N-Doping of thermally polymerizable fullerenes as an electron transporting layer for inverted polymer solar cells

Article abstract of DOI:10.1039/c1jm10214a

A novel [6,6]-phenyl-C₆₁-butyric acid methyl styryl ester (PCBM-S) was synthesized and employed as an electron transporting interfacial layer for bulk heterojunction polymer solar cells with an inverted device configuration. After the deposition of PCBM-S film from solution, the styryl groups of PCBM-S were polymerized by post-thermal treatment to form a robust film which is resistive to common organic solvents. This allows the solution processing of upper bulk heterojunction film without eroding the PCBM-S layer. Additionally, the PCBM-S was n-doped with decamethylcobaltocene (DMC) to increase the conductivity of the film, which resulted in a significantly improved power conversion efficiency from 1.24% to 2.33%. The improved device performance is due to the decrease of series resistance and improved electron extraction property of the n-doped PCBM-S film.

Full text of DOI:10.1039/c1jm10214a

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PAPER
n-Doping of thermally polymerizable fullerenes as an electron transporting
layer for inverted polymer solar cells†
Namchul Cho, Hin-Lap Yip, Steven K. Hau, Kung-Shih Chen, Tae-Wook Kim, Joshua A. Davies,
*
David F. Zeigler and Alex K.-Y. Jen
Received 14th January 2011, Accepted 11th March 2011
DOI: 10.1039/c1jm10214a
A novel [6,6]-phenyl-C61-butyric acid methyl styryl ester (PCBM-S) was synthesized and employed as
an electron transporting interfacial layer for bulk heterojunction polymer solar cells with an inverted
device configuration. After the deposition of PCBM-S film from solution, the styryl groups of PCBM-S
were polymerized by post-thermal treatment to form a robust film which is resistive to common organic
solvents. This allows the solution processing of upper bulk heterojunction film without eroding the
PCBM-S layer. Additionally, the PCBM-S was n-doped with decamethylcobaltocene (DMC) to
increase the conductivity of the film, which resulted in a significantly improved power conversion
efficiency from 1.24% to 2.33%. The improved device performance is due to the decrease of series
resistance and improved electron extraction property of the n-doped PCBM-S film.
electronic properties including (1) appropriate energy level
1. Introduction
matching between the electrodes and the active layer to facilitate
unipolar charge extraction to the corresponding electrode; (2)
high conductivities to minimize the Ohmic loss from series
resistance; (3) high solvent resistance to facilitate multilayer film
deposition from solutions; (4) low optical absorption to maxi-
mize the number of photons reaching the active layer; and (5)
chemically inert to prevent interfacial reactions with electrodes
for achieving long-term stability.
Polymer solar cells (PSCs) are a promising source of low-cost
alternative energy due to their potential scalability and the ease
of fabrication.1–3 Recently, a very encouraging device perfor-
mance of greater than 7% power conversion efficiency (PCE) has
been demonstrated.4,5 The improvement of PCE was realized by
the development of novel donor polymers,6–9 fullerene accep-
tors,10,11 and optimization of the bulk heterojunction (BHJ)
morphology.12–14 Apart from the importance of achieving higher
PCEs, improving the stability of PSCs is equally important. In
general, PSCs with a conventional structure are comprised of
a BHJ active layer sandwiched between an indium tin oxide
(ITO) anode and a low work-function metal cathode. However,
low work-function metal electrodes are quite unstable under
ambient conditions.15 To overcome this problem, PSCs with an
inverted device structure had been developed which enables the
use of stable and printable high work-function metals as hole
collecting top electrodes and low work-function metal oxide as
electron collecting bottom electrodes. This results in significantly
improved stability in an ambient environment.16,17
Several classes of materials had been explored as possible
electron selective materials for inverted PSCs. The most widely
studied ones are n-type inorganic metal oxides including titanium
oxide18 and zinc oxide.19,20 Despite their good electron selective
properties, the sensitivity of their electrical properties to the
surface adsorption of oxygen and UV-irradiation may compli-
cate the use of these materials in PSCs.21–23 Modifications of the
metal oxide surface using fullerene-based self-assembled mono-
layers24,25 or thin films26 had been demonstrated as an efficient
strategy to further improve the interfacial properties between
metal oxide and BHJ film. These have resulted in enhanced
device efficiency and stability. Ultra-thin insulating aluminium
oxide27 and cesium oxide obtained through thermal decomposi-
tion of cesium salt28 have also been used to modify the ITO work
functions to facilitate electron collection in inverted cells.
Although a large number of studies have been performed on
n-type inorganic interfacial materials, only few attempts have
been made on polymer or small molecule based n-type interfacial
materials for inverted PSCs even though they have great
advantages compared to inorganic materials, such as the versa-
tility of tuning their optical, electrical, and physical properties
through organic synthesis. In addition, polymeric materials are
Reversing the polarity of charge-collecting electrodes in
inverted PSC can be achieved by adopting appropriate electron-
transporting and hole-transporting materials between the ITO/
BHJ and BHJ/metal interfaces, respectively. Efficient interfacial
materials should possess several desired physical, chemical, and
Department of Materials Science and Engineering, University of
Washington, Seattle, Washington, 98195, USA. E-mail: ajen@u.
washington.edu; Fax: +1 206 543 3100; Tel: +1 206 543 2626
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1jm10214a
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intrinsically more flexible, making them a better option for
plastic solar cells that are bendable and rugged.
2. Experimental section
Materials synthesis
One of the limiting factors for directly using conjugated
polymers or small molecules as interfacial materials in organic
solar cells is their relatively low conductivities, which results in
high series resistance and poor Ohmic contact with the elec-
trodes.29 As a result, improving the conductivity of interfacial
transport layers through chemical doping is required for the
realization of high performance solar cells. This strategy has been
widely used in vacuum-evaporated small molecule-based solar
cells in which the small molecule semiconductor and dopant can
be co-deposited to achieve controllable and stable doping.29
However, chemical doping of solution processed transport layer
is more challenging. Unlike small molecule-based multilayer
devices that can be fabricated by subsequent deposition through
vacuum processes, polymer-based interfacial materials should
possess good solvent resistance to prevent solvent-induced
erosion during the multi-layer film deposition process. In addi-
tion, concurrent development of stable dopants that are
compatible with solution processing is necessary.
All chemicals were purchased from Aldrich Chemical Corpora-
tion unless otherwise specified. Regio-regular poly(3-hexylth-
iophene) (P3HT) and PCBM were purchased from Rieke Metals,
Inc. and American Dye Source, Inc, respectively, and were used
as received. 4-Vinylbenzyl alcohol (2) and PCBA were synthe-
sized according to literature methods.36,37
Synthesis of PCBM-S
A solution of PCBA (40 mg, 0.045 mmol), 4-vinylbenzyl alcohol
(8.97 mg, 0.067 mmol) and 4-dimethylaminopyrindine-4-tolue-
nesulfonate (19.67 mg, 0.067 mmol) in dichloromethane (30 mL)
in a round bottom flask was stirred for 5 min at room temper-
ature. After 5 min, 1,3-diisopropyl carbodiimide (28.14 mg,
0.225 mmol) was added and the solution was allowed to stir for
12 hours. The coupling reaction was monitored by thin-layer
chromatography. After removal of solvent, the crude product
was purified by column chromatography using toluene/hexane
(1 : 1, v/v). The PCBM-S was collected by filtration and washed
with methanol to give a brown solid (20 mg, 55% yield). 1H NMR
(300 MHz, CDCl3, 25 ꢁC, TMS): d ¼ 2.19 (m, 2H), 2.59 (t, 2H),
2.93 (m, 2H), 5.12 (s, 2H), 5.32 (d, J ¼ 16.8, 1H), 5.80 (d, J ¼
18.3, 1H), 6.68 (q, J ¼ 10.8, 1H), 7.28–7.59 (m, 7H), 7.93(d, J ¼
Thermal- or photo-crosslinkable charge-transporting mate-
rials had been demonstrated as efficient interfacial materials that
provide adequate solvent resistance for the fabrication of poly-
mer light-emitting diodes.30 This class of materials has also been
employed as hole-transporting layers in PSCs to improve the
hole collection property.31–33 Recently, cross-linkable fullerene
materials were developed and employed as n-type interfacial
layers between ZnO and the active layer to enhance the electron
collection property in inverted PSCs.26 However, without ZnO,
fullerene is ineffective as a good electron-transporting material
(ETM) in inverted PSCs because of its low conductivity
compared to metal oxides. Therefore, n-doping of fullerene is
required to enhance the conductivity of the electron-transporting
layer. n-Type doping has been studied in vacuum-deposited small
molecule based solar cells in which the fullerene electron-trans-
porting layer is doped with decamethylcobaltocene (DMC) to
achieve significantly improved PCE compared to the undoped
cells.34
4.8, 2H); 13C NMR (125 MHz, CDCl3, 25 C, TMS): d ¼ 22.4,
ꢁ
33.7, 34.1, 51.9, 66.2, 79.9, 114.4, 128.3, 128.5, 128.6, 132.1,
135.3, 136.3, 137.6, 138.1, 140.8, 141.0, 142.1, 142.2, 142.2, 142.3,
142.9, 143.0, 143.1, 143.8, 144.0, 144.4, 144.5, 144.7, 144.8, 145.1,
145.1, 145.2, 145.9, 147.8, 148.8. Anal. Calcd for C80H20: C,
94.85, H, 1.99. Found: C, 93.28, H, 1.48%.
Absorption, thermal property and morphology measurements
A Perkin-Elmer Lambda-9 spectrophotometer was used to
measure UV-Vis spectra. The thermal property was studied using
differential scanning calorimetry (DSC) (DSC2010, TA instru-
ments) under a heating rate of 10 ꢁC minꢂ1 and a nitrogen flow of
50 mL minꢂ1. The surface morphology of the polymer films was
studied using the tapping mode atomic force microscopy (AFM)
from a Veeco Nanoscope III controller.
A crystallographic study of the fullerene complex with DMC
reported by Konarev et al. showed that the fullerene anion and
DMC cation form a stable three-dimensional framework with
solvent molecules.35 Because the solid-state ionization energy of
DMC (3.3 eV)34 is smaller than the LUMO of PCBM-S
(ꢀ3.7 eV), efficient n-doping of PCBM-S can be achieved
through electron transfer from the HOMO of the dopant to the
LUMO of the fullerene derivative.
Fabrication and characterization of polymer solar cells
To fabricate the inverted solar cells, ITO-coated glass substrates
(15 U per ,) were cleaned in sequential ultrasonic baths of
detergent water, de-ionized water, acetone, and isopropyl
alcohol. Substrates were then treated with the oxygen plasma for
5 min. A thin layer of PCBM-S was prepared by spin-coating the
PCBM-S solution (5 mg mlꢂ1 of PCBM-S in chloroform with
different amounts of DMC) and then annealed at 200 ꢁC for
30 min in the glove box. Afterward, PCBM (American Dye
Source, Inc., 99.0% purity) and P3HT (Rieke Metals, Inc., 4002-
E grade) (0.7 : 1, weight ratio) in a chlorobenzene solution (60
mg mlꢂ1) were spin-coated on the PCBM-S layer in a glove box
The focus of the current study is to demonstrate the improved
device performance of inverted PSCs by incorporating robust
fullerene side-chain polymers that are chemically doped with
DMC to form an efficient electron-transporting interfacial layer.
The fullerene polymer films produced by thermal polymerization
on ITO substrates possess good solvent resistance. Chemical n-
doping of fullerene polymers with DMC is shown to significantly
enhance the PCE. The improved device performance is attributed
to the decreased series resistance and efficient electron extraction
via the n-doped PCBM-S polymer films. We also investigate n-
channel organic thin-film transistors (OTFTs) using n-doped
fullerene polymers to characterize the effect of doping on the
electrical properties of the PCBM-S.
ꢁ
and annealed at 160 C for 10 min to get an active layer with
a thickness of ca. 200 nm. After annealing, a PEDOT:PSS
solution (H. C. Starck, Clevios 4083) was spin-coated onto the
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active layer to get a layer with 50 nm thickness.24 These samples
were annealed for 10 min at 120 ꢁC on a hot plate. A metal
electrode (Ag) was then vacuum-deposited at a base pressure of
2 ꢃ 10ꢂ6 Torr at a rate of 2 A sꢂ1. The J–V characteristics of the
ꢀ
solar cells were tested under ambient conditions using a Keithley
2400 SMU and an Oriel xenon lamp (450 W) with an AM1.5
filter. The device illumination area of 0.0314 cm2 was defined by
using a physical mask to minimize photocurrent generation from
the edge of the electrodes. The light intensity was calibrated to
100 mW cmꢂ2 using a calibrated silicon solar cell with a KG5
filter, which is traced to the National Renewable Energy Labo-
ratory. The performance of the devices was averaged over at least
5 devices for each processing condition.
Fabrication and characterization of OTFTs
Heavily n-doped silicon substrates with a 300 nm thick thermally
grown SiO2 dielectric (from Montco Silicon Technologies, Inc.)
were used to fabricate top contact OTFTs. The substrates were
treated with HMDS by vapor phase deposition in a vacuum oven
ꢁ
(200 mTorr, 100 C, 2 hours) before spin-coating of PCBM-S.
The undoped and doped PCBM-S films were spin-coated in the
glove box. The 1 wt% of PCBM-S in chloroform was used to
obtain a film thickness of 50 nm. Interdigitated source and drain
electrodes (W ¼ 9000 mm, L ¼ 90 mm, W/L ¼ 100) were defined
by evaporating a 100 nm Au at 10ꢂ6 Torr. An Agilent 4155B
semiconductor parameter S6 analyzer was used to characterize
OTFT properties. The electron mobility was calculated in the
saturation regime from the linear fit of (Ids)1/2 vs. Vgs. The Vt was
obtained as the x intercept of the linear section of the plot of
(Ids)1/2 vs. Vgs.
Scheme 1 Synthetic route of fullerene polymer.
3. Results and discussion
Synthesis and characterization
We have designed PCBM-S to match several requirements as
follows. First, the monomer (PCBM-S) should be easily poly-
merizable in the solid state. Second, the size of the reactive units
should be as small as possible to maintain reasonable electron
mobility of the polymer. Finally, the polymer film should be
insoluble in common organic solvents to allow multilayer solu-
tion-processing and the resulting polymer film also need to have
good morphological stability during device processing. The
PCBM-S was synthesized by the esterification of [6,6]-phenyl-
C61-butyric acid (PCBA) with 4-vinyl benzyl alcohol (2), as
shown in Scheme 1. The PCBM-S was characterized by NMR
spectroscopy and elemental analysis.
Fig. 1 DSC thermograms of the PCBM-S with a heating rate of
10 ꢁC minꢂ1
.
polymerization and therefore, we expect that the polymer film
should have good morphological stability.
Absorption properties
Thermal properties
Fig. 2 shows the UV-Vis-NIR absorption spectra of the thin films
of undoped and 6 wt% DMC doped PCBM-S on the glass
substrate measured before and after washing with chloroben-
zene. All films were prepared by spin-coating, then polymerized
by thermal annealing at 200 ꢁC for 30 min under inert conditions.
The small decrease in the absorption intensities observed for
both undoped and 6 wt% DMC doped P-PCBM-S films after
rinsing with chlorobenzene demonstrates effective solvent resis-
tance of the films.
The thermal properties of PCBM-S were studied by DSC. As
shown in Fig. 1,ꢁPCBM-S exhibited a glass-transition tempera-
ture (Tg) of 114 C. After annealing above the Tg, the PCBM-S
showed exothermic peaks at 170 ꢁC and 230 ꢁC, which are
believed to be the thermal polymerization of styrene linkage.38 In
the second scan, these peaks are no longer detectable. This
implies that long-range molecular motion of the polymerized
PCBM-S (P-PCBM-S) is significantly limited after thermal
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Table 1 Summary of device performance with various ETMs
PCE
(%)
Voc
V
/
Jsc/
mA cmꢂ2 FF
Rs/
U cmꢂ2
ETM
No ETM
P-PCBM-S
0.08
1.24
1.46
2.33
2.32
2.16
2.53
0.21
0.54
0.61
0.65
0.66
0.63
0.64
2.60
8.47
7.95
8.96
8.42
8.53
9.06
0.21 315
0.27 1110
P-PCBM-S–DMC (1%)
P-PCBM-S–DMC (3%)
P-PCBM-S–DMC (5%)
P-PCBM-S–DMC (7%)
P-PCBM-S-DMC
(10%)
0.31
0.40
0.42
0.40
0.44
350
40
40
50
50
no significant increase in PCE of the 10 wt% doped device
compared to the 3–5 wt% doped device, we suggest that 3–5 wt%
is an effective doping concentration for present experiments. The
slight variation in PCE of devices doped with higher concentra-
tion of DMC (7 and 10 wt%) can be attributed to the unopti-
mized morphology of the electron transport layers.
Fig. 2 Absorption spectra of the undoped and DMC doped P-PCBM-S
polymer films on glass substrate before and after washing with
chlorobenzene.
Photovoltaic properties
Fig. 3 shows the J–V characteristics of inverted polymer solar
cells with and without modification of ETM. Devices of archi-
tecture ITO/ETM/P3HT:PCBM/PEDOT:PSS/Ag were tested
under simulated AM 1.5G illumination at 100 mW cmꢂ2. The
results show clear evolution of J–V curves as a function of the
doping concentration of DMC in P-PCBM-S films.
Morphology
The ability of controlling the thickness and morphology of the
electron-transporting layer (ETL) is very important for opti-
mizing device performance since the series resistance across the
device is strongly dependent on the thickness of the ETL. In
addition, a smooth and pinhole free ETL film prevents the
leakage current at the interface and plays a crucial role in the
lifetime and efficiencies of PSCs. To investigate how
the morphology of the P-PCBM-S film affects the device
performance, we studied the surface morphology of n-doped
P-PCBM-S films prepared under different spin coating condi-
tions and evaluated the corresponding device performance.
Devices based on 3 wt% DMC doped P-PCBM-S films spin-
coated by 7000 rpm show almost the same device performance
(PCE: 2.30%, Voc: 0.65 V, Jsc: 8.90 mA cmꢂ2; FF: 0.41) with the
devices spin-coated by 4000 rpm (PCE: 2.33%, Voc: 0.65 V, Jsc:
8.96 mA cmꢂ2, FF: 0.40). However, decreasing the spin speed to
2000 rpm results in shorting of devices. This shorting results from
the poor morphology that originates from dewetting between the
P-PCBM-S layer and ITO. The significant morphological
differences of n-doped P-PCBM-S films as a function of the spin
speed were investigated by AFM (Fig. 4).
A summary of the photovoltaic parameters for the devices is
given in Table 1. Note that the PCE increases significantly from
1.24% to 2.53% with an increase in the dopant (DMC) concen-
tration from 0 to 10 wt%. The best performance was obtained
from devices with 10 wt% doped PCBM-S. These improvements
are attributed from the enhanced electrical conductivity and
improved electron extraction efficiency of P-PCBM-S as a result
of the increased doping level. As shown in Table 1, the enhanced
conductivity of the P-PCBM-S causes an overall decrease in
series resistance (Rs) from 1110 U cm2 for undoped P-PCBM-S to
50 U cm2 for 10 wt% doped P-PCBM-S, as well as an increase in
fill factor (FF) from 0.27 to 0.44. There is also a pronounced
enhancement in Jsc, Voc, FF, and thus PCE for 3 wt% and 5 wt%
doped P-PCBM-S relative to the undoped device. Since there is
The DMC doped P-PCBM-S films spin-coated at 2000 rpm
have a very rough surface (9.7 rms) due to dewetting, whereas the
films spin-coated by the speed of 4000 and 7000 rpm give rela-
tively smooth surfaces (4.0 rms and 2.7 rms, respectively). The
surface morphology of the P3HT:PCBM active layer deposited
on the interfacial layers prepared under different spin coating
conditions shows no observable change in the surface
morphology. It is probably due to the relatively thick BHJ film
(ꢀ200 nm) used in our study, making its morphology less
sensitive to the underlying interfacial layer. This further suggests
that the major reason for the poor performance of the device with
the rough P-PCBM-S film prepared at lower spin speed is due to
the potential formation of shorting paths between the ITO
substrate and BHJ film, which resulted in electrical leakage at the
interface.
Fig. 3 J–V curves of inverted polymer solar cells as a function of the
doping concentration of DMC in P-PCBM-S films.
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Fig. 4 AFM images of P-PCBM-S (a), 3% DMC doped P-PCBM-S cast
by different spin speeds: 2000 rpm (b), 4000 rpm (c), and 7000 rpm (d).
There are two plausible mechanisms that could explain the
dewetting of the film spin-coated at the speed of 2000 rpm. One is
that the surface energy of the P-PCBM-S film could be signifi-
cantly changed by DMC compared to the undoped P-PCBM-S
film. Compared to doped P-PCBM-S films, undoped P-PCBM-S
films on ITO show low surface roughness (1–1.5 rms) regardless
of the spin speeds. Another reason for the morphological
difference from different spin speed is that the faster solvent
evaporation rate at higher spin speed may alter the drying
kinetics of the film, which resulted in smoother film by sup-
pressing the dewetting property.39
Fig. 5 (a) Transfer characteristics of OTFTs with undoped and DMC
doped P-PCBM-S as active layers at varying doping concentrations. (b)
Output curves at zero gate voltages for the same devices.
Table 2 OTFT electrical properties for undoped and DMC doped P-
PCBM-S transistors at different doping concentrations measured in the
N2-filled glove box
Organic field-effect transistors
The electrical conductivity of the charge transport materials used
in polymer solar cells is an important factor that governs the
performance of the devices. Thus, we further investigated
the electron-transporting properties of n-doped P-PCBM-S via
the fabrication of n-type OTFTs. The 1, 3, 5, 7 and 10 wt% of
DMC doped PCBM-S solutions were prepared by the same
method used for fabricating solar cells. The n-doped PCBM-S
solutions were spin-coated onto a heavily doped n++ Si wafer
with a 200 nm thick SiO2 layer with a top contact geometry and
Au was used as the source and drain electrodes (see the Experi-
mental section for detailed fabrication protocols).
ETM
m/cm2 Vꢂ1 sꢂ1 Ion/Ioff Vt/V
s/S mꢂ1
P-PCBM-S
1.45 ꢃ 10ꢂ4
103
42.0
6.8
1.65 ꢃ 10ꢂ7
1.53 ꢃ 10ꢂ6
8.54 ꢃ 10ꢂ2
2.84 ꢃ 10ꢂ1
5.96 ꢃ 10ꢂ1
P-PCBM-S–DMC (1%)
P-PCBM-S–DMC (3%)
P-PCBM-S–DMC (5%)
P-PCBM-S–DMC (7%)
P-PCBM-S–DMC (10%)
1.35 ꢃ 10ꢂ5
102
10
2.76 ꢃ 10ꢂ5
ꢂ20.2 1.43 ꢃ 10ꢂ2
—
—
—
<10
<10
<10
—
—
—
With increasing the doping concentration from 0 to 3% in P-
PCBM-S films, the OTFT devices showed well known doping
effects such as the negative shift of threshold voltages and the
decrease of on–off ratio as shown in Fig. 5(a) and Table 2.40 The
device with undoped P-PCBM-S shows field-dependent transfer
characteristics, whereas the devices with highly doped (5–10 wt
%) P-PCBM-S exhibit field-independent properties with small
Ion/Ioff ratio of <10, which is a characteristic of conducting
materials. In addition, Fig. 5(b) clearly shows that the output
current of devices at zero gate bias condition increased upon
increasing the amount of dopant, indicating that the doping
concentration directly affects the amount of charge carriers and
is responsible for the increased electrical conductivity of the
doped P-PCBM-S films.
The conductivity of the films was derived from gated two-
terminal measurements and the conductivity can be calculated
from the equation of s(Vg) ¼ (L/A)(Id/Vd), where L and A are the
channel length and the cross-sectional area of the device,
respectively. The conductivities calculated from the slope of Vd–
Id curves at zero gate voltage are summarized in Table 2. There is
a pronounced enhancement in the conductivity of 1.65 ꢃ 10ꢂ7
S
cmꢂ1 for undoped P-PCBM-S to 5.96 ꢃ 10ꢂ1 S cmꢂ1 for 10 wt%
doped P-PCBM-S. These values are comparable with previously
reported results.40 Moreover, the overall trend of the increased
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output current with doping concentration is well matched with
the decrease of series resistance in the solar cells (Table 1).
For example, there are significant improvements in both the
output current of OTFT devices and series resistance of solar cell
devices from 1% doped P-PCBM-S to 3% doped P-PCBM-S.
However, they are saturated after 3% of the doping concentra-
tion. In particular, the annealed device with 10% of DMC in
P-PCBM-S shown in Fig. 5(b) had more than 6 orders of
magnitude higher drain current than that of the device with
undoped P-PCBM-S. The systematic transition of device char-
acteristics including threshold voltage (Vt), conductivity, and on–
off current ratio (Ion/Ioff) as a function of the doping concen-
tration is obvious and it further elucidates the relationship
between the electrical properties of the n-doped P-PCBM-S
layers and the device performance of solar cells.
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4. Conclusion
18 C. Waldauf, M. Morana, P. Denk, P. Schilinsky, K. Coakley,
S. A. Choulis and C. J. Brabec, Appl. Phys. Lett., 2006, 89, 3.
19 M. S. White, D. C. Olson, S. E. Shaheen, N. Kopidakis and
D. S. Ginley, Appl. Phys. Lett., 2006, 89, 143517.
20 S. K. Hau, H. L. Yip, N. S. Baek, J. Y. Zou, K. O’Malley and
A. K. Y. Jen, Appl. Phys. Lett., 2008, 92, 253301.
21 T. Kuwabara, Y. Kawahara, T. Yamaguchi and K. Takahashi, ACS
Appl. Mater. Interfaces, 2009, 1, 2107.
22 Y. Z. Jin, J. P. Wang, B. Q. Sun, J. C. Blakesley and N. C. Greenham,
Nano Lett., 2008, 8, 1649.
23 D. Ko, J. Tumbleston, M. Ok, H. Chun, R. Lopez and E. Samulski, J.
Appl. Phys., 2010, 108, 083101.
A new thermally polymerizable fullerene derivative has been
synthesized to explore its use as an interfacial electron-trans-
porting material for inverted polymer solar cells. The key
advantage of this fullerene polymer (P-PCBM-S) is that it
enables multi-layer solution processing and facilitates cascade
electron transport for efficient electron collection in PSCs. It was
demonstrated that chemical n-doping of P-PCBM-S almost
double the power conversion efficiency of inverted devices. The
improved device performance is attributed to the decreased series
resistance and efficient electron extraction from the active layer
through the n-doped P-PCBM-S interfacial layer.
24 S. K. Hau, H. L. Yip, O. Acton, N. S. Baek, H. Ma and A. K. Y. Jen,
J. Mater. Chem., 2008, 18, 5113.
25 S. K. Hau, Y. J. Cheng, H. L. Yip, Y. Zhang, H. Ma and A. K. Y. Jen,
ACS Appl. Mater. Interfaces, 2010, 2, 1892.
26 C. H. Hsieh, Y. J. Cheng, P. J. Li, C. H. Chen, M. Dubosc,
R. M. Liang and C. S. Hsu, J. Am. Chem. Soc., 2010, 132,
4887.
Acknowledgements
27 Y. H. Zhou, H. Cheun, W. J. Potscavage, C. Fuentes-Hernandez,
S. J. Kim and B. Kippelen, J. Mater. Chem., 2010, 20,
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28 H. H. Liao, L. M. Chen, Z. Xu, G. Li and Y. Yang, Appl. Phys. Lett.,
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29 A. G. Werner, F. Li, K. Harada, M. Pfeiffer, T. Fritz and K. Leo,
Appl. Phys. Lett., 2003, 82, 4495.
30 F. Huang, Y. J. Cheng, Y. Zhang, M. S. Liu and A. K. Y. Jen, J.
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The authors thank the support from the National Science
Foundation (DMR-0120967), the Department of Energy
(DE-FC3608GO18024/A000), the AFOSR (FA9550-09-1-0426),
the Office of Naval Research (N00014-11-1-0300), and the World
Class University (WCU) program through the National
Research Foundation of Korea under the Ministry of Education,
Science and Technology (R31-21410035). A.K.-Y.J. thanks the
Boeing Foundation for support.
31 A. W. Hains and T. J. Marks, Appl. Phys. Lett., 2008, 92, 023504.
32 A. W. Hains, J. Liu, A. B. F. Martinson, M. D. Irwin and T. J. Marks,
Adv. Funct. Mater., 2010, 20, 595.
33 Y. Sun, X. Gong, B. B. Y. Hsu, H. L. Yip, A. K. Y. Jen and
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34 C. Chan, W. Zhao, A. Kahn and I. Hill, Appl. Phys. Lett., 2009, 94,
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This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 6956–6961 | 6961