Multiple functionalities of polyfluorene grafted with metal ion-intercalated crown ether as an electron transport layer for bulk-heterojunction polymer solar cells: Optical interference, hole blocking, interfacial dipole, and electron conduction

Article abstract of DOI:10.1021/ja303813s

We present a novel electron transport (ET) polymer composed of polyfluorene grafted with a K⁺-intercalated crown ether involving six oxygen atoms (PFCn6:K⁺) for bulk-heterojunction polymer solar cells (PSCs) with regioregular poly(3-hexylthiophene) (P3HT) as the donor and indene-C ₆₀ bisadduct (ICBA) or indene-[6,6]-phenyl-C₆₁-butyric acid methyl ester (IPCBM) as the acceptor in the active layer and with Al or Ca/Al as the cathode. A remarkable improvement in the power conversion efficiency (PCE) (measured in air) was observed upon insertion of this ET layer, which increased the PCE from 5.78 to 7.5% for a PSC with ICBA and Ca/Al (5.53 to 6.63% with IPCBM) and from 3.87 to 6.88% for a PSC with ICBA and Al (3.06 to 6.21% with IPCBM). This ET layer provides multiple functionalities: (1) it generates an optical interference effect for redistribution of light intensity as an optical spacer; (2) it blocks electron-hole recombination at the interface with the cathode; (3) it forms an interfacial dipole that promotes the vacuum level of the cathode metal; and (4) it enhances electron conduction, as evidenced by (1) the increase in total absorption of 1:1 w/w P3HT:ICBA by a factor of 1.3; (2) the reduction in the hole-only current density profile by a factor of 3.3 at 2.0 × 10⁵ V/cm; (3) the decrease of 0.81 eV in the work function of Al from 4.28 to 3.47 eV, as determined by UV photoelectron spectroscopy; and (4) the decrease in the series resistance of PSCs with ICBA and Al by a factor of 4.5, as determined by the current-voltage characteristic under dark conditions; respectively. The PSC of 7.5% is the highest among the reported values for PSC systems with the simplest donor polymer, P3HT.

Full text of DOI:10.1021/ja303813s

Communication
pubs.acs.org/JACS
Multiple Functionalities of Polyfluorene Grafted with Metal Ion-
Intercalated Crown Ether as an Electron Transport Layer for Bulk-
Heterojunction Polymer Solar Cells: Optical Interference, Hole
Blocking, Interfacial Dipole, and Electron Conduction
Sih-Hao Liao, Yi-Lun Li, Tzu-Hao Jen, Yu-Shan Cheng, and Show-An Chen*
Chemical Engineering Department and Frontier Research Center on Fundamental and Applied Sciences of Matters, National
Tsing-Hua University, Hsinchu 30013, Taiwan, ROC
*
S Supporting Information
3
limited performance is due to the low open-circuit voltage (Voc)
ABSTRACT: We present a novel electron transport (ET)
polymer composed of polyfluorene grafted with a K -
of 0.5−0.7 V resulting from the low energy of the lowest
+
unoccupied molecular orbital (LUMO) of PCBM (−3.91 eV), as
intercalated crown ether involving six oxygen atoms
V depends on the difference between the LUMO energy of the
oc
+
(
(
PFCn6:K ) for bulk-heterojunction polymer solar cells
PSCs) with regioregular poly(3-hexylthiophene) (P3HT)
acceptor and the highest occupied MO (HOMO) energy of the
4
donor. To enhance the PCE by design of the active layer, one
as the donor and indene−C bisadduct (ICBA) or
6
0
way is to design an acceptor with a higher LUMO energy,
indene−[6,6]-phenyl-C -butyric acid methyl ester
6
1
increasing V . Some fullerene derivatives have been reported to
oc
(
IPCBM) as the acceptor in the active layer and with Al
give higher V because of their higher LUMO energies, such as
oc
5
or Ca/Al as the cathode. A remarkable improvement in the
power conversion efficiency (PCE) (measured in air) was
observed upon insertion of this ET layer, which increased
the PCE from 5.78 to 7.5% for a PSC with ICBA and Ca/
Al (5.53 to 6.63% with IPCBM) and from 3.87 to 6.88%
for a PSC with ICBA and Al (3.06 to 6.21% with IPCBM).
This ET layer provides multiple functionalities: (1) it
generates an optical interference effect for redistribution of
light intensity as an optical spacer; (2) it blocks electron−
hole recombination at the interface with the cathode; (3) it
forms an interfacial dipole that promotes the vacuum level
of the cathode metal; and (4) it enhances electron
conduction, as evidenced by (1) the increase in total
absorption of 1:1 w/w P3HT:ICBA by a factor of 1.3; (2)
the reduction in the hole-only current density profile by a
PCBM bisadduct (E
PCBM (IPCBM) (E
= −3.7 eV, V = 0.724 V), indene−
LUMO
oc
6
= −3.79 eV, V = 0.72 V), and
LUMO
oc
indene−C60 bisadduct (ICBA) (ELUMO = −3.74 eV, V = 0.84
oc
V). ICBA provided the highest reported PCE with P3HT
7
(
6.48%). The other way is to replace P3HT with lower-band-
gap polymers, allowing the promotion of the PCE from 5 to over
%, as reported for PSCs with donors such as thieno[3,4-
7
b]thiophene/benzodithiophene (PTB7) and poly[N-9″-hepta-
decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzo-
thiadiazole)] (PCDTBT) along with fullerene derivative accept-
ors such as [6,6]-phenyl-C -butyric acid methyl ester (PC BM)
71
71
8
−11
and PCBM.
Other than material design for the active layer, the insertion of
an electron transport layer (ETL) between the active layer and
the metal cathode can also increase the PCE. For example, water-
or alcohol-soluble ETLs based on conjugated polyfluorene
5
factor of 3.3 at 2.0 × 10 V/cm; (3) the decrease of 0.81
12
eV in the work function of Al from 4.28 to 3.47 eV, as
determined by UV photoelectron spectroscopy; and (4)
the decrease in the series resistance of PSCs with ICBA
and Al by a factor of 4.5, as determined by the current−
voltage characteristic under dark conditions; respectively.
The PSC of 7.5% is the highest among the reported values
for PSC systems with the simplest donor polymer, P3HT.
grafted with N,N-dimethylamino (PFN), ammonium salt
13
14
(
WPF-oxy), or phosphonate (PF−EP) have been used in
P3HT:PCBM systems with Al as the cathode, giving PCE
enhancements of 0.08% (from 1.42 to 1.54%), 0.82% (from 2.95
to 3.77%), and 1.4% (from 1.98 to 3.38%), respectively. It was
recently found that the PCE can be enhanced from 5 to 6.5% via
insertion of an interlayer consisting of a conjugated polymer
grafted with ammonium salt (PF2/6-b-P3TMAHT) between the
15
PCDTBT:PC BM active layer and the Al cathode. Also, the
71
1
ulk-heterojunction (BHJ) polymer solar cells (PSCs) with
incorporation of PFN as an interlayer inserted between the active
layer [composed of quinoxaline-containing poly(4,5-ethylene-
2,7-carbazole) (PECz−DTQx), PCDTBT, or PTB7 as the donor
and PC71BM as the acceptor] and the cathode (Al or Ca/Al) was
found to improve the PCE from 3.99 to 6.07% for the first donor
B
an active layer composed of a conjugated polymer as the
donor and a fullerene derivative as the acceptor have attracted
great attention because of their ease of fabrication, promising
flexibility, and capability for large-scale and low-cost production.
Poly(3-hexylthiophene) (P3HT) with high regioregularity and
1
6
(
Al cathode), 4.11 to 6.79% for the second donor (Ca/Al
17
cathode), and 7.13 to 8.37% for the third donor (Ca/Al
[
6,6]-phenyl-C -butyric acid methyl ester (PCBM) are the most
61
representative conjugated polymer donor material and acceptor
material, respectively. PSCs based on these two materials can
reach the power conversion efficiencies (PCEs) of 4−5%. The
Received: April 20, 2012
Published: August 17, 2012
2
©
2012 American Chemical Society
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Journal of the American Chemical Society
Communication
1
7
cathode). The improvements were attributed to better
interfacial contact with the Ca/Al cathode and the formation
17
of an interfacial dipole of PFN with the positively charged end
pointing toward the Al cathode.
Here we employed as ETLs novel polymers composed of
+
polyfluorene grafted with K -intercalated crown ethers involving
+
six, five, or four oxygen atoms, designated as PFCn6:K (Scheme
+
+
1
a), PFCn5:K , and PFCn4:K , respectively, which are soluble in
+
Scheme 1. (a) Chelation of PFCn6 to K ; (b) Schematic
a
Illustration of the Proposed Working Mechanism
2
Figure 1. Performance of PSCs under simulated 100 mW/cm AM 1.5G
illumination. (a) J−V curves and (b) EQE spectra of the devices ITO/
PEDOT:PSS (25 nm)/P3HT:ICBA (1:1 w/w, 180 nm)/ ETL (5 nm or
none)/Al (100 nm). (c) J−V curves and (d) EQE spectra of the devices
identical to those in (a) and (b) except with a Ca/Al cathode. (e) Total
absorption spectra in similar devices with the Al cathode but without the
hole transport layer (PEDOT:PSS), measured in the reflection
geometry. The inset shows the device structure.
a
In (b), the labels on the energy levels are the corresponding energies
7a,18
multiplied by −1,
and Δ denotes the shift in the vacuum level.
characteristic values are listed in Table 1. An investigation of
the effect of the thickness of the ETL on the performance by
varying it from 5 to 20 nm for all the devices showed that thinner
ETLs were better in all cases (Figures S2−S5 and Table S2 in the
SI). For the devices with Al and ICBA, use of PFCn6 as the ETL
gave remarkably increased performance relative to that for the
ethyl acetate/alcohol mixed solvent. The ET polymer provides
multiple functionalities: (1) it generates an optical interference
effect for redistribution of light intensity as an optical spacer; (2)
it blocks electron−hole recombination at the interface with the
cathode; (3) it forms an interfacial dipole that promotes the
vacuum level of the cathode metal; and (4) it enhances the
electron conductivity. Scheme 1b shows the working mechanism
for this ETL. Although these polymers have been successfully
used by us as electron injection layers for high-performance
a
Table 1. Performance of PSCs with or without an ETL
acceptor,
cathode
Voc
Jsc
FF
PCE
(%)
2
ETL
none
(V)
(mA/cm )
(%)
ICBA, Al
0.71
0.87
0.89
0.81
0.74
0.81
0.85
0.87
0.89
0.88
0.87
0.83
0.82
0.82
0.68
0.83
0.85
0.80
0.85
0.86
8.74
10.53
10.96
9.67
62.4
69.4
70.6
65.2
50.0
61.6
65.2
71.2
72.6
71.2
69.5
64.3
65.2
62.5
63.4
68.9
65.2
68.4
70.5
70.6
3.87
6.35
6.88
5.10
2.74
4.90
5.78
6.77
7.50
7.10
6.72
5.34
4.71
5.10
3.06
5.96
6.21
5.53
6.45
6.63
18
deep-blue-emitting polymer light-emitting diode (PLEDs)
ICBA, Al
PFCn6
PFCn6:K+
with Al as the cathode, here they provide functionalities different
from those in PLEDs and work remarkably well in PSCs with an
active layer composed of P3HT as the donor material and ICBA
or IPCBM as the acceptor material [chemical structures for all of
the materials used are shown in Chart S1 in the Supporting
Information (SI)]. Among the three intercalated ET polymers,
b
ICBA, Al
ICBA, Al
PFO
ICBA, Al
PEO
7.41
ICBA, Al
PTCn6
none
9.84
ICBA, Ca/Al
ICBA, Ca/Al
ICBA, Ca/Al
ICBA, Ca/Al
ICBA, Ca/Al
ICBA, Ca/Al
ICBA, Ca/Al
ICBA, Ca/Al
IPCBM, Al
IPCBM, Al
IPCBM, Al
IPCBM, Ca/Al
IPCBM, Ca/Al
IPCBM, Ca/Al
10.43
10.93
11.65
11.33
11.12
10.02
8.81
PFCn6
+
PFCn6:K is the best one, permitting an improvement in the
PFCn6:K+
b
b
b
PCE with ICBA from 5.78 to 7.50% (measured in air), which is
the highest among the values reported for PSCs with the simplest
donor polymer, P3HT. Those next to it are 6.69% for
P3HT:IC BM with Ca as the cathode along with the addition
of methylthiophene but without an ETL and 7.3% for an
inverted PSC of P3HT:ICBA with ZnO as the cathode and cross-
linked fullerene rods (C-PCBSD) as the ETL. For PSCs with
IPCBM instead of ICBA, the PFCn6:K ETL also improved the
PCE from 5.53 to 6.63%.
Figure 1 presents current density versus applied voltage (J−V)
curves and external quantum efficiency (EQE) spectra of the
devices ITO/PEDOT:PSS (25 nm)/P3HT:ICBA (1:1 w/w, 180
nm)/ETL (5 nm or none)/cathode (Al or Ca/Al) under
PFCn5:K+
PFCn4:K+
PFO
70
PEO
19
PTCn6
none
9.97
7.12
20
PFCn6
PFCn6:K+
10.43
11.21
10.12
10.78
10.93
+
b
b
none
PFCn6
PFCn6:K+
a
Device structure: ITO/PEDOT:PSS (25 nm)/P3HT:acceptor (1:1
2
b
simulated 100 mW/cm AM 1.5G illumination, and their
w/w, 180 nm)/ETL (5 nm or none)/cathode. 1:1 mole ratio.
1
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Journal of the American Chemical Society
Communication
device with no ETL, as the PCE increased from 3.87 to 6.35%
crown ether moiety to the PSC performance, we also measured
the performance of the device in which PFCn6 was replaced with
PFO (Table 1). The small additional dipole moment of the
and all the three corresponding parameters [V , the short-circuit
oc
current density (J ), and the fill factor (FF)] were improved
sc
2
from 0.71 V, 8.74 mA/cm , and 62.4%, respectively, to 0.87 V,
crown ether provided enhancements of 0.06 V in V , 0.86 mA/
oc
2
2
1
0.53 mA/cm , and 69.4%, respectively. The improvement in V
cm in J , and 4.2% in FF, resulting in a significant increase of
oc
sc
and J can be attributed mainly to light intensity redistribution
1.25% in the PCE. In addition to dipole contributions by PFO
and PFCn6, electron conductivity provided by the semi-
conducting polyfluorene main chain is also important, as
supported by the drop in performance for the device with a
sc
(
allowing more effective absorption of sunlight), the presence of
the interfacial dipole (causing a rise in the vacuum level of the Al
cathode), and hole blocking along with a minor contribution
from exciton blocking provided by the insertion of the ETL, as
revealed by the following results. With PFCn6, the EQEs from
thin (5 nm) layer of the insulating polymer poly(ethylene oxide)
2
(
PEO) as the ETL (J dropped by 1.33 mA/cm , FF by 12.4%,
sc
4
50 to 625 nm are in the range 60−70%, which are much higher
and PCE by 1.13%), although a slight dipole contribution of 0.03
than those without the ETL (40−55%) (Figure 1b); this is in
agreement with the higher J mentioned above. To investigate
V to V was observed (Table 1).
We further modified the PFCn6 ETL above by intercalating K
oc
+
sc
the origin of the photocurrent improvement, we measured the
ions into its crown ether moieties. This gave further appreciable
improvements in all the three performance parameters,
increasing the PCE by 0.53%. Such improvement mainly resulted
from further promotion of the optical interference effect and
electron conduction, as revealed below, whereas the additional
contribution to the interfacial dipole was insignificant since Voc
increased by only 0.02 V. Moreover, the extent of hole blocking
optical interference (or optical spacer) effect in accordance with
8
the reported procedure and the exciton blocking effect by time-
resolved photoluminescence (PL). The presence of the optical
interference effect is manifested in the reflectance spectra by the
promoted absorption profile of the active layer in the range 450−
650 nm with a slight red shift of ∼10 nm for the device ITO/
P3HT:ICBA (1:1 w/w, 180 nm)/ETL (5 nm)/Al (100 nm)
compared with that without the ETL (Figure 1e). More
specifically, for P3HT:ICBA (1:1 w/w) devices with PFCn6
and poly(9,9-di-n-octylfluorene) (PFO) ETLs, the ratio of the
areas under the absorption curves was promoted by factors of
5
decreased slightly (at 2 × 10 V/cm, the hole current density
dropped by a factor of 3.3, which is less than the factor of 3.6 for
PFCn6, as shown in Figure S15). Surprisingly, the intercalated
+
K
ions provided a remarkable enhancement in the total
absorption, as manifested by the increases in the ratio of the
areas under the absorption curves and the ratio of absorption
maxima of P3HT:ICBA (1:1 w/w) (530 nm) from 1.17 to 1.3
and 1.31 to 1.43 (Figure 1e and Table S8). This indicates that a
strong optical interference effect was further provided by the
1.17 and 1.12, respectively, and the ratio of the absorption
maxima by factors of 1.31 and 1.23, respectively, relative to those
for the device without this layer (Table S8), indicating that the
PF main chains in the ETL do provide a 12% enhancement in
absorption by redistribution of light intensity and that the grafted
crown ether moieties give only a minor enhancement (5% more).
Since the HOMO and LUMO levels of the ETL cover those of
P3HT, one may expect an exciton blocking effect for the active
layer. However, the number of generated excitons that are able to
diffuse toward the cathode is limited because the singlet exciton
lifetime decays from 850 ps to 16−5 ps as 5−50 wt % PCBM is
+
+
intercalated K . PFCn6:K is believed to be the first case of a
polymer optical spacer. The additional electron conduction
enhancement can be manifested by calculations of the series
resistance (R ) and shunt resistance (R ) from the reciprocals of
s
sh
the slopes of the J−V curves under dark conditions at I = 0 and V
23
+
=
0. For the devices without and with PFCn6 and PFCn6:K ,
2
1
these calculations showed a significant drop in R from 18.69 to
s
added (see section S4 in the SI for details). Even though the
ETL−active layer interfacial region might have smaller ICBA
content, as in the case of P3HT:PCBM (1:1 w/w) in which the
average content of PCBM within 20 nm of the interfacial region
2
6
2
.56 and 4.13 Ω cm , respectively, and a drastic rise in R from
sh
2
34 to 780 and 1024 Ω cm , respectively (Figure S6 and Table
S3). In addition, we also measured R from impedance in the dark
s
22
under a bias of 0.8 V. For the devices without and with PFCn6
is reduced to 22%, the number of excitons generated in this
region (13% of the total thickness) that are able to diffuse to the
surface before being dissociated by ET to ICBA is still limited.
Therefore, the contribution to the promoted photocurrent from
exciton blocking by the ETL is secondary relative to that of the
optical interference effect. Since the ETL’s HOMO (−5.8 eV) is
+
and PFCn6:K , these measurements also showed a drop in Rs
from 38.39 to 33.54 and 29.98 Ω (Figure S14 and Table S7).
Furthermore, we also measured the conductivities for PFCn6
+
with and without K intercalation using the four-point probe
method and found that the conductivity increased from 7.6 ×
−3
−2
1
0 to 2.5 × 10 S/cm after the intercalation (Table S6). These
0.6 eV lower than that of P3HT (−5.2 eV), we would expect a
data indicate that the incorporation of the ETL does not increase
hole blocking effect, which was confirmed by the measurement of
the hole-only current density versus electric field for the devices
without and with the PFCn6 ETL (Figure S15). The hole current
density profiles were reduced significantly (e.g., by a factor of 3.6
+
Rs but actually reduces it; the intercalation of K further
promotes the electron conductivity. In the meantime, Rsh is
increased because of reduced leak current, even for the case of the
+
5
crown ether with the intercalated K ions.
at 2.0 × 10 V/cm) relative to that without the ETL. Thus, the
Since the morphology of the bicontinuous phases of donor and
acceptor components in the active layer is desirable for BHJ
PSCs and the crown ether and polyfluorene main chain in
ETL also provides a significant hole blocking effect.
For the interfacial dipole effect provided by PFCn6 (PFO) on
the Al cathode, as measured using UV photoelectron spectros-
copy (UPS), the vacuum level of Al was found to rise by 0.81
+
PFCn6 and PFCn6:K can also form two phases, we used atomic
(
(
0.76) eV (Figure S1 and Table S1) relative to that of bare Al
−4.28 eV), allowing electrons to be collected by the Al cathode
force microscopy in tapping mode to investigate their
morphology, as shown in Figure S12. The surface topographical
image of P3HT:ICBA shows a fine phase separation and
bicontinuous network morphology, and that of PFCn6 shows an+
islandlike morphology that becomes enhanced after K
intercalation (see section S5 for details).
more easily. This indicates that in addition to the polyfluorene
main chain, the crown ether moiety in the former also
contributes to the interfacial dipole, although by only 0.05 eV
(
see section S3 for details). To verify the contribution of the
1
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Journal of the American Chemical Society
Communication
For better electrical contact of the ETL with the cathode, a thin
layer (5 nm) of Ca was inserted, giving the device structure ITO/
PEDOT:PSS/P3HT:ICBA/ETL (5 nm or none)/Ca/Al. As
shown in Figure 1c and Table 1, the trend in the improvement in
performance due to insertion of the ETL was similar to that with
Al alone as the cathode, but the interfacial dipole contribution to
ASSOCIATED CONTENT
Supporting Information
Experimental procedures and additional data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
*
S
AUTHOR INFORMATION
Corresponding Author
sachen@che.nthu.edu.tw
Notes
■
17
the low-work-function Ca cathode was weaker, as expected.
The incorporation of PFCn6 gave slight increases of 0.02 V in Voc
2
(
from 0.85 to 0.87 V), 0.5 mA/cm in J (from 10.43 to 10.93
sc
2
The authors declare no competing financial interest.
mA/cm ), and 6% in FF (from 65.2 to 71.2%), consequently
increasing the PCE by ∼1%. But in the case of the lower-band-
gap ETL (PTCn6) with the same main chain as P3HT, the
device with PTCn6 gave a PCE of 5.10%, which is 0.68% lower
than that without PTCn6 (Table 1), since it also absorbs light in
the same range as P3HT and cannot be used as an optical spacer
and exciton blocking layer for a PSC with P3HT. Thus, the
optical interference effect given by PFCn6 is confirmed.
ACKNOWLEDGMENTS
We thank the National Science Council for financial support
NSC 99-2120-M-007-012 and NSC 100-2120-M-007-009).
■
(
REFERENCES
■
(
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+
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(
2
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sc
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sc
+
Other than the insertion of PFCn6:K , we also prepared
(
1
(
+
+
PFCn4:K and PFCn5:K for use as ETLs in devices with Ca/Al.
The J−V curves are given in Figure S7, and their characteristic
parameters are listed in Table 1. All of the parameters for these
devices are significantly higher than those for devices without an
S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Nat. Photonics 2009, 3,
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+
ETL, and the performance decreased in the order PFCn6:K >
+
+
PFCn5:K >PFCn4:K (see section S6 for details).
Besides ICBA, we also investigated another disubstituted
fullerene derivative, IPCBM, as the acceptor for the same ETL
system with Al or Ca/Al as the cathode. The J−V curves are given
in Figure S8, and their performance parameters are listed in
Table 1. The trends for improvement due to incorporation of
(10) Chen, H. Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.;
Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. Nat. Photonics 2009, 3, 649.
(11) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S. T.; Wu, Y.; Li, G.; Ray, C.; Yu, L.
Adv. Mater. 2010, 22, 1.
(
12) He, C.; Zhong, C.; Wu, H. B.; Yang, R.; Huang, F.; Bazan, G. C.;
Cao, Y. J. Mater. Chem. 2010, 20, 2617.
13) Na, S. I.; Oh, S. H.; Kim, S. S.; Kim, D. Y. Org. Electron. 2009, 10,
+
PFCn6 and PFCn6:K for the devices with Al and Ca/Al
(
cathodes were similar to those for the corresponding devices with
4
96.
ICBA. For the devices with Al, the PCE improved from 3.06 to
(14) Zhao, Y.; Xie, Z. Y.; Qin, C. J.; Qu, Y.; Geng, Y. H.; Wang, L. X. Sol.
+
5
.96 and 6.21% upon incorporation of PFCn6 and PFCn6:K ,
Energy Mater. Sol. Cells 2009, 93, 604.
15) Seo, J.; Gutacker, A.; Sun, Y. M.; Wu, H. B.; Huang, F.; Cao, Y.;
Scherf, U.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2011, 133, 8416.
16) He, Z. C.; Zhang, C.; Xu, X. F.; Zhang, L. J.; Huang, L.; Chen, J.
W.; Wu, H. B.; Cao, Y. Adv. Mater. 2011, 23, 3086.
17) He, Z. C.; Zhang, C.; Huang, X.; Wong, W.-Y.; Wu, H. B.; Chen,
(
respectively, while for the devices with Ca/Al, the PCE improved
from 5.53 to 6.45 and 6.63%, respectively.
(
In conclusion, we have presented the use of polyfluorene
+
+
grafted with a crown ether intercalated with K (PFCn6:K ) as
the electron transport layer in BHJ PSCs with an active layer
composed of P3HT as the donor and the disubstituted fullerene
ICBA as the acceptor. This led to an improvement in the PCE
from 3.87 to 6.88% for the device with Al as the cathode and from
(
L. W.; Su, S. J.; Cao, Y. Adv. Mater. 2011, 23, 4636.
(18) Lu, H.-H.; Ma, Y.-S.; Yang, N.-J.; Lin, G.-H.; Wu, Y.-C.; Chen, S.
A. J. Am. Chem. Soc. 2011, 133, 9634.
(
19) Sun, Y.; Cui, C.; Wang, H.; Li, Y. Adv. Energy. Mater. 2011, 1,
058.
20) Chang, C.-Y.; Wu, C.-E.; Chen, S.-Y.; Cui, C.; Cheng, Y.-J.; Hsu,
1
5.78 to 7.50% for that with Ca/Al as the cathode. Similar trends
(
in improvement were observed when ICBA was replaced with
C.-S.; Wang, Y.-L.; Li, Y. Angew. Chem., Int. Ed. 2011, 50, 9386.
(21) Guo, J.; Ohkita, H.; Benten, H.; Ito, S. J. Am. Chem. Soc. 2010, 132,
IPCBM, but the enhancement was lower by about 0.7−0.9%.
+
6
(
154.
22) Yu, B.-Y.; Lin, W.-C.; Wang, W.-B.; Iida, S.-i.; Chen, S.-Z.; Liu, C.-
The improvement due to PFCn6:K results from its multiple
functionalities of optical interference (by the intercalated metal
ions and the polyfluorene main chain), hole blocking and
formation of an interfacial dipole (by the polyfluorene main
chains and the crown ether), and enhanced electron conduction
Y.; Kuo, C.-H.; Lee, S.-H.; Kao, W.-L.; Yen, G.-J.; You, Y.-W.; Liu, C.-P.;
Jou, J.-H.; Shyue, J.-J ACS Nano 2010, 4, 833.
(23) Moliton, A.; Nunzi, J.-M. Polym. Int. 2006, 55, 583.
(
by the intercalated metal ions).
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dx.doi.org/10.1021/ja303813s | J. Am. Chem. Soc. 2012, 134, 14271−14274