Influence of moisture on the preparation, crystal structure, and photophysical properties of organohalide perovskites

Article abstract of DOI:10.1039/c4cc05231e

The effect of preparing lead-based organohalide perovskites under inert conditions has been investigated. We find that when prepared under anhydrous conditions, only poorly crystalline powders were obtained. On exposure to small amounts of moisture a rapid crystallization into the expected cubic unit cell for CH₃NH₃PbBr₃ and tetragonal cell for CH₃NH₃PbI₃ is observed. While the as-prepared iodide phase is non-emissive, the lifetime of the emission for the bromide is found to be much longer when prepared under atmospheric conditions.

Full text of DOI:10.1039/c4cc05231e

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Page 1 of 4
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DOI: 10.1039/C4CC05231E
Influence of Moisture on the Preparation, Crystal Structure, and Pho-
tophysical Properties of Organohalide Perovskites†
a
a
a
a
a
Kelsey K. Bass, R. Eric McAnally, Shiliang Zhou, Peter I. Djurovich, Mark E. Thompson, Brent
a∗
C. Melot
Received Xth XXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
First published on the web Xth XXXXXXXXXX 200X
DOI: 10.1039/b000000x
ꢡꢝꢜꢘꢡ
The effect of preparing lead-based organohalide per-
ꢇꢚꢋ
ꢞꢛꢘꢗꢖꢘꢕꢉ
ovskites under inert conditions has been investigated. We
find that when prepared under anhydrous conditions, only
poorly crystalline powders were obtained. On exposure to
small amounts of moisture a rapid crystallization into the
ꢚꢘꢛpꢉꢟꢞꢠꢗꢉꢈ
ꢀ
ꢀ
ꢀ
ꢁ
ꢂꢁ
ꢃꢁ
ꢄꢁ
ꢅꢁ
ꢆꢁ ꢀꢏꢎꢂ ꢀꢏꢎꢆ
expected cubic unit cell for CH NH PbBr and tetragonal
ꢖꢉꢖꢛꢚꢊꢎ
3
3
3
cell for CH NH PbI is observed. While the as-prepared
ꢇꢜꢋ
3
3
3
iodide phase is non-emissive, the lifetime of the emission
for the bromide is found to be much longer when prepared
under atmospheric conditions.
ꢀ
ꢀ
ꢀꢁ
ꢂꢁ
ꢃꢁ
ꢄꢁ
ꢅꢁ
ꢆꢁ ꢀꢆ
ꢀꢏ
ꢂθ ꢇꢈꢉꢊꢋpꢌλ ꢍpꢀꢎꢏꢐꢑꢀꢂꢒꢓ
Organo-halide perovskites have attracted considerable atten-
tion for their use in solution processable solar cells with power
Fig. 1 Comparison of laboratory powder diffraction data for (a)
1
–19
conversion efficiencies of over 15%.
Most reports have
CH NH PbBr and (b) CH NH PbI materials
3
3
3
3
3
3
focused on improving device performance whereas the fun-
damental nature of the perovskite phases used in the devices
is less explored. While it has been acknowledged that de-
vices should be prepared in conditions with less than 1% hu-
midity,11 few reports have been made on the influence of
the atmosphere on the resulting product. Niu et al. per-
formed experiments to compare the structure and performance
of CH NH PbI photovoltaic devices before and after expo-
prepared/characterized under inert atmosphere (blue line) and
exposed to ambient atmosphere (red lines). The panels on the right
emphasize the lowest angle peak which splits on distortion from the
cubic to tetragonal phase. The asterisks denote reflections from the
Be window of the air-free XRD cell.
purified further by sublimation to remove a yellow-orange col-
oration that was likely the result of I2 present in the HI. Pow-
3
3
3
sure to 60% humidity, but prepared all of their samples in
ambient conditions.20 To better understand the role of mois-
ture in the preparation of these phases, we have synthesized
CH NH PbBr and CH NH PbI in an air free environment
dered reagents like Pb(NO3) (Acros Organic), CH3NH3Br,
2
and CH3NH3I were weighed and sealed inside of the glove
box before moving to a Schlenk line for the reaction. Ground
glass fittings were preferred over rubber septa due to the re-
activity of the septa with the fumes of the concentrated acids.
A typical synthesis was performed under a constant positive
pressure of nitrogen with stoichiometric amounts of Pb(NO3)2
and CH3NH3X in concentrated HX. The solution was heated
3
3
3
3
3
3
and studied the structure and photophysical properties of the
air free and air exposed powders and films.
All reagents were purchased fresh from the respective sup-
plier and kept inside an Ar-filled glove box with less than
0
.1 ppm of moisture and approximately 0.30 ppm of oxygen.
◦
CH NH Br and CH NH I were prepared by mixing 40%
to 100 C until the powders were fully dissolved before cool-
3
3
3
3
CH NH in water (Spectrum Chemical Mfg. Corp.) with con-
ing to room temperature. The resulting orange (bromide) and
black (iodide) mixture of crystals and powder were washed
with anhydrous diethyl ether and dried under vacuum. We
note that slower cooling from the reaction temperature does
not produce crystalline powders but results in larger, poorly
crystalline particles that crystallize at a slower rate when ex-
posed to moisture.
3
2
centrated HBr (Sigma Aldrich, 48%) or HI (Sigma Aldrich,
2
1
5
7%) as described previously. Prior to use, CH NH I was
3
3
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See DOI:
1
0.1039/b000000x/
a
Department of Chemistry, University of Southern California, Los Angeles,
Fig. 1 shows X-ray diffraction patterns collected on a
CA 90089 E-mail: melot@usc.edu
1
–4 | 1

ChemComm
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ꢏꢎꢐꢝ
ꢀ
ꢀꢎꢅꢝ
ꢏꢎꢝ
ꢏꢎꢐꢝ
ꢀ
ꢀ
ꢁ
ꢂ
ꢃꢄ
ꢃꢅ
ꢃꢀ
ꢀ
ꢁ
ꢂ
ꢃꢄ
ꢃꢅ
ꢃꢀ
ꢅθ ꢆꢇꢈꢉꢊꢋꢌλ ꢍꢋꢄꢎꢀꢃꢏꢁꢐꢃꢑꢒ
ꢅθ ꢆꢇꢈꢉꢊꢋꢌλ ꢍꢋꢄꢎꢀꢃꢏꢁꢐꢃꢑꢒ
Fig. 3 Rietveld refinement of the tetragonal structure of
Fig. 2 Results from the Rietveld refinement of the cubic structure of
CH NH PbI at room temperature against 11-BM data for powders
CH NH PbBr at room temperature against high resolution
3
3
3
3
3
3
which were exposed to the atmosphere. RBragg =5.23%,
synchrotron diffraction data obtained from the 11-BM beam line at
2
R =5.39%, χ =1.09. The inset on the left illustrates the resulting
f
Argonne National Laboratory on powders which were exposed to
2
structure with atoms visualized at the 95% probability level. The
inset on the right emphasizes the splitting of the 100 peak into the
the atmosphere. RBragg =7.48%, R =3.19%, χ =1.38. The inset
f
on the left illustrates the resulting structure with atoms visualized at
the 95% probability level. The inset on the right emphasizes the 100
Bragg reflection of the cubic perovskite.
0
02 and 110 planes of the tetragonal structure.
the methylammonia groups which are known to be highly dis-
3
,23,24
ordered at room temperature.
The tetragonal distortion
Bruker D8 powder diffractometer equipped with a Lynxeye
detector and Co-Kα tube. The pristine patterns were col-
lected using an air-tight stainless steel cell equipped with a
in the iodide is attributed to a slight ordering of these groups,
◦
and results in an alternating 16 rotation of the corner-sharing
2
2
octahedra down the length of the c axis of the unit cell which
generates new reflections at 6.3 and 6.5 in 2θ. There is si-
multaneously a compression of the c/a ratio from the ideal
2
5 mm Be window that has been described previously. Very
weak reflections are seen in the powders prior to exposure
to atmosphere for either the CH NH PbBr [Fig. 1 (a)] or the
◦
◦
3
3
3
1
.50 to 1.43 which splits the [100] Bragg reflection as empha-
CH NH PbI [Fig. 1 (b)] powders. On opening the cell and
3
3
3
sized in the inset.
allowing to sit on the bench top for only a few minutes, peaks
which could be indexed to a cubic unit cell were found for the
bromide and a tetragonal cell for the iodide. The difference in
quality of the patterns is because the iodide had to be scanned
at a much faster rate. Allowing the black powder to sit for
more than a few minutes in air resulted in the powder taking
on a brown tint, indicating a small conversion to the yellow hy-
drated phase. No diffraction peaks were seen for the hydrated
phase, however, so the reaction likely occurred primarily on
the surface without much penetration into the bulk.
To mitigate the conversion to the hydrated phase and ob-
tain a more accurate picture of the structure, the as-prepared
powders were briefly exposed to ambient atmosphere before
sealing in kapton capillaries and sent for synchrotron diffrac-
tion experiments at the 11-BM beam line at Argonne national
To distinguish the effect of oxygen from atmospheric mois-
ture, dry O2 and moist N2 were separately passed over a
small amount of the pristine bromide powder. Importantly,
exposure to the anhydrous gases had absolutely no effect on
the observed diffraction patterns, while the moist nitrogen
was found to rapidly trigger the crystallization into the cu-
bic phase. It is not immediately clear how moisture pro-
motes this rapid crystallization. However, considering these
organohalide perovskites are extremely soluble in aqueous so-
lutions, it is possible that humidity in the air triggers nucle-
ation and crystallization as has been seen in amorphous lactose
2
5,26
particles.
Similar effects also occur with organic solvents
inducing crystallization of Na Au(SO ) and tris(8-hydroxy-
3
3 2
2
7,28
quinoline)aluminum films.
lab. The results of the Rietveld refinement of CH NH PbBr
Since the as-prepared powders are non-emissive, the role
of moisture on the photophysical properties was explored by
dissolving powders which had never been exposed to mois-
ture in anhydrous DMF at a concentration of 10 wt%. Two
sets of films were then cast; one inside an Ar-filled glove
3
3
3
and CH NH PbI structures against the synchrotron data are
3
3
3
shown in Figs 2 and 3 respectively. The high intensity and
resolution of the 11-BM data shows extremely phase-pure
powders with no sign of any secondary phases. Anisotropic
atomic displacement parameters were refined for both phases
and show a significant degree of disorder in the halide ions.
This is probably related to rotational disorder associated with
◦
box (less than 0.01 ppm of moisture and 23.5 C) and the
◦
other in open air (58.2% humidity and 22.6 C). X-ray diffrac-
tion patterns confirmed that films cast under anhydrous con-
2
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DOI: 10.1039/C4CC05231E
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0
.00
.10
1
.00
ꢔꢓꢂ
ꢂꢓꢁ
ꢂꢓꢅ
ꢂꢓꢀ
ꢂꢓꢄ
ꢚꢙꢖꢗꢍꢖꢋꢉ
ꢇꢖꢙꢏꢉꢛꢚꢘꢗꢉꢜ
ꢝꢇꢗꢍꢏꢖꢋꢏꢇꢖꢙ
0
.01
0
500 1000 1500
0.10
pristine
air exposed
cast in air
ꢂꢓꢂ
ꢀ
0
.01
ꢁꢂ
ꢃꢂꢂ
ꢃꢄꢂ
ꢃꢀꢂ
ꢃꢅꢂ
ꢃꢁꢂ
0
10
20
30
Time (ns)
40
50
60
ꢆꢇꢈꢉꢊꢉꢋꢌꢍꢎꢏꢐꢋꢑꢒ
Fig. 4 Optical emission spectra for pristine films of CH NH PbBr
3
before (filled blue circles) and after exposure to atmosphere (unfilled
red squares) as well as those cast in air (unfilled green triangles).
Fig. 5 Time-resolved photoluminescence emission spectra for
3
3
pristine films (filled blue circles) and films which were exposed to
ambient atmosphere (unfilled red squares) for the CH3NH PbBr3.
3
Note the exceptionally longer lifetimes when exposed to air and
recall that all iodide-based phases failed to show any sign of
luminescence.
ditions remained poorly crystalline and that exposure to mois-
ture resulted in rapid crystallization to highly oriented crys-
tallites which predominantly showed diffraction peaks associ-
ated with [00l] Bragg planes much like in the films cast in air.
Analysis using scanning electron microscopy demonstrated
that the morphology of the films cast under Ar versus those
cast in air is different. The films cast in air exhibit smaller
particle sizes, which is counter-intuitive to the enhanced pho-
toluminescence lifetime. Thus, particle size is irrelevant in
these films and does not explain the differences in lifetimes.
The higher crystallinity of the particles in the films cast in air
seems to allow the films to exhibit increased lifetimes. We
note that similar problems with obtaining complete wetting of
the substrate were encountered as previously reported, but
additives to improve coverage were avoided to reduce any po-
tential impact on the performance.
In order to confirm that there were no major differences be-
tween the poorly crystalline and crystalline phases, the optical
emission spectra for both phases were collected. Aside from a
slight difference in peak width, all three films were confirmed
to display emission at the previously reported wavelength of
films cast in air and under inert atmosphere is striking. Films
exposed to air or cast in atmospheric conditions were found
to show increasingly long luminescent lifetimes. Given that
there are no substantial morphological differences between the
films, this result is likely a result of differences in the crys-
tallinity of each film. The pristine films are poorly crystalline
and show the most rapid decay in the emission lifetime. It is
not surprising that only after the films are crystalline that emis-
sion life times begin to reflect what has been reported in the
29,31
literature.
The loss of long-range periodicity in the poorly
crystalline films appears to provide new routes by which the
excited state can decay at a significantly accelerated rate. Our
results then appear to indicate that the presence of a small
amount of moisture is absolutely necessary in order to pro-
duce the highly crystalline films which exhibit the long-lived
excited states that have been seen in the literature, although it
is still not exactly clear how moisture promotes crystallization.
In summary, our results demonstrate that the synthetic con-
ditions under which organohalide perovskites are prepared
plays a critical role in promoting crystallization. Further stud-
ies are needed to understand why exposure to moisture trig-
gers the crystallization of the perovskite phase, but it is clear
from the photoluminescence data that the excited state in the
amorphous films is quenched at a significantly faster rate than
those which were fully crystallized.
2
9
5
40 nm for the bromide phase (Fig. 4), albeit with very low
efficiency (Φ < 0.01). In contrast, no emission was observed
from films prepared from the iodide powders around the ex-
pected wavelength of 770 nm. Very weak emission was ob-
served for the iodide closer to 520 nm, but this is believed to
be associated with a small impurity related to the yellow hy-
drated form. This observation is in keeping with what has
previously been reported for materials prepared using solution
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