Journal of
Materials Chemistry A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
REVIEW
Cite this: DOI: 10.1039/c7ta01798g
View Journal
Additive engineering for highly efficient organic–
inorganic halide perovskite solar cells: recent
advances and perspectives
Taotao Li,a Yufeng Pan,a Ze Wang,a Yingdong Xia,*a Yonghua Chen
and Wei Huang*ab
*a
Organic–inorganic halide perovskite solar cells have recently attracted much attention due to their lowcost fabrication, flexibility, and high efficiency. The power conversion efficiency achieved with such cells
was over 22% in 2016 from an initial 3.81% in 2009, highlighting how the cells are now benefiting from
the highly optimized morphology of perovskite films. To date, a great number of approaches have been
done to improve the morphology of perovskite films, in which additives play an important role in
perovskite crystal growth and in the dynamics of the crystallinity, and thus the performance of perovskite
solar cells. Herein, we review the recent progress on additives, such as polymers, fullerene, metal halide
salts, organic halide salts, inorganic acids, solvents, and nanoparticles, in improving the morphology of
Received 27th February 2017
Accepted 15th May 2017
perovskite films in terms of the crystal growth, crystallization kinetics, and device performance. We also
discuss the importance of further understanding the fundamental homogeneous nucleation process by
DOI: 10.1039/c7ta01798g
rsc.li/materials-a
the use of additives. Further innovation in terms of the additives could help to further develop highperformance devices with long-term stability for future practical applications of perovskite solar cells.
1. Introduction
a
Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM),
Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM),
Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P.
R.
China.
E-mail:
iamydxia@njtech.edu.cn;
iamyhchen@njtech.edu.cn;
iamwhuang@njtech.edu.cn
b
Key Laboratory for Organic Electronics & Information Displays (KLOEID), Institute of
Advanced Materials (IAM), Nanjing University of Posts and Telecommunications,
Wenyuan Road 9, Nanjing 210023, China
Taotao Li received her bachelor
degree from Nanjing Tech
University in Applied Chemistry.
She then continued her postgraduate study under the
supervision of Prof. Wei Huang
and Prof. Yonghua Chen in
organic–inorganic hybrid perovskite solar cells at the Institute
of Advanced Materials, Nanjing
Tech University. Her research
interests are two-dimensional
perovskite materials for solar
cell applications.
This journal is © The Royal Society of Chemistry 2017
With the shortage of fossil fuels and global environmental
problems caused by their use, such as environmental degradation, people are committed to nding clean and renewable
energy sources. As an inexhaustible supply of natural resources,
solar energy has undoubtedly become the rst choice of “new
energy”. In recent years, photovoltaic technologies, which can
directly convert solar energy to electrical energy, have been
Yingdong Xia received her B.E.
degree
in
Macromolecular
Science and Engineering from
Hebei University in 2006 and
her Ph.D. degree in Polymer
Chemistry and Physics from
Changchun Institute of Applied
Chemistry, Chinese Academy of
Sciences, in 2011. She then
conducted postdoc research at
the Georgia Institute of Technology and Wake Forest University during 2011–2013. She is
currently an associate professor at Nanjing Tech University. Her
research interests are focused on organic light-emitting diodes,
perovskite light-emitting diodes, and solar cells.
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
Scheme 1
Review
Classification of the additives used in current perovskite solar cells.
subjected to unprecedented development. Both crystal silicon
solar cells (the rst generation of photovoltaic cells) and thinlm solar cells (the second generation of photovoltaic cells,
including amorphous silicon, CZTS, and CdTe thin-lm solar
cells) can now reach a power conversion efficiency (PCE) of over
20%.1 However, the sophisticated processing and high cost of
current cells limit their future development. Therefore, it is
imperative to nd new photovoltaic materials to replace or to
complete the state-of-the-art photovoltaic technology for the
further development of solar cells.
Recently, a new generation of thin-lm photovoltaic cells
based on hybrid organic–inorganic halide perovskite absorbers
has emerged2–12 that exhibit high efficiencies, now exceeding
22%,13 surpassing by a large margin the organic photovoltaic
and rival established photovoltaic technologies based on Si,
CdTe, CuInGaSe, and GaAs. This new class of hybrid organic–
Yonghua Chen received his B.E.
degree in Chemistry from the
Inner Mongolia University in
2006 and then received his Ph.D.
degree in Polymer Chemistry
and Physics from the Changchun
Institute of Applied Chemistry,
Chinese Academy of Sciences, in
2011. Aer he carried out twoyear postdoctoral research at
Wake Forest University and twoyear postdoctoral research at
Case Western Reserve University. He is currently a full professor in Nanjing Tech University. His
research interests are focused on organic and organic/inorganic
hybrid optoelectronic materials and devices, including organic
light-emitting diodes and eld-induced polymer electroluminescent devices for at panel displays and solid-state lighting, and
polymer and perovskite solar cells for energy conversion.
Wei Huang received his BSc,
MSc, and PhD degrees in
Chemistry from Peking University in 1983, 1988, and 1992,
respectively. In 1993, he began
his postdoctoral research in the
Chemistry Department at the
National University of Singapore, where he participated in
the founding of the Institute of
Materials Research and Engineering (A*STAR). In 2001, he
became a chair professor at
Fudan University, where he founded the Institute of Advanced
Materials. In June 2006, he was appointed as the Deputy President
of Nanjing University of Posts and Telecommunications, where he
founded the Institute of Advanced Materials and the Key Laboratory for Organic Electronics and Information Displays. In July
2012, he was appointed as the President of Nanjing University of
Technology. He is a member of the Chinese Academy of Sciences.
His research interests include organic optoelectronics, nanomaterials, polymer chemistry, plastic electronics, and
bioelectronics.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Review
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Table 1
Journal of Materials Chemistry A
Performance summary of the reported perovskite solar cells with (W) and without (W/O) the respective additives
Additives
Materials
PCE (%) (W/W/O)
Voc (V) (W/W/O)
Jsc (mA cm2) (W/W/O)
FF (W/W/O)
References
Polymer
PEG
Fullerene
PFN-P1
PVP
PAN
PC61BM
12.90/10.47
16.00/8.00
13.20/12.00
7.34/6.34
9.74/3.67
13.60/8.10
12.78/6.90
15.30/12.57
16.00/11.40
14.72/9.83
15.08/11.40
14.20/10.20
20.6/19.6
18.80/14.86
15.14/14.01
15.25/14.01
15.61/14.01
14.18/14.01
3.08/—
17.09/13.18
—
17.55/12.61
19.10/17.10
2.02/3 104
4.00/2.80
17.60/14.30
16.00/3.00
15.76/12.13
7.30/0.40
17.90/16.10
16.20/13.20
16.02/13.08
10.30/7.80
17.81/15.04
9.46/7.26
18.00/0.0063
16.06/12.13
15.41/8.80
13.37/11.13
18.30/14.30
10.20/9.40
18.00/15.60
17.13/16.35
15.20/14.60
11.70/9.60
11.50/9.60
8.40/9.60
2.00/9.60
1.40/9.60
11.60/7.20
16.00/8.80
17.71/14.05
15.10/14.11
11.40/10.70
12.00/10.10
15.20/12.50
15.58/9.83
2.44/5.10
19.50/19.10
13.90/11.90
9.32/6.26
15.66/10.73
0.94/0.88
0.98/—
0.93/0.87
0.94/0.77
1.05/0.85
1.07/0.98
0.90/0.90
0.90/0.88
0.97/0.95
0.90/0.75
1.04/0.90
0.96/0.91
1.19/1.14
1.13/1.11
0.90/0.95
0.99/0.95
0.95/0.95
1.02/0.95
1.05/—
0.96/0.94
—
1.03/0.91
1.10/1.04
0.24/0.01
0.29/0.26
1.10/1.08
1.01/0.65
0.94/0.87
1.09/0.56
1.00/1.00
1.07/1.07
0.99/0.91
0.92/0.90
1.11/1.07
0.85/0.85
1.03/0.10
0.95/0.87
0.98/0.85
1.05/1.02
1.00/0.95
0.86/0.86
1.06/1.02
1.07/1.03
1.08/1.04
0.90/0.90
0.90/0.90
0.89/0.90
0.51/0.90
0.65/0.90
0.84/0.86
1.00/0.86
1.04/0.96
0.99/0.99
1.02/1.04
0.93/0.89
0.97/0.93
0.88/0.87
0.78/0.92
1.14/1.16
—
0.91/0.73
1.02/1.02
19.53/17.28
22.5/—
18.75/18.21
9.43/11.25
15.97/11.81
17.30/14.40
21.86/20.00
22.88/19.04
20.20/16.20
22.67/22.67
19.42/16.60
19.20/16.80
22.5/23.0
23.24/22.28
22.97/21.03
21.81/21.03
22.92/21.03
19.24/21.03
3.76/—
21.51/17.96
—
21.09/19.07
22.40/21.30
22.70/0.19
0.29/0.26
20.50/18.90
22.40/12.00
21.72/19.12
8.50/2.00
23.30/23.0
20.4/17.2
20.88/20.27
15.60/15.00
22.24/22.11
16.90/13.5
20.60/0.33
21.67/19.12
22.90/19.00
18.58/16.15
22.34/20.28
16.80/15.70
21.20/20.07
21.24/21.27
19.30/19.70
19.20/18.80
19.30/18.80
17.80/18.80
15.80/18.80
10.10/18.80
21.10/13.99
22.11/15.60
22.08/22.16
20.90/21.01
16.91/14.76
23.04/22.89
20.80/17.80
22.31/16.02
4.84/11.90
22.70/21.70
—
13.11/—
21.03/18.23
0.70/0.69
0.72/—
0.75/0.75
0.85/0.78
0.68/0.65
0.73/0.65
0.53/0.37
0.74/0.75
0.82/0.73
0.75/0.58
0.75/0.76
0.77/0.67
0.77/0.75
0.72/0.60
0.73/0.70
0.71/0.70
0.72/0.70
0.74/0.70
0.78/—
0.83/0.78
—
0.78/0.72
0.78/0.77
0.37/0.21
0.55/0.50
0.78/0.70
0.71/0.42
0.77/0.72
0.79/0.58
0.77/0.69
0.74/0.72
0.77/0.71
0.71/0.58
0.72/0.63
0.66/0.63
0.85/0.19
0.78/0.73
0.69/0.54
0.69/0.68
0.82/0.74
0.70/0.69
0.79/0.73
0.75/0.75
0.73/0.71
0.68/0.57
0.66/0.57
0.53/0.57
0.25/0.57
0.21/0.57
0.65/0.61
0.75/0.66
0.77/0.73
0.73/0.68
0.67/0.64
0.60/0.50
0.75/0.69
0.80/0.71
0.64/0.49
0.71/0.72
—
0.78/0.60
0.72/0.58
41
43
46
54
55
85
87
88
89
91
95
98
31
97
98
Metal halide salts
PC71BM
1D PC61BM
KCl
NaI
RbI
NaI
CuI
CuBr
AgI
CaCl2
CuBr2
BiI3
InCl3
Al(acac)3
SnF2
Inorganic acids
HI
HBr
Solvent
Organic halide salts
Nanoparticle
Others
HCl
HPA
NH3SO3
DIO
ACN
CN
H2O
MACl
MAI
MABr
EAI
FEAI
GACl
PEI
TPPI
TPPCl
TPPBr
TBPI
TBAI
5-AVA
4-ABPACl
PEAI
PbS
Au@SiO2
MOFs
S-CNTs
I2 (solid)
I2 (liquid)
MAF
MAH2PO2
NH4Cl
BQ
This journal is © The Royal Society of Chemistry 2017
99
100
101
102
103
104
106
107
108
110
111
112
114
116
40
123
124
128
129
130
131
138
139
141
142
143
145
11
149
150
151
157
164
167
168
171
172
175
176
179
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
inorganic perovskite have an ABX3 architecture, where A is
a monovalent organic/inorganic cation, typically Cs+, CH3NH3+
(i.e., MA+), or HC(NH2)2+ (i.e., FA+), while B is a metal cation (i.e.,
Pb2+, Sn2+), and X is a halide anion (i.e., Cl, Br, I or their
mixtures). In a typical perovskite crystal structure, B occupies
the center of an octahedral [BX6]4 cluster, while A is 12-fold
cuboctahedrally coordinated with X anions.1,14 The formability
and stability of this crystal structure can be estimated by its
Goldschmidt tolerance factor t and octahedral factor m, where
m ¼ rB/rX, where rA, rB, and rX are the effective ionic radii for A, B,
and X ions, respectively. Empirically, perovskite can be stabilized when t and m lie in the range of 0.813–1.107 and 0.442–
0.895, respectively. Such a unique structure renders the perovskite with a host of intriguing characteristics, such as high
absorption coefficient,2 wide absorption range,15 tunable
bandgaps,16 low exciton binding energy,17 long electron and
hole diffusion lengths,18,19 high ambipolar charge mobility,20,21
and extended charge carrier lifetime,22 which has led perovskite
to be popular in photovoltaic applications.
In order to achieve high device performance in perovskite
solar cells, great efforts have been made through designing new
device structures,3,8,23 manipulating the interface engineering,24–26 exploring new perovskite materials,27–31 and
controlling the perovskite crystal growth.9,32–38 The perovskite
crystal growth, which is related to the perovskite morphology,
plays an important role in device performance optimization.
Doping with an anti-solvent during spin-coating,38,39 using
a large excess of organic components,3 employing hot-substrate
casting,32 and adding additives40 have all been employed to
improve the perovskite crystal growth. Among these, the use of
additives has proven to be an effective method for facilitating
homogeneous nucleation or for modulating the crystallization
kinetics. The lm morphology can be well controlled by nucleation and crystal growth during lm formation by the inclusion
of additives, including the surface uniformity, surface coverage,
and crystal size, which are expected to reduce the electrical
shunt, the probability of charge recombination, and the bulk
traps during solar cell operation, and thus additives can support
the development of high-performance solar cells.
In this review, we focus on recent advances in additive
engineering on improving the perovskite morphology, charge
transport, and excitonic and optical properties. We systematically discuss the role and the function of the additive components on crystallization during perovskite lm formation.
Specically, we divided the additives into several categories
according to their functionalities (Scheme 1 and Table 1). Some
can help to manipulate the lm morphology, such as improving
the grain size and crystallinity and helping to form a uniform
and smooth surface, while others can help improve the optical
and electrical properties of the perovskite thin lms. The aim
was to open new perspectives for the rational use and design of
additives to realize perovskite solar cells with an unprecedented
improvement in device performance. We believe that our
insights will provide useful information to the solar cell
community and also facilitate a complete understanding of the
use of additives for advancing the understanding of perovskite
semiconductors.
J. Mater. Chem. A
Review
2.
2.1
Additives in perovskite solar cells
Polymer additives
Due to the fact that perovskite, e.g., MAPbI3, consists of organic
cations and inorganic ions, some atoms (e.g., O atoms) in
a polymer could form hydrogen bonds with H atoms in
CH3NH3+ to provide the polymer with resistance to humid
environments for long-time operation. On the other hand, lone
pairs of electrons from some atoms (e.g., S and N atoms) in the
polymer can interact strongly with Pb ions, which in turn
stabilizes the frame structure of perovskite. Moreover, polymers
with good solubility can reduce the contact angle helping the
perovskite precursor spread out smoothly and can also retard
the perovskite growth for large crystal formation, which can
help improve the coverage of perovskite lm on a substrate.
Therefore, polymers have been signicantly explored as additives to promote uniform crystallization, optimize the crystal
growth kinetics, and nally to ultimately improve the device
performance.
Su et al. rst reported poly(ethylene glycol) (PEG) as an
additive in perovskite precursor solution to fabricate highperformance perovskite solar cells.41 They found that the PEG
additive assisted the formation of a smooth lm over a TiO2
substrate compared to the pristine perovskite lm, as shown in
Fig. 1a and b. Moreover, the size and aggregation of perovskite
crystals could be well controlled (Fig. 1c–f). For the pristine
perovskite lm, the crystals grew and aggregated into large
domains with poor coverage, which was consistent with the
previous report.42 However, the lm containing 1 wt% of PEG
exhibited a continuous lm with a decreased size of domains
and no voids. When the PEG content was increased to 3 wt%
and 5 wt%, however, both the size and amount of crystals and
voids increased due to the phase separation between PEG and
perovskite during the perovskite crystal growth. This fullcoverage perovskite crystalline lm could absorb more
sunlight and facilitate charge transport (Fig. 1g). Finally, an
average PCE of 10.47% with a Voc of 0.88 V, Jsc of 17.28 mA cm2,
and an FF of 69.28% was achieved in the control device, while
the device with the PEG additive demonstrated a PCE of 12.90%
with a Voc of 0.94 V, Jsc of 19.53 mA cm2, and an FF of 70.35%
(Fig. 1h).
PEG is a hygroscopic polymer and contains O atoms, which
have a strong interaction with perovskite by hydrogen bonds
with H atoms in CH3NH3+. Zhao et al. demonstrated a polymerscaffold perovskite solar cell with a PCE of over 16% using PEG
as an additive, as shown in Fig. 2a.43 They found that the longchain PEG molecule facilitates the formation of a threedimensional (3D) molecular network in the perovskite, which
guaranteed a uniform thickness and homogeneous morphology
of the polymer-scaffold perovskite layer (Fig. 2b–e), which was
consistent with Su et al.'s report.41 Interestingly, the polymerscaffold perovskite layer showed a self-healing behavior aer
removing it from water vapor due to the excellent hygroscopic
properties of the PEG, as depicted in Fig. 2f. Moreover, strong
humidity resistance was observed because of the strong
hydrogen bonding between PEG molecules and perovskite
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
Journal of Materials Chemistry A
Fig. 1 SEM cross-sectional image of (a) pristine planar heterojunction perovskite solar cell, and (b) planar heterojunction perovskite solar cell
processed with 1 wt% PEG. SEM images of perovskite films: (c) pristine, (d) with 1 wt%, (e) with 3 wt%, and (f) with 5 wt% of PEG additive on top of
the TiO2 nanoparticles layer. (g) UV-vis absorption spectra of perovskite films processed with and without 1 wt% PEG. (h) Current–voltage (J–V)
curves of the devices with different amounts of PEG additive in the perovskite film. Reproduced with permission from ref. 41. Copyright 2015
American Chemical Society.
molecules (Fig. 2g), which ensured a compact moisture barrier
around the perovskite crystal grains, which could exclude water
vapor from the perovskite lm. The device with the polymerscaffold perovskite layer thus exhibited good long-time operation. They nally concluded that the improved stability and selfhealing effect of the polymer-scaffold perovskite devices could
be ascribed to the excellent hygroscopicity of the PEG molecules
and their strong interaction with the perovskite. Recently, they
further demonstrated PEG and [6,6]-phenyl-C61-butyric acid
methyl ester (PC61BM) dual additives in perovskite solution.44
The along-chain insulating polymer PEG still acted as a network
to improve the lm morphology and device stability, while the
inclusion of the small molecule PC61BM in the composite
additives assisted the charge transfer and transport in the
perovskite lm by forming conducting channels. Moreover, the
inclusion of PC61BM in the perovskite lm can passivate trap
states on grain boundaries, and thus the photocurrent hysteresis of the device is signicantly suppressed.45
Although the crystallization kinematics can be substantially
affected by polymer additives in solution-based processes, the
grain size distribution of perovskite lm is still highly limited.
Tripathi et al. introduced a neutral surfactant amine-based
polymer poly[9,9-bis (30 -(N,N0 -dimethylaminol-propyl)-2,7-uorene)-alt-2,7-(9,9-dioctyluorene)] (PFN-P1) into the perovskite
This journal is © The Royal Society of Chemistry 2017
lm to promote uniform crystallization and to control the grain
size.46 They incorporated PFN-P1 by a layer-by-layer approach
instead of directly adding it to the perovskite precursor solution. Because of the solubility of PFN-P1 in DMF, the underlying
PFN-P1 molecules were immediately dissolved in the PbI2 DMF
solution. Some of PFN-P1 molecules stay in the nal perovskite
lm due to the fact that the nitrogen lone pairs in PFN-P1 are
expected to strongly coordinate with Pb2+, whereas other
molecules could be excluded from the perovskite lm and could
be placed on the perovskite lm surface as a surfactant to
passivate the trap states on grain boundaries, as shown in
Fig. 3a and b. Based on this approach, a ripple-like pattern was
observed in the case of PFN-P1 mixed PbI2 lm (Fig. 3c), which
is different from the common morphology of PbI2 on
PEDOT:PSS (Fig. 3d).47 As presented by Fig. 3e and f, the pure
perovskite grains on PEDOT:PSS showed a serious variation in
size while the PFN-P1 mixed perovskite lm gave a compact,
homogeneous, and void-free grain. It was suggested that the
PFN-P1 layer may lead to a at surface and suppress the random
crystallization of the PbI2 layer and promote a more uniform
crystallization. The device with PFN-P1 exhibited an enhanced
Voc compared to that without the polymer due to the reduced
charge carrier recombination at the interfaces (Fig. 3g). Most
importantly, the PFN-P1 devices retained 90% of their initial
J. Mater. Chem. A
View Article Online
Review
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
Fig. 2 (a) Schematic of a polymer-scaffold structured perovskite solar cell. SEM images of the perovskite films (b) without PEG and (c) with PEG.
(d) Photographs of the perovskite films with and without PEG showing the color change evolution after water-spraying for 60 s followed by being
kept in the ambient air for 45 s. (e) Schematic diagram of the hydrogen bonding formation between PEG molecules and MAPbI3. (f) J–V curves of
a device with different PEG concentrations. (g) Mechanisms of the self-healing properties: (1) water absorbs on the perovskite; (2) water causes
the perovskite to hydrolyze into PbI2 and MAI$H2O. (3) MAI restrained by PEG reacts with nearby PbI2 to form perovskite again after the water
evaporates. PEG has a strong interaction with MAI, preventing it from evaporating, and subsequently MAI and PbI2 react in situ to form perovskite
after the film is removed away from the vapor source. Reproduced with permission from ref. 45. Copyright 2016 Nature Publishing Group.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
Journal of Materials Chemistry A
(a) Perovskite formation by using a successive spin-coating method. (b) Schematic of the test device configuration. (c–f) SEM micrographs
of a PbI2 film without PFN-P1, PbI2 film with PFN-P1, perovskite film without PFN-P1, and a perovskite film with PFN-P1. (g) J–V characteristics of
the device with and without PFN-P1. (h) Stability of the encapsulated PFN-P1 devices stored under ambient conditions for 201 days (>6 months).
(i) Stability of aged devices at the maximum power point with tracking under continuous illumination (1 sun). Reproduced with permission from
ref. 46. Copyright 2016 American Chemical Society.
Fig. 3
efficiency over several months (>6 months) (Fig. 3h), while
devices without PFN-P1 showed a 40% loss of their initial efficiency aer 70 days. Fig. 3i illuminates the stability of the aged
devices, which was tested at the maximum power point. Devices
without PFN-P1 degraded very rapidly in less than 50 h, while
the PFN-P1 devices continued to show efficiencies over 7% even
aer 150 h under continuous operation conditions.
Since small crystals are required for a semitransparent
perovskite lm (50–100 nm) for window applications in zeroenergy building technology,48–53 Nakamura et al. fabricated
perovskite solar cells with a polar polymer polyvinylpyrrolidone
(PVP) as an additive to control the crystal size and stability
(Fig. 4a–e).54 A minimum average perovskite grain size of 15–
17 nm was achieved in the PVP-doped perovskite lm, while
a perovskite lm made without PVP doping consisted of crystals
with a minimum grain size of 35–40 nm. Moreover, a much
smoother surface morphology was obtained with an RMS value
of the surface of 14.55 nm and 3.11 nm for the pure perovskite
lm and for the 3.0 wt% PVP-doped lms, respectively, as presented in Fig. 4f–i. Since the O atoms form hydrogen bonds with
H atoms in the perovskite, the polymer PVP can wrap up the
small perovskite crystals and prevent device deterioration under
an air atmosphere. Therefore, the stability of the PVP-doped
devices was further increased, as exhibited in Fig. 4j.
As the established polymer additives in perovskite solar cells
are all insulating polymers, Deng et al. carefully compared the
insulating polymer polyacrylonitrile (PAN) and the conjugated
This journal is © The Royal Society of Chemistry 2017
polymer poly(3-hexylthiophene) (P3HT) as additives in perovskite precursor solution in terms of the perovskite morphology
and device performance.55 As illuminated in the Fig. 5a–d,
compared with the lm without the PAN additive, which
showed a rough surface with branch crystals and a surface
coverage lower than 60%, the lm morphology was gradually
improved with increasing the concentration of PAN additive.
When containing 1.50 wt% PAN, a uniform and compact lm
was formed with only a small number of pores. However, too
much PAN would exert a negative effect on the lm quality; for
instance, the lm with 2.24 wt% PAN exhibited an obvious
phase separation of the MAPbI3 from PAN. Additionally, the
P3HT additive seemed inefficient for improving the device
photovoltaic performance. Although the MAPbI3 lm with P3HT
additive exhibited a high surface coverage and smooth
morphology (Fig. 5e), the device performance was severely lower
than that with the PAN additive (Fig. 5f). As presented in Fig. 5g,
they proposed that the decrease in performance by replacing
the PAN with P3HT could be attributed to the photo-induced
charge transfer between MAPbI3 and P3HT, which lowered the
device performance.
2.2
Fullerene additives
Fullerene and its derivatives have been widely used as electrontransporting layers in organic optoelectronic devices due to
their high electron mobility and impressive accept
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
Review
(a–d) Formation of perovskite nanocrystals in the presence of PVP: (a) precursor solution with or without PVP, (b) perovskite prepared at
100 C without PVP, which takes a cubic lattice at $55 C, (c) formation of tetragonal perovskite at <52 C, (d) perovskite formed with 3 wt% PVP,
which retains a cubic lattice at 5–100 C. (e) Schematic of the device structure. (f–g) SEM and (h–i) AFM images: (f, h) 0 wt% PVP, (g, i) 3 wt% PVP.
(j) Stability of the device with Ag or Au with and without CYTOP. Reproduced with permission from ref. 54. Copyright 2016 Wiley-VCH.
Fig. 4
characteristics, e.g., organic photovoltaic devices,56–69 organic
light-emitting devices,70–72 and organic eld-effect devices.73–75
Therefore, they have also been explored as an electron-
extraction layer in perovskite solar cells.4,23,32,37,76–80 Moreover,
hysteresis of the J–V characteristics,10,22,81 e.g., the scan direction
and scan speed,82–84 can be remarkably reduced by a passivation
Fig. 5 SEM images of the MAPbI3 perovskite layers containing: (a) 0 wt% PAN, (b) 0.76 wt% PAN, (c) 1.50 wt% PAN, (d) 2.24 wt% PAN. (e) SEM
image for the MAPbI3 perovskite layers containing P3HT addition. (f) J–V curves for the perovskite solar cells with PAN and P3HT. (g) Schematic of
the relative energy levels of each layer for the device based on the MAPbI3:P3HT film. Reproduced with permission from ref. 55. Copyright 2016
Wiley-VCH.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
Journal of Materials Chemistry A
Fig. 6 J–V curves of: (a) control device and (b) hybrid device obtained from different scan directions. (c) A schematic of the in situ passivation of
halide-induced deep traps: PC61BM adsorbs on the Pb–I antisite defective grain boundary during perovskite self-assembly. (d) Scheme of
a planar perovskite solar cell using a perovskite–PC61BM hybrid solid as the active absorber; the PC61BM phase is homogeneously distributed at
the grain boundaries throughout the perovskite layer. (e) Absorption spectroscopy of the hybrid solution showing the interaction between
PC61BM and the perovskite ions. Reproduced with permission from ref. 85. Copyright 2015 Nature Publishing Group.
effect and from the electrons and holes recombination
process.45 Accordingly, it is highly desirable to incorporate
fullerene and its derivatives into the perovskite precursor
solution as additives for improved morphology, charge transfer
and transport, and hysteresis performances.
Sargent et al. rst introduced PC61BM into perovskite solution as an additive to form a perovskite/PC61BM hybrid lm
with signicantly reduced hysteresis and recombination loss in
perovskite solar cells.85 As shown in Fig. 6a and b, the devices
with a hybrid lm exhibited a higher PCE with hysteresis-free J–
V characteristics compared to the control devices due to the in
situ passivation of the halide-induced deep traps by PC61BM
adsorbed on the Pb–I antisite defective grain boundary during
the perovskite self-assembly, as illustrated in Fig. 6c. They also
found that the PC61BM was homogeneously distributed
throughout the lm at the perovskite grain boundaries (Fig. 6d).
Fig. 6e depicts the anions (I) induced in the in situ doping of
perovskite by PC61BM was, resulting in PC61BM radical anions
and PC61BM–halide radicals, which can improve the electric
conductivity of hybrid lms and thus the device performance.
Owing to the shorter diffusion length of the electrons
compared to that for the holes in perovskites,86 a mesoporous
electron-transporting scaffold is normally used to effectively
extract and transport electrons for high device performance.
Gong et al. constructed “bulk heterojunction” (BHJ) perovskite
solar cells by mixing PC61BM with MAPbI3 to improve the electron extraction and transport from the PC61BM/MAPbI3 interfaces, as illuminated in Fig. 7(a–c).87 It can be seen from the
device performance that the two BHJ-based devices exhibited
higher current densities than the conventional device (Fig. 7d).
This journal is © The Royal Society of Chemistry 2017
Together with PL quenching (Fig. 7e), enhanced electron
extraction and transport with the addition of PC61BM was
conrmed. Compared to the conventional device that gave a Voc
of 0.90 V, Jsc of 20.0 mA cm2, FF of 37%, and a PCE of 6.90%, the
devices with the PC61BM additive showed a Voc of 0.90 V, Jsc of
26.86 mA cm2, FF of 52.9%, and a corresponding PCE of
12.78%, which represent a two-fold enhancement. Similar to the
polymer additives,43 a retarded reaction between MAI and PbI2 is
expected with the inclusion of the PC61BM additive due to the
interactions between the ester group of PC61BM and MAI to form
an intermediate phase. As shown in Fig. 7f and g, the PC61BM
additive induced a more uniform and dense perovskite lm
compared to the conventional lm. Moreover, they continued to
fabricate bulk heterojunction perovskite solar cells employing
the fullerene derivative A10C60 additive,88 where A is a carboxylic
acid group regiospecically functionalized on the C60 head. A
highest reported FF of 86.7% was observed due to the balance in
the charge carrier extraction efficiency and the enlarged interfacial area between the perovskite and A10C60 additive.
Wu et al. employed [6,6]-phenyl-C71-butyric acid methyl
ester (PC71BM) as an additive to fabricate BHJ perovskite/
PC71BM solar cells with a high FF of 0.82 and a PCE of up to
16.0%.89 The effect concentration of PC71BM on the perovskite
morphology was carefully investigated. As shown in Fig. 8a–f,
with increasing the concentration of PC71BM, the vacancies/
holes in the PbI2 lm were lled, which induced a continuous
and dense BHJ perovskite/PC71BM lm composed of large
grains without observable defects. They recognized that the
PC71BM could move to the surface of the perovskite located at
the grain boundaries and could also ll the empty spaces/holes
J. Mater. Chem. A
View Article Online
Review
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
Fig. 7 (a) The device structure of a conventional planar heterojunction perovskite solar cell. (b) The device structure of a conventional bulk
heterojunction perovskite solar cell. (c) The device structure of a PC61BM-modified bulk heterojunction perovskite solar cell. (d) J–V curves of the
different devices. (e) Photoluminescence (PL) spectra of the different devices. (f, g) SEM images of CH3NH3PbI3 perovskite film and CH3NH3PbI3:PC61BM perovskite film. Reproduced with permission from ref. 87. Copyright 2015 Wiley-VCH.
in the entire lm during the perovskite formation, as exhibited
in Fig. 8g. Therefore, the device showed no signicant photocurrent hysteresis and high current densities (Fig. 8h), corresponding to the results also in previous reports.77,85,88 The
excellent performance is mainly due to the high conductivity,
balanced electron and hole mobility, and long charge diffusion
length of the BHJ perovskite/PC71BM lms. Gong and Wu's
works demonstrated that the BHJ concept used in polymer solar
cells is an effective way for the perovskite layer to achieve a high
device performance.
Compared to PC61BM powder, one-dimensional (1D)
PC61BM nanorods have a higher surface area and transport
J. Mater. Chem. A
channels with higher electron mobility.90 Dai et al. reported that
a controlled amount of 1D PC61BM nanorods incorporated into
a perovskite precursor solution resulted in an enlarged grain
size and a bicontinuous composite structure, yielding an
enhanced PCE without obvious hysteresis.91 The perovskite lm
fabricated with the 1D PC61BM nanorods manifested a wrinklelike surface morphology, which indicated that the incorporation of the 1D PC61BM nanorods could facilitate a uniform
thickness through the whole perovskite lm, as shown in
Fig. 9a–c. The enlarged grain size may derive from the anisotropy of the 1D PC61BM nanorods during the spin-coating
process. Moreover, the 1D PC61BM nanorods help to form
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
Journal of Materials Chemistry A
Fig. 8 SEM surface images of: (a–c) PbI2 film and (d–f) perovskite film without and with 0.01 wt% and 0.1 wt% PC71BM. (g) The mechanism for the
formation of perovskite grains in the absence and presence of PC71BM. (h) J–V curves of the perovskite cell with a PCE of 16.0%. The left was
scanned with various delay times and the right was scanned in two different directions. Reproduced with permission from ref. 89. Copyright 2016
Nature Publishing Group.
a bicontinuous bulk heterojunction structure, which is benecial to the charge separation and transport at the perovskite and
1D PC61BM nanorods interfaces. The best device with the 1D
PC61BM nanorods demonstrated a PCE of 15.3% with no
evident hysteresis, while the control device exhibited a PCE of
12.57%, as presented in Fig. 9d and e. This improvement can be
Fig. 9 SEM images of: (a) perovskite film without 1D PC61BM and (b) with 1D PC61BM. (c) The device structure. (d) J–V curves of a device with and
without 1D PC61BM. (e) J–V curves of the device with 1D PC61BM obtained from different scan directions. Reproduced with permission from ref.
91. Copyright 2016 Royal Society of Chemistry.
This journal is © The Royal Society of Chemistry 2017
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
attributed to the enlarged grain size and uniform thickness
since the enlarged grain size contributes to the reduced interfacial area among the grains, thus suppressing trapping and
leading to a low bulk defect density and high carrier mobility.
Perovskite : fullerene blend lms have great potential for
achieving high-performance perovskite solar cells. First,
fullerene can ll the pinholes and vacancies between perovskite
grains, resulting in a lm with large grains and fewer grain
boundaries.89 The passivation effect of fullerene can signicantly reduce the photocurrent hysteresis of perovskite solar
cells.45,85 Second, the diffusion length of electrons is shorter
than that of holes in perovskite lms.92 Integrating fullerene
into perovskite lms can thus be used to construct electron
transport channels to facilitate electron extraction, further
boosting the efficiency of perovskite solar cells.93 Third, the
photostability of perovskite solar cells can be further improved
by introducing fullerene-based nanostructure additives.81
Therefore, perovskite : fullerene blend lms may open wide
avenues for the development of stable perovskite solar cells.
Moreover, the manufacturing process can be further simplied,
which makes the low-cost and high-throughput manufacturing
of perovskite solar cells possible. All in all, perovskite : fullerene
blend lms open wide avenues for their use in solar cells with
the benets of a simplied manufacturing process, improved
device performance, and enhanced stability.
2.3
Metal halide salt additives
Recently, there have been many reports on small molecule
additives that could have great effect on the perovskite
Review
crystallization process, thin-lm morphology, and thereby
could support a high device performance. Among the small
molecule additives, metal halide salt additives are quite
important additives that could signicantly contribute to
achieving a larger grain size and much smoother perovskite
surface. In fact, alkali metal halide salts, e.g., KCl and NaCl,
have already been used in CZTS and CIGS as additives to
promote the growth of larger grains and to enhance the
conductivity, and hence they have a signicant effect on the lm
morphology.94 On the one hand, metal halide salt additives can
offer template ions to partially replace MA+ or Pb2+ due to the
Fig. 11 (a) J–V curves and (b) EQEs of the best-performing device
without and with 2 mol% Na additive. (c) SEM surface images of the
perovskite film without and with the Na additive. Reproduced with
permission from ref. 96. Copyright 2016 American Chemical Society.
Fig. 10 J–V curves of perovskite solar cells: (a) photo- and dark currents and (b) the best device performance. (c) Normalized performance
decay of perovskite solar cells over time. SEM surface images of: (d–g) PbI2 film and (h–k) perovskite film: (d, h) prepared without any additives, (e,
i) prepared with KCl additive, (f, g) prepared with NaCl additive, (g, k) prepared with LiCl additive. (l) Crystal size of the perovskite films. Reproduced
with permission from ref. 95. Copyright 2016 Royal Society of Chemistry.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
similar ionic radius and can regulate the structure and optoelectronics of perovskite. On the other hand, they play an
important role in helping to modulate the crystallization
kinetics. Moreover, the halide ions can chelate with Pb2+ ions,
resulting in a nanostructured PbI2 lm, and thus could be used
to control the perovskite lm morphology. There is no doubt,
therefore, that metal halide salt additives are extremely
important for producing high-quality perovskite lm and
consequently high-performance perovskite solar cells.
Chu et al. introduced alkali metal halide salts (KCl, NaCl,
and LiCl) as additives in the perovskite precursor solution95 and
Journal of Materials Chemistry A
found that the PCE of the devices with alkali metal halide salt
additives (except for LiCl) were signicantly improved, especially the devices with KCl as an additive, which exhibited a PCE
of 15.08%, showing a 33% improvement compared to the best
PCE of the device without additives (11.4%), as shown in
Fig. 10a and b. They raised the hypothesis that the additives
worked as electrically active impurities and played a key role in
re-crystallizing the small grains and decreasing the grain
boundaries and interface states, resulting in efficient charge
generation and dissociation in the perovskite lm. Furthermore, the stability of devices with KCl as an additive showed
(a) J–V curves of the best device; inset picture presents the scan rate independent maximum power point (MPP) tracking for 60 s,
resulting in a stabilized efficiency of 21.6% at 0.977 V and 22.1 mA cm2. (b) J–V curve of the highest Voc device; inset picture shows the Voc over
120 s, resulting in 1.240 V. (c) UV-vis spectra and PL spectra of MAFA and RbCsMAFA perovskite. (d) XRD spectra of MAFA and RbCsMAFA
perovskite. (e, f) SEM images of the RbCsMAFA perovskite film. (g) Thermal stability test of a perovskite solar cell at 85 C. Reproduced with
permission from ref. 97. Copyright 2016 American Association for the Advancement of Science.
Fig. 12
This journal is © The Royal Society of Chemistry 2017
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
a momentous improvement in air compared to reference
devices, as demonstrated in Fig. 10c. Such advancement can
also be attributed to the fact that the alkali metal halide salt
additives supply halide ions to chelate with Pb2+ and contribute
to the formation of the nanostructure PbI2 lm, as shown in
Fig. 10d–g. The nanostructure PbI2 lm has a continuous
surface with less pinholes and plays an important role in the
formation of high-quality, uniform, and crystalline perovskite
lm, as illuminated in Fig. 10h–k. The perovskite grain size is
enlarged a lot assisted by the inclusion of additives (Fig. 10l),
which suppresses the permeability of oxygen and moisture and
therefore helps improve the stability of the corresponding
device.
Durstock et al. reported a novel way to improve the performance of perovskite solar cells by incorporating a controlled
amount of sodium iodide into the perovskite lm.96 Under
controlled conditions, the cells with the incorporation of
sodium ions gave a PCE of 14.2%, while the best cell without the
sodium iodide additives showed a PCE of 10.2%, as shown in
Fig. 11a. A higher external quantum efficiency (EQE) value of
a device with the sodium additive was also achieved compared
to a control device, which is consistent with the J–V curves
(Fig. 11b). They found that the sodium iodide additives
contributed to the formation of pinhole-free and large-grained
perovskite lms (Fig. 11c). Moreover, a low concentration of
the additive was necessary, which worked as the nucleation sites
Review
and facilitated the large-grain crystal growth, while the iodide
derived from the additive had little inuence on the halogen
concentration in the perovskite thin lm.
Grätzel and his co-worker found that although the tolerance
factor of RbPbI3 does not fall into the range of the “published
perovskites” and RbPbI3 cannot form a stable black phase,31
a slight inclusion of Rb incorporated into the perovskite
precursor solution could result in a dramatic improvement of
the device efficiency, as shown in Fig. 12a. It was demonstrated
that very small non-radiation recombination losses were achieved in pure and defect-free RbCsMAFA-based perovskite, thus
the Voc of the corresponding device could reach 1.240 V with
only a 0.39 V “loss in potential” (Fig. 12b), which is nearly the
lowest value for all PV materials. Recently, Duong et al. also
reported a similar discovery.97 As illustrated in Fig. 12c, MAFAbased perovskite showed several PL peaks in the range of
670 nm to 790 nm, while the RbCsMAFA-based perovskite only
gave a single PL peak in a narrow range, which indicated that
newly formed MAFA-based perovskite lm contains multi
emissive species resulting in an inhomogeneous starting
condition for crystallization, while the RbCsMAFA-based
perovskite comprises pure emissive species, leading to
a homogeneous starting condition for crystallization. Moreover,
a higher crystallinity, as depicted in Fig. 12d, for RbCsMAFAbased perovskite compared to MAFA lms can be observed
and the perovskite crystal size is enlarged with the
Fig. 13 AFM images of perovskite structures: (a) pristine, (b) CuI-, and (c) AgI-based CH3NH3PbI3 deposited on a mesoporous TiO2-coated FTO.
(d) XRD spectra of pristine and additive-based MAPbI3 perovskite grown on a mesoporous TiO2 film. (e) Trends in the Jsc, mh, and me values for
pristine and additive-based perovskite. (f) J–V curves of devices obtained using different monovalent cation halides added to the lead source
solution. Reproduced with permission from ref. 98. Copyright 2016 Wiley-VCH.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
incorporation of Rb (Fig. 12e and f). Most importantly, the
RbCsMAFA-based perovskite device exhibited good thermostability at 85 C for 500 h, which was observed for the rst time at
such a high temperature.
Friend et al. extended the use of monovalent cation halides
with a similar ionic radii to Pb2+, including Cu+ (CuI and CuBr),
Na+ (NaI), and Ag+ (AgI), to investigate the inuence of monovalent
cation halide additives on the optical, excitonic, and electrical
properties of MAPbI3 perovskite.98 They found a continuous
coverage and uniform lm with the use of CuI and AgI additive
lms (Fig. 13a–c) and a better conversion from PbI2 to MAPbI3
perovskite for NaI and CuBr additive lms (Fig. 13d). Moreover,
the hole mobility of the perovskites with additives (except AgI)
exhibited an increase of up to an order of magnitude compared to
the pristine perovskites, as shown in Fig. 13e. Consequentially, the
above aforementioned favorable properties led to the enhancement of PCE to 15.14%, 15.25%, 15.61%, and 14.18% for NaI, CuI,
CuBr, and AgI additive devices, respectively, in comparison with
14.01% for the additive-free control device (Fig. 13f).
While the alkali halide salt additives offer monovalent
cations and anions, bivalent or trivalent metal ions have not yet
been widely explored as additives. Liang et al. reported for the
rst time that the inclusion of a controlled amount of the
nonvolatile chlorinated additive CaCl2 in the perovskite
precursor solution could contribute a lot to the formation of
a dense and uniform perovskite thin lm.99 The lm formed
without CaCl2 additive showed needle-shape crystals and
incomplete, uneven surface coverage, while the lm fabricated
with the CaCl2 additive demonstrated compact and uniform
crystals with signicantly improved surface coverage, as shown
Journal of Materials Chemistry A
in Fig. 14a–c. They explained that the mixture perovskite
precursor solution formed a kind of “solid solution”, which was
unstable yet avoided the formation of a large needle-shape
perovskite. During the annealing process, the excess Cl anions
could help induce a preferred orientation of the MAPbI3 crystal
grains, which was good for the formation of a compact and
dense perovskite lm. However, the devices with CaCl2 additives exhibited a rather low efficiency with serious hysteresis
due to the remaining insulating CaCl2 (Fig. 14d–f).
Cu+-doping has been demonstrated to improve the
morphology of perovskite and to increase the charge-transport
properties.98 In order to investigate the effect of Cu2+-doping
on the perovskite morphology and grain size, Moon et al.
employed CuBr2 as an additive to replace some part of PbI2 in
the MAPbI3 structure by constructing a CuBr2–DMSO2 intermediate.100 Since the CuBr2–DMSO2 intermediate is owable,
the MAI(PbI2)1X(CuBr2)X perovskite could form larger crystalline grains compared to the lm formed from the PbI2–DMSO2
intermediate, which is not owable during the heat-treatment
process due to its simultaneous melting and decomposition,
as shown in Fig. 15a. The conductivity of the MAI(PbI2)1X(CuBr2)X perovskite was much enhanced due to the effective
dopants enhancing the charge carrier density (Fig. 15b) and due
to the remarkably reduced sheet resistance (Fig. 15c). Hence the
device performance was signicantly enhanced, as presented in
Fig. 15d and e.
Metal halide salt additives have primarily been used based
on isovalent metal/halide anions or metal cations, which
promote perovskite crystallization and have a signicant effect
on morphology. By contrast, metal halide salt additives with
Fig. 14 SEM images of perovskite film formed from: (a) pristine precursor solution, (b) unannealed, and (c) annealed perovskite films at 95 C for
20 min obtained from MAI : PbI2 : CaCl2 precursor mixtures with a 1 : 1 : 0.5 molar ratio. J–V curves of devices prepared from MAI : PbI2 : CaCl2
precursor mixtures with: (d) 1 : 1 : 0.5, (e) 1 : 1 : 0.75, and (f) 1 : 1 : 1 molar ratios. Reproduced with permission from ref. 99. Copyright 2015 Royal
Society of Chemistry.
This journal is © The Royal Society of Chemistry 2017
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
Review
Fig. 15 (a) SEM surface images of the MAI(PbI2)1X(CuBr2)X film with different compositional ratios (X ¼ 0, 0.025, 0.050, 0.075, and 0.100). (b)
Carrier density of a complete device obtained by C–V profiling and the corresponding PCE of the MAI(PbI2)1X(CuBr2)X planar perovskite solar
cells with a composition of X ¼ 0, 0.025, 0.050, 0.075, and 0.100. (c) Sheet resistance values of MAI(PbI2)1X(CuBr2)X perovskite films with respect
to the CuBr2 contents. (d) J–V curves and (e) EQEs of MAI(PbI2)1X(CuBr2)X inverted planar perovskite solar cells with different CuBr2 contents (X
¼ 0, 0.025, 0.050, 0.075, and 0.100). Reproduced with permission from ref. 100. Copyright 2016 Elsevier Ltd.
heterovalent ions compared to those in Pb-based perovskites
may have the capacity to tune the electronic structure and thus
can have an impact on the conductivity and concentration of
charge carriers. Bakr et al. for the rst time incorporated BiI3,
AuI3, or InI3 additives in a MAPbBr3 and MAPbI3 perovskite
precursor solution.101 They found that only the BiI3 additives
played an important role in enhancing the quality of the
perovskite crystals (Fig. 16a) and in switching the electronic and
optical characteristics with a reduced bandgap of perovskite
(Fig. 16b and c), which could be attributed to the isoelectronic
structure of Bi3+ (6s2) with Pb2+ (5s2) and thus their similar
chemical behavior. Moreover, Bi has a higher electronegativity
than Pb, which leads to more covalent bonding with bromide,
which may be another reason for the bandgap narrowing. The
Pb vacancies induced by the incorporation of Bi3+ may also
contribute to the bandgap narrowing. Most importantly, the
incorporation of Bi3+ additive could facilitate the transformation of the MAPbBr3 perovskite from a p-type semiconductor to an n-type semiconductor, with a signicantly
J. Mater. Chem. A
improved conductivity and charge carrier concentration, as
shown in Fig. 16d and e.
Indium (In), with a stable oxidation state (In3+), is a posttransition metallic element that can locate diagonally adjacent
to Pb. Liao et al. demonstrated that InCl3 with heterovalent
cations is an effective additive to control the crystallization of
perovskite and can promote a perovskite growth with multiple
ordered crystal orientations.102 As illustrated in Fig. 17a and b,
the InCl3 additive can help to form a perovskite lm with
a uniform crystal, high coverage, fewer pinholes, and a smaller
grain size. The smaller grain size may be induced by the Cl
anions le in the perovskite from the InCl3 additive, while no Cl
anions were observed in the annealed perovskite, which is
consistent with the previous reports.37 Although the grain size
becomes smaller, perovskites with an InCl3 additive exhibit
a good crystal quality with multiple ordered crystal orientations,
as revealed by grazing incidence X-ray diffraction (GIXRD)
technique and as exhibited in Fig. 17c–f. Consequently, the
device with an InCl3 additive demonstrated a PCE of 17.55%, Jsc
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
Journal of Materials Chemistry A
Fig. 16 (a) Photographs showing MAPbBr3 crystals with different concentrations of Bi. (b) Bandgap alignment of MAPbBr3 crystals with different
Bi%. (c) Steady-state absorption spectra of MAPbBr3 crystals with different Bi%. (d) Conductive and (e) majority charge concentration of MAPbBr3
crystals with different Bi/Pb atomic ratios in the crystal. Reproduced with permission from ref. 101. Copyright 2016 American Chemical Society.
of 21.09 mA cm2, Voc of 1.03 V, and an FF of 0.78, while the
control device exhibited a PCE of 12.61%, Jsc of 19.07 mA cm2,
Voc of 0.91 V, and an FF of 0.72 (Fig. 17g). Moreover, the device
with InCl3 showed better stability in air than the control device,
as shown in Fig. 17h.
As shown, the incorporation of metal ions as impurities into
the perovskite precursor solution could provide a more robust
way to inuence the crystallization process and produce
a perovskite lm with a large grain size or multiple ordered
crystal orientations. However, little work has been done on trap
passivation throughout the bulk by metal ions. Very recently,
Snaith et al. introduced aluminum acetylacetonate to the
perovskite precursor solution to rst improve the crystal quality
by reducing the microstrain, but moreover, the PL quantum
efficiency (PLQE) in the polycrystalline lm was remarkably
improved.103 Compared to the control lm, no other noticeable
differences in the morphology or apparent grain size were
observed in the perovskite lms up to doping levels of 0.3
mol%, as demonstrated in Fig. 18a. With further increasing the
Al3+ concentrations at or over 0.75 mol%, the polycrystalline
grains disappeared. The authors proposed a schematic diagram
of the perovskite growth mode and the inuence of the Al3+
doping based on the fact that Al3+ has a much smaller ionic
radius than Pb2+ (Fig. 18f). The Al3+ ion prefers to distribute on
the boundary of grains to passivate traps compared to the pure
This journal is © The Royal Society of Chemistry 2017
lms. Moreover, they observed that the Al3+-doped perovskite
lms had a higher steady-state PL intensity and an almost twofold increase in PL lifetime compared to the control lm, as
presented in Fig. 18b and c. Notably, the PLQE was much higher
for the Al3+-doped samples than for the control lms at all light
intensities, indicating the reduced traps in the lm (Fig. 18d).
Finally, a PCE of 19.1% was achieved in the Al3+-doped device
compared to 17.1% in the control device (Fig. 18e).
As a similar element to lead, tin (Sn) has been reported to be
an alternative to use in lead-free perovskite. However, a bottleneck issue for Sn-based perovskites, e.g., MASnI3 and CsSnI3, is
that Sn2+ is easy to oxidize to Sn4+, which plays a role in p-type
doping in the material, resulting in a limited photoexcited
carrier diffusion length and charge extraction in the device.
Mathews et al. reported that a high photocurrent could be
achieved in CsSnI3 PSC with a SnF2 additive through the
reduction of Sn vacancies.104 Fig. 19a presents the XRD spectra
of lms with 0, 5, 10, 20, and 40 mol% SnF2 in a CsSnI3
perovskite lm, where the similar XRD patterns indicate that
the SnF2 is not incorporated into the CsSnI3 perovskite lattice.
The addition of SnF2 actually eliminates the unknown reections caused by the yellow polymorph phase of the impurities.
The best PCE was nally obtained for 20% SnF2–CsSnI3 with
a Jsc ¼ 22.70 mA cm2, Voc ¼ 0.24 V, and an FF ¼ 0.37, which
resulted in h ¼ 2.02%, as shown in Fig. 19b. Most importantly,
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
Review
Fig. 17 SEM images of perovskite film: (a) without In3+ and (b) with In3+. GIXRD patterns of: (c) MAPbIXCl3X, (d) MAPb0.90In0.10I3Cl0.10, (e)
MAPb0.85In0.15I3Cl0.15, and (f) MAPb0.75In0.25I3Cl0.25 perovskite films. (g) J–V curves of devices with and without In3+; inset picture shows
a histogram of the PCEs of device with the best concentration of In3+. (h) Normalized PCE values as a function of the exposure time in air.
Reproduced with permission from ref. 102. Copyright 2016 Wiley-VCH.
the device with the inclusion of the SnF2 additive showed great
stability for more than 11 days inside a glovebox (Fig. 19c),
which may open up new possibilities for exploiting lead-free
perovskites in terms of their stability. Shortly, the same
authors introduced the same additive SnF2 into the FASnI3
perovskite and the corresponding device achieved a PCE of
2.10% together with an increased stability.105 Such improvements can be attributed to the incorporation of SnF2, which
delays the oxidation of Sn2+ and facilitates the formation of
a more homogeneous and continuous thin lm. Furthermore,
adding SnF2 into the perovskite thin lm suppresses the interfacial states, which could otherwise trap holes or electrons and
have a crucial effect on the PCE of the device.
SnF2 is necessary to stabilize the FASnI3 perovskite and
improve the lm quality; however, a higher amount of SnF2
induces severe phase separation in the lm.105 Seok et al. reported an effective approach employing dual additives of SnF2
and pyrazine, which could remarkably restrict the phase separation induced by the excess SnF2 and reduce the Sn vacancies
effectively.106 The solution without pyrazine gave lms with
plate-like aggregates, as shown in the le-hand side of Fig. 20,
while the additional introduction of pyrazine into the precursor
solution led to a smooth, dense, and pinhole-free FASnI3
perovskite layer (right-hand side of Fig. 20). This implies that
the phase separation was signicantly reduced by the formation
J. Mater. Chem. A
of a SnF2/pyrazine complex and consequently the surface
morphology of the FASnI3 perovskite lm could be ultimately
improved. Moreover, the addition of pyrazine did not change
the crystal structure compared to the structure without pyrazine
since it can be easily removed because of its low boiling point of
115 C during the annealing process. Finally, the highest reported PCE of 4.8% was achieved, which was highly reproducible, as shown in the graph in the middle of Fig. 20.
2.4
Inorganic acid additives
Inorganic acid additives, e.g., hydriodic acid (HI), hydrobromic
acid (HBr), and hydrochloric acid (HCl), as special additives
have recently been receiving increased attention due to their
unique characteristics in the formation of a compact and full
coverage of perovskite lms. There are various reasons for this.
First, the inorganic acid additives help increase the solubility of
perovskite precursors, leading to a higher supersaturation point
that promotes nucleation and leads to a continuous perovskite
lm with a large grain size. Second, inorganic acid additives
prevent the perovskite from decomposition by reducing the
oxidized I2 back into I, thus improving the long-term stability.
Third, inorganic acid additives may react with the PbI2 and
form a pre-crystallized intermediate state to adjust the growth
rate of the perovskite and to control the crystallization
This journal is © The Royal Society of Chemistry 2017
View Article Online
Journal of Materials Chemistry A
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
Fig. 18 (a) SEM images of perovskite without and with metal ion additives. (b) Steady-state photoluminescence spectra of the control (non-
doped) and 0.15 mol% Al3+-doped perovskite thin films. (c) Time-resolved photoluminescence spectra of the control and 0.15 mol% Al3+-doped
perovskite thin films. (d) PLQE of the control and 0.15 mol% Al3+-doped perovskite thin films. (e) J–V curves of the control and 0.15 mol% Al3+doped perovskite solar cells. (f) Schematic diagram of the proposed perovskite polycrystalline thin-film growth and influence of the Al3+ doping,
illustrated with C, N, H, O, I, Pb, and Al atoms as indicated by the colors: black, blue, pink, red, gray, and cyan, respectively; and histograms of the
surface topography and the images (insets) from AFM measurements of the different samples. Reproduced with permission from ref. 103.
Copyright 2016 Royal Society of Chemistry.
This journal is © The Royal Society of Chemistry 2017
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
Review
Fig. 19 (a) XRD patterns of pristine CsSnI3 and samples with 5, 10, 20, and 40 mol% added SnF2, with all the patterns matching the black
orthorhombic phase of CsSnI3. (b) J–V curves of the device without the additive and with different concentrations of SnF2. (c) Device stability
demonstrated in the J–V graphs measured on different days. Reproduced with permission from ref. 104. Copyright 2014 Wiley-VCH.
Fig. 20 J–V curves of the device and SEM images of the perovskite thin film, and the role of pyrazine in the formation of an homogeneous
crystal. Reproduced with permission from ref. 106. Copyright 2016 American Chemical Society.
dynamics of the perovskite. It should be noted that one or
multiple effects can exist in the precursor solution for facilitating the formation of a high-quality perovskite lm.
Im et al. rst used the HI additive in a perovskite precursor
solution to improve the solubility of CH3NH3PbI3 perovskite
with respect to the pure DMF solvent, with the additive
contributing to the formation of a pinhole-free thin lm.107
With the incorporation of HI, the PbI2 could completely
convert to perovskite (Fig. 21a). The control devices showed
a Voc of 1.06 V, Jsc of 18.7 mA cm2, FF of 59%, and a PCE of
11.7% in the forward scan direction, and a Voc of 1.08 V, Jsc of
18.9 mA cm2, FF of 70%, and a PCE of 14.3% in the backward
Fig. 21 (a) XRD spectra of perovskite film with and without HI additive. (b) J–V curves of DMSO-MAPbI3 and HI-MAPbI3 perovskite solar cells. (c,
d) SEM surface images of DMSO-MAPbI3 and HI-MAPbI3 perovskite film. Reproduced with permission from ref. 107. Copyright 2015 Wiley-VCH.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
scan direction (Fig. 21b). The devices with HI additive,
however, exhibited a Voc of 1.1 V, Jsc of 20.4 mA cm2, FF of
75%, and a PCE of 16.8% in the forward scan direction, and
a Voc of 1.1 V, Jsc of 20.5 mA cm2, FF of 78%, and a PCE of
17.6% in the backward scan direction. Moreover, the device
with HI additive showed no obvious hysteresis. Such
improvements, as they explained, could be attributed to the
perovskite lm without the HI additive showing large crystalline domains while the perovskite lm formed with the HI
additive showed no crystalline domains (Fig. 20c and d). This
is a benet for improving the diffusion coefficient, charge
carrier lifetime, charge injection/separation, and collection
efficiency.
Achieving a pure phase of FAPbI3 perovskite is difficult with
the common DMF precursor solution. Consequently, Ho-Baillie
et al. employed HI for the rst time as an additive to improve the
crystallization and morphology of FAPbI3 perovskite.108 The
efficient concentration of HI was highly limited within a narrow
range to facilitate a uniform crystal and lm growth without
pinholes, as shown in Fig. 22a. The low concentration of HI
assisted the formation of sufficient nuclei for crystallization,
while the high concentration of HI, along with more water
added into the perovskite precursor solution, could degrade
and dissolve the perovskite lm. Moreover, HI could also
Journal of Materials Chemistry A
promote the transformation of PbI2 to pure HC(NH2)2PbI3
perovskite with the a-phase (Fig. 22b). With increasing the
concentration of HI, the perovskite lm showed a shortened
time for PL decay (Fig. 22c), because the incorporation of the HI
additive improved the charge carrier extraction. All of the above
improvements had a positive effect on the higher PCE of the
device with the inclusion of the HI additive, as illustrated in
Fig. 22d.
Apart from modifying the crystallization of organic halide
perovskites, HI can also be used as an additive in the crystal
growth of inorganic halide perovskites, as demonstrated by
Eperon et al.109 They used HI as an additive in the CsPbI3
precursor solution and successfully stabilized CsPbI3 in the
black perovskite phase at room temperature, as demonstrated
in Fig. 23a. Furthermore, the perovskite lm formed with the HI
additive could stay in the black phase much longer than that
formed by high-temperature annealing due to the fact that the
black phase was more energetically favorable with the HI
additive. Both the lms with and without the HI additive were
smooth and uniform; however, the lm with the HI additive
showed a much smaller grain size (Fig. 23b). The smaller crystal
size may be due to the HI being more rapidly driven off
compared to the pure DMF, which results in a faster crystallization. Most importantly, the lm with the HI additive exhibited
(a) SEM images, (b) XRD spectra, and (c) PL decay traces of perovskite film without and with different concentrations of HI additive. (d) J–
V curves of the device without and with different concentrations of HI additive. Reproduced with permission from ref. 108. Copyright 2016
American Chemical Society.
Fig. 22
This journal is © The Royal Society of Chemistry 2017
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
a more pronounced orientation (Fig. 23c). Peak splitting at the
[200] peak was observed when incorporating the HI additive in
the perovskite, which indicated the appearance of strain in the
crystal derived from the formation of a smaller crystal, as
depicted in Fig. 23d. Strain in a crystal can induce crystal phase
transitions and completely shi the phase diagram, which may
be the reason why the HI black phase can be formed at a low
temperature.
Review
Huang et al. compared HI, HCl, and HBr as additives in the
perovskite precursor solution and discovered that HBr was the
most efficient additive.110 The lm fabricated with the HBr
additive showed better crystallization, a complete surface
coverage, and fewer pinholes, as shown in Fig. 24a and b.
They explained that an homogeneous solution could lead to
a high-quality morphology. One of the advantages of the
incorporation of HBr is to improve the homogeneity of the
perovskite precursor solution. The device with the HBr
Fig. 23 (a) Without the HI additive, CsPbI3 shows a yellow non-perovskite phase (the left) at room temperature, while CsPbI3 with the HI additive
exhibits a black perovskite phase (the right) at 100 C. (b) SEM images of the perovskite film with and without the HI additive. (c) Comparison of
the XRD spectra of the perovskite with and without the HI additive; the peak marked with # indicated some yellow phase of the perovskite. (d)
Magnification of the100 and [200] peaks. Reproduced with permission from ref. 109. Copyright 2015 Royal Society of Chemistry.
Fig. 24 SEM images of: (a) perovskite film without HBr incorporation and (b) with HBr incorporation. J–V curves of: (c) devices without and with
HBr incorporation and (d) a device with HBr incorporation obtained from different scan directions. Reproduced with permission from ref. 110.
Copyright 2016 Royal Society of Chemistry.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
additive demonstrated a PCE of 15.76%, together with a Voc of
0.94 V, Jsc of 21.72 mA cm2, and an FF of 0.77, while the
control device exhibited a PCE of 12.13%, together with a Voc of
0.87 V, Jsc of 19.12 mA cm2, and an FF of 0.72, as shown in
Fig. 24c. The better results with the HBr additive were because
the Br ion can interact with Pb2+ and form a better immediate
phase in the process of perovskite growth before annealing.
Moreover, the HBr-incorporated perovskite solar cell showed
no signicant hysteresis (Fig. 24d). Such improvements suggested that the compact lm could benet the charge extraction and collection process.
Journal of Materials Chemistry A
HBr was also employed as an effective additive for highquality perovskites, especially for MAPbBr3 perovskite. Im
et al. addressed a novel way to control the formation of dense
MAPbBr3 perovskite thin-lm by inclusion of HBr in the
perovskite precursor solution.111 The perovskite lm without
any additive showed a rough surface with a small grain size,
while that with H2O or HBr solution demonstrated a smooth
surface with a large grain size, as shown in Fig. 25a–i. Such an
improvement could be attributed to the increased solubility of
DMF for the perovskite with the HBr additive since the order of
solubility of the perovskite is: DMF < DMF + H2O < DMF + HBr
solution. The better solubility retards the nucleation time
Fig. 25 SEM surface and section images of the MAPbBr3 perovskite thin film with different solubilities: (a–c) DMF, (d–f) DMF + H2O, and (g–i)
DMF + HBr: DMF ¼ 40 wt% MAPbBr3 in 1 mL of DMF, DMF + H2O ¼ 40 wt% MAPbBr3 in 1 mL of DMF + 0.1 mL of H2O, DMF + HBr ¼ 40 wt%
MAPbBr3 in 1 mL of DMF + 0.1 mL of HBr. (j) J–V curves of perovskite solar cells with different solubilities. Reproduced with permission from ref.
111. Copyright 2014 Wiley-VCH.
Fig. 26 (a) SEM images of the perovskite films fabricated using different hydrochloride concentrations. (b) Scheme of the formation of the Pb–I–Pb
bonds, where chloride acts as an inhibitor in the reaction, which leads to a slower crystallization. (c) J–V curves obtained from the different scanning
directions. (d) Normalized PCE of the device without encapsulation. Reproduced with permission from ref. 113. Copyright 2016 Wiley-VCH.
This journal is © The Royal Society of Chemistry 2017
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
during the spin-coating process. Therefore, the device with the
HBr additive demonstrated a best PCE of 10.4% (Fig. 25j).
Huang et al. also found that the HBr additive could rst improve
the homogeneity of the perovskite precursor solution to lead to
enhanced crystallization, complete surface coverage, and fewer
pinholes.110
As is known, a thermal annealing process (normally 100 C)
is required to facilitate the crystallization of perovskite, but the
additives that are used to control the nucleation and crystal
growth during lm formation may result in pinholes and
cracks in the nal lm.112 Xu et al. recently demonstrated an
approach that involved processing highly smooth and fullcoverage perovskite lms with large crystalline grains by an
HCl-assisted spin-coating method at room temperature.113 As
demonstrated in Fig. 26a, the lm processed without HCl led
to a striped-shape morphology with a low coverage, while the
pinholes became smaller and fewer in number with increasing
the concentration of HCl. Finally, a fully covered lm tortoise
shell-like morphology with a grain size over 1 mm was achieved
in the precursor containing 3 equivalents of HCl. The authors
proposed that the chloride played an important role as an
inhibitor in the crystallization of the perovskite due to the fact
that the formation of Pb–I–Pb is much more difficult when
chloride is coordinated to lead (Fig. 26b), resulting in a deceleration of nucleation and nucleus growth. The bestperforming device had a Jsc ¼ 23.3 mA cm2, Voc ¼ 1.00 V,
FF ¼ 77%, and PCE ¼ 17.9% by reverse scan, and a Jsc ¼ 23.3
mA cm2, Voc ¼ 1.00 V, FF ¼ 69%, and PCE ¼ 16.1% by forward
Review
scan, as exhibited in Fig. 26c. Moreover, the device without
encapsulation exhibited a high air stability with a total of 95%
of the initial PCE retained aer two and a half months
(Fig. 26d).
CH3NH3I was synthesized by mixing CH3NH2 with HI,
where a stabilizer present in HI was the reducing agent,
namely hypophosphorous acid (HPA). Together with the fact
that it was found that an “impurity” may be present in the assynthesized CH3NH3I, which could have a benecial inuence upon the crystallization and optoelectronic properties of
the perovskite, Snaith et al. showed that adding HPA in the
perovskite precursor solution could signicantly improve the
lm quality with an enlarged grain size (Fig. 27a and b), which
leads to highly efficient and reproducible photovoltaic
device.114 Fig. 27c and d show that the lm with the HPA
additive gave an enhanced PL intensity and charge carrier
lifetime, indicating that the electronic quality of the perovskite lm was enhanced in terms of a reduced energetic
disorder and that fewer nonradiative decay pathways are
likely to result from the reduced density of defect sites. This is
because the HPA additive could reduce the oxidized I2 back
into I, as shown in Fig. 27e, which facilitates an improved
stoichiometry in the perovskite crystal and a reduced density
of metallic lead, leading to an improved PCE in the device
from 13.2% to 16.2%.
Based on the chemical formation mechanism of perovskite
elucidated recently,115 Nakamura et al. reported that the accelerated conversion from PbI2 to perovskite and the crystal growth
SEM surface images of perovskite film: (a) without and (b) with the HPA additive, respectively. (c) Steady-state and (d) time-resolved PL
spectra for the perovskite thin films deposited on a glass prepared from the precursor solution without and with HPA. (e) UV-vis absorption
spectra of MAI or I2 dissolved in DMF and absorption quenching of the MAI solution by adding HPA. Reproduced with permission from ref. 114.
Copyright 2015 Nature Publishing Group.
Fig. 27
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Journal of Materials Chemistry A
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
Fig. 28 (a) Proposed six-center catalysis by a zwitterionic acid. (b) SEM images of different wt% films with a thickness of 320 nm prepared from
a 40 wt% precursor solution. (c) Energy levels of 320 nm thick PVs doped with x wt% SA. (d) Dependence of the PL intensity on the NH3SO3doping ratio for a film on glass. (e) J–V curves with SA (blue) and without SA (red) for a 320 nm film. Reproduced with permission from ref. 116.
Copyright 2016 American Chemical Society.
This journal is © The Royal Society of Chemistry 2017
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
to a micrometer size can be achieved by inclusion of a zwitterionic sulfamic acid (NH3SO3) additive.116 As presented in
Fig. 28a, the authors proposed a six-center catalysis from
a zwitterionic acid. With increasing the concentration of
NH3SO3, larger grains were formed, but further increasing the
concentration beyond 2 wt% resulted in the formation of voids
as larger cubic crystals were formed (Fig. 28b). Interestingly,
NH3SO3 could raise the energy levels of the HOMO and LUMO
of the perovskite lm without affecting the band gap, as shown
in Fig. 28c, which facilitates electron transfer to a neighboring
electron-accepting layer in devices. This was further conrmed
by the PL quenching spectrum by PCBM (Fig. 28d). Finally,
a 320 nm thick perovskite lm with 0.7 wt% NH3SO3 showed
a higher PCE of 16.02% compared with the device made without
NH3SO3 (13.08%) (Fig. 28e).
2.5
Solvent additives
Solvent additives, e.g., 1,8-diiodooctane (DIO),117,118 1,8-octanedithiol (ODT),119 diiodohexane (DIH),120 and 1-chloronaphthalene (CN),121 have been widely used in bulk
heterojunction polymer solar cells to control the crystallization of donors and acceptors and to modify the morphology of
the active layer, which could signicantly enhance the device
performance. Based on the successful experience in the use of
additives in polymer solar cells, such solvent additives have
been added to perovskite precursor solutions to control the
kinetics of crystal growth and to improve the performance of
perovskites solar cells, in terms of their stability and
efficiency.
Review
Jen et al. demonstrated for the rst time that the incorporation of a DIO additive into its precursor solution could control
the crystallization rate of perovskite and modulate perovskite
thin lm formation.40 The control device without DIO additive
showed a PCE of 7.8%, Voc of 0.90 V, Jsc of 15.0 mA cm2, and an
FF of 0.58, while the device with the DIO additive exhibited
a signicantly enhanced PCE of 10.3%, Voc of 0.92 V, Jsc of 15.6
mA cm2, and an FF of 0.71, as depicted in Fig. 29a. These
improvements originated from the markedly improved surface
coverage and crystallization of the additive assisted lm
(Fig. 29b and c). The authors also found that the incorporation
of DIO, which is a so Lewis acid, could temporarily chelate
with Pb2+ during the crystal growth, as shown in Fig. 29d. This
contributes to homogeneous nucleation and likely modies the
interfacial energy favorably, ultimately altering the kinetics of
crystal growth. Moreover, they further systematically investigated four more alkyl halide additives with different alkyl chain
lengths and end-groups, e.g., 1,4-diiodionbutane (1,8-DIB), 1,10diiodinedecane (1,10-DID), 1,4-dibromobutane (1,4-DBrB), and
1,4-dichlorobutane (1,4-DClB), to elucidate their inuence on
the kinetics of crystal growth.122 Besides, the alkyl halide additives might participate in perovskite formation via dissociated
carbon–halogen bonds to generate extra halogen ions, while an
excessively long alkyl chain length could result in steric
hindrance for chelation with Pb2+ and an enhanced non-polar
nature of the additives.
The above solvent additives are all strong coordination
molecule; however, Zhou et al. recently found that the weak
coordination acetonitrile (ACN) can also be used as a solvent
additive in PbI2/DMF solution to manipulate the bonding
Fig. 29 (a) J–V curves of the perovskite solar cell with and without the DIO additive. (b) SEM images of the perovskite film with and without the
DIO additive. (c) XRD patterns of the solution-processed perovskite with and without the DIO additive. (d) Schematic diagram for the transient
chelation of Pb2+ with DIO. Reproduced with permission from ref. 40. Copyright 2014 Wiley-VCH.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Journal of Materials Chemistry A
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
Fig. 30 (a) SEM images of the perovskite thin film fabricated with different concentrations of the ACN additive. (b) Size distribution of the PbI2DMF particles. (c) Mechanism for the formation of the PbI2–DMF cluster manipulated by the ACN additive. (d) SEM images of the PbI2 film
fabricated with different concentrations of the ACN additive. (e) Time-resolved PL spectroscopy of the perovskite film fabricated with different
concentrations of the ACN additive. (f) The best device performance with the ACN additive. Reproduced with permission from ref. 123. Copyright
2016 Wiley-VCH.
process of PbI2-DMF complexes, leading to a uniform perovskite
lm.123 Surprisingly, the perovskite crystal size fabricated with
ACN was dramatically enlarged, as depicted in Fig. 30a.
Utilizing dynamic light scattering (DLS) measurements, they
found that with the increase of ACN concentration in the ACN–
PbI2–DMF solution, the radius of the PbI2–DMF particles
decreased from 1042.6 nm to 455.4 nm (Fig. 30b). Fig. 30c
presents the mechanism for the formation of the PbI2–DMF
cluster mediated by the ACN additive, where DMF is partially
replaced by ACN and the ACN–PbI2–DMF particle is much
smaller than the PbI2–DMF particle, resulting in a PbI2 lm with
This journal is © The Royal Society of Chemistry 2017
a smaller crystal size. Because the low boiling point ACN
contributes to a quick vaporization, the PbI2 lm showed many
pores (Fig. 30d), which could serve as nucleation centers for the
newly formed perovskite. The high density and nearly uniform
distribution nucleation centers favored a smooth perovskite
lm with a large grain size. Additionally, they found that the
perovskite lm fabricated with the ACN additive showed a lower
grain boundary and longer carrier lifetime (Fig. 30e). Finally,
the best device fabricated with ACN achieved a PCE of 19.68%,
together with a Voc of 1.15 V, FF of 75.33%, and a Jsc of 22.69 mA
cm2 (Fig. 30f).
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
Review
Fig. 31 SEM images of a perovskite thin film (a) without any additive and (b) with the CN additive. (c) J–V curves of a device with and without the
CN additive. (d) Scheme of the formation of perovskite with and without the CN additive. Reproduced with permission from ref. 124. Copyright
2015 AIP Publishing LLC.
Fig. 32 (a) Photographs of PbI2/DMF solutions containing various amounts of the H2O additive. (b) SEM images of PbI2 film without and with the
H2O additive (top) and perovskite film without and with the H2O additive (bottom). (c) J–V curves of the best performance device with the H2O
additive. (d) Long-term stability of the inverted perovskite solar cell (in a glove box). Reproduced with permission from ref. 128. Copyright 2015
Royal Society of Chemistry.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
Another high boiling point solvent, CN, was used as an
additive in the perovskite precursor solution to regulate the
crystallization transformation kinetics of perovskite to form
high-quality crystal lms.124 The lms with the additive were
smoother and more homogeneous with fewer pinholes and
voids and a better surface coverage than the pristine lms, as
shown in Fig. 31a and b. The authors believed that CN had one
chlorine atom and could also serve as a so Lewis base to
coordinate with Pb2+, similar to the DIO additive (Fig. 31d).
Moreover, CN escaped slowly from the perovskite thin lm due
to its high boiling point (259–263 C), which signicantly
slowed down the crystallization rate of the perovskite crystals.
Finally, a device fabricated from the precursor solution containing the CN additive showed an improved performance with
a PCE of 9.46%, Jsc of 16.9 mA cm2, Voc of 0.85 V, and an FF of
65.8%, while the control device demonstrated a PCE of 7.26%
with a Jsc of 13.5 mA cm2, Voc of 0.85 V, and an FF of 63.4%
(Fig. 31c).
Although H2O has a lower boiling point and higher vapor
pressure compared to DIO and CN, it was demonstrated that
can assist the grain growth of the perovskite lm to dramatically increase the carrier mobility and charge carrier lifetime.24,125–127 Grätzel et al. rst added a small amount of H2O as
an additive into a PbI2/DMF precursor solution and found the
solution became homogeneous, as shown in Fig. 32a.128 The
perovskite thin lm without the additives gave a poor surface
with many small pinholes and voids, while that with the
additives showed an increased grain size and hardly no
Journal of Materials Chemistry A
pinholes from a compact PbI2 lm with 2 wt% H2O (Fig. 32b–
e). The decreased defects and increased lm coverage led to
improved charge transport and light absorption in the perovskite thin lm, resulting in a higher performance of the device.
As reported, the control device showed a PCE of 0.0063%, Jsc of
0.329 mA cm2, Voc of 0.10 V, and an FF of 0.19, while the
device with the H2O additive demonstrated a best PCE of 18%,
Jsc of 20.6 mA cm2, Voc of 1.03 V, and an FF of 0.85 (Fig. 32f).
Moreover, the lower boiling point and higher vapor pressure of
H2O could promote the formation of large crystals. Most
importantly, the stability of the device with the H2O additive
was signicantly improved due to the high-quality perovskite
lms (Fig. 32g).
Controllable perovskite crystallization by the inclusion of
H2O as an additive in a CH3NH3I and lead chloride (PbCl2)
perovskite precursor solution was further demonstrated by Liao
et al. (Fig. 33a).129 The pure perovskite lms exhibited small
pinholes and few voids, while a continuous perovskite lm with
decreased grain boundaries could be observed with 2 wt% H2O
additive, as shown in Fig. 33b and c. The PCE of the device was
remarkably improved from 12.13% in pure perovskite solution
to 16.06% in 2 wt% H2O additive solution (Fig. 33d). In addition, they assumed that CH3NH3PbI3XClX$nH2O hydrated
perovskites were generated during the annealing process, which
are resistant to corrosion by H2O molecules to some extent.
Therefore, the stability of the devices with H2O was signicantly
improved (Fig. 33e).
Fig. 33 (a) Structure of the device. SEM images of: (b) perovskite film without H2O and (c) perovskite film with H2O. (d) J–V curves of a device
fabricated with and without H2O. (e) Device stabilities of DMF- and DMF + 2% H2O-based perovskite solar cells under ambient conditions.
Reproduced with permission from ref. 129. Copyright 2015 Wiley-VCH.
This journal is © The Royal Society of Chemistry 2017
J. Mater. Chem. A
View Article Online
Journal of Materials Chemistry A
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
2.6
Organic halide salt additives
Organic halide salts additives, on the one hand, contain
heteroatoms in organic components, which may form hydrogen
bonds with H atoms in CH3NH3+. The additives could act as
a glue to interconnect separate perovskite crystals to yield
a larger grain size and a continuous surface. On the other hand,
the halides in the additives are easy to integrate into the
perovskite crystal (e.g., Br and I) or can help to form perovskite crystals with a uniform and large grain size (e.g., Cl).
Some organic halide salt additives can react with PbI2 to form
an intermediate phase, which is then slowly converted to the
perovskite in the presence of excess MAI during thermal
annealing. Moreover, low-dimensional or dimensionally mixed
perovskites might be promising when assisted by organic halide
Review
salt additives with large organic components, which can then be
benecial to the formation of a high-quality perovskite lm with
low-dimensional perovskites on the surface of 3D perovskites,
thus their environmental stability can be expected to be
signicantly increased.
Methylamine halide additives were rst suggested for highperformance perovskite solar cells since both the MA+ cation
and halogen anion are the fundamental components in
a typical perovskite structure. Bein et al. used MACl as an
additive in the perovskite precursor solution to signicantly
improve the performance of the corresponding device.130 The
neat triiodine perovskite solar cells showed an average PCE of
5.5%, while that with the MACl additive demonstrated an
average PCE of 10.5% with a best PCE of 15% (Fig. 34a). First,
the incorporation of MACl facilitated the full conversion of PbI2
(a) J–V curves of the device with different concentrations of the MACl additive. (b) Time-resolved PL decay plots of lead halide perovskite
systems for a range of MACl concentrations. Reproduced with permission from ref. 130. Copyright 2014 Wiley-VCH.
Fig. 34
SEM images of: (a) perovskite film without MAI, (b) perovskite film with 0.1 MAI, (c) perovskite film with 0.2 MAI, (d) perovskite film with 0.3
MAI, (e) perovskite film with 0.4 MAI, and SEM cross-sectional image of (f) perovskite film with 0.2 MAI. (g) Steady-state PL spectra of perovskite
films with different concentrations of MAI. (h) J–V curves of a device with different concentrations of MAI. Reproduced with permission from ref.
132. Copyright 2015 American Chemical Society.
Fig. 35
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
to the MAPbCl3XIX perovskite structure, which signicantly
improved the Jsc. Second, they found that the addition of MACl
critically reduced the device series resistance, inducing an
improved FF. Third, as shown in Fig. 34b, the charge diffusion
length became longer as well with the incorporation of MACl,
which also contributed to the improved Jsc. Actually, there was
no MACl le in the nal perovskite, as demonstrated by Zhao
et al.,131 who found that MACl functions as a glue or so
template to control the initial formation of a solid solution with
the main precursor components.
Huang and co-workers also demonstrated methylamine
halide was a good additive in a MAPbI3 perovskite precursor132
when they incorporated MAI into a PbI2 precursor solution to
optimize the rst layer of PbI2 and to fabricate a MAPbI3
perovskite using a two-step dipping method. Interestingly, they
found that the perovskite lm formed with a small amount of
MAI showed a relatively more continuous surface together with
a uniform grain size compared to that formed without any
additive. However, when the concentration of MAI exceeded
0.2 mmol, the lm morphology showed serious pinholes, as
shown in the Fig. 35(a–f). They also found that the PbI2 lm
formed with a small amount of MAI not only had no MAPbI3 but
Journal of Materials Chemistry A
also showed better crystallization. Moreover, they found that
a small amount of MAI incorporation would decrease the
recombination of light-excited electrons and holes (Fig. 35g).
Finally, the control device showed a PCE of 11.13% with a Voc of
1.015 V, Jsc of 16.15 mA cm2, and an FF of 67.9%, while the
device fabricated with the MAI additive demonstrated a best
PCE of 13.37% with a Voc of 1.046 V, Jsc of 18.58 mA cm2, and
an FF of 68.8% (Fig. 35h).
Having shown that MACl and MAI as additives could
improve the perovskite morphology and related device performance, researchers have recently found that Br could
dramatically control the dynamics of perovskite growth as well
as the perovskite morphology.29,111,133 Based on its superiority as
a potential replacement for traditional lead halides,134–137 Zhu
et al. utilized MABr as an additive in the lead acetate (Pb(Ac)2)
and MAI precursor solution, resulting in uniform, compact, and
pinhole-free perovskite lms.138 Based on the perovskite lm
with the MABr additive, a best PCE of 18.3% was achieved with
a Jsc of 22.34 mA cm2, Voc of 1.00 V, and a high FF of 0.82, as
shown in Fig. 36a. By contrast, the control device without the
MABr additive gave a highest PCE of 14.3%, with a Jsc of 20.28
mA cm2, Voc of 0.95 V, and an FF of 0.74. Moreover, the use of
Fig. 36 (a) J–V curves of the best devices obtained without and with 1.5 mol% MABr additive in the precursor solutions. (b) Histograms of the
PCEs for the devices without and with the MABr additive. (c, d) AFM images. (e, f) Time-resolved PL spectra of the perovskite films without and
with MABr. Reproduced with permission from ref. 138. Copyright 2016 Wiley-VCH.
This journal is © The Royal Society of Chemistry 2017
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
MABr as an additive improved the reproducibility of the device
performance (Fig. 36b). It is obvious from the atomic force
microscopy (AFM) images that a much smoother perovskite lm
was produced using the MABr additive (Fig. 36c and d). Moreover, the MABr additive accelerated charge extraction from the
perovskite to the charge collection layers due to the shorter
lifetimes obtained for the perovskite lm with MABr compared
to the pure perovskite lms (Fig. 36e and f). The work indicated
that MABr could be a promising additive to achieve highly
efficient planar heterojunction perovskite solar cells, and
further improvements through chemical engineering are likely
to be feasible.
Methylamine halide additives have been widely investigated due to their unique function as one of the components
in the perovskite structure. If the small MA+ ion is replaced by
a much larger organic primary ammonium cation in
a reasonable design, the 3D perovskite could move to 2D
layered structure owing to steric effects, and the new 2D
structure then exhibits a remarkably improved moisture
stability. Consequently, Cheng et al. tried to use ethylammonium iodine (EAI) as an additive in the perovskite
precursor solution and found that a small amount of EAI
could signicantly affect the perovskite lm morphology and
the corresponding device performance.139 First, they found
that with the incorporation of a little amount of EAI (0.5%),
the perovskite lm surface roughness decreased while the
surface coverage and grain size increased, as shown in
Fig. 37a–c. Moreover, the crystallization of the perovskite was
improved with the incorporation of EAI, resulting in fewer
defects in the crystal. The better perovskite morphology and
Review
crystallization are benecial for a higher resistance shunting
path and available photocurrent, which are important for
improvement of the device performance. Devices with the
incorporation of EAI demonstrated a PCE of 10.2 0.58%,
while the control device showed a PCE of 9.4 0.76%
(Fig. 37d). Most importantly, the device thermal stability was
improved with the incorporation of EAI (Fig. 37e). The nal
perovskite may be a 2D–3D mixed structure, which has been
suggested to be important in the improvement of stability.
However, it has also recently been proven that EA+ can be
incorporated coordinately with MA+ in the lattice of a 3D
perovskite in MAPbI3 perovskite single crystals due to
a balance of opposite lattice distortion strains.140 A higher
crystal symmetry, improved material stability, and markedly
enhanced charge carrier lifetime could thus be achieved. This
crystal engineering strategy may be useful for 3D perovskite
lms to tailor their optoelectronic and environmental
properties.
The dimensionality of the perovskite structure is determined by the value of t as a function of ionic size. For one
thing, the t of EAI-based perovskite (EAPbI3) is 1.05, which falls
out of the range of 0.9–1.0 for cubic perovskites. For the other
thing, the ethyl group is too short to aggregate and induce a 2D
perovskite by van der Waals. Accordingly, Nazeeruddin et al.
used an aliphatic uorinated amphiphilic additive, namely
1,1,1-triuoro-ethyl ammonium iodide (FEAI), as shown in
Fig. 38a, in the perovskite precursor solution and fabricated
a corresponding device with improved performance and
stability.141 They found that the uorinated counterparts of
FEAPbI3 could form 1D 2H perovskite due to the fact that the
Fig. 37 AFM images of a perovskite fabricated: (a) without EAI, (b) with 0.5% EAI and (c) with 1% EAI. (d) J–V curves of a device with various
concentrations of EAI. (e) Long-term performance of perovskite cells prepared with various concentrations of EAI. Reproduced with permission
from ref. 139. Copyright 2015 Royal Society of Chemistry.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
Journal of Materials Chemistry A
Fig. 38 (a) Molecular model of FEAI. (b) SEM images of a perovskite film without FEAI and with 3% FEAI. (c) J–V curves of the best performance
device with 3% FEAI with respect to the forward and reverse scan directions. Aging tests of the device: (d) without FEAI and (e) with 3% FEAI. (f)
Schematic model of the FEAI intercalated on the crystal surface of a 3D perovskite MAPbI3. Reproduced with permission from ref. 141. Copyright
2016 Wiley-VCH.
uorine atom is even larger than hydrogen. Fig. 38b presents
the perovskite lm fabricated with a small amount of FEAI,
which showed a better morphology with a smooth surface,
uniform crystal size, and high continuity. They also found that
the incorporation of FEAI had no inuence on the intrinsic
crystal structure of the pristine material. However, they came
to the conclusion that a small amount of FEAI leads to
a dramatically smaller but more uniform crystallite size. The
best control device exhibited an average PCE of 15.6% with
a Voc of 1.02 V, Jsc of 20.07 mA cm2, and an FF of 0.73, while
the best device fabricated with the FEAI additive demonstrated
a PCE of 18% with a Voc of 1.08 V, Jsc of 21.2 mA cm2, and an
FF of 0.76. The device with FEAI incorporation showed no
obvious hysteresis (Fig. 38c). More importantly, with the
incorporation or FEAI, the stability of the device was greatly
improved (Fig. 38d and e). This may be due to the fact that
This journal is © The Royal Society of Chemistry 2017
FEAPbI3 fabricated from FEAI was found to form a 1D 2H
perovskite, which helps to enhance the moisture resistance
and graing of FEAI on the surface of MAPbI3 crystals
(Fig. 38f).
As well as works about saturated organic amine halide
additives with larger organic cations, unsaturated organic
amine halide have been employed as an additive to improve
the performance of perovskite solar cells. Yang et al. added
a small amount of guanidinium chloride (GACl) into
a perovskite precursor solution and found that the device
with GACl additive exhibited enhanced performance.142
Specically, the device without GACl additive showed a PCE
of 16.35%, with a Voc of 1.025 V, Jsc of 21.27 mA cm2, and an
FF of 75%, while the device with GACl additive demonstrated
a PCE of 17.13%, with a Voc of 1.071, Jsc of 21.24 mA cm2, and
an FF of 75.31% (Fig. 39a). The improved Voc was responsible
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
Review
Fig. 39 (a) J–V curves of the best device with the MA additive and GA additive. SEM images of: (b) perovskite film with the MA additive and (c)
perovskite film with the GA additive. AFM images of (d) perovskite film with the MA additive and (e) perovskite film with the GA additive. (f) Steadystate and (g) time-resolved PL spectra of the perovskite film with the MA additive and GA additive. Reproduced with permission from ref. 142.
Copyright 2016 American Chemical Society.
for the enhancement of the PCE due to the higher grain
continuity with fewer small grain protrusions and less
prominent grain boundaries in the GA-based perovskite lm
(Fig. 39b and c). Moreover, Fig. 39f and g show that the PL
intensity and charge carrier lifetime of the GACl-based
perovskite lm were remarkably enhanced, indicating the
reduced trap density in the lm. They found that GA+ does not
directly replace MA+ into the perovskite crystal lattice due to
the larger ion radius. Actually, GA+ tends to resides at the
grain boundaries and forms hydrogen bonds with undercoordinated iodine species to effectively suppress these
charge trapping/recombination regions. Moreover, the
hydrogen-bonding capability between neighboring grains
may aid in grain growth during the lm formation, thereby
facilitating the higher lm continuity and larger grain
regions. Recently, 2,2,2-triuoroethylamine hydrochloride
(TFEACl), benzenamine hydrochloride (BACl), 3-chloropropylamine hydrochloride (3-CPACl), and diethylamine
hydrochloride (DEACl) were systematically investigated as
additives by Deng.143 These organic cations could accelerate
the perovskite lm formation and consequently high-quality
perovskite lms could be rapidly produced. Therefore, the
approach has a large potential in the production of high-
J. Mater. Chem. A
quality perovskite lms for low-cost, large-scale, and highperformance devices.
Due to the fact that introducing larger organic amine
cations can tune the dynamic of perovskite formation and
improve the lm morphology, much larger organic amine
cations, e.g., polyethylenimine hydriodide (PEI$HI), have been
used as additives to fabricate perovskite thin lms.144 The
incorporation of PEI$HI rst facilitated the growth of 2D
perovskite (PEI)2PbI4 while restraining the growth of the 3D
perovskite crystal, as demonstrated in Fig. 40a. It is interesting
that with the incorporation of 2D perovskite (PEI)2PbI4, the
nal perovskite thin lm showed a decent surface with a dense
grain and no pinholes (Fig. 40(b–d)). Such improvement can be
attributed to the fact that the presence of 2D perovskite
(PEI)2PbI4 slows down the growth and aggregates the perovskite crystal, which leads to a smooth perovskite lm with
appropriate domain sizes. The best device with 2% PEI$HI
demonstrated a PCE of 15.2% with a Jsc of 19.3 mA cm2, Voc of
1.08 V, and an FF of 72.9%, while the control device exhibited
a PCE of 14.6% with a Jsc of 19.7 mA cm2, Voc of 1.04 V, and an
FF of 71.2% (Fig. 40e). Furthermore, because the 2D perovskite
had inherent moisture resistance while the nal perovskite
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
Journal of Materials Chemistry A
Fig. 40 (a) Scheme of the two-step spin-coating procedure for mixed perovskites. SEM images of: (b) perovskite film without the PEI$HI additive,
(c) perovskite film with 2% PEI$HI additive, and (d) perovskite film with 4% PEI$HI additive. (e) J–V curves of the device without and with different
contents of PEI$HI. (f) Stability of devices without and with different contents of the PEI$HI additive. Reproduced with permission from ref. 144.
Copyright 2015 Royal Society of Chemistry.
lm was dense, devices with the incorporation of PEI$HI
showed enhanced moisture stability (Fig. 40f).
Organic ammonium halide additives can signicantly
improve the microstructures of perovskite lms. However,
interfacial properties between perovskites and the charge
transporting layer cannot be ruled out. Yip et al. further
introduced organic phosphonium halides, e.g., tetrabutyl
phosphonium iodide salts (TBPI), and tetraphenylphosphonium iodide (TPPI), bromide (TPPBr), and chloride (TPPCl) as
additives, in the perovskite precursor solution, as shown in
Fig. 41a.145 Tetrabutyl ammonium iodide salts (TBAI) were
also used as a reference. It was found that every perovskite
thin lm with such an additive exhibited a higher surface
coverage with a reduced pinhole size and higher crystallization compared with that without the additive (Fig. 41b). They
speculated that both the alkyl-containing ammonium and
phosphonium cations exert an effect on modulating the lm
morphology by interacting with the perovskite crystallographic plane terminated with the corner-sharing metal
halide octahedrals and formed a layered organic–inorganic
hybrid structure. More importantly, they found that the
phosphonium halide is a good interfacial modier and could
improve the charge extraction in the planar heterojunction
This journal is © The Royal Society of Chemistry 2017
perovskite solar cells. Finally, the controlled device showed
a PCE of 9.6%, while that with tetra-phenylphosphonium
iodide (TPPI) and chloride (TPPCl) additives demonstrated
a PCE of 11.7% and 11.5%, respectively (Fig. 41c). Moreover,
the device with TPPI used as both a processing additive and
interfacial modier reached a PCE of 13% with an FF of 0.73
(Fig. 41d).
The above works are all about organic halide additives with
just one functional group, in fact, alkylcarboxylic acid uammonium bifunctional molecules have also been used as
templates to direct nucleation or crystal growth.146,147 Recently,
Han et al. incorporated 5-ammoniumvaleric acid (5-AVA) iodide
into the perovskite precursor solution and the perovskite lm
was formed by drop-casting the perovskite precursor solution
through a porous carbon lm, as shown in Fig. 42a.11 The
resultant (5-AVA)X(MA)1XPbI3 perovskite lm exhibited
enlarged grain size and better coverage (Fig. 42b and c).
The authors concluded that the 5-AVA cation partially
replaces the MA+ cation as the template cation to form the
(5-AVA)X(MA)1XPbI3 perovskite, in which the COOH groups
form hydrogen bonds with NH3+ groups and I ions from the
PbI6 octahedra. The COOH groups of 5-AVA anchor a monolayer
of the amino acid to the surface of the mesoporous TiO2 and
J. Mater. Chem. A
View Article Online
Review
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
(a) Device configuration and chemical structure of the additives used in the study. (b) SEM images of the perovskite thin film without an
additive and with different additives. (c) J–V curves of the device without an additive and with different additives. (d) J–V curves of pristine
perovskite and TPPI-doped perovskite solar cells with a TPPI interlayer. Reproduced with permission from ref. 145. Copyright 2016 Wiley-VCH.
Fig. 41
ZrO2 lm by coordinatively binding to the exposed Ti(IV) or Zr(IV)
ions. In the adsorbed state, the terminal –NH3+ groups of 5-AVA
face the perovskite solution and hence serves as the nucleation
sites. Moreover, they found newly emerging diffraction peaks
(001) and (111), which indicated that the (5-AVA)X(MA)1XPbI3
crystal grew along a certain dominant orientation (Fig. 42d),
J. Mater. Chem. A
which ensured that the (5-AVA)X(MA)1XPbI3 perovskite had
a better surface contact with the TiO2 surface. The better
morphology resulted in a longer exciton lifetime and a higher
quantum yield to photoinduce charge separation. Accordingly,
the control device showed a PCE of 7.2% with a Voc of 0.855 V, Jsc
of 13.9 mA cm2, and an FF of 0.61, while the device with 5-AVA
This journal is © The Royal Society of Chemistry 2017
View Article Online
Journal of Materials Chemistry A
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
Fig. 42 (a) The structure of the test device. SEM images of a cross-section of the device: (b) without 5-AVA and (c) device with 5-AVA. (d) XRD
patterns of perovskite with and without 5-AVA. (e) J–V curves of the device with and without 5-AVA. (f) J–V curves of the best device obtained
from different scanning directions. (g) Stability test of the 5-AVA-based device in full AM 1.5 simulated sunlight in ambient air over 1008 h without
encapsulation. Reproduced with permission from ref. 11. Copyright 2014 the American Association for the Advancement of Science.
demonstrated a PCE of 11.6% with a Voc of 0.843 V, Jsc of
21.1 mA cm2, and an FF of 0.65 (Fig. 42e). Additionally, with
5-AVA incorporation, the device showed no obvious hysteresis
and a high stability in air (Fig. 42f and g).
It can be seen that 5-AVA was incorporated into the perovskite lattice. In principle, the introduced organic ammonium
cations are better suited to occupy cuboctahedral sites at the
surface of the perovskite crystals without entering into their
lattice.148 Accordingly, Grätzel et al. employed the bifunctional
alkylphosphonic acid u-ammonium chlorides additives, e.g.,
butylphosphonic acid 4-ammonium chloride (4-ABPACl), to
chemically modify the perovskite grain surface.149 They
proposed that the phosphonic acid ammonium additive acted
as a cross-link between neighboring grains in the perovskite
structure, as exhibited in Fig. 43a. Hydrogen bonds were really
This journal is © The Royal Society of Chemistry 2017
important in their design (Fig. 43b), and they proposed that the
NH3+ group of 4-ABPACl occupies an empty A site at the
perovskite surface while the phosphonic acid moiety undergoes
hydrogen bonding with the iodide ions of the inorganic PbI64
octahedra exposed at the surface of neighboring perovskite
crystals. The presence of such hydrogen-bonding interactions is
supported by the results of the SEM analysis (Fig. 43c). An
extremely uniform and full-coverage lm was achieved with the
addition of 4-ABPACl to the perovskite precursor solution. This
enhanced the absorption and photovoltaic performance from
8.8% to 16.7%, as presented in Fig. 43d and e. More importantly, the device with the 4-ABPACl-modied perovskite
lm had signicant resistance to moisture (Fig. 43f). The
introduction of such a bifunctional alkylphosphonic acid uammonium, which can passivate the perovskite surface, control
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
Review
Fig. 43 (a) Schematic illustration of two neighboring grain structures. (b) Representation of the supramolecular hydrogen-bonding interaction in
the CsPbI3 or MAPbI3 precursor solution containing 4-ABPACl. (c) Surface and cross-sectional SEM images of pristine and 4-ABPA-anchored
(MAPbI3–ABPA) perovskite films. (d) UV-vis spectra of pristine (green) and 4-ABPA-anchored (red) perovskites. (e) J–V curves for the bestperforming devices using pristine (green) and 4-ABPA-anchored (red) perovskites. (f) Variation of PCE with time for unsealed heterojunction solar
cells based on the corresponding perovskite films and stored in ambient air at 55% humidity in the dark. Reproduced with permission from ref.
149. Copyright 2015 Nature Publishing Group.
perovskite growth, and cross-link perovskite crystals, may offer
a new and effective tool toward engineering metal halide
perovskites for large-scale applications.
Apart from the small organic amine ion, some researchers
have also investigated aromatic amine ions. Very recently, Jen
and co-workers found that a controlled amount of phenylethylammonium iodide (PEAI) additive could facilitate the
formation of a black phase of FA perovskite, at the same time
enhancing the device ambient stability and improving the
device performance.150 As shown in Fig. 44a, they fabricated
a perovskite thin lm by a two-step solution method and by
doping the PEAI additive into the FAI/IPA solution. Although
the pure PEA+ could only form yellow 2D perovskite,
a different ratio of FAI/PEAI (N) could form black 2D/3D
perovskite (quasi-3D perovskite). The PEA+ might be
J. Mater. Chem. A
presented both on the lattice surface and within the FAPbI3
grain boundaries through a self-assembly process and could
work as a molecular glue to strengthen the intermolecular
interactions of the FAPbI3 crystals to form a quasi-3D perovskite. Fig. 44b illustrates that all of the FAXPEA1XPbI3 lm is
distinct with 2D perovskite, while Fig. 44c further presents
that with the decrease in N, the full width at half maximum
(FWHM) of the peak at z13.8 is also decreased, which
suggests that there are fewer vacancies and better crystallization with the incorporation of PEA+. When utilizing the
calculation model, they found that when FEA+ was introduced
into FAPbI3, the energy barrier of transition from the black dphase to the yellow a-phase was raised (Fig. 44d). Most
importantly, the incorporation of PEA+ yielded a high device
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
Journal of Materials Chemistry A
Fig. 44 (a) Schematic of the deposition process for the FAXPEA1XPbI3 film together with pictures of the FAXPEA1XPbI3 films, where N is the
molar ratio of FA+ to PEA+ in the FAXPEA1XPbI3 crystal. (b) XRD spectra of the FAXPEA1XPbI3 film. (c) Magnified (111) peak from the XRD patterns.
(d) The thermodynamic analysis and kinetic hypothesis. (e) J–V curves of the best FAXPEA1XPbI3 devices. (f) Stability test of the FAXPEA1XPbI3
device. Reproduced with permission from ref. 150. Copyright 2016 Wiley-VCH.
(a) The ligand-exchange process and the corresponding pictures before and after the ligand-exchange treatment, respectively. (b) SEM
surface images of the perovskite thin film without PbS nanoparticles, and with 0.5 wt%, 1.0 wt%, and 1.5 wt% PbS nanoparticles, respectively. (c)
J–V curves of the device without PbS nanoparticles and with different concentrations of PbS nanoparticles. (d) Proposed nucleation and growth
routes of the perovskite crystal thin film without and with PbS nanoparticles. Reproduced with permission from ref. 151. Copyright 2016 Royal
Society of Chemistry.
Fig. 45
This journal is © The Royal Society of Chemistry 2017
J. Mater. Chem. A
View Article Online
Journal of Materials Chemistry A
PCE with no obvious hysteresis and higher stability (Fig. 44e
and f).
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
2.7
Nanoparticles additives
Recently, nanoparticles have become a new family of additives
incorporated into perovskite thin lm. Each class of nanoparticles, however, has a unique effect on the formation of
perovskite thin lm due to its different chemical and physical
characteristics. On the one hand, nanoparticles function as
effective nucleation sites to promote the formation of perovskite
lattice structures. On the other hand, nanoparticles enhance the
crystallinity of perovskites and help achieve large grain size.
Moreover, the photophysical characteristics of perovskites can
be signicantly regulated on a certain scale.
Chen et al. proposed a novel way to tune the lm quality of
perovskite by incorporating a PbS nanoparticle additive as an
effective heterogeneous nucleation site to promote the
Review
formation of the inorganic framework of perovskite lattice
structures.151 By ligand-exchange treatment, as depicted in
Fig. 45a, the MAI-capped PbS nanoparticles were intermixed
with the pristine perovskite precursor solution (MAI and PbCl2)
for device fabrication. As demonstrated in Fig. 45b, large
perovskite crustal grains were achieved with increasing the
concentration of the MAI-capped PbS nanoparticles. The pristine device showed a PCE of 14.11%, Voc of 0.99 V, Jsc of 21.01
mA cm2, and an FF of 0.68, while the device with the MAIcapped PbS nanoparticles additives demonstrated a PCE of
15.10%, Voc of 0.99 V, Jsc of 20.90 mA cm2, and an FF of 0.73
(Fig. 45c). An intermixing-seeded growth technique was nally
proposed by including a small amount of seed-like precursorcapped MAI-capped PbS nanoparticles during lm growth,
which could effectively modulate the nucleation and growth of
the perovskite crystals (Fig. 45d). This work provided a novel
Fig. 46 (a) Structure of the test device. (b) J–V curves of the device with and without Au@SiO2 NPs; inset table shows the performance of the
corresponding devices. (c) Steady-state and (d) time-resolved PL spectra of the device without the additive and with SiO2 and Au@SiO2 NPs. (e)
Time-integrated PL spectra as a function of temperature for the perovskite films on Al2O3-only and on Al2O3 films mixed with Au@SiO2 NPs.
Reproduced with permission from ref. 157. Copyright 2013 Nano Letters.
(a) SEM images of a perovskite thin film without and with different concentrations of MOF-525 nanocrystals. (b) J–V curves of the device
without and with different concentrations of MOF-525 nanocrystals. Reproduced with permission from ref. 164. Copyright 2015 Wiley-VCH.
Fig. 47
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Review
rst time reported that a reduced exciton binding energy was
achieved instead of plasmonic effects by incorporating core–
shell Au@SiO2 nanoparticles into the perovskite lm and hence
an enhanced generation of free charge carriers was achieved, as
presented in Fig. 46a.157 Therefore, the photocurrent of the
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
approach to fabricate high-performance perovskite-based solar
cells with a planar heterojunction.
Many reports have demonstrated the enhancements in light
absorption through plasmonic effects by incorporating metal
nanoparticles into solar cells.152–156 Snaith et al., however, for the
Journal of Materials Chemistry A
Fig. 48 (a) Structure of the test device and schematic diagrams of the incorporation of sulfonated carbon nanotubes (s-CNTs) into perovskite
films and the preparation of s-CNTs from pristine CNTs. (b) Scheme for the formation process of CNT or s-CNTs incorporated into the perovskite
thin film. SEM surface images with different magnifications of: (c and f) the perovskite with CNTs, (d and g) the perovskite without any additives,
and (e and h) perovskite with s-CNTs. (i) J–V curves of the perovskite film without and with different additives. Reproduced with permission from
ref. 167. Copyright 2016 Royal Society of Chemistry.
This journal is © The Royal Society of Chemistry 2017
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
corresponding devices was signicantly improved (Fig. 46b).
From time-resolved and steady-state PL measurements, as
shown in Fig. 46c and d, it could be seen that the metal nanoparticles quench the PL at room temperature. By eliminating
the energy transfer to the metal nanoparticles and by speeding
up the nonradiative or radiative decay due to the enhanced
photocurrent, they concluded that ionization of the exciton and
enhanced charge separation occurred. By tting the integrated
PL, they eventually determined exciton binding energies of 98
meV for the control samples, which dropped to 35 meV for the
samples incorporating Au@SiO2 nanoparticles as additives
(Fig. 46e).
Metal–organic frameworks (MOFs) are a class of 3D porous
crystalline materials constructed of organic linkers and
metal-based nodes,158,159 and have been widely used in many
areas due to their regular nanostructured pores and high
surface area.160–163 The porous MOFs, therefore, could be
suitable to construct mesoporous structure in planar heterojunction perovskite solar cells. Ho et al. introduced MOF
nanocrystals as additives incorporated into the perovskite
precursor solution to enhance the crystallinity of the perovskite thin lm onto porous MOFs.164 They found that the
uniform pore size of MOFs allowed them to penetrate in to
the perovskite precursor solution to facilitate the formation
of uniform grain size perovskite crystals with an ordered
arrangement, which then could render a better surface
coverage, as illustrated in Fig. 47a. Because of the improved
crystallinity of the obtained perovskite thin lm, the device
with MOF additive showed a PCE of 12.0%, Voc of 0.93 V, Jsc of
23.04 mA cm2, and an FF of 0.60, while the control device
showed a PCE of 10.1%, with a Voc of 0.89 V, Jsc of 22.89 mA
cm2, and an FF of 0.50 (Fig. 47b).
While a morphology with homogeneous and highly crystalline characteristics is a prerequisite to achieve highly efficient perovskite solar cells, grain boundaries, always known
as charge trap centers, are also key. Carbon materials have
attracted great attention for their excellent electrical, optical,
chemical, and mechanical properties, which have been highly
suggested for use in perovskite solar cells to facilitate electron
and hole extraction, e.g., carbon nanotubes (CNTs),165 graphene,157 and ultrathin graphene quantum dots.166 Zhang
Review
et al. found that incorporating sulfonate CNTs (s-CNTs) as an
additive into the perovskite solar cells could be a simple way
to enhance the grain size and to reduce the grain boundary of
the perovskite lm, as shown in Fig. 48a.167 Different from
CNTs, the sulfonic acid (–SO3H) groups bonded to the s-CNTs
have a negative charge, which means they have a strong
interaction with MA+ in the solution (–SO3 exchanges I
from MAI). As a result, a number of MAI molecules will gather
around the s-CNTs, and the perovskites near the s-CNTs will
then be crystallized (Fig. 48b). Therefore, a high-quality
perovskite lm with full surface coverage and micron-sized
grains was produced (Fig. 48c–h). Moreover, s-CNTs were lled in the boundary between the perovskite grains. Accordingly, the best device with s-CNTs exhibited a PCE of 15.2%,
Voc of 0.97 V, Jsc of 20.8 mA cm2, and an FF of 75.1%, while
the optimal device with CNTs showed a PCE of 10.3%, Voc of
0.93 V, Jsc of 17.8 mA cm2, and an FF of 62.0%, and the best
device without any additives demonstrated a PCE of 12.5%,
Voc of 0.95 V, Jsc of 19.1 mA cm2, and an FF of 68.8%
(Fig. 48i). The authors thought that such an improvement of
the device with s-CNTs and the decreased performance with
CNTs could be attributed to the change in grain size and grain
boundary of the perovskite lm when incorporating the
additives to the perovskite precursor solution. The pristine
CNTs were only dispersed in the precursor solution, i.e.,
without any interaction with the component. When the DMF
solvent was extracted by chlorobenzene dripping during the
perovskite precursor solution spin-coating process, the CNTs
separated out to suppress the perovskite grain growth, thus
affording small perovskite grains. However, the continuous
presence of MAI could enhance the grain size of the perovskites. Upon heating the lm, the interaction between MAI
and the s-CNTs continued to promote the perovskite grain
growth, and the s-CNTs nally exist in the grain boundary.
As shown above, the origin of the reported improvements
can be classied in three parts: rst, additives can interact
strongly with components in the perovskite solution by chemistry or physics paths, including via chelation, hydrogen bonds,
and physical absorption. Second, additives can passivate the
perovskite lm through the halide-induced deep traps, which
can facilitate charge extraction. Third, additives can regulate
Fig. 49 (a) Average domain size as a function of iodide loading during fabrication. (b) PbMAI3/PbMACl3 peak ratio. (c) J–V curves of the device
with different concentrations of I2 additive. Reproduced with permission from ref. 108. Copyright 2015 American Chemical Society.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
Journal of Materials Chemistry A
Fig. 50 (a) Schematic of the formation of a perovskite film with/without IL. AFM images of MAPbI3 films prepared by spin-coating: (b) without and
(c) with the addition of IL to control the morphology. (d) Schematic of the formation mechanism of MAPbI3 NPs. (e) J–V curves obtained for the
solar cells based on MAPbI3 with and without IL. Reproduced with permission from ref. 171. Copyright 2015 Royal Society of Chemistry.
the crystal structure of the perovskite by replacing anions or
cations due to the similar ion radius to modulate the crystallization kinetics.
2.8
Other additives
Wang et al. used solid iodine as an additive to the perovskite
precursor solution and realized puried MAPbI3 perovskite
crystals.168 The MAPbCl3 phase was always accompanied in
their perovskite precursor system, which is the kinetically
favored product, whereas the MAPbI3 perovskite is the thermodynamically favored product. With the inclusion of I2
additive in the precursor solution, the nal concentration of
MAPbCl3 in the perovskite lm with a decreased grain size was
signicantly reduced, as demonstrated in Fig. 49a and b. In the
presence of I2, MAPbI3 was formed through the proposed
reactions:
3PbI2 + 3MACl / 2PbMAI3 + PbMACl3
5PbI2 + 5MACl + 2I2 / 4PbMAI3 + PbMACl3 + 2ICl(g)[
During the high-temperature annealing process, ICl ultimately escaped, which contributed to the target product being
formed. As a result, the device with the I2 additive showed a PCE
of 15.58%, Voc of 0.877 V, Jsc of 22.31 mA cm2, and an FF of
79.70%, while the device without the I2 additive demonstrated
a PCE of 9.83%, Voc of 0.87 V, Jsc of 16.02 mA cm2, and an FF of
70.51% (Fig. 49c).
It is a good strategy to construct perovskite nanostructures
for perovskite solar cells. MAPbBr3 nanoparticles and MAPbI3
nanowires have previously been synthesized for perovskite solar
This journal is © The Royal Society of Chemistry 2017
(a) Schematic of the proposed perovskite crystal growth
mechanism as controlled by the inclusion of formate anions. (b) Topview SEM images of perovskite films after 1 h annealing at 100 C. Grain
sizes distribution as estimated from the SEM images (right-hand side).
(c) J–V curves of the devices with and without MAF additive. (d)
Maximum PCE as a function of time. Reproduced with permission
from ref. 173. Copyright 2016 Wiley-VCH.
Fig. 51
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
cells with high performance.169,170 Recently, MAPbI3 nanoparticles were prepared by Taima et al. by employing the ionic
liquid (IL) 1-hexyl-3-methylimidazolium chloride as an additive
in the perovskite precursor solution, as depicted in Fig. 50a.171
Smooth uniform nanoparticles were observed by AFM following
the inclusion of the IL (Fig. 50b and c). The formation mechanism for the MAPbI3 nanoparticles is illustrated in Fig. 50d.
During the spin-coating process, small clusters were formed
due to the high boiling point of IL and the interaction with
perovskite, followed by homogeneous nucleation to generate
NPs. However, due to the presence of IL in the lms, the PCE of
the device with the IL was lower than that of the reference device
(Fig. 50e).
Furthermore, Moore et al. demonstrated that methylammonium formate (MAF) can be used as a solvent to
produce high-quality, crystalline perovskite lms.172 Since
HCOO is known to form a metal–organic complex with Pb2+,
Review
the HCOO may slow the crystal growth of the perovskite by
interacting with Pb2+.173 Therefore, Abate et al. used a small
amount of MAF as an additive to control the morphology of
perovskite lms.174 They proposed a mechanism of perovskite
crystal growth from the MAF-based precursor solution, as
shown in Fig. 51a. First, HCOO coordinates with Pb2+ in
solution (STEP 1); second, the complex HCOO–Pb+ is gradually displaced by ion exchange with I under annealing
(STEP 2); third, HCOO is completely replaced by I, thus
enabling the crystal growth (STEP 3). Once the crystallization
is completed, MAF sits at the surface of the perovskite crystals, as schematically presented in the le-hand side of
Fig. 51a. The SEM images of perovskite lms showed that the
average grain sizes are 170 and 325 nm without and with
MAF, respectively, due to the slower crystal growth inducing
a larger average grain size (Fig. 51b). Although MAF additivebased devices only showed and improved Jsc by 1 mA cm2
Fig. 52 SEM images of a perovskite film fabricated with: (a) non-purified MAI, (b) purified MAI, and (c) purified MAI + 2 wt% MAH2PO2. J–V curves
of the corresponding devices: (d) with non-purified MAI, (e) with purified MAI, and (f) with purified MAI + 2 wt% MAH2PO2 obtained from different
scanning directions. Schematic diagram of the perovskite crystallization process: (g) with purified MAI and (h) with purified MAI + 2% MAH2PO2.
Reproduced with permission from ref. 175. Copyright 2016 Royal Society of Chemistry.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
compared to control devices, as demonstrated in Fig. 51c, the
PCE was stabilized aer about 50 s to 18.9% for the control
and 19.5% for the MAF device (Fig. 52d), thus reaching a new
record for planar devices.
It has been frequently reported that the morphology of
perovskite lms tends to be different in different laboratories
even with the same lm preparation procedure. Huang et al.
found that a critical role of the H3PO2 stabilizer in HI has been
largely ignored, yet it introduces MAH2PO2 impurities into the
synthesized MAI (non-puried MAI) by reacting with MA
aqueous solution.175 In order to verify their hypothesis, therefore, the MAH2PO2 impurity was used in the non-puried MAI
in controlling the perovskite lm, as illustrated in Fig. 52a–c. It
is obvious that the perovskite lm fabricated with a controlled
amount of MAH2PO2 almost had the same morphology as that
fabricated with non-puried MAI. The devices fabricated from
non-puried MAI and puried MAI + 2 wt% MAH2PO2 demonstrated almost the same PCE and both of them had no evident
hysteresis (Fig. 52d and e). The control device fabricated with
puried MAI, however, showed a lower PCE together with
evident hysteresis (Fig. 52f). The authors explained that the
incorporation of MAH2PO2 in the perovskite precursor solution
reacted with PbI2 and formed a Pb(H2PO2)2 intermediate phase,
which impeded the formation of MAPbI3 (Fig. 52g and h). The
slower formation of MAPbI3 resulted in a smooth and uniform
perovskite lm, which nally achieved a better device
performance.
Some researchers are concerned with the inorganic
ammine halide additives. Liang et al. reported a facile
method to fabricate high-quality perovskite lm without the
need for an annealing process by the incorporation of an
NH4Cl additive to the perovskite precursor solution, as shown
in Fig. 53a.176 The perovskite lm fabricated with the NH4Cl
Journal of Materials Chemistry A
additive showed a much smoother surface with small-sized
compactly packed crystallites yet no apparent crystal gain
boundaries (Fig. 53b). Such a change may be due to the fact
that ammonia can reversibly intercalate into or form a new
coordination complex with the perovskite crystal lattice or
network, thereby avoiding the formation of elongated large
crystal plates and incomplete coverage.177 The improved lm
morphology also resulted in a higher ll factor, which is
consistent with the previous work by Ding et al.178 The control
device showed a PCE of 6.26%, with a Voc of 0.73 V and an FF
of 60.38%, while the device with the NH4Cl additive demonstrated a PCE of 9.32%, with a Jsc of 13.11 mA cm2, Voc of
0.91 V, and an FF of 78.11%, as shown in Fig. 53c and d.
Additionally, they fabricated the perovskite on the PET
substrate, and the corresponding device showed no obvious
hysteresis (Fig. 53e).
Recently, Adachi et al. used benzoquinone (BQ) as an
additive in a precursor solution to optimize perovskite
lms.179 As depicted in Fig. 54a, during the spin-coating
process, BQ competed with PbI2 to react with MAI and form
a 3D perovskite, thus reducing the speed of the perovskite
crystal formation and resulting in a at, uniform perovskite
lm with an enlarged crystal size (Fig. 54b and c). Moreover,
as shown in Fig. 54d, the perovskite lm incorporated with
BQ become more dense and was connected with neighboring
layers, which is remarkably important for efficient charge
transport. The high-quality perovskite lm led to an
enhanced absorption intensity over the entire wavelength
range and a high device efficiency with no obvious hysteresis
(Fig. 54e and f). As reported, elemental defects of metallic
lead stemming from Frenkel defects are the main source of
carrier traps, which directly affect the device performance. BQ
is a weak oxidant, thus it can reduce the formation of metallic
Fig. 53 (a) Schematic depiction of the test device architecture. SEM images of: (b) perovskite film without NH4Cl, (c) the perovskite film with
NH4Cl. (d, e) J–V curves of the devices without and with NH4Cl treatment. (f) J–V curves of the flexible device on PET substrates; inset shows
a photograph of such a device. Reproduced with permission from ref. 176. Copyright 2016 American Chemical Society.
This journal is © The Royal Society of Chemistry 2017
J. Mater. Chem. A
View Article Online
Review
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
Fig. 54 (a) Proposed perovskite film formation process. SEM images of perovskite film: (b) without BQ additive and (c) with 0.5% BQ additive. (d)
SEM cross-sectional image of a perovskite solar cell with 0.5% BQ additive. (e) Absorption spectra of a perovskite film with and without BQ
additive. (f) J–V curves of the device with and without BQ additive obtained from different scanning directions. (g) Stability test of devices with
and without BQ additive. (h) XPS data for the Pb 4f core-level spectra of the perovskite film with and without BQ additive. The inset summarizes
the relative amounts of metallic and compound lead. Reproduced with permission from ref. 179. Copyright 2016 Wiley-VCH.
J. Mater. Chem. A
This journal is © The Royal Society of Chemistry 2017
View Article Online
Review
lead and can work as a trap passivation agent to reduce the
carrier traps (Fig. 54h). Therefore the BQ-based device
exhibited a higher ambient stability (Fig. 54g).
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
3.
Conclusions and outlook
In this review, we critically reviewed recent advances in the use
of additives on the morphology, stability, excitonic properties,
and optoelectronics of organic–inorganic hybrid perovskite
lms and the performance of related devices fabricated with
such systems. So far, several additives to engineer hybrid
perovskites have been reported, such as polymers, fullerene,
metal halide salts, organic halide salts, inorganic acids,
solvents, and nanoparticles. The interactions between the
perovskite and additives were conducted, on the one hand, by
either chelating with Pb2+ or by forming hydrogen bonds with H
atom in CH3NH3+ or by integrating into the crystal lattice. On
the other hand, the additives could be distributes at the grain
boundary, passivating the perovskite and thus reducing the
nonradiative recombination, consequently improving the
device performance. Moreover, additives may react with PbI2 to
form pre-crystallized intermediate states to adjust the growth
rate of perovskite. As a result, additives are able to signicantly
impact the crystallization kinetics of perovskites, suppress trap
formation, enhance the long-term stability, tailor the electronic
structure, and improve optoelectronic properties.
Perovskite solar cells have reached efficiencies of over 22% in
the small scale and over 20% with large areas (1 cm2), whereas
the state-of-the-art performances still have room for improvement. To further improve the efficiencies, additives have the
great possibility to enable considerable further improvement of
the crystal growth and of the electronic qualities of perovskite
thin lms, and the subsequent devices fabricated with such
lms, through a greater understanding of the role of additives
upon crystallization and the crystal and electronic defects in
perovskite thin lms. Although great effort has been made in
the additive engineering of perovskite photovoltaics, our
current knowledge of the function of additives on perovskite
lm formation is highly limited and needs to be further
explored and challenged. Most of the choices of additives have
been limited to a few existing and well-studied materials.
Exploring new materials as additives with target (e.g., long-term
stability or high efficiency) orientation or by combining these
existing additives with different functions can thus help further
improve the lm quality and should be the next direction for
researchers. Theoretical calculations are still missing, but could
play a major role in further advancing the design of additives.
4. Perspective
Right now, more than 30 additives have been employed but
there is still no general concept as guidance for the rational
selection for the design of appropriate additives, which needs to
be established in the near future. From a view point of hysteresis and FF, the best additive could be fullerene and its derivatives. From a view point of Voc, additives with chloride are
highly recommended. A high Jsc may be related to the
This journal is © The Royal Society of Chemistry 2017
Journal of Materials Chemistry A
morphology with large-scale crystals. Currently, one current
limitation of additives development is how to control the
micromorphology of the perovskite layer and the moistureresistant properties by additives. Although the morphology
can be improved by employing additives, the relationship
between the stability and additive still needs to be further
explored. For further constructing additive systems in perovskite component solution, one challenge is to understand the
crystallization from the composition of the solutions with
additives to the nal crystallized lms, which is still in its
infancy. The other is to understand the fundamental physics
and chemistry of the additives, which could assist creating new
insights in controlling the optical, electronic, and chargetransport properties of perovskite lms. All in all, we believe
that additive engineering has a great future and could offer
insights to help obtain efficient, stable organic–inorganic
hybrid perovskite solar cells for future applications.
Acknowledgements
This work was nancially supported by the National Basic
Research Program of China-Fundamental Studies of Perovskite
Solar Cells (2015CB932200), the Natural Science Foundation of
China (51035063), Natural Science Foundation of Jiangsu Province, China (55135039, 55135040), Jiangsu Specially-Appointed
Professor program (Grant No. 54907024), and Startup from Nanjing Tech University (3983500160, 3983500151, and 44235022).
References
1 T. Leijtens, G.-E. Eperon, N.-K. Noel, S.-N. Habisreutinger,
A. Petrozza and H.-J. Snaith, Adv. Energy Mater., 2015, 5,
1500963.
2 A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, J. Am.
Chem. Soc., 2009, 131, 6050.
3 M.-M. Lee, J. Teuscher, T. Miyasaka, T.-N. Murakami and
H.-J. Snaith, Science, 2012, 338, 643.
4 S.-D. Stranks, G.-E. Eperon, G. Grancini, C. Menelaou,
M.-J.-P. Alcocer, T. Leijtens, L.-M. Herz, A. Petrozza and
H.-J. Snaith, Science, 2013, 342, 341.
5 J. Burschka, N. Pellet, S. Moon, R. Humphry-Baker, P. Gao,
M.-K. Nazeeruddin and M. Graetzel, Nature, 2013, 499, 316.
6 M. Liu, M.-B. Johnston and H.-J. Snaith, Nature, 2013, 501,
395.
7 D. Liu and T.-L. Kelly, Nat. Photonics, 2013, 8, 133.
8 J.-H. Heo, S.-H. Im, J.-H. Noh, T.-N. Mandal, C. Lim,
J.-A. Chang, Y.-H. Lee, H. Kim, A. Sarkar,
M.-K. Nazeeruddin, M. Graetzel and S.-I. Seok, Nat.
Photonics, 2013, 7, 487.
9 J. Im, I. Jang, N. Pellet, M. Grätzel and N. Park, Nat.
Nanotechnol., 2014, 9, 927.
10 M.-A. Green, A. Ho-Baillie and H.-J. Snaith, Nat. Photonics,
2014, 8, 506.
11 A. Mei, X. Li, L. Liu, Z. Ku, T. Liu, Y. Rong, M. Xu, M. Hu,
J. Chen, Y. Yang, M. Grätzel and H. Han, Science, 2014,
345, 295.
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
12 X. Huang, Z. Zhao, L. Cao, Y. Chen, E. Zhu, Z. Lin, M. Li,
A. Yan, A. Zettl, Y.-M. Wang, X. Duan, T. Mueller and
Y. Huang, Science, 2015, 348, 1230.
13 http://www.Nrel.gov/ncpv/images/efficiency
chart.jpg,
access time, January 1, 2017.
14 J. Berry, T. Buonassisi, D.-A. Egger, G. Hodes, L. Kronik,
Y. Loo, I. Lubomirsky, S.-R. Marder, Y. Mastai, J.-S. Miller,
D.-B. Mitzi, Y. Paz, A.-M. Rappe, I. Riess, B. Rybtchinski,
O. Stafsudd, V. Stevanovic, M.-F. Toney, D. Zitoun,
A. Kahn, D. Ginley and D. Cahen, Adv. Mater., 2015, 27,
5102.
15 Y. Ogomi, A. Morita, S. Tsukamoto, T. Saitho, N. Fujikawa,
Q. Shen, T. Toyoda, K. Yoshino, S.-S. Pandey, T. Ma and
S. Hayase, J. Phys. Chem. Lett., 2014, 5, 1004.
16 H.-S. Jung and N. Park, Small, 2015, 11, 10.
17 H.-J. Snaith, J. Phys. Chem. Lett., 2013, 4, 3623.
18 T. Song, Q. Chen, H. Zhou, C. Jiang, H. Wang, Y.-M. Yang,
Y. Liu, J. You and Y. Yang, J. Mater. Chem. A, 2015, 3, 9032.
19 D. Weber, Z. Naturforsch., B: Anorg. Chem., Org. Chem., 1978,
33, 1443.
20 D. Weber, Z. Naturforsch., B: Anorg. Chem., Org. Chem., 1978,
33, 862.
21 T.-C. Sum and N. Mathews, Energy Environ. Sci., 2014, 7,
2518.
22 M. Grätzel, Nat. Mater., 2014, 13, 838.
23 J. Jeng, Y. Chiang, M. Lee, S. Peng, T. Guo, P. Chen and
T. Wen, Adv. Mater., 2013, 25, 3727.
24 H. Zhou, Q. Chen, G. Li, S. Luo, T. Song, H. Duan, Z. Hong,
J. You, Y. Liu and Y. Yang, Science, 2014, 345, 542.
25 J. Jeng, K. Chen, T. Chiang, P. Lin, T. Tsai, Y. Chang, T. Guo,
P. Chen, T. Wen and Y. Hsu, Adv. Mater., 2014, 26, 4107.
26 H. Choi, C. Mai, H. Kim, J. Jeong, S. Song, G.-C. Bazan,
J.-Y. Kim and A.-J. Heeger, Nat. Commun., 2015, 6, 8348.
27 F. Zuo, S.-T. Williams, P. Liang, C. Chueh, C. Liao and
A.-K.-Y. Jen, Adv. Mater., 2014, 26, 6454.
28 P. Liang, C. Chueh, X. Xin, F. Zuo, S.-T. Williams, C. Liao
and A.-K.-Y. Jen, Adv. Energy Mater., 2015, 5, 1400960.
29 N.-J. Jeon, J.-H. Noh, W.-S. Yang, Y.-C. Kim, S. Ryu, J. Seo
and S.-I. Seok, Nature, 2015, 517, 476.
30 D.-P. McMeekin, G. Sadoughi, W. Rehman, G.-E. Eperon,
M. Saliba, M.-T. Hoerantner, A. Haghighirad, N. Sakai,
L. Korte, B. Rech, M.-B. Johnston, L.-M. Herz and
H.-J. Snaith, Science, 2016, 351, 151.
31 M. Saliba, T. Matsui, K. Domanski, J.-Y. Seo,
A. Ummadisingu, S.-M. Zakeeruddin, J.-P. Correa-Baena,
W.-R. Tress, A. Abate, A. Hagfeldt and M. Grätzel, Science,
2016, 354, 206.
32 W. Nie, H. Tsai, R. Asadpour, J.-C. Blancon, A.-J. Neukirch,
G. Gupta, J.-J. Crochet, M. Chhowalla, S. Tretiak,
M.-A. Alam, H.-L. Wang and A.-D. Mohite, Science, 2015,
347, 522.
33 N. Pellet, P. Gao, G. Gregori, T. Yang, M.-K. Nazeeruddin,
J. Maier and M. Grätzel, Angew. Chem., Int. Ed., 2014, 53,
3151.
34 M.-G.-A. Lapotre, R.-C. Ewing, M.-P. Lamb, W.-W. Fischer,
J.-P. Grotzinger, D.-M. Rubin, K.-W. Lewis, M.-J. Ballard,
M. Day, S. Gupta, S.-G. Banham, N.-T. Bridges, D.-J. Des
J. Mater. Chem. A
Review
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
Marais, A.-A. Fraeman, J.-A. Grant, K.-E. Herkenhoff,
D.-W. Ming, M.-A. Mischna, M.-S. Rice, D.-A. Sumner,
A.-R. Vasavada and R.-A. Yingst, Science, 2016, 353, 55.
Q. Chen, H. Zhou, Z. Hong, S. Luo, H. Duan, H. Wang,
Y. Liu, G. Li and Y. Yang, J. Am. Chem. Soc., 2014, 136, 622.
Z. Xiao, C. Bi, Y. Shao, Q. Dong, Q. Wang, Y. Yuan, C. Wang,
Y. Gao and J. Huang, Energy Environ. Sci., 2014, 7, 2619.
J. You, Z. Hong, Y.-M. Yang, Q. Chen, M. Cai, T. Song,
C. Chen, S. Lu, Y. Liu, H. Zhou and Y. Yang, ACS Nano,
2014, 8, 1674.
N.-J. Jeon, J.-H. Noh, Y.-C. Kim, W.-S. Yang, S. Ryu and S. Il
Seol, Nat. Mater., 2014, 13, 897.
M. Xiao, F. Huang, W. Huang, Y. Dkhissi, Y. Zhu,
J. Etheridge, A. Gray-Weale, U. Bach, Y. Cheng and
L. Spiccia, Angew. Chem., Int. Ed., 2014, 53, 9898.
P. Liang, C. Liao, C. Chueh, F. Zuo, S.-T. Williams, X. Xin,
J. Lin and A.-K.-Y. Jen, Adv. Mater., 2014, 26, 3748.
C. Chang, C. Chu, Y. Huang, C. Huang, S. Chang, C. Chen,
C. Chao and W. Su, ACS Appl. Mater. Interfaces, 2015, 7,
4955.
G.-E. Eperon, V.-M. Burlakov, P. Docampo, A. Goriely and
H.-J. Snaith, Adv. Funct. Mater., 2014, 24, 151.
Y. Zhao, J. Wei, H. Li, Y. Yan, W. Zhou, D. Yu and Q. Zhao,
Nat. Commun., 2016, 7, 10228.
J. Wei, H. Li, Y. Zhao, W. Zhou, R. Fu, Y. Leprince-Wang,
D. Yu and Q. Zhao, Nano Energy, 2016, 26, 139.
Y. Shao, Z. Xiao, C. Bi, Y. Yuan and J. Huang, Nat. Commun.,
2014, 5, 5784.
N. Tripathi, Y. Shirai, M. Yanagida, A. Karen and K. Miyano,
ACS Appl. Mater. Interfaces, 2016, 8, 4644.
Y. Wu, A. Islam, X. Yang, C. Qin, J. Liu, K. Zhang, W. Peng
and L. Han, Energy Environ. Sci., 2014, 7, 2934.
J. Huang, G. Li and Y. Yang, Adv. Mater., 2008, 20, 415.
Y. Sun, J.-H. Seo, C.-J. Takacs, J. Seier and A.-J. Heeger,
Adv. Mater., 2011, 23, 1679.
Y. Matsuo, Y. Sato, T. Niinomi, I. Soga, H. Tanaka and
E. Nakamura, J. Am. Chem. Soc., 2009, 131, 16048.
C. Roldán-Carmona, O. Malinkiewicz, R. Betancur,
G. Longo, C. Momblona, F. Jaramillo, L. Camacho and
H.-J. Bolink, Energy Environ. Sci., 2014, 7, 2968.
C. Chen, L. Dou, J. Gao, W. Chang, G. Li and Y. Yang, Energy
Environ. Sci., 2013, 6, 2714.
L.-K. Ono, S. Wang, Y. Kato, S.-R. Raga and Y. Qi, Energy
Environ. Sci., 2014, 7, 3989.
Y. Guo, K. Shoyama, W. Sato and E. Nakamura, Adv. Energy
Mater., 2016, 6, 1502317.
Y. Wang, J. Luo, R. Nie and X. Deng, Energy Technol., 2016,
4, 473.
H. Spillmann, A. Kiebele, M. Stöhr, T.-A. Jung, D. Bonifazi,
F. Cheng and F. Diederich, Adv. Mater., 2006, 18, 275.
X. Gong, M. Tong, F.-G. Brunetti, J. Seo, Y. Sun, D. Moses,
F. Wudl and A.-J. Heeger, Adv. Mater., 2011, 23, 2272.
W. Li, K.-H. Hendriks, W.-S.-C. Roelofs, Y. Kim,
M.-M. Wienk and R.-A.-J. Janssen, Adv. Mater., 2013, 25,
3182.
R.-S. Ashraf, B.-C. Schroeder, H.-A. Bronstein, Z. Huang,
S. Thomas, R.-J. Kline, C.-J. Brabec, P. Rannou,
This journal is © The Royal Society of Chemistry 2017
View Article Online
Review
60
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
T.-D. Anthopoulos, J.-R. Durrant and I. McCulloch, Adv.
Mater., 2013, 25, 2029.
L. Derue, O. Dautel, A. Tournebize, M. Drees, H. Pan,
S. Berthumeyrie, B. Pavageau, E. Cloutet, S. Chambon,
L. Hirsch, A. Rivaton, P. Hudhomme, A. Facchetti and
G. Wantz, Adv. Mater., 2014, 26, 5831.
M.-C. Scharber, D. Mühlbacher, M. Koppe, P. Denk,
C. Waldauf, A.-J. Heeger and C.-J. Brabec, Adv. Mater.,
2006, 18, 789.
A.-J. Heeger, Adv. Mater., 2014, 26, 10.
H. Kang, G. Kim, J. Kim, S. Kwon, H. Kim and K. Lee, Adv.
Mater., 2016, 28, 7821.
G. Yu, J. Gao, J.-C. Hummelen, F. Wudl and A.-J. Heegeer,
Science, 1995, 270, 1789.
C.-H. Peters, I.-T. Sachs-Quintana, J.-P. Kastrop, S. Beaupré,
M. Leclerc and M.-D. McGehee, Adv. Energy Mater., 2011, 1,
491.
F. Liu, W. Zhao, J.-R. Tumbleston, C. Wang, Y. Gu, D. Wang,
A.-L. Briseno, H. Ade and T.-P. Russell, Adv. Energy Mater.,
2014, 4, 1301377.
X. Guo, N. Zhou, S.-J. Lou, J. Smith, D.-B. Tice,
J.-W. Hennek, R. Ponce Ortiz, J.-T. Lopez Navarrete, S. Li,
J. Strzalka, L.-X. Chen, R.-P.-H. Chang, A. Facchetti and
T.-J. Marks, Nat. Photonics, 2013, 7, 825.
Y. Sun, G.-C. Welch, W.-L. Leong, C.-J. Takacs, G.-C. Bazan
and A.-J. Heeger, Nat. Mater., 2012, 11, 44.
Y. Chen, W. Lin, J. Liu and L. Dai, Nano Lett., 2014, 14, 1467.
Y. Chen, D. Ma, H. Sun, J. Chen, Q. Guo, Q. Wang and
Y. Zhao, Light: Sci. Appl., 2016, 5, e16042.
Y. Chen and D. Ma, J. Mater. Chem., 2012, 22, 18718.
Y. Chen, J. Chen, D. Ma, D. Yan, L. Wang and F. Zhu, Appl.
Phys. Lett., 2011, 98, 243309.
H. Sirringhaus, Adv. Mater., 2014, 26, 1319.
S.-J. Kang, Y. Yi, C.-Y. Kim, K. Cho, J.-H. Seo, M. Noh,
K. Jeong, K.-H. Yoo and C.-N. Whang, Appl. Phys. Lett.,
2005, 87, 233502.
H. Yan, T. Kagata and H. Okuzaki, Appl. Phys. Lett., 2009, 94,
023305.
J. Xu, Y. Chen and L. Dai, Nat. Commun., 2015, 6, 8081.
Y. Chen, T. Chen and L. Dai, Adv. Mater., 2015, 27, 1053.
Z. Xiao, Q. Dong, C. Bi, Y. Shao, Y. Yuan and J. Huang, Adv.
Mater., 2014, 26, 6503.
O. Malinkiewicz, A. Yella, Y.-H. Lee, G. Minguez
Espallargas, M. Graetzel, M.-K. Nazeeruddin and
H.-J. Bolink, Nat. Photonics, 2014, 8, 128.
G. Xing, N. Mathews, S. Sun, S.-S. Lim, Y.-M. Lam,
M. Graetzel, S. Mhaisalkar and T.-C. Sum, Science, 2013,
342, 344.
M.-D. McGehee, Nat. Mater., 2014, 13, 845.
H.-J. Snaith, A. Abate, J.-M. Ball, G.-E. Eperon, T. Leijtens,
N.-K. Noel, S.-D. Stranks, J.-T. Wang, K. Wojciechowski
and W. Zhang, J. Phys. Chem. Lett., 2014, 5, 1511.
H. Kim and N. Park, J. Phys. Chem. Lett., 2014, 5, 2927.
E.-L. Unger, E.-T. Hoke, C.-D. Bailie, W.-H. Nguyen,
A.-R. Bowring, T. Heumueller, M.-G. Christoforo and
M.-D. McGehee, Energy Environ. Sci., 2014, 7, 3690.
This journal is © The Royal Society of Chemistry 2017
Journal of Materials Chemistry A
85 J. Xu, A. Buin, A.-H. Ip, W. Li, O. Voznyy, R. Comin, M. Yuan,
S. Jeon, Z. Ning, J.-J. McDowell, P. Kanjanaboos, J. Sun,
X. Lan, L.-N. Quan, D.-H. Kim, I.-G. Hill, P. Maksymovych
and E.-H. Sargent, Nat. Commun., 2015, 6, 7081.
86 E. Edri, S. Kirmayer, A. Henning, S. Mukhopadhyay,
K. Gartsman, Y. Rosenwaks, G. Hodes and D. Cahen,
Nano Lett., 2014, 14, 1000.
87 C. Liu, K. Wang, P. Du, C. Yi, T. Meng and X. Gong, Adv.
Energy Mater., 2015, 5, 1402024.
88 K. Wang, C. Liu, P. Du, J. Zheng and X. Gong, Energy
Environ. Sci., 2015, 8, 1245.
89 C. Chiang and C. Wu, Nat. Photonics, 2016, 10, 196.
90 E. Gracia-Espino, H. Reza Barzegar, T. Shari, A. Yan,
A. Zettl and T. Wagberg, ACS Nano, 2015, 9, 10516.
91 C. Ran, Y. Chen, W. Gao, M. Wang and L. Dai, J. Mater.
Chem. A, 2016, 4, 8566.
92 Y. Wu, X. Yang, W. Chen, Y. Yue, M. Cai, F. Xie, E. Bi,
A. Islam and L. Han, Nat. Energy, 2016, 1, 16148.
93 J. Pascual, I. Kosta, T. T. Ngo, A. Chuvilin, G. Cabanero,
H. J. Grande, E. M. Barea, I. Mora-Seró, J. L. Delgado and
R. Tena-Zaera, ChemSusChem, 2016, 9, 2679.
94 S. Delbos, EPJ Photovoltaics, 2012, 3, 35004.
95 K.-M. Boopathi, R. Mohan, T. Huang, W. Budiawan, M. Lin,
C. Lee, K. Ho and C. Chu, J. Mater. Chem. A, 2016, 4, 1591.
96 S. Bag and M.-F. Durstock, ACS Appl. Mater. Interfaces, 2016,
8, 5053.
97 T. Duong and H.-M. Kumar, Nano Energy, 2016, 30, 330.
98 M. Abdi-Jalebi, M.-I. Dar, A. Sadhanala, S.-P. Senanayak,
M. Franckevičius, N. Arora, Y. Hu, M.-K. Nazeeruddin,
S.-M. Zakeeruddin, M. Grätzel and R.-H. Friend, Adv.
Energy Mater., 2016, 6, 1502472.
99 Y. Chen, Y. Zhao and Z. Liang, J. Mater. Chem. A, 2015, 3,
9137.
100 M. Jahandar, J. H. Heo, C. E. Song, K.-J. Kong, W. S. Shin,
J.-C. Lee, S. H. Im and S.-J. Moon, Nano Energy, 2016, 27,
330.
101 A.-L. Abdelhady, M.-I. Saidaminov, B. Murali, V. Adinol,
O. Voznyy, K. Katsiev, E. Alarousu, R. Comin, I. Dursun,
L. Sinatra, E.-H. Sargent, O.-F. Mohammed and
O.-M. Bakr, J. Phys. Chem. Lett., 2016, 7, 295.
102 Z. Wang, M. Li, Y. Yang, Y. Hu, H. Ma, X. Gao and L. Liao,
Adv. Mater., 2016, 28, 6695.
103 J.-T. Wang, Z. Wang, S. Pathak, W. Zhang,
D.-W.
DeQuilettes,
F.
Wisnivesky-Rocca-Rivarola,
J. Huang, P.-K. Nayak, J.-B. Patel, H.-A.-M. Yusof,
Y. Vaynzof, R. Zhu, I. Ramirez, J. Zhang, C. Ducati,
C. Grovenor, M.-B. Johnston, D.-S. Ginger, R.-J. Nicholas
and H.-J. Snaith, Energy Environ. Sci., 2016, 9, 2892.
104 M.-H. Kumar, S. Dharani, W.-L. Leong, P.-P. Boix,
R.-R. Prabhakar, T. Baikie, C. Shi, H. Ding, R. Ramesh,
M. Asta, M. Graetzel, S.-G. Mhaisalkar and N. Mathews,
Adv. Mater., 2014, 26, 7122.
105 T.-M. Koh, T. Krishnamoorthy, N. Yantara, C. Shi,
W.-L. Leong, P.-P. Boix, A.-C. Grimsdale, S.-G. Mhaisalkar
and N. Mathews, J. Mater. Chem. A, 2015, 3, 14996.
106 S.-J. Lee, S.-S. Shin, Y.-C. Kim, D. Kim, K.-A. Tae, J.-H. Noh,
J. Seo and S.-I. Seok, J. Am. Chem. Soc., 2016, 138, 3974.
J. Mater. Chem. A
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Journal of Materials Chemistry A
107 J.-H. Heo, D.-H. Song, H.-J. Han, S.-Y. Kim, J.-H. Kim,
D. Kim, H.-W. Shin, T.-K. Ahn, C. Wolf, T. Lee and
S.-H. Im, Adv. Mater., 2015, 27, 3424.
108 J. Kim, J.-S. Yun, X. Wen, A.-M. Souani, C.-F.-J. Lau,
B. Wilkinson, J. Seidel, M.-A. Green, S. Huang and
A.-W.-Y. Ho-Baillie, J. Phys. Chem. C, 2016, 120, 11262.
109 G.-E. Eperon, G.-M. Paternò, R.-J. Sutton, A. Zampetti,
A.-A. Haghighirad, F. Cacialli and H.-J. Snaith, J. Mater.
Chem. A, 2015, 3, 19688.
110 J. Huang, M. Wang, L. Ding, Z. Yang and K. Zhang, RSC
Adv., 2016, 6, 55720.
111 J.-H. Heo, D.-H. Song and S.-H. Im, Adv. Mater., 2014, 26,
8179.
112 N. Bassiri-Gharb, Y. Bastani and A. Bernal, Chem. Soc. Rev.,
2014, 43, 2125.
113 J. Pan, C. Mu, Q. Li, W. Li, D. Ma and D. Xu, Adv. Mater.,
2016, 28, 8309.
114 W. Zhang, S. Pathak, N. Sakai, T. Stergiopoulos,
P.-K.
Nayak,
N.-K.
Noel,
A.-A.
Haghighirad,
V.-M. Burlakov, D.-W. DeQuilettes, A. Sadhanala, W. Li,
L. Wang, D.-S. Ginger, R.-H. Friend and H.-J. Snaith, Nat.
Commun., 2015, 6, 10030.
115 Y. Guo, K. Shoyama, W. Sato, Y. Matsuo, K. Inoue,
K. Harano, C. Liu, H. Tanaka and E. Nakamura, J. Am.
Chem. Soc., 2015, 137, 15907.
116 Y. Guo, W. Sato, K. Shoyama and E. Nakamura, J. Am. Chem.
Soc., 2016, 138, 5410.
117 Y. Liang, Z. Xu, J. Xia, S. Tsai, Y. Wu, G. Li, C. Ray and L. Yu,
Adv. Mater., 2010, 22, E135.
118 S.-J. Lou, J.-M. Szarko, T. Xu, L. Yu, T.-J. Marks and
L.-X. Chen, J. Am. Chem. Soc., 2011, 133, 20661.
119 C. Liu, X. Hu, C. Zhong, M. Huang, K. Wang, Z. Zhang,
X. Gong, Y. Cao and A. J. Heeger, Nanoscale, 2014, 6, 14297.
120 M. Su, C. Kuo, M. Yuan, U. Jeng, C. Su and K. Wei, Adv.
Mater., 2011, 23, 3315.
121 J.-S. Moon, C.-J. Takacs, S. Cho, R.-C. Coffin, H. Kim,
G.-C. Bazan and A. J. Heeger, Nano Lett., 2010, 10, 4005.
122 C. Chueh, C. Liao, F. Zuo, S.-T. Williams, P. Liang and
A.-K.-Y. Jen, J. Mater. Chem. A, 2015, 3, 9058.
123 L. Li, Y. Chen, Z. Liu, Q. Chen, X. Wang and H. Zhou, Adv.
Mater., 2016, 28, 9862.
124 X. Song, W. Wang, P. Sun, W. Ma and Z. Chen, Appl. Phys.
Lett., 2015, 106, 33901.
125 J. You, M.-Y. Yang, Z. Hong, T. Song, L. Meng, Y. Liu,
C. Jiang, H. Zhou, W. Chang, G. Li and Y. Yang, Appl.
Phys. Lett., 2014, 105, 183902.
126 K.-K. Bass, R.-E. McAnally, S. Zhou, P.-I. Djurovich,
M.-E. Thompson and B.-C. Melot, Chem. Commun., 2014,
50, 15819.
127 L. Ling, S. Yuan, P. Wang, H. Zhang, L. Tu, J. Wang, Y. Zhan
and L. Zheng, Adv. Funct. Mater., 2016, 26, 5028.
128 C. Wu, C. Chiang, Z. Tseng, M.-K. Nazeeruddin, A. Hagfeldt
and M. Graetzel, Energy Environ. Sci., 2015, 8, 2725.
129 X. Gong, M. Li, X. Shi, H. Ma, Z. Wang and L. Liao, Adv.
Funct. Mater., 2015, 25, 6671.
130 P. Docampo, F.-C. Hanusch, S.-D. Stranks, M. Döblinger,
J.-M.
Feckl,
M.
Ehrensperger,
N.-K.
Minar,
J. Mater. Chem. A
Review
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
M.-B. Johnston, H.-J. Snaith and T. Bein, Adv. Energy
Mater., 2014, 4, 1400355.
Y. Zhao and K. Zhu, J. Am. Chem. Soc., 2014, 136, 12241.
Y. Xie, F. Shao, Y. Wang, T. Xu, D. Wang and F. Huang, ACS
Appl. Mater. Interfaces, 2015, 7, 12937.
M. Yang, T. Zhang, P. Schulz, Z. Li, G. Li, D.-H. Kim, N. Guo,
J.-J. Berry, K. Zhu and Y. Zhao, Nat. Commun., 2016, 7,
12305.
W. Zhang, M. Saliba, D.-T. Moore, S.-K. Pathak,
M.-T. Hoerantner, T. Stergiopoulos, S.-D. Stranks,
G.-E. Eperon, J.-A. Alexander-Webber, A. Abate,
A. Sadhanala, S. Yao, Y. Chen, R.-H. Friend, L.-A. Estroff,
U. Wiesner and H.-J. Snaith, Nat. Commun., 2015, 6, 10300.
D. Forgacs, M. Sessolo and H.-J. Bolink, J. Mater. Chem. A,
2015, 3, 14121.
J. Qing, H. Chandran, H. Xue, Z. Guan, T. Liu, S. Tsang,
M. Lo and C. Lee, Org. Electron., 2015, 27, 12.
J. Qing, H. Chandran, Y. Cheng, X. Liu, H. Li, S. Tsang,
M. Lo and C. Lee, ACS Appl. Mater. Interfaces, 2015, 7, 23110.
L. Zhao, D. Luo, J. Wu, Q. Hu, W. Zhang, K. Chen, T. Liu,
Y. Liu, Y. Zhang, F. Liu, T.-P. Russell, H.-J. Snaith, R. Zhu
and Q. Gong, Adv. Funct. Mater., 2016, 26, 3508.
H. Hsu, C. Chang, C. Chen, B. Jiang, R. Jeng and C. Cheng,
J. Mater. Chem. A, 2015, 3, 9271.
W. Peng, X. Miao, V. Adinol, E. Alarousu, O. El Tall,
A. Emwas, C. Zhao, G. Walters, J. Liu, O. Ouellette, J. Pan,
B. Murali, E.-H. Sargent, O.-F. Mohammed and
O.-M. Bakr, Angew. Chem., Int. Ed., 2016, 55, 10686.
D. Bi, P. Gao, R. Scopelliti, E. Oveisi, J. Luo, M. Grätzel,
A. Hagfeldt and M.-K. Nazeeruddin, Adv. Mater., 2016, 28,
2910.
N.-D. Marco, H. Zhou, Q. Chen, P. Sun, Z. Liu, L. Meng,
E. Yao, Y. Liu, A. Schiffer and Y. Yang, Nano Lett., 2016,
16, 1009.
Y. Wang, N. Song, L. Feng and X. Deng, ACS Appl. Mater.
Interfaces, 2016, 8, 24703.
K. Yao, X. Wang, F. Li and L. Zhou, Chem. Commun., 2015,
51, 15430.
C. Sun, Q. Xue, Z. Hu, Z. Chen, F. Huang, H. Yip and Y. Cao,
Small, 2015, 11, 3344.
N. Mercier, CrystEngComm, 2005, 7, 429.
Y. Li, G. Zheng and J. Lin, Eur. J. Inorg. Chem., 2008, 10,
1689.
R. Ghosh Chaudhuri and S. Paria, Chem. Rev., 2012, 112,
2373.
X. Li, M.-I. Dar, C. Yi, J. Luo, M. Tschumi,
S.-M. Zakeeruddin, M.-K. Nazeeruddin, H. Han and
M. Grätzel, Nat. Chem., 2015, 7, 703.
N. Li, Z. Zhu, C. Chueh, H. Liu, B. Peng, A. Petrone, X. Li,
L. Wang and A.-K.-Y. Jen, Adv. Energy Mater., 2017, 7,
1601307.
S. Li, C. Chang, Y. Wang, C. Lin, D. Wang, J. Lin, C. Chen,
H. Sheu, H. Chia, W. Wu, U. Jeng, C. Liang, R. Sankar,
F. Chou and C. Chen, Energy Environ. Sci., 2016, 9, 1282.
S. Baek, J. Noh, C. Lee, B. Kim, M. Seo and J. Lee, Sci. Rep.,
2013, 3, 542.
This journal is © The Royal Society of Chemistry 2017
View Article Online
Published on 15 May 2017. Downloaded by University of California - San Diego on 26/06/2017 11:32:16.
Review
153 X. Li, W.-C.-H. Choy, H. Lu, W.-E.-I. Sha and A.-H.-P. Ho,
Adv. Funct. Mater., 2013, 23, 2728.
154 M. Notarianni, K. Vernon, A. Chou, M. Aljada, J. Liu and
N. Motta, Sol. Energy, 2014, 106, 23.
155 S. Lin, K. Lee, J. Wu and J. Wu, Sol. Energy, 2012, 86, 2600.
156 D. Derkacs, S.-H. Lim, P. Matheu, W. Mar and E.-T. Yu,
Appl. Phys. Lett., 2006, 89, 093103.
157 W. Zhang, M. Saliba, S.-D. Stranks, Y. Sun, X. Shi,
U. Wiesner and H.-J. Snaith, Nano Lett., 2013, 13, 4505.
158 H. Furukawa, K.-E. Cordova, M. O'Keeffe and O.-M. Yaghi,
Science, 2013, 341, 974.
159 G. Ferey, Chem. Soc. Rev., 2008, 37, 191.
160 O.-K. Farha, A.-O. Yazaydin, I. Eryazici, C.-D. Malliakas,
B.-G. Hauser, M.-G. Kanatzidis, S.-T. Nguyen, R.-Q. Snurr
and J.-T. Hupp, Nat. Chem., 2010, 2, 944.
161 L. Ma, J.-M. Falkowski, C. Abney and W. Lin, Nat. Chem.,
2010, 2, 838.
162 M.-P. Suh, H.-J. Park, T.-K. Prasad and D. Lim, Chem. Rev.,
2012, 112, 782.
163 J. Li, J. Sculley and H. Zhou, Chem. Rev., 2012, 112, 869.
164 T. Chang, C. Kung, H. Chen, T. Huang, S. Kao, H. Lu,
M. Lee, K.-M. Boopathi, C. Chu and K. Ho, Adv. Mater.,
2015, 27, 7229.
165 S.-N. Habisreutinger, T. Leijtens, G.-E. Eperon,
S.-D. Stranks, R.-J. Nicholas and H.-J. Snaith, Nano Lett.,
2014, 14, 5561.
166 Z. Zhu, J. Ma, Z. Wang, C. Mu, Z. Fan, L. Du, Y. Bai, L. Fan,
H. Yan, D.-L. Phillips and S. Yang, J. Am. Chem. Soc., 2014,
136, 3760.
This journal is © The Royal Society of Chemistry 2017
Journal of Materials Chemistry A
167 Y. Zhang, L. Tan, Q. Fu, L. Chen, T. Ji, X. Hu and Y. Chen,
Chem. Commun., 2016, 52, 5674.
168 H. Tsai, W. Nie, P. Cheruku, N.-H. Mack, P. Xu, G. Gupta,
A.-D. Mohite and H. Wang, Chem. Mater., 2015, 27, 5570.
169 L.-C. Schmidt, A. Pertegas, S. Gonzalez-Carrero,
O. Malinkiewicz, S. Agouram, G. Minguez Espallargas,
H.-J. Bolink, R.-E. Galian and J. Perez-Prieto, J. Am. Chem.
Soc., 2014, 136, 850.
170 E. Horvath, M. Spina, Z. Szekrenyes, K. Kamaras, R. Gaal,
D. Gachet and L. Forro, Nano Lett., 2014, 14, 6761.
171 M. Shahiduzzaman, K. Yamamoto, Y. Furumoto,
T. Kuwabara, K. Takahashi and T. Taima, RSC Adv., 2015,
5, 77495.
172 D.-T. Moore, K.-W. Tan, H. Sai, K.-P. Barteau, U. Wiesner
and L.-A. Estroff, Chem. Mater., 2015, 27, 3197.
173 J. Catalano, A. Murphy, Y. Yao, G.-P.-A. Yap,
N. Zumbulyadis, S.-A. Centeno and C. Dybowski, Dalton
Trans., 2015, 44, 2340.
174 J. Seo, T. Matsui, J. Luo, J. Correa-Baena, F. Giordano,
M. Saliba, K. Schenk, A. Ummadisingu, K. Domanski,
M. Hadadian, A. Hagfeldt, S.-M. Zakeeruddin, U. Steiner,
M. Grätzel and A. Abate, Adv. Energy Mater., 2016, 6,
1600767.
175 Z. Xiao, D. Wang, Q. Dong, Q. Wang, W. Wei, J. Dai, X. Zeng
and J. Huang, Energy Environ. Sci., 2016, 9, 867.
176 Y. Chen, Y. Zhao and Z. Liang, Chem. Mater., 2015, 27, 1448.
177 Y. Zhao and K. Zhu, Chem. Commun., 2014, 50, 1605.
178 C. Zuo and L. Ding, Nanoscale, 2014, 6, 9935.
179 C. Qin, T. Matsushima, T. Fujihara and C. Adachi, Adv.
Mater., 2017, 29, 1603808.
J. Mater. Chem. A