Low-Temperature Absorption, Photoluminescence, and Lifetime of CsPbX3 (X = Cl, Br, I) Nanocrystals

Article abstract of DOI:10.1002/adfm.201800945

The absorption and photoluminescence, both steady-state and time-resolved, of CsPbX₃ (X = Cl, Br, I) nanocrystals are reported at temperatures ranging from 3 to 300 K. These measurements offer a unique window into the fundamental properties of this class of materials which is considered promising for light-emitting and detection devices. The bandgaps are shown to increase from low to high temperature, and none of the examined cesium-based perovskite nanocrystals exhibit a bandgap discontinuity in this temperature range suggesting constant crystal phase. Time-resolved measurements show that the radiative lifetime of the band-edge emission depends strongly on the halide ion and increases with heating. The increasing lifetime at higher temperatures is attributed primarily to free carriers produced from exciton fission, corroborated by the prevalence of excitonic character in absorption. The results particularly highlight many of the similarities in physical properties, such as low exciton binding energy and long lifetime, between CsPbI₃ and hybrid organic–inorganic plumbotrihalide perovskites.

Full text of DOI:10.1002/adfm.201800945

FULL PAPER
Semiconductor Nanocrystals
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Low-Temperature Absorption, Photoluminescence,
and Lifetime of CsPbX3 (X = Cl, Br, I) Nanocrystals
Benjamin T. Diroll, Hua Zhou, and Richard D. Schaller*
NC of CsPbX3 (X = Cl, Br, or I) has devel-
oped rapidly both as a potentially more
chemically or thermally stable alternative
material for solar cells,[3,14,15] or others
uses to which the material well-suited,
like high-energy detection[8] or light-emit-
ting devices.[12,16] Although most work
with these materials is focused on room-
temperature behavior, low-temperature
measurements offer the opportunity to
answer fundamental questions about
CsPbX3 materials, including changes in
the bandgap, exciton binding energy, and
electronic structure.
In this work, we examine the optical
properties of pure CsPbX3 (X = Cl, Br,
or I) NCs from 3 to 300 K, using absorp-
tion, photoluminescence (PL), and time-
resolved PL (trPL) measurements. Similar
to earlier reports on selected NCs and
single-crystal samples, both absorption
and PL measurements reveal that the
The absorption and photoluminescence, both steady-state and time-
resolved, of CsPbX3 (X = Cl, Br, I) nanocrystals are reported at temperatures
ranging from 3 to 300 K. These measurements offer a unique window into
the fundamental properties of this class of materials which is considered
promising for light-emitting and detection devices. The bandgaps are
shown to increase from low to high temperature, and none of the examined
cesium-based perovskite nanocrystals exhibit a bandgap discontinuity in
this temperature range suggesting constant crystal phase. Time-resolved
measurements show that the radiative lifetime of the band-edge emission
depends strongly on the halide ion and increases with heating. The
increasing lifetime at higher temperatures is attributed primarily to free
carriers produced from exciton fission, corroborated by the prevalence of
excitonic character in absorption. The results particularly highlight many of
the similarities in physical properties, such as low exciton binding energy and
long lifetime, between CsPbI3 and hybrid organic–inorganic plumbotrihalide
perovskites.
bandgap of CsPbX3 NCs increases upon heating from 3 K,
although the magnitude of the wavelength shift in CsPbCl3 is
very small. Unlike bulk materials,[13] no phase transition from
the orthorhombic phase stable at room temperature is observed
optically from 3 to 300 K in this work, or even to 500 K in ear-
lier work.[17] This is confirmed by X-ray diffraction over a more
limited number of temperatures. Absorption measurements at
low temperature reveal a clear bound excitonic feature which
decreases in strength with increasing temperature, permitting
an estimate of the exciton binding energy. Time-resolved PL
data show that the radiative rate of CsPbX3 species is fastest
at low temperature, reaching a decay lifetime of just 30 ps
for CsPbCl3 NCs, confirming that the lowest excitonic state is
optically allowed. This is distinct from the apparent excitonic
structure of both FAPbI3 and MAPbI3 NCs, which show long
PL lifetimes at low temperatures.[18,19] Several potential mecha-
nisms to explain this behavior are discussed and compared
with experimental data. The temperature-dependent changes in
lifetime of the CsPbX3 NCs are best explained as a combina-
tion including exciton fission and additional thermally induced
changes in the radiative rate, which is particularly important
for CsPbI3.
1. Introduction
The tremendous and rapid success of plumbotrihalide pero-
vskite materials as single crystals, thin films, and nanocrystals
(NCs) in solid-state devices including solar cells,[1–4] light-emit-
ting diodes (LEDs),[5–7] detectors,[8,9] and lasers[10–12] has led to
great interest in their fundamental properties. The successful
implementation of methylammonium (MA) and formadinium
(FA) lead iodide in solution-processes solar cells inspired
renewed interest in the long-known cesium analogs,[13] which
share many of the advantageous optoelectronic properties of
organolead trihalides. The synthesis of both single-crystal and
Dr. B. T. Diroll, Dr. R. D. Schaller
Center for Nanoscale Materials
Argonne National Laboratory
9700 S. Cass Avenue, Lemont, IL 60439, USA
E-mail: schaller@anl.gov
Dr. H. Zhou
Advanced Photon Source
Argonne National Laboratory
9700 S. Cass Avenue, Lemont, IL 60439, USA
Dr. R. D. Schaller
Department of Chemistry
Northwestern University
2145 Sheridan Road, Evanston, IL 60208, USA
2. Results and Discussion
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/adfm.201800945.
At least one aspect of the appeal of plumbotrihalide perovskites
is that changing the halide composition permits a change in
DOI: 10.1002/adfm.201800945
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taken to isolate NCs without antisolvent precipitation of any
kind, since common polar solvents readily dissolve the polar
halide perovskite compounds.[3] Typical optical spectra of the
CsPbX3 NCs are shown in Figure 1a with accompanying trans-
mission electron microscopy (TEM) images in Figure 1b–d.
The absorption edges and PL spectra span the visible spectrum
as the halide composition is changed. This size of the approx-
imately cubic NCs used in this study is 9.1 nm for CsPbCl3,
15.0 nm for CsPbBr3, and 16.6 nm for CsPbI3, which is above
the estimated Bohr radii for the chloride (5 nm), bromide
(7 nm), and iodide (12 nm).[20] Thus, quantum confinement is
not anticipated to play a significant role in the observed optical
properties. Multiple samples were examined for each compo-
sition to confirm the same observed trends, indicating relative
insensitivity to small changes in size. As the cubic perovskite
“black” phase of CsPbI3 is metastable and the family of CsPbX3
is known to be quite sensitive to air, all samples were measured
soon after purification (<24 h) under high-vacuum conditions
(≈10−7 Torr). The use of NCs, which present distinct phase sta-
bility from bulk materials,[3] enables the study of the metastable
CsPbI3 perovskite phase in a manner which has proven diffi-
cult to achieve with single crystals or thin films; this study rep-
resents, to our knowledge, the first examination of absorption
and PL of cubic CsPbI3 < 80 K.
Figure 1. a) Absorption and photoluminescence spectra of solution of
CsPbX3 nanocrystals. TEM images of the b) CsPbI3, c) CsPbBr3, and
d) CsPbCl3 NC samples.
Temperature-dependent PL and absorption collected from
3 to 300 K for the CsPbX3 samples are shown in Figure 2.
Both PL and absorption data show that bandgap increases
with temperature, which is atypical for most semiconductors
but widely reported for plumbotrihalides. Similar results have
the bandgap energy through the visible spectrum. In this work,
pure-halide compositions were synthesized as NCs following
literature recipes for hot injection synthesis.[20] Special care was
Figure 2. Normalized steady-state PL of a) CsPbI3, b) CsPbBr3, and c) CsPbCl3 NC samples collected from 3 to 300 K. Absorption measurements of
d) CsPbI3, e) CsPbBr3, and f) CsPbCl3 NC samples collected over the same temperature range.
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Figure 3. a) Temperature dependence of the bandgap of CsPbX3 NCs based upon measurements of absorption (solid circles), derived from Tauc plots,
PL (solid diamonds), derived from the center wavelength of the emission, and excitonic absorption maximum (open circles), derived from second
derivative analysis of the absorption. Solid lines represent Varshni fits to the data. b) Temperature dependence of the PL linewidth for CsPbX3 NCs
(open symbols) fitted in solid lines with a model for thermal broadening. c) Temperature dependence of the relative oscillator strength of the excitonic
absorption feature for the CsPbX3 NC samples. Solid fit lines represent a function of the form I(T)= I0(1+ AeE /kT )−1.
b
been observed previously in low-temperature measurements
of selected plumbotrihalide perovskites.[16,21–23] The magnitude
of increase follows the trend of CsPbCl3 < CsPbBr3 < CsPbI3.
As expected, the samples display thermal broadening of PL at
higher temperatures. Unlike the same bulk materials,[13] no
phase transition is apparent in the optical data collected here
or in measurements up to 500 K.[17] Consistent with these
optical observations, a more limited sampling of temperature-
dependent X-ray measurements, presented in Figure S1 (Sup-
porting Information) as well as in prior literature,[24] shows no
change in the crystal structure with temperature, although the
unit cell expands slightly at higher temperature, as is expected
for most materials.
The trends in bandgap and spectral broadening are tracked
quantitatively in Figure 3. Figure 3a relates three measures
of the semiconductor bandgap as a function of temperature:
measured from fitted center of PL energy (open circles), second
derivative minima representing the energy of the excitonic
absorption (triangles), and Tauc plot measurement extrapo-
lations (solid circles). With the exception of the PL energy of
the CsPbCl3 NC sample at temperatures > 250 K, the bandgaps
increase with increasing temperature. This small blueshift
followed by a small redshift with increasing temperature for
CsPbCl3 was reproducible for NCs synthesized in several
batches. Thermal changes in bandgap determined by each
method for each composition were fitted using the empirical
Varshni relation, which describes the temperature-dependent
bandgap of semiconductors according to Equation (1)
for Si and 8.9 × 10−4 K−2 for GaAs),[25] with especially small
thermal bandgap changes for CsPbCl3.[25] This indicates that
thermo-optic changes in PL color are a significantly smaller
factor in light-emitting technology that operates at elevated
temperatures.
PL broadening arises from intrinsic inhomogeneity (Γinh),
contributions of exciton–acoustic phonon coupling (Γac), and
exciton-longitudinal optical (LO) phonon coupling (ΓLO at ELO
)
−1
LO /kT
Γ
(
T
)
= Γinh + ΓACT + ΓLO eE
− 1
(2)
(
)
Unlike single crystals, NCs may show an inhomogeneous
linewidth due to variable environments (frequently observed as
spectral diffusion in single particle measurements) and size dis-
persion, arising from quantum confinement effects, although
these are expected to be weak for the NCs measured here.[20]
We do not observe any clear secondary features or shoulder of
the PL at any temperature, despite earlier reports.[26,27] Thermal
broadening of PL at higher temperatures, plotted in Figure 3b,
is accounted for using a Voight fit. Raman measurements of
LO phonon energies (ELO) are 46 meV for CsPbCl3,[28] 16[29] or
19 meV[30] for CsPbBr3, and values for CsPbI3 are not reported,
but the value obtained by fitting the PL line broadening here is
16 meV.
The most noteworthy change observed in the absorption
profile is the increasing prominence, at low temperature, of
distinctive excitonic absorption features in the samples. These
absorption features are associated with bound excitonic states
in the materials and have also been identified at low tempera-
Egap = Egap, 0K −αT2 / β +T
(1)
[31,32]
(
)
ture in bulk films for CsPbBr3
and stand in stark contrast
to our previous measurements of the absorption spectrum of
MAPbI3 and FAPbI3 NCs, which show very weak excitonic
absorption.[19] These results are at least qualitatively consistent
with earlier work that has described the exciton binding energy,
based upon modeling of relative PL intensity versus tempera-
ture, to be 20 meV for CsPbI3,[20] 37–44 meV CsPbBr3,[16,20,33]
and 57–75 meV for CsPbCl3.[20,33,34] Calculations based upon
a hydrogenic Wannier–Mott model place the exciton binding
energy between 50 and 80 meV for CsPbX3, but also claim
that predicted exciton binding energies follow the trend of
I < Cl < Br for CsPbX3 in contradistinction to other trihalide
perovskites.[35] Our absorption data allow a direct experimental
in which α is the coefficient of change and β is the Debye tem-
perature for the material.[25] The plots of the bandgap change
for each of the materials are close to linear, with fitted β values
from the Varshni relation close to zero, suggestive of low Debye
temperatures. α values for all the samples are negative, but
increase in absolute magnitude from chloride to bromide to
iodide. These values range from −0.5 × 10−5 K−2 for CsPbCl3,
−(1.3 to 1.9) × 10−4 K−2 for CsPbBr3, and −(2.0 to 2.6) × 10−4 K−2
for CsPbI3 (see Table S1, Supporting Information). Thermal
changes in bandgap are opposite in sign and smaller in mag-
nitude than most semiconductors (e.g., α is 7.0 × 10−4 K−2
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estimation of the exciton binding energy,
which is a critical parameter which underlies
performance of optoelectronic devices. This
methodology also improves on other indi-
rect methods for estimating exciton binding
energy, which are particularly fraught in
nanomaterials.
Exciton binding energy estimates using
temperature-dependent PL intensity meas-
urements, which typically show thermally
activated quenching of PL,[16,34] assume that
exciton fission must result in nonradia-
tive recombination. It is far from clear that
exciton fission should have a similar impact
on NCs as on single crystals; in some cases
many thermally activated processes can occur
simultaneously to quench PL;[34] and sample
preparation may yield distinct thermal sta-
bility.[16,17,34] Here, we modify this procedure
by observing the absorption spectrum directly
as a function of temperature and estimating
the exciton binding energy by fitting the
temperature dependence of the intensity (I)
of the bound excitonic absorption of CsPbX3
samples normalized to the higher-energy
continuum absorption, which corrects for
any changes in the scale of absorption from
sample heterogeneity. Because absorption
is less sensitive to impurities than PL and
represents a direct measure of oscillator
strength of various transitions, this meas-
urement offers a more reliable measure
Figure 4. Normalized trPL data for a) CsPbCl3, b) CsPbBr3, and c) CsPbI3 samples collected
from 2.7 to 300 K. d) Lifetimes obtained from fits to the experimental trPL data.
of exciton binding energy. The exciton binding energy, Eb, is
halide composition is strongly predictive of the lifetime, with
Cl < Br < I.[20,41] In particular, the radiative rate at 2.7 K is dictated
by recombination of bound excitons, which in turn depends
strongly on the dielectric screening of the different materials.[42]
This relationship holds at all temperatures, with a change in
halide composition increasing the lifetime by a factor of 3–10
from CsPbCl3 to CsPbBr3 and from CsPbBr3 to CsPbI3. For the
CsPbCl3 sample, a more clearly biexponential decay is observed
beginning at 120 K. It is not clear if this is trap-related recom-
bination, as it appeared in all NC samples measured and also
appears in single crystals.[34] As highlighted in previous work,
bromide and iodide samples showed substantially maintained
integrated PL intensity upon cooling from room temperature,
while chloride samples brighten significantly.[17] This is also
reflected in the biexponential character of PL decay in the chlo-
ride sample in Figure 4c. Consequently, the CsPbCl3 NC data
were fitted with a biexponential decay at higher temperatures
(plotting the averaged lifetime),[43] but the CsPbBr3 and CsPbI3
samples exhibited trPL dominated by single-exponential decays
at all temperatures. The trPL measurements at 2.7 K confirm
that the lowest energy states of all CsPbX3 samples are bright
and well-described by a single exponential decay, with lifetimes
of 30 ps (CsPbCl3), 120 ps (CsPbBr3), and 2.0 ns (CsPbI3).
As shown in Figure 4d, the fitted lifetimes of the samples
increase from 3 to 300 K. An increase in lifetime with tem-
perature elevation has been observed in other measurements
of trihalide perovskites.[17,21–23,44–46] Importantly, the CsPbI3
b /kT −1
obtained according to I (T) = I0 (1+ AeE ) ,[16,34] in which A is
a scaling factor. Although exciton binding energies may change
with temperature, the CsPbX3 samples do not undergo a phase
transition and therefore the assumption of comparable Eb over
the experimental temperature range is less problematic than
for the organolead halide perovskites.[36,37] The result, plotted
in Figure 3a, yields values of 67 meV for CsPbCl3, 47 meV for
CsPbBr3, and 25 meV for CsPbI3, close to most predicted theo-
retical values. Compared to organolead trihalides, the estimated
exciton binding energies of the CsPbX3 samples are larger as
these materials cannot demonstrate the same plastic crystal
properties associated with free cation rotation.[36,38] Although
the excitonic absorption of CsPbI3 is weaker at room temper-
ature (and above) than in the chloride and bromide samples,
second derivative analysis still reveals a small excitonic contri-
bution to the band-edge absorption (Figures S2 and S3, Sup-
porting Information). As has already been demonstrated,[3]
these results confirm that the “black” CsPbI3 phase is a more
promising solar cell material because it not only has a more
ideal bandgap, but also that absorption leads to a substantially
larger fraction of free carriers and performance is uncon-
strained by excitonic effects that may reduce carrier lifetime, as
contemplated in the following section.[39,40]
Figure 4 shows time-resolved photoluminescence measure-
ments collected for the CsPbX3 NC samples with the fitted expo-
nential lifetimes plotted in Figure 4d. As found previously, the
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Figure 5. Superimposed average lifetime data (closed circles) with the excitonic absorption feature (open circles) for a) CsPbI3, b) CsPbBr3, and
c) CsPbCl3.
NC trPL lifetime at room temperature is comparable to FAPbI3
and MAPbI3 NCs of similar size, underlining that the organic
cation of hybrid perovskites is not essential for longer carrier
lifetimes.[40] The extensive literature on cesium- and organolead
halide perovskites provides several, sometimes contradictory,
mechanisms for the observed behavior. Primarily, the focus
of literature on perovskites has been to explain long room-
temperature lifetime which is advantageous in many optoelec-
tronic devices, but these data show that these materials may
also have relatively short PL decay times at lower temperature.
Several mechanisms to explain temperature-dependent lifetime
behavior include (i) dark-state occupation, (ii) direct-to-indirect
bandgap through Rashba splitting, and (iii) exciton fission, with
the most evidence supporting the last of these.
Combined with previous studies, our data argue strongly
against dark states, direct-to-indirect bandgaps, and changes
in oscillator strength for NC forms of these materials. Temper-
ature-dependent occupation of singlet and triplet character of
PL is observed in several inorganic and organic/biological sys-
tems.[47,48] However, single NC magneto-optical measurements
demonstrate multiple closely spaced (<5 meV) bright states at
2 K, but no clear evidence of singlet-triplet mixing (i.e., dark
states) in the spectrally resolved or time-resolved data under
strong magnetic fields.[49] These measurements suggest that
if dark excitonic states exist above a bright ground state, they
remain inaccessible with a high magnetic field (7 T) and there-
fore are unlikely to influence dynamics < 100 K. Several works
have explained the comparatively long lifetime and enhanced
photovoltaic performance for organolead trihalide perovskites
as attributable to an indirect bandgap transition as the lowest
energy state.[50,51] Band-edge Rashba splitting has been deter-
mined computationally in methylammonium lead iodide
(MAPbI3)[51–54] and CsPbI3.[55] But the application of this theory
to the family of CsPbX3 materials is less clear, as these sam-
ples can exhibit very short lifetimes (particularly in the case of
CsPbCl3 and CsPbBr3 at cryogenic temperatures) and small
Stokes shifts which are uncorrelated with PL lifetime, which
make this unlikely (see Figure S9, Supporting Information).
Absorption data in Figure 2 showing relatively small
increases (up to a factor of 2) in the sample extinction suggest
that gross changes in oscillator strength of CsPbX3 at low tem-
perature do not explain the observed lifetime trend. However,
as described above, the band edge absorption feature, which
reflects bound excitons, changes more significantly than absorp-
tion into continuum states. Previously, the increased lifetime of
plumbotrihalide perovskites has been described as originating
from exciton fission.[16,36,37] Under this explanation, exciton fis-
sion would be expected to slow electron–hole recombination.
Consistent with this picture, as the size of CsPbBr3 NCs is
increased, and compared with a single crystal (see Figure S4,
Supporting Information), lifetime also increases, which sug-
gests that the spatial extent of delocalization controlled by the
crystal size can play a role in dictating the radiative rate in
NCs. Qualitatively, an exciton fission model is consistent with
decreasing excitonic absorption features, such as in Figure 2d–f
as well as the absolute lifetimes of the CsPbX3 samples and
the fitted exciton binding energies. Comparison of lifetime
and excitonic absorption data in Figure 5 for the CsPbX3 sam-
ples show that although the temperature dependence of both
parameters is similar for the chloride and bromide samples,
CsPbI3 shows divergent behavior of the lifetime and excitonic
absorption. The CsPbI3 sample does present a significant
increase in lifetime as thermal energy enables exciton fission
to free carriers, but an even larger increase at higher temper-
atures > 200 K where excitons are comparatively rare. In this
regard, CsPbI3 is similar to organic–inorganic hybrid perovs-
kites, which show that the lifetime of PL continues to increase
without significant bound exciton populations.[17,23] Although
long lifetime in organolead halide perovskites has been attrib-
uted by others to free rotation of the organic cation which raises
the dielectric function and enhances charge screening, this has
more recently been demonstrated to be nonessential, as CsPbI3
can also exhibit long lifetimes in the bulk.[40] Another specific
example of thermally activated process which may explain
the continued elongation of PL lifetimes above which exciton
fission can occur is the formation of polar domains,[56] which
are more prominent at higher temperature. Diffraction meas-
urements[24] and low-dose TEM imaging[57] have each found evi-
dence of polytypism within individual NCs, although without
explicit reference to polar domains, which may also result in
slower recombination.
3. Conclusion
In conclusion, we have presented the temperature-dependent
absorption, PL, and PL lifetime of colloidal CsPbX3 NCs. Com-
bined with previous work at high temperature, these measure-
ments show the distinct phase stability of NCs versus bulk
materials, particularly in the stability of “black” CsPbI3, with
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no phase change detected in optical measurements from 2.7
to 500 K. Absorption and PL measurements confirm the unu-
sual bandgap blueshift with increasing temperature which has
also been observed in other plumbotrihalides. Low-temper-
ature absorption measurements enable a direct estimation of
the exciton binding energy from the temperature-dependent
behavior of the bound excitonic absorption feature, in contrast
to previous indirect measurements based upon temperature-
dependent PL intensity. The decrease in the exciton binding
energy which results from heavier halides in the crystal, related
to the concomitant change in the dielectric function of CsPbX3,
translates into large changes in the temperature-dependent PL
lifetime dictated primarily by exciton fission with increasing
temperature.
Acknowledgements
This work was performed, in part, at the Center for Nanoscale
Materials, U.S. Department of Energy Office of Science User
a
Facility under Contract No. DE-AC02-06CH11357. This research used
resources of the Advanced Photon Source, a U.S. Department of
Energy (DOE) Office of Science User Facility operated for the DOE
Office of Science by Argonne National Laboratory under Contract No.
DE-AC02-06CH11357.
Conflict of Interest
The authors declare no conflict of interest.
Keywords
absorption, excitons, lifetime, perovskites, photoluminescence
4. Experimental Section
Received: February 5, 2018
Revised: April 19, 2018
Published online:
Materials: Oleic acid (90%), oleylamine (70%), octadecene
(ODE, 90%), tricotylphosphine (97%), lead bromide (>98%), lead
chloride (98%), lead iodide (99%), cesium carbonate (99.9%), and poly-
butylmethacrylate-co-polyisobutylmethacrylate (354 kDa) were purchased
from Sigma-Aldrich. Oleic acid and oleylamine were dried and stored in
a nitrogen filled glove box. All solvents were ACS grade or higher.
Synthesis: Synthesis of perovskite NCs followed the procedure of
Protesescu et al.[20] Typically, a reaction flask containing 4 mL ODE and
0.19 mmol PbX2 was heated to 120 °C under vacuum and held 1 h. Once
the reaction flask was under vacuum 1 h, it was flushed with nitrogen
and 0.5 mL of dry and degassed oleylamine and oleic acid were added.
The temperature was maintained until the reaction flask contained a
homogeneous solution. To synthesize CsPbCl3, an additional 1 mL of
trioctylphosphine was added. The flask was then heated under nitrogen
flow to 160 °C for synthesis of CsPbI3 and 180 °C for CsPbCl3 and
CsPbBr3. Separately, to prepare the Cs-oleate precursor, 407 mg Cs2CO3,
1.25 mL oleic acid, and 20 mL ODE were heated under vacuum to 120 °C
and held 1 h, then heated under nitrogen to 150 °C and held until the
Cs2CO3 completely dissolved. The Cs-oleate precursor was maintained
as a homogeneous clear solution by heating prior to injection. Once
the flask reached the set-point temperature, 0.4 mL of hot Cs-oleate
solution was rapidly injected, the reaction proceeded for 5 s, then was
cooled using a water bath. Samples were isolated by centrifugation of
the reaction media at 15 000 rpm for 15 min.
Steady-State Optical Measurements: Steady-state absorption
measurements were collected using a white light source focused on to
the sample within a Janis helium cryostat to a spot size of ≈0.5 mm. The
transmitted light was collected into a fiber and directed into a calibrated
Ocean Optics spectrometer. Steady-state PL measurements were
performed by exciting the samples with either a 350 nm 2 kHz source
(CsPbCl3 and CsPbBr3) generated by an OPA driven by a Spectra-Physics
Ti: sapphire laser or a 100 kHz PicoQuant 405 nm diode laser (CsPbI3).
The choice was driven both by wavelength considerations for absorption
and ability to synchronize the laser outputs with appropriate time-resolved
detection (Hamamatsu streak camera for CsPbCl3 and CsPbBr3; PicoQuant
avalanche photodiode for CsPbI3). To ensure that the results reflect
low-fluence single-exciton regime behavior, the laser was not focused
and power dependence was measured in time-resolved experiments,
which showed power-independent dynamics. PL was collected on the
opposite sample side of excitation and fiber coupled through a Princeton
Instruments spectrometer to a Princeton Instruments CCD.
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2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim