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: jcrochet@lanl.gov (J.C.C.); amohite@lanl.gov (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

Materials and Methods

MM1. Crystals and thin films preparation

Two dimensional Ruddlesden-Popper perovskite (RPP or 2D perovskite) crystal samples:

the crystal structures of the RPPs, (BA)2(MA)n-1PbnI3n+1, is composed of an anionic layer

{(MA)n-1PbnI3n+1}2-

, derived from bulk methylammonium lead triiodide perovskites (MAPbI3),

which is sandwiched between n-butylammonium (BA) spacer cations (Fig. 1A). RPPs with n

ranging from one to five, corresponding to perovskite layer thickness d between 0.641 and 3.139

nm, were synthetized and purified following previously reported method (4, 11). More

precisely, the raw crystals were prepared by combining PbO, MACl and BA in appropriate molar

ratios in a HI/H3PO2 solvent mixture. The precursor solutions were prepared with 0.225 M of

Pb2+

concentration and stirred at room temperature overnight. Phase-purity of each sample was

established by monitoring the position and number of the low angle peaks using X-ray

diffraction.

Thin films and exfoliated crystals: Samples for this study were prepared as both thin films

and exfoliated crystals. In the latter case, the raw RPP crystals obtained after synthesis were

mechanical exfoliated onto either glass substrates for absorption measurements or silicon

substrates with a few-nm SiO2 top layer, resulting in exfoliated crystals of few layers thick. The

thin films were prepared using the hot-casting method described in details in ref. (12), where

films were reported to have excellent crystallinity with preferential orientation of the perovskite

layers perpendicular to the substrate. We also verified that the phase-purity was preserved during

the hot-casting process (12). Examples of thin films are displayed in Fig. S1C.

MM2. Solar cells

The planar solar cells and LEDs are in the device structure of ITO/PEDOT:PSS/

RPP/PCBM/Al. All devices were prepared following the method described in details in ref. (12).

The active layers are deposited by the “hot-casting” method described in ref. (12). Specifically,

the substrates were preheated to 110ºC in the argon filled and subsequently transferred to spin

coater chunk. The spin process was started instantaneously after the precursor solution was

dropped on the substrate without ramping to 5000 rpm for 20 Sec. The film color turned brown

right after the solvent evaporates. After the RPP thin film is formed, an in-stock solution

containing 20mg PCBM in 1ml chlorobenzene was spin coated on top of the perovskite layer

serving as electron transport/injection layer. The devices were completed by aluminium

deposition by thermal evaporation in a vacuum chamber in the same glove-box. The solar cells

were mounted into a cryostat and pumped down to 10-5

torr for current-voltage measurements

under AM1.5G white light solar simulator.

MM3. X-ray diffraction characterization of the 2D perovskite phase

The layered perovskite crystals were examined by powder X-ray diffraction system (PXRD)

(Siemens D5000) with Cu(Ka) radiation (l = 1.5406 Å) at 0.05 per step with a holding time of 5s

per step under the operation conditions of 40 kV and 35 mA. The phase purity of the RPP for a

given n-value can be monitored by PXRD system with the number of peaks in the low angle

regime between 2θ = 2º-12º. Details can be found in ref. (4).

2

MM4. Optical absorption, photoluminescence (PL), time-resolved photoluminescence (TRPL)

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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