www.sciencemag.org/cgi/content/full/science.aal4211/DC1 Supplementary Materials for Extremely efficient internal exciton dissociation through edge states in layered 2D perovskites J.-C. Blancon, H. Tsai, W. Nie, C. C. Stoumpos, L. Pedesseau, C. Katan, M. Kepenekian, C. M. M. Soe, K. Appavoo, M. Y. Sfeir, S. Tretiak, P. M. Ajayan, M. G. Kanatzidis, J. Even, J. J. Crochet,* A. D. Mohite* *Corresponding author. Email: [email protected] (J.C.C.); [email protected] (A.D.M.) Published 9 March 2017 on Science First Release DOI: 10.1126/science.aal4211 This PDF file includes: Materials and Methods Supplementary Text Figs. S1 to S14 Tables S1 and S2 References
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Supplementary Materials for · 2017. 3. 8. · Two dimensional Ruddlesden-Popper perovskite (RPP or 2D perovskite) crystal samples: the crystal structures of the RPPs, (BA) 2 (MA)
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Supplementary Materials for Extremely efficient internal exciton dissociation through edge states in layered
2D perovskites
J.-C. Blancon, H. Tsai, W. Nie, C. C. Stoumpos, L. Pedesseau, C. Katan, M. Kepenekian, C. M. M. Soe, K. Appavoo, M. Y. Sfeir, S. Tretiak, P. M. Ajayan, M. G. Kanatzidis, J. Even,
Spatially-resolved PL, TRPL and optical absorption were performed with an in-lab-built
confocal microscopy system focusing a CW or monochromatic 6-ps-pulsed laser (repetition rate
between 0.5 and 2 MHz) close to the diffraction limit. PL, reflection, and transmission spectral
responses were obtained through a spectrograph (Spectra-Pro 2300i) and a CCD camera
(EMCCD 1024B) yielding a maximum error of 2 nm. PL data in the main text were measured for
light excitation at 561 nm or 440 nm, depending on the PL response of each RPP, and the
excitation intensity was typically maintained below 1000 mW/cm2 to prevent any sample
degradation. PL intensity maps were obtained by rastering a tightly focused laser beam (below
1μm resolution) onto the sample surface by means of a fast steering mirror.
TRPL measurements were performed by means of a time correlated single photon counting
module (PicoHarp 300) combined with an Avalanche Photo-Diode (MPD-SPAD) through a
spectrograph to remove all laser light excitation.
Samples were measured under vacuum (10-5
-10-6
torr) under ambient conditions of
temperature if not mentioned otherwise.
MM5. Absolute absorption and photoluminescence quantum yield (PLQY)
Absolute absorption and PLQY of the thin films were measured by means of an integrating
sphere, following ref. (35), in air under ambient conditions. Measurements were acquired
directly after taking the samples out of vacuum to minimize effects of air exposure which were
found to be negligible for few hours (12), and some of the samples were encapsulated for cross-
check of the data. Alternatively, we performed these measurements in the microscope and verify
a meaningful set of data points using the integrating sphere.
3
Supplementary Text
ST1. Exciton binding energy in RPPs
The values of exciton binding energy measured in RPPs with n=1 to 5 are larger than 200
meV (Fig. S3). A limiting value of 150 meV was obtained alternatively from an analysis
including a Tauc plot for determining the energy of the continuum (Fig. S4). This is in stark
contrast with the exciton binding energy reported in 3D organic-inorganic hybrid lead
perovskite, which was estimated to be less than 50 meV (19) (and reference therein), with recent
measurements reporting 25 meV at room temperature in MAPbI3 (36) or even smaller (37).
Moreover, the intensity ratio between the exciton resonance state and the continuum contribution
is much larger than reported in the literature for the 3D perovskite MAPbI3, where the exciton
resonance is very difficult to separate from the continuum. At room temperature excitons in the
3D perovskite MAPbI3 are easily ionized. More precisely, the exciton binding energy in 3D
perovskite MAPbI3 is of the order of kBT at room temperature or of the LO phonon energy ~16
meV, which mandates the use of the low-frequency dielectric constant (εS ~ 30) in the excitonic
picture. This results in strong screening of the exciton and its ionization in 3D perovskites at
room temperature (see for example discussion in (38)). The situation in RPPs of n = 5 and
smaller is fundamentally different, because the exciton Bohr radius (~2.2 nm from our
preliminary calculations (9)) is comparable to the QW thickness. This leads to strong quantum
confinement effects resulting in exciton binding energy larger than 200 meV, requiring the use of
ε∞~ 6.1. In other words, even for n=5 the RPP layers behave as thin QWs.
Comparison to lead salt (LS) materials (e.g. PbS and PbSe) shows that the exciton binding
energies found in RPPs with n=1 to 5 are comparable to LS quantum dots (20), LS nanorods
(21), and LS nanosheets (22). However, we note that the high frequency dielectric constant in
RPPs (ε∞ = 6.1) is significantly smaller than that of LS materials (ε∞ = 17 − 23), which
explains the larger contribution of dielectric confinement effects in LS systems while quantum
confinement is important in RPPs.
Finally, in the Table S2 we compare the exciton binding energy in RPPs and low-
dimensional LS systems for similar confinement parameters (size and surrounding medium
dielectric constant 𝜀env). We note that the case n=1 corresponds to an extremely thin QW with a
sub-nanometric thickness. Very small confinement lengths were explored for LS nanorods (21)
showing indeed an increase by at least a factor of two of the binding energy for sub-nanometric
confinement lengths, consistent with our observations.
ST2. Potential cause of layer-edge states (LES) formation
LES are only observed for n>2 and their density-of-states increases with n (Fig. S7). This
strongly suggests that LES find their origin in surface states associated with the perovskite
octahedra not directly in contact with the (BA) organic spacers and located at the edges of the
anionic perovskite layers (Fig. 2E and 3D). Potential causes of LES formation include distortion
of the octahedra, exciton self-trapping, dangling bonds, and adsorption of molecules forming
hybrid surface states. Based on the experimental observation, exciton self-trapping and octahedra
distortion at the surface are a possible physical mechanism, as the variation of the octahedral
tilting angles has been reported to lead to band gap variations of several hundreds of meV (Fig. 2
of Ref. (9)). Alternatively or concomitantly, a mechanism purely of exciton self-trapping and
octahedra distortion origin is hardly compatible with the almost-constant energy of LES. On the
other hand, a mechanism of chemical origin, i.e. related to dangling bonds of the perovskite
4
octahedra or to adsorbed molecules at the perovskite layer edges, is also consistent with the
properties of the LES reported here.
5
Fig. S1. Principle of the microscopy experiment probing the exfoliated crystals (A)
and the thin films (B). (C) Photography of the thin films mounted in the cryostat. The
preferential orientation of the 2D perovskite layers was normal to the substrate in the thin
films (12) and parallel to the substrate in the exfoliated crystals.
Fig. S2. Stokes shift in both the RPP thin films and the exfoliated crystals as a function of n.
Derived from the optical band gap data in Fig. 1.
6
Fig. S3. Quantum and dielectric confinement effects in 2D perovskites. (A) Absorption of
the exfoliated crystals. ■ indicates the main exciton peak and ● the position of the band-to-band
absorption edge (also called continuum, which looks like a step-like function in 2D systems). (B)
Energy of band-to-band absorption threshold (EG) as a function of the 2D perovskite layer (QW)
thickness d (corresponding to n=1-5). (C) EG versus 1/d2. For 2D quantum confinement effect
with infinite confinement barriers, one expects a linear dependence. Deviation from classic
behavior, even for finite confinement barriers, has been reported previously (7) and is a
consequence of significant dielectric confinement that should be taken into account for low n-
values (small QW thickness d). (D) Exciton binding energy (Eb) for different n-value at room
temperature, estimated from the difference between the main exciton and the onset of the band-
to-band absorption in 2D (5), see details in Fig. S4. The error bar accounts for the relatively
important uncertainty on the position of the band-to-band absorption edge at room temperature.
The value and error bar for n=5 was derived from the temperature dependence of the absorption
presented in E,F. (E) Optical density (OD) response (absorption) as a function of temperature for
RPP n=5. We note the absence of phase transition in contrast to previous reports in n=1 RPPs (5,
6). (F) Temperature dependence of the energy position of the exciton peak (■) and band-to-band
absorption edge (●). We note that uncertainty on these energy positions increases with
temperature, which is a consequence of increased thermal fluctuations resulting in broadening of
both the exciton and the continuum features. (inset) Exciton binding energy derived from the
difference between the exciton peak position and the continuum in E. The average exciton
binding energy in n=5 over the temperature range 10 K to 295 K is 0.235 eV with mean
uncertainty 0.019 eV, with maximum error 0.080 eV close to room temperature.
A B C
D
2.8
2.6
2.4
2.2
2.0
1.8
En
erg
y (
eV
)
3.02.52.01.51.00.5
d (nm)
54321n
Continuum
Exciton peak
450
400
350
300
250
200
Eb (
me
V)
3.02.52.01.51.0
d (nm)
54321n
Ab
so
rpti
on
(a.u
.)
2.82.42.01.6Energy (eV)
n = 1
n = 5
OD
(a.u
.)
2.82.42.0
Energy (eV)
10K
35K
80K
150K
220K
295K2.10
2.05
2.00
1.95
1.90
1.85
1.80
En
erg
y (
eV
)
3002001000
Temperature (K)
0.26
0.24
0.22
0.20
0.18
Eb (
eV
)
3002001000
Continuum
Exciton peak
n = 5n = 5 n = 5
2.8
2.6
2.4
2.2
EG (
eV
)
2.52.01.51.00.50.0
1/d2 (nm
-2)
E F
7
Fig. S4. Methods for determining the exciton binding energy in exfoliated crystals. (A) Step
function method where the continuum energy is determined by identifying the position of the
step-like function characteristic of the band-to-band absorption profile in 2D systems (5). At
room temperature the step function is broadened and we locate the step-function profile using
both linear fit of the absorbance (red dashed line) and the second derivative of the absorbance (in
red, maxima and minima gives the inflection points of the step profile), which is between 2.074
and 2.140 eV in the case of n = 5 here. We retain 2.074 eV as the onset of the continuum and
estimate the error of this value by taking the width of the step profile (2.074-2.140=0.066 eV
here). (B) Tauc plot method where we use the Tauc plot to estimate the position of the
continuum by fitting the band-to-band transition with a linear function (dashed red line) and the
crossing with the zero energy axis provides the onset of the continuum (2.039 eV in the case of
n=5 here). We note that the Tauc plot method is an approximate method in the case of an
excitonic system. (C,D) Summary of the results for RPPs with n = 1 to 5 at room temperature;
energy of the continuum (C) and exciton binding energy (D). The measured exciton binding
energy is larger than 150 meV independent of the method used to determine the continuum
position, which confirms that excitons are strongly bound at room temperature in RPPs n=1 to 5.
B A
2.8
2.6
2.4
2.2
2.0
En
erg
y (
eV
)
54321
n
Continuum Step function Tauc plot
Exciton peak
D C 450
400
350
300
250
200
150
Eb (
me
V)
54321
n
Tauc plot
Step function
Step function method Tauc plot method
8
Fig. S5. Optical absorption anisotropy in 2D perovskite thin films and exfoliated crystals.
(A) Principle of the anisotropy measurement where θpol is the angle between the incident light
polarization and the RPP layers. In-plane polarization �⃗⃗� ⊥ �̂� corresponds to θpol=0 ̊. Out-of-plane
polarization �⃗⃗� ∥ �̂� takes place for θpol=90 ̊. (B) Polarization-dependent reflectance for a single
crystal of 2D perovskite of n=4. Significant anisotropy effect in crystals is observed, for example
the feature around 1.9 eV is almost completely damped for out-of-plane polarization. This is in
agreement with previous reports on crystals (5, 14). (C) Polarization-dependent absorption in 2D
perovskite thin films. The thin films demonstrate little anisotropy effects, meaning all absorption
optical transitions are preserved for all polarization direction (θpol). We observe minor changes of
ratio of the exciton-like features (between 2.0 and 2.2 eV), and small changes of the high energy
absorption continuum (>2.2 eV) up to 20% in n=4 probably related to interference effects
depending on incident light angle. We also investigated polarization-dependent PL and do not
observe any change of the optical band gap in the thin films.
9
Fig. S6. Microscopic origin of the low energy band gap in 2D perovskite thin films
with n=4. (A) Principle of the micro-PL experiment. (B) Intensity map of an exfoliated
crystal n=4, probed at 1.90 eV and 1.68 eV. (left) Microscopy image showing exposed
crystal edges at the center. (C,D) Comparison of the PL in the crystal, at the edges of the
crystal, and in the corresponding thin film. For the exfoliated crystal (C), the color-coded
spectra correspond to the positions identified by colored dots on the surface map in (B).
(E) PLE integrated signal of the LES by monitoring the PL at the crystal edge (red curve
in C,D) for different light excitation energy between 1.8 and 2.65 eV. For information the
PL profile of the LES is plotted in light red color. X indicates the position of the main
exciton in crystals. (F) TRPL of the PL features X and LES.
10
Fig. S7. Optical absorption and photoluminescence properties of 2D perovskite thin films
with n=1-5. (Left panels) Absorbance and (inset) PL at ~100 mW/cm2 (black) and higher
intensity (red). (Right panels) Light excitation intensity (I0) dependence of the integrated PL
signal for the layer-edge-state (LES) and exciton (X) features. Dashed lines are fits to the data
(see details in main text).
11
Fig. S8. Temperature dependence of the optical band gap in RPP thin films with n =
1 to 5. (A-E) Waterfall plots of the PL spectra in thin films as a function of temperature
from 5K to 290K. Dashed lines are guides for the eyes indicating the energy of the main
PL peak (corresponding to the optical band gap) as a function of temperature. (F) Optical
band gap energy shift as a function of temperature for n = 1 to 5. The temperature
variations of the optical band gap in RPPs with n>2 (corresponding to the LES) is about
identical, yielding 210 μeV/K and corresponding to a blue-shift. This value is similar to
that observed in 3D perovskite MAPbI3 in its tetragonal phase and attributed to the
thermal expansion of the lattice (30). On the other hand, RPPs with n=1,2 (corresponding
to exciton X-states at the band gap) demonstrate either negative (red-shift) or negligible
energy shift of their optical band gap with increasing temperature, supporting the
different origin of these electronic states as compared to n>2 and further identified as
exciton from previous studies (5, 6).
12
Fig. S9. Temperature dependence of the excitonic features in RPP thin films with
n>2. (A) Example of 2D perovskite with n = 3. (left) Absorption traces showing the
exciton X features (also identified in Fig. S7). (right) Second derivative of the absorption
spectra allowing for a better identification of the three features and their temperature
dependence. Dashed lines are guides for the eyes. (B) Energy shift of the three features as
a function of temperature in RPP with n=3 to 5. All peaks yield either negative (red-shift)
or negligible energy shift with increasing temperature, identified as exciton-type
absorption transitions in previous studies (5, 6).
13
Fig. S10. GIWAXS images and structure orientation of RPP thin films. Illustration for thin
film n=3 of Fig. 3, see additional details in ref. (12). (A) Grazing incidence wide-angle X-ray
scattering (GIWAXS) image with Miller indices of the most prominent peaks. Color scale is
proportional to X-ray scattering intensity. (B) Linear X-ray diffraction of the thin film (along qz-
direction). The intense, sharp, discrete Bragg spots in GIWAXS indicate near single crystalline
orientation of the RPPs in thin films. Following our method detailed in ref. (12), we indexed the
observed Bragg reflections in the GIWAXS diffraction pattern (in white in A) and concluded that
the perovskite layers have preferential orientation along the (101) planes that is perpendicular to
the substrate (qz) as illustrated in (C).
Fig. S11. Thin film morphology for n=2 and n=4. Scanning electron microscopy of the thin
film surfaces (top panels) and cross-sections (bottom panels). Scale bars, 1 µm. From surface
images and previous reports (12), the apparent grain characteristic size is ~200-400 nm in both
samples. Cross-section images suggested that the majority of the grains had a thickness of about
the thin film thickness (typically ~200-300 nm). The difference of contract between the two
samples came from the lower conductivity in n=2 thin films.
1.0
0.8
0.6
0.4
0.2
0.0XR
D i
nte
ns
ity
(N
orm
.)
5040302010
2 (°)
(111)
(202)
(313)
B A
n=3
C
Substrate
n = 2 n = 4
14
Fig. S12. Temperature dependence of the LES feature in RPPs with n>2. Peak energy shift
(main) and integrated intensity (inset) derived from PL data. The LES peak energy position blue
shifts with increasing temperature at a rate of 0.21±0.01 meV/K (linear fit to the data is the black
dash-line). The PL intensity decreases with temperature and can be fitted using the Arrhenius
law IPL(T) = I0*(1+a*exp(-Ea/kbT)), see the black dash-line in the inset. I0 is the zero-temperature
intensity (normalized to 1 here, and two data points at lower temperature are not shown on this
graph for sake of clarity). a reflects the ratio between radiative and non-radiative recombination
(a is of the order of 103 for LES). Ea represents an activation energy which was about 130 meV
in this measurement. The origin of Ea is still unclear at this point, one possibility being that this
Ea derived from the PL of the LES is related to deep electronic impurities in the band gap. One
might also attribute this Ea to a potential barrier between the LES and the RPP crystal for the
diffusion of carriers.
Fig. S13. Estimation of the diffusion length of exciton before conversion to LES in RPPs
with n>2. Difference between rise times of the X-states and LES (to achieve maximum
population) as a function of absorbed light density nabs; the data are derived from TRPL
measurements similar to Fig. 3C. Dashed lines are exponential fits to the data, where the
population of LES (nLES) yields 𝐧𝐋𝐄𝐒̇ = 𝝉𝐞𝐱−𝟏𝐧𝐚𝐛𝐬 (we neglected the depopulation of X-states
during Δt), where 𝝉𝐞𝐱 is the transfer time constant or time for energy funnelling from the X-state
to the LES. Fitting the solution to the results yield 𝝉𝐞𝐱 of 100 ps, 126 ps, and 96 ps for RPPs with
n=3,4,5, respectively. These values suggest an average diffusion of the order of 10 nm to 100 nm
from the location of the photo-generated exciton to the LES, depending on mobility values
considered in the range 0.6 cm2/V.s (3, 16) to 10 cm2/V.s (39). These quantities define a typical
size of 2D perovskite layers in thin films and are consistent with the preferential vertical
alignment of perovskite layers in our 200-nm-thick films (12).
15
Fig. S14. PL quantum yield (PLQY) as a function of light excitation intensity in RPP thin
films with n = 1 to 5. Error bars reflect the dispersion of PLQY measured on three different
batches of samples, for each value of n, around the average PLQY values.
20
15
10
5
0
PL
QY
(%
)
101
102
103
104
Light intensity (mW/cm2)
n=1 n=2 n=3 n=4 n=5
16
n
Ruddlesden-
Popper
perovskite phase
Perovskite
layer
thickness d
(nm)a
Optical
band gap
crystals
(eV)b
Optical
band gap
thin films
(eV)c
1 (BA)2Pb1I4 0.641 2.420 2.490
2 (BA)2(MA)Pb2I7 1.255 2.170 2.179
3 (BA)2(MA)2Pb3I10 1.892 2.039 1.733
4 (BA)2(MA)3Pb4I13 2.511 1.924 1.711
5 (BA)2(MA)4Pb5I16 3.139 1.851 1.682
∞ MAPbI3 ∞ 1.610d 1.659
aOrganic spacing layer thickness is typically 0.710 nm (4).
bMaximum error 5 meV.
cMaximum error 15 meV due to
slight variation in hot-casting solution processing during thin film fabrication. dFrom ref. (40).
Table S1. Details of the 2D perovskite structural parameter and measured optical band
gap.
RPPs
(Fig. S3)
Lead salts
quantum dots
(20)
nanorods
(21)
nanosheets
(22)
Exciton binding energy in
sub-nanometric systems
383±70 meV
(n=1) NA
> 400 meV
(𝜺𝐞𝐧𝐯 = 𝟐) NA
Exciton binding energy in
systems with size ~ 3.1 nm
(corresponding to QW
thickness in n=5 RPPs)
235±19 meV
(n=5) ~ 450 meV
~ 140 meV
(𝜺𝐞𝐧𝐯 = 𝟐)
~ 80 meV
(𝜺𝐞𝐧𝐯 = 𝟐)
Table S2. Comparison of exciton binding energy in RPPs and in lead salt materials of low
dimensionality.
17
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