Identification of physical origin behind disorder, heterogeneity, and reconstruction and their

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1
Identification of physical origin behind disorder, heterogeneity, and reconstruction and their
correlation with photoluminescence lifetime in hybrid perovskite thin film
Taame Abraha Berhe1,#
, Ju-Hsiang Cheng2,#
, Wei-Nien Su1,
*, Chun-Jern Pan2
, Meng-Che Tsai2
, Hung-Ming
Chen2
, Zhenyu Yang3
, Hairen Tan3
, Ching-Hsiang Chen2
, Min-Hsin Yeh2
, Andebet Gedamu Tamirat2
, Shin-Fu
Huang2
, Liang-Yih Chen2
, Jyh-Fu Lee4
,Yen-Fa Liao4
, Edward H. Sargent3,
*, Hongjie Dai5
and Bing-Joe
Hwang2,4,
*
1
NanoElectrochemistry Laboratory, Graduate Institute of Applied Science and Technology, National Taiwan University
of Science and Technology, Taipei 106, Taiwan
2
NanoElectrochemistry Laboratory, Department of Chemical Engineering, National Taiwan University of Science and
Technology, Taipei 106, Taiwan
3
Department of Electrical and Computer Engineering, University of Toronto, 35 St George Street, Toronto, Ontario M5S
1A4, Canada
4
National Synchrotron Radiation Research Center, Hsin-Chu 30076, Taiwan
5
Department of Chemistry, Stanford University, Stanford, CA, California 94305-4401, USA
*Corresponding Authors: E-mail: bjh@mail.ntust.edu.tw (B. J. Hwang), wsu@mail.ntust.edu.tw (W. N. Su) and
ted.sargent@utoronto.ca (Edward H. Sargent)
#
These authors have equal contribution in this work
Abstract
Organolead halide perovskites are an impressive and relatively recent class of light-absorbing
materials for solar cells and light-emitting devices. It has been reported that exposure of the
perovskites materials to light has negative impacts on device performance. Also, surface
recombination has been reported as a major obstacle to the total carrier lifetime in perovskite
polycrystalline thin films. Herein, we explored the role played by Nanosecond pulsed UV laser-
irradiation on carrier dynamics in perovskites thin films. Steady-state and time-resolved
photoluminescence measurements revealed the interplay of disorder and heterogeneity on
photoexcited carrier dynamics, while in-situ micro Raman and Angle dispersive X-ray diffraction
showed the mechanisms of crystal phase reconstruction. Exposures to laser light leads to rapid
crystal phase reconstruction and hence, unexpectedly, extend PL lifetime by fourfold instead of
promoting degradation. This verifies nanosecond pulsed laser irradiation plays a beneficial role in
improving in optoelectronic material parameters. Our findings reveal that pulsed laser irradiation is a
new approach to the reconstruction of microstructure and offers beneficial effects in the manufacture
of perovskites solar cells. Moreover, this work provides a clear insight towards identifying the
physical origin behind the disorder, heterogeneity, film reconstruction and nano-structuring as well
as their correlation with improved PL lifetime.
Keywords: perovskites, pulsed laser irradiation, reconstruction, disorder, heterogeneity,
photoluminescence
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Introduction
Solar cells using organic-inorganic hybrid perovskite (OIHPs) materials are revolutionizing
photovoltaics. Their power conversion efficiencies are now 22.1%,1
a fivefold increase in cell
efficiency over the past six years.2
This rapid increase in efficiency is due to the excellent properties
of the perovskite polycrystalline films, which exhibit very high absorption coefficients as evidenced
by their sharp absorption edges, direct bandgaps, long charge carrier diffusion lengths, low carrier
effective masses, and a tolerance to defects.3-6
These allow them to be incorporated into a wide
variety of device architectures such as mesoporous,7
planar8
and inverted structures,9
while
overcoming the conventional limitations of solution-processed semiconductors.10
The reasons underlying the long carrier lifetime of CH3NH3PbI3 perovskite films remain unclear.2, 11
Theoretical investigations have indicated that the dynamic position of the conduction band minimum
could be the reason for the prolonged carrier lifetime:12
the conduction band minimum and the
valence band maximum originate in the 6p orbital of Pb and the 5p orbital of iodine, since absorption
takes place at the symmetric vibrations of PbI3
-
octahedral site. Thus, the bandgap in CH3NH3PbI3 is
altered by changing the Pb-X bond length, where X denotes the halides.13
It has been reported that
the optical and electronic bandgaps vary in orthorhombic, tetragonal and cubic structures14, 15
and
that changes in Raman peaks, morphology and x-ray patterns have been reported as light-induced
degradation16-18
and light-induced structural transformations.19
Although CH3NH3PbI3 is a promising light harvesting material, its structural fluctuations and
instability impede understanding and technological applications. The influences of dynamic
structural disorder,20
heterogeneity and phase transition phenomena are not clearly understood.
Hence, the in-situ interplay of these factors on the lifetime and relaxation processes as well as device
performance during operation requires more exploration.
Recent experiments have indicated that carrier recombination kinetics can be described as a
combination of trap-assisted, monomolecular (first-order), and bimolecular (second-order)
recombination.21
Although most studies agree that radioactive bimolecular recombination dominates
at high initial carrier densities (n0 > 1017
cm−3
)22, 23
, other reports at lower excitation densities
(relevant to solar cell operation)24
range from single-exponential6, 25
to biexponential26, 27
or to
stretched-exponential24, 28
functions with varying levels of fidelity. These distributions have in turn
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been explained in terms of unintentional doping29
and charge trapping.30
Furthermore, perovskite
synthesis and growth conditions,7, 31
as well as post-deposition treatments,25, 32
greatly alter the
CH3NH3PbI3 structure, carrier lifetime, and device performance, yet the underlying relationships
between these parameters remain unanswered. Understanding these issues will help the design of
laser devices, high performance solar cells and ultrafast optoelectronic devices. To this end the
structural characterization of CH3NH3PbI3 can increase our understanding of the in-situ dynamic
structural evolution; variations in morphology disorder and heterogeneity as a function of laser
irradiation and their further impact on carrier decay times and relaxation processes as well as charge
carrier mobility. In particular, the physical origin of the disorder, heterogeneity and their correlation
with optical properties of hybrid perovskites materials is not clearly known.
Micro-Raman, Angle dispersive X-ray diffraction(ADXRD), X-ray absorption spectroscopy (XAS),
atomic force microscopy (AFM) and time-resolved photoluminescence (TRPL) are powerful tools to
reveal the structural and morphological disorder,33, 34
the relationships of photoexcited carriers
dynamics and, the presence of disorder and heterogeneity.35
Herein we combine these techniques to
study the presence/absence of structural disorder and film heterogeneity on the CH3NH3PbI3
perovskite thin films , as well as the impacts on the dynamics of photoexcited carriers. We found that
increased in temperature induces structural disorder and morphological heterogeneity, together with
the in-situ transformation of the tetragonal CH3NH3PbI3 to the cubic CH3NH3PbI3 phase. These
changes have a significant impact on the dynamics of photoexcited carriers, diminishing the decay
time to less than 1 ns. In contrast, laser irradiation can effectively mitigate the impacts of structural
disorder, and morphology heterogeneity present in the CH3NH3PbI3 films on the dynamics of the
photoexcited carriers. Nanosecond pulsed laser irradiation boosts the decay time fourfold, i.e., from 7
ns to 29 ns with increasing pulsed laser irradiation times. These findings point out the importance of
Nanosecond pulsed laser irradiation during the manufacture of perovskite films for the perovskite
solar cells and other ultrafast optoelectronic devices and equally important to understand and
consider light and temperature associated phenomena taking place during the device operation. Light
exposure is associated with rapid and dynamic photothermal induced structural evolution,
morphological changes and reduction of non-radiative recombination centers in CH3NH3PbI3 films
and further improving the PL lifetime, indicating that they are not as benign as has been suggested in
previous literature.16-18
In particular, our results provide an insight towards exploring the mechanism
of temperature, nanosecond pulsed laser and continuous laser light irradiation induced surface
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recombination in perovskite film with think of developing a materials model for what happens with
temperature and laser light exposure during manufacturing of perovskites thin film.
Experimental procedures
CH3NH3PbI3 Film preparation
2.0 x 2.5 cm microscopic glass slides were washed using acetone, 2-propanol and DI water, each
twice and then dried using N2 gas. Finally, we treated it using UV visible light for 30 min to activate
its surface. CH3NH3PbI3 precursor solution was prepared using the method reported elsewhere.36
The
CH3NH3PbI3 powder was prepared using acetonitrile as a solvent by mixing PbI2 (99.99%, Sigma-
Aldrich) and CH3NH3I (>98%, Uniregion Biotech or UR- CH3NH3I). CH3NH3PbI3 films were
prepared in DMF and then spin-coated onto a microscopic glass slide (2.0 x 2.5 cm) at 2000 RPM for
30 seconds, which was subsequently annealed at 80 °C for 15 minutes in a nitrogen-filled glovebox.
Interestingly, we have observed that treating the glass surface using UV/visible light before the spin-
coating is essential to step to obtain uniform film coverage. The power of this UV lamp was 60 W
and its wavelength is 185 nm. Each substrate was treated for 30 minutes after washing and quickly
transferred in the glovebox. The as-synthesized CH3NH3PbI3 powder and film were characterized by
in-situ micro-Raman combined with in-situ angular dispersive x-ray diffraction and in-situ x-ray
absorption spectroscopy. Impacts of in-situ structural evolution and transformation on the decay time
of photoexcited state were studied using combined steady state and time-resolved
photoluminescence, together with atomic force microscopy.
Characterization techniques
Steady State Photoluminescence (PL) and Time-Resolved PL (TRPL) Measurement: Samples
were encapsulated in the in-situ device (Fig. S1a, in supplementary information, SI) filled with inert
gas and excited face-on (not through the substrate) for both steady state and TRPL measurements. PL
and TRPL spectra were acquired with a dark chamber, Micro PL and TRPL system (UniNanoTech
Co. Ltd, 5 ns pulse width). Steady-state PL measurements were performed with the 266 nm several
hundred nanoseconds pulsed UV laser. The spectra were corrected for the optical transfer function of
the system. PL measurements were carried out in a 90° configuration with the PL signal detected in
the same direction of the reflected excitation beam. The maximum power density of this technique is
20 mW/cm2
but we used an optimum power density of 10 mW/cm2
for this study. A 470 nm pulsed
diode laser (PDL-800 LDH-P-C-470B, 250 ps pulse width) was used for excitation with repetition
rates between 625 kHz and 80 MHz. The emission was filtered through a long pass filter.
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Photoluminescence from the sample was directed by photon counting PMT mounted with S193
spectrometer for time-resolved PL and the steady-state PL measurements is detected by CCD with
1024 x 256 pixels. For steady-state PL and TRPL measurement, CH3NH3PbI3 thin film on the glass
substrate was encapsulated in in-situ device inside glovebox and PL measurement using 266 nm laser
and TRPL measurement using 470 nm lasers were conducted, respectively, before and after each
laser exposure in order to observe the effect of laser light on CH3NH3PbI3 structure and impact of
structural changes on the decay time photoexcited carrier. Three spectra were acquired to take
averages from different locations for each sample to ensure representative sampling and incorporate
spot-to-spot variability in the signal.
In-situ Micro-Raman Spectroscopy: Raman spectra of the CH3NH3PbI3 powder and film were
collected on a micro-Raman spectrometer with laser excitation of 532 nm. The data acquisition time
and exposure time were 10 s and 5 s, respectively. The peak intensities of the samples were
normalized to those of a silicon wafer at 520 cm-1
. A thermoelectrically cooled charge-coupled
device (CCD) with 1024 x 256 pixels operating at -60 °C was used as the detector with 1 cm-1
resolution. The laser line was focused onto the sample in a backscattering geometry, using an
Olympus 50x objective with a numerical aperture of 0.55, providing the scattering area of ~0.25
mm2
. The laser excitation intensity was kept from 1% to 30% at an estimated laser spot size of ~0.8
– 1.0 mm, and integration times were increased accordingly. The CH3NH3PbI3 powder and film were
encapsulated in the in-situ device or sample stage (Fig. S2) filled with N2 gas and excited face-on
(not through the substrate).
In Situ synchrotron X-ray diffraction during heating process: During the heating process, In-situ
XRD measurements were carried out by using a synchrotron light source at Beamline 01C1 at
National Synchrotron Radiation Research Center (NSRRC), Hsin-Chu, Taiwan. The electron storage
ring was operated at 1.5 GeV with a beam current of 360 mA. An image plate detector was used for
data collection. The wavelength was moved to 0.774901 Å and calibrated by silicon-LaB6 mixtures.
A typical amount of sample powder (1−2 mg) was put into a flow cell (quartz capillary tube with a
diameter of 0.8 mm) in the Ar-filled glove box in advance. The samples were then heated by a hot-
air gun from room temperature to 100 o
C. Thus, any structural transformations observed were not
because of decomposition or oxidation of the material upon air exposure. The 2Theta scan was
performed in the range of 5 - 100 degree with the step angle size of 0.02 degree. The obtained XRD
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patterns, then were converted to 1.5418 Å as the energy of Cu Kα1 in order to facilitate the
comparison with in-house XRD results and other literature.
Angle Dispersive X-ray diffraction (ADXRD): ADXRD measurements were performed using the
beamline BL12B1 at SPring8, Japan. The wavelength is 0.688917 Å (18 keV). To find d-values from
the IP data, both the sample–detector distance and the monochromatic beam energy must be
determined before the 2D-ADXD measurement by measuring a standard sample (e.g., CeO2) whose
d-values are well known. In this system, we use CeO2 as the standard sample to calibrate the energy
of the monochromatic beam and to determine the sample–detector distance. For calibration of the
monochromatic beam energy, CeO2 is measured using the monochromatic beam A program (General
Structure Analysis System, GSAS)37
with an editor (graphical user interface, GUI, EXPUGI) was
adopted to control the progress of the Rietveld-type fit to obtain unit cell parameters and volumes.
In-situ X-ray Absorption Spectroscopy: The X-ray absorption spectra were recorded at the
Beamline BL17C1 (NSRRC, Hsin-Chu, Taiwan) with an operation current of 360 mA. The electron
storage ring was operated at 3 GeV. A double Si(111) crystal monochromator was used for energy
selection. Three gas-filled ionization chambers were employed in series to measure the intensities of
the incident beam (I0), the beam transmitted by the sample (It), and the beam subsequently
transmitted by the reference foil (Ir) as represented with a general setup in Fig. S3a. The third ion
chamber was used in conjunction with the reference sample, which was a Pb foil for Pb L3-edge
measurements. The CH3NH3PbI3 powder was prepared in a metal plate and sealed with Kepton
tape inside the glovebox to avoid air contact during measurement. Then the metal plate containing
the sample was placed in a stainless steel holder in order to control the temperature change during
XAS measurement. The sample with stainless steel holder was then placed between the incident
beam detector and the reference beam detector. Prior to the XAS measurements, the perovskite
sample was treated in the holder with pure Ar gas for 30 min with a flow rate of 30 cm3
/min. The in-
situ XAS scanning was performed at various temperatures 25, 80, and 100°C). In order to investigate
the radiation effect, two temperature range was applied for the measurement: the in-situ XAS scan
was firstly conducted at 25 °C, and increased to 100 °C and then returned back to 25 °C by natural
cooling. All in-situ measurement steps, including the sample preparation, control of parameters for
extended x-ray absorption fine structure (EXAFS) measurements, and data collection modes, were
following the guidelines set by International XAFS Society Standards and Criteria Committee.38, 39
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Atomic Force Microscopy: The surface morphology of perovskite thin film was imaged with
tapping-mode atomic force microscopy (AFM, Asylum MFP-3D-BIO).36
An Al reflex coated AFM
probe (Olympus AC240TS) with a spring constant of 2 Nm-1
and tip radius of 9 nm was used.
Temperature controller and 532 nm laser source were installed together with the AFM to study the
temperature and laser irradiation-dependent morphology evolutions and grain size change.
Parameters such as drive amplitudes, set point, scan size and resolution, and feedback gains were all
tuned to yield high-quality scans with minimal noise or damage to the sample and tip.
Results and Discussion
Physical origin behind disorder and film heterogeneity
A reliable crystallization method for perovskites thin film, compatible with fast continuous process
over large-area flexible substrates is crucial for high performance solar cell production. We
investigated laser light irradiation induced reconstruction, crystallization and associated material
property changes in solution processed organolead halide hybrid perovskites thin film. Moreover, we
mainly explored the impact played by the Nanosecond pulsed laser irradiation of hybrid perovskites
and mechanisms of laser induced processes, which require an in-depth study. We applied UV pulse
laser for thin film treatments.
Laser irradiation vs thermal induced photoexcited carrier dynamics in CH3NH3PbI3 thin film are
compared and experimental TRPL results are shown in Fig. 1a and 1b. According to the previous
time-resolved studies in the CH3NH3PbI3 thin film, a controversial decay features of the photoexcited
state have been reported as nonexponential24, 26-28
and exponential6, 25
decay features. In contrast to
previous reports that showed light-induced degradation of CH3NH3PbI3 crystalline,16-18
we
investigated that laser irradiation can extend photoexcited carrier decay times. The decay equation,
as shown in Equation 1, can have a single-exponential form the fast process and stretched
exponential decay from the slow process.40, 41
The type of exponential form depends on the β values.
These values are less than unity and the stretching exponential dominated over the single
exponential. The controversial reports from previous studies did not include important parameters:
(1) the role of disorder and heterogeneity of the perovskite thin film and (2) the dispersion factor, β.
Therefore, considering these two parameters allows to determine the exponential form in perovskite
thin film. Based on our findings, the type of exponential is stretched exponential and the decay has
only slow component.
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R(t) = Aexp(-t/τ)β
, (Equation 1)
where, R(t) is the normalized PL intensity (i.e., relaxation phenomena) during the decay process. The
decay time τ and the dispersion factor β are characteristic wavelength-dependent decay constants.
The parameter β (0 < β < 1) is related to the curvature of the decay, i.e., the decay time distribution.41
This decay law is often encountered in disordered and heterogeneous systems and is considered as a
consequence of the dispersive diffusion of the photoexcited carriers.42
A stretched exponential (β <
1) is often used to describe the heterogeneous dynamics of disordered materials. Hence, diffusion
heterogeneities are often quantified by the exponent β of a stretched exponential function, which in
the homogeneous case is 1 (single exponential function) and for heterogeneous systems is less than
1. The heterogeneities manifested themselves in the apparent randomness in the morphology of
materials.43
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Fig. 1 | Laser irradiation vs thermal induced photoexcited carrier dynamics and heterogeneity.
(a) and (d) schematic representation of the experimental setup for laser irradiation and thermal
treatment, (b) and (e) Bulk time-resolved PL extended decay time after excitation of the
CH3NH3PbI3 film at 266 nm laser and treated at a different temperature, respectively, fitted to a
stretched exponential function. (c) and (f) Dynamics of the time-dependent decay time and β values,
respectively.
Note that we used the term disorder to explain position and orientation disorder or lattice distortion
and thermal fluctuation, while the term heterogeneity was used to describe thin film surface
inhomogeneties related to presence of traps, defects, impurities resulting in none-uniform film
morphology and local structural inhomogeneties related to phase coexistence in the sample or
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spatially heterogeneous polycrystalline morphology of these perovskites based materials, which arise
from the presence of crystalline grains with remarkably different size, shape, and possibly distinct
level of defects unless otherwise stated. Therefore, we proposed that the Nanosecond pulsed laser
irradiation induced reduction of randomness in the morphology, dynamic disorder and heterogeneity
extended the lifetime of photoexcited state in the CH3NH3PbI3 film as illustrated by the extension of
the decay time from 7 ns to 29 ns after 266 nm laser pulse irradiation (Fig. 1b). The best result we
obtained was due to both reduced disorder and heterogeneity as indicated by the increase in β values,
from 0.45 to 0.68 as shown in Fig. 1b, indicating laser induced reduction of disorder, defects and its
related non-radiative recombinations such as Auger and Shockley-Read-Hall (SRH) recombination
pathways in the CH3NH3PbI3 film, respectively. we used the term reconstruction process that related
to removal of disorder and heterogeneities and improving the surface (morphology) of the thin film,
for instance, our nanosecond pulsed laser reconstructed the original and randomly networked
morphology into 1D or nano-pillar like morphology, with improved properties as well as crystal
phase reconstruction by reorganization of the atoms in the crystal unless otherwise stated.
Mechanisms of microstructure reconstruction and nano-structuring-Steady state PL: The
occurrence of small β values (β < 1) in our experimental results (Fig. 1) indicated presence of
dispersive exciton motions that confirm influence from the disorder and the heterogeneity at room
temperature. The β exponent in Equation 1 is determined only by the dispersive motion.42
Indeed, in
the absence of disorder and heterogeneity, the electron diffusion will not modify the decay line
shape, at most it will change the decay time constant. Previous reports based on free carrier model
and exciton energy analysis have indicated that a single exponential has been used to interpret time-
resolved studies and the recombination of free electrons and holes, dominated by the relaxation
process of photoexcited states.29, 30
In our work, however, we have observed a slower decay process.
This attributes to slow carrier recombination processes resulting from more ordered morphologies or
decreases in heterogeneity as indicated by the increase in β values from 0.45 to 0.68 in Fig. 1c. This
indicated that pulse nanosecond laser irradiation cleans the disordered and heterogeneities present in
the perovskites thin film surface. This verified that pulse nanosecond laser treatment of perovskites
thin film surface is a powerful surface cleaning tool to reconstruct and restructure the surface,
improve the morphology and reduce non-radiative recombination centers. All these are proved by the
increase in PL intensity shown in Fig. 2a. The evolution of the PL intensity clearly verified the
removal of non-radiative recombination centers present in the thin film before (after) the laser light
treatment. Therefore, the PL lifetime increased in Figs. 1b and 1c is due to the increase the total PL
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intensity (Fig. 2a). On the other hand, as the temperature increased, both τ and β (Fig. 1e and 1f)
decreased and thus the morphological disorder and/or degree of heterogeneity increases, indicating
even higher-order recombination effects present.44
The corresponding reduction in PL lifetime (Fig.
1e, Fig. 2b and Fig. 2c), is impacted by the disorder and disorder induced trap states and
heterogeneities such as defects, which verified increase in non-radiative recombination rate at higher
temperatures. As the temperature increases, the PL intensity decreases, with a thermal quenching
activation energy equal to the exciton binding energy.45
Thus, at elevated temperatures, charge
carriers are no longer bound together, and it is faster for electrons and holes to find heterogeneous
sites such as structural and/or interface defects and quickly recombine non-radiatively. The low
carrier radiative recombination rate does not change as function of temperature but the non-radiative
recombination rate increases as the temperature increases.46
Hence, nanosecond pulsed laser
irradiation of CH3NH3PbI3 film mitigated the impact of structural disorder, morphology
heterogeneity and in-situ transformation to the cubic CH3NH3PbI3 phase, which boost the decay time
almost fourfold (i.e., from 7 ns to 29 ns) with increasing laser irradiation times. This indicates the
extreme importance of laser light treatment during perovskites film manufacture for perovskites solar
cells and other ultrafast optoelectronic devices applications.
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Fig. 2│ Laser light and thermal treatment of CH3NH3PbI3 thin film. PL intensity versus a)
nanosecond pulsed 266 nm laser exposure time, b) temperature and c) at continuous 266 nm laser
irradiation of perovskites thin film. Note that while the pulse laser exposure was dominated by
photon energy (enhancing the PL intensity), the continuous laser irradiation was dominated by
thermal energy (quenching the PL intensity). Excitation and Emission wavelength were 470 nm and
765 nm, respectively.
Low-temperature in-situ PL spectra, taken using 266 nm UV laser excitation (~10 W/cm2
) and aging
times ranging from 0 to 57 minutes continuously, are shown in Figs. S5b, S5c, and S5d. The fast
intensity decay (Figs. S5b and S5c) indicated either laser induced surface traps and/or structural
transformation induced crystal and interfacial defects. Previous reports16-18
showed changes in
morphology and XRD patterns which were simply assigned to light-induced degradations. However,
all of those changes are associated with light-induced structural transformations and changes in
morphology of the perovskite film rather than light-induced degradations under an inert atmosphere.
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This is due to continuous laser irradiation induced high photon flux, which enhanced the thermal
induced effects and this verified an increased non-radiative recombination rate. This was the reason
for the reduction of total PL intensity in Fig. S5b and S5c. We performed in-situ Raman after
extended irradiation of CH3NH3PbI3 using the UV laser and found that the shape of the Raman
spectra, obtained after steady-state PL measurement using 266 nm laser excitation, were similar to
those for CH3NH3PbI3 irradiated with longer exposure times using a 532 nm laser (Fig. S6a). Figs.
S6b and S6c also indicated, the material undergone phase transformation verified by the shift in the
band edge luminescence position from 1.61 to 1.59 eV. The effect of 266 nm laser intensity was
applied to get an optimum intensity with the optimum laser intensity of 10 mW/cm3
(Fig. S7a and
S7b). We noticed that this effect is the laser wavelength dependent: longer wavelength (e.g., 470 nm,
weak irradiation) laser irradiation treatment shows no remarkable extension of lifetime decay (Fig.
S7c and S7d) compared to 266 nm laser strong excitation, attributed to a rapid relaxation of the
carriers to defect states. 266 nm laser strong excitation induced higher PL lifetime due to strong
defect deactivation and disorder reduction on the surface of the thin film. In general, excitons could
be trapped by structural disorder, impurities or defects. In disordered polycrystalline thin films such
as CH3NH3PbI3, a greater number of trap states are likely to be formed. Thus, an increase in
temperature and continuous laser irradiations of the thin film enhance all these non-radiative
recombination centers and result in reduction of both PL intensity and PL lifetime. The TRPL
analysis confirmed that laser light extended the decay time and enhanced the carrier mobility by
reducing the recombination in the perovskites thin film. This suggested that laser induced surface
reconstruction and in-situ photo-thermal dynamic microstructural evolution plays a role to extend the
decay time dynamics. In order to understand the mechanisms of these two key processes in the thin
film of CH3NH3PbI3, we used in-situ micro-Raman spectroscopy and ADXRD. Notably, as observed
in Figs. 3b and 3c, the Raman signal intensity gradually evolved as a function of laser irradiation
time. This evolution can be summarized using two Raman spectral patterns, i.e. tetragonal and cubic
CH3NH3PbI3. Note that the detailed discussions for integrated peak intensity, integrated peak area
and full-width half maxima (Figs. S9 and S10 in SI).
Mechanisms of microstructure reconstruction and nano-structuring-micro Raman and Angle
dispersive X-ray diffraction: In α-CH3NH3PbI3 (Fig. 3b), it can be observed that two dominant
Raman active peaks are assigned to the vibrations of Pb-I bond (87 cm-1
) and vibrations of I-I
interaction (120 cm-1
): these vibration modes progressively shifted to 89 cm-1
and 123 cm-1
,
respectively. During the laser exposure period (500 seconds), the peak intensity (especially I-I
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signal) increased. After that, a discontinuity appeared with a Raman peak blue shift from 120 to 139
cm-1
accompanied by the appearance of a new broad peak, assigned to the methyl ammonium-MA at
280 cm-1
, possibly in a disordered state. Raman spectra for cubic CH3NH3PbI3, shown in Fig. 3c,
evolve as follows: (i)peak splitting results in two dominant peaks with the first at a lower
wavenumber shifting from 89 to 85 cm-1
, red shifted, and the second at the higher wavenumber
shifting from 120 to 139 cm-1
; (ii)peak intensity increased to a maximum and then decreased; (iii)the
decrease in peak intensity was compensated by peak broadening; (iv)a final 10 cm-1
peak position
red shifted from 280 to 270 cm-1
. A blue shift observed from 120 to 139 cm-1
, while the appearance
of a broad peak at 280 cm-1
indicated the presence of high structural distortion in CH3NH3PbI3. This
peak distortion activated new Raman modes (see Table S1 in SI) as indicated by the appearance of a
new broad peak at 280 cm-1
and, the peak split, shift and appearance of the more intense peak of the
Raman I-I band indicating phase transition and in-situ formation of new microstructure. The phase
transition from tetragonal CH3NH3PbI3 to cubic one was indicated by the discontinuity, appearance
of new peaks and the evolution of the Raman intensities at 500 seconds as shown in Fig. 3b. After
650 seconds (Fig. 3c), the Raman intensity started to decrease up to 1000 seconds showing crystal
defect formation during the disappearance of tetragonal and appearance of cubic phases. Then the
intensity starts to increase up to 1200 seconds indicating laser induced reconstruction of the
microstructure of the cubic or defect passivation. The final decrease in Raman intensity may be due
to laser induced ablation.
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Fig. 3 Laser-induced in-situ microstructure reconstruction. (a) Schematic representation of in-
situ Raman experimental set up; structure transformation of (b) tetragonal CH3NH3PbI3 to (c) cubic
CH3NH3PbI3 phase at continuous laser exposure using 532 nm with 30% laser power under
atmosphere controlled atmosphere, where α and β represent tetragonal and cubic phases.
Therefore, we noticed laser induced reconstruction and microstructure evolutions in the perovskite
thin film surface with increasing laser irradiation times. The main incidence of this topic lies in the
use of pulsed-laser processing in the fabrication and, improving surface and microstructure
properties of perovskite thin film. Laser processing has been demonstrated to be a powerful surface
tool: it can clean semiconductor surfaces very efficiently and very rapidly, and it can induce original
surface structures, which cannot be obtained by other means.47
Laser crystallization of hybrid
perovskite solar cells using near-infrared (NIR) laser (λ = 1064 nm) has been reported as an
alternative strategy to achieve higher device performances than conventional thermal annealing.48
632.8 nm and 785 nm lasers were, therefore, applied to study the effect of longer wavelength laser on
the structure of this material as shown in Fig. S11 in SI. We observed that except the change in
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Raman intensity, no structural changes occurred during the given irradiation time. This suggested the
photon energy of these two types of laser may not be enough to induce the change of the chemical
bonding or crystal phase change, consistent with what we observed from the aformentioned TRPL
measurements. Therefore, the laser induced changes depend on the laser wavelength, power and
energy density of the laser sources. High laser power and extended laser irradiation time can induce
thermal effect due to the high photon flux of the laser. These thermal effects induce local heating,
which promotes structural transformation. Hence, the local coordination of iodine surrounding Pb2+
and the conversion of tetragonal-CH3NH3PbI3 to cubic-CH3NH3PbI3 result in the rearrangement of
PbI3
-
octahedral and phase transition, as indicated by the appearance of new Raman peak, Raman-
active peak shifts as well as a gradual split of peaks at tetragonal-CH3NH3PbI3 at 500 seconds. It is
important to note that these laser-induced transformations are primarily raised on the surface rather
than in the bulk.
In order to clearly understand the laser induced reconstruction of microstructure and transformations
observed by micro-Raman, we further applied ADXRD (wavelength and energy of 0.688917 Å and
18 keV, respectively) available at BL12B1 Materials X-ray Study beamline in Spring-8 Japan. As a
result of laser irradiation, we observed three processes in Fig. 4a-4b: (1) The laser induced more
intense X-ray diffraction patterns compared to the pristine CH3NH3PbI3 indicating laser induced re-
crystallization and reconstruction of the tetragonal phase; (2) after 800 seconds of laser irradiation,
peaks at 2Theta values of 13.9, 28.6, 31.67 and 40.7° disappeared verifying another new phase
started to appear and (3) the X-ray diffraction patterns of the new phase become even more intense
indicating that laser induced crystallization and reconstruction of the material increase with laser
irradiation time even after phase transition. 30 mW/cm2
of homemade laser power (Fig. 4a) resulted
in slower reconstruction, crystallization efficiency and phase transformations as compared with these
processes at higher laser power (90 mW/cm2
). As shown in Fig. 4b, the homemade laser power of 90
mW/cm2
resulted in strong diffraction peaks and fast phase transformation into the cubic structure.
This is evidenced by the diffraction peaks after 480 seconds (Fig. 4b) and the diffraction peaks at 500
seconds (Fig. 4a). This indicated the laser irradiation has two advantages: 1) induces crystallization
and then 2) crystal structuring, which both verifying crystal reconstructions in perovskites materials.
The 3rd
one is a phase transition, which is free of crystal phase coexistence and cannot be achieved
by using temperature. The formation of highly crystalline cubic phase is evidenced by the similarity
of the laser induced diffraction peaks at 480 seconds (Fig. 4b) and temperature induced diffraction
peaks at 100 °C (Fig. S11). The intense diffraction peaks in Fig. 4b are verifying that pulsed laser
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irradiation can achieve better crystallization of hybrid perovskite than temperature. Laser power of
90 mW/cm2
can resolve this structural coexistence within 100 seconds and this verifies special
advantage of laser treatment of perovskites over temperature treatment. Therefore, the reason for the
extended decay time after laser treatment of perovskite film in the previous discussions is due to
laser induced reconstruction of perovskite thin film and achieving better crystallization efficiency
during laser irradiation. It is important to note that the use of a high-powered laser and extended laser
irradiation leads to degradations as shown in Fig. 4b at 720-960 seconds. A pulsed laser power
greater than 100 mW/cm2
can directly lead to degradation. The signals at 9.68 and 12.53° (Fig. 4b),
which are readily assigned to CH3NH3I and PbI2, respectively, clearly show the degradation of
perovskite films under laser irradiation.
Fig. 4 Laser irradiation induced microstructure reconstruction. Microstructure evolutions and
transformations of tetragonal phase under laser irradiation time (a) from 0-900 seconds at 30
mW/cm2
and (b) from 0-960 seconds at 90 mW/cm2
laser power of 532 nm homemade laser source
during laser irradiation time under the atmosphere controlled environment. Note that MGL-FN-532 -
300 mW/cm2
, 12060578-DPSSL DRIVER movable laser source was used in this study. Its operating
mode is CW and output power is up to 300 mW/cm2
.
In-situ XAS study of local disorder and structural evolutions in CH3NH3PbI3 thin films: The
detailed dynamic behavior of CH3NH3PbI3 can be extracted from in-situ micro-Raman and ADXRD
studies. In order to understand the thermal and nanosecond pulsed laser effects, we applied
temperature dependent in-situ XAS for the first time. The local structures around Pb (II) atoms were
studied by Pb L3-edge extended X-ray absorption fine structure (EXAFS) to examine structural
distortion of the octahedron and local structural changes. Fig. 5 shows an in-situ XAS study of local
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structural changes in CH3NH3PbI3. The peak intensity decreased when the temperature increased as
shown in Figs. 5a and 5b. Changes of the fine structure (Figs. 5a and 5b) and atomic distortions are
observed in Table 1. Changes in the XANES are not observed in the temperature range used in this
study (Fig. 5c). Note that, except the fine structure of CH3NH3PbI3 in EXAFS spectra, the oxidation
state of Pb in CH3NH3PbI3 does not show any change with changes in temperature in the XANES
spectra, indicating that it is in the +2 state similar to PbI2. In order to ensure the reliability of the
structural information, we fitted multiple shells of atoms (two shell-fitting for the room temperature
structure, and five shell-fitting for the high-temperature metastable structure) resulting in ‘well-
fitted’ results are obtained as shown from the experimental spectra of Pb L3-edge, χ(k), shown in
Fig. 5. The fitting parameters are r and σ2
for each shell, giving a total coordination number of 6. At
room temperature, two kinds of atom pairs, longer Pb-I (1) bond (3.218 Å) and shorter Pb-I (2) bond
(3.102 Å) are analyzed independently. All these values correspond to the tetragonal structure.
However, at higher temperatures, above the phase transition, we found five types of atom pairs as
shown in Table 1. From these values, we estimated the distortion of Pb atoms with I atoms in the
tetragonal and cubic structures, respectively. Our fittings used the standard r factor and fitting results
from the Fourier transform window shown in Fig. S14 in SI. In very rare cases, high values of the
Debye–Waller factor are indicative of a distortion of the iodine environment of the Pb atoms, which
is due to deformation and rotations of the PbI3
-
octahedral.49
Thus, the local environment of Pb(II) is
drastically disordered when the temperature was increased as indicated by the larger value of Debye-
Waller factor, which increased with temperature. Based on the fitting results we have developed a
theoretical model for both the room temperature and high-temperature as shown in Figs. 15a and
15b. CH3NH3PbI3 crystals have a tetragonal symmetry of the room temperature phase as shown in
Fig. 15a and Table S3. For this phase, iodide anions are ordered, which resulted in the lower
symmetry. Site occupancies are 1/4 for C and N for the tetragonal CH3NH3PbI3. While the
CH3NH3PbI3 crystals have a cubic symmetry for the high temperature phase, the CH3NH3
+
cation is
polar and has C3v symmetry, which should result in disordered cubic phase.8
In addition to the
disordering of the CH3NH3
+
cation, the halogen ions are also disordered in the cubic phase shown
in Fig. S16b and Table S4 in SI. Site occupancies are 1/4 for I atom and 1/12 for C and N atoms.
Those CH3NH3
+
cations occupy those 12 equivalent orientations of the C2-axis, and hydrogen atoms
have two sets of configurations on the C2-axis. Then, the total degree of freedom is 24.50
In order to
understand the effect of radiation on the local environment of Pb(II), we measured EXAFS spectra at
room temperature and then after heating to 100 o
C and finally cooled to room temperature to make
sure the material was not decomposed. The calculated values after cooling are similar to the values
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calculated at room temperature except for higher distortion values, due to the reversible phase
transition from cubic back to tetragonal structure and greater degree of distortion after treatment, as
indicated by the distortion values and Pb-I (1) bond length in Table 1. This confirmed that
CH3NH3PbI3 does not decompose, but only phase change occurred inside the thin films. The pulse
laser irradiation did not lead to a phase transition, but it played two roles: a) crystallization efficiency
increased after pulse laser treatment indicated by the increased peak intensity compared to the
thermal treated one (Fig. 5), which is observed to intensity decrease; b) the distortion values
decreased after laser treatment as shown in Table S2. These results verified that the structural
disorder was well minimized while crystallinity was well enhanced. The phenomena induced by the
two conditions clearly reason out why the PL lifetime and total PL intensity changes in Error!
Reference source not found.a & 1c and Error! Reference source not found.a, respectively, with
temperature and laser light. In order to understand the effect of radiation on the local environment of
Pb (II), we measured EXAFS after cooling the temperature back to room temperature and we found
better crystallization efficiency, indicating there was no degradation during heating.
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Fig. 5 In-situ XAS study of Local structural change CH3NH3PbI3 powder. Pb L3-edge (a) and (c)
EXAFS spectra, (b) and (d) XANES spectra and (e) and (f) the k-weighted XAFS, k3
χ(k)
CH3NH3PbI3 at room temperature, 80 and 100 o
C obtained by a transmission mode. All
measurements have been conducted in an atmosphere controlled system using a sealed metal plate.
Experimental χ(k) spectra of Pb L3-edge for CH3NH3PbI3 at various temperatures and Nanosecond
pulse laser irradiation time. Note that a weighting scheme of k3
compensates for the attenuation in
EXAFS amplitude at higher k values, and prevents the domination of larger amplitude oscillations in
the determination of interatomic distances. In addition, this weighting scheme minimizes the
influence of chemical and multiple scattering effects on the signal, which occurs mainly in the low-k
region of the spectrum.
Based on this information, we conclude that thermal post-treatment results in structural disorder on
CH3NH3PbI3 as shown in Table 1. This distortion results from the difference between the shortest
and longest nearest neighboring spacing for different Pb-I distances within the different CH3NH3PbI3
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structures. EXAFS spectra obtained from samples annealed at different temperatures also reflect that
the large local atomic distortions occur during the thermal process.
Table 1 Structural parameters of CH3NH3PbI3 obtained from EXAFS spectra
Sample Shell N R (Å) σ2
(Å
2
) *10
-3
r-factor Structure
Start at room
temp.
Pb-I(1) 4 3.219 ± 0.018 17.880 ±3.280
0.008 TetragonalPb-I(2) 2
3.103 ± 0.011
10.600 ±1.070
Treatment at
80˚C
Pb-I(1) 1 3.102 ± 0.014 15.469 ± 2.128
0.008 Cubic
Pb-I(2) 2 3.102 ± 0.007 11.832 ± 0.746
Pb-I(3) 1 3.199 ± 0.014 15.469± 2.128
Pb-I(4) 1 3.240 ± 0.014 15.469 ± 2.128
Pb-I(5) 1 3.274 ± 0.014 15.469 ± 2.128
Treatment at
100˚C
Pb-I(1) 1 3.114 ± 0.020 17.372 ± 3.530
0.015 Cubic
Pb-I(2) 2 3.104 ± 0.011 12.343± 0.121
Pb-I(3) 1 3.210 ± 0.021 17.372 ± 3.530
Pb-I(4) 1 3.251 ± 0.021 17.372± 3.530
Pb-I(5) 1 3.286 ± 0.021 17.372± 3.530
Return to
room temp.
Pb-I(1) 4 3.224 ± 0.026 21.750±1.720
0.009 TetragonalPb-I(2) 2 3.104 ± 0.011 10.840± 0.740
CH3NH3PbI3 film morphology evolution: The significant morphological improvements of
CH3NH3PbI3 film has been reported using the intermixing-seeded growth technique by intermixing
precursor-capped inorganic nanoparticles of PbS.51
We used UV pulse laser irradiation to reconstruct
and improve the morphology of CH3NH3PbI3 film. AFM (tapping mode) topographic imaging was
used to characterize and visualize the surface morphological changes in the CH3NH3PbI3 films
treated by laser irradiation and thermal heating. Fig. 6A (a, c, e, and g) shows the morphology
changes with increasing laser irradiation times. The pristine film has a surface roughness (Rq) of
15.9 nm (5 µm × 5 µm scan). The Rq of CH3NH3PbI3 film increased from 15.9 to 74.1 nm as the
laser irradiation time increased from 0 to 6 minutes (360 seconds). Similarly, the AFM measured
grain size of the film increased from 111 to 610 nm with the corresponding laser irradiation time.
Higher film roughness correlates directly to larger grains. This increase in roughness and grain size is
related to the growth of 1D morphology. The microscopic mechanisms of nucleation and growth of
these of 1D morphology may need further investigation. As shown in Fig. 6A (a), the morphology
displays valleys and hills. These changes of the valleys and hills in Fig. 6A (c) is the surface
morphology of coalesced grains, suggesting that the laser irradiation stimulated the migration of
grain boundaries and may cause the coalescence of more grains upon a time. This can also happen
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not only with increasing laser irradiation time, but also with increasing laser power: (1) As a result,
the randomness in morphology/structural disorder and film heterogeneity observed before laser
irradiation becomes changed into a compact, uniform and dense film with a narrow grain distribution
of particle interconnections. (2) The growth of 1D morphology should have fascinating properties
and extending the decay time. The fascinating properties, which these 1D nanostructures exhibit, are
based on their atomic scale structures and their 1D morphology e.g. their length and their diameter
dimensions on the nanoscale. This potential relies on the subtle control of their physical properties
such as their density of states and the transport of electrons and photons. As shown in Fig. 6B (b, d, f
and h), however, the growth of 1D morphology is not observed after thermal heating except various
valleys and hills, verifying the increased morphological heterogeneities and degree of morphological
randomness. The temperature induced roughness and measured grain size changes are far less than
those resulting from laser excitation. The main reason for the extended PL lifetime and PL intensity
was pulse laser induced following phenomena. 1) surface photo-brightening or cleaning of the
disordered (randomly networked) of the surface or morphology and cleaning of heterogeneities
related to size, shape and defects as well as coexisting crystal phases. 2) Growth of 1D morphology
i.e. pulse laser induced structuring of the randomly networked surface and morphology of the
perovskites thin film into 1D morphology as shown in Fig. 6 and Fig. S17 and S18 in SI. Phenomena
1) and 2) collectively increased the probability of radiative recombination over non-radiative
recombination. However, for the temperature treated and continuously irradiated thin film, higher
probability of non-radiative recombination over radiative recombination was observed due to thermal
activated quenching of PL intensity and PL lifetime. An analysis of the temperature dependence of
the spectra and PL decays suggests that the observed reduction of PL lifetimes and PL intensity can
be attributed to increase of impurity or defect concentrations. Intrinsic impurities and deep level
defects were shortening the PL lifetime and reduce the PL intensity. Additionally, surface disorder
induced by the processing temperature and continuous laser irradiation can have a significant effect
on the luminescence intensity for non-radiative recombination centers can be generated. Unlike the
above physical phenomena, possibility of surface chemical composition changes were not
happening. This was supported by the increase in the X-ray Diffraction intensity and EXAFS data,
verifying increase in crystallization efficiency and decrease in distortion. However, at extended time
exposure and higher laser power, it is confirmed by the XRD data that degradation was taking place.
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Fig. 6 Comparison of laser irradiation and thermally induced morphology variations in CH3NH3PbI3
film surfaces. Variations of morphology disorder and heterogeneity with variations in (A) laser light
irradiation time and (B) temperature. Laser power used was 30 mW/cm2
.
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To this end, substantial differences in the microstructure and morphology of the laser treated and
temperature treated films are clearly observed from the 3D AFM images shown in Fig. 6 and 2D
images of surface morphology as shown in Fig. S16 in SI. Thus, we concluded that nanosecond
pulsed laser processing is a powerful surface tool that it can clean hybrid perovskite surfaces very
efficiently and very rapidly and it can induce 1D morphology structures, which cannot be obtained
by other means such as temperature as shown in the SEM images of Fig. S17 and S18 in SI. Monte
Carlo simulations reveal that Auger recombination results from charge carrier congregations in
‘Auger hot spots’: lower-energy sites that are present because of energy disorder. Disorder-
enhanced Auger recombination is a general effect that is expected to be active in all disordered
materials.52
The reduced heterogeneity leads to overcome recombination at defect sites like SRH
recombination, while the reduced randomness in the morphology and disorder present in the
perovskite thin film overcomes the Auger recombination as shown in Scheme 1a. This was achieved
by the proposed mechanisms of morphological and structural transformations into the oriented 1D
morphology structure shown in Scheme 1a after nanosecond pulsed laser irradiations when compared
with morphology in Scheme 1b after thermal treatment. Thus, nanosecond pulsed laser irradiation
changed the randomly packed particle network to regularly orient 1D packed particle network, which
further reduces the disorder and heterogeneity present in the thin film. Nanosecond pulsed laser
irradiation can be a strategy to make extended decay time in perovskites thin film by reducing the
morphological disorder, heterogeneity, and dimensionality of the morphological network and
mitigating the non-radiative recombination processes. The results obtained from the AFM study
along with our structural investigations can help understanding the role of nanosecond pulsed laser
light. Thus, this nanosecond pulsed laser treatment during and post-synthesis crystallization of
CH3NH3PbI3 film have been shown high efficiencies of 17.8%53
and 11.3%48
, respectively, verifying
nanosecond pulsed laser treatment of hybrid perovskites during synthesis results in better
crystallization efficiency. Thus, both film homogeneity and Pulse laser excitation-driven non-thermal
transformation of random network of morphology into highly ordered 1D morphology reduced the
non-radiative recombination centers in CH3NH3PbI3 thin film thereby were extending the decay time
(Figs. 1b and 1c). Moreover, this work provides a clear insight towards identifying the physical
origin behind the disorder, heterogeneity, film reconstruction and nano-structuring as well as their
correlation with improved PL lifetime. More importantly, our results provide an insight towards
exploring the mechanism of temperature and light-induced surface recombination in perovskite film
with the think of developing a material model for what happens with temperature and laser light.
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Scheme 1 Schematic representation of the role of a) nanosecond pulsed laser irradiation by 1P
excitation UV laser and b) thermal heating of organolead halide hybrid perovskite thin films.
Conclusion
We found that thermal post-treatment caused morphology/structural disorder, film heterogeneity, and
thermally induced phase transition in perovskites films. All these parameters strongly diminished the
decay time of the photoexcited carrier and phonon lifetime. In contrast to this, laser treatment
cleaned morphology/structural disorder, film heterogeneity and overcome problems related to phase
transitions by introducing rapid crystal phase reconstruction and 1D morphology. This rapid
reconstruction varies with the laser irradiation time, laser power and/or laser intensity and laser
wavelength. Compared to 633 nm and 785 nm wavelength laser sources, a better crystal phase
reconstruction using 532 nm lasers at constant laser power and 266 nm UV pulse laser is explored.
Nanosecond pulsed UV laser can derive the reconstruction process more rapidly compared to visible
and near infrared pulse lasers. Interestingly, nanosecond pulsed laser induced cleaning further is a
useful approach to reduce the density of recombination centers of perovskites film, and strongly
enhanced the phonon lifetime, the decay time of the photoexcited carrier and charge carrier mobility.
Thus, light irradiation is directly associated with rapid and dynamic photo-thermal induced
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morphology reconstruction, structural evolution, electro-optical modifications and phase
transformation of CH3NH3PbI3 perovskites, but not necessarily linked with degradation, though
extended laser irradiation time and high laser power lead to degradations. Furthermore, our findings
suggest that by using nanosecond pulsed laser light treatment during manufacturing, it is likely that
we will see boosting the lifetime and performance of perovskites solar cell. Our work highlights the
importance of nanosecond pulsed laser cleaning of morphological disorder, heterogeneity, and
various imperfections in developing high-performance perovskites based optoelectronic devices.
Thus, Nanosecond pulsed laser irradiation is a new approach to the reconstruction of perovskites film
microstructure and will create a new approach towards future processing of high quality organolead
halide hybrid perovskites materials. Moreover, this work provides a clear insight towards identifying
the physical origin behind disorder, heterogeneity, film reconstruction and structuring as well as their
correlation with improved PL lifetime.
Acknowledgements: The financial support from the Ministry of Science and Technology (MoST)
(104-3113-E-011-001, 104-ET-E-001-ET, 103-2221-E-011-156-MY3, and 102-2221-E-011-157),
the Top University Projects 100H45140), and the Global Networking Talent 3.0 plan (NTUST
104DI005) from the Ministry of Education of Taiwan. We would like to thank the facilities support
from National Taiwan University of Science and Technology (NTUST).
Supplementary information
Contents about experimental setup, spectroscopic analysis results – XRD, XAS, Raman, PL as well
as AFM, SEM images are also available at: http://
Authors’ Contribution
B. J. Hwang and W. N. Su conceived the project and designed the experiments. T. A. Berhe, C. H.
Chen, L. Y. Chen and H. M. Chen performed material preparation, structural characterization,
Raman measurements and lifetime measurements. T. A. Berhe, J. H. Cheng, C. J. Pan, M.H. Yeh, S.F.
Huang, J. F. Lee, Yen-Fa Liao and B. J. Hwang performed the X-ray absorption and angular X-ray
diffraction measurement and M. C. Tsai performed the computational calculation. T. A. Berhe, J. H.
Cheng, W. N. Su, B. J. Hwang and H. Dai analyzed the data. T. A. Berhe, W. N. Su, and B. J.
Hwang wrote the paper. Z. Yang, H. Tan and E. H. Sargent performed all rounds of feedback and
direct-editing and general oversight.
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