Non-conjugated water/alcohol soluble polymers with different oxidation states of sulfide as cathode interlayers for high-performance polymer solar cells

Article abstract of DOI:10.1039/c6tc00906a

A series of non-conjugated water/alcohol soluble copolymers containing diphenyl sulfide, diphenyl sulfoxide or diphenyl sulfone in the main chain were synthesized. The surfactant-like functionalities in side chains endow the polymers with outstanding solubility in highly polar solvents and excellent electron extraction abilities, while the limited conjugation length of main chains renders the polymers weak optical absorption. These unique characteristics make the polymers excellent cathode interfacial layers, which contribute to highly efficient polymer solar cells (PSCs) with power conversion efficiencies of 8-9%. More importantly, some of them can yield highly efficient PSCs in a relative large interlayer thickness range of 2-20 nm. The results suggest that these polymers are promising candidates for use as cathode interfacial layers in single-junction PSCs and as recombination layers in multi-junction PSCs.

Full text of DOI:10.1039/c6tc00906a

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Non-conjugated water/alcohol soluble polymers with different
oxidation states of sulfide as cathode interlayers for high-
performance polymer solar cells
Received 00th January 20xx,
Accepted 00th January 20xx
Xi Liu,†,‡ Rongguo Xu,†,‡ Chunhui Duan,*,† Fei Huang,*,† and Yong Cao†
DOI: 10.1039/x0xx00000x
A series of non-conjugated water/alcohol soluble copolymers containing diphenyl sulfide, diphenyl sulfoxide or diphenyl
sulfone in main chain were synthesized. The surfactant-like functionalities in side chains endow the polymers outstanding
solubility in highly polar solvents and excellent electron extraction abilities, while the limited conjugation length of main
chains renders the polymers with weak optical absorption. These unique characteristics make the polymers excellent
cathode interfacial layers, which contribute to highly efficient polymer solar cells (PSCs) with power conversion efficiencies
of 8–9%. More importantly, some of them can yield highly efficient PSCs at a relative large interlayer thickness range of 2–
20 nm. The results suggest that these polymers are promising candidates for use as cathode interfacial layers in single-
www.rsc.org/
junction
PSCs
and
as
recombination
layers
in
multi-junction
PSCs.
good stability and unique solubility.25, 26, 28-31, 55
1. Introduction
For example, amino-group-functionalized conjugated
polymers based on fluorene and their ionic derivatives have
Bulk-heterojunction (BHJ) polymer solar cells (PSCs) have
attracted considerable attention in recent years because of its
great potential of producing light-weight, and flexible
photovoltaic modules via low-cost large-scale fabrication.1-5
The power conversion efficiencies (PCEs) of PSCs in both
single-junction devices and multi-junction devices have
exceeded 10%6-8 owing to combined efforts including the
development of new photoactive polymers,9-15 morphology
control,16-20 device engineering,6, 21-24 interlayer modification25-
32, 34, 50, 51, 56-59
been successfully applied as CILs in PSCs.27,
These polymers have good solubility in polar solvents such as
alcohol and water, which make them attractive for the
fabrication of multilayer PSC devices through orthogonal
solvent processing. Moreover, these polymers can significantly
enhance the performance of PSCs via boosting one or multiple
device parameters (open-circuit voltage (Voc), short-circuit
current density (Jsc), and fill factor (FF)). For example,
poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-
2,7-(9,9-dioctylfluorene)] (PFN)60 had been refreshing the PCE
records of PSCs repeatedly.27, 56
31
and so on. Now it is well recognized that the control of
active layer/electrode interface to form Ohmic contact is
pivotal to maximize the performance of PSCs. Hence, lots of
efforts were devoted to the development of novel interfacial
materials for electrode modification, especially for cathode
modification.8, 32-39 Several kinds of materials have been widely
used as cathode interfacial layer (CIL) in PSCs, which are based
on different lines, spanning from inorganic compounds such as
LiF,40 TiOx,41 CsCO3,42 and ZnO43-45 to organic small molecules38,
39, 46-49
50-54
and conjugated/non-conjugated polymers.37,
Among various CIL materials, conjugated water/alcohol soluble
polymers (CWSPs) that consist of conjugated main chain and
polar/ionic side chain have received growing attention
because of its outstanding intrinsic optoelectronic properties,
*
Scheme 1 Device architecture of PSCs used in this study, as well as the chemical
structures of NWSPs, and the chemical structures of active layer PTB7:PC71BM
employed in this study.
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Herein, we report a series of nonconjugated water/alcohol promising candidates cathode interfacial layers in single-
soluble polymers (NWSPs) that derived from fluorene and junction PSCs and recombination layers in multi-junction PSCs.
diphenyl sufide with different oxidation states (Scheme 1) for
use as CILs to achieve highly efficient PSCs. These novel NWSPs
2. Results and Discussion
(PF6NPS, PF6NPSO, PF6NPSO2, PF6NPSBr, PF6NPSOBr,
PF6NPSO2Br) comprises rigid nonconjugated main chain and
2.1. Design, Synthesis, and Characterization
surfactant-like side chains. The neutral amino groups or ionic
Scheme 2 shows the chemical structures and synthetic routes
ammonium groups render the polymers with good solubility in
of the relevant monomers and polymers. Monomer 4,4’-
polar solvents (such as methanol) for multilayer solution
dibromo-diphenylsulfide
(
1
)
was obtained by treating
processing and good electron collection ability in metal
cathodes. The conjugation of the main chains of these
polymers was interrupted by the sulfur atoms, which can be
further oxidized to different states, sulfoxide and sulfone. This
design renders the new interfacial materials with several
unique advantages. First, the limited conjugation will endow
the polymers weak optical absorption and high transparency in
visible region. Second, the introduction of sulfoxide and
sulfone unit can deepen the frontier molecular orbital energy
levels of the polymers, and hence improve electron collection
ability and hole blocking ability of the resulting PSCs.
Benefitting from these novel CILs, highly efficient PSCs with
PCEs of 8–9% were fabricated on the basis of the photovoltaic
active layer of poly[4,8-bis(2-ethylhexyloxyl)benzo-[1,2-b:4,5-
b']dithiophene-2,6-diyl-alt-ethylhexyl-3-uorothithieno-[3,4-
b]thiophene-2-carboxylate-4,6-diyl] (PTB7)61 and [6,6]-phenyl-
C71-butyricacid methyl ester (PC71BM) (see Scheme 1 for
chemical structures and device architecture). More
importantly, some of these new interfacial materials can yield
highly efficient PSCs at a relative large interlayer thickness
range of 2–20 nm. The excellent cathode modification abilities
combined with weak optical absorption render these polymers
diphenylsulfide with bromine and 30% hydrogen peroxide in a
mixed solution of dichloromethane and water under argon.62
Monomer 4,4’-dibromo-diphenylsulfoxide (
2) and monomer
4,4’-dibromo-diphenylsulfone (3) were obtained by treating
1
°
with 30% hydrogen peroxide in glacial acetic acid at 80 C
under argon with different duration, and the products were
purified by silica gel column chromatography. The diboronic
ester
dioxaborolane-2-yl)-9,9-bis(6-(N,N-diethylamino)-
hexyl)fluorene ( ) was synthesized according to previous
monomer
2,7-bis(4,4,5,5-tetramethyl-1,3,2-
4
work.63 The three neutral amino-group containing polymers
(PF6NPS, PF6NPSO and PF6NPSO2) were synthesized by the
standard Pd-catalyzed Suzuki cross-coupling polymerization
with equivalent molar ratios of dibromide monomers (
1, 2 or 3)
and diboronic ester monomer The polymers were
4.
characterized with 1H NMR spectroscopy. The number-average
molecular weights (Mn) estimated by gel permeation
chromatography (GPC) using polystyrene as the standard and
THF as the eluent are 17.1 kg mol-1 (with a polydispersity (PDI)
of 1.9) for PF6NPS, 14.7 kg mol-1 (with a PDI of 1.9) for
PF6NPSO, and 16.8 kg mol-1 (with a PDI of 2.1) for PF6NPSO2,
respectively.
Scheme 2 Chemical structures and synthetic routes of the monomers and polymers. Reagents and conditions: (i) Br2, hydrogen peroxide 30% aq., H2O, CH2Cl2, ambient
°
temperature, 8 h, argon; (ii) hydrogen peroxide 30% aq., CH3COOH, 80 C, 6 h, argon; (iii) Pd(PPh3)4, tetraethylammonium hydroxide 20% aq., toluene, THF, 90 °C, 24 h, argon; (iv)
bromoethane, THF/MeOH, 40 °C, 3 d, argon.
2 | J. Name., 2012, 00, 1-3
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The three ammonium polymers (PF6NPSBr, PF6NSOBr and
Fig. S2 shows the ultraviolet-visible (UV-vis) absorption
PF6NPSO2Br) were obtained by treating the corresponding spectra of the polymers in solutions (chloroform for PF6NPS,
neutral amino-group polymers with excess bromoethane in PF6NPSO, and PF6NPSO2; methanol for PF6NPSBr, PF6NPSOBr,
°
THF-methanol solvent mixture at 40 C for 3 days in argon and PF6NPSO2Br) with concentration of 1 × 10-5 g mL-1 and in
atmosphere. The conversions of chemical structures from thin films. The results are summarized in Table 1. In
1
neutral polymers to ammonium polymers were verified by H chloroform, PF6NPS show absorption peak (λabsmax) at 344 nm,
NMR spectroscopy.60 The ionic side chains endow the while both PF6NPSO and PF6NPSO2 show absorption peaks at
ammonium polymers with excellent solubility in high polarity 350 nm. In solid state, PF6NPS, PF6NPSO, and PF6NPSO2 show
solvents, such as methanol, ethanol, N,N-dimethylformamide absorption peak at 346, 350, and 354 nm, respectively. The
(DMF) and dimethyl sulfoxide (DMSO). The three amino- ammonium polymers PF6NPSBr, PF6NPSOBr, and PF6NPSO2Br
containing neutral polymers are soluble in common non-polar exhibit the absorption peak in methanol at 350, 354 and 367
solvent, such as tetrahydrofuran (THF), chloroform and nm, respectively. In thin films, PF6NPSBr, PF6NPSOBr and
toluene, but cannot be dissolved in highly polar solvents. PF6NPSO2Br show the absorption peak at 353, 357 and 375
Interestingly, these neutral polymers are soluble in methanol nm, respectively, which are slightly red-shifted to their
by adding a trace amount of acetic acid because of the weak absorption in solutions. The absorption onsets of PF6NPS,
interaction between the basic nitrogen atoms and protons.64 PF6NPSO, PF6NPSO2, PF6NPSBr, PF6NPSOBr, and PF6NPSO2Br
The excellent solubility in highly polar solvents make these in thin films are 388, 393, 400, 392, 402 and 414 nm,
polymers good candidates for multilayer organic photovoltaic respectively, which correspond to an optical band gaps (Eg) of
devices fabrication from orthogonal solvents. The thermal 3.19, 3.15, 3.10, 3.16, 3.08 and 2.99 eV, respectively.
stability of the polymers was investigated by thermal gravity
analysis (TGA) under a nitrogen atmosphere, and the results
are presented in Fig. S1. PF6NPS, PF6NPSO and PF6NPSO2
(a)
showed good thermal stability with 5% weight-loss
temperatures (Td) of 367, 365 and 387 °C, respectively. In
contrast, PF6NPSBr, PF6NPSOBr and PF6NPSO2Br showed
much lower Td of 223, 214 and 233 °C, respectively.
PF6NPS
PF6NPSO
2.2. Optical and electrochemical properties
Fig.
1 shows the ultraviolet-visible (UV-vis) transmission
spectra of the polymers in solid state for PF6NPS, PF6NPSO,
PF6NPSO2, PF6NPSBr, PF6NPSOBr, and PF6NPSO2Br. The film
thicknesses are around 50 nm. For comparison, the
transparency of PFN thin film was studied as well. The
transmittances of the newly synthesized NWSPs reach 90–100%
over 400 nm, while the PFN lost its transparency under 430 nm.
The high transparency of these CIL materials indicates that the
active layer of the resulting PSCs, especially for the PSCs with
inverted device structures, will have more efficient photon-
harvesting and higher quantum efficiencies in high energy
region.
PF6NPSO2
0.0
0.5
1.0
1.5
2.0
Potential (V)
(b)
TBAB
PF6NPSBr
PF6NPSOBr
PF6NPSO2Br
100
90
PFN
PF6NPS
80
70
60
50
40
30
20
PF6NPSO
PF6NPSO2
PF6NPSBr
PF6NPSOBr
PF6NPSO2Br
0.0
0.5
1.0
1.5
2.0
Potential (V)
400
450
500
550
600
Fig. 2 Cyclic voltammetry (CV) curves of NWSPs. (a) PF6NPS, PF6NPSO and PF6NPSO2 in
films; (b) PF6NPSBr, PF6NPSOBr and PF6NPSO2Br in films. The measurements were
performed in 0.1 M Bu4NPF6 acetonitrile solution under nitrogen atmosphere at a scan
Wavelength (nm)
Fig. 1 UV-vis transmission spectra of the polymers and PFN in solid state. The films of
PFN, PF6NPS, PF6NPSO and PF6NPSO2 were spin-coated from CHCl3 solutions, and the
films of PF6NPSBr, PF6NPSOBBr and PF6NPSO2Br were spin-coated from MeOH
solutions.
rate of 50 mV s-1
.
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of PFN is also shown. It is clear that the HOMO and LUMO
energy levels of the NWSPs polymers decreased
simultaneously along with the increase of oxidation degree
from diphenylsulfide to diphenylsulfoxide and further to
diphenylsulfone. Moreover, all of these NWSPs show deeper
HOMO and LUMO energy levels than those of PFN, suggest
these new CIL materials potentially have better hole blocking
ability and electron transport ability.
Fig. 3 Schematic energy level diagram of the PSCs based on different CILs. The work
functions of ITO electrode, Al electrode and PEDOT:PSS, as well as the HOMO and
LUMO energy levels of PTB761, PCBM30 and PFN60 adopted the values reported in
2.3. Interface modification abilities
literature. The HOMO energy levels of NWSPs were calculated from CV measurements, To investigate the interface modification abilities of these
the LUMO energy levels of NWSPs were calculated from their HOMO levels and Eg.
NWSPs in PSCs, bulk-heterojunction PSCs with a conventional
device structure of ITO/PEDOT:PSS/PTB7:PC71BM/NWSPs/Al
The cyclic voltammetry (CV) measurements were
performed in 0.1 tetrabutylammonium
that use PTB7:PC71BM blend as the active layer were
fabricated and tested. The chemical structures of the active
layer materials PTB7 and PC71BM, as well as the device
structure are shown in Scheme 1. A thin layer (approximate 5
nm) was spin-coated on top of the active layer from MeOH
solution to form a CIL. For comparison, a control device
without any interlayer was also fabricated and tested. To
exclude the possible influence caused by methanol, the MeOH
rinsed control devices were also fabricated and tested.
M
hexafluorophosphate (Bu4NPF6) acetonitrile solution under
nitrogen atmosphere at the scan rate of 50 mV/s at room
temperature. The polymer thin films coated on a platinum
electrode were used as the working electrode. A Pt wire was
used as the counter electrode, and Ag/Ag+ (0.1 M of AgNO3 in
acetonitrile) electrode was used as the reference electrode.
The CV curves of the polymers are presented in Fig. 2. The
onset oxidation potential (Eox) for PF6NPS, PF6NPSO and
PF6NPSO2 are 1.14, 1.45 and 1.55 V, respectively. The highest
occupied molecular orbital (HOMO) energy levels of PF6NPS,
PF6NPSO, and PF6NPSO2 are calculated to be -5.52, -5.83, and
-5.93 eV, respectively, with ferrocene value of 4.8 eV below
the vacuum energy level as the internal standard.65 The onset
reduction potential (Ered) for PF6NPS, PF6NPSO and PF6NPSO2
are -2.06, -1.91 and -1.83 V, respectively. The lowest
unoccupied molecular orbital (LUMO) energy levels of PF6NPS,
PF6NPSO, and PF6NPSO2 are calculated to be -2.32, -2.47, and
-2.55 eV, respectively. The (Ered) for PF6NPSBr, PF6NPSOBr and
PF6NPSO2Br are 1.29, 1.50 and 1.64 V, respectively. The
oxidation process at around 0.70 V can be attributed to the
oxidation of the quaternary ammonium bromide in the
polymer side chain, which is confirmed by the oxidation
process of tetrabutyl ammonium bromide (TBAB). The HOMO
energy levels of PF6NPSBr, PF6NPSOBr and PF6NPSO2Br are
calculated to be -5.70, -5.91, and -6.05 eV, respectively. The
LUMO energy levels of PF6NPSBr, PF6NPSOBr, and
PF6NPSO2Br are calculated to be -2.54, -2.83, and -3.06 eV
from their HOMO energy levels and optical band gaps. The
values of HOMO and LUMO energy levels are summarized in
Table 1. Based on the data, we depicted the energy level
diagram of the PSCs in Fig. 3. For comparison, the energy level
Fig. 4a shows the corresponding current density-voltage (J–
V) characteristics of the PSCs under AM1.5G irradiation (100
mW cm-2). The photovoltaic properties of the PSCs including Jsc,
Voc, FF and PCE are listed in Table 2. The bare Al solar cells
exhibited an average PCE of 5.2%, with a Jsc of 16.7 mA cm-2, a
Voc of 0.57 V and an FF of 0.55. After inserting a thin layer of
NWSPs between the active layer and Al as CIL, all of the PSCs
devices exhibit significant performance enhancement. The
devices with PF6NPS, PF6NPSO, PF6NPSO2, PF6NPSBr,
PF6NPSOBr and PF6NPSO2Br as interlayer offered the PCE of
8.7%, 8.5%, 7.9%, 8.8%, 9.2% and 7.9%, respectively. The Voc of
the corresponding PSCs increased from 0.57 V for the bare Al
devices to approximate 0.75 V for CIL/Al devices. The Voc
enhancements upon inserting CIL between active layer and
electrode may originate from the lowered work function of Al
electrode due to the formation of dipoles in the active layer/Al
interface, which consequently result in enhanced built-in
potential and attainable Voc in PSCs.19 The FF of the
corresponding PSCs enhanced from 0.55 for the bare Al
devices to approximate 0.70 for the sulfide- and sulfoxide-
containing polymers incorporated devices, and to approximate
0.65 for the sulfone polymer inserted devices. The Jsc of the
CIL/Al devices are similar to that of the bare Al device.
Table 1 UV-vis absorption, and electrochemical properties of the polymers in solid state.
λabsa (nm)
346
Polymer
Thickness (nm)
λonsetb (nm)
388
Egc (eV)
3.19
3.15
3.10
3.16
3.08
3.00
Eoxd (V)
1.14
EHOMOe (eV)
-5.52
ELUMOf (eV)
-2.33
PF6NPS
45
58
49
51
46
PF6NPSO
PF6NPSO2
PF6NPSBr
PF6NPSOBr
PF6NPSO2Br
350
393
1.45
-5.83
-2.68
354
400
1.55
-5.93
-2.83
353
392
1.29
-5.70
-2.54
357
402
1.50
-5.91
-2.83
57
375
414
1.64
-6.05
-3.05
b
c
d
a Absorption peak. Absorption onset. Optical band gap. Onset voltage of oxidation process during cyclic voltammetry. e Calculated according to EHOMO (eV) = -e│Eox - E1/2
(ferrocene) + 4.8│. f ELUMO = Eg + EHOMO (eV).
4 | J. Name., 2012, 00, 1-3
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Table
ITO/PEDOT:PSS/PTB7:PC71BM/interlayer/Al.
2
Photovoltaic properties parameters of the PSCs with various interlayers under AM1.5G irradiation (100 mW cm-2). The device structure is
Cathode
PCE (%)a
8.3 ± 0.4
8.2 ± 0.3
7.8 ± 0.2
8.5 ± 0.3
8.9 ± 0.3
7.4 ± 0.5
5.3 ± 0.2
5.1 ± 0.2
Jsc (mA cm-2)a
16.1 ± 0.3
16.3 ± 0.3
16.2 ± 0.2
16.6 ± 0.2
16.8 ± 0.1
16.4 ± 0.3
16.2 ± 0.3
16.6 ± 0.1
Voc (V)a
FFa
Rs (Ω cm2)b
5.1
μe (cm2 V-1 s-1)c
5.3 × 10-4
5.8 × 10-4
4.2 × 10-4
5.9 × 10-4
5.3 × 10-4
3.6 × 10-4
2.1 × 10-5
1.7 × 10-5
PF6NPS/Al
PF6NPSO/Al
PF6NPSO2/Al
PF6NPSBr/Al
PF6NPSOBr/Al
PF6NPSO2Br/Al
MeOH/Al
0.75 ± 0.01
0.75 ± 0.02
0.74 ± 0.01
0.75 ± 0.01
0.75 ± 0.02
0.73 ± 0.02
0.59 ± 0.01
0.57 ± 0.01
0.70 ± 0.01
0.67 ± 0.03
0.65 ± 0.01
0.68 ± 0.02
0.71± 0.02
0.62 ± 0.03
0.55 ± 0.04
0.54 ± 0.01
5.1
5.7
4.1
4.4
7.4
8.0
Bare Al
8.5
a Average values and standard deviation of device statistics from 8 devices with device area of 0.16 cm2. b Series resistances (Rss) are acquired from J-V curves of the best devices.
c Electron mobilities are acquired from electron-only devices with device structure of ITO/Al/active layer/CIL/Al.
No considerable "MeOH effect" 66, 67 was observed. Hence, the
performance enhancement of the PSCs with the resulting
(a)
0
NWSPs as CIL originates mainly from interfacial polymer
PF6NPS
PF6NPSO
themselves.
PF6NPSO2
PF6NPSBr
PF6NPSOBr
PF6NPSO2Br
Bare Al
-4
-8
To obtain an in-depth understanding on the performance
enhancement of PSCs upon inserting the resulting NWSPs as
CILs, the J–V characteristics (Fig. 4b) of the PSCs were
measured under dark. The devices with NWSPs as CILs show a
turn-on voltage of approximate 0.90 V, which is much higher
than those (0.69 V) of the bare Al device and the MeOH-
treated device. In the range from 0.30 to 0.90 V, the leakage
currents of the bare Al device and the MeOH-treated device
are much higher than those of the NWSP/Al devices, agreeing
with the FF of the corresponding devices. The series resistance
(Rs) acquired from the best NWSP/Al devices (Table 2) are
about 4–5 Ω cm2, which is significantly lower than those of the
bare Al device and the MeOH-treated device (approximate 8 Ω
cm2). Although the Voc and FF of the PSCs devices with the
resulting NWSPs as CILs increase significantly compared to the
control devices, no apparent change in Jsc of all these devices is
observed, which is further confirmed by the external quantum
efficiency (EQE) spectra of the devices. As shown in Fig. 5, the
EQE curves of the NWSPs/Al devices overlap almost
completely with the control devices.
Methanol
-12
-16
0.0
0.2
0.4
Voltage (V)
0.6
0.8
100
10
(b)
1
0.1
0.01
1E-3
1E-4
1E-5
1E-6
1E-7
PF6NPS
PF6NPSO
PF6NPSO2
PF6NPSBr
PF6NPSOBr
PF6NPSO2Br
No interlayer
Methanol
-2.0 -1.5 -1.0 -0.5
0.0
0.5
1.0
1.5
2.0
Voltage (V)
To further understand the improvement of solar cell
performances upon inserting the synthesized polymers as CILs,
the work function (WF) of Al cathode with different treatment
were measured by Kelvin probe, which is a useful tool to study
the energy level at a metal/organic interface.68, 69 The results
are summarized in Table S1. The WF values of neat Al and
MeOH-treated Al are 4.28 eV and 4.26 eV, respectively. Upon
depositing a thin layer of the synthesized polymer, the
resulting Al/polymer surfaces show an around 0.3 eV work
function reduction with the value of 3.95 eV for PF6NPS/Al,
3.96 eV for PF6NPSO/Al, 3.99 eV for PF6NPSO2/Al, 3.96 eV for
PF6NPSBr/Al, 3.99 eV for PF6NPSOBr/Al and 4.03 eV for
PF6NPSO2Br/Al, respectively. The reduced work function of Al
cathode can reduce the barrier for electron collection and can
achieve Ohmic contact at the organic/metal interface, which
thereby contributes to the enhancement of Voc and FF of the
PSCs.
Fig. 4 The current density-voltage (J–V) curves of the
80
70
60
PF6NPS
PF6NPSO
50
PF6NPSO2
40
PF6NPSBr
PF6NPSOBr
PF6NPSO2Br
No interlayer
Methanol
30
20
10
0
300
400
500
600
700
800
Wavelength (nm)
Fig. 5 The external quantum efficiency (EQE) spectra of the PSCs devices with different
interlayer.
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Table 3 Photovoltaic properties of the PSCs with different CIL thickness under AM1.5G irradiation (100 mW cm-2
).
Interlayer
PF6NPSBr
Thickness (nm)
3
PCE (%)a
8.5 ± 0.2
Jsc (mA cm-2)a
Voc (V)a
FFa
Rs (Ω cm2)b
16.2 ± 0.4
0.74 ± 0.01
0.71 ± 0.03
4.9
6
7.7 ± 0.3
7.4 ± 0.1
7.9 ± 0.2
7.3 ± 0.2
15.4 ± 0.3
16.1 ± 0.6
15.5 ± 0.3
16.3 ± 0.5
0.73 ± 0.01
0.74 ± 0.01
0.73 ± 0.02
0.73 ± 0.00
0.69 ± 0.02
0.62 ± 0.01
0.68 ± 0.01
0.61 ± 0.02
6.4
5.7
7.1
4.9
10
13
21
3
7.6 ± 0.2
16.9 ± 0.1
0.73 ± 0.01
0.62 ± 0.01
6.1
7
8.1 ± 0.4
8.5 ± 0.4
8.0 ± 0.3
5.6 ± 0.5
17.1 ± 0.2
17.1 ± 0.1
17.1 ± 0.4
16.7 ± 0.5
0.74 ± 0.02
0.74 ± 0.01
0.74 ± 0.01
0.75 ± 0.00
0.64 ± 0.02
0.66 ± 0.02
0.63 ± 0.01
0.44 ± 0.03
5.3
5.7
11
14
18
PF6NPSO2Br
8.3
25.1
a Average values and standard deviation of device statistics from 8 devices with device area of 0.16 cm2. b Series resistance (Rs) is acquired from the J-V curves of the best devices.
(1.7 × 10-5 cm2 V-1 s-1). Upon inserting the NWSPs as CILs, the
4
electron mobilities of all devices are enhanced one order of
(a)
magnitude. Moreover, the difference in electron mobilities
0
among the devices with different NWSP as CIL are clearly
observed (5.3 × 10-4 cm2 V-1 s-1 for PF6NPS, 5.8 × 10-4 cm2 V-1 s-1
3nm
6nm
10nm
13nm
-4
for PF6NPSO, 4.2 × 10-4 cm2 V-1 s-1 for PF6NPSO2, 5.9 × 10-4 cm2
-8
V-1 s-1 for PF6NPSBr, 5.3 × 10-4 cm2 V-1 s-1 for PF6NPSOBr, 3.6 ×
10-4 cm2 V-1 s-1 for PF6NPSO2Br), which are in agreement with
21nm
-12
-16
the trend of FF as listed in Table 2.
Up to date, most solution processable organic/polymeric
interfacial materials can yield efficient PSCs only with very thin
interlayer films because of their low electric conductivities.27, 34
Clearly, this delicate thickness dependence renders the
0.0
0.2
0.4
0.6
0.8
Voltage (V)
production of PSCs via large-scale roll-to-roll processing highly
challenging.70,
71
4
0
Interestingly, some of our new interfacial
(b)
materials allow for the fabrication of efficient PSCs in a relative
wide range of CIL thickness. The results are shown in Table 3
and Fig. 6. For example, the PCEs exceeding 7% were
maintained for the PF6NPSBr/Al devices in the interlayer
thickness range of 3–20 nm. High PCEs of 7.8%–8.9% are also
observed for the PF6NPSO2Br/Al devices in the CIL thickness
range of 3–14 nm. The excellent performances of PSCs
operated at such a large thickness range of CILs indicate the
high electric conductivities of CILs. Previously, Jen's group have
proved that several kind of anions such as bromide anions and
iodide anions can n-dope fullerene and thereby produce
enhanced electric conductivities for the resulting films.72 Very
recently, our group also disclosed the n-doping effects and
conductivity-enhancing properties in naphthalene diimide-
based conjugated polymers due to the electron transfer from
the electron-rich species to the naphthalene diimide-based
polymer main chain.73, 74 We thus speculate that similar n-
doping effects may also exist in PF6NPSBr and PF6NPSO2Br
when considering the high electron affinities of the polymers
due to their strong electron-withdrawing sulfone unit.
3nm
7nm
11nm
13nm
18nm
-4
-8
-12
-16
0.0
0.2
0.4
Voltage (V)
0.6
0.8
Fig. 6 J-V characteristics of the PSCs with different interlayers thickness under AM1.5G
irradiation (100 mW cm-2): (a) with PF6NPSBr interlayer, and (b) with PF6NPSO2Br
interlayer.
Despite all the resulting NWSPs can improve the
performance of PSCs, considerable PCE differences in PSCs
exist among these polymers, which are mainly due to the
fluctuation in FF. The electron mobilities of PSCs with these
polymers were thus measured in electron-only devices with a
device structure of ITO/Al/PTB7:PC71BM/CIL/Al by fitting the
data with space-charge-limited-current model. The J1/2-V plots
of the devices are shown in Fig. S4, and the mobility values are
listed in Table 2. The electron mobility of the MeOH-treated
device (2.1 × 10-5 cm2 V-1 s-1) is similar to the Al-only device
3. Conclusions
6 | J. Name., 2012, 00, 1-3
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16 Y. Huang, E. J. Kramer, A. J. Heeger and G. C. Bazan, Chem.
Rev., 2014, 114, 7006.
17 W. L. Ma, C. Y. Yang, X. Gong, K. Lee and A. J. Heeger, Adv.
Funct. Mater., 2005, 15, 1617.
18 G. Li, V. Shrotriya, J. S. Huang, Y. Yao, T. Moriarty, K. Emery
To summarize,
a
series of neutral amino-containing
nonconjugated polymers with different sulfide oxidation states
in main chain were synthesized by Suzuki polymerization, and
their ammonium products were prepared through post-
treatment with bromoethane. All of these polymers have good
solubility in methanol, making them attractive candidates for
fabricating multilayer PSCs via orthogonal solution processing.
The PSCs employed the new interfacial polymers exhibit PCEs
of 8–9%. Moreover, some of these polymers (PF6NPSBr and
PF6NPSO2Br) allow for the fabrication of efficient PSCs in a
wide range of CIL thickness. The outstanding interface
modification abilities, good solubility in polar solvent, high
transparency in both near-infrared region and visible region,
low thickness sensitiveness make these new polymers
promising candidates for use as CIL in single-junction PSCs and
as recombination layer in multi-junction devices.
and Y. Yang, Nat. Mater., 2005,
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6
20 J. K. Lee, W. L. Ma, C. J. Brabec, J. Yuen, J. S. Moon, J. Y. Kim,
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24 F. Guo, N. Li, F. W. Fecher, N. Gasparini, C. O. R. Quiroz, C.
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Acknowledgements
26 C. Duan, K. Zhang, C. Zhong, F. Huang and Y. Cao, Chem. Soc.
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27 Z. He, C. Zhong, S. Su, M. Xu, H. Wu and Y. Cao, Nat.
This work was financially supported by the Ministry of Science
and Technology (No. 2014CB643501), the Natural Science
Foundation of China (No. 21520102006, 21490573, and
51361165301) and Guangdong Natural Science Foundation
(Grant No. S2012030006232).
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Text and Graphic for the table of contents
A series of non-conjugated water/alcohol soluble copolymers containing diphenyl
sulfide, diphenyl sulfoxide or diphenyl sulfone are synthesized and applied in polymer
solar cells.