Polyacetylene derivatives in perovskite solar cells: From defect passivation to moisture endurance

Article abstract of DOI:10.1039/d0ta12509a

The last decade has witnessed the exploration of exceptional optoelectronic and photovoltaic properties of perovskite solar cells (PSCs) at the laboratory scale. Unfortunately, their sensitivity to moisture causes bulk degradation, hindering the commercialization of PSC devices. Despite the numerous strategies that have been developed to date in this field, effective passivation against moisture remains highly challenging. Here, we report a novel approach based on the incorporation of polyacetylene derivatives into the perovskite active layer to yield perovskite films with larger grains, lower defect density, and excellent robustness with respect to moisture. Moreover, it is revealed that the reduced trap-state density of these films is most likely due to the efficient coordination between the carboxylate moieties in the polymer and the undercoordinated Pb2+ in the perovskite. Upon adopting the polymer-doped perovskite as an active layer in inverted planar heterojunction PSCs with all-inorganic charge extraction layers, the power conversion efficiency (PCE) is improved to 20.41%, which is the highest value reported to date for this type of PSC to the best of our knowledge. Most importantly, the optimized device retained 90% of its initial PCE after aging in ambient air for 60 days due to its dual mechanism of moisture resistance. This work highlights an approach for developing high-performance PSCs with improved moisture stability and paves the way for their potential commercialization. This journal is

Full text of DOI:10.1039/d0ta12509a

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Polyacetylene derivatives in perovskite solar cells:
from defect passivation to moisture endurance†
Cite this: J. Mater. Chem. A, 2021, 9,
3220
1
a
a
a
a
Jiexuan Jiang,‡ Xianhua Lang, ‡ Qiugui Zeng, M. Bilal Faheem,
a
b
ac
Shanshan Rong, Hui Zhao and Yanbo Li
*
The last decade has witnessed the exploration of exceptional optoelectronic and photovoltaic properties of
perovskite solar cells (PSCs) at the laboratory scale. Unfortunately, their sensitivity to moisture causes bulk
degradation, hindering the commercialization of PSC devices. Despite the numerous strategies that have
been developed to date in this field, effective passivation against moisture remains highly challenging.
Here, we report a novel approach based on the incorporation of polyacetylene derivatives into the
perovskite active layer to yield perovskite films with larger grains, lower defect density, and excellent
robustness with respect to moisture. Moreover, it is revealed that the reduced trap-state density of these
films is most likely due to the efficient coordination between the carboxylate moieties in the polymer
2+
and the undercoordinated Pb in the perovskite. Upon adopting the polymer-doped perovskite as an
active layer in inverted planar heterojunction PSCs with all-inorganic charge extraction layers, the power
conversion efficiency (PCE) is improved to 20.41%, which is the highest value reported to date for this
type of PSC to the best of our knowledge. Most importantly, the optimized device retained 90% of its
initial PCE after aging in ambient air for 60 days due to its dual mechanism of moisture resistance. This
work highlights an approach for developing high-performance PSCs with improved moisture stability
and paves the way for their potential commercialization.
Received 28th December 2020
Accepted 30th April 2021
DOI: 10.1039/d0ta12509a
rsc.li/materials-a
17–22
in the overall device performance.
Among the different
Introduction
proposed device architectures, PSCs with all-inorganic selective
In recent years, organic–inorganic hybrid perovskites have CTLs show a unique advantage in improving the device stability
attracted extensive attention due to their exceptional optoelec- by replacing the commonly used but relatively unstable organic
23
tronic properties such as high absorption coefficients, long charge-transporting materials. This type of device was rst
carrier diffusion lengths, suitable band gap and energy levels, demonstrated by Yang et al. who used a NiO hole transporting
Hybrid layer (HTL) and a ZnO electron transporting layer (ETL) and
perovskites are widely utilized in a variety of optoelectronic demonstrated a maximum power conversion efficiency (PCE) of
x
1–5
and their facile and low-cost synthesis processes.
24
devices such as photodetectors, light-emitting diodes, and solar 16.1%. We recently achieved a maximum PCE of over 19%
6
,7
cells. The efficiency of perovskite solar cells (PSCs) has been using electron-beam-deposited NiO
x
and Nb
2 5
O as the HTL and
8–12
25,26
increased remarkably from 3.8% to 25.5% in only a decade,
ETL, respectively.
However, in this type of device, the overall
making them promising competitors to silicon-based photo- device efficiency and stability are still signicantly affected by
voltaics and some of the most competitive next-generation the perovskite active layer. Due to the existence of under-
13–16
energy-harvesting material candidates.
A PSC device consists of a perovskite active layer, selective the perovskite active layer,
coordinated ions on the surface and at the grain boundaries of
27–32
further improvement in device
charge (electron and hole) transporting layers (CTLs), and efficiency and stability is still necessary. Meanwhile, a high
carrier collection layers (front-electrode or substrate and surface defect density within the perovskite layer may also
33–36
metallic counter-electrode). Each layer plays an important role contribute to a large current density–voltage (J–V) hysteresis.
It was shown that by increasing the perovskite grain size and
using passivation techniques, the solar cell performance can be
a
Institute of Fundamental and Frontier Sciences, University of Electronic Science and
37–40
considerably enhanced.
It has been found that perovskite
Technology of China, Chengdu 610054, China. E-mail: yanboli@uestc.edu.cn
b
thin lms with improved morphology and better crystallinity
School of Chemical Engineering, Sichuan University, Chengdu 610065, China
c
can be achieved by incorporating appropriate polymers into the
Engineering Research Center of Advanced Functional Material Manufacturing of
41–43
Ministry of Education, Zhengzhou University, Zhengzhou 450001, China
perovskite precursor solution.
In particular, polymers con-
†
1
‡
Electronic supplementary information (ESI) available. See DOI: taining weak acids, amino groups, and ionic groups with elec-
0.1039/d0ta12509a
tron lone pairs were found to be effective in passivating the trap
These authors contributed equally to this work.
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states within perovskites, enabling enhanced carrier transport
Results and discussion
Through the incorporation of semi-
44–47
and collection.
conductor polymer dopants with good electrical conductivity, Polyacetylene-derivative polymers were used to improve the
48
49,50
Liu et al. and Han et al.
realized greatly improved device chemistry of the perovskite precursor solution and to control
performance for PSCs. On the other hand, Zhao et al. reported the nucleation and crystallization of the deposited perovskite
that only chemically inert polymers can be utilized to achieve thin lms. Moreover, we fabricated inverted planar hetero-
51
52
effective carrier transport. Very recently, Li et al. discovered junction PSCs with the ITO/NiO /FA0.85MA0.15PbI2.55Br0.45
/
x
that doping of a graphdiyne derivative into the perovskite layer Nb O /Ag device structure, as shown in Fig. 1a. A representative
2
5
can increase the size of the crystalline grains. Motivated by cross-sectional scanning electron microscopy (SEM) image of
these results, we propose to further improve the efficiency and the device structure is presented in Fig. 1b. Four polymer
stability of the all-inorganic CTL-based PSCs by doping the semiconductor materials, namely polyacetylene, penta-
perovskite with polyacetylene-based polymers. To date, these uorophenyl (PFP) ester-functionalized poly(phenylacetylene)s,
0
polymers have not been used for perovskite doping and this poly(4 -triuoromethylphenyl acetylene), and triphenylamine
strategy has not been applied to all-inorganic CTL-based PSCs. (TPA)-functionalized polyacetylenes (denoted as P1, P2, P3, and
In this work, small amounts of highly conductive poly- P4, respectively) were used as additives to dope the perovskite
acetylene derivatives are added to the perovskite precursor active layer. Fig. 1c shows the molecular structures of poly-
solution, leading to a signicant improvement in the overall acetylene and its derivatives. The successful synthesis of the
device performance. The polyacetylene derivatives affect the polyacetylene derivatives was conrmed by nuclear magnetic
crystallization of the perovskite layer to yield high-quality, resonance (NMR) spectroscopy (Fig. S1–S6†). All of the polymers
uniform lms with larger grain sizes. In addition, it is found can be well-dissolved in N,N-dimethylformamide (DMF), form-
that the polyacetylene derivative additives not only improve the ing clear solutions without precipitates (Fig. S7†). The device
carrier dynamics and passivate the perovskite surface defects, performance characteristics for different polymer-doped PSCs
but also enhance the PSC device stability. Upon adopting the were compared to identify the best polymer dopant. Fig. 1d
perovskite doped with a polyacetylene derivative as the active presents the J–V curves for the reference PSC device and for the
layer in inverted planar heterojunction PSCs with all-inorganic PSCs with different polymer additives. The photovoltaic
CTLs, a champion PCE of 20.41% is achieved, together with parameters of the PSC devices are summarized in Table 1. All of
a signicantly improved device stability against moisture the devices with the polymer-doped perovskite absorber show
ingress. The improved efficiency and stability of the PSC is signicantly increased short-circuit current density (JSC) and ll
attributed to the increased grain size, decreased trap-state factor (FF), particularly for the P2-doped PSC that showed
density, more favorable band alignment for charge extraction, a champion performance.
and good hydrophobicity of the polyacetylene-derivative-doped
perovskite active layer.
To further optimize the PSC device, the concentration of the
P2 dopant was varied between 0.2 and 0.6 mM. Fig. 1e shows the
Fig. 1 (a) Schematic of the perovskite solar cell. (b) Cross-sectional SEM image of the PSC device. (c) Molecular structures of the polyacetylene-
derivative polymers used in this study. (d) J–V characteristics of the PSCs doped with different polymers. (e) J–V characteristics of the PSCs
doped with different P2 concentrations. (f) EQE spectra and integrated JSC with and without P2.
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Table 1 The key J–V parameters of the PSCs doped with different magnication SEM images in Fig. S8† show that the P2-
polymers
modied perovskite lms have fewer pin-holes than the pris-
ꢀ
2
tine lm. The corresponding grain size distributions are
statistically summarized by histograms in Fig. 2e–h. The
average grain size of the four perovskite lms was found to be
240, 291, 349, and 279 nm, for the pristine lm and the lms
with 0.2 mM, 0.4 mM and 0.6 mM P2, respectively. This illus-
trates the benecial effect of P2 incorporation on the nucleation
and growth of perovskite crystalline grains. Moreover, it was
also observed that a small number of platelets with bright
contrast were distributed on the surface of the P2-doped lms.
Polymer (0.4 mM)
V
OC (V)
J
SC (mA cm
)
FF (%)
PCE (%)
Reference
1.083
1.104
1.098
1.099
1.094
22.66
23.38
23.59
23.32
23.11
74.5
74.1
78.8
75.8
78.1
18.28
19.13
20.41
19.42
19.74
P1
P2
P3
P4
J–V curves for the PSCs fabricated with different P2 concentra-
tions, with the key photovoltaic parameters summarized in
Table 2. The pristine PSC device (without P2) delivered a PCE of
8.28%, with an open-circuit voltage (VOC) of 1.083 V, JSC of
2.66 mA cm , and FF of 74.5%. With the incorporation of
53–56
According to previous reports,
the bright platelets are most
likely PbI that can passivate the grain boundaries and surface
2
of perovskite lms to improve the device performance.
1
2
To study the structural and optical properties of the perov-
skite lms, four representative samples with different P2
concentrations (0, 0.2, 0.4, and 0.6 mM) were prepared and
characterized. Fig. 2i presents the X-ray diffraction (XRD)
patterns showing that the incorporation of P2 made a key
contribution to the improved crystallization of perovskite lms
ꢀ2
a small amount of P2 (0.2 mM), the performance of the PSC was
signicantly improved. The PCE was raised to 19.13%, with the
ꢀ2
corresponding VOC, JSC, and FF of 1.091 V, 23.06 mA cm , and
6.0%, respectively. It is clear that the P2-modied perovskite
absorber dramatically enhances the device performance. When
the P2 concentration was further increased to 0.4 mM, an
impressive rise in the PCE (20.41%) was observed, with
7
and the peak of the PbI
2
phase was also slightly enhanced,
which is consistent with the SEM observations. This may be
+
+
ascribed to the interaction between P2 and the FA and MA
cations that caused a small amount of PbI to escape from the
ꢀ2
enhanced VOC, JSC, and FF of 1.098 V, 23.59 mA cm , and
8.8%, respectively. It is important to note that both JSC and FF
2
7
perovskite surface. Fig. 2j shows the UV-vis absorption spectra
for the pristine perovskite lms and the lms with different P2
concentrations deposited on quartz glass substrates. The
incorporation of P2 results in enhanced light absorption that is
attributed to the increased light-harvesting capability of the
perovskite lms with larger grain size.
were increased substantially through the optimization of the P2
content by increasing its concentration. This noticeable rise in
photovoltaic parameters is attributed to the good crystallinity of
the perovskite grains, suppressed Shockley–Read–Hall recom-
bination, and smooth carrier transport from the perovskite bulk
that were achieved by P2 doping, as will be discussed later.
When the P2 concentration was further increased to 0.6 mM,
the device performance showed a decline in the PCE to 18.67%
with VOC, JSC, and FF of 1.093 V, 23.37 mA cm , and 73.1%,
respectively, showing that the perovskite properties are optimal
at a P2 additive concentration of 0.4 mM. Fig. 1f presents the
external quantum efficiency (EQE) spectra for the optimized
and reference PSCs which also conrm the J–V scans taken
under simulated sunlight. Compared to the pristine device, the
EQE of the optimized devices was enhanced dramatically with
Hysteresis is a notorious phenomenon that degrades the
overall device performance and operational stability, particu-
larly for planar PSCs. Fig. 3a shows the typical J–V curves of the
devices measured in both reverse and forward scan directions,
with important device parameters listed in Table 3. It is
observed that J–V hysteresis was substantially reduced by the
addition of P2, while the pristine device showed pronounced
hysteresis. For optimized PSCs, the J–V curves for reverse and
forward scans almost overlap. The PCE measured under the
reverse scan is 20.41%, while the PCE remains at 19.99% when
measured under the forward scan. We believe that the signi-
cantly reduced hysteresis is attributed to the reduced defect
density and efficient carrier transport from within the perov-
skite absorber layer achieved by P2 incorporation. The hyster-
ꢀ2
ꢀ
2
the integrated JSC improved to 23.03 mA cm , which was
signicantly higher than the integrated JSC of 21.86 mA cm
ꢀ2
obtained for the pristine device.
To further investigate the origin of the improved PSC
performance, the morphology of the perovskite lms was
characterized. Fig. 2a–d show the top-view SEM images of the
different P2 concentration modied perovskite lms. Low-
57
esis index of the devices can be dened as
PCEreverse ꢀ PCEforward
Hysteresis index ¼
(1)
PCEreverse
The hysteresis index for the optimized device is 0.020, which is
noticeably lower than the value (0.133) for the pristine device.
To ensure the validity of the J–V measurements, a steady-
Table 2 The key J–V parameters of the PSCs with different P2
concentrations
state photocurrent output was measured, as presented in
PCE (%) Fig. 3b. Steady-state photocurrent densities of 22.92 and 21.58
Polymer P2
ꢀ2
(mM)
V
OC (V)
J
SC (mA cm
)
FF (%)
ꢀ
2
mA cm
were achieved at 0.923 and 0.862 V for the P2-
0
0
0
0
1.083
1.091
1.098
1.093
22.66
23.06
23.59
23.37
74.5
76.0
78.8
73.1
18.28
19.13
20.41
18.67
optimized and reference devices, yielding stabilized power
conversion outputs of 20.01% and 17.53%, respectively. It is
observed that the steady-state power output for the P2-
.2
.4
.6
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Fig. 2 (a–d) Surface topographic SEM images, and (e–h) grain size distributions of the FA0.85MA0.15PbI2.55Br0.45 films doped with different P2
concentrations. The crystal structure and optical properties of the perovskite films with different amounts of P2 (0, 0.2, 0.4, and 0.6 mM): (i) XRD
patterns and (j) UV-vis absorption spectra.
F
optimized device maintained good stability, while the steady- excitation energy (21.22 eV). Therefore, the Fermi levels (E ) lie
state power output for the pristine device decayed slightly 4.22 and 4.00 eV below the vacuum level (EVAC) for the reference
aer 100 s. The steady-state photocurrent of the P2-optimized and P2-optimized perovskite lms, respectively. From the
device was also recorded under simulated sunlight with 20 s valence band spectra in Fig. 4b, the valence band positions of
light on/off cycles, as shown in Fig. S9.† An extremely fast and the reference and P2-optimized perovskite lms were found to
smooth photoresponse with stabilized performance character- be 1.41 and 1.49 eV below their Fermi levels, respectively.
istics was observed. These results demonstrate that the incor- Combined with the band gaps obtained from the Tauc plots of
poration of P2 improves the operational stability of the PSC, the absorption spectra (Fig. S10†), the detailed energy levels of
most likely due to the more efficient charge carrier transport the reference and P2-optimized perovskite lms are shown in
within and at the interfaces of the perovskite absorber layer. To Fig. 4c. According to the band positions of the NiO
x
HTL and
25
demonstrate the reproducibility of this work, 50 individual Nb O ETL obtained in our previous study, the schematic
2 5
devices were fabricated using the same procedure, with the energy diagrams of the reference and optimized devices are
statistical distributions of the key J–V parameters (JSC, VOC, FF, shown in Fig. 4d. These results reveal that P2 modication
and PCE) presented in Fig. 3c–f. All of the key parameters shis the conduction band edge (CBE) and valence band edge
exhibited relatively narrow distributions, indicating that the (VBE) upward, resulting in better alignment of the CBE and VBE
process was reproducible.
To investigate the origin of the improved device perfor- particular, the CB of the P2-optimized perovskite becomes
mance, the energy band positions of the perovskite lms with higher than that of the Nb ETL, eliminating the barrier for
and without P2 modication were analyzed by ultraviolet electron extraction. This most likely accounts for the increased
photoelectron spectroscopy (UPS). From the UPS results pre- OC of the P2-optimized device. Moreover, the E of the P2-
energies with those of the NiOx HTL and Nb O5 ETL. In
2
2 5
O
V
F
sented in Fig. 4a, the work functions (WFs) of the reference and optimized perovskite comes closer to the CB, suggesting an
P2-optimized perovskite lms were determined by subtracting improved conductivity of the perovskite lm that can contribute
the cut-off energies of the secondary electrons from the He I to the improved FF of the P2-optimized device.
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Fig. 3 (a) J–V characteristics in reverse and forward scan directions. (b) Steady-state photocurrent and power conversion output of the cor-
responding PSCs. Statistical distributions of the J–V parameters for 50 P2-optimized and reference devices: (c) JSC, (d) VOC, (e) FF, and (f) PCE.
Steady-state and time-resolved photoluminescence (TRPL) (FWHM) of the spectra decreased from 47.0 to 44.4 nm. These
was carried out to investigate the effect of P2 incorporation on results indicate that P2 incorporation may passivate defects in
the charge transfer and recombination kinetics of the perov- the perovskite lm. For the perovskite lms deposited on the
skite lms. Comparison of the perovskite lms deposited on NiO -coated ITO substrates, the PL intensity was quenched to
x
a quartz glass substrate shows that the PL intensity was a greater extent in the P2-optimized sample, suggesting more
enhanced by over 30% upon P2 incorporation (Fig. 5a). Mean- effective hole extraction at the interface between the NiO and
x
while, with P2 incorporation the peak position blue-shied the P2-optimized perovskite lm. In agreement with the PL
from 772 to 765 nm and the full-width-at-half-maximum data, the TRPL results in Fig. 5b show that with P2
Table 3 The key J–V parameters and hysteresis index of the reference and P2-optimized PSCs
Hysteresis
ꢀ2
Device
Scanning mode
VOC (V)
JSC (mA cm
)
FF (%)
PCE (%)
index
0.020
0.133
Optimized
Reference
Reverse
Forward
Reverse
Forward
1.098
1.086
1.083
1.074
23.59
23.51
22.66
22.63
78.8
78.3
74.5
65.2
20.41
19.99
18.28
15.84
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Fig. 4 UPS spectra of perovskite films without and with the addition of 0.4 mM P2 (a) near the cutoff energy, and (b) near the valence band edge.
c) Comparison of energy levels of perovskite films. (d) Schematic energy diagram.
(
incorporation the average lifetime increased from 1.489 to 1.815 of the frequencies at the peaks of the semicircles in the Cole–
59
n
ms for the samples deposited on quartz glass substrates, while it Cole plots. The s of the optimized device (6.34 ms) is consid-
decreased from 0.344 to 0.212 ms for the samples deposited on erably longer than that of the reference cell (3.99 ms), indicating
the NiO -coated ITO substrates (Table S1†). These results that photogenerated carriers were transported for a longer time
x
demonstrate that P2 incorporation reduces the trap density and to the electrodes prior to possible recombination. Therefore, the
improves charge transfer within the bulk and at the NiO
x
/
above results conrm that P2 incorporation effectively passiv-
perovskite interface via effective coordination between the ates the trap states in the perovskite lms and promotes the
2
+
carboxylate moieties and the undercoordinated Pb at the transport of charge carriers, resulting in improved photovoltaic
grain boundaries and on the surfaces, leading to improved performance.
photovoltaic performance in the P2-optimized PSCs.
The space charge limited current (SCLC) method was
To further elucidate the role of P2 in the improved PSC employed to determine the trap density in the perovskite lms.
performance, the recombination and transport resistances were Fig. 5d shows the I–V curves of the electron-only perovskite
58
investigated by electronic impedance spectroscopy (EIS), as devices with the ITO/Nb O /perovskite/Nb O /Ag architecture.
2
5
2 5
shown in Fig. 5c. The Nyquist plots for the pristine and P2- The current increases linearly in the low-bias-voltage region and
optimized devices were measured in the dark. The equivalent a non-linear sharp increase is observed when the bias voltage
circuit model comprising the series resistance (R
s
), the recom- exceeds the kink point that is dened as the trap-lled limit
bination resistance (Rrec) and capacitance (Crec) is depicted in voltage (VTFL). The trap-state density (n
t
) can be calculated
60
the inset of Fig. 5c. The tted parameters for the Nyquist plots according to the following equation
are summarized in Table S2.† The value of R decreased from
.85 to 5.36 U aer the P2 modication, indicating that P2
s
0 TFL
233 V
n
t
¼
(2)
9
eL2
incorporation facilitates carrier transport. A more pronounced
change from 150.2 U in the reference cell to 358.3 U in the
optimized device was found for Rrec. The much larger Rrec in the
optimized device indicates the effective suppression of charge
recombination that may originate from the passivation of the
where e is the elementary charge, L is the thickness of the lm,
and 3 and 3 are the vacuum permittivity and relative dielectric
values of
the pristine and P2-optimized perovskite lms were measured
to be 0.601 and 0.224 V, respectively. The corresponding trap-
state densities for the pristine and P2-optimized perovskite
0
constant of perovskite (3 ¼ 28.8), respectively. The V
TFL
n
traps by P2 modication. Meanwhile, the effective lifetimes (s )
were extracted from the complex plane curves as the reciprocals
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Fig. 5 (a) Steady-state PL spectra and (b) TRPL decay curves of the reference and P2-optimized perovskite films deposited on different
substrates. (c) Nyquist plots of the reference and P2-optimized PSCs. (d) Dark I–V plots of the electron-only perovskite device depicted in the
inset.
1
5
ꢀ3
15
ꢃ
perovskite lm showed a small contact angle of 57 , while the P2-

lms were calculated to be 5.32 ꢁ 10 cm and 1.98 ꢁ 10
ꢀ
3
ꢃ
ꢃ
cm , respectively, clearly demonstrating the reduction of the modied lms presented larger contact angles of 63 , 67 , and
ꢃ
trap-state density by a factor of ꢂ2.7.
74 for various P2 concentrations. It is clear that the contact
The improvement of the moisture endurance of the perov- angle increased with increasing P2 concentrations, demon-
skite lm by the incorporation of the P2 polymer may be due to strating that the addition of P2 can effectively prevent water
the following mechanisms depicted in Fig. 6a. First, the high molecules from entering the perovskite lm. Therefore, the P2
hydrophobicity of the uoride ion within P2 effectively prevents incorporation into the perovskite layer acts as an effective barrier
the external moisture ingress into the perovskite lms. against humidity, enhancing the overall device stability. To
Furthermore, the C]O group of P2 can form hydrogen bonds probe the interaction between P2 and the perovskite, we
1
with the cation in the perovskite, and the formation of such measured the H NMR (Fig. S11†) and Fourier transform
hydrogen bonds balances the electron cloud density of the infrared (FTIR) spectra (Fig. S12†) of the perovskite precursor
organic cation, hindering the octahedral tilting of the corner- solution in the absence and presence of P2 in deuterated
4
ꢀ
sharing [PbI
6
]
black phase through the reduction of the dimethyl sulfoxide-d6 (DMSO-d6). The NMR spectra in Fig. 6c
stretching vibration frequency. This will improve the intrinsic show a single broad peak at d ¼ 8.585 ppm for the pristine
stability of the perovskite lm. Even with moisture inltration perovskite sample that is ascribed to the FA amine hydrogen (NH
62
1
2
at high humidity, P2 in the perovskite can absorb some of the and NH ). In contrast, peak splitting of the H NMR peak was
water molecules to form carboxylic acid groups through the observed in the precursor solution with the addition of P2 and
hydrolysis of the side-chain ester groups, which can also these peaks showed a downeld shi. This is most likely due to
+
passivate the surface electronic trap states via efficient coordi- the C]N double bond vibration in the FA cation that causes the
61
nation between the carboxylate moieties. The passivation FA amine hydrogen atoms (NH and NH
decreases the chemical activity of the perovskite surface to chemical environments so that only one broad peak at
suppress the hydration reaction. 8.585 ppm is observed. However, the P2 addition leads to the
2
) to be in similar
To provide physical evidence for the above mechanism, the splitting of the NMR peak of the amine hydrogen and imine
contact angle of the water droplet on the perovskite lms was hydrogen in FA. This indicates that the carbonyl group (C]O) in
measured with the results shown in Fig. 6b. The pristine P2 forms a hydrogen bond with the amine hydrogen (NH) in FA,
13226 | J. Mater. Chem. A, 2021, 9, 13220–13230
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Fig. 6 (a) Proposed mechanism for the enhanced stability of P2-modified perovskite films. (b) Static contact angle measurements with water
1
droplets on top of the perovskite films with different P2 concentrations. (c) H NMR spectra of the perovskite precursor solution with and without
P2. (d) Core-level XPS spectra of Pb 4f for perovskite films with and without P2. (e) Stability tests for PSCs stored in air with 55 ꢄ 10% RH for 60
days at room temperature.
signicantly reducing the vibration of the FA double bond and passivate the iodine and cation vacancies, resulting in the
changing the environment of the FA amine hydrogen atoms (NH remarkably improved device performance and stability.
and NH ). This change leads to the observed peak splitting. The
Based on the above discussion, it is concluded that the P2-
2
FTIR spectra presented in Fig. S12† show that the C]N modied perovskite lms can not only improve the passiv-
stretching vibration peak slightly shied to a lower wavenumber ation and stability but also strongly inhibit the reaction between
in the P2-doped perovskite lm, indicating a change of the C]N the ambient water molecules and perovskite lms. Therefore,
double bond vibration likely due to the formation of a hydrogen we studied the moisture stability of the perovskite lms and
bond. X-ray photoelectron spectroscopy (XPS) wide-scan results PSC devices with and without P2 doping. The perovskite lms
of the perovskite lms did not show any signicant change and PSC devices without any encapsulation were tested upon
following the P2 doping (Fig. S13†). No F signal was detected in exposure to the ambient environment (in air at a relative
the P2-doped perovskite, most likely because its concentration is humidity of 55 ꢄ 10% at 25 ꢃC). The color of the pristine
below the detection limit. Nevertheless, the core-level XPS perovskite lm changed from dark brown to yellowish aer 30
spectra of Pb 4f in Fig. 6d reveal that the presence of metallic Pb days, indicating the decomposition of the perovskite into PbI
2
,
ꢃ
on the surface of the perovskite is suppressed by P2 doping. The as evidenced by the intense PbI
2
peak at 12.68 in the XRD
appearance of metallic Pb in the pristine perovskite lm may be pattern of the sample (Fig. S14a†). In contrast, the color of the
due to the existence of iodine and cation vacancies that have P2-modied perovskite lm remained almost unchanged and
2
been recognized as the most important non-radiative recombi- the intensity of the PbI diffraction peak increased only slightly
63
nation centers. In contrast, there were no detectable signals for within the same period (Fig. S14b†), indicating a signicant
metallic Pb in the P2-modied perovskite, indicating that the improvement of moisture stability. Consequently, the P2-
undercoordinated Pb atoms have been effectively reduced aer optimized device retained 90% of its initial PCE value aer
P2 doping. Meanwhile, the peak position of the Pb 4f shied storage in air for 60 days, as shown in Fig. 6e. In contrast, the
noticeably, indicating the formation of new chemical coordina- PCE of the pristine device decreased to 68% of its initial value
tion bonds on the perovskite surface. Therefore, the reaction aer the same aging. The stability tests demonstrated that the
between ambient water molecules and the perovskite lms may P2-optimized devices have the potential for showing stability in
be inhibited and the formed organic terminal groups can harsh environments.
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Paper
Deposition of perovskite lms. Perovskite lms were
deposited using a simple one-step spin-coating method. The
Conclusions
In summary, we propose a novel strategy for the synthesis of NiO -coated ITO substrates were transferred into an N -lled
x
2
stable and highly efficient inverted perovskite solar cells with glovebox and the FA0.85MA0.15PbI2.55Br0.45 precursor solutions
all-inorganic charge extraction layers exceeding 20% PCE with or without the polyacetylene derivative polymers were spin-
through the incorporation of a polyacetylene derivative into the cast onto the substrate at a speed of 4000 rpm for 40 s and CB
perovskite active layer. The interaction of the P2 polymer with (100 mL) was dropped onto the spinning substrate during the
the perovskite precursor not only effectively passivates the spin-coating step 20 s before the end of the procedure. Then, the
ꢃ
defects in the perovskite but also improves the charge extraction lms were heated on a hotplate at 150 C for 15 min.
efficiency and the water resistance of the perovskite lms. As
2 5
Final PSC device assembly. Nb O thin lm with a thickness
a result, the P2-optimized device exhibits the highest PCE of of 60 nm was deposited onto the perovskite layer by EB evapo-
ꢀ
1
2
0.41% reported to date for inverted PSCs based on all- ration at a deposition rate of approximately 1 A˚ s . Finally, a Ag
inorganic charge extraction layers. More importantly, the electrode (99.99%, 100 nm) was deposited on top of the Nb O5
2
device shows excellent long-term stability, retaining 90% of its ETL through a metal shadow mask by EB evaporation at a base
ꢀ
6
initial PCE aer exposure for 60 days to air with a relative pressure of ꢂ5 ꢁ 10 Torr and a deposition rate of approxi-
ꢀ1
˚
humility of 55 ꢄ 10%. This work offers an approach for devel- mately 2 A s as the back contact. The active area of the solar
2
oping high-performance PSCs and provides another potential cells was 0.160 cm and the unencapsulated devices were stored
route toward PSC commercialization.
under dry conditions and characterized under an ambient
atmosphere.
Experimental section
Materials preparation
Materials characterization
The surface morphology was characterized using a scanning
electron microscope (SEM, ZEISS Crossbeam 340). X-ray
diffraction (XRD; Thermo Scientic ARL EQUINOX 1000) was
measured using a Cu Ka radiation source operated at 40 kV and
All the chemicals were used as received without further puri-
cation. Lead iodide (PbI
Aesar; lead bromide (PbBr
2
, 99.9985%) was purchased from Alfa
, 99.999%) was purchased from
2
Aladdin. Dehydrated solvents of chlorobenzene (CB, 99.9%),
dimethyl sulfoxide (DMSO, 99.9%) and N,N-dimethylforma-
mide (DMF, 99.9%) were purchased from Sigma-Aldrich. Silver
30 mA. The absorption spectra of the samples were measured by
UV-visible spectroscopy (UV-Vis; UV-1900, SHIMADZU) and
2
calculated via Tauc analysis according to the equation (ahn) ¼
(
(
Ag, 99.999%), nickel oxide (NiO, 99.9%), and niobium oxide
Nb , 99.9%) were purchased from ZhongNuo Advanced
A(hn ꢀ E ). The photocurrent density–voltage (J–V) curves were
g
2 5
O
measured under the reverse scan (open-circuit bias to short-
circuit bias) using an AAA class solar simulator (SAN-EI ELEC-
Material (Beijing) Technology. The chemicals and detailed
procedures for the synthesis of the polyacetylene derivative
polymers are given in the ESI.†
Perovskite solution preparation. FA0.85MA0.15PbI2.55Br0.45
solution (1.2 M) was prepared in a mixed solvent of DMF and
DMSO with a volume ratio of 4 : 1. The molar ratios for PbI2/
PbBr2 and FAI/MABr were both xed at 0.85 : 0.15, and the
ꢃ
TRIC, XES-40S2) as the light source at 25 C. Before the use of
the light source, the light intensity was calibrated to 1 sun (100
ꢀ2
mW cm ) using a certied reference cell (Konica Minolta AK-
200). The spectral response was obtained using an EQE
measurement system (QEX10, PV Measurements), which is
equipped with a monochromator, a lock-in amplier, a Xe
2 2
molar ratio of (FAI + MABr)/(PbI + PbBr ) was xed at 1 : 1.
lamp, and
a current–voltage amplier. The frequency-
Prepared solutions were ltered through a 0.22 mm pore PTFE
dependent capacitance (C-f) of the device was measured using
a potentiostat (Bio-Logic SP-200). The AC voltage perturbation
was 20 mV, and a constant zero bias was maintained. Each
spectrum was measured at frequencies ranging from 1 kHz to 1
MHz. Regarding nuclear magnetic resonance (NMR) spectros-
ꢃ
syringe lter aer stirring at 60 C for 24 h and were stored in
a dry nitrogen atmosphere. For polyacetylene derivative doping,
the polymers were directly added to the perovskite precursor
solution at desired concentrations.
1
copy, H NMR spectra were recorded on a JNM-ECZ400S/L1 400
Perovskite solar cell fabrication
MHz NMR spectrometer. Deuterated dimethyl sulfoxide
(
DMSO-d6) and deuterated chloroform (CDCl ) were used as
3
Preparation of the NiO hole transport layer. Pre-patterned
x
ꢀ
1
solvents. XPS measurements were performed using a Kratos-
Axis Supra system with a monochromatized Al Ka X-ray source
under a pressure of 5.0 ꢁ 10 Pa. UPS measurements were
carried out with a Thermo Fisher Scientic ESCALAB 250 Xi
using the He I line (21.22 eV) as the excitation source.
ITO/glass substrates (Yaoke Optoelectronics, 10 U sq ) with
2
sizes of 1.4 ꢁ 1.6 cm were ultrasonically cleaned with a 1%
ꢀ
7
Alconox detergent diluted in deionized water, deionized water,
acetone, and isopropanol (IPA) in succession each for 15 min,
and dried under a nitrogen stream. Then, a 30 nm thick NiO
lm was deposited onto the ITO substrates by electron beam
EB) evaporation (Angstrom Engineering, AMOD) at a base
x

(
Author contributions
ꢀ
6
ꢀ1
˚
pressure of <5 ꢁ 10 Torr and a deposition rate of ꢂ1 A s
.
Post-annealing of the NiO
x
thin lms was carried out in a tube JJ and XL contributed equally to this work. JJ, XL, HZ, and YL
ꢃ
furnace in air at 300 C for 1 h.
conceived the idea and designed the experiments. JJ and XL
13228 | J. Mater. Chem. A, 2021, 9, 13220–13230
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Journal of Materials Chemistry A
performed the experiments and analyzed the data. JJ, MBF, and
YL wrote the manuscript. All authors reviewed and commented
on the manuscript.
M. D. Mensi, S. Kinge and M. K. Nazeeruddin, Energy
Environ. Sci., 2020, 13, 1222–1230.
17 H. Ren, S. Yu, L. Chao, Y. Xia, Y. Sun, S. Zuo, F. Li, T. Niu,
Y. Yang, H. Ju, B. Li, H. Du, X. Gao, J. Zhang, J. Wang,
L. Zhang, Y. Chen and W. Huang, Nat. Photonics, 2020, 14,
Conflicts of interest
154–163.
1
8 N. K. Noel, S. N. Habisreutinger, A. Pellaroque, F. Pulvirenti,
B. Wenger, F. Zhang, Y.-H. Lin, O. G. Reid, J. Leisen,
Y. Zhang, S. Barlow, S. R. Marder, A. Kahn, H. J. Snaith,
C. B. Arnold and B. P. Rand, Energy Environ. Sci., 2019, 12,
The authors declare no competing nancial interests.
Acknowledgements
3063–3073.
This work was supported by the National Natural Science
Foundation of China (No. 21872019) and Sichuan Science and
Technology Foundation (No. 2018JY0137).
1
9 E. Shi, B. Yuan, S. B. Shiring, Y. Gao, Akriti, Y. Guo, C. Su,
M. Lai, P. Yang, J. Kong, B. M. Savoie, Y. Yu and L. Dou,
Nature, 2020, 580, 614–620.
2
0 D. Bogachuk, S. Zouhair, K. Wojciechowski, B. Yang,
V. Babu, L. Wagner, B. Xu, J. Lim, S. Mastroianni,
H. Pettersson, A. Hagfeldt and A. Hinsch, Energy Environ.
Sci., 2020, 13, 3880–3916.
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