Thermally induced recrystallization of MAPbI3 perovskite under methylamine atmosphere: An approach to fabricating large uniform crystalline grains

Article abstract of DOI:10.1039/c6cc04521a

A liquid to solid phase transition of methylammonium lead triiodide (MAPbI₃) under methylamine (MA) atmosphere at elevated temperatures was discovered, and used to form high quality and uniform thin films containing large, low defect crystal grains tens of microns in size.

Full text of DOI:10.1039/c6cc04521a

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Thermally Induced Recrystallization of MAPbI3 Perovskite Under
Methylamine Atmosphere: An Approach to Fabricating Large
Uniform Crystalline Grains
Received 00th January 20xx,
Accepted 00th January 20xx
Daniel L. Jacobs, Ling Zang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A liquid to solid phase transition of methylammonium lead to the micron scale or less.9, 10 The hot casting technique
triiodide (MAPbI3) under methylamine (MA) atmosphere at developed by Nie et al has reported ultra large grains up to the
elevated temperatures was discovered, and used to form high millimeter scale,7 however, subsequent detailed studies of this
quality and uniform thin films containing large, low defect crystal method suggest that these domains are not single crystals,
grains tens of microns in size.
rather the large area domains are compositions of smaller
crystal grains formed in a Volmer-Weber thin film growth
process.11, 12 Thus, techniques to fabricate large area single
crystal grains uniformly across the surface remains a significant
barrier to further improving device performance.
Hybrid organic-inorganic photovoltaic devices, particularly
those based on methylammonium lead triiodide (MAPbI3),
have shown unprecedented performance enhancement in less
than a decade, with the most recent record efficiencies above
20%.1 However, controlling the deposition and crystallization
of MAPbI3 has been a significant challenge in reproducibly
fabricating high efficiency devices. Early devices were typically
fabricated from a simple 1-step spin coating deposition
followed by thermal annealing to recrystallize the film.
However, these films suffered from large and non-uniform
grains due to the fast crystal growth kinetics during the
annealing step, which also leads to the formation of significant
pinhole area that limit device performance.2 Major
breakthroughs in MAPbI3-based solar cells have stemmed from
new deposition techniques capable of forming large area,
Among the many promising physical properties of organo-
metallic hybrid perovskite materials, the reported room
temperature solid to liquid phase transition under
methylamine (MA) gas atmosphere is particularly unique. Zhou
et al first reported the phenomenon and found that short
exposure of a MAPbI3 thin film to MA gas at room temperature
causes an immediate color change from dark brown to
completely transparent, and when removed from the MA
atmosphere, the process quickly reversed.13 The morphology
of the MA treated films show ultra smooth surfaces across the
entire sample with enhanced crystallinity leading to improved
photovoltaic performance. However, the grain sizes may
actually decrease by up to an order of magnitude after
exposure to MA.14 The interaction between MA and MAPbI3 is
believed to be a result of polar MA molecules disrupting the
ionic bonding of the Pb-I lattice causing a collapse of the
crystal structure and forming a MAPbI3•xMA liquid phase, with
x being estimated at ~3.5.13 When the film is removed from
the MA atmosphere, the complex formation quickly reverses
as the gas molecules are expelled and MAPbI3 is recrystallized.
Compared to a solvent phase crystallization process, which
requires the liquid solvent molecules to evaporate before
crystallization can occur, the gas phase MA molecules are
expelled from the film uniformly and at a much faster rate.
This effectively quenches molecular transport, and thereby
crystal growth, leading to formation of fine grained thin films
dominated by a nucleation process.8
uniform, smooth and pin hole-free thin films. Such techniques
include the 2-step deposition methods,3,
anti-solvent
4
6
methods,5, and the hot casting method.7 The common
underlying mechanism behind the enhanced surface coverage
from these techniques is based on increasing the nucleation
kinetics during crystallization, which results in small uniform
grains across the substrate.8 However, this comes at the cost
of increased grain boundary area, which can also limit the
photovoltaic performance. While there have been several
techniques developed to increase the grain size through
traditional post deposition techniques, such as thermal or
solvent annealing, the resulting grains are still typically limited
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Figure 1. Temperature dependent annealing of MAPbI3 under MA gas atmosphere.
Shown here are the images of seven MAPbI3 coated glass substrates exposed to
nitrogen or MA atmosphere maintained at a constant temperature. All images in a
column represent the same sample at a constant temperature at different times
throughout the experiment. Top Row: Images of the as deposited film after five
minutes under flowing nitrogen atmosphere at the corresponding temperature.
Middle Row: Image of the films immediately after exposure to MA atmosphere.
Bottom Row: Images of the samples after 30 seconds under continual flowing MA
atmosphere. The schematic at the bottom shows the proposed 1-D phase diagram
of MAPbI3 under a constant MA atmosphere.
Figure 2. Experimental procedure of the TIRMA process. a) Proposed 2-D phase
diagram showing the experimental path of TIRMA. b) in situ images of MAPbI3 thin films
during the TIRMA process. The MAPbI3 film as preparedin ambient air (25˚C) was an
opaque dark brown color (state 1). Immediately after exposure to MA atmosphere at
room temperature, the film transformed to a colorless liquid state (State 2). The film
(after exposed to MA for 30 sec) was heated up to 100˚C, and during the 120 sec
heating it started to recrystallize, eventually forming the crystalline phase with a semi-
transparent brown color (State 3). After complete recrystallization, the MA atmosphere
was purged with nitrogen for 2 minutes at 100˚C (State 4) followed by cooling to room
temperature to form the final state. No significant difference after State 3 through the
final state was found regarding either structure or morphology.
In this work, the above reported gas phase annealing
process was further investigated at elevated temperatures,
25˚C - 100˚C, as shown in Figure 1. It was found that above
55˚C, the liquid phase of MAPbI3 under MA atmosphere
becomes unstable, and starts transitioning back to the solid
state. At 100˚C, no liquid intermediate phase exists. This liquid-
solid phase transition at higher temperatures, as illustrated in
the one-dimensional (1-D) phase diagram (Figure 1), is similar
to the inverse temperature crystallization phenomenon as
observed in solution phase for many of the organo-metallic
halide perovskites. For example, MAPbI3 can precipitate out of
a heated solution in γ-butyrolacetone, which can be used to
rapidly grow large single crystals through controlled heating of
the solution.15 Following a similar approach, high quality and
uniform large area thin films of MAPbI3 with large, highly
crystalline grains were formed from the thermally induced
recrystallization under methylamine atmosphere (TIRMA)
process introduced herein.
MAPbI3 films were prepared in air on glass substrates via a
1-step spin coating with antisolvent drip technique6. The
experimental path of the TIRMA process related to changes in
the substrate temperature and the gas atmosphere is shown
on the 2-D phase diagram in Figure 2a. Figure 2b shows
snapshots of the MAPbI3 films during TIRMA treatment and a
full video can be found in the electronic supplementary
Figure 3. SEM image of a) pristine and b) TIRMA treated MAPbI3 showing the
formation of large crystalline grains (scale bars are 20µm). c) Cross sectional SEM image
of TIRMA treated film (scale bar is 3 µm). Cross sectional SEM image of the pristine film
is shown in Figure S2 for comparison. d) XRD spectra of a pristine and a TIRMA treated
MAPbI3 film. The inset shows the isolated pristine MAPbI3 film spectrum for
comparison. e) Absolute absorption and normalized photoluminescence spectra of a
pristine and a TIRMA treated MAPbI3 film.
information (ESI). Samples were loaded into
a sealed
homemade chamber at room temperature and purged with
nitrogen for 5 minutes. MA gas was introduced by routing the
nitrogen flow to a sealed 100 mL flask containing 5 mL of a 33
wt% solution of MA in ethanol to carry the headspace gas to
the sample chamber. After being exposed to MA, the opaque
dark brown MAPbI3 film was immediately transitioned into the
transparent colorless liquid state of MAPbI3•xMA. After 30 s
under constant MA atmosphere, the chamber was placed on a
hot plate preheated to 100˚C. Upon heating, the film began to
recrystallize, initially from the edges, followed by massive
growth throughout the film until complete recrystallization
occurred. When the recrystallization was complete, the system
was switched back to the nitrogen atmosphere and kept on
the hot plate for 2 minutes. Finally, the chamber was removed
from the hotplate and cooled to room temperature. No
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Figure 4. Schematic of proposed TIRMA mechanism leading to large crystalline grain film. a) As deposited MAPbI3 film under nitrogen at room temperature. b)
Exposure to MA at room temperature leading to MAPbI3•xMA liquid phase. c) Increasing substrate temperature leading to partial expulsion of MA molecules. d)
Nucleation of MAPbI3 at the film edge followed by crystal growth via cellular precipitation as more MA molecules are expulsed from the film. e) Stable crystalline
phase reached when all excessive MA molecules are released from the film.
structural or morphology change of the MAPbI3 film was scattering the light within the film, resulting in an increase in
observed during the nitrogen purge or the cooling steps. the optical path length. In comparison, the TIRMA treated
Detailed experimental methods can be found in the ESI.
films have grains over an order of magnitude larger than the
The TIRMA process resulted in exceptional film coverage wavelength of incident light, which should decrease light
and film quality as summarized in Figure 3. The grain sizes of scattering and therefore the path length of light within the
the TIRMA treated samples increased by about 2 orders of film.
magnitude compared with the pristine film (Figure 3a, a high
For comparison, the fast annealing technique in MA vapor
magnification SEM image in Figure S1, and cross sectional SEM at room temperature (MART) used in previously reported
image in Figure S2) with an average grain size of the TIRMA studies13,14 was performed and characterized. In short, the as-
treated film estimated to be about 15 µm (Figure 3b). The SEM deposited film was brought into the headspace of a vial
image of the TIRMA treated sample in Figure 3b shows a highly containing the MA solution at room temperature for a short
crystalline morphology with crystal twin boundaries clearly amount of time (~ 1 s), and rapidly brought back into air. Such
observed. The sample showed uniform morphology across the MART treated film was characterized using UV-vis absorption
surface and no significant pin-holing was observed as and PL spectra, SEM imaging and XRD as shown in Figure S6.
evidenced by the cross sectional SEM image (Figure 3c) and The MART treatment did not result in any significant change in
large area SEM image (Figure S3) (a note on the observed dark grain size compared to the pristine sample, but did cause
lines in the SEM images of the TIRMA sample is provided in the observable morphology change with the MART film showing
supporting information). The enhanced crystallinity of the film less defined and distinct grain boundaries. This implies that
is supported by X-ray diffraction (XRD) studies. As seen in MART treatment helps healing the grain boundary defects. The
Figure 3d, the pristine MAPbI3 film showed mostly (110) lattice XRD measurement supports this with a drastic increase (2
orientation in addition to several minority orientations. orders of magnitude) in the peak intensity and much more
However, the TIRMA treated samples showed an increase in uniform crystal orientation compared to the pristine film. The
the peak intensity by over 2 orders of magnitude with strong FWHM was calculated to be 0.093˚ for the MART film, similar to
preferred lattice orientation only in the (110) direction. The the pristine, which supports the observation of no significant
full width half max (FWHM) of the (110) reflection at 14˚ change in grain size. The UV-Vis absorption and PL spectra also
decreased 36% from 0.106˚ to 0.068˚, signifying a significant show a similar trend to the TIRMA films, with the PL showing a
increase in grain size. To further show the improved decrease in FWHM to 48.5 nm and a blue shift in the
crystallinity and phase purity of the TIRMA treated samples, absorption band edge to about 1.6 eV. While the MART film
photoluminescence (PL) spectral measurement was performed shows significant improvement in the film quality compared to
for the MAPbI3 film before and after TIRMA treatment (Figure the pristine films, it is less than what is achieved with the
3e). The TIRMA treated sample showed a 10 nm blue shift of TIRMA treatment regarding the grain size and crystallinity.
the PL peak from 785 to 775, and a decrease in the FWHM
The unprecedented enhancement in crystallinity of the
from 74 nm to 46 nm, indicating a reduction of low lying bulk TIRMA treated MAPbI3 film is proposed to be a result of the
14, 16
and grain boundary defects.10,
This PL peak shift is slow growth controlled crystallization as opposed to the fast
accompanied with a blue shift of the absorption band edge nucleation driven process. Figure 4 shows a schematic of the
seen in the absorption spectrum in Figure 3e. Tauc plot (Figure proposed mechanism of the TIRMA process. When the MAPbI3
S5) shows the band edge shifting from 1.59 eV for the pristine is exposed to the MA gas at room temperature, the colorless
film to 1.62 eV for the TIRMA treated sample, which converts liquid phase MAPbI3•xMA is immediately formed. When the
to about a 15 nm shift, similar to the PL shifting. The chamber is put on the hot plate, a vertical temperature
absorption spectra in Figure 3e show a decrease in the overall gradient is created up through the MAPbI3 film. As the
absorption after TIRMA treatment. This correlates with the substrate temperature reaches the critical temperature
apparent increase in transmission during the TIRMA process (>55˚C), the colorless liquid phase begins to show a darker
visible in the supporting movie and in Figure 2b, which is yellow color as seen in Figure 2b at 60 s and the supporting
attributed to the significant change in grain size. The pristine video. This yellow color can also be seen in images in Figure 1,
film contains grain sizes on the order of the wavelength of particularly at 55˚C, 65˚C and 75˚C samples. This yellow film is
visible light, and should therefore be more efficient in proposed to be an intermediate phase, MAPbI3•(x-y)MA,
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2.
3.
4.
G. E. Eperon, V. M. Burlakov, P. Docampo, A. Goriely and
resulted from the partial expulsion of MA molecules from the
room temperature equilibrium state MAPbI3•xMA. The
equilibrium molar ratio (x-y) depends on the temperature, and
more work is needed to characterize this intermediate state.
When the substrate temperature rises above the critical
temperature, continued expulsion of MA molecules leads to
supersaturation, causing MAPbI3 to nucleate on the surface. By
maintaining a constant elevated temperature throughout the
process, the MAPbI3 nuclei grow by continuous expulsion of
MA. Such phase growth process can be considered as a typical
case of cellular precipitation, γ à α + β, where the γ
represents the liquid phase of MAPbI3•(x-y)MA, α represents
the solid precipitate of MAPbI3, and β is the gas phase of MA.
As the whole system remains in the MA atmosphere, the α and
β are in equilibrium, i.e., precipitation of MAPbI3 and release
of MA gas occur simultaneously at the expense of the original
supersaturated phase of MAPbI3•(x-y)MA. The diffusion
distance remained short and unchanged during the phase
growth process as seen from the approximately constant
moving rate of the recrystallization front in the video in the
ESI. Because the precipitation of MAPbI3 is driven by expulsion
of MA gas, the phase transition occurs at higher temperature,
reverse to the other cellular precipitation processes, which are
usually triggered by decreasing temperatures.
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In conclusion,
a
liquid-solid phase transition of
MAPbI3•xMA into MAPbI3 at elevated temperatures was
revealed under constant MA gas atmosphere. This
phenomenon was used to directly crystallize MAPbI3 thin film
with crystal grains on the order of tens of micrometers. The
resultant film showed significantly enhanced crystallinity and
reduced bulk crystal defects. This TIRMA process can be
interpreted as a typical cellular precipitation, which allows for
kinetic control (via temperature) of the phase growth in order
to form large uniform crystalline domains with minimal
defects. The films thus formed also afford complete and
compact coverage across the substrate. Future studies to
control the nucleation and growth kinetics through more
precise control of temperature and MA concentration, along
with appropriate surface modification of the substrate is
anticipated to further increase the size and quality of crystal
grains and the uniformness of the film, which combined will
facilitate the fabrication of high efficiency photovoltaic
devices.
14.
15.
16.
This work was supported by the University of Utah Seed
Grant and the State of Utah USTAR Program. D.L.J.
acknowledges support from the NSF IGERT (DGE0903715). The
Authors would like to acknowledge Michael A. Scarpulla for his
helpful discussions.
Notes and references
1.
M. Saliba, T. Matsui, J.-Y. Seo, K. Domanski, J.-P. Correa-
Baena, M. K. Nazeeruddin, S. M. Zakeeruddin, W. Tress, A.
Abate, A. Hagfeldt and M. Gratzel, Energy &
Environmental Science, 2016, DOI: 10.1039/C5EE03874J.
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