N-Type Water/Alcohol-Soluble Naphthalene Diimide-Based Conjugated Polymers for High-Performance Polymer Solar Cells

Article abstract of DOI:10.1021/jacs.5b12664

With the demonstration of small-area, single-junction polymer solar cells (PSCs) with power conversion efficiencies (PCEs) over the 10% performance milestone, the manufacturing of high-performance large-area PSC modules is becoming the most critical issue for commercial applications. However, materials and processes that are optimized for fabricating small-area devices may not be applicable for the production of high-performance large-area PSC modules. One of the challenges is to develop new conductive interfacial materials that can be easily processed with a wide range of thicknesses without significantly affecting the performance of the PSCs. Toward this goal, we report two novel naphthalene diimide-based, self-doped, n-type water/alcohol-soluble conjugated polymers (WSCPs) that can be processed with a broad thickness range of 5 to 100 nm as efficient electron transporting layers (ETLs) for high-performance PSCs. Space charge limited current and electron spin resonance spectroscopy studies confirm that the presence of amine or ammonium bromide groups on the side chains of the WSCP can n-dope PC₇₁BM at the bulk heterojunction (BHJ)/ETL interface, which improves the electron extraction properties at the cathode. In addition, both amino functional groups can induce self-doping to the WSCPs, although by different doping mechanisms, which leads to highly conductive ETLs with reduced ohmic loss for electron transport and extraction. Ultimately, PSCs based on the self-doped WSCP ETLs exhibit significantly improved device performance, yielding PCEs as high as 9.7% and 10.11% for PTB7-Th/PC₇₁BM and PffBT4T-2OD/PC₇₁BM systems, respectively. More importantly, with PffBT4T-2OD/PC₇₁BM BHJ as an active layer, a prominent PCE of over 8% was achieved even when a thick ETL of 100 nm was used. To the best of our knowledge, this is the highest efficiency demonstrated for PSCs with a thick interlayer and light-harvesting layer, which are important criteria for eventually making organic photovoltaic modules based on roll-to-roll coating processes.

Full text of DOI:10.1021/jacs.5b12664

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Article
n-Type Water/Alcohol-Soluble Naphthalene Diimide-Based
Conjugated Polymers for High-Performance Polymer Solar Cells
Zhihong Wu, Chen Sun, Sheng Dong, Xiao-Fang Jiang, Siping
Wu, Hongbin Wu, Hin-Lap Yip, Fei Huang, and Yong Cao
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b12664 • Publication Date (Web): 22 Jan 2016
Downloaded from http://pubs.acs.org on January 24, 2016
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Journal of the American Chemical Society
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n-Type Water/Alcohol-Soluble Naphthalene Diimide-Based Conju-
gated Polymers for High-Performance Polymer Solar Cells
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Zhihong Wu,† Chen Sun,† Sheng Dong,† Xiao-Fang Jiang, Siping Wu, Hongbin Wu, Hin-Lap
Yip,* Fei Huang,* Yong Cao
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Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and De-
vices, South China University of Technology, Guangzhou 510640, P. R. China.
ABSTRACT: With the demonstration of small-area, single-junction polymer solar cells (PSCs) with power conversion effi-
ciencies (PCEs) over the 10% performance milestone, the manufacturing of high-performance large-area PSC modules is
becoming the most critical issue for commercial applications. However, materials and processes that are optimized for
fabricating small-area devices may not be applicable for the production of high-performance large-area PSC modules.
One of the challenges is to develop new conductive interfacial materials that can be easily processed with a wide range of
thicknesses without significantly affecting the performance of the PSCs. Toward this goal, we report two novel naphtha-
lene diimide–based, self-doped, n-type water/alcohol-soluble conjugated polymers (WSCPs) that can be processed with a
broad thickness range of 5 to 100 nm as efficient electron transporting layers (ETLs) for high-performance PSCs. Space
charge limited current and electron spin resonance spectroscopy studies confirm that the presence of amine or ammoni-
um bromide groups on the side chains of the WSCP can n-dope PC71BM at the bulk heterojunction (BHJ)/ETL interface,
which improves the electron extraction properties at the cathode. In addition, both amino functional groups can induce
self-doping to the WSCPs, although by different doping mechanisms, which leads to highly conductive ETLs with reduced
ohmic loss for electron transport and extraction. Ultimately, PSCs based on the self-doped WSCP ETLs exhibit significant-
ly improved device performance, yielding PCEs as high as 9.7% and 10.11% for PTB7-Th/PC71BM and PffBT4T-OD/PC71BM
systems, respectively. More importantly, with PffBT4T-OD/PC71BM BHJ as an active layer, a prominent PCE of over 8%
was achieved even when a thick ETL of 100 nm was used. To the best of our knowledge, this is the highest efficiency
demonstrated for PSCs with a thick interlayer and light-harvesting layer, which are important criteria for eventually mak-
ing organic photovoltaic modules based on roll-to-roll coating processes.
INTRODUCTION
mobility/conductivity that can work efficiently under
thick film conditions, which can provide a better
processing window to achieve high surface coverage and
good film homogeneity.3
Polymer solar cells (PSCs) based on the photoactive layer
of a conjugated polymer donor and fullerene acceptor
have attracted enormous attention in the past decades
due to their various advantages in applications, including
low-cost, light-weight, mechanical flexibility, and the
potential of developing into large-area modules by
solution processing.1 Recently, much progress has been
made in improving the performance of PSCs, with power
conversion efficiencies (PCEs) of over 10% and 11% for
Water/alcohol-soluble conjugated polymers (WSCPs)
have been successfully used as the interlayer materials in
opto-electronic devices due to their many unique
advantages.4 They are usually processed from
environmentally friendly solvents (such as alcohol), which
is important for future industrial applications and can
offer orthogonal solvent processability to prevent the
solvent erosion problem during multilayer device
fabrication. Moreover, the polar functional groups on the
side chains of the polymers endow them with excellent
interfacial modification capability, which can enhance the
charge collection from the active layer to the electrode.5
Doping is typically required to improve the charge
transport property of interlayers. A good example is the
small-area,
single-junction
and
tandem
PSCs,
respectively.2 To advance PSCs to a practical technology,
however, several scientific and technological challenges
must be overcome. It is well known that large-area
manufacturing in combination with high-throughput
processing such as roll-to-roll (R2R) methods are the key
advantages of PSCs, but the high PCEs demonstrated in
the laboratory have not yet been successfully translated to
large-area devices that can be fabricated via R2R
processes. One of the main issues is the relative low
commonly
used
poly(3,4-
ethylenedioxythiophene):poly(styrene
sulfonate)
charge
mobility/conductivity
of
the
organic
(PEDOT:PSS) hole transport layer, in which the PEDOT is
p-doped by a proton transfer mechanism and stabilized
by the PSS polyanions and the doping concentration can
be tuned to obtain films with varied conductivity.6
Recently, a novel self-doping mechanism was introduced
to develop p-type narrow-band-gap conjugated
polyelectrolyte WSCPs with cyclopenta-[2,1-b;3,4-b’]-
semiconductors in PSCs compared to traditional
inorganic semiconductors, which limit the use of
relatively thin active layers (~100 nm) and interlayers (~5-
50 nm) in most of the champion devices. However, to be
compatible with R2R coating processes, it is critical to
develop
new
materials
with
good
charge
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Journal of the American Chemical Society
dithiophene-alt-4,7-(2,1,3-benzothiadiazole) (CPDT-alt-
BT) backbones and they were successfully applied as
efficient hole transport layers in state-of-the-art single-
junction and tandem PSCs.7 Besides the development of
new hole transport materials, more efforts have been
dedicated to the development of WSCP-based ETLs to
improve electron collection in PSCs.8 One classic example
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is
poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-
fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN),9 which has
been widely used as the cathode interlayers in
optoelectronic devices, including all-printed polymer
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light-emitting
displays,10
state-of-the-art
single-
junction/tandem PSCs,2h,11 and high-efficiency perovskite
solar cells.12 Despite their successful application as
cathode interlayer materials, most of the WSCPs are
based on polymers with an electron-rich conjugated
backbone, which show relatively low electron mobility
and therefore these WSCPs only work efficiently when a
very thin film (<10 nm) is applied. Ideally, WSCPs based
on an electron-deficient conjugated backbone with good
electron transport properties are more suitable for use as
efficient ETL for PSCs, but only a very few studies have
reported on this type of WSCP.13 Therefore, designing new
n-type WSCPs and understanding their structure-
property relationships are very important for the
development of a new generation of cathode interfacial
materials that is required to overcome the ultrathin
interlayer bottleneck currently encountered in high
performance PSCs. Recently, we have developed a new
naphthalene diimide (NDI) based polymer as an efficient
ETL to replace fullerenes in p-i-n planar-heterojunction
perovskite solar cells, and it was found that a wide range
of thicknesses of the new ETL can be applied to produce
devices with high PCE owing to the good electron
transport property of the polymer,14 which triggered us to
further design new polymer electron transporting materi-
als as ETL for PSCs in this study.
Chart 1. Chemical structures of PffBT4T-OD, PTB7-
Th, PNDIT-F3N, PNDIT-F3N-Br, PNDIT-F8 and
PC71BM.
synthesized WSCPs possess a doping effect on PC71BM.
Furthermore, the ESR study results also reveal a self-
doping effect among PNDIT-F3N and PNDIT-F3N-Br,
which results in greatly improved electron transporting
capability compared to that of their analogue copolymer
poly[2,7-(9,9’-dioctylfluorene)-alt-5,5’-bis(2,2’-thiophene)-
2,6-naphthalene-1,4,5,8-tetracaboxylic-N,N’-di(2-
ethylhexyl)imide] (PNDIT-F8, Chart 1) without the amino
or ammonium pendant group. More interestingly,
photoconductivity studies indicate an obviously different
doping mechanism between PNDIT-F3N and PNDIT-F3N-
Br. PNDIT-F3N-Br exhibits the same high conductivity
despite being in a dark or illuminated condition, whereas
PNDIT-F3N displays distinct photo conductivity and
exhibits further enhanced conductivity under light
soaking. Both PNDIT-F3N and PNDIT-F3N-Br were used
as the ETLs in PSCs with poly[[2,6’-4,8-di(5-
ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene][3-
fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-
Herein, we report the design and synthesis of two novel
n-type
WSCPs
poly[(9,9-bis(3’-(N,N-
b]thiophenediyl]] (PTB7-Th) or poly[(5,6-difluoro-2,1,3-
benzothiadiazol-4,7-diyl)-alt-(3,3’’’-di(2OD)-
dimethylamino)propyl)-2,7-fluorene)-alt-5,5’-bis(2,2’-
thiophene)-2,6-naphthalene-1,4,5,8-tetracaboxylic-N,N’-
di(2-ethylhexyl)imide] (PNDIT-F3N) and poly[(9,9-bis(3’-
((N,N-dimethyl)-N-ethylammonium)propyl)-2,7-
fluorene)-alt-5,5’-bis(2,2’-thiophene)-2,6-naphthalene-
1,4,5,8-tetracaboxylic-N,N’-di(2-
2,2’;5’,2’’;5’’,2’’’-quaterthiophen-5,5’’’-diyl)] (PffBT4T-OD)
(Chart 1) as the donor material and PC71BM as the
acceptor. Due to their excellent interface modification
and electron transporting capabilities, the resulting
devices exhibit promising performance with PCEs as high
as 9.7% and 10.11% for PTB7-Th/PC71BM and PffBT4T-
ethylhexyl)imide]dibromide
(PNDIT-F3N-Br).
Both
PNDIT-F3N and PNDIT-F3N-Br contain a NDI-based
conjugated main chain, with highly polar amino- or
ammonium-functionalized side chains, respectively
(Chart 1). The NDI based conjugated main chains endow
the polymers with good electron mobility,15 which have
been used in transistors,16 all polymer solar cells,17 for
diode transport,18 battery,19 and recently in perovskite
solar cell.14 The amino or ammonium functionalized side
chains endow them with good processability from alcohol
and also a unique interface modification capability to
enhance electron collection in PSCs.8e,20 Besides, SCLC
and ESR studies indicate that both of these newly
OD/PC71BM
systems,
respectively.
Moreover,
investigation of the dependence of device performance on
the ETL thickness shows both the open-circuit voltage
(Voc) and the fill factor (FF) of the resulting devices are
largely maintained with a wider range of film thickness,
and the PffBT4T-OD/PC71BM-based PSCs exhibit
a
prominent PCE of 8.04%, even when the ETL thickness is
increased to 100 nm. Our results show that these NDI-
based WSCPs are promising interlayer materials for PSCs
and can potentially be used in R2R processed large-area
optoelectronic devices that required a better processing
window. More importantly, understanding the interesting
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Journal of the American Chemical Society
and different doping behavior of these NDI-based WSCPs,
caused by their different pendant side-chain groups, may
provide more insights to further develop new
generation of self-doped n-type semiconductors for
optoelectronic devices.
dibromo monomer (4) with monomer (6), whereas its
1
2
3
4
5
6
7
8
ammonium-functionalized derivative PNDIT-F3N-Br was
obtained by quaternizing the precursor polymer PNDIT-
F3N with excess bromoethane in DMF solution at room
temperature in darkness for 2 days. For comparison, their
analogous polymer PNDIT-F8 with the same conjugated
main chain but a different side chain was also made by
the Suzuki copolymerization of monomers (4) and (7).
The molecular structures of all monomers and
copolymers were confirmed by 1H nuclear magnetic
resonance (NMR) spectroscopy. For the neutral polymer
PNDIT-F3N in CDCl3 solution, the signals corresponding
to the methyleneamino and methylamino groups mixed
together with the peaks for the methylene (-CH2-) groups
adjacent to the 9-position carbon atoms of the fluorene
rings in the region around 2.15-2.02. After quaternizing
with bromoethane, the signals for the methylene (-CH2N,
-NCH2CH3) and methyl (-NCH3) groups adjacent to the
nitrogen atoms shifted to the lower field region in DMSO-
d6
a
RESULTS AND DISCUSSION
Synthesis and Characterization. The synthesis of the
new n-type copolymers is shown in Scheme 1. Monomers
N,N’-bis(2-ethylhexyl)-2,6-bis(5-bromo-thiophen-2-yl)-
9
10
11
12
13
14
15
16
17
18
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22
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1,4,5,8-naphthalenediimide21
(4)
2,7-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolane-2-yl)-9,9-bis(6-(N,N-
dimethylamino)-propyl)fluorine (6) and 2,7-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene
(7) were synthesized and purified according to the
published procedures.22b The new n-type NDI based
amino-functionalized WSCP PNDIT-F3N was synthesized
by the palladium-catalyzed Suzuki copolymerization of
Br
HO
N
OH
O
N
O
Br2
N
N
HN
N
LiOH, H2O
Br
C2H5
N
C2H5
N
C2H5
N
OH
O
C4H9
O
C4H9
O
C4H9
O
Br
O
O
O
O
O
O
O
O
O
O
O
O
O
N
O
S
Sn(Bu)3
Br
HN
N
Br
Br
Br
EH-NH2
S
S
NBS
O
concentrated
sulfuric acid
S
S
Toluene
CH3COOH
CHCl3
Br
Br
O
Br
O
O
N
O
O
O
N
O
O
N
O
1
C4H9
C4H9
C4H9
C2H5
C2H5
C2H5
2
3
4
N
N
N
N
O
O
O
3-dimethylaminopropylchloride hydrochloride
50wt%NaOH/H2O, DMSO,TBAB
n-BuLi/THF
Br
Br
B
B
Br
Br
O
B
O
O
6
5
C2H5
N
C2H5
N
C4H9
C4H9
O
N
N
O
O
N
N
O
O
Br
S
Pd(PPh3)4, Na2CO3 (aq.,2M)
S
O
O
O
O
B
B
+
S
A336, degassed Toluene,
S
Br
n
95 , 3h
N
O
O
N
O
C4H9
C4H9
C2H5
C2H5
PNDIT-F3N
C2H5
C2H5
Br
Br
N
C4H9
O
N
C4H9
N
N
O
O
N
O
N
O
S
S
, THF
Br
DMF, rt., in darkness
S
S
n
n
N
O
O
N
O
C4H9
C4H9
C2H5
C2H5
PNDIT-F3N-Br
C2H5
N
C2H5
C4H9
O
C4H9
O
N
O
O
S
Br
Pd(PPh3)4, Na2CO3 (aq.,2M)
S
O
O
O
B
B
+
S
A336, degassed Toluene,
S
O
Br
n
95 , 3h
7
O
N
O
O
N
O
C4H9
C4H9
C2H5
PNDIT-F8
C2H5
Scheme 1. Synthesis of n-type WSCPs PNDIT-F3N, PNDIT-F3N-Br and n-type semiconducting
polymer PNDIT-F8.
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Table1. UV-vis absorption and electrochemical properties of the polymers
1
2
3
Polymers
λabsa [nm]
λabsb [nm]
Egapc [eV]
Ere [V]
Eox [V]
HOMOd [eV]
LUMOe [eV]
4
5
6
7
PNDIT-F3N
PNDIT-F3N-Br
PNDIT-F8
384,586
385,593
387,582
390,630
391,643
391,625
1.63
1.56
1.63
-0.41
-0.31
-0.43
1.22
1.14
1.30
-5.55
-5.47
-5.63
-3.91
-4.18
-3.89
8
9
[a] solution, [b] film, [c] absorption onset, [d] EHOMO = -(Eox+4.33) eV, [e] ELUMO = -(Ere + 4.33) eV.
10
11
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60
solution and could be distinguished from the methylene
(-CH2-) groups adjacent to the 9-position carbon atoms of
the fluorene rings. Both PNDIT-F3N and PNDIT-F8 are
readily soluble (solubility > 30 mg/mL) in common
organic solvents such as tetrahydrofuran (THF),
chloroform, and toluene but insoluble in water and
DMSO. The molecular weights of the copolymers were
evaluated by gel permeation chromatography (GPC)
measurement, which was carried out with THF as the
eluent using linear polystyrene standards. The number-
average molecular weight (Mn) of PNDIT-F3N and
PDNIT-F8 was estimated to be 12.3 and 27 kg mol−1 with
polydispersity index in the range of 1.7 and 1.3,
respectively. The solubility of the quaternized polymer
PNDIT-F3N-Br is different from its neutral precursor,
polymer PNDIT-F3N-Br is insoluble in THF, chloroform,
toluene and water, but exhibits limited solubility in
ethanol (about 1mg/mL), good solubility in DMSO
(solubility > 20 mg/mL), methanol (> 15 mg/mL), and
DMF (> 20 mg/mL). It is worth noting that the amino-
functionalized copolymer PNDIT-F3N is soluble in
methanol and ethanol (both > 12 mg/mL) in the presence
of a small amount of acetic acid (about 0.1% in volume)
due to a weak interaction between the nitrogen atoms in
amino groups in the side chain and the acetic acid.9,23
Consequently, both PNDIT-F3N and PNDIT-F3N-Br are
good candidates as ETLs in PSCs, as they offer orthogonal
solvent processability to prevent the solvent erosion
problem during multilayer device fabrication. The
thermal behavior of these copolymers was evaluated by
differential scanning chromatography (DSC) and
thermogravimetric analysis (see Figure S4-5). The
temperature was ramped from 40 to 800℃at 10℃/min in
a nitrogen atmosphere. As depicted in Figure S4, the
copolymer PNDIT-F8 shows a main decomposition with
the onset temperature at 380℃, while the two WSCPs
exhibit at least two main degradation processes with the
onset degradation temperature at 248 and 228 ℃,
respectively. The main decomposition of the copolymer
PNDIT-F8 occurred when the side chains began to cleave,
whereas for PNDIT-F3N, the first mass loss may due to
the cleavage of the amino groups as the mass loss ratio is
closed to the weight ratio of amino groups in the polymer.
For polymer PNDIT-F3N-Br, the first mass loss is
chains, which is similar with previous reports.22 As shown
in Figure S5, DSC revealed no distinct exothermal
transition for these copolymers in the second heating
cycle over the temperature range of 30-180℃, revealing
that no crystalline behavior or phase transition occurred
during this temperature section.
Photophysical and Electrochemical Properties. The
photophysical and electrochemical properties of the
resulting copolymers are summarized in Table 1, and the
corresponding UV-Vis absorption spectra are shown in
1.2
PNDIT-F3N-THF
PNDIT-F3N-Br-Methanol
(a)
1
PNDIT-F8-THF
PNDIT-F3N-Film
0.8
0.6
0.4
0.2
0
PNDIT-F3N-Br-Film
PNDIT-F8-Film
-0.2
400
600
800
1000
1200
Wavelength (nm)
PNDIT-F3N
PNDIT-F3N-Br
PNDIT-F8
(b)
-0.5
0
0.5
1
1.5
2
Potential (V vs. Ag/AgCl)
attributed to the decomposition of
a quaternized
ammonium group with release the ethyl bromide
group.22a Thereafter both WSCPs underwent the second
major degradation process owing to the cleavage of side
Figure 1. UV-vis spectra (a) and cyclic
voltammograms (b) of the polymers.
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ground-state electronic structure by the ion’s Coulombic
Figure 1a. All of the copolymers show two similar
1
2
3
4
5
6
7
8
absorption bands, in which the first absorption peak
below 500 nm is attributable to the characteristic
absorbance of π−π* of the polymer backbones, and the
low-energy absorbance in the region of 500−800 nm, is
attributable to the intramolecular charge transfer (ICT)
effects corresponding to the fluorene unit and electron-
withdrawing NDIs. The amino-functionalized copolymer
PNDIT-F3N exhibited almost the same absorption spectra,
both in solution and in thin film, as its analogous
copolymer PNDIT-F8, whereas the ICT absorption peak of
the ammonium-functionalized copolymer PNDIT-F3N-Br
was obviously red-shifted compared with PNDIT-F3N and
PNDIT-F8. It is common for the conjugated
polyelectrolytes to exhibit red-shifted absorption spectra
in solution compared to their neutral precursor polymers,
which may be due to the electrostatic perturbation by
ions in close proximity to the optically active segment,
solvatochromic effects arising from the polar solvent,24 or
multichain aggregation.9 The thin-film absorption spectra
were very similar to the solution spectra, but the longer
wavelength (ICT) absorption bands were red-shifted and
enhanced due to the increased conjugation and
intermolecular interactions in the solid state. Moreover, a
similar trend was observed with a red-shift in λmax and
λonset for the polyelectrolyte relative to its neutral
precursor, and because no solvent was present, it is
reasonable to speculate that the red-shifted absorption in
the film was due to electrostatic modification of the
field, in spite of possible differences in coil structure and
degree of aggregation cannot be ruled out.25
Cyclic voltammetry (CV) measurements were used to
determine the frontier molecular orbitals of the
copolymers in the thin films. The measurements were
carried out under an inert atmosphere by using tetra-n-
butylammoniumhexafluorophosphate (n-Bu4NPF6, 0.1M
in acetonitrile) as the supporting electrolyte with a glass
carbon working electrode, a platinum wire counter
electrode, and an Ag/AgCl electrode as the reference
electrode. The ferrocene/ferrocenium (Fc/Fc+) reference
was used as an internal standard, which was assigned an
absolute energy of −4.8 eV vs vacuum level.26 Under the
same experimental conditions, the onset potential of
Fc/Fc+ was measured to be 0.47 V with respect to the
Ag/AgCl reference electrode. Based on the onset
reduction potential with respect to the standard oxidation
potential of the Fc/Fc+, the lowest unoccupied molecular
orbital energy levels (ELUMO) and the highest occupied
molecular orbital energy levels (EHOMO) of the copolymers
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
were calculated as ELUMO = -e (Ere + 4.33) (eV) and EHOMO
=
-e (Eox+4.33) (eV) respectively,27 where Ere (or Eox) is the
onset of the reduction (or oxidation) potential vs the
Ag/AgCl reference electrode. The respective LUMO
values of PNDIT-F8, PNDIT-F3N, and PNDIT-F3N-Br
were estimated to be -3.89, -3.91 and -4.18 eV (Table 1).
The relative lower LUMO value of PNDIT-F3N-Br may
5
PNDIT-F3N-5nm
PNDIT-F3N-22nm
PNDIT-F3N-50nm
PNDIT-F3N-Br-5nm
PNDIT-F3N-Br-20nm
PNDIT-F3N-Br-50nm
PNDIT-F3N-5nm
PNDIT-F3N-22nm
(a)
100
(b)
PNDIT-F3N-50nm
0
-5
PNDIT-F3N-Br-5nm
PNDIT-F3N-Br-20nm
PNDIT-F3N-Br-50nm
80
60
40
20
0
-10
-15
-20
-0.2
0
0.2
0.4
0.6
0.8
1
300
400
500
600
700
800
Voltage (V)
Wavelength (nm)
5
0
80
70
60
50
40
30
20
10
0
(e)
(d)
PNDIT-F3N-5nm
PNDIT-F3N-40nm
PNDIT-F3N-100nm
PNDIT-F3N-Br-5nm
PNDIT-F3N-Br-41nm
PNDIT-F3N-Br-102nm
-5
-10
-15
-20
PNDIT-F3N-Br (102nm)
PNDIT-F3N-Br (5nm)
PNDIT-F3N (100nm)
PNDIT-F3N (5nm)
-0.2
0
0.2
0.4
0.6
0.8
1
300
400
500
600
700
800
Voltage (V)
Wavelength (nm)
Figure 2. (a) J-V curves of PSC devices using PTB7-Th:PC71BM as the active layer with different ETLs in various
thickness. (b) EQE of the devices using PTB7-Th:PC71BM as the active layer with different ETLs in various
thickness. (c) Device structure of the PTB7-Th:PC71BM BHJ based PSCs. (d) J-V curves of PSC devices using
PffBT4T-2OD: PC71BM as the active layer with different ETLs in various thickness. (e) EQE of the devices using
PffBT4T-2OD: PC71BM as the active layer with different ETLs in various thickness. (f) Device structure of the
PffBT4T-2OD: PC71BM BHJ based PSCs.
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Journal of the American Chemical Society
Page 6 of 13
1
2
3
Table 2. Device parameters of the PSCs based on PTB7-Th/PC71BM with different ETLs in various
thicknesses under the illumination of AM 1.5G, 100 mW cm-2
4
5
ETL
Voc(V)
Jsc(mA/cm2)
FF (%)
PCE(%)
Best PCE (%)
6
7
8
PNDIT-F3N
5 nm
0.79±0.01
0.79±0.01
0.78±0.01
16.32±0.42
14.52±0.40
10.98±0.41
69.42±1.35
66.56±1.32
68.24±1.62
8.8±0.2
7.7±0.1
5.8±0.2
9.04
7.79
6.03
9
22 nm
50 nm
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
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32
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37
38
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
PNDIT-F3N-Br
5 nm
0.79±0.01
0.78±0.01
0.78±0.01
16.62±0.37
14.33±0.38
11.41±0.40
71.14±1.10
70.62±1.25
70.18±1.27
9.5±0.2
7.7±0.4
6.3±0.2
9.70
8.08
6.45
20 nm
50 nm
Table 3. Device parameters of the PSCs based on PffBT4T-OD/PC71BM with different ETLs in various
thicknesses under the illumination of AM 1.5G, 100 mW cm-2
ETL
Voc(V)
Jsc(mA/cm2)
FF (%)
PCE(%)
Best PCE (%)
PNDIT-F3N
5 nm
0.75±0.00
0.76±0.01
0.76±0.00
18.38±0.49
16.58±0.39
16.12±0.55
65.91±1.15
64.31±2.54
62.31±0.84
9.1±0.3
8.1±0.2
7.6±0.2
9.42
8.33
7.97
40 nm
100 nm
PNDIT-F3N-Br
5 nm
0.77±0.00
0.77±0.00
0.77±0.00
17.64±0.27
16.48±0.35
14.28±0.30
72.29±1.20
72.85±0.41
71.48±1.24
9.8±0.2
9.3±0.2
7.9±0.2
10.11
9.61
8.04
41 nm
102 nm
have been caused by electrostatic and anion-induced
polarization due to the anion-π interaction between the
bromine anion and the electron deficient aromatic
units.28 The low-lying LUMOs of polymers PNDIT-F3N
and PNDIT-F3N-Br were very close to those of PCBM and
PC71BM, which can facilitate electron transportation from
the fullerene acceptor to the cathode side. From the onset
oxidation potential, the HOMO energy levels of PNDIT-
F3N and PNDIT-F8 were determined to be -5.55 eV and -
5.63 eV, respectively. It is worth noting that no signal for
the oxidation of the pendant amino group was detected
for PNDIT-F3N, which may be explained by the lone pair
electrons on the nitrogen atom being prone to transfer
onto the electron-deficient NDIs and leading to the
potential of around 0.5 V before the oxidation of a
conjugated main chain, which was verified by the
oxidation of tetrabutylammonium bromide at the same
applied potential in solution (see Figure S6), indicating
that the bromine anions had much better electron-
donating ability than the nitrogen atoms. Therefore, the
HOMO level of the conjugated main chain of PNDIT-
F3N-Br was calculated from the second onset oxidation
potential and estimated to be -5.47 eV. These low-lying
HOMO levels of the conjugated main chains indicate that
the two WSCPs possess hole-blocking properties with
respect to various donor materials in the photovoltaic
devices.30
formation of charge transfer complexes,
phenomenon was also observed in previous reports.5,29
Polymer PNDIT-F3N-Br exhibited clear oxidation
process of the bromine anion with an onset oxidation
a similar
Performance of Bulk Heterojunction Polymer Solar
Cells. To evaluate the performance of the new ETLs in
PSCs, the devices were first investigated by fabricating
and evaluating diodes with the configuration of indium
a
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Journal of the American Chemical Society
tin oxide (ITO)/PEDOT:PSS/PTB7-Th:PC71BM (100
nm)/ETL/Ag (Figure 2c) with different cathode interfacial
layers (PFN, PNDTI-F3N, and PNDIT-F3N-Br), which
Table 4. Scanning Kelvin probe microscopy
(SKPM) of different ETLs treated Ag electrodes
1
2
3
contain
all-solution
processed
organic
layers.
4
5
6
7
8
9
Electrode
Ag
Thickness of ETLs/Work Function
-4.66 eV
Furthermore, to investigate the solvent effect on device
performance,31 reference devices with methanol treatment
of the active layer were conducted, with all devices tested
under a simulated AM 1.5G illumination at 100 mW/cm2.
As illustrated in Table S1, the performance of these
devices increased from a PCEmax of 5.58% (short-circuit
current density (Jsc) = 16.42 mA/cm2, open-circuit voltage
(Voc) = 0.67 V, fill factor (FF) = 54.42%) for methanol, to
8.98% (Jsc = 16.58 mA/cm2, Voc = 0.80 V, FF = 69.32%) for
PFN, to 9.04% (Jsc = 16.74 mA/cm2, Voc = 0.80 V, FF =
Ag/PFN
5 nm/-4.19 eV
5 nm/-
4.20 eV
5 nm/-
3.93 eV
50 nm/-
4.04 eV
50 nm/-
3.93 eV
100 nm/-
3.95 eV
100 nm/-
3.94 eV
Ag/PNDIT-F3N
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
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42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Ag/PNDIT-
F3N-Br
70.77%) for PNDIT-F3N, and eventually to 9.7% (Jsc
=
16.99 mA/cm2, Voc = 0.80 V, FF = 72.24%) for PNDIT-F3N-
Br, demonstrating that the NDI-based WSCPs are a
promising type of ETL. Moreover, the dependence of
device performance on ETL thickness was investigated by
spin-coating polymer solutions of varying concentrations
onto an active layer to produce ETLs over a thickness
range of 5 nm to nearly 100 nm. The current density-
voltage (J−V) characteristics of devices are provided in
Figure 2a, and the photovoltaic characteristics are
summarized in Table 2. As Figure 2a shows, although the
performance of the device declines gradually with
increasing ETL thickness, no S-shaped J-V kink is present,
indicating no evident enhancement of the series
resistance Rs of the devices.29d This is further evidenced by
the dark-current J-V curves as the curves remain
unchanged despite the change in the thickness of the
ETLs (see Figure S7-8). From Table 2, it can be clearly
determined that the degradation of the PCEs is mainly
due to the decline of Jsc, whereas Voc and FF are basically
unchanged.
light being absorbed in the active layer (see Figure 2e).
These results indicate that the newly synthesized n-type
WSCPs are a promising type of ETL for the R2R
processing of PSCs. Consequently, a prominent PCE of
8.04% (Jsc = 14.58 mA/cm2, Voc = 0.77 V, FF = 72.72%) with
an ETL thickness of 100nm was achieved and an
outstanding PCE of 10.11% (Jsc = 17.91 mA/cm2, Voc = 0.77
V, FF = 73.49%) with optimized ETL thickness was
realized. To the best of our knowledge, this is the first
report of WSCP ETLs that can be processed in 100nm
thickness and in which the PCE still surpasses 8%.
The surface topographic features of the various ETLs
were examined by atomic force microscopy (AFM). All
samples were spin-coated from the polymer solutions in
methanol onto active layers under the same condition as
used in device fabrication. The AFM spectra are provided
in the Supplementary Information (see Figures S11-S12).
As the thickness increased, the root-mean-square values
of both of the WSCP films exhibited the same decreasing
trend within a small range of amplitudes, indicating the
thick-film ETLs can smooth the surface of active layers,
which is advantageous for improving device performance.
In addition, compared to the PffBT4T-OD/PC71BM BHJ
We further investigated the external quantum
efficiency (EQE) of devices with various ETL thicknesses.
In Figure 2b, a major decline in the EQE can be observed
in the 500-800 nm section, which is exactly covered by
the ICT absorption band of PNDIT-F3N. Thereby, we
supposed that the suppressed reflectance in the 500–800
nm range for devices with thicker ETLs might be
responsible for the reduced Jsc. Nevertheless, the thick
BHJ composite layers play a more significant role in R2R
manufacturing processing, and thus the potential of these
ETLs was further evaluated for broader device application.
We then more intensively investigated another BHJ
system (PffBT4T-OD/PC71BM), which could be processed
in a wide thickness to minimize the absorption loss in the
active layer. As shown in Table 3, there was less than 20%
loss of PCE for devices fabricated with ETL thicknesses
ranging from 5 nm to 100 nm for PNDIT-F3N-Br, and for
PNDIT-F3N, the corresponding loss of PCE was around
15%. In the PTB7-Th/PC71BM system, the loss is around
50%, and moreover, the EQE spectrum data of the
PffBT4T-OD/PC71BM-based devices further demonstrate
that with the thicker film of the active layer the effect of
the thickness of the ETL on Jsc will be reduced due to the
104
(a)
(b)
Al
1000
ETL(X nm)
100
10
PC71BM
1
PC71BM
Al
PNDIT-F3N
PNDIT-F3N-Br
0.1
ITO
0.01
0.01
1
10
Vap0p.1l-Vbi-Vs (V)
104
1000
100
10
104
(c)
(d)
1000
100
10
1
1
PC71BM
0.1
PC71BM
PC71BM/PNDIT-F3N5nm
0.1
PC71BM/PNDIT-F3N 100nm
PC71BM/PNDIT-F3N-Br 100nm
PC71BM/PNDIT-F3N-Br 5nm
0.01
0.01
0.01
0.1
1
10
0.01
0.1
1
10
Vappl-Vbi-Vs (V)
Vappl-Vbi-Vs (V)
Figure 3. The structure of electron only devices
(a).and J-V curves of electron only devices.
ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 8 of 13
system, films on the PTB7-Th/PC71BM BHJ layer were 100 nm, at which point the WF of the Ag cathode is
1
2
3
4
5
6
observed to be slightly rougher but showed no significant
difference, implying that the surface morphologies of
these devices have a negligible influence on device
performance.
similar for both cases, the PSCs based on the two different
ETLs showed comparable performance. The flexibility of
using a relatively thick ETL and also a more stable and
possibly printable Ag as the cathode paves the way for
eventually using all-printing processes to fabricate the
PSCs.
Scanning Kelvin Probe Microscopy Study. The effect
of the thickness of the WSCPs on the work function (WF)
of the Ag electrode was studied by scanning Kelvin probe
microscopy, and the results are summarized in Table 4.
The WF of the bare Ag was measured to be -4.66 eV, and
after deposition of an ultrathin layer (5 nm) of different
ETLs, including PFN, PNDIT-F3N, and PNDIT-F3N-Br,
the WF decreased to -4.19, -4.20, and -3.93 eV,
respectively. The variation in WFs among the WSCP-
treated electrodes resulted from the different polar nature
of the terminal groups on the WSCP. This suggests that
the quaternary ammonium derivatives could form larger
interfacial dipoles at the electrode when compared to the
amine group, leading to better ohmic contact at the
cathode. To further investigate the effect of thickness of
the ETLs on the WF tuning ability of the Ag electrodes,
various thicknesses of the ETLs from 5 nm to 100 nm were
applied, and the corresponding WFs of the modified
electrodes are summarized in Table 4. We observed that
the WFs of the PNDIT-F3N-Br-modified electrodes were
independent of the thickness of the ETL, whereas the
WFs of the PNDIT-F3N-modified electrodes reduced
subsequently from -4.20 to -3.95 eV with increasing film
thickness. A similar effect was also reported in the cases
of other amine-functionalized conjugated polymers.32 The
performance of the PSCs showed dependency on the WF
of the Ag cathode, and at low thickness (5 nm), PSCs
based on the PNDIT-F3N-Br ETL showed obviously
higher PCEs than the PNDIT-F3N-based devices (Tables 2
and 3). However, when the ETL thickness increased to 50-
7
8
9
Space Charge Limited Current and Electron Spin
Resonance Spectroscopy Study. To study the charge
transport properties of the ETLs, electron-only devices
based on the architectures of ITO/Al/ETL/Al were
prepared and the electron mobilities of the ETLs were
extracted by fitting the data using the SCLC model.33 The
electron mobility of sole PC71BM, PNDIT-F3N, and
PNDIT-F3N-Br calculated from the SCLC curves shown in
Figure 3b are at the same orders of magnitude and were
determined to be 7.1×10−4, 2.5×10−4, and 2.6×10−4 cm2 V−1s−1,
respectively. Because a few reports have suggested that
amines and ammonium salts may possibly n-dope
fullerenes with enhanced conductivity,29a we further
evaluated the contact properties between the PC71BM and
the ETLs by testing the electron-only devices with the
structure of ITO/Al/PC71BM/ETL/Al, as shown in Figure
3a. In comparison with the sole layer of PC71BM, the
current densities of the PC71BM/ETL (5 nm) bilayer films
at low voltages were two to three orders of magnitude
higher (Figure 3c), indicating that the electrical
conductivities of the films were improved by possible n-
doping of the PC71BM at the PC71BM/ETL interface. In
addition, the SCLC electron mobilities of the devices were
determined to be 1.2×10−3 and 2.0×10−3 cm2 V−1s−1 for the
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
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40
41
42
43
44
45
46
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48
49
50
51
52
53
54
55
56
57
58
59
60
20
100
(b)
(a)
15
10
50
5
0
-50
0
-5
4000
PNDIT-F8 (Dark)
PNDIT-F3N
PNDIT-F8 (Light)
PNDIT-F8 dark
PNDIT-F3N (Dark)
PNDIT-F3N (Light)
PNDIT-F3NBr (Dark)
PNDIT-F3NBr (Light)
-10
-15
-20
PNDIT-F3N-Br
PNDIT-F8
PNDIT-F8 light
3000
PNDIT-F8 (30% F3N) dark
PNDIT-F8 (30% F3N) light
PNDIT-F3N/PC71BM
-100
PNDIT-F8/PC71BM
-1
-0.5
0
0.5
1
2000
-1
-0.5
0
0.5
1
Voltage (V)
Voltage (V)
8
7
6
5
4
3
2
1
0
7
(c)
PNDIT-F3N-dark
PNDIT-F3N-light
PNDIT-F3N-light-dark
1000
0
(d)
PNDIT-F3N
6
5
PNDIT-F3N-light-dark-light
4
3
-1000
-2000
-3000
-4000
Light off
2
1
0
-1
-0.2
0
20 40 60 80 100 120 140
0
0.2
0.4
0.6
0.8
1
Time (min)
Voltage (V)
Figure 5. (a) Dark and illuminated J−V characteristics
of schottky-junction devices based on different ETL
layers. (b) J-V curves of devices for the blend film
PNDIT-F8/F3N in dark and under illumination. (c)
Photo current respond of the PNDIT-F3N film. (d)
Dark and illuminated J-V curves of the devices for
the PNDIT-F3N film after the irradiation.
3220
3230
3240
3250
3260
Magnetic Field (G)
Figure 4. ESR spectra of different polymers and
blends in solid state.
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Journal of the American Chemical Society
PC71BM/PNDIT-F3N and PC71BM/PNDIT-F3N-Br films,
respectively, which are much higher than those of the
1
2
3
4
5
6
7
8
1.2
pure films. The improved mobilities may be due to the
removal of the PC71BM surface traps through n-doping,
which reduces the transient time of the electrons through
the film.20c It is noteworthy that this improvement in the
electrical properties was also observed when the thickness
of the ETLs was increased to 100 nm (Figure 3d). The
current density of the devices remained ten times higher
than that of the sole PC71BM device under the equivalent
voltage V (V = Vappl-Vbi-Vs). These results suggest that
achieving electrically coherent contact between the
PC71BM and ETL through the interfacial n-doping is an
important factor that contributes to the overall
improvement of the performance of the PSCs.5,29b,c
PNDIT-F3N 5nm
(a)
1
0.8
0.6
0.4
0.2
0
PNDIT-F3N-Br 5nm
PNDIT-F3N 50nm
PNDIT-F3N-Br 50nm
PNDIT-F3N 100nm
PNDIT-F3N-Br 100nm
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
-0.2
-1
0
1
2
3
4
Time（μs）
To provide further evidence on the electron transfer
from the amino groups or bromide anions to PC71BM, an
ESR spectroscopy study was performed. A reference
polymer PNDIT-F8 with pure alkyl side chains was also
tested as it is composed of the same backbone structure
as PNDIT-F3N and PNDIT-F3N-Br, and all samples were
studied in solid form and in darkness. We first
investigated the sole component of PNDIT-F8, PNDIT-
F3N, and PNDIT-F3N-Br. No ESR signal was detected for
PNDIT-F8, but for PNDIT-F3N and PNDIT-F3N-Br, weak
ESR signals from the unpaired electron with a g value of
2.002 were observed (Figure 4), revealing that self-doping
may occur in both of the WSCPs. Blended samples of
PC71BM/PNDIT-F8 and PC71BM/PNDIT-F3N with the
same 1:1 weight ratio were also tested to study the charge
transfer properties between the two components. The
samples were prepared by dissolving the same quantity of
PC71BM and polymers in chloroform followed by drying
under vacuum at 40 ℃. As shown in Figure 4, no
diagnostic anionic fullerene absorption could be observed
in the blend of PC71BM/PNDIT-F8, suggesting no electron
transfer from PNDIT-F8 to PC71BM. However, in the
PC71BM/PNDIT-F3N blend, strong resonance peaks with g
values of 2.000 were detected. This result clearly suggests
that efficient electron transfer occurred from the amine
groups on the side chain of the PNDIT-F3N to PC71BM.
Owing to the poor compatibility between the two
materials in the PC71BM/PNDIT-F3N-Br blend, we could
not identify a suitable solvent system to prepare a
thoroughly mixed blend, and therefore no accurate ESR
study could be performed. However, Jen et al. have
proved that bromide anions can n-dope PC71BM even
under a mild condition, suggesting that the PNDIT-F3N-
Br in our case may also n-dope the PC71BM.34 The results
from both the SCLC and ESR studies provide evidence
that the amine groups and the bromide anions of the
ETLs could n-dope PC71BM at the BHJ/ETL interface,
leading to enhanced PSC device performance.
1.2
1
(b)
PNDIT-F3N-FB 5nm
PNDIT-F3N-Br 5nm
PNDIT-F3N-FB 50nm
PNDIT-F3N-Br 50nm
PNDIT-F3N-FB 100nm
PNDIT-F3N-Br-FB 100nm
0.8
0.6
0.4
0.2
0
-0.2
-1
0
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2
3
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7
Time (μs)
Figure 6. Transient photocurrent of different BHJ
systems PTB7-Th:PC71BM (a) and PffBT4T-
OD/PC71BM (b) based PSC devices with different
ETLs.
through photo-induced electron transfer between an
electron donor and an electron acceptor, which was
recently exploited in the preparation of ETLs for highly
efficient PSCs.36 To study the underlying electron
transport mechanism of our ETLs, we conducted
electrical measurements under both dark and illuminated
conditions based on an ITO/ETL/Ag device structure
(Figure 5a) to evaluate whether photo-induced doping
was present in our systems. There was no difference in
the I-V characteristic of the reference polymer PNDIT-F8
when the device was measured under darkness or
illumination at AM 1.5G 100 mW/cm2, indicating that no
photo-induced doping had occurred. However, for the
PNDIT-F3N based devices, a distinct change in I-V
characteristic was observed. When the device was
illuminated, the current increased significantly by about
25-fold when compared to the current in darkness at a
bias of 1 V (Figure 5a). The high photoconductivity of
PNDIT-F3N can be explained by the photo-induced
intramolecular electron transfer from amino groups to the
conjugated backbone of the polymer.37 The proposed
Photoconductivity Measurement. It is quite unusual
for PSCs with such a thick ETL (100 nm) to operate
properly unless the ETL possesses high conductivity
through doping.35 One possible doping mechanism is
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mechanism is described as follows: when the PNDIT-F3N very similar decay curves, indicating that the charge
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is illuminated, one electron is photo-excited from the
HOMO to the LUMO of the polymer, thus leaving a hole
in the HOMO. As the amine contains an electron lone
pair and the oxidation potential of amine (~ -5.1 eV) is
smaller than the HOMO of PNDIT-F3N (-5.5 eV), electron
transfer from the amine to the polymer HOMO is
energetically favorable, and therefore the hole on the
polymer is backfilled by the electron transferred from the
amine group. As a result, a free electron remains in the
LUMO of the polymer and is stable by the positively
charged ammonium as a result of the photo-induced n-
doping process. The stabilized n-doping effect was
revealed in the I-V response of the device after the
removal of light as shown in Figure 5c. It was observed
that when the light was on, the current increased
significantly and then slowly saturated at ~ 7.2 µA after
about 20 min of light soaking. When the light was turned
off, a rapid drop in current was observed before it slowly
stabilized with increasing time. After 2 hours in darkness,
the current (4.6 µA) maintained over 60% of the original
magnitude before the light was turned off, suggesting that
the n-doping effect was stabilized within the polymer.
When the light was applied again to the sample, the
current could be recovered as shown in Figure 5d,
indicating that the photo-induced n-doping is a reversible
process. To prove that the n-doping originated from the
photo-induced electron transfer from the amine, we
mixed the reference polymer PNDIT-F8 with the 2,7-
dibromo-9,9-bis(3’-(N,N-dimethylamino)propyl)-fluorene
(F3N) molecule, which is the fluorine unit with the
amine-functionalized side chains building block for
PNDIT-F3N, and studied the current response of the
PNDIT-F8/F3N (molar ratio = 1:1) blend film. As can be
seen in Figure 5b, upon light soaking, the conductivity of
the blend film greatly increased, and it resembled that of
the PNDIT-F3N. This finding further validated the
proposed photo-induced doping mechanism for the
PNDIT-F3N polymer. For the PNDIT-F3N-Br, the
corresponding devices exhibited high conductivity under
both the dark and illuminated conditions, indicating that
the doping mechanism is different from that in the
PNDIT-F3N and it is very likely, that self-doping had
occurred within the material. We hypothesized that the
electron from the bromine anion could transfer to the
polymer and n-doped the conjugated backbone. Indeed, a
similar effect was previously observed in other n-type
organic semiconductors, including fullerene20c,34 and
perylene diimide molecules38 for generating highly
conductive organic films.
extraction time is similar among all of the devices
regardless of the difference in thickness of the ETLs, and
this result is consistent with the high FF observed in all of
the devices. The current decay times of the PffBT4T-
OD/PC71BM-based devices were also quite similar among
the devices with different ETLs and thicknesses, but in
general, they were longer than those found in the PTB7-
Th/PC71BM-based devices. The longer carrier lifetime
9
found
in
the
PffBT4T-OD/PC71BM
system
is
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understandable as higher charge mobilities were observed
in the PffBT4T-OD/PC71BM system,2a which allowed the
use of a thicker active layer to produce PSCs with very
high efficiency. Nevertheless, these results suggest that
the thickness of the ETLs had a negligible influence on
the charge extraction rate of the devices.
CONCLUSIONS
In summary, two novel n-type NDIs based WSCPs were
explored for high performance PSCs. Both WSCPs possess
good solubility in polar solvent, providing a facile way to
process ETLs of a range of thickness with orthogonal
solvents for multilayer fabrication. The π-delocalized
planar structure with high electron affinity of the NDI
unit enables the WSCPs to have high electron mobility
and appropriate energy level alignment, which is
desirable for ETLs. Both the amino and bromide-
quaternized ammonium groups allow these WSCPs to
possess WF tuning effects at the interface with the metal
electrode and n-doping effects at the interface with the
acceptor of the active layer. Interestingly, the amine-
functional n-type conjugated polymer PNDIT-F3N
exhibits
a
photo-induced
conductivity-enhancing
property, whereas the bromide-quaternized polymer
PNDIT-F3N-Br can be self-doped with no need of light
excitation. Additional studies suggested that photo-
induced electron transfer from the amino groups to the
NDI-based main chain plays a key role in the dramatic
photo conductivity behavior of PNDIT-F3N. These results
may bring new insights into the design of novel
thickness-insensitive ETLs. Eventually, a high PCE of
10.11% in the PffBT4T-OD/PC71BM system with
conventional device structure and a prominent PCE of
8.04% with an ETL thickness of 100nm are achieved. The
specific properties of these kinds of WSCPs make them
promising candidates as ETLs for use in high-
performance large-area PSC modules in the future.
Transient Photocurrent. Finally, transient photocurrent
measurements were used to study the competition
between carrier sweep-out by the internal field and
recombination during the operation of the BHJ solar
cells.39 As illustrated in Figure 6, the transient
photocurrent of the PSCs with different ETLs of various
thicknesses was measured at 0 bias. Devices based on the
PTB7-Th/PC71BM BHJ blend with different ETLs showed
ASSOCIATED CONTENT
Supporting Information. Complete experimental section
including synthetic procedures, device fabrication and char-
acterization details, additional UV-vis absorption spectra,
cyclic voltammetry, thermal properties and atomic force
micrographs. The supporting information is available free of
charge via the Internet at http://pubs.acs.org.
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Journal of the American Chemical Society
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Corresponding Author
*msfhuang@scut.edu.cn
*msangusyip@scut.edu.cn
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Author Contributions
†These authors contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENT
The work was financially supported by the Ministry of Sci-
ence and Technology (No. 2014CB643500), the Natural Sci-
ence Foundation of China (No. 51521002, 21520102006,
21490573, 51323003, and 21125419), Guangdong Innovative
Research Team Program of China (No. 201101C0105067115)
and Guangdong Natural Science Foundation (Grant No.
S2012030006232).
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