Double fullerene cathode buffer layers afford highly efficient and stable inverted planar perovskite solar cells

Article abstract of DOI:10.1016/j.orgel.2020.105726

Fullerene derivatives especially [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) with strong electron-accepting abilities have been commonly implemented as indispensable cathode buffer layers (CBLs) of inverted (p-i-n) planar perovskite solar cells (iPS

Full text of DOI:10.1016/j.orgel.2020.105726

Organic Electronics 82 (2020) 105726
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: http://www.elsevier.com/locate/orgel
Double fullerene cathode buffer layers afford highly efficient and stable
inverted planar perovskite solar cells
,
,**
Lingbo Jiaa, Bairu Lia, Yanbo Shanga, Muqing Chena *, Guan-Wu Wangb
,
,
***
Shangfeng Yanga
a Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and
Engineering, Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei, 230026, China
b CAS Key Laboratory of Soft Matter Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Chemistry for Energy
Materials (iChEM), Department of Chemistry, University of Science and Technology of China, Hefei, 230026, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Fullerene derivatives especially [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) with strong electron-
accepting abilities have been commonly implemented as indispensable cathode buffer layers (CBLs) of inver-
ted (p-i-n) planar perovskite solar cells (iPSCs) to facilitate electron transport. However, only a single fullerene
CBL is typically used in iPSC devices, resulting in interfacial energy offset between fullerene CBL and metal
cathode and consequently insufficient electron transport. Herein, we synthesized a novel bis-dimethylamino-
functionalized fullerene derivative (abbreviated as PCBDMAM) and applied it as an auxiliary fullerene inter-
layer atop of PCBM to form a PCBM/PCBDMAM double fullerene CBL, leading to dramatic enhancement of both
efficiency and ambient stability of iPSC devices. Incorporation of PCBDMAM interlayer facilitates the formation
of interfacial dipole layer between PCBM and Ag cathode, resulting in decrease of the work function of the Ag
cathode. As a result, the CH3NH3PbI3 (MAPbI3) iPSC devices based on PCBM/PCBDMAM double fullerene CBL
exhibit the highest power conversion efficiency (PCE) of 18.11%, which is drastically higher than that of the
control device based on single PCBM CBL (14.21%) and represents the highest value reported for double
fullerene CBL-based iPSC devices. Moreover, due to the higher hydrophobicity of PCBDMAM than PCBM, iPSC
devices based on PCBM/PCBDMAM double fullerene CBL shows an enhanced ambient stability, retaining 67% of
the initial PCE after storage 1440 h exposure under the ambient atmosphere without any encapsulation, whereas
only 43% retaining was achieved for the control device based on single PCBM CBL.
Perovskite solar cells
Fullerene derivative
Interfacial engineering
Cathode buffer layer
Work function
1. Introduction
commercialized crystalline-Si and inorganic semiconductor thin film
solar cells promises vast applications of PSCs.
Organic-inorganic hybrid perovskite materials exemplified by
CH3NH3PbX3 (MAPbX3, X ¼ I, Br, Cl) have been drawing intense at-
tentions in photoelectric conversion due to their tunable optical
bandgaps [1–3], long electron-hole diffusion lengths [4], large absorp-
tion coefficients and excellent charge carrier transport properties
[5–10]. During the past decade, extensive studies on perovskite solar
cells (PSCs) employing organic-inorganic hybrid perovskite as light
absorbers have been reported toward enhancement of device perfor-
mance, enabling ever-increasing power conversion efficiency (PCE)
reaching up to 25.2% [11]. Such a competitive PCE to those of the
Among the popularly used planar heterojunction architectures of
PSC device, inverted (p-i-n) planar PSCs (iPSCs) with the advantages of
the low-temperature solution processibility and negligible current-
voltage hysteresis have attracted more and more interests in recent
years [12–20]. To ensure efficient electron transport, a cathode buffer
layer (CBL) is typically required, which is incorporated between the
perovskite layer and cathode [21]. Fullerene derivatives especially [6,
6]-phenyl-C61-butyric acid methyl ester (PCBM) with strong
electron-accepting abilities have been commonly implemented as
indispensable CBLs of iPSC devices, fulfilling efficient electron transport
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: mqchen@ustc.edu.cn (M. Chen), gwang@ustc.edu.cn (G.-W. Wang), sfyang@ustc.edu.cn (S. Yang).
https://doi.org/10.1016/j.orgel.2020.105726
Received 7 January 2020; Received in revised form 7 March 2020; Accepted 14 March 2020
Available online 21 March 2020
1566-1199/© 2020 Elsevier B.V. All rights reserved.

L. Jia et al.
Organic Electronics 82 (2020) 105726
from the perovskite layer to cathode along with trap states passivation of
perovskite surface [12,22,23]. However, only a single fullerene CBL is
typically used for iPSCs, resulting in the existence of an interfacial en-
ergy offset between fullerene CBL and metal cathode since the Fermi
level of the cathode such as Ag (À 4.3 eV) is much lower than the lowest
unoccupied molecular orbital (LUMO) of PCBM (À 4.0 eV) [24–27]. This
consequently leads to insufficient electron transport and inferior PCE. To
promote electron transport from the perovskite layer to metal cathode
for iPSC devices, substituting PCBM by other fullerene derivatives or
incorporating an auxiliary interlayer between PCBM and cathode has
been employed, and the latter strategy appears more feasible since no
change in the fabrication procedure of the PCBM CBL is needed [19,28,
29]. So far most reported interlayer materials incorporated between
PCBM and cathode are commercially available organic small molecules
or polymers such as BCP, PEIE [24,30–37], which have nevertheless
relatively low tolerance to moisture and/or oxygen and are thus detri-
mental to the ambient stability of iPSC devices. Alternatively, fullerene
derivatives with high stability owing to the hydrophobic nature of the
parent fullerene have been also applied as interlayers between PCBM
and metal cathode, demonstrating enhanced efficiency via inhibiting
interfacial charge recombination loss and improved device stability
[38–42]. For instance, an amine functionalized fullerene derivative
(DMAPA-C60) composed of mixtures bearing different number of
dimethylamino groups was applied by Azimi and Zhang et al. as an
interlayer modifying PCBM CBL, leading to a dramatic increase of PCE
from 9.4% to 13.4% for CH3NH3PbI3-xClx iPSC device, which is attrib-
uted to the formation of an interfacial dipole layer rendering optimum
energy level alignment at the perovskite/PCBM interface [43]. Later on,
Lei and Yang et al. applied another dimethylamino-modified fullerene
derivative (PCBDAN) containing only one dimethylamino group as an
alternative interlayer of CH3NH3PbI3-xClx iPSC devices, which exhibited
an enhanced PCE of 17.2% due to decreased energy barrier between
perovskite and Ag cathode accomplished by lowering the work function
of Ag cathode [44]. Likewise, Russell et al. synthesized a full-
eropyrrolidine derivative (C60–N) bearing three dimethylamino groups
and applied it as an interlayer modifying PCBM CBL of FA0.5MA0.5PbI3
iPSCs, achieving a high PCE of 15.5% which is much higher than that of
the control device without interlayer (7.5%) due to the decreased work
function of Ag cathode induced by C60–N interfacial modification [25].
These studies reveal the importance of the amine group in not only
making the hydrophobic fullerene derivative into alcohol-soluble ma-
terial so as to realize orthogonal solvent processing of the interlayer but
also facilitating the formation of dipole layer [36,45–49]. Noteworthy,
most of these interlayer fullerene derivatives contain only one end
amine group, limiting the interactions between fullerene derivatives
with the atop metal cathode. Therefore, it is highly desirable to develop
novel alcohol-soluble amine-functionalized fullerene derivatives
bearing multiple amine groups as more efficient interlayers of iPSC
devices.
2. Experimental section
2.1. Materials
Fluorine-doped tin oxide (FTO)-coated glass substrates were pur-
chased from NSG Group, Japan, with a sheet resistance of 13 � 1.5 Ω/sq.
PbI2 was bought from Tokyo Chemical Industry Co., Ltd and MAI was
purchased from Xi’an Polymer Light Technology Corp. Dime-
thylformamide (DMF, 99.8%), dimethylsulfoxide (DMSO, 99.8%) and
chlorobenzene (99.9%) were obtained from Sigma-Aldrich. Isopropanol,
ethanol, and acetone were all bought from Sinopharm Chemical Reagent
Co., Ltd. C60 was purchased from Suzhou Dade Carbon Nanotechnology
Co. Ltd. PCBM (99.5%, Solenne Bv, Holand). Bathocuproine (BCP) was
obtained from Alfa Aesar. All reactants and solvents were used as
received without further purification.
2.2. Synthesis of bis (4-(2-(dimethylamino) ethoxy) phenyl) methanone
(DMAM)
Synthesis of compound DMAM was referred to the method reported
before [50]. The product is a white solid with the yield of 44.5%. 1H
NMR (400 MHz, CDCl3, δ): 7.77 (d, J ¼ 8.8 Hz, 2H), 6.98 (d, J ¼ 8.8 Hz,
2H), 4.15 (t, J ¼ 5.7 Hz, 2H), 2.78 (t, J ¼ 5.7 Hz, 2H), 2.36 (s, 6H). 13
C
NMR (101 MHz, CDCl3, δ): 194.45, 162.09, 132.19, 130.77, 114.01,
66.19, 58.14, 45.93.
2.3. Synthesis of bis (4-(2-(dimethylamino) ethoxy) phenyl) methanol
(DMAM-OH)
Compound (DMAM) (400 mg, 1.12 mmol) was dissolved in 15 mL
THF and then was cooled to 0 �C by ice bath. Later, LiAlH4 (63.8 mg,
1.68 mmol) was added into the solution. Then, the reaction proceeded
overnight with magnetic stirrer at room temperature under a nitrogen
atmosphere. Finally, the reaction was quenched with water, and organic
phase was extracted three times with dichloromethane. The organic
solvent collected was dried over anhydrous MgSO4. After removing the
solvent, the product purified by silica gel column chromatography
afford light yellow viscous liquid with the yield of 61.4%. 1H NMR (400
MHz, CDCl3, δ): 7.26 (d, J ¼ 8.4 Hz, 4H), 6.85 (d, J ¼ 8.7 Hz, 4H), 5.74
(s, 1H), 4.02 (t, J ¼ 5.8 Hz, 4H), 3.47 (s, 1H), 2.71 (t, J ¼ 5.8 Hz, 4H),
2.32 (s, 12H). 13C NMR (101 MHz, CDCl3, δ): 157.96, 136.99, 127.71,
114.37, 75.08, 65.76, 58.15, 45.78.
2.4. Synthesis of PCBDMAM
To a solution of [6,6]-Phenyl-C61-butyric acid (PCBA) (0.2 g, 0.22
mmol), compound DMAM-OH (0.2 g, 0.26 mmol), p-toluenesulfonic
acid (p-TSA) (0.0424 g, 0.22 mmol), 4-dimethylaminopyridine (DMAP)
(0.0272 g, 0.22 mmol), and diisopropylcarbinol (DIPC) (0.0197, 0.31
mmol) in 25 mL o-Dichlorobenzene were added. After that, the mixed
solution was stirred by magnetic stirrer at room temperature under N2
atmosphere for 24 h. Then the solvent was removed under reduced
pressure. The crude mixture was purified by silica gel column chroma-
tography, affording a black solid product with the yield of 50.6%. 1H
NMR (400 MHz, CDCl3, δ): 7.93–7.86 (m, 2H), 7.59–7.42 (m, 4H), 7.20
(d, J ¼ 8.7 Hz, 4H), 6.86 (d, J ¼ 8.8 Hz, 4H), 4.04 (t, J ¼ 5.7 Hz, 4H),
2.90–2.84 (m, 2H), 2.73 (t, J ¼ 5.7 Hz, 4H), 2.59 (t, J ¼ 7.4 Hz, 2H), 2.34
(s, 12H), 2.23–2.16 (m, 2H). 13C NMR (101 MHz, CDCl3, δ): 172.08,
158.34, 148.77, 147.67, 145.77, 145.12, 145.07, 145.04, 144.97,
144.75, 144.72, 144.64, 144.61, 144.44, 144.31, 143.95, 143.70,
143.68, 142.98, 142.96, 142.92, 142.87, 142.19, 142.12, 142.08,
142.06, 140.91, 140.67, 138.01, 137.50, 136.66, 132.61, 132.08,
128.37, 114.44, 79.81, 65.79, 58.16, 51.85, 45.80, 34.35, 33.59, 22.35.
MALDI-TOF m/z for PCBDMAM: [M]þ calcd for C92H40N2O4, 1236.31;
found, 1236.82.
Herein, we synthesized a novel PCBM-like bis-dimethylamino-func-
tionalized fullerene derivative (abbreviated as PCBDMAM) via a facile
esterification reaction. Upon applying PCBDMAM as an auxiliary
fullerene interlayer atop of PCBM to form a PCBM/PCBDMAM double
fullerene CBL, MAPbI3 iPSC devices exhibit dramatic enhancements of
both PCE and ambient stability relative to the control devices based on
single PCBM CBL. The influence of PCBDMAM interlayer on the work
function of Ag cathode deposited atop of PCBM was investigated,
revealing that incorporation of PCBDMAM interlayer induced decrease
of the work function of Ag cathode and facilitated the formation of
interfacial dipole layer. Furthermore, involvement of PCBM/PCBDMAM
double fullerene CBL was found to improve the ambient stability of the
iPSC devices due to the higher hydrophobicity of PCBDMAM than
PCBM.
2

L. Jia et al.
Organic Electronics 82 (2020) 105726
Scheme 1. Synthetic route and chemical structure of PCBDMAM. (i) Dimethylaminoethyl chloride hydrochloride, K2CO3, acetone. (ii) LiAlH4, THF. iii) HCl, HOAc,
toulene. (iv) DMAM-OH, p-toluenesulfonic acid (PTSA), diisopropylcarbinol (DIPC), 4-dimethylaminopyridine (DMAP), o-DCB, N2.
2.5. Device fabrication
Tencor P6 surface profilometer. The steady-state photoluminescence
(PL) spectra were determined through an Edinburgh Instruments
FLS920 fluorescence spectrometer with an excitation wavelength of 460
nm. The time-resolved photoluminescence (TRPL) spectra were per-
formed via the time-correlated single-photon counting method with a
Picoquant Gmbh Solea Supercontinuum laser. Synchrotron radiation
photoelectron spectra (HR-SRPES) measurements were carried out at
the Catalysis and Surface Science Endstation at the BL11U beamline in
the National Synchrotron Radiation Laboratory (NSRL), Hefei. The
secondary electron cutoff spectra and valence band edge spectra were
measured using synchrotron radiation light as the excitation source with
photon energy of 40 eV. A sample bias of À 5 V was applied to observe
the secondary electron cutoff. Impedance spectroscopy measurements
(EIS) were measured in the dark by using an electrochemical worksta-
tion (Autolab 320, Metrohm, Switzerland) in a frequency range of 1 Hz
to 1 MHz under 1.0 V. Alternating current (AC) 20 mV perturbation was
applied with a frequency from 1 MHz to 1 Hz. The obtained impedance
spectra were fitted with ZView software (v2.8b, Scribner Associates).
Water contact angles were obtained by using a CAM instrument (Data
Physics, Germany).
The FTO glass substrate was cleaned using detergent, deionized
water, acetone, and isopropanol for 15 min and then dried under vac-
uum at 60 �C overnight. After the ultraviolet-ozone treatment for 15
min, the NiOx precursor solution prepared by sol-gel process (0.1 mmol
was dissolved in 10 mL ethanol and 60 μL ethanolamine, then stirred at
70 �C overnight) was spin-coated onto the FTO substrate in ambient
atmosphere at 4000 rpm for 30 s and then dried at 280 �C for 40 min. In
the following step, the FTO substrate with NiOx film was transfer to a
nitrogen-filled glovebox to fabricate MAPbI3 perovskite layer. Perov-
skite precursor solution composed of PbI2 (1.1 M), MAI (1 M) in DMF
and DMSO (7:3 vol/vol) was dropped onto FTO/NiOx substrate by spin-
coating at 1000 rpm for 10 s and 4000 rpm for 45 s. After that, 80 μL
chlorobenzene was drop-coated onto perovskite layer during the 25 s,
the devices were annealed on a hot plate at 100 �C for 10 min. Later,
PCBM (20 mg/mL in chlorobenzene) and PCBDMAM (0.03 mg/mL in
isopropanol solution) were successively spin-coated onto MAPbI3
perovskite film at 2000 rpm for 30 s and 1000 rpm for 30s, respectively,
to form the ETL and CBL. Finally, Ag electrode (100 nm) was thermally
evaporated onto the PCBDMAM layer under a pressure of about 10À 5
Torr. Through the shadow mask, the active area of device was defined as
0.1 cm2.
3. Results and discussion
3.1. Synthesis and characterization of PCBDMAM
2.6. Measurements and characterization
The novel bis-dimethylamino-functionalized fullerene derivative
(PCBDMAM) was synthesized via a facile esterification reaction of [6,
6]-phenyl-C61-butyric acid (PCBA) with bis(4-(2-(dimethylamino)
ethoxy) phenyl) methanol (DMAM-OH) as illustrated in Scheme 1. The
detailed synthetic procedures are depicted in the experimental section
(see Supporting Information S1). The solubility of PCBDMAM in chlo-
robenzene is ca. 20 mg/mL, smaller than that of PCBM, indicating that
the involvement of bis-dimethyamino moieties deteriorates the solubi-
lity of PCBM due to increased molecular polarity. However, the exis-
tence of the bis-dimethylamino group makes PCBDMAM alcohol-soluble
with a solubility of ca. 0.1 mg/mL in isopropanol. This enables the
feasibility of orthogonal solvent processing of PCBDMAM layer atop of
PCBM without destroying PCBM layer. Considering that synthesis
mono-dimethylamino group fullerene derivative is difficult to exclude
the effect of additional hydroxyl group, and it is reported that the more
exposed amine groups, the more effective of WF’s altering [45].
Therefore, we did not design and synthesize a single amine-functional
fullerene derivative here for comparison.
1H NMR spectrum and 13C NMR spectrum was recorded on a Bruker
AV 400 MHz NMR spectrometer and tetramethylsilane (TMS) was used
as internal standard. MALDI-TOF Mass spectra were collected on a
Bruker Autoflex Speed mass spectrometer. UV–vis spectra were recorded
on UV–vis–NIR 3600 spectrometer (Shimadzu, Japan).
The current density-voltage (J-V) characterizations were detected by
a Keithley 2400 source measurement unit under simulated AM 1.5
irradiation (100 mW cm-2) with a standard xenon-lamp-based solar
simulator (Oriel Sol 3A). The simulator illumination intensity was cali-
brated with a mono-crystalline silicon reference cell (Oriel P/N 91150 V,
with KG-5 visible color filter) calibrated by the National Renewable
Energy Laboratory (NREL). Around 50 devices were prepared and
detected independently to make sure that the results were reliable. The
EQE measurements were carried out on an Oriel Intelligent Quantum
Efficiency (IQE) 200TM measurement system with a tunable light
source. SEM measurements were measured via a field-emission scanning
electron microscope (Zeiss Gemini SEM 500, Germany). AFM measure-
ments were obtained by using a XE-7 scanning probe microscope in
noncontact mode (Park Systems, Korea). XPS measurements were per-
formed on a Thermo ESCALAB 250 instrument with a monochromatized
The chemical structure of PCBDMAM was determined by 1H and 13
C
NMR spectroscopies, FT-IR spectroscopy and matrix-assisted laser
desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry.
According to the 1H NMR spectrum of PCBDMAM, proton signals of
Al K
α X-ray source. The film thicknesses were measured by a KLA-
3

L. Jia et al.
Organic Electronics 82 (2020) 105726
Fig. 1. (a) The device structure of iPSCs based on
fullerene derivative CBL and the chemical structures
of PCBDMAM. (b) Top-view SEM image of MAPbI3
perovskite film grown on NiOx. (c) Cross-section
SEM image of the iPSC device base on PCBDMAM
CBL. (d) J-V curves of the control and PCBDMAM
CBL-based device measured under illumination of
AM 1.5G 100 mW/cm2 in air. The scanning direction
is from open-circuit voltage to short-circuit (reverse)
voltage, and the scan speed is 100 mV/s (e) EQE
spectra of the control and PCBDMAM CBL-based
device measured in air.
phenyl, dimethylamino moieties are observed in the range of 6.85–7.91
ppm and at 2.34 ppm, respectively (see Supporting Information Fig. S5).
In the 13C NMR spectrum, signals in the range of 132.1–148.8 ppm is
assigned to the sp2-carbons of C60 cage along with the phenyl ring
moiety attached onto the bridgehead carbon (see Supporting Informa-
tion Fig. S6). The other sp2-carbon signals at 114.4 and 128.4 ppm are
thermal stability of PCBDMAM was further evaluated by thermogravi-
metric analysis (TGA), revealing its excellent thermal stability with the
decomposition temperature beyond 392.3 �C (5% weight loss, see Sup-
porting Information Fig. S9).
3.2. Photovoltaic performance of iPSC devices based on PCBM/
PCBDMAM double fullerene CBL
assigned to the phenyl ring within the DMAM moiety. In addition, the
13
–
C NMR signal at 172.1 ppm is assigned to carbonyl (–O–C O) group.
–
Furthermore, the sp3-carbon signals located at 45.8 ppm is assigned to
the methyl group within the dimethylamino moiety.
In order to incorporate PCBDMAM layer without destroying the
underneath PCBM layer, we spin-coated PCBDMAM dissolved in iso-
propanol with variable concentrations (0.01–0.1 mg/mL) onto PCBM
layer at 1000 rpm. The optimized concentration of PCBDMAM in iso-
propanol was determined to be 0.03 mg/mL according to its perfor-
mance in device efficiency enhancement (see Supporting Information
Fig. S10). Given that the concentration of PCBDMAM in isopropanol is
quite low, it is necessary to verify the existence of PCBDMAM film atop
of PCBM layer. We carried out X-ray photoelectron spectroscopy (XPS)
to probe the characteristic nitrogen signal of PCBDMAM. According to
the comparison of the XPS survey and high-resolution N1s XPS spec-
troscopic results, the N1s signal at 399.6 eV is clearly observed for
PCBM/PCBDMAM double fullerene CBL, which is however absent in the
single PCBM film (see Supporting Information Fig. S11). This confirms
MALDI-TOF mass spectrometry of PCBDMAM shows a dominant
ionic peak at m/z ¼ 1236, which is coincident with the molecular weight
of PCBDMAM, confirming the proposed chemical structure (see Sup-
porting Information Fig. S7). Furthermore, in the FT-IR spectrum of
–
–
PCBDMAM, the C C bending vibrations of the phenyl ring were
observed at 1607, 1511, 1460 cmÀ 1, respectively. The characteristic
vibrational peaks at 1738, 1240, and 1180 cmÀ 1 was assigned to the
–
–
stretching vibrations of C O, O–C and C–N bonds, respectively. Besides,
the characteristic vibration peak of C60 cage at 526 cmÀ 1 was clearly
observed (see Supporting Information Fig. S8). The simultaneously ex-
istence of the characteristic vibrational peaks corresponding to C60 cage
and the addends confirm the chemical structure of PCBDMAM. The
Table 1
Summary of photovoltaic parameters of perovskite solar cells with or without PCBDMAM as CBL obtained under AM 1.5G simulated sun light.
b
Device
CBL
Voc
Jsc
FF
PCE (%)
Averagea
Rsb
Rsh
(V)
(mA/cm2)
(%)
Best
(Ω⋅cm2)
(Ω⋅cm2)
A
B
PCBM
PCBM/PCBDMAM
1.01 � 0.02
1.03 � 0.01
20.31 � 1.22
21.51 � 0.72
61.52 � 2.47
76.67 � 1.57
12.58 � 0.85
17.12 � 0.50
14.21
18.11
21.9
4.0
1566.8
2164.8
a
b
Averaged over 50 devices fabricated independently.
Rs and Rsh are obtained by the PCE measurement system.
4

L. Jia et al.
Organic Electronics 82 (2020) 105726
Fig. 2. (a) Ag 3d XPS spectra and b) synchrotron radiation photoelectron spectra (HR-SRPES) of the Ag electrode and PCBDMAM modified Ag electrode. (c)
Schematic diagram of iPSCs with a CBL and the cathode interaction process. (d) The energy-level diagram of the iPSCs.
the existence of PCBDMAM film atop of PCBM layer.
dramatic PCE enhancement of device based on PCBM/PCBDMAM dou-
ble fullerene CBL is indeed contributed by PCBDMAM interlayer.
Noteworthy, the best PCE of 18.11% achieved in our present work
represents the highest value reported for double fullerene CBL-based
iPSC devices (see Supporting Information Table S3 and Fig. S14).
Interestingly, the PCE enhancement upon incorporation of
PCBDMAM interlayer is primarily due to the increase of FF (from
61.52% to 76.67%, ~19.7% enhancement), whereas the increases of
both Jsc (from 20.31 to 21.51 mA/cm2, ~4.5% enhancement) and Voc
(from 1.01 to 1.03 V, ~1.9% enhancement) are much smaller. This is
verified by analyzing the statistical photovoltaic parameters (PCE, FF,
Jsc, and Voc) based on 50 devices fabricated independently (see Sup-
porting Information Fig. S15 for the box plots and Fig. S16 for the his-
tograms of PCE). Besides, the slight increase of Jsc is consistent with the
external quantum efficiency (EQE) measurement results, revealing that
the EQE responses in the broad visible-light region of 400–750 nm for
PCBM/PCBDMAM double fullerene CBL-based device are slightly higher
than those for the control device based on single PCBM CBL (Fig. 1e).
To unveil the reasons responsible for the increase of FF upon incor-
poration of PCBDMAM interlayer, we carried out a series of morpho-
logical and spectroscopic characterizations. The influence of PCBDMAM
interlayer on surface morphology of the underneath PCBM layer was
investigated by atomic force microscopy (AFM). According to the
comparison of the AFM height images of PCBM layers with and without
PCBDMAM deposition, while the single PCBM film exhibits a smooth
surface with the root mean square (RMS) roughness of 0.69 nm, depo-
sition of PCBDMAM interlayer atop leads to a slight increase of the RMS
roughness to 0.79 nm (see Supporting Information Fig. S17). Moreover,
some irregular particles appear (see also Supporting Information
Fig. S17 for the phase image), which were likely resulted from the ag-
gregation of PCBDMAM molecules with amphipathic feature. However,
these aggregates are too thin to affect the overall surface morphology of
the MAPbI3/PCBM film according to scanning electron microscopic
(SEM) results (see Supporting Information Fig. S18). On the other hand,
incorporation of PCBDMAM interlayer is found to impose negligible
influence on the optical absorption of MAPbI3 as well (see Supporting
Information Fig. S19).
Based on PCBM/PCBDMAM double fullerene CBL, we fabricated
iPSC devices in glove box with the structure of FTO/NiOx/CH3NH3PbI3/
PCBM/PCBDMAM/Ag (Fig. 1a), in which a thin film of NiOx (~20 nm)
prepared by sol-gel process was used as hole transport layer (HTL) [51,
52]. A compact and dense perovskite film of ~300 nm without any
pinhole was prepared by one-step method through spin-coating the
precursor solution of PbI2:MAI (1.1:1, molar ratio) onto the surface of
FTO/NiOx film in nitrogen glovebox (Fig. 1b). For comparison, control
devices based on the single PCBM CBL with and without isopropanol
treatment were also fabricated. The current density-voltage (J-V) curves
of the iPSC devices with and without PCBDMAM interlayer measured
under one sun illumination are compared in Fig. 1d, and the corre-
sponding photovoltaic parameters including open-circuit voltage (Voc),
short-circuit current (Jsc), fill factor (FF), PCE, series resistance (Rs), and
shunt resistance (Rsh) are summarized in Table 1. The control device
based on single PCBM CBL gives a Voc of 1.01 V, a Jsc of 20.31 mA/cm2,
FF of 61.52%, and an average PCE of 12.58%. Upon incorporating
PCBDMAM interlayer to form PCBM/PCBDMAM double fullerene CBL,
the average PCE increases dramatically to 17.12% calculated from a Voc
of 1.03 V, a Jsc of 21.51 mA/cm2, and a FF of 76.67%, and the best PCE
reaches 18.11%, which is enhanced by ~27% relative to that of the
control device based on single PCBM CBL (14.21%). To confirm the
reliability of the iPSC device with PCBM/PCBDMAM double fullerene
CBL, steady-state photocurrent output measurements at the maximum
power point was performed. The iPSC device with PCBM/PCBDMAM
double fullerene CBL exhibits sensitive light response with a stabilized
PCE of 17.88% during the illumination period of 300 s, confirming the
considerable reliability of the iPSC device with PCBM/PCBDMAM
double fullerene CBL, which is much higher than that of the control
device based on single PCBM CBL (see Supporting Information Fig. S12).
No obvious hysteresis of J-V curves is observed for both devices (see
Supporting Information Fig. S13 and Table S2). To rule out the influence
of the isopropanol solvent, we fabricated control devices based on the
single PCBM CBL treated with isopropanol by spin-coating isopropanol
at 1000 rpm onto PCBM layer, and found that the average PCE decreases
dramatically to 9.85% due primarily to the decrease of FF (see Sup-
porting Information Fig. S10 and Table S1). This result confirms that the
It has been reported that the lone-pair electrons of nitrogen (N)
5

L. Jia et al.
Organic Electronics 82 (2020) 105726
Fig. 3. (a) The steady-state PL spectra and (b) time-
resolved PL (TRPL) spectra of MAPbI3, MAPbI3/
PCBM, MAPbI3/PCBM/PCBDMAM films on glass
substrate. (c) The dependence of Voc on different
light intensities for the devices with and without
PCBDMAM CBL. (d) Nyquist plots (symbols) and
fitting curves (solid lines) of the ac impedance
spectra of the devices with and without PCBDMAM
measured in dark under a reverse potential of 1.0 V.
Inset is the equivalent circuit model employed for
the fitting of the impedance spectra.
within the amine group can interact with Ag to form Ag–N bonds which
are beneficial for electron extraction from PCBM to Ag cathode [53,54].
To monitor whether the dimethylamino groups of PCBDMAM interact
with Ag either, we measured XPS spectra of ITO/Ag films with and
without PCBDMAM deposition, and found that the Ag 3d5/2 and 3d3/2
signals of ITO/Ag film at 368.7 eV and 374.8 eV negatively shifted
negatively to 368.3 eV and 374.3 eV respectively upon PCBDMAM
deposition (Fig. 2a), suggesting the strong interaction between
PCBDMAM and Ag electrode resulting in the formation of Ag–N bonds
[53,54].
(see supporting information S15). Relative to the control MAPbI3 film
with a lifetime of 34.11 ns, the perovskite film after depositing PCBM
single layer exhibits a remarkable decreased lifetime of 4.89 ns and even
smaller lifetime of 4.12 ns after depositing PCBM/PCBDMAM double
fullerene CBL, suggesting the more effective charge transport from
MAPbI3 to PCBM/PCBDMAM double fullerene CBL [58,59]. In addition,
we studied the relationships between Voc and incident light intensity
(Plight) to unveil the origin of PCE enhancement after depositing
PCBDMAM interlayer atop PCBM layer (Fig. 3c) by plotting Voc as a
function of incident light intensity according to the following formula:
We further used synchrotron radiation photoelectron spectra (HR-
SRPES) to monitor the influence of PCBDMAM on the work function of
Ag electrode. According to the secondary electron cutoff of synchrotron
radiation photoelectron spectra (HR-SRPES) (Fig. 2b) [55], the work
functions of Ag electrodes before and after depositing PCBDMAM
interlayer were calculated to be 4.38 eV and 3.75 eV, respectively (see
Supporting Information S14), revealing the obvious decrease of the
work function of Ag electrode after depositing PCBDMAM. This suggests
that PCBDMAM interlayer can act as an interfacial dipole layer via the
formation of Ag–N bonds as discussed above (Fig. 2c), inducing the
decrease of the work function of Ag electrode. As a result, the work
function of Ag electrode decreased to 3.75 eV, consequently the inter-
facial energy offset between PCBM CBL and Ag cathode is minimized
(Fig. 2d), facilitating electron transport from PCBM CBL to Ag [30,56].
In order to study the influence of PCBDMAM CBL on the electron
transport process and interfacial charge recombination dynamics of
iPSCs, steady-state photoluminescence (PL) and time-resolved photo-
luminescence (TRPL) characterizations were performed [57]. As shown
in Fig. 3a, a strong PL characteristic emission peak of the pristine
perovskite film appears at 765 nm, which was quenched obviously after
depositing PCBM single layer. The further quenching was observed upon
depositing PCBDMAM interlayer onto PCBM layer, revealing the more
efficient electron transfer from perovskite to PCBM/PCBDMAM double
fullerene CBL. Additionally, we further performed the TRPL measure-
ments to monitor the charge transfer kinetics change of perovskite layer
after depositing PCBM single layer and PCBM/PCBDMAM double
fullerene CBL with the architecture as glass/MAPbI3/PCBM/PCBDMAM,
glass/MAPbI3/PCBM and glass/MAPbI3 (Fig. 3b) and corresponding
TRPL data fitted by the bi-exponential decay function were summarized
À
�
nkT
q
Voc∝
ln Plight
(1)
where k is Boltzmann constant, T is absolute temperature, q is electron
charge, and n is the ideal factor related to the dominant charge
recombination mechanism [60,61]. Slope larger than kT=q demon-
strates that monomolecular recombination was occurred at the inter-
face, indicating the existence of trap states. The n value corresponding to
monomolecular recombination process shown that the iPSC device
based on PCBM/PCBDMAM double fullerene CBL have lower value of
1.03 than that of the control device with PCBM single CBL (n ¼ 1.22),
implying the enhanced trap state passivation ability of
PCBM/PCBDMAM double fullerene CBL toward perovskite layer and the
improved Jsc and FF as well as the PCE enhancement of iPSC device.
Moreover, we evaluated the influence of PCBDMAM interlayer on
the electron conductivity (σ0) of iPSC devices based on PCBM/
PCBDMAM double fullerene CBL and PCBM single CBL with the archi-
tectures of ITO/PCBM/PCBDMAM/Ag and ITO/PCBM/Ag. According to
the following function:
I ¼
σ
0ALÀ 1
V
(2)
Where A is area of the device, L is thickness of the films [33,62,63]. The
higher σ0 (7.96 � 10À 3 mS cmÀ 1) of device based on PCBM/PCBDMAM
double fullerene CBL than that of device with only PCBM single CBL
(3.04 � 10À 3 mS cmÀ 1) indicates the higher electron transport ability
and less interfacial defect in the device with the aid of PCBDMAM
interlayer (see Supporting Information S16 for detailed analysis). We
further studied the influence of PCBDMAM interlayer on the electron
6

L. Jia et al.
Organic Electronics 82 (2020) 105726
Fig. 4. (a) Stabilities of devices with and without PCBDMAM stored in ambient condition without encapsulation (temperature: 20 �C, relative humidity: ~35%) for
1440h. (b) Water contact angles on MAPbI3/PCBM and MAPbI3/PCBM/PCBDMAM. (c) Photographs of perovskite film of PCBM and PCBM/PCBDMAM after water
was dropped on the surface for several minutes.
mobility of the iPSC device by space charge limited current (SCLC)
measurements based on electron-only device with the configuration of
ITO/TiOx/perovskite/PCBM/PCBDMAM/Ag. Based on the Mott-Gurney
equation (Supporting Information S17 for detailed analyses), the elec-
tron mobility (μe) of the device with PCBM/PCBDMAM double fullerene
CBL was calculated to be 1.08 � 10À 4 cm2 VÀ 1 SÀ 1 which is higher than
that of the control device with PCBM single CBL (μe ¼ 4.52 � 10À 5 cm2
VÀ 1 SÀ 1), suggesting the decreased non-radiative recombination at
interface of perovskite and cathode as well as the reduced series resis-
tance (Rs) which facilitate the increase of FF and Jsc. (as shown in
Table 1).
FTO/NiOx/perovskite/PCBM/PCBDMAM film than that of FTO/NiOx/
perovskite/PCBM film (74.6�, Fig. 4b). In order to verify the improved
stability of iPSC device after incorporating PCBDMAM interlayer, water
was dropped directly onto the FTO/NiOx/perovskite/PCBM films with
and without PCBDMAM to observe visually the color change of perov-
skite film (Fig. 4c). Obviously, the color of MAPbI3 perovskite film
without PCBDMAM interlayer quickly changed from black to yellow
within only 3 min, indicating the rapid decomposition of the perovskite.
However, with the existence of PCBDMAM interlayer on PCBM, the
MAPbI3 perovskite film still keeps the pristine black color after 3 min
and gradually turns into yellow color after 13 min, indicating the
improved device stability.
To unveil the charge transport dynamics of iPSC devices, we carried
out the electrochemical impedance spectroscopy (EIS) measurements in
dark under a reverse potential of 1.0 V. Fig. 3d compares the Nyquist
plots of the iPSC devices with PCBM/PCBDMAM double fullerene CBL
and single PCBM CBL and their corresponding fitted curves using an
equivalent circuit model were also illustrated [64]. The fitted parame-
ters obtained from Nyquist plots including series resistance (Rs), inter-
face charge transfer resistance (Rct), and the constant phase element
(CPE) are related to the interfacial ohmic contact and non-ideal chem-
ical capacitances of the device, respectively (Summarized in Supporting
Information Table S5). The iPSC device based on PCBM/PCBDMAM
double fullerene CBL exhibits significantly lower Rs (13.19 Ω cm2) and
Rct (157.9 Ω cm2) than those of the control device with only PCBM single
CBL with the value of Rs (27.53 Ω cm2) and Rct (1081.1 Ω cm2), indi-
cating the improved charge transport capability of iPSC device upon the
involvement of PCBDMAM interlayer, which is beneficial to the Jsc and
FF of the iPSC devices.
4. Conclusions
In summary, a novel bis-dimethylamino-functionalized fullerene
derivative (PCBDMAM) was synthesized via a facile esterification re-
action, and corresponding molecular structure was confirmed by 1H and
13C NMR, FT-IR spectroscopies, and MALDI-TOF mass spectrometry.
PCBDMAM was applied as an auxiliary fullerene interlayer atop of
PCBM to form a PCBM/PCBDMAM double fullerene CBL in MAPbI3 iPSC
devices. Incorporation of PCBDMAM interlayer facilitates the formation
of interfacial dipole layer between PCBM and Ag cathode, resulting in
decrease of the work function of the Ag cathode. Consequently the
interfacial energy offset between PCBM CBL and Ag cathode is mini-
mized, facilitating electron transport from PCBM CBL to Ag cathode. As
a result, iPSC devices based on PCBM/PCBDMAM double fullerene CBL
exhibit the highest PCE of 18.11%, which is drastically higher than that
of the control device based on single PCBM CBL (14.21%) and represents
the highest value reported for double fullerene CBL-based iPSC devices.
Moreover, due to the higher hydrophobicity of PCBDMAM than PCBM,
iPSC devices based on PCBM/PCBDMAM double fullerene CBL shows an
enhanced ambient stability, retaining more than 67% of the initial PCE
after storage 1440 h exposure under the ambient atmosphere without
any encapsulation, whereas only 43% retaining was achieved for the
control device based on single PCBM CBL. With the effectiveness of
double fullerene cathode buffer layers in enhancing both the efficiency
and stability of iPSC devices, our strategy paves the way to realizing
practical applications of PSCs.
3.3. Stability of iPSC devices based on PCBM/PCBDMAM double
fullerene CBL
We further evaluated the ambient stability of the iPSC devices by
storing them without any encapsulation in dark under the ambient at-
mosphere (temperature: 20 �C, relative humidity: ~35%). As shown in
Fig. 4a, the iPSC devices based on PCBM/PCBDMAM double fullerene
CBL still retained approximately 67% of the initial PCE after 1440 h
exposure. While the control device with single PCBM CBL only retained
43% of the initial PCE, indicating the improved device stability after
inserting PCBDMAM interlayer. Moreover, the monitored photovoltaic
parameters including Voc, Jsc and FF of the devices during 1440 h storage
are compared in Supporting Information Fig. S22. The improved
ambient stability of the iPSC device based on PCBM/PCBDMAM double
fullerene CBL was attributed to the improved hydrophobicity of
PCBDMAM interlayer, which protects perovskite layer from the erosion
of water and oxygen. Indeed, the improved hydrophobicity of
PCBDMAM was confirmed by the larger water contact angle of 85.1� for
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
7

L. Jia et al.
Organic Electronics 82 (2020) 105726
[21] C. Cui, Y. Li, Y. Li, Fullerene derivatives for the applications as acceptor and
cathode buffer layer materials for organic and perovskite solar cells, Adv. Energy
Mater. 7 (2017), 1601251.
Acknowledgements
This work was partially supported by the National Key Research and
Development Program of China (2017YFA0402800), the National Nat-
ural Science Foundation of China (51572254, 51602097), and the
Fundamental Research Funds for the Central Universities
(WK2060140025).
[22] Q. Wang, Y. Shao, Q. Dong, Z. Xiao, Y. Yuan, J. Huang, Large fill-factor bilayer
iodine perovskite solar cells fabricated by a low-temperature solution-process,
Energy Environ. Sci. 7 (2014) 2359–2365.
[23] S. Sun, T. Salim, N. Mathews, M. Duchamp, C. Boothroyd, G. Xing, T.C. Sum, Y.
M. Lam, The origin of high efficiency in low-temperature solution-processable
bilayer organometal halide hybrid solar cells, Energy Environ. Sci. 7 (2014)
399–407.
[24] H. Lei, X. Chen, L. Xue, L. Sun, J. Chen, Z. Tan, Z.G. Zhang, Y. Li, G. Fang,
A solution-processed pillar[5]arene-based small molecule cathode buffer layer for
efficient planar perovskite solar cells, Nanoscale 10 (2018) 8088–8098.
[25] Y. Liu, M. Bag, L.A. Renna, Z.A. Page, P. Kim, T. Emrick, D. Venkataraman, T.
P. Russell, Understanding interface engineering for high-performance fullerene/
perovskite planar heterojunction solar cells, Adv. Energy Mater. 6 (2016),
1501606.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.orgel.2020.105726.
[26] H. Dong, J. Xi, L. Zuo, J. Li, Y. Yang, D. Wang, Y. Yu, L. Ma, C. Ran, W. Gao, B. Jiao,
J. Xu, T. Lei, F. Wei, F. Yuan, L. Zhang, Y. Shi, X. Hou, Z. Wu, Conjugated molecules
“bridge”: functional ligand toward highly efficient and long-term stable perovskite
solar cell, Adv. Funct. Mater. 29 (2019), 1808119.
References
[1] J.H. Noh, S.H. Im, J.H. Heo, T.N. Mandal, S.I. Seok, Chemical management for
colorful, efficient, and stable inorganic-organic hybrid nanostructured solar cells,
Nano Lett. 13 (2013) 1764–1769.
[27] L. Meng, J. You, T.F. Guo, Y. Yang, Recent advances in the inverted planar
structure of perovskite solar cells, Acc. Chem. Res. 49 (2016) 155–165.
[28] Y. Xing, C. Sun, H.L. Yip, G.C. Bazan, F. Huang, Y. Cao, New fullerene design
enables efficient passivation of surface traps in high performance p-i-n
heterojunction perovskite solar cells, Nano Energy 26 (2016) 7–15.
[29] Z. Luo, F. Wu, T. Zhang, X. Zeng, Y. Xiao, T. Liu, C. Zhong, X. Lu, L. Zhu, S. Yang,
C. Yang, Designing perylene diimide/fullerene hybrid as effective electron
transporting material in inverted perovskite solar cells with enhanced efficiency
and stability, Angew. Chem. Int. Ed. 58 (2019) 8520–8525.
€
[2] E.L. Unger, L. Kegelmann, K. Suchan, D. Sorell, L. Korte, S. Albrecht, Roadmap and
roadblocks for the band gap tunability of metal halide perovskites, J. Mater. Chem.
A 5 (2017) 11401–11409.
[3] A. Dubey, N. Adhikari, S. Mabrouk, F. Wu, K. Chen, S. Yang, Q. Qiao, A strategic
review on processing routes towards highly efficient perovskite solar cells,
J. Mater. Chem. A 6 (2018) 2406–2431.
[4] Q. Wu, W. Zhou, Q. Liu, P. Zhou, T. Chen, Y. Lu, Q. Qiao, S. Yang, Solution-
Processable ionic liquid as an independent or modifying electron transport layer for
high-efficiency perovskite solar cells, ACS Appl. Mater. Interfaces 8 (2016)
34464–34473.
[30] H. Zhang, H. Azimi, Y. Hou, T. Ameri, T. Przybilla, E. Spiecker, M. Kraft, U. Scherf,
C.J. Brabec, Improved high-efficiency perovskite planar heterojunction solar cells
via incorporation of a polyelectrolyte interlayer, Chem. Mater. 26 (2014)
5190–5193.
[5] J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M.K. Nazeeruddin,
€
M. Gratzel, Sequential deposition as a route to high-performance perovskite-
[31] C.-Y. Chang, Y.-C. Chang, W.-K. Huang, K.-T. Lee, A.-C. Cho, C.-C. Hsu, Enhanced
performance and stability of semitransparent perovskite solar cells using solution-
processed thiol-functionalized cationic surfactant as cathode buffer layer, Chem.
Mater. 27 (2015) 7119–7127.
sensitized solar cells, Nature 499 (2013) 316–319.
[6] C.C. Stoumpos, M.G. Kanatzidis, Halide Perovskites: poor Man’s high-performance
semiconductors, Adv. Mater. 28 (2016) 5778–5793.
[7] Y.-C. Wang, J. Chang, L. Zhu, X. Li, C. Song, J. Fang, Electron-transport-layer-
assisted crystallization of perovskite films for high-efficiency planar heterojunction
solar cells, Adv. Funct. Mater. 28 (2018), 1706317.
[32] C.-Y. Chang, W.-K. Huang, J.-L. Wu, Y.-C. Chang, K.-T. Lee, C.-T. Chen, Room-
temperature solution-processed n-doped zirconium oxide cathode buffer layer for
efficient and stable organic and hybrid perovskite solar cells, Chem. Mater. 28
(2015) 242–251.
[8] X. Liu, P. Li, Y. Zhang, X. Hu, Y. Duan, F. Li, D. Li, G. Shao, Y. Song, High-efficiency
perovskite solar cells based on self-assembly n-doped fullerene derivative with
excellent thermal stability, J. Power Sources 413 (2019) 459–466.
[9] H. Zhou, Q. Chen, G. Li, S. Luo, T.-b. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu,
Y. Yang, Interface engineering of highly efficient perovskite solar cells, Science 345
(2014) 542–546.
[33] J. Li, T. Jiu, C. Duan, Y. Wang, H. Zhang, H. Jian, Y. Zhao, N. Wang, C. Huang,
Y. Li, Improved electron transport in MAPbI3 perovskite solar cells based on dual
doping graphdiyne, Nano Energy 46 (2018) 331–337.
[34] W. Chen, L. Xu, X. Feng, J. Jie, Z. He, Metal acetylacetonate series in interface
engineering for full low-temperature-processed, high-performance, and stable
planar perovskite solar cells with conversion efficiency over 16% on 1 cm2 scale,
Adv. Mater. 29 (2017), 1603923.
[10] P.S. Chandrasekhar, A. Dubey, Q. Qiao, High efficiency perovskite solar cells using
nitrogen-doped graphene/ZnO nanorod composite as an electron transport layer,
Sol. Energy 197 (2020) 78–83.
[35] X. Jia, L. Zhang, Q. Luo, H. Lu, X. Li, Z. Xie, Y. Yang, Y.Q. Li, X. Liu, C.Q. Ma, Power
conversion efficiency and device stability improvement of inverted perovskite solar
cells by using a ZnO:PFN composite cathode buffer layer, ACS Appl. Mater.
Interfaces 8 (2016) 18410–18417.
[11] Research cell efficiency records. www.nrel.gov/pv/assets/pdfs/best-research-cell-
efficiencies.20191106.pdf (National Renewable Energy Laboratory).
[12] Y. Shao, Z. Xiao, C. Bi, Y. Yuan, J. Huang, Origin and elimination of photocurrent
hysteresis by fullerene passivation in CH3NH3PbI3 planar heterojunction solar
cells, Nat. Commun. 5 (2014) 5784.
[36] Q. Xue, Z. Hu, J. Liu, J. Lin, C. Sun, Z. Chen, C. Duan, J. Wang, C. Liao, W.M. Lau,
F. Huang, H.-L. Yip, Y. Cao, Highly efficient fullerene/perovskite planar
heterojunction solar cells via cathode modification with an amino-functionalized
polymer interlayer, J. Mater. Chem. A 2 (2014) 19598–19603.
[13] Y. Hou, W. Chen, D. Baran, T. Stubhan, N.A. Luechinger, B. Hartmeier, M. Richter,
J. Min, S. Chen, C.O.R. Quiroz, N. Li, H. Zhang, T. Heumueller, G.J. Matt, A. Osvet,
K. Forberich, Z.-G. Zhang, Y. Li, B. Winter, P. Schweizer, E. Spiecker, C.J. Brabec,
Overcoming the interface losses in planar heterojunction perovskite-based solar
cells, Adv. Mater. 28 (2016) 5112–5120.
[37] J.Y. Jeng, Y.F. Chiang, M.H. Lee, S.R. Peng, T.F. Guo, P. Chen, T.C. Wen,
CH3NH3PbI3 perovskite/fullerene planar-heterojunction hybrid solar cells, Adv.
Mater. 25 (2013) 3727–3732.
[14] H.-C. Liao, P. Guo, C.-P. Hsu, M. Lin, B. Wang, L. Zeng, W. Huang, C.M.M. Soe, W.-
F. Su, M.J. Bedzyk, M.R. Wasielewski, A. Facchetti, R.P.H. Chang, M.G. Kanatzidis,
T.J. Marks, Enhanced efficiency of hot-cast large-area planar perovskite solar cells/
modules having controlled chloride incorporation, Adv. Energy Mater. 7 (2017),
1601660.
[38] P.W. Liang, C.Y. Liao, C.C. Chueh, F. Zuo, S.T. Williams, X.K. Xin, J. Lin, A.K. Jen,
Additive enhanced crystallization of solution-processed perovskite for highly
efficient planar-heterojunction solar cells, Adv. Mater. 26 (2014) 3748–3754.
[39] X. Liu, W. Jiao, M. Lei, Y. Zhou, B. Song, Y. Li, Crown-ether functionalized
fullerene as a solution-processable cathode buffer layer for high performance
perovskite and polymer solar cells, J. Mater. Chem. A 3 (2015) 9278–9284.
[40] X. Liu, M. Lei, Y. Zhou, B. Song, Y. Li, High performance planar p-i-n perovskite
solar cells with crown-ether functionalized fullerene and LiF as double cathode
buffer layers, Appl. Phys. Lett. 107 (2015), 063901.
[15] E. Castro, T.J. Sisto, E.L. Romero, F. Liu, S.R. Peurifoy, J. Wang, X. Zhu,
C. Nuckolls, L. Echegoyen, Cove-edge nanoribbon materials for efficient inverted
halide perovskite solar cells, Angew. Chem. Int. Ed. 129 (2017) 14840–14844.
� �
[16] W. Chen, Y. Wu, J. Fan, A.B. Djurisic, F. Liu, H.W. Tam, A. Ng, C. Surya, W.K. Chan,
D. Wang, Z.-B. He, Understanding the doping effect on NiO: toward high-
performance inverted perovskite solar cells, Adv. Energy Mater. 8 (2018),
1703519.
[41] Z. Zhu, C.C. Chueh, F. Lin, A.K. Jen, Enhanced ambient stability of efficient
perovskite solar cells by employing a modified fullerene cathode interlayer, Adv.
Sci. 3 (2016), 1600027.
[17] W. Chen, Y. Zhou, L. Wang, Y. Wu, B. Tu, B. Yu, F. Liu, H.-W. Tam, G. Wang, A.
[42] V.V. Duzhko, B. Dunham, S.J. Rosa, M.D. Cole, A. Paul, Z.A. Page,
C. Dimitrakopoulos, T. Emrick, N-doped zwitterionic fullerenes as interlayers in
organic and perovskite photovoltaic devices, ACS Energy Lett 2 (2017) 957–963.
[43] H. Azimi, T. Ameri, H. Zhang, Y. Hou, C.O.R. Quiroz, J. Min, M. Hu, Z.-G. Zhang,
T. Przybilla, G.J. Matt, E. Spiecker, Y. Li, C.J. Brabec, A universal interface layer
based on an amine-functionalized fullerene derivative with dual functionality for
efficient solution processed organic and perovskite solar cells, Adv. Energy Mater.
5 (2015), 1401692.
� �
B. Djurisic, L. Huang, Z. He, Molecule-doped nickel oxide: verified charge transfer
and planar inverted mixed cation perovskite solar cell, Adv. Mater. 30 (2018)
1800515.
[18] X. Liu, B. Li, N. Zhang, Z. Yu, K. Sun, B. Tang, D. Shi, H. Yao, J. Ouyang, H. Gong,
Multifunctional RbCl dopants for efficient inverted planar perovskite solar cell with
ultra-high fill factor, negligible hysteresis and improved stability, Nano Energy 53
(2018) 567–578.
[19] Q. Xue, Y. Bai, M. Liu, R. Xia, Z. Hu, Z. Chen, X.-F. Jiang, F. Huang, S. Yang,
Y. Matsuo, H.-L. Yip, Y. Cao, Dual interfacial modifications enable high
performance semitransparent perovskite solar cells with large open circuit voltage
and fill factor, Adv. Energy Mater. 7 (2017), 1602333.
[44] J. Xie, X. Yu, X. Sun, J. Huang, Y. Zhang, M. Lei, K. Huang, D. Xu, Z. Tang, C. Cui,
D. Yang, Improved performance and air stability of planar perovskite solar cells via
interfacial engineering using a fullerene amine interlayer, Nano Energy 28 (2016)
330–337.
[20] F. Wu, B. Bahrami, K. Chen, S. Mabrouk, R. Pathak, Y. Tong, X. Li, T. Zhang,
R. Jian, Q. Qiao, Bias-dependent normal and inverted J-V hysteresis in perovskite
solar cells, ACS Appl. Mater. Interfaces 10 (2018) 25604–25613.
[45] T. Jia, C. Sun, R. Xu, Z. Chen, Q. Yin, Y. Jin, H.-L. Yip, F. Huang, Y. Cao,
Naphthalene diimide based n-type conjugated polymers as efficient cathode
8

L. Jia et al.
Organic Electronics 82 (2020) 105726
interfacial materials for polymer and perovskite solar cells, ACS Appl. Mater.
liquid-like behavior: surface-induced secondary grain growth of photovoltaic
perovskite thin film, J. Am. Chem. Soc. 141 (2019) 13948–13953.
[56] Q. Chen, L. Yuan, R. Duan, P. Huang, J. Fu, H. Ma, X. Wang, Y. Zhou, B. Song,
Zwitterionic polymer: a facile interfacial material works at both anode and cathode
in p-i-n perovskite solar cells, Sol. RRL 3 (2019), 1900118.
Interfaces 9 (2017) 36070–36081.
[46] S. van Reenen, S. Kouijzer, R.A.J. Janssen, M.M. Wienk, M. Kemerink, Origin of
work function modification by ionic and amine-based interface layers, Adv. Mater.
Interfaces 1 (2014), 1400189.
[47] Q. Chen, W. Wang, S. Xiao, Y.B. Cheng, F. Huang, W. Xiang, Improved performance
of planar perovskite solar cells using an amino-terminated multifunctional
fullerene derivative as the passivation layer, ACS Appl. Mater. Interfaces 11 (2019)
27145–27152.
[57] G. Wang, J. Liu, K. Chen, R. Pathak, A. Gurung, Q. Qiao, High-performance carbon
electrode-based CsPbI2Br inorganic perovskite solar cell based on poly(3-
hexylthiophene)-carbon nanotubes composite hole-transporting layer, J. Colloid
Interface Sci. 555 (2019) 180–186.
[48] Y. Zhang, P. Wang, X. Yu, J. Xie, X. Sun, H. Wang, J. Huang, L. Xu, C. Cui, M. Lei,
D. Yang, Enhanced performance and light soaking stability of planar perovskite
solar cells using an amine-based fullerene interfacial modifier, J. Mater. Chem. A 4
(2016) 18509–18515.
[58] Y. Lin, L. Shen, J. Dai, Y. Deng, Y. Wu, Y. Bai, X. Zheng, J. Wang, Y. Fang, H. Wei,
W. Ma, X.C. Zeng, X. Zhan, J. Huang, Pi-conjugated lewis base: efficient trap-
passivation and charge-extraction for hybrid perovskite solar cells, Adv. Mater. 29
(2017).
[49] J. Xie, X. Yu, J. Huang, X. Sun, Y. Zhang, Z. Yang, M. Lei, L. Xu, Z. Tang, C. Cui,
P. Wang, D. Yang, Self-organized fullerene interfacial layer for efficient and low-
temperature processed planar perovskite solar cells with high UV-light stability,
Adv. Sci. 4 (2017), 1700018.
[59] G. Xu, R. Xue, W. Chen, J. Zhang, M. Zhang, H. Chen, C. Cui, H. Li, Y. Li, Y. Li, New
strategy for two-step sequential deposition: incorporation of hydrophilic fullerene
in second precursor for high-performance p-i-n planar perovskite solar cells, Adv.
Energy Mater. 8 (2018), 1703054.
[50] Z. Liu, W. Li, R. Peng, W. Jiang, Q. Guan, T. Lei, R. Yang, A. Islam, Q. Wei, Z. Ge,
Benzophenone-based small molecular cathode interlayers with various polar
groups for efficient polymer solar cells, J. Mater. Chem. A 5 (2017) 10154–10160.
[51] J.-l. Wu, W.-K. Huang, Y.-C. Chang, B.-C. Tsai, Y.-C. Hsiao, C.-Y. Chang, C.-T. Chen,
C.-T. Chen, Simple mono-halogenated perylene diimides as non-fullerene electron
transporting materials in inverted perovskite solar cells with ZnO nanoparticle
cathode buffer layers, J. Mater. Chem. A 5 (2017) 12811–12821.
[52] M. Li, X. Xu, Y. Xie, H.-W. Li, Y. Ma, Y. Cheng, S.-W. Tsang, Improving the
conductivity of sol–gel derived NiOx with a mixed oxide composite to realize over
80% fill factor in inverted planar perovskite solar cells, J. Mater. Chem. A 7 (2019)
9578–9586.
[60] J. Duan, Y. Zhao, B. He, Q. Tang, Simplified perovskite solar cell with 4.1%
efficiency employing inorganic CsPbBr3 as light absorber, Small 14 (2018),
1704443.
[61] W. Chen, H. Chen, G. Xu, R. Xue, S. Wang, Y. Li, Y. Li, Precise control of crystal
growth for highly efficient CsPbI2Br perovskite solar cells, Joule 3 (2019) 191–204.
[62] G. Kakavelakis, I. Paradisanos, B. Paci, A. Generosi, M. Papachatzakis,
T. Maksudov, L. Najafi, A.E. Del Rio Castillo, G. Kioseoglou, E. Stratakis,
F. Bonaccorso, E. Kymakis, Extending the continuous operating lifetime of
perovskite solar cells with a molybdenum disulfide hole extraction interlayer, Adv.
Energy Mater. 8 (2018), 1702287.
[63] Z. Wang, D.P. McMeekin, N. Sakai, S. van Reenen, K. Wojciechowski, J.B. Patel, M.
B. Johnston, H.J. Snaith, Efficient and air-stable mixed-cation lead mixed-halide
perovskite solar cells with n-doped organic electron extraction layers, Adv. Mater.
29 (2017), 1604186.
[53] H. Back, G. Kim, J. Kim, J. Kong, T.K. Kim, H. Kang, H. Kim, J. Lee, S. Lee, K. Lee,
Achieving long-term stable perovskite solar cells via ion neutralization, Energy
Environ. Sci. 9 (2016) 1258–1263.
[54] X. Meng, C.H.Y. Ho, S. Xiao, Y. Bai, T. Zhang, C. Hu, H. Lin, Y. Yang, S.K. So,
S. Yang, Molecular design enabled reduction of interface trap density affords
highly efficient and stable perovskite solar cells with over 83% fill factor, Nano
Energy 52 (2018) 300–306.
[64] P. Zhou, Z. Fang, W. Zhou, Q. Qiao, M. Wang, T. Chen, S. Yang, Nonconjugated
polymer poly(vinylpyrrolidone) as an efficient interlayer promoting electron
transport for perovskite solar cells, ACS Appl. Mater. Interfaces 9 (2017)
32957–32964.
[55] J. Xue, R. Wang, K.L. Wang, Z.K. Wang, I. Yavuz, Y. Wang, Y. Yang, X. Gao,
T. Huang, S. Nuryyeva, J.W. Lee, Y. Duan, L.S. Liao, R. Kaner, Y. Yang, Crystalline
9