-2,7carbazole-alt-5,5-(4´,7´-di-2-thienyl-2´,1´,3´-benzothiadiazole)
(PCDTBT) to 6.5% via incorporation of a conjugated polyelectrolyte interlayer.[25] Although it is indispensable to obtain simultaneous enhancement in VOC, JSC, and FF in order to push the
PCE of PSCs towards their theoretical limit (≈10%),[26] there are
few methods so far in the literature that are able to simultaneously improve all three parameters towards realization of the
high efficiency of 10%.
In this study, we apply our method of incorporating an
alcohol/water-soluble conjugated polymer, poly [(9,9-bis(3´-(N,Ndimethylamino) propyl)-2,7-fluorene)-alt-2,7-(9,9–dioctylfluorene)]
(PFN, see Scheme S1, Supporting Information) as a novel
cathode interlayer in polymer:fullerene bulk heterojunction solar
cells based on [6,6]-phenyl C71-butyric acid methyl ester (PC71BM)
and promising low bandgap donor materials such as PCDTBT
(Scheme S1, Supporting Information)[27] and thieno[3,4-b]thiophene/benzodithiophene (PTB7, Scheme S1, Supporting
Information).[4] The synthesis of PFN has been described elsewhere[28] and the applications of this material for highly efficient
polymer light-emitting devices (PLEDs) and other optoelectronic
devices have been reported in previous publications by our
group[29,30] and other groups.[31] The PSCs incorporating the PFN
interlayer show significant and simultaneous enhancement in
JSC, VOC, and FF, which leads to a PCE of 6.79% and 8.37% for
PCDTBT and PTB7 devices, respectively, and is the best reported
in literature to date for PSCs. Due to drastic improvement in efficiency and easy utilization, this method opens new opportunities for PSCs from various material systems to improve towards
10% efficiency. Similar to all-printed red-green-blue (RGB) emission PLEDs demonstrated previously,[32] the fabrication of these
high-performance PSCs is compatible with low-cost, roll-to-roll
processing,[3] which is now on the eve of a critical development
where round-robins or interlaboratory studies are possible.[33]
Figure 1a presents the current density versus voltage (J–V)
characteristics of the best PCDTBT PSCs with and without
the PFN interlayer between the active layer and the Al cathode
under 1000 W m−2 air mass 1.5 global (AM 1.5 G) illumination. For each type, over 200 devices were made from over
© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 1. The effect of the PFN interlayer on PCDTBT:PC71BM solar cell performance. a) Current density versus voltage (J–V) characteristics of devices
with (solid circles) and without the PFN interlayer (open circles) measured
under 1000 W m−2 AM 1.5 G illumination. The solid lines represent simulated J–V characteristics obtained from numerical modeling. b) J–V characteristics of devices with (solid circles) and without the interlayer (open
circles) in the dark. c) External quantum efficiency (EQE) spectra of solar
cells with (solid circles) and without the PFN interlayer (open circles).
40 individual films. A high PCE of 6.55 ± 0.09% was achieved
in optimized devices with interlayer (with the highest value
of 6.73%), while a 3.90 ± 0.17% (with the maximum value of
4.02%) was obtained for the control devices without interlayer.
Incorporation of the PFN interlayer leads to a simultaneous
enhancement in VOC (0.90 V vs. 0.70 V), JSC (12.1 mA cm−2 vs.
11.4 mA cm−2) and fill factor (62% vs. 49%), resulting in a
drastic improvement in the PCE by 68% (6.55% vs. 3.90%) over
the control devices.
Conventional cathode interlayers using low work function
metal such as Ca are widely applied to form ohmic contact with
the active layer and improve electron extraction.[13] We also prepared control devices using 3 nm thick Ca film as the cathode
interlayer. It shows that Ca interlayer only results in moderate
improvement in JSC (12.4 mA cm−2 vs. 11.4 mA cm−2), but
no improvement in FF (46%) and VOC (0.72 V), resulting in a
PCE of 4.11%. Application of the PFN interlayer between the
active layer and Ca not only retains the JSC (12.7 mA cm−2) but
also improves FF (59%) and VOC (0.90 V), leading to a PCE of
6.79%. Thus we believe the functions of PFN must involve a
mechanism different from the Ca interlayer.
Under AM 1.5G illumination, the device with a thin (≈5 nm)
PFN interlayer shows high VOC of 0.91 ± 0.02V, which is significantly higher than that of the control devices (0.71 ± 0.02 V).
In dark, the device with PFN interlayer exhibits a turn-on
voltage of about 0.8–1.0 V while it is only 0.5–0.6 V for the
control device (Figure 1b), implying that the built-in potential
(Vbi) across the device, which is the upper limit of the attainable VOC in PSCs,[20,26] drastically increases upon utilization of
the PFN cathode interlayer. A similar increase in Vbi has been
observed in PLEDs devices utilizing PFN interlayers. An interfacial electrical dipole due to the strong interaction and alignment of polar amino group at the side chain of PFN with the
active layer and a consequent vacuum level shift at the interface was identified between PFN and active layer of PLEDs by
using small angle X-ray diffraction and electroabsorption techniques.[30,34,35] We suspect similar interfacial dipole between
PFN and PCDTBT:PCBM active layer could be the origin of
enhanced Vbi in our devices.
To visualize the interfacial dipole upon the incorporation
of the PFN interlayer, we directly probe the surface potential
change using scanning Kelvin probe microscopy (SKPM) with
the experimental setup depicted in Figure 2c. The surface potential is uniform over both the active layer and the interlayer areas,
but the surface potential of interlayer area is about 300 mV
more positive than that of the active layer without interlayer
(Figure 2b,d). At the edge of the interlayer area, the topography
shows an abrupt bump because of accumulated material, but
the surface potential remains the same as the rest of the interlayer, therefore, the difference in surface potential is due to
chemical and/or physical nature of the interlayer rather than
the height contrast. This indicates a microscopic electric dipole
moment with the positive charge end pointing toward the Al
electrode and the negative charge end pointing toward the
active layer. The direction of this dipole moment is aligned with
the built-in potential originated from the asymmetric contact at
the electrodes; therefore, the actual built-in potential across the
device is reinforced as a result of the superposition (Figure 2f).
For an order-of-magnitude rough estimate, the electrical field
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(a)
Height
0.5 µ m
(c)
LU
MO
(e)
LU
MO
Al
PFN Interlayer
Active Layer
0
V
HO
MO
PEDOT:PSS
ITO
Anode
Glass
LU
MO
-0.5
(b)
Surface
Potential
(D
)
Tip
Vac+ Vdc
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(d)
(f)
LU
MO
(A
)
(D
)
Metal
Cathode
(A
)
(D
)
0.8 V
LU
MO
HO
MO
0.4
Anode
LU
MO
(A
)
(D
)
Metal
Cathode
(A
)
µ
0
Figure 2. Electric dipole moment of the PFN interlayer probed by SKPM. Topographic image (a) and surface potential image (b) of device active layer
area partially covered with the interlayer. Scale bars in both images represent 10 mm. c) Schematic illustration of the experimental setup. d) Crosssectional line profile of the topographic and surface potential images in (a) and (b). The energy band structure of the devices without (e) and with
(f) the interlayer under short-circuit conditions (with zero external applied bias) constructed from the observed SKPM results.
due to the interfacial dipole layer is about 6 × 105 V cm−1 (0.3 V
potential difference over a thickness of 5 nm) (Figure 2f), while
the built-in field in devices without the PFN interlayer under
short-circuit condition (with zero external applied bias) is about
7 × 104 V cm−1 (0.7 V potential difference over a thickness of
100 nm) (Figure 2e). Therefore, the incorporation of the PFN
interlayer not only enhances Vbi, but also exerts a strong electrical field at the active layer/cathode interface that may strongly
influence charge transport and extraction. This new approach
of increasing Vbi and VOC of the device by exploiting an interfacial dipole layer does not directly change the energy levels of
active materials and their bandgap. It retains the solar spectral
range utilized by the active material and does not sacrifice light
absorption. Therefore, the trade-off between VOC and JSC can
be avoided and we even observe simultaneous improvement as
discussed in following sections.
In addition to the enhancement of Vbi, the PFN interlayer
also avoids Fermi level pinning between the metal cathode and
the acceptor material PCBM in the active layer.[35] Therefore,
the PFN/Al cathode can be expected to be an electron-selective
electrode in nature. Indeed, the electrical leakage for the devices
with an interlayer is considerably suppressed (Figure 1b). The
superior diode quality to that of the control devices (characterized with much higher rectification ratio at ±2 V, i.e., 283 000 vs.
5000) indicates that the interlayer is efficient in blocking holes
and collecting electrons. As a consequence of reduced leakage
current, a slightly higher VOC can be expected according to the
Shockley equation.[21] In addition, a reduced series resistance in
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the devices with an interlayer can further ensure a reduced VOC
loss due to potential drop at the bulk of the active layer and the
interface with the electrodes.
Besides the significant improvement in VOC, an increase of
JSC from 11.4 mA cm−2 to 12.1 mA cm−2 (by an extra increase of
5%) was observed upon the incorporation of the interlayer. The
external quantum efficiency (EQE) spectra of the devices illuminated by monochromatic light are shown in Figure 1c. An
average EQE of 70% in range of 350–650 nm are observed for
both the devices with and without the interlayer. Note that theoretical JSC obtained by integrating the product of the EQE data
in Figure 1c and the AM 1.5G solar spectrum (12.0 mA cm−2
and 11.4 mA cm−2 for the devices with the interlayer and
without the interlayer, respectively) perfectly matches the
values measured from J–V characteristics (Figure 1a; 12.1 and
11.4 mA cm−2, respectively).
Simultaneously with the enhancement in VOC and JSC, the
FF of the optimized devices is found to be 62%, much higher
than that of the control devices (49%), suggesting that the
charge transport properties are substantially improved. In order
to make a realistic evaluation on the apparent charge carrier
mobility in the active layer, we measure J–V characteristics of
single charge carrier devices and then fit the results using the
space-charge-limited current (SCLC) model (Figure 3) and the
Mott–Gurney law that includes
field-dependent mobility, given
2
V −Vbi [36]
bi )
by J = 98 g rg 0: 0 (V −V
exp
(
$
), where g r g0 is the dielecL
L3
tric permittivity of the active layer, L is the thickness of the active
layer, : 0 the zero-field mobility, and $ the field activation factor
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is used (Figure S2 and Table S1, Supporting Information). At a
typical electric field of 105 V cm−1(corresponding to an applied
voltage of 1 V across the bulk of a 100 nm device), apparent hole
mobilities of 1.7 × 10−3 cm2 V−1 s−1 and 3.2 × 10−4 cm2 V−1 s−1 and
apparent electron mobilities of 5.8 × 10−3 cm2 V−1 s−1 and 3.3 ×
10−3 cm2 V−1 s−1 have been determined for the devices with and
without the PFN interlayer, respectively. In other words, upon
the incorporation of the PFN interlayer, both the hole and electron mobilities increase and a more balanced charge transport
in the devices can be achieved. While the microscopic origin
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Figure 3. The effect of the PFN interlayer on charge transport properties. J–V characteristics of hole-only device in configuration of indium
tin oxide (ITO)/poly(3,4-ethylene-dioxythiophene) (PEDOT)/active layer/
MoO3/Au (a) and electron-only device in configuration of ITO/Al/active
layer/Ca/Al (b). The solid circles and open circles are for the device with
and without the interlayer, respectively. The solid lines represent the best
fitting from the SCLC model that includes field-dependent charge carrier
mobility with the fitting parameters summarized in Table S1 (Supporting
Information).
for the enhanced mobility remains uncertain at this point, we
speculate that the increased Vbi across the device and the strong
local field at the active layer/cathode interface due to the PFN
interlayer may facilitate charge carriers to escape shallow traps
and thus improve their mobility (Figure 2f).
The PFN interlayer is also found to prevent the build-up
of space charge under device working conditions. Figure 4a,b
show the experimental power law dependence of photocurrent
on incident light intensity. At a high effective voltage, Veff, of
2.1 V, whereVeff = V0 – V, and V0 is the compensation voltage
at which net photocurrent Jph = 0 and V is the applied voltage;
Veff = 2.1 V corresponds to negative biases of –1.4 V and –1.2 V
for devices with and without the PFN interlayer, respectively.
The power exponents are both 0.90, much higher than the 0.75
exponent for the space-charge-limited scenario,[37,38] indicating
no build-up of net space charges in either devices even at the
highest illumination intensity; this is due to the high electric
field as well as high and balanced charge carrier mobilities, as
confirmed by the SCLC measurements. However, at a low effective voltage of 0.1 V,which corresponds to an external bias of
0.6 V for devices without the PFN interlayer, the power exponent decreases to 0.84 (Figure 4b) for the device without the
PFN interlayer, implying a buildup of space charge and the subsequent bimolecular recombination.[38] In contrast, the power
exponent for devices with the PN interlayer remains 0.90 at Veff =
0.1 V, corresponding to an external bias of 0.8 V for devices
with the PFN interlayer (Figure 4a) and indicating no space
charge buildup. The results clearly identify that the PFN interlayer effectively prevents space charge build-up even under low
effective voltages, at which maximal power output condition of
PSCs usually appears.
The incorporation of the PFN interlayer also improves the
charge collection efficiency under working conditions. Figure 4c
shows the J–V characteristics in a wide reverse bias range under
AM 1.5G illumination. The results are plotted as the net photocurrent ( J ph = J L − J D ) dependence on the effective applied
voltage Veff, where J L and J D are the current density under
illumination and in the dark, respectively, V is the applied
voltage, and V0 is the compensation voltage at which Jph = 0.[37]
At a large reverse voltage (i.e., Veff = 2 V), Jph reaches saturation for both devices, suggesting all photogenerated excitons
are dissociated into free carriers and all carriers are collected
at the electrodes without any bimolecular recombination.[13,14,39]
In this case saturation current density Jsat is only limited by
the amount of absorbed incident photon flux (Nphoton), thus
the maximal obtainable exciton generation rates (Gmax, given
by J sat = e G max L where L is the thickness of the active layer)
are essentially the same (9.4 × 1027 m−3 s−1) in both devices
under AM 1.5G illumination. At the slightly lower effective
voltage range (0.5 V < Veff < 2 V), Jph – Veff characteristics for
both devices approach saturation and overlap with each other.
For example, at the short current density conditions, the photocurrent densities Jph are 94% and 90% of the saturation current
density Jsat (Jph at Veff = 2 V) for devices with and without the
interlayer, respectively. Interestingly, in the low effective voltage
range (Veff < 0.5 V), Jph – Veff characteristics for the two devices
show large differences. For example, at the maximal power
output condition (corresponding to Veff = 0.2 V for both devices),
Jph/Jsat is 74% for the device with the PFN interlayer while it
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Figure 4. The effect of the PFN interlayer on PCDTBT:PC71BM solar
cell incident light intensity dependence. Double logarithmic plot of
photocurrent density as a function of the incident light intensity for
PCDTBT:PC71BM solar cells with (a) and without (b) the PFN interlayer
measured under effective voltage of 2.1 V (solid black squares) and 0.1 V
(open circles). The lines represent the best power fitting. c) Photocurrent
density versus effective voltage (Jph – Veff ) characteristics of devices with
(solid circles) and without the interlayer (open circles) under constant
incident light intensity (AM 1.5G, 1000 W m−2). Solid lines represent the
simulation results.
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is only 61% for the control device without the PFN interlayer.
Since the ratio Jph/Jsat is essentially the product of exciton dissociation efficiency and charge collection efficiency,[40] a decreased
Jph/Jsat suggests either a reduced exciton dissociation efficiency
or charge collection efficiency, with the latter suggesting that
bimolecular recombination begins to dominate and usually
lead to a lower FF.[39] The recombination in devices is manifested in the deviation of the photocurrent from the square-root
dependence on effective voltage, which is one of the signatures
of recombination-limited photocurrent in PSCs.[37] The superior Jph – Veff characteristics from devices with interlayer clearly
identifies the effect of interlayer on reducing bimolecular
recombination at the low effective voltage, at which maximal
power output condition of PSCs usually appears.
Such reduced bimolecular recombination in devices with
an interlayer can be attributed to the observed higher charge
mobility therein. Since charge transport is described by driftdiffusion equations and diffusion-controlled photocurrent is
dominant in the low effective voltage regime,[40] the diffusion
coefficients of the hole and electron, which are associated with
mobility and given by the Einstein relation D = kBe T : where
D is the diffusion coefficient, kB is the Boltzmann’s constant,
T is the temperature, e is the electron charge, and μ is the
mobolity) will increase in the device with interlayer. Therefore,
the enhancement in charge diffusion and charge transport is
responsible for the distinctly different Jph/Jsat when compared
with the control device.
To further investigate the effect of the interlayer on photocurrent generation and collection and fully quantify the device
enhancement, we applied two independent numerical models
to adequately describe the characteristics of the devices. The
first model is based on the geminate recombination theory of
Onsager,[40,41] including drift and diffusion of charge carriers
and a field- and temperature-dependent photocurrent generation rate. The second model involves a field-independent charge
pair generation rate and a charge-density-dependent recombination coefficient that can explain the dependence of the
photocurrent on the bias in the operating regime, as proposed
by Shuttle et al.[39] (see Supporting Information for details of
the modeling). Using input parameters from Table S2 (Supporting Information) for charge mobility and most of the other
remaining the same (Table S3, Supporting Information) for the
devices with and without the PFN interlayer, the experimental
J–V characteristics for the actual solar cells under AM 1.5 G
illumination and in dark are consistently described by the both
models, as shown in Figure S3 (Supporting Information).
The lines in Figure 4c denote the simulation results made
with the same set of input parameters, which are in good agreement with the experimental data. It is clear that higher mobilities in the devices with the interlayer are responsible for the
enhanced Jph/Jsat in the low field regime. In a high field regime,
both simulation and experimental data indicate that Jph/Jsat
becomes field-independent and thus weakly dependent on
charge carrier mobility, which can be employed to explain the
nearly identical Jph/Jsat in the high field regime for both devices.
It is worth mentioning that this approach was also successfully applied in other material systems. Simultaneous
enhancement in the JSC, VOC, and FF was also demonstrated in
PTB7:PC71BM PSCs when the PFN interlayer was incorporated
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(CPVT) for certification, and a certified PCE of 8.37% was
obtained (Figure 5b and Figure S4, Supporting Information).
Independent certification by Solarmer Energy Inc. yielded a
slightly lower PCE of 7.65%, which can be attributed to lag time
(18 d) between two the laboratories.
In conclusion, we have successfully demonstrated that
simultaneous enhancement in the open-circuit voltage, shortcircuit current density, and fill factor can be achieved in highly
efficient polymer solar cells by simply incorporating a thin
layer of alcohol/water-soluble polymer as the cathode interlayer,
resulting in a power conversion efficiency up to 6.79% and a
certified 8.37% for PCDTBT and PTB7 devices, respectively.
The effects of the interlayer on the improvement of device performance is shown to be threefold: an enhanced built-in potential across the device due to the existence of interface dipole,
improved charge-transport properties, elimination of the
buildup of space charge, and reduced recombination loss due
to the increase in built-in field and charge carrier mobility. Further optimization is conceivable because the interfacial dipole
of thicker interlayer can result in as large as 0.5 V enhancement in built-in potential. The approach reported here provides
a simple and versatile method to optimize polymer solar cells
and may set the efficiency of devices towards the goal of 10%
from various material systems.
Experimental Section
Figure 5. The effect of the PFN interlayer on PTB7:PC71BM solar cell performance. a) J–V characteristics of devices with Al (open circles), PFN/Al
(solid circles), Ca/Al (open squares), PFN/Ca/Al (solid squares) cathode
measured under 1000 W m−2 AM 1.5G illumination, which give PCE of
5.00%, 8.04%, 7.13%, and 8.22%, respectively. b) CPVT-certified J–V
characteristics of PTB7:PC71BM solar cells with a PCE of 8.370%. c) EQE
spectra of the solar cells with a PFN/Ca/Al cathode.
(Figure 5a). We achieved a high PCE of 8.22% for devices by
incorporating the PFN cathode interlayer. After encapsulation,
the devices were sent to the National Center of Supervision &
Inspection on Solar Photovoltaic Products Quality of China
Adv. Mater. 2011, 23, 4636–4643
Fabrication of Polymer Solar Cells: PCDTBT and PTB7 were purchased
from 1-material Chemscitech Inc. (St-Laurent, Quebec, Canada) and
used as received. The device structure was ITO/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/polymer:PC71BM
blend/Al. The PCDTBT:PCBM (1:4 by weight) active blend layer with a
thickness of 90 nm was prepared by spin-coating the chlorobenzene
solution at 1000 rpm for 2 min. The blend ratio of PTB7:PC71BM iwa
1:1.5 by weight and the active layer was spin-cast from mixed solvent
of chlorobenzene/1,8-diiodoctane (97:3 vol%).[4,14] The interlayer
material was dissolved in highly polar solvents such as methanol under
the presence of small amount of acetic acid and its solution was spincoated on the top of the obtained active layer to form a thin interlayer
of 5 nm. Determination of the thickness of the interlayer followed a
previously published methods[29] and thickness control was adjusted by
the concentration of the solution and the spin speed between 600 rpm
to 2500 rpm. The thickness of the interlayer was determined by a surface
profiler (Alfa Step-500, Tencor), in combination with extrapolation from
an absorbance–thickness curve that assumed a linear dependence of
absorbance at 380 nm on film thickness. Al electrode (100 nm) was
evaporated through a shadow mask to define the active area of the
devices (2 × 8 mm2).
Characterization and Measurement: SKPM measurements were carried
out on an Agilent 5500 atomic force microscope (Agilent technologies,
USA) equipped with a homemade vacuum glove box. The samples were
transferred into a glove box and the active layer/interlayer boundary was
identified using an optical field monitor on the atomic force microscope
and then imaged under nitrogen gas environment using the standard
SKPM mode. Conducting atomic force microscopy (AFM) tips (NSC19/
Ti-Pt, Mickomasch, San Jose, CA) used for this study had a typical
spring constant of 0.68 N m−1 and a resonance frequency of 80 kHz.
Typical scan line frequency was 0.3 Hz and each image contained
512 × 512 pixels. The AC modulation for the Kelvin probe scan was 3 V in
amplitude and 10 kHz in frequency. The resulted topographical images
were plane fit with Picoimage software but the surface potential images
were unprocessed original data.
The PCE was determined from J–V curve measurement (using a
Keithley 2400 sourcemeter) under 1 sun, AM 1.5G (air mass 1.5 global)
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spectrum from a solar simulator (Oriel model 91192) (100 mW cm−2).
Masks made from laser beam cutting technology with well-defined area
size of 2.00, 3.14, or 10.0 mm2 were attached to define the effective
area for accurate measurement. All of the masked and unmasked tests
gave consistent results with relative errors within 3%. Solar simulator
illumination intensity was determined by a monocrystal silicon reference
cell (Hamamatsu S1133, with KG-5 visible color filter) calibrated by
the National Renewable Energy Laboratory (NREL). The theoretical JSC
obtained by integrating the product of the EQE with the AM 1.5G solar
spectrum perfectly agreed with the measured value to within 1%. The
spectral mismatch factors (M) were calculated according to standard
procedure,[42] and M values of 0.977 and 1.011 were used to obtain
correct photocurrent and efficiency for PCDTBT and PTB7 devices,
respectively.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors thank Dr. Min Yun of the National Center of Supervision &
Inspection on Solar Photovoltaic Products Quality and Dr. Gang Li of
Solarmer Energy Inc. for device performance verification. H.-B.W. thanks
the National Basic Research Program of China (No. 2009CB623602),
National Nature Science Foundation of China (No. 50990065, 61177022
and 60906032), and Program for New Century Excellent Talents in
University and Fundamental Research Funds for the Central Universities
(2009ZZ0047) for financial support. L. C. thanks the National Basic
Research Program of China (2010CB934700), the Natural Science
Foundation of China (20973122), the Hundred Talents program and the
Knowledge Innovation Program (No. KJCX2-YW-H21) of the Chinese
Academy of Science for financial support. W.-Y.W. acknowledges financial
support from a General Research Grant and a Collaborative Research
Grant from the Hong Kong Research Grants Council (Grant no: HKBU
202607 and HKUST2/CRF/10), a FRG grant from Hong Kong Baptist
University (FRG2/09-10/091), and the Areas of Excellence Scheme,
University Grants Committee of HKSAR, P.R. China (AoE/P-03/08).
Received: August 5, 2011
Revised: August 10, 2011
Published online: September 9, 2011
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