# Mapping the Pathways of Photo-induced Ion Migration in Organic-inorganic
Hybrid Halide Perovskites

Taeyong Kim  Department of Mechanical Engineering, University of California,
Santa Barbara, CA 93106, USA  Department of Mechanical Engineering, Seoul
National University, Seoul, 08826, Republic of Korea  Soyeon Park  National
Renewable Energy Laboratory, Golden, CO 80401, USA  Vasudevan Iyer  Center for
Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN
37830, USA  Qi Jiang  National Renewable Energy Laboratory, Golden, CO 80401,
USA  Usama Choudhry  Department of Mechanical Engineering, University of
California, Santa Barbara, CA 93106, USA  Gage Eichman  Center for Nanophase
Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN 37830, USA
Ryan Gnabasik  Department of Mechanical Engineering, University of California,
Santa Barbara, CA 93106, USA  Benjamin Lawrie<EMAIL_ADDRESS>Center for
Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN
37830, USA  Materials Science and Technology Division, Oak Ridge National
Laboratory, Oak Ridge, TN 37830, USA  Kai Zhu<EMAIL_ADDRESS>National
Renewable Energy Laboratory, Golden, CO 80401, USA  Bolin Liao<EMAIL_ADDRESS>Department of Mechanical Engineering, University of California, Santa Barbara,
CA 93106, USA

(August 27, 2024)

###### Abstract

Organic-inorganic hybrid perovskites (OIHPs) exhibiting exceptional
photovoltaic and optoelectronic properties are of fundamental and practical
interest, owing to their tunability and low manufacturing cost. For practical
applications, however, challenges such as material instability and the
photocurrent hysteresis occurring in perovskite solar cells under light
exposure need to be understood and addressed. While extensive investigations
have suggested that ion migration is a plausible origin of these detrimental
effects, detailed understanding of the ion migration pathways remains elusive.
Here, we report the characterization of photo-induced ion migration in OIHPs
using in situ laser illumination inside a scanning electron microscope,
coupled with secondary electron imaging, energy-dispersive X-ray spectroscopy
and cathodoluminescence with varying primary electron energies. Using
methylammonium lead iodide (MAPbI3), formamidinium lead iodide (FAPbI3) and
hybrid formamidinium-methylammonium lead iodide as model systems, we observed
photo-induced long-range migration of halide ions over hundreds of micrometers
and elucidated the transport pathways of various ions both near the surface
and inside the bulk of the OIHPs, including a surprising finding of the
vertical migration of lead ions. Our study provides insights into ion
migration processes in OIHPs that can aid OIHP material design and processing
in future applications.

hybrid perovskites, ion migration, electron microscopy, energy dispersive
X-ray spectroscopy, cathodoluminescence

## I. Introduction

Rapid progress has been made in the past decade in improving the power
conversion efficiency of photovoltaic cells based on organic-inorganic hybrid
perovskites (OIHPs). Recent reports have described power conversion
efficiencies exceeding 25% Jung  _et al._ (2019); Jiang  _et al._ (2022); Kim
_et al._ (2020); Park and Zhu (2022). The remarkable performance has been
attributed to OIHPs’ unique optoelectronic properties such as a direct band
gap and sharp absorption edge near visible wavelengths Baikie  _et al._
(2013); Stranks and Snaith (2015); Yin  _et al._ (2015), large photocarrier
diffusion lengths Shi  _et al._ (2015); Guo  _et al._ (2017) and high
tolerance to defects and grain boundaries Ball and Petrozza (2016); Brenes
_et al._ (2017), which are key factors for photovoltaic cells Kojima  _et al._
(2009); Grancini and Nazeeruddin (2019); Green  _et al._ (2021). Additionally,
OIHPs have shown other interesting properties, such as their tunability
Kulkarni  _et al._ (2014), moderate mobilities of charge carriers Shi  _et
al._ (2015) and ions Bakulin  _et al._ (2015), strong polaronic coupling
Srimath Kandada and Silva (2020); Grancini and Nazeeruddin (2019), and a
prominent photostriction effect Zhou  _et al._ (2016). Coupled with their
cost-effective low-temperature solution processability Song  _et al._ (2017),
these unusual optoelectronic properties suggest wide-ranging practical
optoelectronic applications of OIHPs beyond solar cells.

In addition to investigations of device efficiency optimization, extensive
prior studies have focused on understanding and improving the stability of
OIHPs Boyd  _et al._ (2019), which is a crucial limiting factor for long-term
practical applications. It has been well established experimentally that many
extrinsic factors can lead to degradation in these hybrid perovskite materials
Bella  _et al._ (2016); Boyd  _et al._ (2019), including exposure to moisture
Bella  _et al._ (2016); Christians  _et al._ (2015) and oxygen Bryant  _et
al._ (2016), thermal decomposition Conings  _et al._ (2015), and light-induced
chemical reactions Domanski  _et al._ (2017); Burschka  _et al._ (2013).
Corresponding strategies have been developed to counter some of the
degradation factors, e.g. OIHP encapsulation to avoid exposure to moisture or
oxygen Lee  _et al._ (2018); Cheacharoen  _et al._ (2018).

Many of the instability factors are related to the weak chemical bonds Ma  _et
al._ (2019) and strongly anharmonic lattice dynamics Yaffe  _et al._ (2017) in
OIHPs that also lead to ultralow thermal conductivity Haque  _et al._ (2020)
and facile ion migration induced by temperature Lai  _et al._ (2018), electric
field Xiao  _et al._ (2015a), or light Kim  _et al._ . Since temperature
gradients, electric fields, and light exposure are unavoidable in the normal
operation of solar cells, it is recognized that ion migration processes play
an essential role in OIHP-based devices Yuan and Huang (2016); Bi  _et al._
(2021). In this light, recent studies have observed ion-migration occurring in
OIHPs and its impact on the material morphology Xiao  _et al._ (2015a),
composition and device stability Burschka  _et al._ (2013); Snaith  _et al._
(2014); Yuan and Huang (2016). In addition, these studies have reported that
the ion migration might cause the abnormal photocurrent hysteresis effect,
which, in turn, affects fill factors and solar cell device performance Snaith
_et al._ (2014). Unlike extrinsic factors such as moisture and oxygen, ion
migration is intrinsic in OIHPs, likely linked to certain strongly anharmonic
lattice vibration modes involving the mobile ions Mizusaki  _et al._ (1983),
and, therefore, requires more sophisticated mitigation strategies Bi  _et al._
(2021). On the other hand, ion migration represents a possible mechanism for
the large static dielectric constant in OIHPs Lin  _et al._ (2015) that can
effectively screen charged defects and benefit charge transport. Some studies
have also suggested that the redistribution of ions as a result of migration
can lead to optimized p-i-n structures Deng  _et al._ (2015) or healed charge
trapping centers Ghosh  _et al._ (2017) and improve the device performance.
Besides, efficient ion migration in OIHPs also implies potential applications
of these materials as ionic conductors Kim  _et al._ . These examples
highlight the importance of understanding the ion migration processes in OIHPs
Yuan and Huang (2016).

Previous works have reported light-induced migration of ions such as the
halide anion (I-), the metal cation (Pb2+), and the organic cations (MA+ =
Methylammonium, FA+ = Formamidinium) Kim  _et al._ ; Zhao  _et al._ (2017);
Shirzadi  _et al._ (2022); Barbé  _et al._ (2018). In parallel, prior
computational studies have shown that the migration rate is determined by the
activation energy and that I- is more likely to migrate owing to its lowest
activation energy among other constituent ions (I-: 0.08 - 0.58 eV in Refs.
Azpiroz  _et al._ (2015); Eames  _et al._ (2015); Haruyama  _et al._ (2015);
Futscher  _et al._ (2019); MA+: 0.46 - 0.84 eV in Refs. Azpiroz  _et al._
(2015); Eames  _et al._ (2015); Haruyama  _et al._ (2015); Futscher  _et al._
(2019); Pb2+: 0.8 - 2.31 eV in Refs. Azpiroz  _et al._ (2015); Eames  _et al._
(2015)). Experimental measurements have corroborated this, with the
diffusivity of I- ($\sim 10^{-12}$ to $\sim 10^{-9}$ cm2 s-1 Azpiroz  _et al._
(2015); McGovern  _et al._ (2021); Futscher  _et al._ (2019)) far exceeding
that of organic cations (MA: $\sim 10^{-16}$ to $\sim 10^{-11}$ cm2 s-1
Azpiroz  _et al._ (2015); McGovern  _et al._ (2021); Futscher  _et al._
(2019)). In most previous experimental studies, the Pb2+ ion was assumed to be
immobile up to the available detection limits Yuan  _et al._ (2016a). Despite
the abundant previous literature on photo-induced ion migration in OIHPs, the
detailed microscopic migration pathways of various ions remain unclear. In
particular, ion migration both parallel to the sample surface (lateral ion
migration) and perpendicular to the sample surface (vertical ion migration)
can occur given the light intensity variation along the lateral direction and
the finite optical absorption depth into the sample. Therefore, experimental
tools that are sensitive to the distribution of ions along both the lateral
and vertical directions are highly desirable.

In this work, we focus on photo-induced ion migration in OIHPs. We
incorporated a laser source in situ into a scanning electron microscope (SEM)
and combined secondary electron imaging (SEI), energy-dispersive X-ray
spectroscopy (EDS) and cathodoluminescence (CL) with different primary
electron (PE) energies to directly map the three-dimensional (3D) ion
migration pathways in two archetypal OIHPs, MAPbI3, FAPbI3 and their hybrid.
We demonstrated that both lateral and vertical migrations of various ion
species driven by light exposure can be mapped in this manner, which further
correlate to the optoelectronic properties of OIHPs. This information is
critical for designing more stable and efficient devices based on OIHPs.

## II. Results

### II.1. Probing ion distribution within different depths

Our experimental setup for the SEI and EDS measurements is hosted at the
University of California Santa Barbara (UCSB) and is identical to that
described in Ref. Kim  _et al._ (2021), and the principle of operation is
schematically illustrated in Fig. 1A. An optical beam from a fiber laser
(Clark-MXR IMPULSE, photon energy: $\sim 2.4$ eV; repetition rate: 5 MHz) was
coupled into the SEM sample chamber and focused onto the sample, with a beam
diameter of $50\leavevmode\nobreak\ \mu$m. To examine the impact of the photo-
induced ion migration, the optical power and the exposure time were varied,
ranging from $\sim 2.5$ to $\sim 6.7$ mW (optical fluence: $\sim 39.8-106.6$
$\mu$J cm-2), and from 5 to 180 minutes, respectively. These optical
parameters for the SEI and EDS measurements were chosen to minimize the laser-
induced heating effect, and we estimated that the highest laser-induced
temperature rise in our experiment was within 30 K (see Supporting Information
Sec. V). Then, changes in the secondary electron (SE) emission and the
elemental composition induced by the optical exposure were mapped in situ with
concurrent SEI and EDS mapping. The depth-dependent ion distribution was
probed by comparing the response to PEs with different kinetic energies (30
keV and 5 keV) and different penetration depths into the sample. The CL
measurements coupled with in situ light-exposure were conducted at the Center
for Nanophase Materials Sciences (CNMS) at the Oak Ridge National Laboratory
(ORNL), and the corresponding experimental details are given in Methods. The
procedure for sample fabrication is also described in Methods. The structure
of the sample stack is illustrated in the inset of Fig. 1D. A high vacuum of
$1\times 10^{-6}$ torr was maintained inside the SEM chambers for all
measurements reported here, thus minimizing the influence of moisture or
oxygen. Representative EDS spectra of a pristine FAPbI3 sample deposited on a
fluorine-tin-oxide (FTO) coated glass are given in Fig. 1B (the MAPbI3 spectra
is given in the Supporting Information Sec. I). In the EDS spectra, peaks
corresponding to characteristic X-ray lines of various chemical elements can
be seen. The intensity of these characteristic peaks also depends on the PE
kinetic energy used. With 30-keV PEs, peaks near 1.74 keV, 2.35 keV, 3.44 keV,
and 3.94 keV were prominent, which were characteristic X-ray emissions from
silicon (Kα band), lead (Mα band), tin (Lα band) and iodine (Lα1 band) Mandal
_et al._ (2020); Cheng  _et al._ (2016); Maity and Ghosh (2018); Lin  _et al._
(2011). Lead and iodine were from the FAPbI3 layer, while tin and silicon only
existed in the substrate, signaling a deep penetration of the 30-keV PEs. In
contrast, with 5-keV PEs, we found that the tin and iodine bands were
suppressed, the lead band remained, while other lower-energy peaks emerged
near 277 eV, 392 eV, and 525 eV, corresponding to the characteristic X-ray
emissions from carbon, nitrogen and oxygen Goldstein  _et al._ (2017).
Additional EDS spectra with intermediate PE energies (10 keV and 20 keV) are
shown in the Supporting Information Sec. I.

To quantitatively understand the probing depth of PEs with different kinetic
energies, we conducted Monte Carlo simulations of the PE trajectories with
30-keV and 5-keV kinetic energies and the results were shown in Fig. 1C and D.
The detailed parameters used in our simulation were described in the
Supporting Information Sec. II. As shown in Fig. 1C and D, many of the 30-keV
PEs penetrated the entire FAPbI3 layer and into the substrate, while those at
5 keV were mostly confined within 200 nm near the surface of FAPbI3. This
indicates that the EDS spectra obtained with PEs with different kinetic
energies are sensitive to ion distributions at different depths into the
sample. Additionally, we note here that the self-absorption depth of the X-ray
photons emitted inside the samples was comparable to or greater than the
sample thickness, except for those X-ray photons emitted from carbon, nitrogen
and oxygen (see the Supporting Information Sec. IV for further details),
indicating that our measurement was sensitive to the PE penetration depth
rather than the X-ray absorption depth except for carbon, nitrogen and oxygen.

Figure 1:  Schematic illustration of the experimental setup and sample
structure. (A) Illustration of the SEM experimental platform incorporating an
in situ laser beam with SEI and EDS detectors. (B) The measured EDS spectra of
FAPbI3 without light exposure at the primary electron energies of 30 keV (blue
line) and 5 keV (red line). The corresponding elements are annotated near each
characteristic peak. Also shown are the simulated trajectories of primary
electrons with energies of (C) 30 keV, and (D) 5 keV. While the primary
electrons can penetrate the entire sample stack at 30 keV, those at 5 keV are
mostly confined within 200 nm near the FAPbI3 surface. The interfaces between
FAPbI3 and fluorine doped tin oxide (FTO), and between FTO and the glass
substrate are labeled with gray lines. The inset in (D) shows the structure of
the FAPbI3 sample stack.

Our experimental configuration permitted us to characterize the photo-induced
ion movement along with their impact on the SE yield in an in situ manner.
Figure 2 shows several SE images of FAPbI3 and MAPbI3 samples, which were
taken after the laser beam exposure (optical fluence: $106.6$ $\mu$J cm-2 for
both FAPbI3 and MAPbI3) at the center of the field of view for variable laser
dwell times. As shown in Fig. 2A, for FAPbI3, the SE image contrast became
brighter after the laser exposure for 5 minutes. Further exposure of up to 20
minutes led to the bright contrast expanding beyond the laser spot size. Under
prolonged optical exposure of $\gtrsim 30$ minutes, an area with dark contrast
emerged near the center of the beam area, forming a ring-shaped profile that
was surrounded by bright contrast at the outer region. We also observed that
additional optical exposure caused further expansion of the bright ring, while
the surrounding bright contrast area far exceeded the size of the optical
beam. In contrast, the SE images of MAPbI3 after light exposure exhibited a
slower temporal evolution of the bright contrast, likely due to a larger
bandgap in MAPbI3 and, thus, a weaker optical absorption. The size of the area
with bright contrast after longer exposure times was observed to be comparable
to the optical beam size. At lower optical fluences, qualitatively similar
contrast in SE images was seen, although the required exposure time was much
longer (see the Supporting Information Sec. I for additional data). We can
further correlate the observed bright SE contrast with the ion distribution
measured in situ by EDS. In particular, the X-ray counts corresponding to the
characteristic iodine line at 3.94 keV (probed by 30-keV PEs) and lead line at
2.35 keV (probed by 5-keV PEs) were measured along a horizontal line across
the center of the area exposed to the optical beam and were given as red
dashed lines in Fig. 2A and B. In general, we observed a decrease of iodine
counts within the area exposed to light, and the area with an iodine
deficiency increased with prolonged light exposure. Particularly in FAPbI3
(Fig. 2A), the iodine concentration profile agreed well with the spatial
extent of the bright contrast, suggesting long-ranged migration of iodine ions
far outside the illuminated region. However, the iodine concentration profile
is less consistent with the bright ring contrast observed in FAPbI3. In
contrast, although the lead concentration profile did not correlate to the
overall spatial range of the bright contrast, a two-peak structure developed
in FAPbI3 after 30-minute light exposure that matches the bright ring-shaped
contrast, indicating a lead-rich phase was responsible for the significantly
enhanced SE emission from the ring-shaped area. With our current technique,
however, the precise composition of this lead-rich phase could not be
determined, since the 30-keV PEs probed the iodine concentration inside the
entire thickness of the sample, not only at the surface.

Figure 2:  Secondary electron images with line profiles of iodine and lead
distribution after optical beam exposure. (A) SE images of FAPbI3 after laser
illumination for 5 to 60 minutes. (B) SE images of MAPbI3 after laser
illumination for 10 to 90 minutes. The red dashed lines (orange solid lines)
indicate the X-ray counts for iodine (lead) at 3.94 keV (2.35 keV) across the
illuminated area as measured using EDS. The iodine line was probed by 30-keV
PEs and the lead line was probed by 5-keV PEs. For all the data presented
here, an optical fluence of $\sim 106.6$ $\mu$J cm-2 was used.

### II.2. Quantification of ion migration processes

We can quantify the ion migration processes by carefully examining the ion
distributions mapped by EDS after different light exposure times. We first
focus on the iodine ions, which are known to be the most mobile species in
OIHPs. In Fig. 3A, we show the changes in the characteristic iodine X-ray
photons at 3.94 keV from FAPbI3 near the illuminated region. The changes in
the counts were calculated relative to the value measured in the pristine
region without light exposure. The changes in counts were measured using EDS
with 30-keV PEs and plotted as a function of the distance from the center of
the optical beam and after different light exposure durations. A reduction of
the iodine X-ray counts was observed in the illuminated region, signaling a
deficiency of iodine as a result of light exposure. Furthermore, the iodine
deficiency kept increasing and its spatial distribution kept expanding as a
function of exposure time. Since the 30-keV PEs probed the entire sample
thickness, the observed iodine deficiency indicated the amount of iodine
inside the sample was reduced, likely due to the decomposition of FAPbI3 and
the formation of I2 vapor escaping to the vacuum Albrecht and Green (1977). In
addition, the expanding spatial profile of the iodine deficiency signals
migration of iodine ions over a lengthscale of a few hundred micrometers,
significantly exceeding the optical beam size, driven by the iodine
concentration gradient due to the I2 escaping from the surface. From the time
dependence of the spatial profile of the iodine deficiency, we can estimate
the diffusivity of iodine ions, as shown in Fig. 3B. The extracted iodine ion
diffusivity is around $3.5\times 10^{-10}$ cm2 s-1, which is within the range
of previously reported values Azpiroz  _et al._ (2015); McGovern  _et al._
(2021); Futscher  _et al._ (2019). A detailed analysis of the diffusion
process was described in the Supporting Information Sec. III.

Next, we focus on the lead ions, whose distribution can be mapped by PEs with
both 30-keV and 5-keV kinetic energies. As shown in Fig. 3C and D, the lead
ion distribution probed by 30-keV PEs remained almost unchanged as a result of
light exposure, while its distribution probed by 5-keV PEs showed a
significant increase with light exposure. Since the 30-keV PEs can probe the
entire sample thickness and the 5-keV PEs only probe near the sample surface,
this surprising finding implied that the migration of lead ions mainly
occurred along the vertical direction, namely moving from inside the sample
towards the sample surface. More interestingly, the lead distribution near the
surface as probed by the 5-keV PEs showed a double-peak structure after a long
exposure, consistent with the bright ring contrast observed in the SE images
shown in Fig. 2A. As discussed before, this suggests that a lead-rich phase is
responsible for the enhanced SE emission in the bright ring region.

Another surprising finding was the observed migration of silicon and tin ions
from the substrate, as shown in Fig. 3E and F. While a small trace of silicon
was detected with the 5-keV PEs (as shown in Fig. 1B), the silicon
distribution did not change appreciably due to the light exposure (See the
Supporting Information Sec. I for additional data). However, the silicon and
tin distributions as probed by the 30-keV PEs showed a significant increase
with prolonged light exposure, suggesting increased silicon and tin
concentration deep inside the bulk of the FAPbI3 sample. This can be induced
by the isovalent lead vacancies created deep inside the bulk of the FAPbI3
sample after the lead ions migrated towards the sample surface. This
observation implies that ion migration can lead to unwanted interactions
between the OIHPs and the charge transport layers that can be detrimental to
solar cells based on OIHPs Li  _et al._ (2019). Lastly, we also observed a
decrease of nitrogen, carbon and oxygen concentration near the sample surface
as probed by 5-keV PEs due to light exposure (See the Supporting Information
Sec. I for additional data). However, we could not quantify the diffusivity of
the organic ions due to the relatively low EDS sensitivity and, thus, a low
signal-noise ratio.

Figure 3:  EDS elemental analysis of FAPbI3 after optical exposure. An optical
fluence of $\sim 106.6$ $\mu$J cm-2 was used for this dataset. (A) The change
in the X-ray counts corresponding to iodine as probed by 30-keV PEs. The
change in counts was calculated relative to the value measured from pristine
areas without light exposure. (B) The corresponding fitted radii of the iodine
deficiency distributions versus the exposure times measured by 30-keV PEs. The
error bar indicates numerically determined 95% confidence intervals. Also
shown are the measured EDS linescan datasets for (C) lead measured by 30-keV
PEs, (D) lead measured by 5-keV PEs, (E) tin measured by 30-keV PEs, and (F)
silicon measured by 30-keV PEs.

Based on the observations described above, we summarize the microscopic
picture of photo-induced ion migration in FAPbI3 schematically in Fig. 4.
Photo-induced decomposition of FAPbI3 into PbI2 and, further, into I2 vapor
leads to an iodine deficiency near the surface, which creates an iodine
concentration gradient that drives the diffusion of iodine ions towards the
surface. The lateral iodine concentration gradient created by the Gaussian
distribution of the laser beam intensity further induces iodine ion migration
towards the center of the laser illuminated area. In the meantime, lead ions
also migrate from the bulk towards the surface. Since lead cannot escape the
sample surface in vapor form, the accumulation of lead near the center of the
illuminated area starts to drive lead ions to move outwards near the surface
after longer light exposure, creating the double-peak lead concentration
profile that corresponds to the bright ring contrast in SE images. As a
consequence of lead ion migration, the lead vacancies left behind deep inside
the sample bulk induce the migration of isovalent tin and silicon ions from
the substrate into the sample, creating new defects and phases that might be
detrimental to OIHP-based devices.

Figure 4:  Schematic illustration of the photo-induced ion migration pathways
in OIHPs. Iodine ions migrate from the bulk towards the surface due to photo-
induced decomposition and I2 escaping to the vacuum. Lead ions migrate
vertically toward the surface. At the same time, elements in the substrate
such as tin and silicon penetrate across the sample/substrate interface. Right
panel indicates schematically the PE penetration depths through the specimen,
which depends on the PE kinetic energy. Figure 5:  Cathodoluminescence
analysis of 97% FAPbI3/3% MAPbBr3 after in situ pulsed laser exposure.
Correlated SE images (first row) and CL NMF decomposition (row 2 to 4) for (a)
the pristine sample and the same sample after (B) 5 minutes of optical
exposure, (C) 10 minutes of optical exposure, (D) 20 minutes of optical
exposure, (E) 30 minutes of optical exposure, and (F) 60 minutes of optical
exposure. The two spectral components of each NMF decomposition are
illustrated in the second row, and the intensity maps associated with each of
those components are illustrated in the third and the fourth row,
respectively. The color bars represent the absolute CL counts and highlight
the increase in band-edge CL intensity as a function of increased laser
exposure time.

### II.3. Correlating ion migration with cathodoluminescence

We note here that Barbé et al. observed a light-induced ring-shaped area with
enhanced photoluminescence Barbé  _et al._ (2018) in MAPbI3 and they
attributed it to the formation of a thin layer ($<20$ nm) of PbI2 using Raman
spectroscopy. However, no photoluminescence near the bandgap of PbI2 ($\sim
520$ nm to 540 nm) Ferreira da Silva  _et al._ (1996) was observed. To further
examine the correlation between light exposure and the optoelectronic
properties of OIHPs, we conducted in situ CL measurements of a hybrid OIHP,
97% FAPbI3 with 3% MAPbBr3. For this measurement, a femtosecond laser with 495
nm wavelength and 1.25 mW average power was focused to a 5 $\mu$m spot
(optical fluence: 80 $\mu$J cm-2) on the sample using a parabolic mirror in
the SEM. The laser-induced temperature rise in this case was around 90 K due
to the small beam size (see Supporting Information Sec. V for details).
Despite similar optical fluences, the higher steady-state temperature rise in
this case could also induce ion migration thermally. The same parabolic mirror
was used to collect CL generated by the perovskite film. Notably, substantial
research efforts in recent years have probed ion migration and perovskite
decomposition under optical and electron-beam excitation and in the presence
of various environmental factors Shirzadi  _et al._ (2022); Milosavljević  _et
al._ (2016); Yuan  _et al._ (2016b); Xiao  _et al._ (2015b). However, under
the electron-beam conditions used here (5-keV PE and 7 pA of beam current), we
have not observed any electron-beam induced degradation Taylor  _et al._
(2022). While environmental degradation limited CL characterization of the
pure FAPbI3 and MAPbI3 films (see Supporting Information Sec. I for more
data), the hybrid film exhibited a bright CL band near 800 nm consistent with
near band-edge luminescence.

The correlated SE and CL images acquired as a function of laser exposure time
are shown in Fig. 5. We used non-negative matrix factorization (NMF) to
decompose the hyperspectral CL images into representative spectral components.
The calculated explained variance ratio (EVR) is greater than 90% with two
spectral components for all of the spectrum images, and it is greater than 99%
with two spectral components for all of the images acquired after laser
exposure. The two spectral components for each image are shown in the second
row of Fig. 5, while the spatial distribution of CL corresponding to the two
spectral components are presented in row 3 and 4.

A few critical features are visible in Fig. 5. First, the surface morphology
exhibits substantial microstructuring after laser exposure, and the size of
the microstructured area increases with increasing laser exposure time.
Second, CL NMF decomposition component 1 is consistent with excitonic
luminescence, and component 2 is consistent with near-band-edge CL from an
intermediate phase that results from partial degradation of the film Xiao  _et
al._ (2015b); Taylor  _et al._ (2022). Critically, we did not observe any
substantial CL band from PbI2 near 520 nm to 540 nm, and we did not observe
any statistically significant increase in the intensity of the intermediate
phase CL as a function of laser exposure or position. However, while the band-
edge luminescence is completely suppressed in the microstructured area, the
band-edge CL intensity increases by up to a factor of 20 in the bright ring
surrounding the laser spot (with a monotonic increase in CL intensity as a
function of laser exposure time). Thus, we can conclude that (a) the ion
migration induced by laser exposure does not introduce any near-surface PbI2
or partially decomposed hybrid perovskite phases with any statistical
significance, (b) the ion migration completely suppresses band-edge
luminescence inside the bright ring, and (c) the ion migration substantially
enhanced the luminescence intensity within the bright ring in a manner that is
determined by the laser exposure time. We observed similar results using a
continuous-wave laser source at 532 nm for light exposure (see Supporting
Information Sec. I for further details). Interestingly, we also observed a
partial recovery of the excitonic CL emission near 800 nm in an
environmentally degraded FAPbI3 sample after light exposure (see Fig. S10 in
the Supporting Information), suggesting a beneficial impact of light-induced
ion migration in degraded OIHP samples.

### II.4. Summary

In summary, we utilized an in situ laser source incorporated into an SEM to
directly map lateral and vertical photo-induced ion migration pathways in
OIHPs with SEI, EDS and CL. We observed long-ranged iodine migration far
beyond the illuminated region, a surprising vertical migration of lead ions
and cross-boundary migration of silicon and tin ions from the substrate. CL
analysis indicated significantly enhanced band-edge emission in a bright ring
surrounding the laser spot with complete suppression of band-edge emission
within the ring. No increase in luminescence due to partially degraded
perovskite phases or to PbI2 band-edge emission was observed. These results
highlight the ubiquity of photo-induced ion migration processes in OIHPs and
their potential impact on the material quality and device performance. While
this understanding of photo-induced ion migration is certainly critical to the
development of mitigation strategies to suppress unwanted ion migration, the
observation of 20-fold enhancement in band-edge luminescence also highlights
an opportunity to improve perovskite device performance through controlled ion
migration. Our work also demonstrated that multi-modal optoelectronic
spectroscopies that combine laser and converged electron-beam excitations in
situ with variable PE energies and interaction volumes can be a valuable tool
to study microscopic ion transport in emerging materials.

## III. Methods

### III.1. Sample Fabrication

This section presents the method of fabrication for the samples considered in
this study. The FTO-coated glass substrates were washed with the detergent
aqueous solution, deionized water, and 2-propanol in the ultrasonic bath for
15 mins sequentially. Then the FTO substrate was treated with UV-ozone for 20
mins just before the perovskite precursor deposition. To prepare the 1.5M
MAPbI3 precursor solution, equal molar ratios of methylammonium iodide (MAI,
GreatCell Solar, 99.99%) and PbI2 (TCI, 99.99%) were dissolved in
dimethylformamide (DMF, Sigma-Aldrich, 99.8%) and dimethyl sulfoxide (DMSO,
Sigma-Aldrich, 99.8%) with a volume ratio of 4:1. To prepare the 1.4M FAPbI3
precursor solution, equal molar ratios of formanidium iodide (FAI, GreatCell
Solar, 99.99%), and PbI2 were dissolved in DMF and DMSO with a volume ratio of
8:1. The perovskite precursor solution was spin-coated onto the FTO substrates
at 4000 rpm for 30 s, and 0.6 ml of diethyl ether was dripped onto the
substrate at the time of 15 s to the end. The as-spun films were annealed at
100 ∘C for 10 mins and 150 ∘C for 20 mins for MAPbI3 and FAPbI3, respectively.

### III.2. SEM and EDS Measurement

Secondary electron imaging and EDS measurements reported in this work was
conducted inside a ThermoFisher Quanta 650 FEG SEM hosted at the University of
California Santa Barbara. The secondary electrons were detected using a
standard Everhart-Thornley detector, while the EDS spectra were collected
using a ThermoFisher UltraDry 30 EDS detector and interpreted using the
ThermoFisher Pathfinder software. A high vacuum of $1\times 10^{-6}$ torr was
maintained inside the SEM chamber for all measurements. The laser beam was
generated from a Yb-doped-fiber laser (Clark-MXR IMPULSE) with a fundamental
wavelength of 1030 nm, an average pulse width of 150 fs and a repetition rate
of 5 MHz. The fundamental wavelength was converted to 515 nm using a BBO
crystal and used as the source for light exposure. The laser beam was fed into
the SEM chamber through a transparent viewport on the chamber wall.

A fresh area on the sample was used for every light exposure test. After the
sample was exposed to light with a controlled intensity for a given amount of
time, a secondary electron image was taken with an accelerate voltage of 5 keV
and a beam current of 300 pA. A low beam current was used to minimize
electron-beam-induced damage to the sample. A dwell time of 1 $\mu$s at each
pixel was used to form the image. Then EDS line scans across the center of the
light-exposure area at every $\sim 7$ $\mu$m were taken with a beam current of
300 pA. 100 linescans (100 ms per scan) were taken and averaged to minimize
electron-beam induced damage, while enhancing the signal-to-noise ratio. The
change in the absolute count of X-ray photons emitted from the exposed area
was calculated as compared to the pristine background area.

### III.3. Cathodoluminescence

Cathodoluminescence (CL) measurements were performed with a Delmic Sparc CL
collection module in a ThermoFisher Quattro environmental SEM hosted in the
Center for Nanophase Materials Sciences (CNMS) at the Oak Ridge National
Laboratory. A parabolic mirror was used to collect CL generated under
electron-beam excitation and to deliver the laser to the sample. A retractable
mirror was inserted to direct free-space coupled laser sources to the
parabolic mirror and retracted to allow for in situ CL characterization using
an Andor Kymera spectrograph. Either a variable power 532 nm Cobolt CW laser
source or a 495 nm pulsed laser source generated from the second harmonic of a
Mai Tai Ti:Sapphire laser with 100 fs pulse duration, 80 MHz repetition rate,
and 1.25 mW average power were focused to a spot size of 5 microns by the
parabolic mirror. After variable laser exposure times with the retractable
mirror inserted, the mirror was retracted, and CL spectrum images were
acquired with the electron-beam conditions described in the manuscript.
Because the SEM and the adjacent optics table are independently floated, an
Aligna active beam stabilization system was used to dynamically correct for
any movement of the SEM relative to the optics table. Blind non-negative
matrix factorization (NMF) performed with scikit-learn Pedregosa  _et al._
(2011) was used to track changes in the CL spectrum images as a function of
laser exposure time.

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###### Acknowledgements.

The work conducted at University of California Santa Barbara (SEI and EDS) was
based on research supported by US Department of Energy, Office of Basic Energy
Sciences, under the award number DE-SC0019244 (for the development of the
laser-coupled SEM) and by the US Army Research Office under the award number
W911NF-19-1-0060 (for studying photo-induced physics in emerging materials).
Cathodoluminescence microscopies were conducted as part of a user project at
the Center for Nanophase Materials Sciences (CNMS), which is a US Department
of Energy, Office of Science User Facility at Oak Ridge National Laboratory.
The sample fabrication at National Renewable Energy Laboratory (NREL) was
supported by the US Department of Energy under Contract No. DE-AC36-08GO28308
with Alliance for Sustainable Energy, Limited Liability Company (LLC), the
Manager and Operator of NREL. The authors at NREL also acknowledge the support
on perovskite sample preparation from DE-FOA-0002064 and DE-EE0008790, funded
by the US Department of Energy, Office of Energy Efficiency and Renewable
Energy, Solar Energy Technologies Office. The views expressed in the article
do not necessarily represent the views of the DOE or the U.S. Government.

## Author Contributions

B. Liao, K.Z. and B. Lawrie conceived and supervised the project. S.P., Q.J.
and K.Z. prepared the samples. T.K. conducted the SEI and EDS measurements at
UCSB. U.C. and R.G. contributed to the instrument development at UCSB. V.I.
and B. Lawrie conducted SEI and CL measurements at ORNL. G.E. and B. Lawrie
conducted the CL data analysis. T.K., B. Liao and B. Lawrie drafted the
manuscript. All authors have commented and edited the manuscript.

## Competing Interests

The authors declare no competing interests.