Formation of organic-inorganic mixed halide perovskite films by thermal evaporation of PbCl2 and CH3NH3I compounds

Article abstract of DOI:10.1039/c4ra17316c

Organic-inorganic hybrid perovskites were prepared by thermal evaporation from precursor materials PbCl₂ and CH₃NH₃I (MAI). The structures of the perovskite films with various ingredients were characterized by X-ray diffraction (XRD), UV-vis absorption, and scanning electron microscopy (SEM). We found that with the addition of MAI material the evaporated PbCl₂ films were initially transformed to PbI₂, then to standard a stoichiometric perovskite and finally to the MAPbI₃·xMAI phase. The film composition ingredients strongly affect the device performance. An unmatched PbCl₂ to MAI ratio in the evaporated films resulted in reduced conversion efficiency and higher moisture sensitivity. The planar perovskite solar cells with organic charge layers showed negligible photocurrent hysteresis and delivered a power conversion efficiency of 10.5%.

Full text of DOI:10.1039/c4ra17316c

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DOI: 10.1039/C4RA17316C
Formation of organic-inorganic mixed halide perovskite films by thermal
evaporation of PbCl and CH NH I compounds
2
3
3
a,b
, b
b
b
b
Cheng Gao, Jiang Liu,* Cheng Liao, Qinyan Ye, Yongzheng Zhang, Xulin He,
b
, a
b
b
Xiaowei Guo,* Jun Mei, Woonming Lau
a
School of Optoelectronic Information, University of Electronic Science and
Technology, Chengdu, 610054, China
b
Chengdu Green Energy and Green Manufacturing Technology R&D Centre,
Southwest Airport Economic Development Zone, Shuangliu, Chengdu, 610207, China
*
Corresponding author
E-mail address: 546jiang@163.com (J. Liu); guosnu@gmail.com (X. Guo)
Abstract
Organic-inorganic hybrid perovskites were prepared by thermal evaporation from
precursor materials PbCl and CH NH I (MAI). The structures of the perovskite films
2
3
3
with various ingredients were characterized by X-ray diffraction (XRD), UV-vis
absorption, and scanning electron microscopy (SEM). We found that with the addition
of MAI material the evaporated PbCl films was initially transformed to PbI then to
2
2,
standard stoichiometric perovskite and finally to MAPbI •xMAI phase. The film
3
composition ingredients strongly affect the device performance. Unmatched PbCl to
2
MAI ratio in the evaporated films resulted in reduced conversion efficiency and
higher moisture sensitivity. The planar perovskite solar cells with organic charge
layers showed negligible photocurrent hysteresis and delivered a power conversion
efficiency of 10.5%.

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DOI: 10.1039/C4RA17316C
Introduction
Solar photovoltaic, which converts sunlight directly into electricity, is considered
as a clean, sustainable, promising renewable energy conversion technology. Recently
the organic-inorganic halide perovskite, with the features of excellent optical
1
2
3,
properties , ambipolar charge transport and very long electron-hole diffusion length
4
,
are attracting much attention, and has achieved tremendous progress. In 2009, lead
halide perovskites were first reported as photosensitizers in liquid-type sensitized
5
TiO solar cells. All-solid-state perovskite solar cells were then developed to achieve
2
good stability and high efficiency using spiro-OMeTAD as hole transporting
6
, 7
material.
To date, the power conversion efficiency of over 16% has been
demonstrated by several groups in mesostructured and planar perovskite solar
8
-10
cells. It was initially assumed that a mesostructured (or nanostructured) matrix may
be necessary for efficient charge transport and collection in perovskite solar cell, but
11, 12
many studies
confirmed planar device structure is also very suitable for perovskite
solar cell due to their long charge diffusion lengths.
1
1, 13-15
Various processing techniques, including thermal evaporation
, vapor
1
6
6, 7, 10
9, 12, 17
assisted solution process , one-step
and two-step solution process
, have
been used for the preparation of high quality perovskite absorbers. For the solution
process, a great deal of effort has been put to control the morphology of the resulting
1
8, 19
20-22
8
film by adjusting the annealing condition
, using additives
and mixed solvents ,
etc. In contrast, evaporation can offer inherent advantages in obtaining perovskite film
1
1
with excellent coverage, high material purity and good thickness control. Liu et al
first demonstrated vapor-deposited perovskite solar cell with a simple planar device
structure. Their findings showed great promise for preparing efficient planar
1
5
perovskite solar cell by evaporation. Malinkiewicz et al reported an inverted planar
perovskite by evaporation, sandwiched between two organic charge transport layers.
This inverted structure allows a relative low processing temperature for fabricating

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perovskite solar cells. In addition, the evaporation process is free of organic
D
s
O
o
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l
:
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e
.1
n
039/C4RA17316C
t
,
which may erode the underlying charge layer, thus allowing more choice of organic
2
3
charge materials .
In this paper, we present a detailed study of the evaporated perovskite film under
3
, 24
various evaporation conditions. Based the previously reported results
that the
addition of chloride could dramatically increase the photoluminescence lifetime of the
photoexcited species and charge diffusion lengths in the absorber, we adopted the
PbCl and MAI as the precursor materials. The effect of PbCl to CH NH I (MAI)
2
2
3
3
ratio on structural, optical and electrical properties of evaporated perovskite films was
investigated. Using the planar device structure with low-temperature processed
organic charge layers, we obtained an optimal device with an efficiency of 10.5%
which didn’t show obvious hysteresis.
Experimental section
CH NH I Synthesis
3
3
7
The material CH NH I (MAI) was synthesized based on the literature . In short,
3
3
to prepare MAI, hydroiodic acid (10 mL, 57 wt % in water, Sigma-Aldrich) and
methylamine (10 mL, 33 wt% in absolute ethanol, Sigma-Aldrich) were reacted in a
round bottomed flask in ice-cold water with stirring for 60 min. Raw CH NH I was
3
3
obtained by removing the solvent at 50 °C on a rotary evaporator. The material was
then washed in diethyl ether and filtered several times. The precipitate was then dried
in a vacuum oven overnight at 50 °C and then stored in nitrogen-filled glove box.
Perovskite film evaporation
The evaporation was carried out in Angstrom Engineering EvoVac deposition
system with integrated Innovative Technology glovebox. The PbCl and MAI
2
materials were both evaporated from ceramic crucibles. The evaporation rate was
monitored by the quartz crystals. The quartz crystals could be reused after dipping in
DMF solution and then washing in ethanol. New quartz crystal was used for each

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evaporation. The undulating evaporated CH NH I film makes it difficult to mDO
easu
.10re39/C4RA17316C
I:
1
0
3
3
the accurate thickness. To avoid the tedious calibration process for the evaporation (or
deposition) rate of each material, we set the tooling factor (which is a ratio of the film
deposition rate monitored by sensor to that on the substrate) to be 100% during all the
co-evaporation process. Initially, the evaporator sources were heated slowly to an
level that the evaporation rate was just around the desired range, the process were
then carried out in auto mode. A desired evaporation rate can be obtained via heating
control system from the feedback of their sensors. After the evaporation rate were
stabilized for a prolonged time, the PbCl film and MAI film were then co-evaporated
2
onto the substrates with substrate baffle open. Various MAI evaporation rate (0–8 Å/s)
were employed. The evaporation rate of PbCl material was maintained at constant
2
monitored rate of 2 Å/s.
Solar cell fabrication
Solar cells with FTO/PEDOT:PSS/Perovskite/PCBM/Ag planar device structure were
prepared. Before deposition of other films, the FTO-coated glass substrates were
ultrasonically cleaned with detergent, deionized water, acetone and ethanol
sequentially, and were then blow-dried in nitrogen. After that, PEDOT:PSS solution
(Clevious Al 4083) was spin-coated on FTO-coated substrates at 2000 r.p.m for 45 s,
and subsequently the PEDOT:PSS films were annealed at 150 °C for 20 min. After
cooling down, perovskite absorbers were evaporated on PEDOT:PSS films. The
evaporated perovskite films were then annealed at 100 °C for 45 min on a hot plate.
3
0 mg/ml PCBM (FEM. Inc.) solution in chlorobenzene (Sigma-Aldrich) was then
spin-coated at 1000 r.p.m for 45 s. Finally, Ag top contact was deposited by
-6
evaporation through an aperture mask under a base pressure of 7×10 Torr. The
2
active device area is 0.12 cm .
Measurement and characterization
X-ray diffraction (XRD) analysis was performed on a D/max-RB diffractometer
o
(
Rigaku) using Cu Ka radiation at a scan rate of 6 /min. The energy-dispersive X-ray
EDX) compositions and spectra were performed using energy-dispersive
(
spectroscopy (EDS) combined with a field-emission scanning electron microscope

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(SEM, Hitachi S4500). SEM images were obtained using Hitachi S4500 and
D
H
OI
i
:
t
1
a
0
c
.1039/C4RA17316C
h
i
S5200. Film absorbance spectra were measured by Shanghai UV-VIS SP-752
spectrometer. Current-voltage measurements and power conversion efficiencies were
obtained using Keithley 2400 at room temperature under AM 1.5G illuminations
2
(
1000 W/m ) from a solar simulator which was calibrated using a standard silicon
solar cell device.
Results and discussion
The CH NH PbI Cl perovskite films were prepared by co-evaporation method
3
3
3-x
x
with various MAI evaporation rate but at a fixed PbCl evaporation rate. The details
2
of the co-evaporation are described in experimental section. The deposition condition
and EDX composition data of the prepared perovskite thin films are shown in Table 1,
where the evaporation rate ratio of CH NH I to PbCl is referred to as y . All EDX
3
3
2
rt
analysis was performed at electron accelerating voltage of 10 kV. It can be seen that
when the evaporation rate was increased from 0 to 8Å/s, the Cl content in the films
dropped greatly from 64.45% down to 0.18%, while the Pb content only from 35.55%
to 19.86%. Therefore, it is easy to conclude that the loss of Cl occurred in the
evaporating process. In the solution-processed perovskite film employing PbCl and
2
MAI with a molar ratio of 1:3, many studies have showed that Cl tended to get lost
during the annealing, which accords well with our results. Cl was believed to sublime
18
in the form of CH NH Cl , thus the loss of Cl increased with the incorporation of
3
3
MAI. Although there is a negligible amount of Cl content inside fully annealed
perovskite films, it is still considered the addition of chloride play a key role in
2
0, 21, 24, 25
improving film growth and charge transport
. Particular attention was paid to
the I to Pb ratio in evaporated film. When the process condition y equals 2, the I to
rt
Pb ratio is about 2, which can be attributed to the PbI formation. For y = 3, the I to
2
rt
Pb ratio is about 3, which is close to that of the standard stoichiometric MAPbI Cl
x
3
-x
perovskite (Pb/I=1/3).

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DOI: 10.1039/C4RA17316C
Table 1 Composition variation (measured by EDX) of the evaporated films at
different deposition conditions
Evaporation rate (Å/s)
EDX composition
Pb(at%) I (at%) Cl (at%)
Evaporation
rate ratio yrt
Sample
CH NH I
PbCl2
3
3
S1
S2
S3
S4
0
2
3
4
0
2
2
2
2
35.55
32.18
25.96
19.86
0
64.45
5.72
0.60
0.18
4
6
8
62.10
73.43
79.96
The evaporation rate ratio of CH NH I to PbCl is referred to as y , and y =0 means
3
3
2
rt
rt
only PbCl film was deposited.
2
(a)
(b)
PbCl2
2.0
PbCl2
yrt = 0
Insufficient MAI
Moderate
Excess MAI
Insufficient MAI yrt = 2
Moderate yrt = 3
1.5
Excess MAI yrt = 4
Exposure to air yrt = 4
yrt = 4
1
.0
yrt = 3
yrt = 2
0.5
yrt = 0
0
.0
4
00
500
600
700
800
1
0
15
20
25
30
35
40
Wavelength (nm)
2
 (degree)
(c) yrt = 0
(d) yrt = 2
(e) yrt = 3
(f)
yrt = 4
(g)
yrt = 4
After exposure
to moisture air
Figure 1. (a) XRD spectra, (b) UV-vis absorbance and (c-g) photographs of the
evaporated films at different deposition conditions.
In order to identify the phase constitution, X-ray diffraction (XRD) analysis was
conducted to study the structural evolution of evaporated perovskite films at different
deposition conditions. Figure 1a shows the XRD patterns of four samples listed in
Table 1. Much difference can be observed between the samples deposited with
different amounts of MAI. When only PbCl (y = 0) is deposited, the sample exhibits
2
rt

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peaks at 19.9°, 23.0°, 25.1° and 32.4°, which is in agreement with PbCl XRDO
D
I: 1
d
0.
a
1039/C4RA17316C
t
a
2
according to JCPDS card 26-1150. For the sample with deposition condition y = 2,
rt
the peak positions of the x-ray reflection are comparable to the powder-diffraction
standard data (JCPDS 07-0235) for PbI , indicating the formation of PbI from the
2
2
reaction of PbCl and MAI. It can be also seen that the PbCl peaks disappear entirely
2
2
although there was a little Cl (5.72 at%) contained in the film, which can be explained
by low crystallinity of possible remaining PbCl or limited determination precision of
2
XRD used. In this case, no peaks related to perovskite MAPbI phase are found from
3
the XRD pattern, indicating the amount of evaporated MAI is not sufficient. As the
amount of MAI was further increased (y = 3), two strong peaks appear at 14.2° and
rt
2
8.5°, which correspond to the perovskite CH NH PbI (110) and (220) crystalline
3 3 3
planes, respectively. Compared with the mesostructured perovskite layers in other
1
7
literatures , the evaporated planar perovskite film shows an extremely preferred
orientation along (110) plane, perhaps due to lack of the restrictions from oxide
scaffolds in the formation of the perovskite layer. For the samples with deposition
condition y = 4, some extra peaks appeared, which means that the amount of
rt
evaporated MAI is too-much. In contrast to the X-ray diffractogram of powdered
MAI in Figure S3, we found those extra peaks don’t originate from the MAI material
and may be attributed to the possible compound MAPbI •xMAI.
3
In Figure 1b and c-g we present UV-vis absorption spectra and photographs of the
samples with different deposition rate ratios. The PbCl film shows white color and an
2
extremely low absorbance. When the amount of MAI is insufficient, the sample
exhibits a yellow color due to the presence of PbI . When amount of MAI is moderate
2
or excess, the samples after annealing exhibit a dark brown color due to the formation
of perovskite phase, and the corresponding absorbance spectra also display an onset
7
of drop at 780 nm, which is in agreement with previously reported results . Additional
features related to the stability must be also mentioned. The perovskite film with
moderate MAI demonstrated good stability, as evidenced by the absorbance spectra
change over time (Figure S4). In contrast, the perovskite film with excess MAI
exhibits rapid degradation in absorbance and the film color varies from dark brown to

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white (Figure 1g) after exposing to the air (relative humidity more than 60%)
D
fo
O
r
I: 1
o
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.1039/C4RA17316C
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y
2
0-30 minutes. It is noted that the degradation is probably different from the
hydrolyzation of perovskite to yellow PbI . However, the transition is reversible. The
2
brown dark perovskite phase will recover again upon heating. Similar results shown
in Figure S2 were also found for the samples with surface composition gradient. We
believe that the excess MAI in the perovskite film affects the morphology and
crystalline phase, thus reducing the environmental stability.
To further investigate the morphology of the films, we performed SEM analysis.
Figure 2 shows the SEM micrographs of the evaporated films deposited with different
MAI to PbCl ratios. It can be seen that all the films have dense microstructures and
2
show a 100% coverage which is hard to realize in solution-processed films. When the
film was deposited with either PbCl (y = 0) or insufficient MAI (y = 2), the grain
2
rt
rt
size of the films is within a small range. For the film with y = 3, considerable grain
rt
growth with smooth surface can be observed. Smooth films could lower the dark
2
6
current and reduce the density of interface states , which is essential for fabricating a
high-quality planar solar cell. When y was increased to 4, loose porous surface
rt
morphology was formed. The significant difference can be contributed to the presence
of excess MAI due to its high surface activity and hygroscopic nature.

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DOI: 10.1039/C4RA17316C
Insufficient MAI
(
(
a)
(b)
yrt = 0
PbCl2
yrt = 2
5
00 nm
500 nm
c)
(d)
yrt = 3
Moderate
yrt = 4
Excess MAI
5
00 nm
500 nm
Figure 2. SEM -images of the evaporated films with different deposition ratios
Table 2. Effect of the amount of CH NH I on photovoltaic performance of perovskite
3
3
solar cells. The mean values and standard deviations were attained from 8 cells.
Evaporation
rate ratio yrt
2
Voc (V)
J (mA/cm )
FF (%)
PCE (%)
sc
3
2
2
.0
.9
.8
0.87±0.03
0.87±0.07
0.78±0.08
9.08±1.86
14.68±2.52
15.89±2.68
0.50±0.05
0.59±0.02
0.52±0.06
3.87±0.61
7.61±1.60
6.48±1.18
The above structural and optical results predict that the perovskite film with y =
rt
3
should yield good photovoltaic performance. To find the optimal deposition
condition, detailed optimizations of evaporation rate ratios around y = 3 were carried
rt
out. Three perovskite solar cells with FTO/PEDOT:PSS/Perovskite/PCBM/Ag planar
structures were prepared. Figure 3 shows a cross-sectional SEM image of the
complete device. The thicknesses of PEDOT:PSS, Perovskite and PCBM layers are
about 30 nm, 400 nm and 100 nm, respectively. The average device parameters with
the standard deviations are summarized in Table 2. It is observed that three sets of

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cells demonstrated a low statistical spread in open circuit voltage (V ), whichDO
c
I
a
: 1
n
0.1039/C4RA17316C
b
e
oc
explained by good coverage of the evaporated perovskite films. The main differences
in the solar cells performance result from fill factor (FF) and short circuit current
density (J ). For the solar cells with y = 3, the average short circuit current density
sc
rt
2
(
J ) is only 9.08 mA/cm , which may be attributed to the non-exactly-matching of
sc
MAI to PbCl ratio. When the deposition ratio y reaches 2.9, the mean power
2
rt
conversion efficiency (PCE) of 7.61% was achieved with increased J and FF.
sc
Reducing y to 2.8 results in decreased PCE. Therefore, the optimal deposition ratio
rt
yrt determined in this work is 2.9.
Recently, there has been increasing discussion as to the photocurrent hysteresis in
2
7, 28
perovskite devices.
To understand the phenomenon of the fabricated perovskite
solar cells, the PCE differences were investigated under between the forward and
reverse scans. 20 devices at the process condition of y =2.9 were prepared. Their
rt
performances were characterized under forward (-0.2 V → 1.2 V) and reverse (1.2 V
→
-0.2 V) scans. The results are shown in Table 3 where detailed photovoltaic
parameters are listed in Table S1. Figure 4 shows the current-voltage (J-V) curves for
the best performing device measured under forward and reverse scans. The champion
2
device (Figure 4a) exhibited a J of 17.3 mA/cm , FF of 0.63, V of 0.97 V, and PCE
sc
oc
2
of 10.50% under forward scans, and a J of 17.0 mA/cm , FF of 0.63, V of 0.97 V,
sc
oc
and PCE of 10.32% under reverse scans. Figure 4b shows the external quantum
efficiency (EQE) curve for one optimized device. The EQE curve starts increasing
rapidly at 800 nm, which is related to the optical absorption of the perovskite absorber,
reaches a maximum value of 71% at 560 nm, and then drops slowly at about 400 nm.
From Figure 4a, the champion device doesn't show obvious hysteresis of photocurrent.
However, the statistics result (see Figure 5) shows the devices under reverse scans
have a 0.66% PCE higher than that under forward scans. The higher efficiency under
reverse scans mainly results from a slightly higher V by 0.03 V and J by 1.08
oc
sc
2
8
mA/cm . Compared to TiO -based perovskite planar solar cell in other literatures , the
2
difference is rather small, which could be explained by the difference of perovskite
preparation process or charge separation interfaces. There is still great room to further

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increase the conversion efficiency. The best device in this study shows a V cl
D
o
O
s
I
e
: 10to.10
1
39/C4RA17316C
oc
V, and its performance is mainly limited by the relatively low FF and J , which may
sc
indicate that the charge transport and collection for the solar cells are not still very
efficient. Further understanding and optimization of the device structure, apart from
the process control, may help improve the device performance.
Table 3 Photovoltaic performance of perovskite solar cells with the process condition
yrt =2.9 under forward and reverse scan directions. The mean values and standard
deviations were attained from 20 cells.
Evaporation
rate ratio yrt
Scan
direction
2
Voc (V)
J (mA/cm )
FF (%)
PCE (%)
sc
Forward
Reverse
Best
0.85±0.11 13.63±3.29
0.88±0.09 14.71±2.67
0.58±0.10
0.57±0.04
0.63
6.91±2.47
7.57±2.06
10.50
2
.9
0.97
17.30
Ag
PCBM
Perovskite
PEDOT
FTO
1
μm
Figure 3. Cross-sectional SEM image of a complete
FTO/PEDOT:PSS/CH NH PbI Cl /PCBM/Ag perovskite device (inset shows a
3
3
3-x
x
photograph of the perovskite device).

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DOI: 10.1039/C4RA17316C
(
a) 15
(b)100
Forward
Reverse
2
2
1
0
5
0
5
Jsc = 17.3 mA/cm Jsc = 17.0 mA/cm
8
6
4
2
0
0
0
0
0
Voc = 0.97 V
FF = 0.63
Voc = 0.97 V
FF = 0.63
 =10.32%

=10.50%
-
Forward
Reverse
-
-
-
10
15
20
-
0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
400 450 500 550 600 650 700 750 800
Wavelength (nm)
Voltage (V)
Figure 4. (a) J-V curves for the best performing perovskite solar cell measured under
forward and reverse scans. (b) External quantum efficiency of a perovskite device
with deposition ratio y =2.9.
rt
(
a) 10
(b) 10
Forward
PCE(%)
9
8
7
6
5
4
3
2
1
0
Reverse
9
8
7
6
5
4
3
2
1
0
PCE(%)
7
.14±2.27
7.79±1.83
2
3
4
5
6
7
8
9
10 11 12
2
3
4
5
6
7
8
9 10 11 12
Efficiency (%)
Efficiency (%)
Figure 5. Device performance statistics for 20 cells measured under (a) forward and (b)
reverse scans.
Conclusion
In summary, we present a detailed study of the evaporated perovskite film under
various evaporation conditions. The effects of PbCl to CH NH I ratio on structural,
2
3
3
optical and electrical properties of evaporated perovskite films were investigated. We
found the deposition condition plays an important role in device performance and
optimized the evaporation process. The solar cells utilizing the optimized evaporation
process and low temperature processed organic charge layer yielded a power
conversion efficiency of 10.5% with negligible hysteresis.

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DOI: 10.1039/C4RA17316C
ACKNOWLEDGMENTS
This work is supported by the National Nature Science Foundation of China
51202227), the Science and Technology Development Foundation of China academy
(
of Engineering Physics (2014A0302015 and 2014B0302054), the National High
Technology Research and Development Program of China (2012AA050704), Sichuan
International Cooperation Research Project (No. 2014HH0068) and the Fundamental
Research Funds for the Central Universities of China (No.2672012ZYGX2012J065).
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