Fabrication of high-performance and low-hysteresis lead halide perovskite solar cells by utilizing a versatile alcohol-soluble bispyridinium salt as an efficient cathode modifier

Article abstract of DOI:10.1039/c7ta04995a

A novel alcohol-soluble conjugated bispyridinium salt (FPyBr) is developed and used as a cathode modifier to improve the cathode interface of planar heterojunction perovskite solar cells (PHJ PVSCs). The excellent electron-withdrawing ability of bispyridinium rings endows FPyBr with a favorable energy level alignment with phenyl-C₆₀-butyric acid methyl ester (PCBM) and the cathode (e.g., Al), which leads to an ideal ohmic contact and efficient electron transport and collection. The deep-lying highest occupied molecular orbital energy level of FPyBr can also effectively block hole carriers and thus decrease leakage current and hole-electron recombination at the cathode interface. In addition, FPyBr can n-dope PCBM through an anion-induced electron transfer process, which increases the electron mobility of PCBM drastically, thereby diminishing interfacial resistance and promoting electron transport. As a result, by incorporating an FPyBr cathode interlayer with ethanol solvent, high-performance and low-hysteresis PHJ PVSCs with a maximal power conversion efficiency (PCE) of 19.61% can be realized. In contrast, reference devices without any cathode interlayer display a distinctly worse performance, with a PCE of 16.97%. Therefore, this excellent cathode modifier provides a new opportunity to fabricate high performance multilayer PVSCs using lowerature solution processing without interfacial erosion/mixing.

Full text of DOI:10.1039/c7ta04995a

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DOI: 10.1039/C7TA04995A.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
rsc.li/materials-a

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High-Performance and Low-Hysteresis Lead Halide Perovskite
Solar Cells by Utilizing a Versatile Alcohol-Soluble Bispyridinium
Salt as an Efficient Cathode Modifier
Received 00th January 20xx,
Accepted 00th January 20xx
Guiting Chen,†a Fan Zhang,†b Meiyue Liu,a Jun Song,*b Jiarong Lian,b Pengju Zeng,b Hin-Lap Yip,*a
DOI: 10.1039/x0xx00000x
Wei Yang,a Bin Zhang*ab and Yong Caoa
www.rsc.org/
A novel alcohol-soluble conjugated bispyridinium salt (FPyBr) is developed and used as a cathode modifier to improve the
cathode interface of planar heterojunction perovskite solar cells (PHJ PVSCs). The excellent electron-withdrawing ability of
bispyridinium rings endows FPyBr with a favorable energy level alignment with phenyl-C60-butyric acid methyl ester
(PCBM) and the cathode (e.g., Al), which leads to an ideal ohmic contact and efficient electron transport and collection.
The deep-lying highest occupied molecular orbital energy level of FPyBr can also effectively block hole carriers and thus
decreases leakage current and hole-electron recombination at the cathode interface. In addition, FPyBr can n-dope PCBM
through an anion-induced electron transfer process, which increases the electron mobility of PCBM drastically, thereby
diminishing interfacial resistance and promoting electron transport. As a result, by incorporating a FPyBr cathode
interlayer with ethanol solvent, high-performance and low-hysteresis PHJ PVSCs with a maximal power conversion
efficiency (PCE) of 19.61% can be realized. On the contrary, the reference devices without any cathode interlayer display a
distinctly worse performance, with a PCE of 16.97%. Therefore, this excellent cathode modifier provides a new
opportunity to fabricate high performance multilayer PVSCs using low-temperature solution processing without interfacial
erosion/mixing.
classified as mesoporous and planar heterojunction (PHJ)
(regular and inverted) structures.12,13 As the device fabrications
for mesoporous architectures usually involve high-
Introduction
Organic-inorganic hybrid perovskite solar cells (PVSCs) have
attracted significant attention due to their unique
advantages,1–4 including light weight, low-cost solution
processability, mechanical flexibility and high efficiency close
to traditional solar cell technologies, such as crystalline silicon,
copper indium gallium selenide/sulfide and cadmium
telluride.5 Since perovskite semiconductors possess excellent
optoelectronic properties,6–8 including tunable band gaps, high
absorption coefficients, long exciton and charge carrier
diffusion lengths and lifetimes, low exciton binding energies
and ambipolar charge transport, the performance of PVSCs has
been improved drastically in a short time and the power
conversion efficiency (PCE) values have already surpassed
20%.5,9–11
temperature (> 450 °C) sintering processes for TiO2 scaffolds,
which increase the cost and are incompatible with flexible
substrates, PHJ structures are therefore considered to be more
desirable for the realization of the large-scale production of
flexible devices using a continuous and cost-effective solution-
printing process.14 Alternatively, although regular PVSCs have
shown slightly higher PCEs than the inverted ones,
photocurrent hysteresis was found to be more serious for
regular PVSCs. Indeed, inverted PVSCs often display lower
hysteresis that is attributed to the better passivation of
electronic trap states at the perovskite/electron transporting
layer (ETL) interface by electron transporting materials (ETMs)
(e.g., fullerene derivatives).4 Therefore, it is urgent to improve
the PCEs of inverted PHJ PVSCs in order to obtain high-
performance PVSCs with negligible hysteresis.
Generally, the device configurations of PVSCs can be
One of the most commonly used device architectures for
inverted
PHJ
PVSCs
is
ITO/poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS)/perovskite/phenyl-C60-butyric acid methyl ester
(PCBM)/cathode, in which PEDOT:PSS and PCBM play the roles
of the hole transporting layer (HTL) and ETL, respectively.
Although inverted PHJ PVSCs based on PEDOT:PSS HTL have
achieved impressive PCEs,14 there are still challenges for
PEDOT:PSS to serve as the HTL.13,15,16 For instance, perovskite
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films grown directly on PEDOT:PSS exhibit incomplete surface for better contact with the metal cathode and improve the
coverage and apparent pinholes, which result in poor device charge transfer rate at the cathode interface.
performance.15,16 To solve this problem, Huang et al.
introduced non-wetting
In addition to inorganic CILs, amine-functionalized polymers
poly(bis(4-phenyl)(2,4,6- and small molecules, which have been demonstrated to be an
a
trimethylphenyl)amine) (PTAA) layer as the HTL, and obtained effective class of CILs for organic photovoltaics (OPVs),41–43
perovskite grains with a high average aspect ratio.17 As a were also successfully used to adjust the interfacial properties
result, these PVSCs exhibited excellent performance with a PCE of PCBM/cathode in PVSCs.38,44–47 Since these CILs can be
as high as 18.1% due to the reduced perovskite grain boundary processed at low temperature from alcohols, low-cost and
area and increased crystallinity. In addition to improving the solution-processed multilayer PVSCs can be realized without
perovskite morphology, the shallower lowest unoccupied interfacial mixing or erosion, which is highly critical for high-
molecular orbital energy level (ELUMO) and deeper highest performance devices. For example, Xue et al. employed an
occupied molecular orbital energy level (EHOMO) of PTAA alcohol-soluble and amino-functionalized polymer, PN4N, to
enables it to efficiently block electrons and increase the built- improve the PCBM/Al interface, and the PCE was enhanced
in potential (Vbi) within the device, respectively.18 Regarding from 12.4% (without any CIL) to 15.0% (with PN4N CIL).46
the choice of ETM, PCBM has been proven as one of the most Zhang et al. demonstrated that the PCE of a PVSC device based
effective ETMs for inverted PHJ PVSCs due to its relatively high on ITO/PEDOT:PSS/MAPbI3-xClx/PCBM/CIL/Ag could be
electron mobility, suitable energy level, and room- improved from 10.0% (without any CIL) to 14.0% by
temperature and orthogonal solvent processability.19,20 While incorporating a methanol-processed perylenediimide CIL.38
other ETMs, such as n-type conjugated polymers21,22 and One of the probable working mechanisms for these CILs is that
discotic molecules,23 have also been explored as efficient the ionic or polar groups within the CILs can interact with high-
cathode interfacial layers for p-i-n PHJ PVSCs.
WF cathodes, leading to the generation of oriented interfacial
Several strategies have been developed to improve the dipoles that can reduce the WFs of the cathodes.48 The
performance of inverted PHJ PVSCs, such as the introduction decreased WFs of cathodes can increase the Vbis across PVSC
of advanced methods for fabricating perovskite films,24–26 devices, which is beneficial for charge transport and
compositional engineering of perovskite films27,28 and extraction, enhancing Rshs and finally boosting the open circuit
interfacial engineering.29–32 The latter method is of particular voltage (Voc) and FF. A second mechanism may be that the
interest to material chemists as new materials with tailored lone-pair electrons or anions within CILs can n-dope the PCBM,
interfacial properties can be rationally designed to improve the which increases the electron mobility of PCBM layers,
interfaces and device performance of PVSCs. At the diminishes the device Rs values and improves the circuit
perovskite/PCBM/cathode side, although low work-function current density (Jsc) and FF.49 The other function of CILs
(WF) metal cathodes, such as Ca (WF of ~2.9 eV), can achieve possibly lies in improving the hydrophilicity of PCBM and thus
large Vbi values that are favorable for exciton dissociation, as its compatibility with metal cathodes.42 Therefore, in order to
well as charge transport and extraction,33 their extremely high achieve high-performance inverted PHJ PVSCs fabricated by
reactivity to oxygen and moisture in air often leads to low-temperature solution processing technologies, it is
insufficient device stability. On the contrary, relatively stable important to modify the cathode interfaces by introducing
metals, such as Al (WF of ~4.3 eV), Ag (WF of ~4.7 eV) and Au surfactant-like CILs.
(WF of ~5.2 eV), are more desirable for realizing long-term
One of the existing problems of reported CILs is that their
stable devices.34–36 However, the relatively large mismatch ELUMO values are typically higher than those of fullerene ETLs.
between the WFs of these metals and the ELUMO of PCBM Mismatches in ELUMO may affect the charge transfer properties
produces small Vbi values and inferior device performance.37 between the ETL/CIL interfaces. Ideally, a CIL with a deep ELUMO
Moreover, as PCBM shows limited conductivity, only relatively (~−4.0 eV) that matches well with that of the fullerene ETL will
thin films of it can be used, which therefore may lead to be beneficial for improving the electron transport from the ETL
incomplete coverage on the rough perovskite film. In such a to the metal cathode. Alternatively, pyridinium salts, which
situation, direct contact of the metal atoms (e.g., Al or Ag) with show deep ELUMO and have been proven to be efficient CILs for
the perovskite film could happen, resulting in poor device OPVs,48,50 may be even more efficient CILs for PVSCs.
performance and stability.38 A versatile approach to address
Herein, we present high-performance inverted PHJ PVSCs,
these issues is to insert a cathode interfacial layer (CIL) to based on an ITO/PEDOT:PSS/PTAA/MAPbI3/PCBM/CIL/Al
further modify and improve the interface between the PCBM system, by introducing a newly developed bispyridinium salt
and metal cathode. For instance, Snaith et al. developed good-
(FPyBr) as the CIL. The deep-lying ELUMO of FPyBr offers good
performance and flexible PVSCs containing a low temperature energy level alignment with both PCBM and Al, and its high
processed n-type TiOx layer between the PCBM/Al interface.39 ionization potential can effectively block the undesired hole
The TiOx CIL could serve as a hole-blocking layer and also carriers from reaching the cathode. In addition, FPyBr can
prevent metal diffusion into the PCBM, thus improving the efficiently n-dope PCBM by the Br- anion, which not only
PVSC performance. Yuan and coworkers also demonstrated increases the electron mobility of PCBM, but also creates more
that a solution processed bathocuproine (BCP) CIL40 could efficient conductive pathways for electron transport. As a
reduce leakage current and improve the fill factor (FF) of result, an almost hysteresis-free PCE of 19.61% is obtained
PVSCs since BCP can planarize the perovskite/PCBM surface under 100 mW cm−2 illumination, compared to the bare-Al
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device with a PCE of 16.97%, suggesting that FPyBr is a spectrum in solution, a slightly spectral red shift is observed in
promising CIL for improving the performance of PVSCs.
the film as a result of weak intermolecular aggregation in the
solid state. In the molecular structure of FPyBr, probably it
exists torsion angle between two adjacent pyridinium rings
due to the exclusion effect between the two N+ ions, which
reduces the planar conformation of conjugated backbone. The
decreased planar conformation as well as the short
conjugation could induce weaker intermolecular aggregation
in solid state. The optical band gap (Egopt) estimated from the
band edge of the absorption spectrum in the film is 2.46 eV. In
addition, the PL spectra display similar profiles both in solution
and film form, with the same peaks at 553 nm.
Results and discussion
Synthesis and characterization
The synthetic route for the bispyridinium salt FPyBr is outlined
in Scheme 1. 5-Bromo-[2,2']bipyridinyl (
reacting 5-bromo-2-iodopyridine
1
) was prepared by
with 2-
tributylstannanylpyridine under the conditions of a palladium-
catalyzed Stille cross-coupling reaction. The palladium-
catalyzed Suzuki cross-coupling reaction between 2,7-
bis(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-9,9-dioctyl-
Electrochemical properties
fluorene and
1 gave 2,7-bis-([2,2']bipyridinyl-5-yl)-9,9-dioctyl-
Cyclic voltammetry (CV) was employed to investigate the
electrochemical properties of FPyBr, as shown in Figure 3. The
oxidation potential of ferrocene/ferrocenium (Fc/Fc+) was
measured to be 0.42 eV relative to the reference electrode
(saturated calomel electrode, SCE), and the redox potential
fluorene (FPy) as a colorless oil. Then, the bispyridinium salt
FPyBr was obtained by intermolecular cyclization between FPy
and 1,2-dibromoethane in 125 °C. The chemical structures of
compound 1, FPy (see the Supporting Information, Figure S1)
1
and FPyBr (Figure S2) were verified by H NMR, 13C NMR and
elemental analysis. FPy is readily soluble in low-polarity
organic solvents (e.g., toluene, CHCl3 and tetrahydrofuran
[THF]) and cannot be dissolved in highly polar solvents (e.g.,
dimethyl sulfoxide [DMSO], methanol and ethanol). However,
after quaternization, FPyBr shows excellent solubility in high-
polarity solvents, including DMSO, methanol, ethanol and
dimethyl formamide (DMF), but is insoluble in solvents with
lower polarity (e.g., toluene, CHCl3 and THF). This unique
dissolution characteristic offers orthogonal solution
processability for depositing FPyBr on non-polar PCBM films
without eroding the layers underneath, thereby forming a
robust CIL for inverted PHJ PVSCs.
(
Ered) of FPyBr was tested to be -0.30 eV under the same
experimental conditions. The ELUMO was calculated to be −4.08
eV according to the equation of ELUMO = - Ered + 4.80 – 0.42)
e
(
(eV), which means a good energy level alignment with both
PCBM and Al, and thereby is beneficial for electron extraction
at the cathode side. The EHOMO was estimated to be −6.54 eV
according to the equation of EHOMO = ELUMO - Egopt. This deep-
lying EHOMO is in favor of blocking hole carriers from the
perovskite layer into the cathode, which can reduce leakage
current and charge recombination loss at the cathode
interface.
Photovoltaic performance of perovskite solar cells
Thermal properties
To investigate the cathode modification ability of FPyBr, we
fabricated inverted PHJ PVSCs with the structure of
ITO/PEDOT:PSS/PTAA/MAPbI3/PCBM/FPyBr/Al (Figure 4a). The
PTAA layer was used to improve the interface between
PEDOT:PSS and MAPbI3 layers, providing better EHOMO
matching and an electron blocking effect for more efficient
hole extraction at the anode.17,18 For comparison, reference
PVSCs without the FPyBr CIL were also fabricated. The detailed
procedures for preparing the MAPbI3 film and other functional
layers (except FPyBr layer) can be found in a previous report.51
The X-ray diffraction (XRD) pattern of MAPbI3 agrees well with
the crystalline structures of MAPbI3 in previous reports (Figure
S3a).52,53 The CILs with a thickness of about 4 nm were
deposited on PCBM layers by spin coating a FPyBr solution (0.3
mg mL−1 in ethanol) at 6000 rpm for 20 s. A schematic of the
energy level of each layer is shown in Figure 4b.
The thermal properties of FPyBr were investigated by thermal
gravimetric analysis (TGA) and differential scanning
calorimetry (DSC), as shown in Figure 1. From the TGA study,
FPyBr shows a 5% weight loss temperature (Td) of 295 °C,
confirming its high thermal stability. The DSC curve reveals the
well-defined glass transition temperature (Tg) of 134 °C, which
suggests that FPyBr is amorphous in nature.
Optical properties
The UV-Vis absorption and photoluminescence (PL) spectra of
FPyBr in methanol and film were measured, as shown in Figure
2, and the optical parameters are summarized in Table 1.
FPyBr exhibits dominant absorption peak at 431 nm in
methanol, which is assigned to the π-π* transition of the
conjugated backbone. Compared with the absorption
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Scheme 1. Synthetic route for FPyBr.
Table 1. Optical properties of FPyBr.
Solution
Film
Compound
Egopt (eV)
λ
abs (nm)
431
λ
PL (nm)
553
λ
abs (nm)
435
λ
PL (nm)
553
FPyBr
2.46
Figure 3. CV curve of FPyBr as solution measured in 0.1 M Bu4NPF6 in acetonitrile.
The top-view scanning electron microscopy (SEM) image of
the MAPbI3 film shows a uniform and compact morphology
with a large crystal grain size (Figure 4c). The cross-sectional
SEM image of the PVSC device without a cathode displays a
well-defined layered structure with sharp interfaces (Figure
4d). It can be observed that the MAPbI3 grain size in the
vertical direction is comparable to the perovskite film
thickness, which is the ideal condition for carrier transport and
collection, since the chance of charge carriers being quenched
at grain boundaries can be minimized.51 Although the ultrathin
FPyBr layer is not entirely obvious in the cross-sectional SEM
image, an X-ray photoelectron spectroscopy (XPS)
measurement on the ITO/PCBM/FPyBr surface confirms its
existence on PCBM, since the N 1s (398.2 and 398.6 eV) and Br
3d (63.8 eV) peaks can all be detected (Figure S3b).
Figure 1. a) TGA and b) DSC curves of FPyBr.
Current density versus voltage (J–V) characteristics of the
inverted PHJ PVSCs with or without FPyBr CIL are shown in
Figure 5a, and their photovoltaic performance parameters are
listed in Table 2. The reference device without the FPyBr layer
exhibits a PCE of 16.97%, with a Voc of 1.11 V, Jsc of 22.50 mA
cm−2 and FF of 67.8% under reverse scanning conditions.
Interestingly, after introducing the FPyBr layer, the PCE
increases to 19.61%, with a Voc of 1.11 V, Jsc of 23.19 mA cm−2
and FF of 76.0% under reverse scanning conditions. As a
comparison, we also studied the FPyBr layer effect on
photovoltaic performance by varying the concentration of
FPyBr in ethanol, and the related J-V curves and photovoltaic
parameters are presented in Figure S4 and Table S1,
respectively. It can be found that the best processing
concentration for FPyBr is 0.3 mg mL−1. The external quantum
efficiency (EQE) of the FPyBr-based device shows values higher
than 80% over the whole visible region, and the maximum
value reaches over 90% (Figure 5b). The calculated Jsc from
integrating the EQE spectrum is 22.77 mA cm−2, with a
difference of about 2% relative to the measured Jsc, suggesting
Figure 2. a) UV-vis absorption and b) PL spectra of FPyBr.
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an accurate measurement in this study. Alternatively, the measured by applying a constant bias voltage at the maximum
FPyBr-based PVSC displays a PCE as high as 19.32% (with Voc
=
power point. As shown in Figure 5d, we obtain a steady-state
1.11 V, Jsc = 23.11 mA cm−2 and FF = 75.4%) under forward PCE of 19.4% for the FPyBr-based device at a constant bias
scanning conditions (Figure 5c and Table 2). The PCE difference voltage of 0.92 V, which is in good agreement with the average
between various scanning directions is smaller than 1.5%, PCE of the J-V curves with both reverse and forward scanning
indicating a low hysteresis effect, which is important for directions. To check the reproducibility of the FPyBr-based
achieving high-performance and stable PVSCs. In order to devices, a histogram of PCE values obtained from 43 devices is
evaluate the stability of the power output of the FPyBr-based summarized in Figure S5, which implies that most of the PVSCs
PVSCs, the steady-state photocurrent and efficiency were can achieve high photovoltaic performance.
Figure 4. a) Configuration of the inverted PHJ PVSC. b) Schematic of the energy level of each layer. c) Top-view SEM image of MAPbI3 film on the surface of ITO/PEDOT:PSS/ PTAA
films. d) Cross-sectional SEM image of the PVSC device without cathode.
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Figure 5. Photovoltaic performance characteristics under AM 1.5G solar irradiation. a) J-V curves of the PVSCs with or without the FPyBr CIL. b) EQE spectrum and integrated
photocurrent curve of the FPyBr-based PVSC. c) J-V curves of the FPyBr-based PVSC with different scanning directions. d) Steady-state PCE and photocurrent of the FPyBr-based
PVSC measured at the maximum power point.
Al/PCBM/FPyBr/Al, and the electron mobilities were extracted
Table 2. Photovoltaic performance parameters of the PVSCs with or
by fitting the data with the space charge limited current (SCLC)
model (Figures 6b and S6).59 The deposition conditions of
without FPyBr CIL under AM 1.5G irradiation.
Jsc
Rs
(ꢀ
Rsh
(ꢀ
PCBM and FPyBr were the same as those used for the PVSC
Voc
FF
PCEmax
(%)
PCEavg
(%)
CIL
(mA
cm−2
fabrication. In comparison with the Al/PCBM/Al device with an
(V)
(%)
10−4 cm2 V−1 s−1, the
)
cm2)
cm2)
electron mobility of 4.0
Al/PCBM/FPyBr/Al device exhibits a much higher electron
mobility of 1.1
10−3 cm2 V−1 s−1. The enhanced electron
×
w/o
FPyBr
1.11
22.50
67.8
16.97
16.35
7.6
990.1
×
(reverse)
FPyBr
mobility probably results from the removal of the PCBM
surface traps via n-doping by FPyBr, which decreases the
transient time of the electrons through the film.54 Note that
the increase in electron mobility is also consistent with the
result of reduction in Rs of the PVSC after inserting the FPyBr
1.11
1.11
23.19
23.11
76.0
75.4
19.61
19.32
18.93
18.72
5.7
5.3
2809.9
1339.9
(reverse)
FPyBr
(forward)
layer, which leads to a higher FF and Jsc
.
The improvement of PVSC performance after inserting the
FPyBr CIL is attributed to the significantly increased FF and
slightly enhanced Jsc. Compared to the bare-Al device, the
introduction of FPyBr with a suitable ELUMO can induce better
energy alignment and ohmic contact between PCBM and Al,
which facilitate electron transport and extraction, and
decrease interfacial recombination at the cathode side, leading
to a higher Jsc and FF, which have been reported in the case of
OPVs.54,55 Moreover, FPyBr with its deep-lying EHOMO can serve
as an efficient hole-blocking layer to improve the charge
selectivity, which reduces leakage current, as well as electron-
Steady-state PL and time-resolved PL (TRPL) decay
measurements were performed to study the charge transfer
properties at the MAPbI3/PCBM interfaces (Figures 7 and S7).
All
the
samples
(MAPbI3,
MAPbI3/PCBM
and
MAPbI3/PCBM/FPyBr) exhibit steady-state PL peaks at 775 nm
originating from MAPbI3. The PL intensity of the perovskite
layer coated with PCBM is obviously lower than that of the
pristine perovskite layer. Note that depositing the FPyBr layer
on the PCBM surface induces more significant PL quenching,
which implies that FPyBr can increase the degree of charge
transfer at the MAPbI3/PCBM interface.27 From the TRPL
curves, the MAPbI3/PCBM/FPyBr sample displays a shorter
fluorescence lifetime than the MAPbI3/PCBM film sample,
which means that the electron-hole separation at the
MAPbI3/PCBM interface is more efficient after introducing a
FPyBr layer.60 The improvement in charge transfer properties
by FPyBr is probably due to the increased electron mobility of
the PCBM layer by FPyBr, which decreases the trap-assisted
recombination of electrons and holes at the MAPbI3/PCBM
interface.61 The slightly deeper ELUMO of FPyBr than that of
PCBM may also make electron injection from MAPbI3 into
PCBM easier.
hole recombination loss at the cathode interface, and finally
30
improves the FF and Jsc
.
The obviously increased Rsh from
990.1
ꢀ
cm2 for the bare-Al device to 2809.9 cm2 for the
ꢀ
FPyBr-based device confirms that the leakage current and/or
charge recombination can be reduced after introducing the
FPyBr CIL.46
It is well known that for both PVSCs and OPVs, the
introduction of alcohol-soluble ionic polymers56 or small
molecules44 can often decrease the WFs of cathodes and also
introduce interfacial n-doping to PCBM, facilitating better
electron transport from the active layers to cathodes. In our
case, the Voc values of the bare-Al and FPyBr-based devices
show the same value, which suggests that the WF matching
effect may only play a minor role in improving device
performance. Therefore, we further investigate the n-doping
effect induced by the FPyBr CIL to PCBM. Amine- or
ammonium salt-functionalized CILs are known to n-dope PCBM
via the lone-pair electrons or anion-induced electron transfer
process, which induce more efficient conductive pathways to
improve electron transport and extraction.54,57,58 We
We used atomic force microscopy (AFM) to study the
influence of the FPyBr layer on the morphology of the
ITO/PCBM films (Figure 8a-c). The pristine and ethanol-treated
PCBM films both show smooth surfaces with root mean square
(rms) roughnesses of 1.13 and 1.00 nm, respectively.
Dramatically, after depositing the FPyBr layer on top of PCBM,
the surface becomes smoother with a rms roughness of 0.711
nm, probably because FPyBr molecules fill the voids in the
PCBM surface. The improved surface smoothness and
uniformity of the PCBM/FPyBr bilayer may form better contact
with the Al cathode, thus reducing leakage current at the
cathode.62 Moreover, FPyBr may also serve as a protective
layer to reduce the diffusion of Al atoms into the
MAPbI3/PCBM layers, owning to the strong metal coordination
property of the bispyridinium unit in FPyBr. In addition,
performed electron
paramagnetic
resonance
(EPR)
spectroscopy to study the electron transfer phenomenon in
our PVSCs (Figure 6a). The sole component of PCBM or FPyBr
shows no detectable EPR signal, while the blend of PCBM and
FPyBr (1:1, w/w) displays intense anionic fullerence absorption,
which suggests electron transfer from Br- to PCBM occurred.
The influence of the FPyBr layer on the charge transport
properties of the PCBM layer was studied through the
electron-only devices with the structures of Al/PCBM/Al and
measurements of the static water contact angle (θ) were
performed to study the wetting properties of the ITO/PCBM
6 | J. Name., 2012, 00, 1-3
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ARTICLE
films with different treatments (Figure 8d-f). The pristine hydrophilicity significantly, with a
PCBM film displays an intensely hydrophobic surface with a improved hydrophilicity contributes to better wettability and
of 105.3 , which is not beneficial for contact with the metal compatibility with the top metal cathode, which is favorable
cathode. After spin-coating ethanol on top, the PCBM surface for decreasing the contact resistance at the cathode interface,
shows a slight change in wetting ( = 103.2
). Interestingly, the thus increasing the FF.63
θ value of 79.8°. The
θ
°
θ
°
deposition of FPyBr atop the PCBM film enhances the
Figure 6. a) EPR spectra of different components in the solid state. b) J1/2-V characteristics of the electron-only devices with or without the FPyBr CIL (device structure:
Al/PCBM/CIL/Al).
Figure 7. a) Steady-state PL and b) fitted TRPL decay measurements of different samples. The films were deposited on ITO/PEDOT:PSS/PTAA surfaces and the deposition conditions
of all layers were the same as those for the PVSC fabrication.
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Journal Name
Figure 8. AFM images of different films: a) ITO/PCBM, b) ITO/PCBM/ethanol, and c) ITO/PCBM/FPyBr; static water contact angle images of different films: d) ITO/PCBM, e)
ITO/PCBM/ethanol, and f) ITO/PCBM/FPyBr. The deposition conditions of all the films were the same as those for the PVSC fabrications.
bought from Heraeus (Hanau, Germany). PCBM was purchased
from Nichem Fine Technology Co. Ltd. (Hsinchu County,
Conclusions
Taiwan). The solvents were further purified by general
procedures and were distilled before used. All air and water
sensitive synthetic manipulations were performed under a dry
In summary, we demonstrate high-performance and low-
hysteresis inverted PHJ PVSCs by inserting a FPyBr layer
between PCBM and Al. Compared to the bare-Al device with a
PCE of 16.97%, the PVSC with the FPyBr CIL achieves
simultaneous enhancements in FF and Jsc, and thus a boosted
PCE as high as 19.61%. The improvements in FF and Jsc mainly
result from the following contributions: (1) the suitable ELUMO
of FPyBr offers good energy level alignment with both PCBM
and Al, and the high ionization potential can block holes from
transporting into the cathode; (2) the n-doping to PCBM by Br-
within FPyBr induces efficient conductive pathways for
electron transport; (3) the increased electron mobility of the
PCBM layer by FPyBr modification leads to more efficient
charge transfer and transport; (4) the smooth and uniform
FPyBr layer serves as a protector to prevent Al atoms from
contacting with PCBM and the perovskite, thus decreasing the
leakage current; (5) the increased hydrophilicity of the PCBM
film by FPyBr improves the wettability and compatibility with
the metal cathode, and consequently lowers the contact
resistance. Therefore, we conclude that this kind of novel
bispyridinium salt is a promising cathode modifier for high-
performance and low-hysteresis organic-inorganic hybrid lead
halide perovskite solar cells. Our demonstration can also
provide important guidelines for designing new generation
interfacial materials for highly efficient perovskite solar cells.
argon
atmosphere.
2,7-Bis(4,4,5,5-tetramethyl-
[1,3,2]dioxaborolan-2-yl)-9,9-dioctyl-fluorene was synthesized
according to a published paper.64
General measurements and characterization
1H NMR and 13C NMR measurements were carried out on a
Bruker AV-500 spectrometer (Bruker, Rheinstetten, Germany)
operating at 500 MHz (for 1H) and 75 MHz (for 13C) with
tetramethylsilane as the internal reference. APCI-MS analyses
were obtained from an Acquity Waters UPLC equipped with a
Waters Acquity TQ detector (Thermo Finnigan LCQ Fleet
system; Waters, Milford, MA, USA). Elemental analyses were
performed on a Vario EL elemental analysis instrument
(Elementar Co., Langenselbold, Germany). TGA was conducted
using a NETZSCH TG 209 at a heating rate of 10 °C min−1 under
N2 flow. DSC was measured on a Netzsch DSC 204 under N2
flow at a heating rate of 10 °C min−1 and a cooling rate of 20 °C
min−1. UV-vis absorption spectra were performed on a HP 8453
spectrophotometer. PL spectra were recorded on an Instaspec
IV CCD spectrophotometer (Oriel Co., Stradford, CT, USA). CV
was characterized on a CHI600D electrochemical workstation
with a standard three electrode cell based on a Pt wire counter
electrode and a platinum (Pt) working electrode, against a
saturated calomel electrode as the reference electrode at a
scan rate of 50 mV s−1 within a nitrogen-saturated anhydrous
solution
of
0.1
mol
L−1
tetrabutylammonium
Experimental section
hexafluorophosphates (Bu4NPF6) in acetonitrile, versus
ferrocene/ferrocenium (Fc/Fc+) as the internal reference.
Scanning Kelvin probe microscopy tests were implemented on
a KP Technology SKP5050.
Materials
All reagents, unless otherwise specified, were obtained from
Aladdin Chemical Co. (Shanghai, China), Sigma Aldrich
Chemical Co. (St. Louis, MO, USA), Alfa Aesar Chemical Co.
(Ward Hill, MA, USA) or J&K Chemical Co. (Beijing, China), and
were used as received. Clevios PVP AI 4083 PEDOT:PSS was
Synthesis
8 | J. Name., 2012, 00, 1-3
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ARTICLE
5-Bromo-[2,2']bipyridinyl (1). Into a 50 mL of three-neck flask (m, 20H), 0.76-0.56 (m, 10H). 13C NMR (75 MHz, DMSO-d6, δ):
were added 5-bromo-2-iodopyridine (2.0 g, 7.0 mmol), 2- 153.08, 147.81, 147.64, 145.80, 143.55, 142.91, 141.20, 140.33,
tributylstannanylpyridine (2.6 g, 7.0 mmol), 20 mL of 138.06, 132.22, 130.27, 130.13, 128.16, 127.67, 122.77, 122.67,
anhydrous toluene and Pd(PPh3)4 (0.58 g, 0.50 mmol) under 56.30, 52.79, 52.65, 40.58, 31.52, 29.81, 29.16, 29.04, 24.10,
argon flow. The reaction was heated to 120 °C and stirred 22.44, 14.36. Anal. calcd for C53H62Br4N4: C 59.23, H 5.81, N
overnight with argon protection. The mixture was cooled to 5.21; found: C 59.09, H 5.97, N 5.10.
room temperature and evaporated. The crude product was
purified by column chromatography to afford compound
1
as a Solar cell fabrication and characterization
white solid (1.3 g, 79%). H NMR (500 MHz, CDCl3, δ): 8.71 (s,
1H), 8.66-8.65 (d, = 5 Hz, 1H), 8.37-8.36 (d, = 5 Hz, 1H), 8.32-
8.31 (d, = 5 Hz, 1H), 7.93-7.92 (d, = 5 Hz, 1H), 7.82-7.80 (t,
1
The MAI crystal was synthesized upon reaction of
a
J
J
concentrated aqueous solution of hydroiodic acid (HI) (15.0 mL,
57 wt.% in water, Aldrich) with methylamine (CH3NH2) (13.5
mL, 40 wt.% in aqueous solution, Aldrich) at 0 °C for 2 h with
constant stirring under a nitrogen atmosphere. The precipitate
was recovered by evaporation at 70 °C until completely dry.
MAI was dissolved in ethanol at 90 °C, recrystallized from the
supersaturated solution at room temperature, and then dried
at 60 °C in a vacuum oven for 12 h. The perovskite precursor
solution was prepared by mixing 922.0 mg of PbI2 (99.999%,
Alfa Aesar) and 349.8 mg of MAI in 0.9 mL of DMF and 0.1 mL
of DMSO, with a PbI2: MAI molar ratio of 1:1.1.
J
J
J
= 5 Hz, 1H), 7.33-7.30 (m, 1H). 13C NMR (75 MHz, CDCl3, δ):
154.67, 150.38, 150.30, 148.47, 139.70, 137.98, 124.24, 122.71,
121.58, 121.47. MS (APCI)
m/z
: [M + H]+ calcd for C10H7BrN2,
235.1; found, 236.0. Anal. calcd for C10H7BrN2: C 51.09, H 3.00,
N 11.92; found: C 51.28, H 3.19, N 11.71.
2,7-Bis([2,2']bipyridinyl-5-yl)-9,9-dioctyl-fluorene (FPy). 2,7-
Bis(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-9,9-dioctyl-
fluorene (0.57 g, 0.89 mmol), compound
1 (0.49 g, 2.1 mmol),
10 mL of toluene, K2CO3 aqueous solution (2M, 2.7 mL) and
Pd(PPh3)4 (0.10 g, 0.087 mmol) were mixed in a 50 mL of
three-neck flask under argon. The reaction was heated to 80 °C
and stirred overnight under argon atmosphere. Then the
mixture was cooled to room temperature and mixed with 20
mL of distilled water. The aqueous layer was extracted with
CH2Cl2 for three times. The combined organic layer was
washed with distilled water and brine before dried over
anhydrous MgSO4. The organic solvent was distilled and the
crude product was purified by column chromatography to
The PEDOT:PSS solution was spun on top of the glass/ITO
and annealed at 125 °C for 10 min. Then, the PTAA solution
(2.5 mg mL−1 in toluene) was spin-coated on the PEDOT:PSS
layer at 6000 rpm and annealed at 100 °C for 5 min. The
perovskite precursor solution (70 °C) was spin coated on the
PTAA layer at 6000 rpm and, after 7 s of delay time, the wet
film was washed with 200 μL of sec-butyl alcohol by uniformly
dropping sec-butyl alcohol in the center of the substrate. After
the spin stopped, the film was annealed on a hot plate at
100 °C for ~30 s. The film was then soaked with 200 μL of sec-
butyl alcohol for 10 s, and subsequently dried by spin-coating
at 6000 rpm for 30 s. The spin-coating processes were typically
conducted in a glove box under a nitrogen atmosphere and
with a real-time humidity of ~1 ppm. Finally, the film was
subjected to a post-annealing treatment. In this step, the
perovskite film was transferred onto a hot plate, first annealed
in ambient air (real-time humidity of 55–65%) at 100 °C for 15
min, and then annealed in a DMSO atmosphere at 100 °C for
15 min. For the DMSO atmosphere, 50 μL of DMSO solvent
was dropped into a glass Petri dish, which was then used to
cover the sample. Subsequently, PCBM (20 mg mL−1,
chlorobenzene) was spun on the perovskite layer at 2500 rpm.
For the FPyBr sample, the FPyBr solution (0.3 mg mL−1 in
ethanol) was spin-coated on the PCBM layer at 6000 rpm for
20 s. Finally, the device was completed by evaporating a 100-
nm-thick aluminum film as the electrode (active area of 12
mm2).
1
afford FPy as a colorless oil (0.38 g, 61%). H NMR (500 MHz,
CDCl3, δ): 9.02-9.01 (d,
8.53-8.52 (d, = 5 Hz, 2H), 8.49-8.47 (d,
8.10 (dd, = 7.5 Hz, = 2.5 Hz, 2H), 7.88-7.84 (m, 4H), 7.68-7.67
(dd, = 5 Hz, = 2.5 Hz, 2H), 7.65-7.64 (d, = 5 Hz, 2H), 7.35-
J
= 5 Hz, 2H), 8.73-8.72 (d,
J = 5 Hz, 2H),
J
J
= 10 Hz, 2H), 8.12-
J
J
J
J
J
7.33 (m, 2H), 2.10-2.07 (m, 4H), 1.19-1.09 (m, 20H), 0.80-0.77
(m, 10H). 13C NMR (75 MHz, CDCl3, δ): 155.84, 154.64, 152.16,
149.21, 147.71, 140.70, 137.07, 136.92, 136.66, 135.22, 126.15,
123.74, 121.44, 121.14, 121.05, 120.60, 55.52, 40.33, 31.73,
29.97, 29.18, 29.17, 23.88, 22.58, 14.04. MS (APCI)
m/z: [M +
H]+ calcd for C49H54N4, 699.0; found, 699.7. Anal. calcd for
C49H54N4: C 84.20, H 7.79, N 8.02; found: C 84.03, H 7.88, N
8.09.
The bispyridinium salt (FPyBr). Compound FPy (0.20 g, 0.29
mmol) was dissolved in 15 mL of 1,2-dibromoethane under
argon, and the solution was heated to 125 °C and stirred for 24
h. After being cooled to room temperature, the precipitate
was collected by filtration. The solid was washed with
tetrahydrofuran and acetone successively in
a soxhlet
SEM images and XRD patterns of the films were obtained
with a Zeiss Supra 55 microscope and a Bruker QUANTAX 200
diffractometer, respectively. A sun simulator (Zolix Sirius-SS)
was used to provide the simulated solar irradiation (AM 1.5G,
extractor, and then dissolved in 5 mL of methanol and
precipitated from 80 mL of ethyl acetate. The target
compound was got by filtration to give a red solid (0.26 g, 82%).
1H NMR (500 MHz, DMSO-d6, δ): 10.09 (s, 2H), 9.43-9.41 (dd,
100 mW cm−2).
Keithley 2400 source meter. The output of the light source was
adjusted using calibrated silicon photodiode (ABET
technology). The J-V curves were measured by backward—
−0.5 V)
J-V characteristics were measured using a
J
= 7.5 Hz,
J
= 2.5 Hz, 2H), 9.40-9.38 (d,
J
= 10 Hz, 2H), 9.24-9.23
(d, J = 5 Hz, 4H), 8.99-8.96 (t,
J
= 7.5 Hz, 2H), 8.48-8.45 (t, J =
a
7.5 Hz, 2H), 8.41 (s, 2H), 8.31-8.29 (d,
(dd, = 10 Hz, = 2.5 Hz, 2H), 5.46-5.45 (t,
5.36 (t, = 2.5 Hz, 4H), 2.29-2.26 (t, = 7.5 Hz, 4H), 1.14-1.00
J
= 10 Hz, 2H), 8.23-8.21
J
J
J
= 2.5 Hz, 4H), 5.37-
forward bias (1.2 V) → negaꢀve bias (
—or forward—
J
J
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ARTICLE
Journal Name
12 W. R. Erwin, H. F. Zarick, E. M. Talbert and R. Bardhan,
Energy Environ. Sci., 2016, , 1577.
13 S. S. Mali and C. K. Hong, Nanoscale, 2016,
14 W. Yan, S. Ye, Y. Li, W. Sun, H. Rao, Z. Liu, Z. Bian and C.
Huang, Adv. Energy Mater., 2016, , 1600474.
15 S. Bai, Y. Jin and F. Gao, in Advanced Functional Materials
(Eds: A. Tiwari, L. Uzun), Wiley, Hoboken, NJ, America 2015,
Chapter 1310.1002/9781118998977.ch.13.
negative bias (−0.5 V) → forward bias (1.2 V)—scan. The EQE
was measured using a power source (Zolix Sirius-SS) with a
monochromator (Zolix Omni-λ) and a source meter (Keithley
2400). The steady-state PL spectra were measured by the
fluorescence spectrophotometer (OmniPl-microS, Zolix, China),
an excitation light of 325 nm came into the films from the air
side. The time-resolved PL spectra were measured at 775 nm
on the excitation at 600 nm by the time-correlated single
photon counting (TCSPC) spectrometer (FLIM, Becker & Hickl
GmbH, Germany). The device area (12 mm2) was determined
by the overlap of cathode and anode. Notably, the presence of
DMSO in the perovskite precursor solution probably alters the
charge transport properties of the PEDOT:PSS layer. To avoid
overestimation of the photocurrent, an aperture size of 12
mm2 was used to define the light absorption area. All devices
were measured immediately after fabrication without
encapsulation in a super-clean lab with a controlled humidity
of 55–65%.
9
8, 10528.
6
16 J. You, Y. Yang, Z. Hong, T. B. Song, L. Meng, Y. Liu, C. Jiang,
H. Zhou, W. H. Chang, G. Li and Y. Yang, Appl. Phys. Lett.,
2014, 105, 183902.
17 C. Bi, Q. Wang, Y. Shao, Y. Yuan, Z. Xiao and J. Huang, Nat.
Commun., 2015,
18 C.-C. Chueh, C.-Z. Li and A. K.-Y. Jen, Energy Environ. Sci.,
2015, , 1160.
6, 7747.
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19 X. Liu, F. Lin, C.-C. Chueh, Q. Chen, T. Zhao, P.-W. Liang, Z.
Zhu, Y. Sun and A. K.-Y. Jen, Nano energy, 2016, 30, 417.
20 C. Cui, Y. Li and Y. Li, Adv. Energy Mater., 2016, 1601251.
21 C. Sun, Z. Wu, H.-L. Yip, H. Zhang, X.-F. Jiang, Q. Xue, Z. Hu, Z.
Hu, Y. Shen, M. Wang, F. Huang and Y. Cao, Adv. Energy
Mater., 2016,
22 W. Wang, J. Yuan, G. Shi, X. Zhu, S. Shi, Z. Liu, L. Han, H.-Q.
Wang and W. Ma, ACS Appl. Mater. Interfaces, 2015, , 3994.
6, 1501534.
7
23 D. Zhao, Z. Zhu, M.-Y. Kuo, C.-C. Chueh and A. K.-Y. Jen,
Angew. Chem. Int. Ed., 2016, 55, 8999.
24 J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao,
M. K. Nazeeruddin and M. Gratzel, Nature, 2013, 499, 316.
Conflicts of interest
There are no conflicts of interest to declare
25 M. Liu, M. B. Johnston and H. J. Snaith, Nature, 2013, 501
395.
,
Acknowledgements
26 Q. Chen, H. Zhou, Z. Hong, S. Luo, H.-S. Duan, H.-H. Wang, Y.
Liu, G. Li and Y. Yang, J. Am. Chem. Soc., 2014, 136, 622.
27 L. Zhao, D. Luo, J. Wu, Q. Hu, W. Zhang, K. Chen, T. Liu, Y. Liu,
Y. Zhang, F. Liu, T. P. Russell, H. J. Snaith, R. Zhu and Q. Gong,
Adv. Funct. Mater., 2016, 26, 3508.
28 N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu, J. S. Seo
and S. H. Seok, Nature, 2015, 517, 476.
29 O. Malinkiewicz, A. Yella, Y. H. Lee, G. M. Espallargas, M.
Gratzel, M. K. Nazeeruddin and H. J. Bolink, Nature Photon.,
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This work was financially supported by the National Nature
Science Foundation of China (nos. 51273069, 91333206,
61505123, 51573057, 51323003, 21761132001), Ministry of
Science and Technology (No. 2014CB643505), Science and
Technology
Program
of
Guangzhou,
China
(No.
201607020010), General Financial Grant from the China
Postdoctoral Science Foundation (no. 2015M582409) and
30 Q. Xue, G. Chen, M. Liu, J. Xiao, Z. Chen, Z. Hu, X.-F. Jiang, B.
Zhang, F. Huang, W. Yang, H.-L. Yip and Y. Cao, Adv. Energy
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31 C. Sun, Q. Xue, Z. Hu, Z. Chen, F. Huang, H.-L. Yip and Y. Cao,
Small, 2015, 11, 3344.
32 S. F. Volker, S. Collavini and J. L. Delgado, ChemSusChem,
Shenzhen
Fundamental
R&D
Program
(nos.
JCYJ20160520175355048,
JCYJ20160427161937700).
JCYJ20150525092940976,
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Journal of Materials Chemistry A
Page 12 of 12
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DOI: 10.1039/C7TA04995A
Graphical Abstract
A novel bispyridinium salt (FPyBr) is designed as cathode modifier to achieve high-performance
and low-hysteresis fullerene/perovskite solar cells with a maximal PCE of 19.61%. The suitable
ELUMO of FPyBr induces ideal ohmic contact at the cathode, and the deep EHOMO provides efficient
hole-blocking ability. FPyBr can increase the electron mobility of PCBM via n-doping, which
improves charge transfer and transport.