Highly efficient inverted polymer solar cells based on an alcohol soluble fullerene derivative interfacial modification material

Article abstract of DOI:10.1021/cm300824h

A phosphate group-containing bisadducts fullerene derivative bis-[6,6]-phenyl-C₆₀-pentyl phosphoric diethyl ester (B-PCPO) has been developed as an indium tin oxide (ITO) interlayer material in inverted polymer solar cells. The B-PCPO possessed good alcohol solubility, n-type nature, good electron transporting ability, and ITO modification function simultaneously, and it was utilized as an ITO interlayer in solution-processed multilayer inverted bulk-heterojunction polymer solar cells to improve the electron transporting and collection. It was found that the B-PCPO interlayer could effectively decrease the work function of ITO and thereby enhance the electron collection of ITO electrode. The device studies showed that the resulting solar cells' efficiencies were significantly increased from 4.83% to 6.20% by using a B-PCPO interlayer, benefiting from the dramatic enhancement in open circuit voltage and moderate increase in fill factor. Our results provide a promising approach to improve the performance of inverted polymer solar cells.

Full text of DOI:10.1021/cm300824h

Article
pubs.acs.org/cm
Highly Efficient Inverted Polymer Solar Cells Based on an Alcohol
Soluble Fullerene Derivative Interfacial Modification Material
Chunhui Duan,† Chengmei Zhong,† Chunchen Liu, Fei Huang,* and Yong Cao
State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China
University of Technology, Guangzhou 510640, P. R. China
S
* Supporting Information
ABSTRACT: A phosphate group-containing bisadducts fullerene derivative bis-[6,6]-phenyl-C60-pentyl phosphoric diethyl ester
(B-PCPO) has been developed as an indium tin oxide (ITO) interlayer material in inverted polymer solar cells. The B-PCPO
possessed good alcohol solubility, n-type nature, good electron transporting ability, and ITO modification function
simultaneously, and it was utilized as an ITO interlayer in solution-processed multilayer inverted bulk-heterojunction polymer
solar cells to improve the electron transporting and collection. It was found that the B-PCPO interlayer could effectively decrease
the work function of ITO and thereby enhance the electron collection of ITO electrode. The device studies showed that the
resulting solar cells' efficiencies were significantly increased from 4.83% to 6.20% by using a B-PCPO interlayer, benefiting from
the dramatic enhancement in open circuit voltage and moderate increase in fill factor. Our results provide a promising approach
to improve the performance of inverted polymer solar cells.
KEYWORDS: inverted polymer solar cells, alcohol soluble fullerene derivative, interfacial modification, ITO interlayer
structures,14−16 the incorporation of effective interlayers,17−21
and so on.
INTRODUCTION
Bulk-heterojunction polymer solar cells (PSCs) based on a
■
Compared to conventional PSCs, inverted PSCs (i-PSCs)
nanoscale phase separated blend of semiconducting conjugated
have attracted increasing attention because of their greatly
polymer electron donor and fullerene derivative electron
improved device stability by using air stable metals as the hole
acceptor is one of the most promising photovoltaic
technologies to directly convert the terrestrial solar radiation
into electricity, owing to their mechanical flexibility and their
collecting electrode and ITO as the electron collecting
electrode.15,16 To achieve highly efficient i-PSCs, it is critical
to improve the electron collection of ITO electrode, as well as
suppress its hole collection, since ITO is commonly used as the
electron collecting electrode in i-PSCs. To realize this, many
compatibility with low-cost, large-scale fabrication by solution
processing.1−3 The most widely used device configuration of
PSCs is the so-called sandwich structure, where the blended
efforts were thus devoted to modify the interface between ITO
and organic active layer,22−30 including the insertion of Cs2CO3
organic active layer was sandwiched between a low work
to reduce the ITO work function,25 and the incorporation of
function metal electron collecting electrode and a hole-
collecting poly(3,4-ethylenedioxylenethiophene):poly-
(styrenesulfonic acid) (PEDOT:PSS) coated indium tin oxide
(ITO) electrode.4 In recent years, significant improvements in
power conversion efficiencies (PCEs) of PSCs with this
conventional device structure have been achieved by the
combined efforts from various aspects including the develop-
ment of new narrow-band gap conjugated polymers and new
fullerene-based acceptor materials,5−10 the usage of new device
processing methods,11−13 the invention of innovative device
semiconducting n-type metal oxides such as zinc oxide
(ZnO)26−29 and titanium oxide (TiOx)30 to selectively collect
electrons. However, the requirement of high vacuum for the
deposition of a Cs2CO3 layer is not compatible with the large-
scale solution processing techniques, while the electrical
Received: January 11, 2012
Revised: April 20, 2012
Published: April 21, 2012
© 2012 American Chemical Society
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a
Scheme 1. Chemical Structure and Synthetic Route of B-PCPO
a
Reagents and conditions: (i) EtONa, EtOH, reflux, 6 h; (ii) NaOH, EtOH, reflux, 4 h; (iii) p-toluene-sulfonyl hydrazide, MeOH, reflux, 10 h; (iv)
MeONa, pyridine, 30 min; C60, 75 °C, 16 h; and then reflux 24 h; (v) BBr3, o-DCB, 0 °C, then room temperature, overnight; (vi) triethyl phosphite,
o-DCB, reflux, 24 h.
properties of an n-type metal oxide layer are sensitive to the
surface adsorption of oxygen, UV-irradiation, and processing
conditions, which limited the widespread application of these
materials in i-PSCs.31−33
a single layer of B-PCPO on top of ITO. Consequently, the
resulting devices' PCEs were significantly enhanced from 4.83%
to 6.20% when a carbazole-derived copolymer PCDTBT47 and
[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) were used
as donor and acceptor, respectively. Our results showed that
alcohol soluble fullerene material B-PCPO is promising as an
independent ITO interlayer for i-PSCs.
Alternatively, organic interface modification materials have
also attracted great attention for i-PSCs, mainly due to their
solution processability and successful applications in polymer
light-emitting diodes as electron injection layers.20,34−41 For
example, hydrophilic polymers (HPPs) (such as poly(ethylene
oxide) (PEO)38,39 and alcohol soluble polyfluorenes and
polycarbazoles39−41), which have orthogonal solubilities relative
to the above active layer to allow for the fabrication of solution-
processed multilayer devices without interface mixing, were
used as an independent interlayer or TiOx/HPP complex
interlayer on top of ITO in i-PSCs. Moreover, the work
function of ITO can be effectively decreased by spin-coating a
thin layer of HPPs on top of it,41−43 which finally led to better
device performances. Alternatively, these HPPs are not ideal
candidates for ITO modification in i-PSCs in principle because
of the insulating nature of PEO, and the p-type nature of
alcohol soluble polyfluorenes and polycarbazoles, which are
unfavorable for electron transporting and collection in the ITO
electrode. However, cross-linked fullerene materials, which are
n-type in nature, were reported as effective electron trans-
porting layers (ETLs) on top of ZnO or TiOx in i-PSCs.44−46
Despite of having a remarkably high PCE of 6.2% achieved by
this mean, these cross-linked fullerene materials cannot be used
as an independent ITO interlayer, and a layer of n-type
semiconducting metal oxides is indispensable due to the poor
alignments between the LUMO levels of these fullerene-based
materials (∼3.9 eV) and the work function of ITO (∼4.3−4.7
eV), which complicated the device fabrication.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The chemical structure
and synthetic route of B-PCPO were demonstrated in Scheme
1. To ensure sufficient alcohol solubility of the resulting
fullerene-based material, bisadduct C60 derivative containing
two pendant phosphate groups was designed and synthesized.
Note that bisadduct C60 derivative is a mixture of a series of
regioisomers, which have similar electronic properties and can
be directly used for device fabrication without time-consuming
separation.48,49 Compound 5, which is a key intermediate to
provide 1,3-dipoles for the following [3 + 2] cycloaddition
reaction to C60, was synthesized by a three-step procedure, i.e.,
nucleophilic substitution from compound 1 and 2 to yield
compound 3, elimination from β-benzoylacetic ester 3 to yield
ketone 4, which was converted to tosylhydrazone 5 via an
addition−elimination process. The introduction of side chains
onto C60 was performed by using [3 + 2] cycloaddition from
compound 5 to C60 and then underwent a thermal isomer-
ization to yield compound 6. The protect groups of p-cresoxyl
in compound 6 were cleaved by BBr3 under mild conditions to
give side chain bromo-functionalized intermediate 7 in a high
yield. The target compound B-PCPO was thus obtained by
treating compound 7 with triethyl phosphite in o-DCB at 140
°C, and the product was purified by column chromatography.
In this contribution, we developed a new alcohol soluble
fullerene material, bis-[6,6]-phenyl-C60-pentyl phosphoric
diethyl ester (B-PCPO), which integrated the benefits of
both HPPs (good alcohol processability and ITO modification
function) and fullerene-based interfacial materials (n-type
nature and good electron transporting ability), as an
independent ITO interlayer in i-PSCs. It was found that the
work function of ITO was effectively decreased by spin-coating
1
The product was characterized by H NMR, and fast atom
bombardment mass spectrometry (FAB-MS). The B-PCPO
was readily soluble in high polarity solvent, such as methanol,
ethanol, i-propanol, N,N-dimethylformamide (DMF), dimethyl
sulfoxide (DMSO), etc. Hence, B-PCPO would be a suitable
candidate as ETL material for fabricating multilayer i-PSCs
devices with orthogonal solvents.
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Electrochemical Properties. The electrochemical prop-
erty of B-PCPO thin film was studied by cyclic voltammetry,
and the resulted cyclic voltammogram is shown in Figure 1. For
donor polymer (PCDTBT) and acceptor (PC71BM), the device
configuration, and the energy levels of the component materials
in the device are presented in Scheme 2. A 5 nm layer of B-
Scheme 2. (a) Device Structure of Inverted Solar Cell Using
B-PCPO as ETL; (b) Energy Level Diagram of the
a
Component Materials Used in Device Fabrication; (c)
Chemical Structures of PCDTBT and PC71BM
a
The energy level values for B-PCPO, PC71BM, and PCDTBT were
determined by cyclic voltammetry in our lab under the same
conditions. The work function of ITO was estimated by XPS. The
work function of Al and the band structure of MoO3 were from refs 51
and 52.
Figure 1. Cyclic voltammograms of (a) PC61BM, (b) PC71BM, and
(c) B-PCPO as thin films.
PCPO was deposited on top of ITO from its ethanol solution
as an interlayer. For comparison, control devices using sol−gel
processed ZnO interlayer26 with thickness of 40 nm and
without interlayer were also fabricated under the same
conditions.
The current density−voltage (J−V) curves of the solar cells
measured under AM 1.5G irradiation (84 mW cm−2) are
presented in Figure 2. The corresponding Jsc, Voc, FF, and PCE
values as determined from the J−V curves are summarized in
Table 1. The best i-PSC with bare ITO electrode showed a
PCE value of 4.83%, with a Jsc of 10.2 mA cm−2, a Voc of 0.71 V,
and a FF of 56.1%. After inserting a thin layer of sol−gel
processed ZnO as ETL, the device’s PCE reached 5.31% with a
significantly enhanced Voc of 0.95 V, a fairly improved FF of
comparison, the cyclic voltammogram of [6,6]-phenyl C61
butyric acid methyl ester (PC61BM) and PC71BM were also
recorded under the same conditions. PC61BM and PC71BM
exhibited three well-defined n-doping processes in the scanning
range from 0 to −1.8 V, while B-PCPO exhibited two well-
defined reduction processes in the scanning range from 0 to
−1.6 V. From the reduction curves, the onset potentials of each
reduction process were evaluated to be −0.73, −0.96, and
−1.40 V for PC61BM, −0.79, −0.97, and −1.28 V for PC71BM,
and −0.63 and −0.97 V for B-PCPO, respectively. The
reference electrode was calibrated by the ferrocene/ferroce-
nium (Fc/Fc+) (4.8 eV below vacuum level)50 to obtain
accurate energy levels. The LUMO levels of PC61BM, PC71BM,
and B-PCPO were thus calculated to be −3.75, −3.69, and
−3.85 eV, respectively. Notably, the LUMO level of B-PCPO is
slightly lower than those of PC61BM and PC71BM. This
phenomenon is different from the fact that bisadduct C60
derivatives usually possess an elevated LUMO level compared
to monoadduct C60 derivatives,10,48,49 suggesting that the polar
side chains may have some influence on the electrochemical
properties of the resulting fullerene derivatives. Nevertheless,
the well-aligned LUMO energy levels between PC61BM/
PC71BM and B-PCPO indicate that B-PCPO would be a
good electron transporting material in i-PSCs.
Photovoltaic Performances and Interfacial Modifica-
tion Functions. I-PSCs were fabricated with the configuration
of ITO/interlayer/PCDTBT:PC71BM/MoO3/Al to study the
interfacial function of B-PCPO. The chemical structure of
Figure 2. J−V characteristics of inverted PCDTBT:PC71BM solar cells
without interlayer, and with ZnO or B-PCPO interlayer.
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Table 1. Photovoltaic Performance Parameters of Best Inverted PCDTBT:PC71BM Solar Cells without Interlayer and with ZnO
a
or B-PCPO Interlayer
interlayer
B-PCPO
Jsc (mA cm−2
)
Voc (V)
FF (%)
PCE (%)
Rsh (Ω cm2)
Rs (Ω cm2)
9.50 (9.32 0.23)
8.14 (8.55 0.58)
10.2 (9.01 0.90)
9.86 (9.53 0.24)
0.89 (0.87 0.02)
0.95 (0.87 0.06)
0.71 (0.64 0.04)
0.65 (0.60 0.04)
61.7 (58.6 0.2)
57.8 (55.9 0.3)
56.1 (53.2 0.4)
57.4 (52.8 0.3)
6.20 (5.65 0.35)
5.31 (4.96 0.32)
4.83 (3.66 0.73)
4.31 (3.58 0.53)
4.2 × 104
9.1 × 104
7.2 × 104
6.0 × 104
3.3
3.8
11
ZnO
no interlayer
ethanol washed
15
a
Values inside the parentheses were average values, and the corresponding standard deviations were calculated over eight devices.
57.8%, but a noticeably decreased Jsc of 8.14 mA cm−2.
Interestingly, the i-PSC using B-PCPO as the ITO interlayer
delivered comparable performance as the device using ZnO as
the interlayer. The B-PCPO/ITO electrode device offered a
higher PCE of 6.20%, with a Jsc of 9.50 mA cm−2, a Voc of 0.89
V, and a FF of 61.7%. Notably, Voc and FF of the ITO/B-
PCPO device were simultaneously enhanced relative to the
bare ITO electrode device, which led to higher overall
performance even though Jsc was slightly decreased. It is
interesting that B-PCPO exhibited comparable performance as
an ETL material relative to the solution processed ZnO, making
it a good alternative ETL material in i-PSC device design. To
distinguish between the solvent effect on ITO and the effect on
ITO introduced by B-PCPO ETL alone, the reference i-PSC
devices using ethanol washed ITO electrode were fabricated,
and the results are also listed in Table 1. The statistical data
(average values and standard deviations) of photovoltaic
performances calculated from eight repeating devices for each
kinds of i-PSCs are also listed in Table 1. It was clearly shown
that the ITO/B-PCPO devices exhibited significantly improved
device performances compared to bare ITO devices, while the
EtOH solvent did not play a role on the improved device
performances of the resulting devices. In addition to routine
device characterizations, the light soaking effect, which is a
common problem in i-PSCs, was also investigated in this
study.53,54 This effect typically leads to great changes in device
performances after UV irradiation. However, it was found that
the light soaking effect was insignificant for the i-PSC devices
using ZnO or B-PCPO as ETL in our study. We irradiated the
as-prepared ITO/ZnO and ITO/B-PCPO i-PSC devices with
365 nm UV light for 10 min, and little difference in device
performance was observed. The results are shown in the Table
S1 in Supporting Information.
of three i-PSC devices agreed well with the integration of the
corresponding EQE spectra. The shape of the EQE curve of the
ITO/B-PCPO device was almost identical with that of the bare
ITO device across the entire wavelength range, despite that a
slight decrease in values was observed. However, the ITO/ZnO
device exhibited significant loss in EQE in two wavelength
regions (400−435 nm and 480−620 nm), which was consistent
with the relative low Jsc of ITO/ZnO devices with respect to the
other two kinds of devices. Considering the good transparence
of ZnO in the wavelength range over 400 nm, the EQE loss of
the ITO/ZnO device is possibly caused by the charge carrier
recombination at the interface or the change in active layer
morphology due to the insertion of the ZnO interlayer. The
surface of metal oxides processed by the sol−gel method
usually has hydroxyl groups,55 which can introduce charge
trapping at the metal oxide/active layer interface, leading to
high interface charge recombination. Besides, it is well-known
that the underlying interfacial layer has significant influence on
the overlayer BHJ morphology.17,56−60 For example, Jen’s
group has reported that the use of metal oxide directly or after
being modified by a fullerene-based self-assembled monolayer
as ITO interlayers in i-PSCs led to different surface roughness
for the above active layer, as well as different BHJ phase
separation and crystallinity.60 To study the influence of the
interfaical layers on the morphology of overlying BHJ layer as
well as to better understand the difference of photocurrent and
EQE spectra of different solar cell devices, atomic force
microscope (AFM) under the tapping mode was used to track
the surface morphologies of the PCDTBT:PC71BM layer on
different substrates, and the corresponding images were
presented in Figure 4. The surface of the PCDTBT:PC71BM
layer on top of bare ITO or ITO/B-PCPO were both very
smooth and homogeneous, with near root-mean-square (rms)
roughnesses of 0.79 and 0.78 nm, respectively. On the contrary,
the film of PCDTBT:PC71BM on top of ITO/ZnO exhibited
significantly larger surface roughness due to the zigzag features
of the underlying ZnO layer. Correspondingly, the rms
roughness of the PCDTBT:PC71BM layer was 3.35 nm,
which is much higher than those of the films on top of bare
ITO and ITO/B-PCPO. The more uniform film of
PCDTBT:PC71BM on top of bare ITO and ITO/B-PCPO
were beneficial for obtaining as large as heterojunction interface
areas, which is also consistent with the Jsc and EQE spectra
observed for the related solar cell devices.
To gain an in-depth understanding of the improvement of
device performance after inserting the B-PCPO ETL, the
external quantum efficiency (EQE) spectra (see Figure 3) of
the i-PSCs both with and without ETLs were measured. The Jsc
The J−V characteristics of the devices obtained under dark
conditions are shown in Figure 5. Clearly, the ITO/B-PCPO
device and ITO/ZnO device exhibited similar J−V character-
istics under dark conditions. In the region from −2 to 0.95 V,
the reverse and leakage currents of the inverted devices using B-
PCPO and ZnO as interlayer were significantly suppressed
compared to that of the bare ITO electrode device. Such
reduced dark leakage current is beneficial for Voc and PCE
improvements in PSC devices.61 While in the region over 0.95
Figure 3. EQE spectra of inverted PCDTBT:PC71BM solar cells
without interlayer and with ZnO or B-PCPO interlayer.
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Figure 4. Tapping mode surface topographic AFM images of the PCDTBT:PC71BM (1:4) layer on top of (a) bare ITO, (b) ITO/B-PCPO, and (c)
ITO/ZnO.
Figure 6. Comparison of XPS secondary cutoff of ITO substrates with
Figure 5. Dark currents of inverted PCDTBT:PC71BM solar cells
a B-PCPO layer and without thin film on top.
without interlayer and with ZnO or B-PCPO interlayer.
V, the B-PCPO and ZnO interlayer devices showed higher
injection currents compared to that of the bare ITO electrode
device, which is indicative of increased electron injection from
the ITO electrode and is consistent with the decreased series
resistance (Rs) (11, 3.8, and 3.3 Ω cm2 for bare ITO, ITO/
ZnO, and ITO/ZnO devices, respectively) (see Table 1).62 The
improved diode characteristics of the ITO/B-PCPO device and
the ITO/ZnO device compared to that of the control bare ITO
electrode device suggested that B-PCPO is as efficient as ZnO
in blocking holes and collecting electrons, and thereby
contributed to the improved photovoltaic performance. In
addition, the reduced leakage currents and series resistances in
devices with an interlayer are beneficial for obtaining a higher
Voc due to the reduced potential drop across the devices.
To gain an in-depth understanding of the origin of the
significant enhancement in Voc of i-PSCs with a B-PCPO
interlayer compared to that of the control bare ITO electrode
device, the work function or surface potential of ITO substrates
before and after modification by B-PCPO were measured by X-
ray photoelectron spectroscopy (XPS).63,64 The XPS secondary
electron cutoff of the ITO substrates without O2 plasma
treatment as used in actual i-PSC devices are presented in
Figure 6. The work functions were measured to be 4.3 and 3.9
eV for a pristine ITO substrate and B-PCPO coated ITO,
respectively. Clearly, the work function of ITO can be
decreased by ∼0.4 eV via the introduction of a thin B-PCPO
layer, which may be caused by the dipole formation in the
ITO/active layer interface as the function of many other
hydrophilic polymers in optoelectronic devices.41,43 The
lowered work function of ITO is beneficial for enhancing the
built-in potential in i-PSCs, which in turn contributed to the
enhanced Voc.65,66 The decreased work function of ITO by B-
PCPO interlayer can also offer better energy level alignment
with the LUMO level of PC71BM and thereby facilitate the
electron transporting and collection, which in turn transform to
the enhancement in FF. To distinguish between the solvent
effect on ITO and the effect on ITO introduced by B-PCPO
ETL alone, the XPS work function measurements for bare ITO,
EtOH covered ITO, and B-PCPO covered ITO after O2 plasma
were also conducted. As the results shown in Figure S2 in the
Supporting Information, no work function change can be
observed.
CONCLUSIONS
■
In conclusion, an n-type, alcohol soluble, electron-transporting
fullerene bisadduct B-PCPO containing polar side chains was
synthesized and utilized as an efficient ITO interlayer to
improve the electron transporting and collection in high
performance i-PSCs. By using B-PCPO as ETL on top of ITO,
the PCE values of the i-PSCs can be increased from the initial
4.83% to 6.20%, benefiting from the dramatic enhancement in
Voc and moderate increase in FF. The improvement in device
performances were revealed to be mainly caused by the
decrease of the ITO work function. Much higher PCE values
would be achievable if a more efficient photoactive system was
used to fabricate solar cells. Moreover, our results indicated that
B-PCPO is comparable to sol−gel processed ZnO as ETL in i-
PSCs based on the same BHJ system. Combining ideal energy
levels, excellent electron transporting/collection ability, and
good alcohol solubility, hydrophilic fullerene derivatives would
be a promising family of interfacial materials for highly efficient
i-PSCs.
EXPERIMENTAL SECTION
■
Instruments and Measurements. 1H and 13C NMR spectra
were recorded on a Bruker AV-300 spectrometer operating at 300 or
75 MHz, respectively, in deuterated chloroform or deuterated
dimethyl sulfoxide solution at room temperature and were referred
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to tetramethylsilane. Mass spectrometry (MS) data of compounds 3−
5 were obtained on a Bruker Esquire HCT PLUS with atmospheric
pressure chemical ionization resource (APCI). The remaining MS data
were collected with fast atom bombardment (FAB) MS on a MAT
95XP (Thermo). UV−vis absorption spectra were recorded on an HP
8453 spectrophotometer. Cyclic voltammetry (CV) was performed on
a CHI800C electrochemical workstation with a platinum working
electrode and a Pt wire counter electrode at a scan rate of 30 mV s−1
against a reference electrode of saturated calomel electrode (SCE)
with an argon-saturated anhydrous solution of 0.1 mol L−1
tetrabutylammonium hexafluorophosphate (Bu4NPF6) in acetonitrile.
The films of fullerene materials for electrochemical measurements
were coated from their dilute solution.
Materials. Benzoylacetic ester (1) was purchased from Aladdin
Reagent Inc. p-Cresoxypropyl bromide (2) was prepared according to
a literature reported method.67 The other chemical reagents, unless
otherwise specified, were purchased from Alfa Aesar or Sigma-Aldrich
and used as received. All the solvents used were further purified prior
to use.
Synthesis of 3-(p-Cresoxypropyl)benzoylacetic Ester (3). Benzoyl-
acetic ester (1) (20 g, 52.1 mmol) and p-cresoxypropyl bromide (2)
(5.96 g, 26 mmol) were dissolved in 15 mL of absolute alcohol. After
degassing with argon for 15 min, sodium ethoxide (0.177 g, 26 mmol)
was added into the solution. The reaction mixture was refluxed for 6 h
under argon atmosphere. After cooling to room temperature, the
solvent was removed under reduced pressure, water was added, and
the mixture was extracted with dichloromethane three times. The
combined organic layers were washed with water and brine
sequentially and then dried over anhydrous magnesium sulfate. After
removing the solvent under reduced pressure, the residue was purified
using column chromatography (silica gel, petroleum ether/ethyl
acetate (15/1)) to afford compound 3 as a colorless oil (4.4 g,
50%). 1H NMR (CDCl3, 300 MHz), δ (ppm): 8.02−7.99 (d, J = 7.17
Hz, 2H), 7.60−7.56 (t, J = 4.14 Hz, 1H), 7.50−7.45 (t, J = 7.8 Hz,
2H), 7.08−7.05 (d, J = 8.31 Hz, 2H), 6.79−6.75 (d, J = 6.63 Hz, 2H),
4.46−4.41 (t, J = 7.2 Hz, 1H), 4.19−4.12 (q, J = 7.08 Hz, 2H), 4.00−
3.96 (t, J = 6.42 Hz, 2H), 2.28 (s, 3H), 2.25−2.17 (m, 2H), 1.91−1.86
(m, 2H), 1.20−1.15 (t, J = 7.11 Hz, 3H). 13C NMR (CDCl3, 75
MHz), δ (ppm): 195.14, 169.90, 156.67, 136.18, 133.54, 129.88,
128.76, 128.66, 114.30, 67.32, 61.43, 53.78, 27.15, 25.83, 20.47, 14.02.
MS (APCI, +MS): m/z = 341.2.
Synthesis of 4-(p-Cresoxybutyl)phenyl Ketone (4). To a 250 mL
two-necked round-bottom flask, compound 3 (13.2 g, 38.8 mmol),
sodium hydroxide (1.86 g, 46.6 mmol), and absolute alcohol (110 mL)
were added under argon atmosphere. The reaction mixture was stirred
for 4 h under reflux. After cooling to room temperature, the mixture
was poured into water and extracted with dichloromethane three
times. The combined organic phases were washed with water and
brine, sequentially, and then dried over anhydrous magnesium sulfate.
After removing the solvent under reduced pressure, the residue was
purified using silica gel column chromatography and then recrystal-
lized from methanol to yield compound 4 as colorless needles (7.5 g,
72%). 1H NMR (CDCl3, 300 MHz), δ (ppm): 7.98−7.96 (d, J = 7.05
Hz, 2H), 7.59−7.53 (t, J = 6.06 Hz, 1H), 7.49−7.43 (t, J = 6.21 Hz,
2H), 7.08−7.06 (d, J = 4.44 Hz, 2H), 6.80−6.78 (d, 2H), 4.01−3.97
(t, J = 6.09 Hz, 2H), 3.08 (t, J = 6.81 Hz, 2H), 2.28 (s, 3H), 1.96−1.85
(m, 4H). 13C NMR (CDCl3, 75 MHz), δ (ppm): 200.02, 156.87,
137.04, 132.94, 129.85, 128.57, 128.04, 114.39, 67.67, 38.12, 29.69,
28.87, 20.98, 20.44. MS (APCI, +MS): m/z = 269.1.
300 MHz), δ (ppm): 10.71 (s, 1H), 7.70−7.78 (d, J = 8.25 Hz, 2H),
7.61−7.58 (m, 2H), 7.41−7.33 (m, 5H), 7.07−7.05 (d, J = 8.16 Hz,
2H), 6.80−6.76 (d, J = 8.55 Hz, 2H), 3.91−3.87 (t, J = 6.36 Hz, 2H),
2.76−2.71 (t, J = 7.83 Hz, 2H), 2.36 (s, 3H), 2.21 (s, 3H), 1.74−1.67
(m, 2H), 1.53−1.48 (m, 2H). 13C NMR (CDCl3, 75 MHz), δ (ppm):
156.93, 156.18, 143.76, 137.01, 136.75, 130.23, 129.96, 129.72, 129.49,
128.89, 127.89, 126.60, 114.77, 67.46, 28.92, 26.66, 22.75, 21.46,
20.52. MS (APCI, +MS): m/z = 436.6.
Synthesis of Bis-[6,6]-phenyl-C61-pentyl p-Tolyl Ether (6). To a
100 mL two-necked round-bottom flask, compound 5 (700 mg, 1.6
mmol), sodium methoxide (90 mg, 1.67 mmol), and dry pyridine (10
mL) were added under argon atmosphere. The mixture was stirred at
room temperature for 30 min. To the mixture, a solution of C60 (925
mg, 1.28 mmol) in o-DCB (45 mL) was added, and the reaction
mixture was stirred at 75 °C under argon for 16 h. Afterward, the
solution was further heated, and the reaction mixture was allowed to
be stirred and refluxed for 24 h under argon atmosphere. After cooling
to room temperature, the reaction mixture was precipitated from
methanol. After drying, the resulted precipitates were dissolved in
toluene and reabsorbed on silica gel. The unreacted C60, [6,6]-phenyl-
C61-butyl p-tolyl ether, and bis-[6,6]-phenyl-C61-butyl p-tolyl ether
were separated by column chromatography (silica gel, petroleum
ether/toluene (5/1)). The title compound 6 was precipitated with
methanol, centrifuged, and decanted. The product was treated with the
same manner for several times to yield a brown solid (483 mg, 31%).
1H NMR (CDCl3, 300 MHz), δ (ppm): 8.01−7.80 (m, 4H), 7.55−
7.43 (m, 6H), 7.05−7.03 (m, 4H), 6.82−6.70 (m, 4H), 4.04−3.76 (m,
4H), 3.14−2.78 (m, 4H), 2.28−2.24 (m, 6H), 2.06−1.95 (m, 8H). MS
(FAB): m/z = 1224.
Synthesis of Bis-[6,6]-Phenyl-C61-pentyl Bromide (7). To a
solution of compound 6 (510 mg, 0.417 mmol) in o-DCB (40 mL)
at 0 °C was added BBr3 (3 mL) dropwise under argon atmosphere.
The reaction mixture was slowly warmed to room temperature and
kept stirred for 4 h. Then, water was added to quench the reaction.
After washing with water for several times, the separated organic phase
was precipitated from methanol. The collected precipitate was purified
using column chromatography (silica gel, petroleum ether/toluene (5/
1)). The solution of pure title compound 7 was concentrated and
precipitated from methanol to a brown solid (453 mg, 93%). 1H NMR
(CDCl3, 300 MHz), δ (ppm): 8.13−7.76 (m, 4H), 7.69−7.40 (m,
6H), 3.57−3.42 (m, 4H), 3.09−2.74 (m, 4H), 2.18−2.00 (m, 8H). MS
(FAB): m/z = 1171.
Synthesis of Bis-[6,6]-phenyl-C61-pentyl Phosphoric Diethyl Ester
(8). To a 50 mL two-necked round-bottom flask, compound 7 (300
mg, 0.26 mmol), the excess triethyl phosphite (1 mL), and o-DCB (15
mL) were added under argon atmosphere. The reaction mixture was
heated to 140 °C for 24 h. After cooling to room temperature, the
mixture was subjected to a silica gel column and eluted with toluene/
alcohol (10/1). The solution of title compound 8 was concentrated
and precipitated from hexane to yield a brown solid (273 mg, 83%).
1H NMR (CDCl3, 300 MHz), δ (ppm): 8.12−7.74 (m, 4H), 7.65−
7.38 (m, 6H), 4.07 (m, 8H), 3.07−2.55 (m, 4H), 1.75 (m, 12H),
1.36−1.26 (m, 12H). MS (FAB): m/z = 1285.
Device Fabrication and Characterization. The inverted
polymer solar cell devices were fabricated by the following process.
First, prepatterned ITO glass substrates were cleaned by subsequent
ultrasonication in acetone, detergent, deionized water, and isopropyl
alcohol. Then, the ITO was modified to function as a transparent
electron collecting electrode by spin-coating a 20 nm thick B-PCPO
layer onto the ITO surface. Approximately 5 nm of B-PCPO was
generally left after deposition of the active layer from o-DCB/CB = 3:1
solvent mixture according to the absorption spectra. The active layer
was then spin-coated onto the B-PCPO layer (PCDTBT/PC71BM =
1:4 with a total concentration of 15 mg mL−1 in o-DCB/CB = 3:1
solvent mixture) with a thickness of 80 nm. After the spin-coating of
the active layer, the devices were transferred into a thermal
evaporation chamber with a vacuum level of 3 × 10−6 mbar. To
complete the device fabrication, 10 nm of MoO3 and 100 nm of Al
were then thermally evaporated onto the active layer to form the top
electrode of the device. The devices using sol−gel processed ZnO as
Synthesis of 4-(p-Cresoxybutyl)benzoyl p-Tosylhydrazone(5). To
a 250 mL two-necked round-bottom flask, compound 4 (4.87 g, 18.2
mmol), p-toluene-sulfonyl hydrazide (4.06 g, 21.8 mmol), catalytic
amount of HCl, and methanol (100 mL) were added under argon
atmosphere. The reaction mixture was stirred and refluxed for 10 h
under argon atmosphere. After cooling to room temperature, the
reaction mixture was placed into a fridge and frozen at −25 °C
overnight. The resulting white precipitate was collected by filtration
and washed with cool methanol. The crude product was further
purified by recrystallizing from methanol twice to afford the title
1
compound as colorless needles (7.04 g, 89%). H NMR (d6-DMSO,
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Chemistry of Materials
Article
the interlayer were fabricated according to the literature reported
procedures,26 and the thickness of ZnO layer was 40 nm. The effective
area of a single device was 0.16 cm2.
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ASSOCIATED CONTENT
■
S
* Supporting Information
High-resolution mass spectrometry of the fullerene derivatives
and UV−vis absorption spectra of the fullerene materials; XPS
work function data of ITO substrates with/without B-PCPO
layer and ITO substrate subject to ethanol washing; i-PSC
device performance before and after light-soaking. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: msfhuang@scut.edu.cn.
Author Contributions
†These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The work was financially supported by the Natural Science
Foundation of China (No. 21125419, 50990065, 51010003,
51073058, and 20904011) and the Ministry of Science and
Technology, China (MOST), National Research Project (No.
2009CB623601 and 2009CB930604).
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