Alcohol-Soluble n-Type Conjugated Polyelectrolyte as Electron Transport Layer for Polymer Solar Cells

Article abstract of DOI:10.1021/acs.macromol.5b01137

(Chemical Presented). A novel alcohol-soluble n-type conjugated polyelectrolyte (n-CPE) poly-2,5-bis(2-octyldodecyl)-3,6-bis(thiophen-2-yl)-pyrrolo[3,4-c]pyrrole-1,4-dione-alt-2,5-bis[6-(N,N,N-trimethylammonium)hexyl]-3,6-bis(thiophen-2-yl)-pyrrolo[3,4-c]pyrrole-1,4-dione (PDPPNBr) is synthesized for applications as an electron transport layer (ETL) in an inverted polymer solar cells (PSCs) device. Because of the electron-deficient nature of diketopyrrolopyrrole (DPP) backbone and its planar structure, PDPPNBr is endowed with high conductivity and electron mobility. The interfacial dipole moment created by n-CPE PDPPNBr can substantially reduce the work function of ITO and induce a better energy alignment in the device, facilitating electron extraction and decreasing exctions recombination at active layer/cathode interface. As a result, the power conversion efficiency (PCE) of the inverted devices based poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C₆₁ butyric acid methyl ester (PC₆₁BM) active layer with PDPPNBr as ETL achieves a value of 4.03%, with 25% improvement than that of the control device with ZnO ETL. Moreover, the universal PDPPNBr ETL also delivers a notable PCE of 8.02% in the devices based on polythieno[3,4-b]-thiophene-co-benzodithiophene (PTB7):(6,6)-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM). To our best knowledge, this is the first time that n-type conjugated polyelectrolyte-based cathode interlayer is reported. Quite different from the traditional p-type conjugated and nonconjugated polyelectrolytes ETLs, n-CPE PDPPNBr as ETL could function efficiently with a thickness approximate 30 nm because of the high conductivity and electron mobility. Furthermore, the PDPPNBr interlayer also can ensure the device with the improved long-term stability. The successful application of this alcohol solution processed n-type conjugated polyelectrolyte indicates that the electron-deficient planar structure with high electron mobility could be very promising in developing high performance and environmentally friendly polymer solar cells.

Full text of DOI:10.1021/acs.macromol.5b01137

Article
pubs.acs.org/Macromolecules
Alcohol-Soluble n‑Type Conjugated Polyelectrolyte as Electron
Transport Layer for Polymer Solar Cells
†
†
§
†
†
†
,†,‡
Lin Hu, Feiyan Wu, Chunquan Li, Aifeng Hu, Xiaotian Hu, Yong Zhang, Lie Chen,*
and Yiwang Chen†,‡
†
‡
§
College of Chemistry/Institute of Polymers, Jiangxi Provincial Key Laboratory of New Energy Chemistry, and Department of
Electronic Information Engineering, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China
*
S Supporting Information
ABSTRACT: A novel alcohol-soluble n-type conjugated polyelec-
trolyte (n-CPE) poly-2,5-bis(2-octyldodecyl)-3,6-bis(thiophen-2-yl)-
pyrrolo[3,4-c]pyrrole-1,4-dione-alt-2,5-bis[6-(N,N,N-trimethyl-
ammonium)hexyl]-3,6-bis(thiophen-2-yl)-pyrrolo[3,4-c]pyrrole-1,4-
dione (PDPPNBr) is synthesized for applications as an electron
transport layer (ETL) in an inverted polymer solar cells (PSCs)
device. Because of the electron-deficient nature of diketopyrrolo-
pyrrole (DPP) backbone and its planar structure, PDPPNBr is
endowed with high conductivity and electron mobility. The interfacial
dipole moment created by n-CPE PDPPNBr can substantially reduce
the work function of ITO and induce a better energy alignment in the
device, facilitating electron extraction and decreasing exctions
recombination at active layer/cathode interface. As a result, the
power conversion efficiency (PCE) of the inverted devices based
poly(3-hexylthiophene) (P3HT):(6,6)-phenyl-C61 butyric acid methyl ester (PC BM) active layer with PDPPNBr as ETL
61
achieves a value of 4.03%, with 25% improvement than that of the control device with ZnO ETL. Moreover, the universal
PDPPNBr ETL also delivers a notable PCE of 8.02% in the devices based on polythieno[3,4-b]-thiophene-co-benzodithiophene
(
PTB7):(6,6)-phenyl-C -butyric acid methyl ester (PC BM). To our best knowledge, this is the first time that n-type
71 71
conjugated polyelectrolyte-based cathode interlayer is reported. Quite different from the traditional p-type conjugated and
nonconjugated polyelectrolytes ETLs, n-CPE PDPPNBr as ETL could function efficiently with a thickness approximate 30 nm
because of the high conductivity and electron mobility. Furthermore, the PDPPNBr interlayer also can ensure the device with the
improved long-term stability. The successful application of this alcohol solution processed n-type conjugated polyelectrolyte
indicates that the electron-deficient planar structure with high electron mobility could be very promising in developing high
performance and environmentally friendly polymer solar cells.
1
. INTRODUCTION
the charge extraction and transportation but also promote
interfacial stability for a high-efficiency and stable device
Bulk heterojunction (BHJ) polymer cells (PSCs) have achieved
tremendous progress in the past decade due to their advantages
such as low-cost manufacturing, mechanical flexibility, and easy
fabrication for large-scale production.
conversion efficiencies (PCE) of the state-of-the-art devices have
already exceeded 10%.
structure PSCs are of particular interest for the superior
efficiency and stability compared with the conventional
devices.
coated between the bottom cathode of indium tin oxide (ITO)
and top anode of a high work function (WF) metal (Ag or
Au).
common and successful strategy is developing an original
photoactive layer material and fabricating a desirable device
structure. Nevertheless, the interface engineering in the device
also plays a significant role for a breakup in the superior efficiency
and stability. An excellent interlayer can not only build a tunable
energy level alignment between the contact interface to facilitate
17,18
eventually.
In the inverted devices, the inorganic metal
oxides such as titanium oxide (TiO ) or zinc oxide (ZnO) have
x
1
−5
always been incorporated as cathode interlayers or electron
transport layers (ETLs) to modify ITO cathode due to their high
Recently, the power
1
9,20
6
−11
charge mobility and electron selective properties.
However,
Especially, the so-called inverted
these inorganic ETLs are not well compatible with the organic
active layer. Moreover, most of them typically require a high-
temperature annealing process to yield high charge carrier
mobility, which do not meet with the high throughput roll-to-roll
manufacturing techniques for commercial application.
In order to overcome the weakness of the inorganic metal
oxides, a series of organic interlayer materials have already been
explored, including small molecule-based organic interlayers and
1
2−14
For the inverted PSCs, the BHJ active layer is
1
5,16
To obtain a more brilliant performance, the most
Received: May 27, 2015
Revised: August 2, 2015
©
XXXX American Chemical Society
A
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a
Scheme 1. Synthesis Routes of the Copolymer PDPPBr and n-CPE PDPPNBr
a
The inset is the photographs of the copolymer PDPPBr and n-CPE PDPPNBr dissolved in methanol solution.
polymer-based organic interlayers. The successful small-
molecular interlayers are the fullerene derivatives, perylenedii-
materials. What is more, most of reported PEs are based on p-
type conjugated or nonconjugated polymer backbones, such as
polythiophenes, polyfluorenes, polycarbazoles, and polyethyle-
21,22
mide salt, and so on.
However, it is limited due to the
29−32
difficulty of product purification and an unfavorable film of the
interlayer. The typical polymer-based interlayers are mainly
polyelectrolytes (PEs), such as poly [(9,9-bis(3′-(N,N-
dimethylamino)propyl)-2,7- fluorene)-alt-2,7-(9,9-dioctylfluor-
ene (PFN) and polyethylenimine ethoxylated (PEIE), which
have been paid more attention for the reasons of good film
nimine.
Theoretically, these p-type conjugated or non-
conjugate backbone as ETLs would hamper the electron
extraction and transportation, so that the thickness of these
PEs interlayers is extremely limited to only a few nanometers
33
(usually less than 10 nm), which is not helpful to high
throughput commercial application in the near future. Although
metal-containing conjugated polymers interlayer, e.g. PFEN-Hg,
with a thickness ∼20 nm demonstrated a good device
11,23−25
forming properties and easy to preparation.
It has been
reported the PEs were introduced as interlayers to afford
34
remarkable performance improvement in the PSCs, and the
performance, the toxic metal Hg has been incorporated.
26
power conversion has broken through 10%. The ionic or polar
pendant groups of the PEs side chain render the conjugated
polymers soluble in water, methanol, dimethylformamide, and
other polar solvents, which can efficiently avoid the erosion from
the active layer dissolved in the chlorinated solvents. More
importantly, the different polar groups in these PEs can form a
strong interfacial dipole moments in the cathode interface,
resulting in a shift of the vacuum level at the PE/cathode
interface and an adjusted work function of cathode electrodes
In this study, we concentrate in the PEs backbone to design
and synthesize a novel n-type conjugated polyelectrolyte (n-
CPE) poly-2,5-bis(2-octyldodecyl)-3,6-bis(thiophen-2-yl)-
pyrrolo[3,4-c]pyrrole-1,4-dione-alt-2,5-bis-[6-(N,N,N-
trimethylammonium)hexyl]-3,6-bis(thiophen-2-yl)-pyrrolo-
[3,4-c]pyrrole-1,4-dione (PDPPNBr) (Scheme 1). We find that
the feature of the conjugated backbone of the polyelectrolyte has
a significant impact on the properties. Because of the electron-
deficient nature and strong interactions of diketopyrrolopyrrole
(DPP) backbone, this novel n-CPE shows high conductivity and
(
such as Cu, Ag, and Au). Therefore, electron transport and
35
injection from the active layer to the cathode can be effectively
electron mobility. Moreover, the introduced ammonium salt
polar pendant groups in the n-CPE PDPPNBr can not only
create favorable dipole moments to provide better energy
alignment in solar cells but also endow n-CPE PDPPNBr with
2
7,28
regulated.
However, the present research about the PEs
mainly focuses on the pendent polar groups but neglects the
influence of PEs backbone on the characteristics of the interlayer
B
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good solubility in methanol and other environmentally friendly
solvents. Subsequently, the n-CPE PDPPNBr as ETL has been
successfully applied in the inverted device with a structure of
ITO/n-CPE/P3HT:PC BM/MoO /Ag, and a desirable PCE of
6
1
3
4.03% is obtained, which is about 24% enhancement than the
value obtained from the control device with ZnO ETL. Replacing
the active layer with the low-bandgap donor polythieno[3,4-b]-
thiophene-co-benzodithiophene (PTB7) and acceptor [6,6]-
phenyl-C -butyric acid methyl ester (PC BM), a prominent
71
71
PCE as high as 8.02% is achieved. It is worthy to note that this is
the first time that n-type conjugated polyelectrolyte-based
interlayer is reported. Inspiringly, quite different from p-type
conjugated or nonconjugated PEs, the high electron mobility and
conductivity benefitted from the n-type feature and solid-state
packing can effectively alleviate the thickness limitation and make
PDPPNBr capable of functioning efficiently in a wide thickness
range of 6 to ∼30 nm. This characteristic of the solution
processed n-CPE ETL provides the feasibility of large-area roll-
to-roll manufacturing techniques. Additionally, the PDPPNBr
interlayer also can ensure the device with an improved long-term
stability. These findings indicate this unprecedented alcohol-
soluble n-type CPE could be a desirable candidate for the
interfacial layer materials in highly efficient and stable PSCs.
Figure 1. UV−vis absorption spectra of PDPPBr in chloroform solution
and PDPPNBr in methyl alcohol solution as well as the corresponding
thin films.
(
∼150 nm) relative to that of their corresponding solutions,
resulting from intermolecular interactions of the diketopyrrolo-
pyrrole backbone favoring the strong solid-state packing. The
frontier molecular orbital energy of copolymer PDPPBr and the
conjugated polyelectrolyte PDPPNBr was calculated from the
cyclic voltammetry (CV), which is presented in Figure S4. Based
on the reduction potential (−0.78 eV) and the oxidation
potential (0.75 eV), the lowest unoccupied molecular orbital
(LUMO) and the highest occupied molecular orbital (HOMO)
of PDPPNBr are estimated to be −3.62 and −5.15 eV,
respectively. The physiochemical properties of the copolymer
are summarized in Table S1. The energy levels are well consistent
with density functional theory (DFT) calculations with the
Gaussian 09 at the B3LYP/6-31g(d) level for copolymer
PDPPBr, as shown in Figure S5. After optimizing the geometrical
structures with a fixed repeating structure, the calculated HOMO
and LUMO energy levels are −4.89 and −2.96 eV, respectively.
Furthermore, the torsion angle between the two thiophene rings
is only 6° tilted from the planar geometry, which would be in
2. RESULTS AND DISCUSSION
The synthesis and structure of the n-type conjugated
polyelectrolyte PDPPNBr are shown in Scheme 1 (synthesis
details are provided in the Supporting Information). Compound
1
reacted with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxobor-
olane to afford DPP-based compound 2. Then, the copolymer
PDPPBr was first prepared by the Suzuki coupling polymer-
ization between compounds 2 and 3. The copolymer PDPPBr
was purified by precipitation in methanol followed by Soxhlet
extraction using methanol, acetone, and chloroform. The final
dark-purple solid was easily soluble in the common organic
solvents such as chloroform, chlorobenzene, and toluene at room
temperature. The ionized copolymer PDPPNBr was obtained
through an almost quantitative quaternization by treating the
copolymer with a trimethylamine in tetrahydrofuran (THF)
37,38
favor of the charge transfer.
24,36
solutions at low temperature.
Because of the existence of the
The polar side chain in the polyelectrolyte is preferable to form
an aligned dipole moment at metal/organic material interface,
which can modify the work function (WF) of the electrode by
ammonium salt polar pendant groups, the resulted n-CPE
PDPPNBr can be readily dissolved in polar solvents including
methanol, dimethylformamide (DMF), and dimethyl sulfoxide
39
inducing a vacuum-level shift. To characterize the WF change
of PDPPNBr-modified ITO electrode, ultraviolet photoelectron
spectroscopy (UPS) was carried out. A thin layer of PDPPNBr
with a thickness of approximately 12 nm was deposited on the
ITO substrate. As shown in Figure 2a, the bare ITO exhibits a
(
DMSO). The comparison photographs of the copolymer
PDPPBr and its corresponding n-CPE PDPPNBr dissolved in
methanol solution are shown in Figure 1. It can be observed that
the copolymer PDPPBr presents a large amount of precipitate at
the bottom of the bottle, while the n-CPE PDPPNBr shows a
clearly blue methanol solution, thanks to the ammonium salt
polar groups. In addition, all the intermediates and final product
have been thoroughly purified, and the chemical structures of the
products were characterized by spectroscopic methods, from
which satisfactory analysis data were obtained (for the
17
typical WF value of 4.84 eV, in accordance with the literature,
while the WF moves to 4.12 eV by 0.72 eV shift when ITO is
modified with PDPPNBr. Upon deposition of PDPPNBr on
ITO, the negative bromide counterions tend to be located near
the hydrophilic ITO, whereas the hydrophobic conjugated
polymer backbone preferentially locates on the surface (as shown
1
13
39,40
corresponding H NMR and C NMR of the intermediates
and final product see the Experimental Section in Figures S1−S3
of the Supporting Information).
schematically in Figure 2b),
which would spontaneously give
rise to the formation of interfacial dipole at the ITO/PDPPNBr
interface. The dipole moment pointing away from the ITO
surface to active layer makes the substantial reduction of WF
The ultraviolet−visible (UV−vis) absorption spectra of the
copolymer PDPPBr and the conjugated polyelectrolyte
PDPPNBr in solution and solid states are shown in Figure 1.
Owing to the same backbone of PDPPBr and PDPPNBr, the
copolymer and conjugated polyelectrolyte solutions maintain
similar absorption bands. However, the maximum absorption
wavelength (λmax) of both thin films displays an obvious red-shift
through the swift down of the vacuum level (E ) of the
vac
substrates closer to that of the active layer. Consequently, the
interfacial dipole formation at ITO/PDPPNBr interface can
lower the Schottky barrier and favor a better energy alignment to
facilitate the electron injection and transport from the photo-
harvest layer to ITO.
C
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Figure 2. (a) Work function of the bare ITO and PDPPNBr modified ITO by ultraviolet photoelectron spectroscopy (UPS). (b) Schematic of the
schematically proposal formation of interfacial dipole at ITO/PDPPNBr interface in the inverted devices. (c) Work function error graph of the bare ITO
and ITO coated with ZnO and PDPPNBr, respectively. (d) Schematic energy level diagram of the different investigated electrode interlayers in various
photoactive layer devices.
Figure 3. AFM morphology images (5 μm × 5 μm) of (a) bare ITO, (b) ITO/ZnO, (c) ITO/PDPPNBr, (d) ITO/P3HT:PC BM, (e) ITO/ZnO/
61
P3HT:PC BM, and (f) ITO/ZnO/PDPPNBr/P3HT:PC BM.
61
61
In order to further confirm the electronic property variation
obtained from the ultraviolet photoelectron spectroscopy, the
Kelvin probe method was employed to determine the work
function of ITO/PDPPNBr, and result of the commonly efficient
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Figure 4. (a) Chemical structures of the photoactive layer used in the devices fabrication, (b) structure of the inverted device, (c) the J−V curves of the
PSCs devices ITO/ETLs/P3HT:PC BM/MoO /Ag with various ETLs, and (d) the corresponding EQE spectra of the PSCs devices with various
61
3
ETLs.
4
1
19
composite electrode ITO/ZnO (sol−gel ZnO nanoparticles
ITO/ZnO, and ITO/PDPPNBr. As shown in Figure 3, the bare
ITO exhibits a roughness root-mean-square (rms) of 3.29 nm.
After being spin-coated with a thin layer ZnO nanoparticles, the
surface becomes smoother with a decreased rms of 1.23 nm,
while upon deposition of the PDPPNBr on ITO substrate, a
relatively rougher surface with a rms of 3.59 nm is obtained. Since
the surface wettability and morphology of electrode have been
altered by covering a layer of PDPPNBr, it is expected to
influence the deposition of the active layer. Thereby, AFM was
conducted to investigate the morphology of active layer
deposited on bare ITO, ITO/ZnO, and ITO/PDPPNBr. The
P3HT:PC BM active layer was spin-coated on these substrates
deposited on ITO substrate) was also presented for comparison.
Figure 2c demonstrates that the bare ITO and the reference
ITO/ZnO exhibit the work function value of −4.70 and −4.41
eV, respectively, in agreement with the literature. Coating a
layer of PDPPNBr on ITO leads to a remarkable reduction of
WF compared with the bare ITO (−4.25 eV vs −4.70 eV). Note
that the WF of ITO/PDPPNBr is even lower than that of ITO/
ZnO (−4.25 eV vs −4.41 eV). The reduced WF of ITO/
PDPPNBr is very close to the LUMOs of PC BM and PC BM,
17
6
1
71
which can form an ohmic contact with the active layer and
smoothly transport the electrons from fullerene acceptor to the
ITO cathode (Figure 2d).
61
and then annealed at 150 °C for 10 min. Seen from the images in
Figure S6 displays the hydrophilic alteration of bare ITO,
ITO/ZnO, and ITO/PDPPNBr. The bare ITO exhibits a water
contact angle of ca. 73°. After a layer of the so-gel ZnO
nanoparticles spin-coated on the ITO, the contact angle to
deionized water of ITO/ZnO decreases to ca. 60°. Replacing the
ZnO to PDPPNBr leads to the surface of ITO/PDPPNBr with
an increased water contact angle of ca. 89°. The more
hydrophobic surface of ITO/PDPPNBr again gives a direct
proof to verify that the negative bromide counterions tend to
locate at the hydrophilic ITO, whereas the hydrophobic
conjugated polymer backbone preferentially locates at the side
of organic active layer (Figure 2b). The increased hydrophobic
surface of ITO/PDPPNBr allows for the more imitated contact
with upper active layer. Moreover, the calculated surface energy
of ITO/PDPPNBr (30.67 mN/m) is closer to PC BM and
Figure 3, the P3HT:PC BM deposited on bare ITO and ITO/
61
ZnO substrates exhibits a roughness rms of 3.1 and 1.8 nm,
respectively. It is worthy to note that the PFN, a well-known
interlayer for state-of-art PSCs, is not suitable for P3HT:PC BM
6
1
devices due to the poor compatibility between interlayer and
active layer. Delightfully, deposition of P3HT:PC BM on the
6
1
ITO/PDPPNBr develops a robust and uniform film with a
roughness rms of 4.3 nm, suggesting a good contact of ITO/
PDPPNBr with P3HT:PC BM. The difference in roughness of
61
P3HT:PC BM is caused from the morphology variation of the
61
substrates. It has been reported a rougher surface of the active
layer would facilitate the interfacial adhesion between sub-
sequent hole extraction layer and the active layer; therefore, an
43,44
improved charge transport can be anticipated.
In addition, a
uniform microphase separation of P3HT:PC BM is well
61
61
PC BM (28.1 mN/m) than P3HT (23.8 mN/m), which would
be beneficial to a vertical phase separation of the active layer.
developed on the ITO/PDPPNBr substrate, which would be
very helpful for charge separation and transportation.
71
42
Additionally, atomic force microscopy (AFM) was also
employed to determine the morphology of the bare ITO,
To evaluate the interfacial modification function of the novel
n-type conjugated polyelectrolyte PDPPNBr as ETL, bulk
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Table 1. Device Parameters of the Devices ITO/PDPPNBr/P3HT:PC BM/MoO /Ag with PDPPNBr Interlayer of Various
6
1
3
Thicknesses under the Illumination of AM 1.5G, 100 mW cm−
2
Jsc (mA/cm2)
FF (%)
PCE (%)
R (Ω cm )
2
Rsh (Ω cm )
2
active layer
interlayer (thickness)
Voc (V)
s
P3HT:PC BM
0.37
0.62
0.61
0.65
0.64
0.66
0.46
0.73
0.75
7.63
7.70
30.21
68.03
67.38
69.29
68.96
65.83
52.8
0.85
3.25
2.89
4.03
3.68
3.41
2.83
7.15
8.02
41.24
8.90
1.95
2.14
3.46
13.28
12.26
4.16
2.04
66.94
592.48
2748
61
ZnO
PDPPNBr (6 nm)
PDPPNBr (12 nm)
PDPPNBr (20 nm)
PDPPNBr (29 nm)
7.02
8.95
1537
8.35
1665
7.84
1579
PTB7:PC BM
11.67
14.29
15.10
301.42
2271.55
2690.82
71
ZnO
68.5
PDPPNBr (12 nm)
70.8
heterojunction (BHJ) solar cells with an inverted structure of
ITO/PDPPNBr/active layer/MoO /Ag were fabricated. The
3
details of the devices fabrication process are provided in the
Supporting Information. A schematic device structure and the
chemical structures of the active materials employed in this work
are shown in Figure 4. First of all, the effect of thickness (6, 12,
20, and 29 nm) of the PDPPNBr on the device performance has
been determined, and the optical transmittance spectra of the
bare ITO and ITO coated with various thicknesses of PDPPNBr
are also investigated. As displayed in Figure S7, the trans-
mittances of ITO/PDPPNBr with different thickness are almost
identical to that of the bare ITO, inferring that this n-CPE is
suitable as interlayer. Figure 4 shows the illuminated current
density−voltage (J−V) curves of devices based on
P3HT:PC BM with PDPPNBr as the ETL, and the detailed
61
photovoltaic characteristics are summarized in Table 1. The best
PCE of 4.03%, together with an open circuit voltage (V ) of 0.65
oc
2
V, a short circuit current density (J ) of 8.95 mA/cm , and a fill
sc
factor (FF) of 69.29%, is achieved from the device based on
PDPPNBr with an optimized thickness of ∼12 nm (Figure 5a).
The device with the ultrathin PDPPNBr cathode interlayer (6
nm) delivers the lowest PCE of 2.9%, presumably due to the
incompletely covered interlayer on the ITO electrode. More
intriguingly, when the thickness of PDPPNBr increases to 20 and
2
3
9 nm, the PCEs of the devices still remain at the high values of
.68% and 3.41%, respectively. As we all know, most
polyelectrolytes (usually based on p-type conjugated or non-
conjugated backbone) as interlayers are very sensitive to the
thickness, and the thickness limitation is always less than 10 nm.
For instance, the optimized thickness of the well-known cathode
33
interlayer PFN is found to be 5−6 nm, and further increasing
the interlayer thickness results in a sharply decreased device
efficiency. This is because the p-type conjugated or non-
conjugated backbone is not beneficial for electron transport;
therefore, the electron extraction from active layer to cathode
45
mainly relies on the electron tunneling effect. In order to
further confirm this opinion, the control device based non-
conjugated polyelectrolyte polyethylenimine ethoxylated (PEIE)
ETL was fabricated; the contrastive parameters are shown in
Table S2. It is obvious that the PEIE could also tune the interface
energy alignment so that a superior V was obtained, whereas the
oc
inferior J was probably caused by the nonconjugated backbone
sc
with a poor conductivity. Additionally, increasing the interlayer
thickness will inevitably increase the bulk resistance of the
interlayer, consequently leading to the sharply decreased
electron extraction and transportation. For the PDPPNBr
Figure 5. (a) J−V characteristics of the devices ITO/PDPPNBr/
P3HT:PC BM/MoO /Ag with PDPPNBr interlayer of various
6
1
3
−2
thicknesses under the illumination of AM 1.5G, 100 mW cm , (b)
−V characteristics of electron-only devices ITO/ETLs/
P3HT:PC BM/LiF/Al with different ETLs, and (c) I−V curves for
0
.5
J
61
ETL, when the thickness increases from 6 to 29 nm, the series
2
electron-only transfer devices ITO/ETLs/Au.
resistance (R ) only slightly increases from 1.95 to 13.28 Ω cm .
s
To explore the reason behind the unique PDPPNBr ETL, the
F
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Figure 6. (a) J−V curves of the PSCs devices ITO/ETLs/PTB7:PC BM/MoO /Ag with various ETLs, (b) the corresponding EQE spectra of the
71
3
PSCs based on PTB7:PC BM with various ETLs, (c) measured V of cells with ZnO and PDPPNBr interlayer plotted against light intensity on a
71
oc
logarithmic scale (symbols), together with linear fits to the data (solid lines), and (d) measured J of cells with ZnO and PDPPNBr interlayer plotted
sc
against light intensity on a logarithmic scale. Fitting a power law (solid lines) to these data yields α. The devices are based on PTB7:PC BM as active
7
1
layer.
2
electron mobility of PDPPNBr in the solar cell devices is
estimated using the space-charge limited current (SCLC)
0.37 to 0.65 eV), J (from 7.63 to 8.95 mA/cm ), and FF (from
sc
30.2% to 69.3%), suggesting a more favorable charge injection
and collection are achieved at the electrodes. The simultaneously
increased Voc and Jsc values are related to the better energy
alignment induced by the dipole moments created from the
PDPPNBr with polar groups, which is in good agreement with
the UPS and Kelvin probe observation. As revealed by Table 1,
compared with bare ITO and ITO/ZnO-based devices,
incorporation of PDPPNBr ETL into the device can significantly
reduce the series resistance (R ) and increase the shunt resistance
(R ), indicating a good interfacial modification to boost charge
injection and suppress charge recombination. As a result, a
distinctly improved Jsc and FF are achieved. Moreover, the
desirable morphology of the active layer and favorable interface
contact observed from AFM and water contact angle measure-
46−48
method according to the Mott−Gurney equation.
As
plotted in Figure 5b, the device with bare ITO shows an electron
−5
2
−1 −1
mobility of 2.08 × 10 cm V s . After incorporation of
PDPPNBr as ETL, the electron mobility is remarkably enhanced
to 5.70 × 10− cm V s , which is even higher than that of ZnO
4
2
−1 −1
−
4
2
−1 −1
ETL (3.78 × 10 cm V s ) (Table S2). In addition, the
conductivity of the ZnO and PDPPNBr interlayer was also
investigated and compared. As shown in Figure 5c, PDPPNBr
displays more superior conductivity over ZnO (9.98 × 10
cm vs 7.89 × 10 S cm ), and its conductivity is even 3 orders
s
−6
S
sh
−1
−6
−1
−9
of magnitude higher than that of PFN interlayer (6.9 × 10
S
−
1
49
cm ). The high electron mobility and conductivity of
PDPPNBr interlayer mainly originate from the n-type character
and the dense packing of planar DPP units, which should be
responsible for the highly maintained performance devices with
the thick ETL. This novel interlayer could provide the feasibility
of large-area roll-to-roll manufacturing techniques.
ment should also account for the J and FF enhancement. The
highest performance promoted by the PDPPNBr ETL can be
further elucidated by the most restrained leakage current in the
sc
dark J−V curve (Figure S8). The theoretical J obtained by
sc
Superiority of the PDPPNBr ETL can be clearly verified by
comparing the devices performances with ITO/PDPPNBr,
ITO/ZnO, and bare ITO. From Figure 4 and Table 1 we can
see that all of the devices with PDPPNBr ETLs show significantly
improved PCE than the control device with bare ITO (0.85%),
and PDPPNBr with a thickness of 12, 20, and 29 nm even
achieves a higher PCE than the widely used ZnO ETL (3.24%).
And the highest efficiency obtained from PDPPNBr (4.03%)
shows 374% and 24% improvement over those obtained from
bare ITO and ZnO interlayers, respectively. The remarkable
integrating the corresponding external quantum efficiency
(EQE) data (in Figure 4d) is also well consistent with the
current values from the J−V characteristic.
To further confirm and testify the universality of this novel
PDPPNBr ETL, the inverted PSCs devices based on the low-
bandgap polymer polythieno[3,4-b]-thiophene-co-benzodithio-
phene (PTB7) as a donor and (6,6)-phenyl-C -butyric acid
7
1
methyl ester (PC BM) as a acceptor were fabricated. The
71
corresponding J−V characteristics and devices parameters are
summarized in Figure 6a and Table 1, respectively. As expected,
with respect to the bare ITO and ZnO modified ITO electrode,
improvement in PCE is a result of the overall improved V (from
oc
G
DOI: 10.1021/acs.macromol.5b01137
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
the PDPPNBr modified ITO cathode yields the best PCE as high
as 8.02% with a V of 0.75 V, a J of 15.10, and a FF of 70.8%.
oc
sc
Similar to the device with P3HT:PC BM, the notable efficiency
6
1
of device with PTB7: PC BM is also mainly attributed to the
7
1
overall improved device parameter, including V , J , and FF.
oc sc
The external quantum efficiency (EQE) curves of the devices are
presented in Figure 6b. It can be seen that the PDPPNBr-based
device shows the highest EQE value, in accordance with the
values obtained from the J−V characteristics in Figure 6a.
Table 1 demonstrates that all of the PDPPNBr-based devices
show the dropped R and enhanced R than the bare ITO and
s
sh
ITO/ZnO-based devices, regardless of the active layer. It can be
concluded that incorporation of PDPPNBr ETL into the devices
can significantly improve charge injection and suppress charge
recombination. To gain deeper insight into the influence of
PDPPNBr interlayer on the recombination mechanism in the
devices, an additional measurement with respect to the V and J
Figure 7. Air stability test of the unencapsulated device based
P3HT:PC BM active layer with various electron transport layers for
61
30 days under ambient conditions.
oc
sc
of the ZnO and PDPPNBr based devices at various light
−2
intensities from 23 to 100 mW cm was carried out. We plotted
Voc versus the logarithm of light intensity following the equation
electron mobility. The interfacial dipole moment created by n-
CPE PDPPNBr can substantially reduce the work function of
ITO and induce a better energy alignment in the device,
facilitating electron extraction and decreasing exctions recombi-
nation at active layer/cathode interface. Consequently, the
inverted device with n-CPE PDPPNBr as ETL achieves a notable
PCE as high as 8.02%. Quite different from the traditional p-type
conjugated and nonconjugated polyelectrolytes ETLs, n-CPE
PDPPNBr as ETL could function efficiently with a thickness
approximate 30 nm, thanks to the high conductivity and electron
mobility. The success of PDPPNBr implies that alcohol-soluble
n-type conjugated polyelectrolyte could be particularly useful in
developing high-performance organic interlayer materials. In
addition, though the PDPPNBr would absorb more light with the
thickness of the PDPPNBr increase, consequently slightly
reduced short circuit current of the devices, the photovoltaic
devices still maintained a superior performance due to a desirable
conductivity and electron mobility of the novel polyelectrolytes.
Therefore, it is instructive for us to design and synthesis the n-
CPE interface materials with less light absorption in the future.
Voc ∝ nkT/q ln(P)
(1)
where k, T, q, and P are the Boltzmann constant, temperature in
kelvin, the elementary charge, and the light intensity,
5
0
respectively. Fitting the curves in Figure 6c yields a value of
.49kT/q of the cell with ZnO interlayer, while that of cell with
1
PDPPNBr interlayer is 1.37kT/q, indicating the device with
PDPPNBr has less recombination at the interface between the
51,52
active layer and ITO anode.
On the other hand, the
dependence of J upon illumination intensity is plotted on a log−
sc
log scale and fitted to a power law according the equation
J ∝ Iα
sc
(2)
53,54
where I is the light intensity and α is the exponential factor.
As shown in Figure 6d, the value α = 0.913 and α = 0.933 for the
PSCs with ZnO and PDPPNBr interlayer, respectively. The
result elucidates that the bimolecular recombination is obviously
reduced in the device based on PDPPNBr interlayer. The
suppressed charge recombination at the interface of active layer
and cathode subsequently causes the increase of J and FF.
sc
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.5b01137.
■
The air stability of polymer solar is also a crucial factor for their
practical application; therefore, the air stability of the
unencapsulated inverted devices based on P3HT:PC BM with
*
S
61
or without the cathode interlayer was periodically measured for
0 days (Figure 7). Obviously, the device without any cathode
3
Detailed synthesis procedure and corresponding NMR
spectra, UV, CV, and molecular orbital diagram of the
polymer and polyelectrolyte, and the specific device
fabrication and material (PDF)
buffer layer retained a terrible long-term stability with 28% of its
original PCE after being exposed to ambient conditions. In
contrast, the device based on PDPPNBr interlayer can still
maintained 64% of the initial efficiency after being exposed to
ambient conditions for 30 days, which is even higher than the
device based on ZnO with 60% of the initial value. The improved
long-term stability presumably results from the imitate interface
compatibility between the ITO and active layer induced by the
novel alcohol conjugated polyelectrolyte.
AUTHOR INFORMATION
Corresponding Author
*Tel +86 791 83968703; fax +86 791 83968830; e-mail chenlie@
ncu.edu.cn (L.C.).
■
Notes
3. CONCLUSIONS
The authors declare no competing financial interest.
In conclusion, a novel alcohol-soluble conjugated polyelectrolyte
PDPPNBr based on n-type backbone is first synthesized and
applied as ETL in high performance PSCs. The feature of the
conjugated backbone of the polyelectrolyte has significantly
impact on the properties. The planar DPP backbone with n-type
nature endows the PDPPNBr with high conductivity and
ACKNOWLEDGMENTS
■
This work was financially supported by the National Science
Fund for Distinguished Young Scholars (51425304), National
Natural Science Foundation of China (51263016, 51473075 and
21402080), National Basic Research Program of China (973
H
DOI: 10.1021/acs.macromol.5b01137
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
Program 2014CB260409), and Graduate Innovation Fund
Projects of Jiangxi Province (YC2014-S041).
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DOI: 10.1021/acs.macromol.5b01137
Macromolecules XXXX, XXX, XXX−XXX