Luminescent and Photoconductive Layered Lead Halide Perovskite Compounds Comprising Mixtures of Cesium and Guanidinium Cations

Article abstract of DOI:10.1021/acs.inorgchem.7b01204

Interest in hybrid organic-inorganic lead halide compounds with perovskite-like two-dimensional crystal structures is growing due to the unique electronic and optoelectronic properties of these compounds. Herein, we demonstrate the synthesis, thermal and optical properties, and calculations of the electronic band structures for one- and two-layer compounds comprising both cesium and guanidinium cations: Cs[C(NH₂)₃]PbI₄ (I), Cs[C(NH₂)₃]PbBr₄ (II), and Cs₂[C(NH₂)₃]Pb₂Br₇ (III). Compounds I and II exhibit intense photoluminescence at low temperatures, whereas compound III is emissive at room temperature. All of the obtained substances are stable in air and do not thermally decompose until 300 °C. Since Cs⁺ and C(NH₂)₃⁺ are increasingly utilized in precursor solutions for depositing polycrystalline lead halide perovskite thin films for photovoltaics, exploring possible compounds within this compositional space is of high practical relevance to understanding the photophysics and atomistic chemical nature of such films.

Full text of DOI:10.1021/acs.inorgchem.7b01204

Article
pubs.acs.org/IC
Luminescent and Photoconductive Layered Lead Halide Perovskite
Compounds Comprising Mixtures of Cesium and Guanidinium
Cations
Olga Nazarenko,†,‡,§ Martin Robert Kotyrba,†,‡,§ Michael Worle,† Eduardo Cuervo-Reyes,‡
̈
Sergii Yakunin,†,‡ and Maksym V. Kovalenko*,†,‡
†ETH Zurich, Department of Chemistry and Applied Biosciences, CH-8093 Zurich, Switzerland
̈
‡Empa-Swiss Federal Laboratories for Materials Science and Technology, CH-8600 Dubendorf, Switzerland
̈
S
* Supporting Information
ABSTRACT: Interest in hybrid organic−inorganic lead halide
compounds with perovskite-like two-dimensional crystal
structures is growing due to the unique electronic and optoele-
ctronic properties of these compounds. Herein, we demon-
strate the synthesis, thermal and optical properties, and
calculations of the electronic band structures for one- and two-
layer compounds comprising both cesium and guanidinium
cations: Cs[C(NH2)3]PbI4 (I), Cs[C(NH2)3]PbBr4 (II), and
Cs2[C(NH2)3]Pb2Br7 (III). Compounds I and II exhibit
intense photoluminescence at low temperatures, whereas com-
pound III is emissive at room temperature. All of the obtained
substances are stable in air and do not thermally decompose
+
until 300 °C. Since Cs+ and C(NH2)3 are increasingly utilized in precursor solutions for depositing polycrystalline lead halide
perovskite thin films for photovoltaics, exploring possible compounds within this compositional space is of high practical
relevance to understanding the photophysics and atomistic chemical nature of such films.
lone-pair effects.30−33 The orientation of the alkylammonium
cations between the inorganic layers allows the ammonium
groups (−NH3) to form hydrogen bonds with neighboring
halogen ions.
INTRODUCTION
■
Organic−inorganic layered lead halide compounds with
perovskite-like structures are widely studied due to their
fascinating electronic and optical properties and their rich
These A2PbX4 compounds can therefore be considered
structural chemistry.1−21 Fundamental to the construction of
strongly quantum confined systems, i.e., quantum wells, wherein
perovskites is the formation of PbX6 octahedra (X = Cl−, Br−,
electron motion is limited to two dimensions. An interlayer
I− and mixtures thereof) and their condensation into crystals
organic entity acts as the potential wall. The variation in the
that possess two- or three-dimensional (2D and 3D, respec-
carbon chain length in (RNH3)2PbI4 (from 4 to 14 carbon
tively) corner sharing. Such extended 2D and 3D delocalization
of electronic bands imparts semiconductive properties. Com-
atoms) has a rather modest effect on the observed band gap
energies. For example, the energies of the photoluminescence
pounds with 3D arrangements and the generic formula of
(PL) maxima are 2.35 eV for C4 and 2.4 eV for C10, con-
APbX3 are formed when very small A-site cations are utilized
firming a highly localized quantum-well-like behavior.4,5,31
Engineering the chemistry of these compounds at the interlayer
space is of increasing interest,18 with examples including
intercalations of molecules such as hexane, dichlorobenzene,
iodine, etc.34,35 However, engineering the inorganic slab remains
the principal approach to adjusting the absorption edges and
PL maxima. For example, band gaps of 3.75, 3.17, and 2.55 eV are
found for X− = Cl−, Br−, I−, respectively, in (C10H21NH3)2PbX4.36
Another strategy to tune the electronic structure is to increase
the thickness of the lead halide slabs from a single layer to
multiple layers, thereby tuning the band gap energies between
22−29
(A+ = Cs+, CH3NH3 , CH(NH2)2 ).
In contrast, larger
+
+
A-site cations do not fit within the highly confined space
between the octahedra. When a larger A-site organic cation is
employed, the most likely outcome is the formation of 2D
compounds, such as the compounds with the generic for-
mula A2PbX4 consisting of single layers of PbX6 octahedra
sharing four of their corners, with the A cations in the interlayer
space. Most of the known layered (2D) perovskite derivatives
have mono- or diammonium cations, i.e., (RNH3)2MX4 and
(NH3RNH3)PbX4, respectively, where R is commonly an alkyl
+
chain. The nonspherical geometry of RNH3 cations plays a
significant role in the crystal symmetry of (RNH3)2MX4
compounds, in addition to distortions and tilting of the PbX6
octahedra, which are often caused by Jahn−Teller and/or
Received: May 15, 2017
© XXXX American Chemical Society
A
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the two limiting cases of 2D A2PbX4 and 3D APbX3. Creating
thicker slabs must involve at least two A-site cations, wherein
the larger cation remains as an interlayer cation and the second
cation fills in the smaller interslab cavities (which are of similar
size as in the limiting case of APbX3 compounds). Structurally,
these compounds are analogues to the Ruddlesden−Popper
phase, as in the case of (RNH3)2(CH3NH3)n−1PbnX3n+1 (R =
C9H19−, Ph−CH2CH2−; X− = Br−, I−).37 Herein, a small
locations and for filling the voids within the slab. In contrast,
the guanidinium ion can be accommodated only in the inter-
+
layer space, despite being only slightly larger than CH3NH3
and CH(NH2)2 .
56
+
We succeeded in obtaining three layered perovskite com-
pounds with n = 1, 2, Cs[C(NH2)3]PbI4 (I), Cs[C(NH2)3]-
PbBr4 (II), and Cs2[C(NH2)3]Pb2Br7 (III), all of which con-
tain (2D) slabs of corner-sharing octahedra and possess
satisfactory thermal stability (up to 300 °C). Compounds I
and II are luminescent upon moderate cooling, and compound
III is emissive at room temperature. Photoresponsivities in the
range of 1−10 mA W−1 and with narrow-band characteristics were
measured for compounds I and III, confirming their extended
semiconductor nature.
+
CH3NH3 fills voids within the lead halide slabs, and the long
alkyl chain ammonium ions are situated between the slabs, thus
determining the interslab distance and defining the crystallo-
graphic orientation of the perovskite sheets. These phases can
be seen as 2D slices of a 3D APbX3 structure with various cry-
stallographic orientations. For further crystallographic details of
such compounds, we refer readers to recent review and research
articles.7,38,39
METHODS
■
In (C4H9NH3)2(CH3NH3)n−1PbnI3n+1, the band gap energies
change from 2.43 to 1.91 eV as the thickness is adjusted from
n = 1 to n = 4.5 Due to such tunability, these 2D systems are
presently being intensively investigated for their optical
properties and applications in light emission and in solar
cells.1,4,31,40−42 In particular, the archetypical 3D compound
CH3NH3PbI3 (n = ∞) had been reported recently as an out-
standing photovoltaic material with power conversion efficien-
cies repeatedly exceeding 20%.28,43−45 However, CH3NH3PbI3
is known to undergo irreversible degradation due to the volatile
nature of the decomposition products (methylamine, hydrogen
iodide (HI), etc.), which is accelerated upon exposure to moi-
sture, as in ambient air.46 Higher chemical stability can be found
within the family of (PEA)2(CH3NH3)n−1PbnI3n+1 (PEA =
C6H5(CH2)2NH3+), wherein compounds with n = 1−3 have
been synthesized and characterized in detail.47,48 (PEA)2(CH3-
NH3)2Pb3I10 (n = 3, band gap of 2.1 eV) exhibits higher chemical
stability than CH3NH3PbI3.48 The relevance of such structural
motifs extends beyond lead-based halides; for example, in the
tin iodide compounds (C4H9NH3)2(CH3NH3)n−1SnnI3n+1, for
n > 3 metallic behavior is observed, while compounds with
n < 3 are semiconductors.40 However, tin-based compounds are
highly prone to oxidation to Sn4+, limiting their practical utility.
In this work, we aimed to fully eliminate protonated ammo-
nium salts, such as those in primary, secondary, or ternary
alkylamines, from the construction of layered lead halide
perovskites. To create monolayer A2PbX4-like compounds, we
Chemicals, Reagents, and Synthesis Procedures. Lead(II)
iodide (PbI2, 99%) and cesium bromide (CsBr, 99.9%) were
purchased from Sigma-Aldrich. Lead(II) bromide (PbBr2, 98+%),
hydrobromic acid (HBr, 48% water solution), and guanidinium
carbonate (99+%) were purchased from Acros. Cesium iodide (CsI,
99.9%) and hydriodic acid (HI, 57%, stabilized with 1.5%
hypophosphorous acid, H3PO2) were received from ABCR. Diethyl
ether (>99%) was dried over molecular sieves. All chemicals were used
as received without further purification.
Cs[C(NH2)3]PbI4 (I), Cs[C(NH2)3]PbBr4 (II), and Cs2
[C(NH2)3]Pb2Br7 (III) were synthesized from concentrated hydriodic
and hydrobromic acids. The compounds were yellow (II, III) and red
(I) microcrystalline powders (Figure S1 in the Supporting Information).
A large excess (≥6-fold) of guanidinium ions was found to be necessary
to obtain the desired compounds.
Cs[C(NH2)3]PbI4 (I). CsI (1.55 mmol), [C(NH2)3]2CO3 (6 mmol),
and PbI2 (1.55 mmol) were loaded into a 25 mL round- bottom flask,
and 10 mL of HI acid (57% in water) was added carefully. Strong gas
evolution was observed, and a red precipitate immediately formed.
The precipitate was redissolved upon heating in a glycerol bath, yielding
a clear yellow solution. Next, the flask was cooled under a cold water
stream, causing the formation of a red crystalline precipitate of I.
The solution remained undisturbed for another few hours to allow
further precipitation. The precipitate was separated by vacuum filtration,
washed with diethyl ether, and dried in a vacuum oven at 60 °C.
A yield of 72% was estimated relative to Pb. To obtain larger crystals
of I for photoconductivity measurements (0.5−2 mm3), the precursor
solution was slowly evaporated at 60 °C instead of cooling.
Cs[C(NH2)3]PbBr4 (II). CsBr (2 mmol), [C(NH2)3]2CO3 (6 mmol),
and PbBr2 (2 mmol) were loaded into a 25 mL round- bottom flask, and
10 mL of HBr (48%) acid was carefully added. Strong gas evolution
was observed, and a yellow precipitate formed. Subsequent
manipulations of the solution were identical with the procedure for
compound I. A yield of 63% was estimated relative to Pb.
+
replaced the unstable CH3NH3 with a mixture of Cs+ and
guanidinium [C(NH2)3]+ ions. Guanidinium is an ideal alterna-
tive to the primary ammonium ions due to its thermodynamic
stability, high basicity (pKa = 13.6), and strong hydrogen-
bonding capabilities.49,50 The high basicity of the guanidine
molecule can be understood by considering a resonance stabi-
lization of the guanidinium ion by 6−8 kcal/mol.51 The
guanidinium cation has been previously incorporated into a
variety of perovskite-like salts with the formate (HCOO−)
anion, [C(NH2)3][MII(HCOO)3], where M is Mn, Co, Fe, Ni,
Cu, Zn, or Cd.52,53 This suggests that guanidinium facilitates
stable, highly symmetric structures. Although stable [C(NH2)3]2-
Pb(Sn)I4 compounds have already been reported (with
corrugated structure),54,55 our interest was to test three further
possibilities: (i) whether it is possible to mix such a stable organic
cation with an inorganic cation at the interlayer and what
structural effects accompany this combination, (ii) whether this
mixture can also enable the formation of slabs thicker than one
monolayer, and (iii) whether a noncorrugated structure can
form. The small Cs ion is universally suited both for interlayer
Cs2[C(NH2)3]Pb2Br7 (III). CsBr (1 mmol), [C(NH2)3]2CO3 (3 mmol),
and PbBr2 (2 mmol) were used for this synthesis. Upon addition of
12 mL of HBr (48%) acid, strong gas evolution was observed, and a
yellow precipitate formed. Subsequent manipulations for the solution
were identical with the procedure for compound I. A yield of 27% was
estimated relative to Pb. To obtain larger crystals of III for photo-
conductivity measurements (0.5−2 mm3), the precursor solution was
slowly evaporated at 60 °C.
(n-C4H9NH3)2PbI4 (IV). This compound was prepared according to
Stoumpous et al.5
[C(NH2)3]2PbI4 (V). This compound was obtained as yellow plate-
lets from a hot acidic solution using [C(NH2)3]2CO3 (3 mmol), PbI2
(2 mmol) and 7.5 mL of HI (57%). The crystals were separated by
vacuum filtration, washed with diethyl ether and dried in a vacuum
oven at 60 °C.
CsPbBr3 (VI). This compound was crystallized from aqueous HBr
using the corresponding bromides in stoichiometric quantities (0.15 M
PbBr2, 0.15 M CsBr). Upon cooling to room temperature, orange
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Figure 1. (a) Crystal structure of 2D Cs[C(NH2)3]PbI4 (I). (b) Hydrogen bonding between protons of guanidinium cations and iodine atoms of
two 2D Pb−I layers. (c) Distorted Pb−I octahedral coordination.
crystals precipitated. Subsequent manipulations of the solution were
identical with the procedure for compound I.
Supporting Information). The obtained crystal structures of I, II, III
and I′ were deposited as CIF files into the CCDC database with the
numbers 1552605 (I), 1552602 (III), 1552603 (II), and 1552604 (I′).
UV−vis absorbance spectra of the microcrystalline powders were
collected using a Jasco V670 spectrophotometer equipped with
deuterium (190−350 nm) and halogen (330−2700 nm) lamps and an
integrating sphere (ILN-725, working wavelength range of 220−2200
nm).
(n-C4H9NH3)2PbBr4 (VII). The synthesis of VII from hydrobromic
acid was analogous to that of IV, using n-butylamine and PbO (in
stoichiometric quantities, 0.5 M PbO, 1 M C4H9NH2). Upon cooling
to room temperature, colorless transparent plates crystallized. Sub-
sequent manipulations of the solution were identical with the pro-
cedure for compound I.
CsPb2Br5 (VIII). This compound was crystallized from stoichio-
metric quantities of the corresponding bromides in hydrobromic acid.
Typically, CsBr (0.7 mmol) and PbBr2 (1.4 mmol) were dissolved in
6 mL of hot HBr acid. Upon cooling to room temperature, colorless
crystalline plates precipitated, followed by vacuum filtration, washing
with diethyl ether, and drying in the vacuum oven at 60 °C.
[C(NH2)3]2PbBr4 (IX). This compound was grown from a hot acid
solution. [C(NH2)3]2CO3 (4 mmol) and PbBr2 (4 mmol) were loaded
into a 25 mL round-bottom flask with 10 mL of HBr (48%) acid.
The precipitate was redissolved upon heating in a glycerol bath, yielding
a yellowish solution. Upon cooling to room temperature, colorless trans-
parent crystals appeared within a few hours.
The absorbance spectra were estimated from reflectance and transmittance
spectra collected from a thin layer of powder dispersed in an optically
transparent Teflon grease and by diffuse reflectance transformed into
absorbance using the Kubelka−Munk relation. The band gaps were
estimated from the Kubelka−Munk function (F(R∞)), by subtracting
excitonic peaks from the absorption edge. First, the excitonic transition
was fitted with the Gaussian function. The residual spectrum was used
to calculate [F(R∞)·hν]2 (where hν is the incident photon energy).
Plotting [F(R∞)·hν]2 versus energy (hν) gave a spectrum for estima-
tion of band gaps. Although the fitting with a Gaussian function does
not give an impeccable fit for the sub band gap region of the Urbach
tail, this fit gives the possibility of estimating the band gap values.
A better understanding of the spectral region at the Urbach tail can be
attained by modeling according to the Elliott approach.57,58 PL spectra
were measured in a Joule−Thomson cryostat (MMR Technologies)
operated in the temperature range of 78−300 K. PL emission was
recorded with a heating rate of approximately 5 K/min. A 355 nm
excitation source (a frequency-tripled, picosecond Nd:YAG laser, the
Duetto model from Time-Bandwidth) and a CW diode laser with an
excitation wavelength of 405 nm were used. Scattered laser emission
was filtered out using dielectric long-pass filters with edges at 400 and
450 nm, respectively. The emission from the samples was collimated
to an optical fiber and recorded at 1 K intervals with an SP-2300
spectrograph from Princeton Instruments coupled with a CCD array
(LC100/M from Thorlabs). PL spectra were corrected to the spectral
sensitivity of the setup using Planck irradiation from a calibrated
halogen lamp. Time-resolved photoluminescence (TR-PL) measure-
ments were performed using a time-correlated single photon counting
(TCSPC) setup, equipped with a SPC-130-EM counting module
CsPbI3 (X). This compound was crystallized in the form of orange
needles from HI acid using the corresponding iodides in stoichio-
metric quantities as precursors (0.1 M PbI2, 0.1 M CsI).
Characterization. Powder X-ray diffraction (XRD) patterns were
collected in the transmission mode (Debye−Scherrer geometry) using
a STADI P diffractometer (STOE& Cie GmbH) equipped with a
silicon strip MYTHEN 1K Detector (Fa. DECTRIS) with a curved
Ge (111) monochromator (Cu Kα1 = 1.54056 Å). Single-crystal XRD
measurements were conducted using a Bruker Smart Platform diffrac-
tometer equipped with an Apex I CCD detector and a molybdenum
(Mo Kα = 0.71073 Å) sealed tube as an X-ray source. The crystals
were tip-mounted on a micromount with paraffin oil. The data were
processed using APEX3 (Bruker software), and the structure solu-
tion and refinement were performed with SHELXT and SHELXL
programs embedded in the Olex2 software package. For twin indexing,
the cell_now algorithm (a part of the Bruker APEX2 software
package) was used (see the crystallographic data section in the
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Figure 2. (a, b) Crystal structures of 2D Cs[C(NH2)3]PbBr4 (II) and Cs2[C(NH2)3]Pb2Br7 (III) perovskites. (c) Hydrogen−halogen bonds
between protons of guanidinium cations and bromides of 2D layers of Cs2[C(NH2)3]Pb2Br7. (d) Distortion of Pb−Br octahedral coordination in III
and bromide split position in the crystal structure of Cs2[C(NH2)3]Pb2Br7.
(Becker & Hickl GmbH) and an IDQ-ID-100-20-ULN avalanche
photodiode (Quantique) for recording the decay traces. The emission
was excited by a frequency-tripled (λ 355 nm), picosecond Nd:YAG
laser, the Duetto model from Time-Bandwidth, externally triggered at
a 78 kHz repetition rate. PL emission from the samples passed through
sets of long- and short-pass optical filters selecting the wavelength
ranges 500−600 and 650−800 nm. Thermal analysis (thermogravim-
etry, TG, and differential thermal analysis, DTA) was performed using
a NETZSCH STA 409 C/CD instrument in an alumina crucible under
argon flow (40 mL/min) with a heating rate of 10 K/min.
Photoconductivity. Measurements were performed with illumi-
nation from a tungsten lamp dispersed by an Acton SP2150 (Roper
Scientific) spectrograph/monochromator. The light was modulated by
a mechanical chopper at a frequency of 13 Hz. The sample was gently
pressed from two opposite sides by conductive rubber contacts in a
custom-made holder and biased at 50 V through these contacts using a
Keithley 236 SMU apparatus. The bias voltage was adjusted to obtain
stable dark currents in the range of 1−10 nA. The signal, amplitude, and
phase were measured across a series resistance by a Stanford Research
830 lock-in amplifier. The light intensity was controlled by a calibrated
power detector (UM9B-BL, Gentec-EO). The current−voltage (I−V)
characteristics were measured using a Keithley 236 SMU apparatus.
Electronic Structure Calculations. Two implementations of
density functional theory (DFT) were employed. For the calculations
of the band structure and density of states, we used the Dmol3
package within the Materials Studio suite.59,60 For the real space
representation of the bonding, we computed the electron locali-
zation function (ELF)61,62 using Savin’s implementation within the
TB-LMTO-ASA code developed at MPI Stuttgart.63 The ELF plots
contain information on both ELF and electron density over a given plane
and were obtained using a code developed in-house. ELF values and
electron density values are shown as the color of the pixels and the
number of colored pixels, respectively, over a black background. Scalar-
relativistic corrections were included in all calculations because they
are expected to play an important role in the presence of heavy
elements. The LMTO is an all-electron method, and in Dmol3 we
chose to also include all electrons equally in the scalar-relativistic
calculation, meaning that no pseudopotential was used. Within Dmol3,
the tolerance for self-consistency was set to about 10−6 Ha for the total
energy, using the number of k points that results in a grid with
separation smaller than 0.03 Å−1 in reciprocal space. We employed the
BOP (Becke one parameter) exchange-correlation functional, with
which we have had in general better outcomes for the gaps in com-
parison with the commonly used PBE. For the LMTO calculations, we
employed the Langreth−Mehl−Hu exchange-correlation functional.
The self-consistency tolerance was set to 10−5 Ry for the total energy
and 10−5 e for the atomic charges. Taking advantage of the speed of
the LMTO code, the k space was sampled over 64-time denser grids
than those used within Dmol3. The investigation of the effects on the
band gaps due to spin−orbit coupling and nonlocal exchange inter-
actions, which require the extensive use of semiempirical methods,
should be the subject of future (and more computationally oriented)
works.
RESULTS AND DISCUSSION
■
Crystal Structures of I−III. The synthesized compounds
were characterized by single-crystal XRD (Table 1 and Figures 1
and 2), powder XRD, thermal analysis (Figures 3), optical
absorption, and PL spectroscopy (Figures 4 and 5). In addi-
tion, electronic structures were calculated by DFT methods.
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Figure 3. X-ray diffraction powder patterns of (a) Cs[C(NH2)3]PbI4, (b) Cs[C(NH2)3]PbBr4, (c) Cs2[C(NH2)3]Pb2Br7: comparison with simulated
data from single-crystal analysis. Thermal stability analysis of I (d), II (e), and III (f).
The crystal structures were solved with direct methods; light
elements (C, N) were located in the difference Fourier map,
and hydrogen atoms were placed at calculated positions. Some
of the crystals under investigation were twinned. Twin indexing
and integration was performed with the Bruker Apex3 program
software package. The compounds crystallize in the ortho-
rhombic crystal system (centrosymmetric space groups Pnnm,
Imma, and Cmmm for I−III, respectively; Table 1). The crystal
structures of I and II feature single layers (n = 1) of PbX6
octahedra, whereas compound III consists of a bilayer arrange-
ment of the octahedra (Figures 1 and 2). To obtain the com-
pound with n = 2, i.e., Cs2[C(NH2)3]Pb2Br7, the Cs:C(NH2)3:Pb
molar ratio was adjusted to 1:6:2 during the synthesis. Further
varying the molar ratios or thermal growth profiles during the
crystallization did not yield compounds other than I−III. 2D
layers were separated with a mixture of organic (guanidinium)
and inorganic (cesium) cations in a 1:1 ratio, alternating in a
periodic matter. As is typical for layered A2BX4 systems (n = 1),
neighboring layers are shifted by half of the octahedra with
respect to each other (Figure 1a), maximizing the X−X dis-
tance for minimal electrostatic repulsion.64 In all three com-
pounds, the PbX6 octahedra tilt, e.g., they deviate from the
idealized Pb−X−Pb angle of 180°, to better accommodate the
small Cs+ and large C(NH2)3+ cations. Guanidinium ions closely
pack, with intermolecular d(C···N) values in the range of
3−3.2 Å for all three compounds (Table S1 in the Supporting
Information), and form hydrogen−halogen bonds with neigh-
boring halogen ions (bromine or iodine, Figures 1b and 2c
and Tables S5, S8, S11, S14, and S17 in the Supporting
Information). Cs[C(NH2)3]PbI4 (I, Figure 1) exhibits some-
what higher structural disorder and stronger distortions in com-
parison to those of its bromide analogue. Both bridging iodides
(along the c axis) of PbI6 octahedra can be represented as split
into two positions (Figure 1c). All Pb−I bond lengths range
from 3.101(2) to 3.226(1) Å. The average Pb−I distance is
3.168 Å, which is slightly longer than that in the cubic structure
of CsPbI3, where d(Pb−I) = 3.1447 Å,65 but smaller than that
in the closely related (C4H9NH3)2PbI4, where d(Pb−I) = 3.184 Å
was estimated.31 Below 250 K, the crystals of I undergo extensive
cracking into multiple new domains, prohibiting structural
description of the newly formed phase(s). However, at exactly
250 K, crystal structure refinement is still possible, and certain
conclusions can be drawn from the electron density map (this
compound is denoted by I′). In particular, the disorder of the
bridging iodides (c direction) is reduced, and they can be
described by a single atomic position (Figure 1c). The Pb−I
distances in I′ are in the range of 3.145(1)−3.228(1) Å.
Compound II is a 2D compound with n = 1 (Figure 2a).
Bridging bromides (Br3) (along the b direction) are disordered
at two positions in the Pb−Br octahedra (Figure S2a in the
Supporting Information). The distortion of the lead bromide
octahedra could be described by Pb−Br3−Pb angles, which
are equal to 157.488(3)°. Two apical and two bridging (along the
a axis) bromides can be depicted as lying in the same plane
with ideal 90° Br−Pb−Br′ angles: d(Pb−Br1) = 3.030(1) Å
and d(Pb−Br2) = 3.002(1) Å. Bridging bromide ions (along
the b direction) are asymmetrically tilted at 92.7° with respect to
that plane (Figure S2a) and have equal Pb−Br distances of
2.953(1) Å. Guanidinium ions form hydrogen bonds with halo-
gen atoms of two different layers: d(H−Br) = 2.7445−2.869 Å
(Figure S2b). The cesium ions occupy one type of position
with a coordination number of 10 in the smaller voids between
the Pb−Br octahedra.
Compound III represents a succession of Cs[C(NH2)3]-
PbBr4 with n = 2 (Figure 2b). The crystal structure of III con-
sists of double 2D layers of Pb−Br octahedra with an average
Pb−Br distance of 2.977 Å, which is slightly longer than that in
cubic CsPbBr3 (d(Pb−Br) = 2.937 Å).66 Cesium occupies two
F
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types of positions in the structure: in the cavities within the
double layers and in the space between the layers. The cor-
responding coordination numbers for the two positions are
8 and 10, respectively, with d(Cs−Br) = 3.775(1)−4.272(1) Å.
Guanidinium is situated in the void created by the Pb−Br
octahedra of the two 2D layers and forms hydrogen bonds with
the neighboring Br ions of these two layers with d(H−Br)=
2.804−2.847 Å, generating a 3D supramolecular network
(Figure 2c). Bridging bromide ions (along the c direction)
are disordered at a single position (Figure 2d).
Powder XRD patterns of the synthesized compounds were
compared to a theoretical pattern (Figure 3a−c) for testing the
overall purity of the substances. For compound I, no impurities
were detected in the powder XRD pattern (Figure 3a). Com-
pounds II and III often contain a few percent of each other as
an impurity.
Thermogravimetric analysis was performed under inert con-
ditions under an argon atmosphere (40 mL/min, ambient
pressure), in alumina (Al2O3) crucibles, at a heating rate of
10 K/min. Compounds I−III exhibit high thermal stabilities
(Figure 3d−f). Decomposition starts at approximately 300 °C.
Before decomposition, an endothermic process without a mass
loss occurs for all three compounds: at 213 °C for II and III
and at 283 °C for I, which is closer to the decomposition point.
These endothermic processes indicate phase transitions, which
could be melting or another structural transformation, which in
the case of compound I is followed by the decomposition of the
compound.
Absorption Spectra. Having established the crystal struc-
tures of compounds I−III, we examined their electronic struc-
ture using optical absorption spectroscopy (Figure 4a,b).
In particular, we sought to compare the band gap energies
with those for the known compositionally and/or structurally
related compounds IV−X (Figure 4a−c and Figure S3 in the
Supporting Information). These reference compounds were
previously reported by other groups: IV,5,11,31 V,54 VI,66,67
VII,1,68,69 VIII,70 IX,71 and X.66 In this work, compounds IV−X
were grown from acidic solutions (see Methods). The purity of
compounds I−X was assessed by comparing their powder XRD
patterns with the simulated powder patterns generated using
single-crystal data (see Figure 3a−c and Figures S4−S10 in the
Supporting Information). Single-crystal data for some reference
compounds were taken from ICSD cards 92045 (for V), 97851
(for VI), and 27979 (for X) as well as from ref 31 (for IV) and
from ref 68 (for VII). Compounds VIII and IX, namely,
CsPb2Br5 and [C(NH2)3]2PbBr4, had not been structurally
characterized prior to this study. Therefore, we determined their
crystal structures by single-crystal XRD (for crystallographic
details see Figure S3 and Table S2 in the Supporting Information
and CCDC entries 1552606 and 1552601) and found good
agreement with the powder patterns (Figures S8 and S9 in the
Supporting Information). CsPb2Br5 (VIII, Figure S3b) is a 2D
compound with layers constructed of Pb−Br face-sharing
distorted square antiprisms and cesium ions located in the inter-
layer space. This compound is often reported for Cs−Pb−Br
systems, along with the 0D perovskite Cs4PbBr6 and 3D
perovskite CsPbBr3.67,72,73 Our single-crystal diffraction data for
CsPb2Br5 are consistent with a powder diffraction pattern reported
in ref 70 and in PDF2 (#00-025-0211). [C(NH2)3]2PbBr4 (IX,
Figure 4. (a, b) Kubelka−Munk function F(R∞) = (1 − R∞)2/
R∞ (R∞ - diffusive reflectance) of I−IX. (c) Table of compounds and
their band gaps determined from [F(R∞)hυ]2 (after subtraction of the
excitonic peak from the Kubelka−Munk function), where F(R∞) is in
Kubelka−Munk units and hν is the incident photon energy.
number of Pb(II) is 6, including the lone 6s2 pair of electrons.
The guanidinium cations are located between the inorganic
chains and form hydrogen−halogen bonds, resulting in a 3D
supramolecular structure. This structural analysis underlines the
importance of Cs ions for building a stable 2D perovskite of
compound II. Interestingly, the iodide compositional analogue
[C(NH2)3]2PbI4 (V) does form a 2D lattice, but unlike I, it is
Figure S3a) crystallizes in the triclinic P1 space group with a
̅
structural motif similar to that of [C(NH2)3]2SnCl4, as reported
by Szafran
corner-sharing [PbBr5] square pyramids, wherein the coordination
́
ski et al.74 [C(NH2)3]2PbBr4 consists of 1D chains of
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Figure 5. (a−c) Normalized temperature-dependent PL spectra. The color bar (on the right) represents the intensity for all PL matrices.
(d−f) Spectra at 77 K for Cs[C(NH2)3]PbI4 (I), Cs[C(NH2)3]PbBr4(II), and Cs2[C(NH2)3]Pb2Br7 (III).
made of corrugated (zigzag) layers (see the structural com-
parison among I, V, and IV in Figure S11a−c in the Supporting
Information).31,54
have a more significant influence on the band gap than the
out-of-plane deviations.75,76 When (n-C4H9NH3)2PbI4 and
Cs[C(NH2)3]PbI4 are compared (Figure S15 in the Supporting
Information), the difference in the band gaps is consistent with
a degree of distortion of Pb−I octahedra. The in-plane devia-
tion of the Pb−I−Pb angle from 180° for (n-C4H9NH3)2PbI4 is
as significant as 24.36°, while for Cs[C(NH2)3]PbI4 the maxi-
mum angle of deviation is 18.43°.
The Kubelka−Munk absorption functions of 2D perovskites
of variable slab thickness II and III point to a smaller band gap,
as expected, for thicker Pb−Br slabs of III. In addition, the
effect of the interlayer spacing is clearly seen in the approxi-
mately 0.35 eV wider gap of VII (flat 1D sheets) in comparison
to that of II. Both II and III exhibit intermediate band
gap values in comparison to those of compounds with higher
and lower dimensionalities: i.e., CsPbBr3 (VI, 3D) and
[C(NH2)3]2PbBr4 (IX, 1D), respectively.
Photoluminescence. Optical emission spectra (Figure 5)
were recorded in the range of 77−300 K for I−III. Only com-
pound III exhibits a measurable PL intensity at room tem-
perature (Figure 5c). All three compounds are brightly emissive
to the naked eye under UV light at 77 K. For this temperature,
Figure 5d−f present the luminescence spectra, while the entire
temperature dependence is plotted as color maps in Figure 5a−c.
The presence of minor emission peaks in Figure 5b,c suggests
that compounds II and III contain small impurities of each
other, as also concluded from the powder XRD patterns. For
the PL spectrum of I, in addition to the main emission peak at
535 nm, a broader, low-intensity feature at 650−800 nm was
detected.
In order to understand the origin of this feature, we perfor-
med time-resolved photoluminescence measurement at various
temperatures (Figure S16 in the Supporting Information).
It can be seen that while the emission at 535 nm decays
very quickly, within 10 ns, the long-wavelength band relaxes
much more slowly (subμs range) and the relaxation becomes
even slower upon cooling to 200 K. These features of the
spectrally broad and long-lived emission is consistent with the
Figure 4a,b and Figure S12 in the Supporting Information
present the normalized Kubelka−Munk (KM) functions,
F(R∞), for all 10 compounds. This function is derived from
the diffuse reflectance spectra of the powdered samples. This
function is estimated as F(R∞) = α/S = (1 − R∞)2/R∞, where α
is the absorption coefficient, S is the scattering coefficient, and
R∞ is the reflectance of an infinitely thick layer. Band gap
energies were estimated by plotting [F(R∞)·hν]2 (where F(R∞)
is in Kubelka−Munk units and hν is the incident photon
energy) versus energy (hν). The excitonic peaks were sub-
tracted from the Kubelka−Munk function for the band gap
determination (Figure S13 in the Supporting Information).57
Conventional optical absorption spectra for all compounds I-X
are presented in Figure S14 in the Supporting Information.
The most important comparison is between (n-C4H9NH3)2PbX4
(IV and VII) and Cs[C(NH2)3]PbX4 (I and II) because these
compounds consist of flat sheets of PbX6 (unlike the corrugated
sheets in V). The band gaps for the Cs guanidinium based
compounds are smaller by 0.1−0.35 eV. This observation
correlates well with the smaller interlayer ions reducing the
average thickness of one layer (center to center distance) and,
hence, also reducing the electronic isolation of the PbX6 layers.
For the case of iodides, the average thickness of one layer is
9.78 Å in I (13.8 Å in IV); correspondingly, the specific density
of I is 1.54 g cm−3 greater than that of IV (4.162 g cm−3 vs
2.702 g cm−3), whereas their molecular weights are similar
(907.79 and 863.109 g mol−1, respectively), pointing to much
smaller molar volumes obtained with smaller interlayer cations.
Interestingly, despite its similar density, compound V shows
a wider band gap of 2.63 eV (Figure 4c), arguably due to
the corrugated 2D structure (Figure S11c in the Supporting
Information). Another important factor is a distortion of
the Pb−X octahedra. It can be described by a deviation of
Pb−X−Pb angles from 180°. Moreover, in-plane (in the plane
of propagation of inorganic layers) distortions were shown to
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phenomenon of self-trapped excitons that was recently
observed in 2D layered perovskites.35,76,77 Often, the emission
of self-trapped excitons accompanies narrow-band shorter-
wavelength excitonic emission, such as in the present case.35,76
Thus, we believe that we observed an excitonic emission at 535
nm and self-trapped excitons at 650−800 nm.
as an example for the following detailed discussion. The quan-
titative differences such as the band gaps and their relation to
composition are addressed below. The density of states (DoS)
per primitive cell is plotted in Figure 7. The solid black line
Photoconductivity. Semiconductive properties of the
obtained compounds are manifested in the production of free
charge carriers under light excitation and in the photocon-
ductivity effects. The dark specific resistivity values are approxi-
mately 3 × 108 and 5 × 109 Ω cm for compounds I and III,
respectively. The photoconductivity spectra (Figure 6) of two
Figure 7. Electronic density of states (DoS) per primitive cell per eV
in Cs[C(NH2)3]PbBr4 (II).
represents the sum of all valence states, and the contributions
of selected species, i.e., the partial density of states (PDoS), are
represented by colored areas.
Figure 6. Room-temperature photoconductivity of Cs[C(NH2)3]PbI4
(I) and Cs2[C(NH2)3]Pb2Br7 (III) single crystals.
The interaction of bromine with the other atoms is rather
ionic, as can be inferred from the nearly 100% coverage
(magenta areas) of the DoS in the regions where bromine
contributes. The main contribution of lead to the occupied
states originates from the electrons in the 6s orbitals at approxi-
mately 8 eV below the Fermi level, represented by the dashed
gray area. The remaining valence electrons belong to guani-
dinium cations. The bonding, nonbonding, and antibonding
combinations of the CN3 π system are labeled in Figure 7.
These states are not stabilized by the interaction with the
protons and appear higher in energy. The nonbonding section
has a negligible contribution from carbon (cyan area) as expected
for the nearly ideal D3h symmetry of guanidinium within this
structure. The π contribution to the C−N bonds is also visible in
the electron localization function (ELF, shown in Figure 8),61
where the bc cut (left side) and the ac cut (right side) show
different widths of the bonding attractor: i.e., the C−N bond
cross section has an oval shape. The ELF also confirms the
dominant ionic interaction between all Pb−Br and Cs−Br pairs,
with the formation of nearly spherical closed shells. In the
present case, the guanidinium−Br hydrogen bond is a proton
bridge type of bond. Due to the deficit of one electron in the
cation, the electron cloud is pulled by nitrogen and carbon
toward the center, leaving the positive protons as an outer
layer to interact with the bromine atoms. Accordingly, the very
small contribution of hydrogen to the DoS (not highlighted in
Figure 7) shows negligible mixing with bromine states. Note
that the closed-shell-like ELF attractor of Br is distorted in a way
that “avoids” the protons, which is typical for ionic interactions.
All three compounds feature guanidinium cation stacking in
alternating orientations along a crystallographic axis, which
suggests a stabilizing interaction between neighboring cations.
We find that this stacking allows van der Waals interactions
involving the easily polarizable π electron cloud of guanidinium
cations. This conclusion is corroborated by the small contribution
single crystals of compounds I and III exhibit similar features.
First, a shoulder on the shorter wavelength side corresponds to
band gap absorption. Second, sharp peaks appear at approxi-
mately 1 mA W−1 for Cs[C(NH2)3]PbI4 (I) and 9 mA W−1 for
Cs2[C(NH2)3]Pb2Br7 (III). Such photopeaks have been recently
reported for APbX3 perovskite single crystals72,78,79 and can be
explained by the thickness (depth)-dependent absorption and
charge transport phenomena. Two competing effects determine
the photocurrent in this case. First, the charges produced in the
crystal volume are more effectively collected on the electrodes,
while the charges produced on or close to the surface demon-
strate decreased charge transport characteristics, likely due to
the trapping of surface charges. When this trapping effect is
combined with the absorption spectrum, a peak is expected at
the onset of the absorption, not at the absorption peak.
The wavelength of the peak, which is essentially equal to the
band gap energy, is the wavelength at which sufficiently intense
light propagates into the bulk region. At higher energies
(shorter wavelengths), the higher absorption coefficient will
limit the absorption to the near-surface regions. A third feature
that differentiates compounds I and III form APbX3 perovskite
single crystals72,78,79 is the long-wavelength photocurrent tail at
energies well below the band gap; this tail is most pronounced
for compound I. This region may be attributed to the photo-
current involving the midgap trap states. Interestingly, the
charges produced in the SC volume (sharp peak and shoulder
at longer wavelengths) are delayed in comparison to charges
from the surface, corresponding to a decrease in the phase of
the photoresponse (Figure S17 in the Supporting Information).
This may be due to the moderate charge mobility in these
compounds.
Electronic Structure. Compounds I−III have qualitatively
similar electronic structures. We chose Cs[C(NH2)3]PbBr4 (II)
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in comparison to the generally flat band structure of such
systems. The alternate stacking allows the small energy cost for
polarizing the shells to be overcompensated by the interaction
between the induced dipoles, resulting in higher stability.
Compounds I−III are all semiconductors. DFT predicts that
the highest occupied and lowest empty bands predominantly
consist of halide p orbital and Pb p orbital contributions,
respectively. The calculated band gaps are approximately 2.7 eV
for Cs[C(NH2)3]PbBr4, 2.2 eV for Cs2[C(NH2)3]Pb2Br7, and
2.0 eV for Cs[C(NH2)3]PbI4. This trend is consistent with the
measured absorption spectra (Figure 4) and can be understood
in terms of basic quantum-mechanical principles. The band gap
increases with the confinement, i.e., with the decreasing lead
halide slab thickness (from n = 2 to n = 1) or on moving from
heavier iodide to lighter bromide. Bromine is more electro-
negative than iodine, and therefore, the center of the valence
and conduction bands should be closer in energy for the
iodides (compound I), resulting in a smaller gap in this case.
CONCLUSIONS
■
In conclusion, we presented the synthesis and characterization
of three layered lead halide perovskite compounds, containing a
mixture of two stable interlayer cations in their structure (cesium
and guanidinium): Cs[C(NH2)3]PbI4 (I), Cs[C(NH2)3]PbBr4
(II), and Cs2[C(NH2)3]Pb2Br7 (III). The two interlayer cations
act synergistically to enable stable 2D perovskites through the
van der Waals interactions between guanidinium cations and
through hydrogen−halogen bonds between guanidinium
cations and halide ions of different layers. Cs+ ions fill the
small voids within the Pb−Br layer in III, in addition to also
occupying the interlayer space. All three compounds were
shown to possess high thermal stability, up to above 300 °C.
The compounds exhibit clear signatures of strong quantum
confinement. Narrow-band photoconductive response was
observed, indicating a balance between the optical absorption
effects and the varying charge transport efficiency in the bulk
and at the surface of the material.
Figure 8. ELF planar cut at x = 0 (left) and y = 0 (right), each
covering 2 × 2 unit cells of Cs[C(NH2)3]PbBr4. Crystallographic sites
are labeled with the name of the corresponding elements.
of the carbon states to the nonbonding peak just below −2.5 eV
(see Figure 7). The latter can appear only if the D3h symmetry
is broken and the π electron cloud is polarized. At the same
time, the π-bonding band appearing at −6.8 eV (see Figure 9)
disperses along the W−R segment in reciprocal space. This
band dispersion of approximately 0.7 eV is significant, especially
These findings have strong relevance to perovskite thin-film
photovoltaic research. In particular, recent studies have increas-
ingly departed from the archetypical APbX3-like compound
CH3NH3PbI3 to multicationic and multi-ionic formulations
such as that in ref 80, wherein four A-site cations were used
(CH3NH3+, CH(NH2)2 , Cs+, Rb+). Moreover, adding guanidi-
+
nium ions has been reported to improve solar cell character-
istics.81,82 Clearly, structural studies within this compositional
space, as shown here for mixed-cation Cs−guanidinium
systems, might unravel the complexities of such optoelectronic
systems.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.7b01204.
Photographs of compounds I and III under daylight and
UV illumination (at 77 K), crystallographic data for
I−III, VIII, and IX, absorption spectra for I−X, powder
XRD for IV−X, comparison of crystal structures for I, IV,
and V, excitonic peak subtraction from the Kubelka−
Munk function of I, time-resolved PL spectra at various
temperatures for I, and photoresponce for single crystals
of I and III (PDF)
Figure 9. Electronic band structure of Cs[C(NH2)3]PbBr4 (II) from
DFT calculations. The flatness of the majority of the occupied bands
indicates the localized character of the states. Note the exceptional
dispersion along the W−R segment of the π-bonding guanidinium
states (in cyan at approximately −6.8 eV).
J
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(12) Billing, D. G.; Lemmerer, A. Synthesis, characterization and
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Accession Codes
CCDC 1552601−1552606 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(14) Stoumpos, C. C.; Frazer, L.; Clark, D. J.; Kim, Y. S.; Rhim, S. H.;
Freeman, A. J.; Ketterson, J. B.; Jang, J. I.; Kanatzidis, M. G. Hybrid
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(16) Mao, L.; Tsai, H.; Nie, W.; Ma, L.; Im, J.; Stoumpos, C. C.;
Malliakas, C. D.; Hao, F.; Wasielewski, M. R.; Mohite, A. D.;
Kanatzidis, M. G. Role of organic counterion in lead- and tin-based
two-dimensional semiconducting iodide perovskites and application in
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AUTHOR INFORMATION
Corresponding Author
*M.V.K.: e-mail, mvkovalenko@ethz.ch; tel, +41 044 633 4156.
ORCID
Sergii Yakunin: 0000-0002-6409-0565
Maksym V. Kovalenko: 0000-0002-6396-8938
■
Author Contributions
§O.N. and M.R.K. contributed equally to this work:
Notes
The authors declare no competing financial interest.
(17) Stoumpos, C. C.; Mao, L.; Malliakas, C. D.; Kanatzidis, M. G.
Structure−band gap relationships in hexagonal polytypes and low-
dimensional structures of hybrid tin iodide perovskites. Inorg. Chem.
2017, 56, 56−73.
(18) Smith, I. C.; Smith, M. D.; Jaffe, A.; Lin, Y.; Karunadasa, H. I.
Between the sheets: postsynthetic transformations in hybrid perov-
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ACKNOWLEDGMENTS
■
This work was financially supported by the European Union
through the FP7 (ERC Starting Grant NANOSOLID, GA No.
306733) and in part by the Swiss Federal Commission for
Technology and Innovation (CTI-No. 18614.1 PFNM-NM).
The authors thank Christian Mensing for thermal analysis of
the synthesized compounds (DTA/TG) and Prof. Masanori
Koshimizu for providing crystallographic information file of
(n-C4H9NH3)2PbBr4.
(19) Blancon, J.-C.; Tsai, H.; Nie, W.; Stoumpos, C. C.; Pedesseau,
L.; Katan, C.; Kepenekian, M.; Soe, C. M. M.; Appavoo, K.; Sfeir, M.
Y.; Tretiak, S.; Ajayan, P. M.; Kanatzidis, M. G.; Even, J.; Crochet, J. J.;
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