Enhanced Out-of-Plane Conductivity and Photovoltaic Performance in n = 1 Layered Perovskites through Organic Cation Design

Article abstract of DOI:10.1021/jacs.8b03659

Layered perovskites with the formula (R-NH₃)₂PbI₄ have excellent environmental stability but poor photovoltaic function due to the preferential orientation of the semiconducting layer parallel to the substrate and the typically insulating nature of the R-NH₃⁺ cation. Here, we report a series of these n = 1 layered perovskites with the form (aromatic-O-linker-NH₃)₂PbI₄ where the aromatic moiety is naphthalene, pyrene, or perylene and the linker is ethyl, propyl, or butyl. These materials achieve enhanced conductivity perpendicular to the inorganic layers due to better energy level matching between the inorganic layers and organic galleries. The enhanced conductivity and visible absorption of these materials led to a champion power conversion efficiency of 1.38%, which is the highest value reported for any n = 1 layered perovskite, and it is an order of magnitude higher efficiency than any other n = 1 layered perovskite oriented with layers parallel to the substrate. These findings demonstrate the importance of leveraging the electronic character of the organic cation to improve optoelectronic properties and thus the photovoltaic performance of these chemically stable low n layered perovskites.

Full text of DOI:10.1021/jacs.8b03659

Article
pubs.acs.org/JACS
Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Enhanced Out-of-Plane Conductivity and Photovoltaic Performance
in n = 1 Layered Perovskites through Organic Cation Design
James V. Passarelli,†,∇ Daniel J. Fairfield,‡,∇ Nicholas A. Sather,‡ Mark P. Hendricks,⊥ Hiroaki Sai,⊥
Charlotte L. Stern,†,§ and Samuel I. Stupp*,†,‡,⊥,∥,#
‡
§
†Department of Chemistry, Department of Materials Science and Engineering, Integrated Molecular Structure Education and
∥
Research Center, and Department of Biomedical Engineering, Northwestern University, Evanston, Illinois 60208, United States
⊥Simpson Querrey Institute for BioNanotechnology, and Department of Medicine, Northwestern University, Chicago, Illinois
#
60611, United States
S
* Supporting Information
ABSTRACT: Layered perovskites with the formula (R−NH3)2PbI4
have excellent environmental stability but poor photovoltaic function
due to the preferential orientation of the semiconducting layer
parallel to the substrate and the typically insulating nature of the R−
+
NH3 cation. Here, we report a series of these n = 1 layered
perovskites with the form (aromatic-O-linker-NH3)2PbI4 where the
aromatic moiety is naphthalene, pyrene, or perylene and the linker is
ethyl, propyl, or butyl. These materials achieve enhanced
conductivity perpendicular to the inorganic layers due to better
energy level matching between the inorganic layers and organic
galleries. The enhanced conductivity and visible absorption of these
materials led to a champion power conversion efficiency of 1.38%,
which is the highest value reported for any n = 1 layered perovskite,
and it is an order of magnitude higher efficiency than any other n = 1 layered perovskite oriented with layers parallel to the
substrate. These findings demonstrate the importance of leveraging the electronic character of the organic cation to improve
optoelectronic properties and thus the photovoltaic performance of these chemically stable low n layered perovskites.
remarkable results with a reported power conversion efficiency
(PCE) of 12.5% for a layered perovskite where n = 4, R is
butylammonium iodide, and A is methylammonium iodide.1 In
this example, the inorganic layers were oriented perpendicular
to the bottom contact through a hot-casting technique to
mitigate the negative impact of the insulating butyl groups on
conductivity between the inorganic layers.1 Even though the
introduction of hydrophobic cations into these n > 1 layered
perovskites has been shown to improve stability as compared to
three-dimensional perovskites, most examples still utilize
methylammonium iodide as the A-site cation, which is known
to be intrinsically unstable under atmospheric conditions.9
Furthermore, it is still unclear if the orientational control of the
hot-casting technique is applicable to all cations in layered
perovskites or for low n layered perovskites in general.
In this work, we study layered perovskites of the n = 1 variety
where X = I. These n = 1 layered perovskites do not contain an
A-site cation and are thus considerably more stable than the n >
1 variety. These materials, however, present new challenges due
to their wide bandgap and preferential orientation with
inorganic layers parallel to the substrate. This leads to
considerably lower PCE than devices with higher n active
INTRODUCTION
■
Two-dimensional organic−inorganic layered perovskites based
on the lead halide framework have demonstrated many of the
attractive optoelectronic properties and translational possibil-
ities of their parent three-dimensional perovskites while
achieving better stability.1−3 The structure of these layered
perovskites can be generally viewed as the periodic splitting of
the three-dimensional structure along a particular crystallo-
graphic plane, most commonly along the (100) plane, to form a
layered structure. Chemically, this is accomplished by
+
substituting a bulky organic cation (R−NH3 ) for the
interstitial A-site cation according to the formula (R−
4,5
NH3)2PbnX3n+1An−1
.
In these structures, n corner-sharing
2−
PbX4 octahedra span the thickness of the inorganic layer
where n is modulated by tuning the ratio of the small interstitial
A site cation to the bulky organic cation (n = 1 shown in Figure
1A). These 2D semiconducting inorganic layers are separated
by bulky organic cations, which are typically insulating and thus
isolate the inorganic layers both physically and electroni-
cally.6−8
In the modern photovoltaic literature, the role of the organic
cation has been primarily structural, and the optoelectronic
properties of these materials have been designed through the
compositional tuning of inorganic layers (varying X and n)9 and
processing conditions.10,11 To date, these studies have achieved
Received: April 4, 2018
© XXXX American Chemical Society
A
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respect to the substrate surface in photovoltaic devices and
result in a more generally applicable strategy for low-n layered
perovskite devices.
Recent layered perovskite studies have utilized well-known
organic moieties such as butylammonium,2 benzylammo-
nium,12 and phenylethylammonium,3 or other commercially
available molecules where the desired optoelectronic properties
were achieved through tuning of the inorganic lattice and
postsynthetic modification.14−16 However, preceding research
of layered perovskites as photovoltaic materials, a number of
investigators studied optoelectronic properties and functions
with cations containing conjugated moieties such oligothio-
phene,17 pyrene,18 anthryl,19 naphthalene,19 and phenyl.20
Much of this earlier work investigated the charge transfer of
photoexcited carriers between the inorganic and organic layers
and explored electroluminescent properties.17,18 Their work
showed the contribution that the organic chromophores can
have on the optoelectronic properties and applications of these
materials. They also demonstrated how the alignment of the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) energy levels of the
organic and inorganic layers can affect charge transfer between
them in the hybrid.18 Because this work predated interest in
these materials as photovoltaic active layers, it primarily
considered charge transfer from inorganic-to-organic layers or
vice versa as a single event. However, charge transfer in a
photovoltaic active layer requires many charge transfer events
across many organic−inorganic interfaces through the thickness
of the device. The predominant model for these materials has
followed a multiquantum-well architecture where the electronic
properties of the organic and inorganic layers have been
considered separately.5,18,19 Recent theoretical work has
suggested that the orbitals of the organic moieties, particularly
those that are aromatic, may contribute to the band structure of
the inorganic.12,21 Using inspiration from this work in organic−
inorganic layered perovskites as well as our own work on hybrid
systems where the organic components serve both structural
and electronic functions,22−24 we report here on efforts to
enhance photovoltaic performance in n = 1 layered perovskite
through organic cation design.
In this work, we utilize the electronic and structural
properties of custom-synthesized organic cations to achieve
enhanced optoelectronic properties and device performance.
We have synthesized organic cations containing various
aromatic moieties and have explored how the electronic
properties and structural arrangement of the cation within the
organic galleries contribute to overcoming the chief limitation
of n = 1 layered perovskites, which is their poor out-of-plane
conductivity. Specifically, we have synthesized n = 1 layered
perovskites with the formula (aromatic-O-linker-NH3)2PbI4 for
which bulky cations are synthesized to contain the aromatic
groups naphthalene, pyrene, or perylene and the linkers ethyl,
propyl, or butyl conjugated through an ether bond (Figure 1A
and B). We have investigated the impact of the HOMO and
LUMO alignment of the organic cation and the inorganic layer
as well as the structural arrangement of the organic cations
within the organic layer on out-of-plane conductivity and
optical properties of these layered perovskite materials. Guided
by our findings, we fabricated photovoltaic devices and
analyzed their performance.
Figure 1. (a) n = 1 layered perovskites of the form (R−NH3)2PbI4
showing alternating organic and inorganic layers that align parallel to
the substrate. (b) Modular molecular design of the ammonium iodide
cation consisting of an aromatic core joined by an ether bond to an
alkyl linker of variable length. (c) Energy levels of typical n = 1 layered
perovskites (gray) with values specific to (butyl-NH3)2PbI4 as
compared to the energy levels of the conjugated organic ammonium
iodide salts of interest: perylene-O-ethyl-NH3I, pyrene-O-propyl-
NH3I, and naphthalene-O-propyl-NH3I. HTL and ETL refer to hole
and electron transport layers.
layers. The highest-performance n = 1 device with parallel
orientation previously reported had a PCE of 0.12%.12 Higher
PCE has been achieved in one rare example where the
inorganic layers prefer perpendicular orientation (η = 1.12%)12
and in another example where a conducting mesoporous
scaffold likely resulted in an isotropic orientation and reduced
charge transport distance in the active layer (η = 1.08%).13 The
favored parallel orientation of n = 1 layered perovskites requires
conduction of excited-state carriers perpendicular to the
inorganic layers and through typically insulating organic layers
(out-of-plane conductivity) to reach the electrodes. If enhanced
out-of-plane conductivity can be achieved in these materials, it
would obviate the need to reorient the inorganic layers with
B
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Table 1. Crystal Data for n = 1 Layered Perovskites of the Form (Aromatic-O-linker-NH3)2PbI4
(1) Energy of S1 exciton determined by optical absorption of thin films in transmission mode. (2) Energy of S1 exciton determined by optical
reflectance spectroscopy on finely powdered crystals. Structures solved with direct methods using SHELXL and refined with least-squares
minimization using the SHELXS refinement package. Additional crystallographic details are provided in the Supporting Information.
facilitates the study of how the energy levels of the aromatic
moieties and packing of the organic cations affect the
optoelectronic properties of the resulting layered perovskites.
Specifically, we synthesized six aromatic ammonium iodide salts
with the general form aromatic-O-linker-NH3I where the
aromatic moiety is naphthalene, pyrene, or perylene and the
linker is ethyl, propyl, or butyl. The aromatic moieties were
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Organic Salts. We
report here on a modular synthetic approach, which enabled
the development of a homologous series of organic ammonium
iodide salts that varied in both electronic character of the
aromatic moiety and length of aliphatic linker. This approach
C
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Figure 2. (a) Energy of the S1 exciton versus average distance between the nitrogen cation and the peripheral iodides of the inorganic layers (dN−I
)
(left); (naphthalene-O-propyl-NH3)2PbI4 structure showing with dashed lines the four different N−I distances used to calculate dN−I (right). Crystal
structures showing intramolecular hydrogen bonding (if present) for (b) (pyrene-O-propyl-NH3)2PbI4, (c) (naphthalene-O-propyl-NH3)2PbI4·
(C4H6O2)0.5, and (d) (naphthalene-O-propyl-NH3)2PbI4. Note that the presence of an intramolecular hydrogen bond in the crystal structure acts to
increase dN−I and reduce ES1.
chosen on the basis of reported HOMO and LUMO levels for
the nonfunctionalized aromatic core in relation to the HOMO
and LUMO levels reported for typical n = 1 lead iodide layered
perovskites with aliphatic cations. To synthesize these
ammonium iodide salts, hydroxyl-naphthalene, pyrene, or
perylene were conjugated to either the protected bromo-
ethyl, propyl, or butyl amine under basic conditions. Following
purification, the protecting groups were cleaved and the
resulting amine protonated with hydroiodic acid to form the
final ammonium iodide salt (see the Supporting Information
for synthetic details). Once synthesized, the HOMO and
LUMO levels of the ammonium iodide salts were determined
by ultraviolet photoelectron spectroscopy (UPS) and optical
absorption experiments. These are plotted with respect to the
HOMO and LUMO levels reported for the reported n = 1
Crystal Growth and Optical Properties. The six aromatic
cations with structure aromatic-O-linker-NH3I were successfully
crystallized with lead(II) iodide into the desired (100) type n =
1 layered perovskite. Single crystals were prepared through
vapor diffusion of dichloromethane (DCM) into γ-butyrolac-
tone (GBL) solutions of 2:1 molar ratio of organic ammonium
iodide salt:lead(II) iodide.25 We will refer to the resulting
perovskites using an abbreviated formula of the general type
(aromatic-O-linker-NH3)2PbI4. Here, we report on seven n = 1
layered perovskites including (naphthalene-O-ethyl-NH3)2PbI4,
(naphthalene-O-propyl-NH3)2PbI4, (naphthalene-O-propyl-
NH3)2PbI4·(C4H6O2)0.5, (pyrene-O-ethyl-NH3)2PbI4, (pyrene-
O-propyl-NH3)2PbI4, (pyrene-O-butyl-NH3)2PbI4, and (pery-
lene-O-ethyl-NH3)2PbI4. The structures of these layered
perovskites were determined using single-crystal X-ray
diffraction. Crystallographic details are provided in Table 1
2
layered perovskite (CH3(CH2)3NH3)2PbI4 in Figure 1C.
D
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and Table S1. Note that naphthalene-O-propyl-NH3I can form
two distinct layered perovskites. The first has the anticipated
form and stoichiometry (naphthalene-O-propyl-NH3)2PbI4,
and the second contains GBL within the lattice (naphthalene-
O-propyl-NH3)2PbI4·(C4H6O2)0.5. This was the only structure
in this work that contained solvent molecules within the unit
cell, and this structure could not be produced in thin film. All
other layered perovskites were crystallized in thin film by spin-
casting and annealing the same stoichiometric ratios dissolved
in a 1:1 volume ratio of dimethylformamide (DMF):dimethyl
sulfoxide (DMSO). Grazing-incidence wide-angle X-ray scatter-
ing (GIWAXS) was used to verify that the same crystallo-
graphic phase was obtained in thin film as observed in single
crystal (Figures S1 and ST1).
The structural features of the organic cation must dictate dN−I
and thus influence the resulting electrostatic interaction.
Understanding and harnessing the structural properties of the
organic cation to influence ES1 and the bandgap of the layered
perovskite will be crucial to the further development of these
materials.
As mentioned previously, the ES1 values observed for the
seven layered perovskites investigated here span 0.3 eV, a
significant range for n = 1 layered perovskites. In most of these
layered perovskites, the alkyl linker to the ammonium cation
adopts an extended conformation within the organic galleries.
However, this does not occur in (pyrene-O-propyl-NH3)2PbI4
where we observe an intramolecular hydrogen bond between
the ammonium cation and the ether linkage (Figure 2B). This
intramolecular hydrogen bond hinders the extended con-
formation of the alkyl linker, resulting in just a 0.22 Å increase
in the inorganic-to-inorganic layer spacing from (pyrene-O-
ethyl-NH3)2PbI4 to (pyrene-O-propyl-NH3)2PbI4. This is
compared to a 1.77 Å spacing increase from (pyrene-O-
propyl-NH3)2PbI4 to (pyrene-O-butyl-NH3)2PbI4. The intra-
molecular hydrogen bond observed in (pyrene-O-propyl-
NH3)2PbI4 also appears to affect the dN−I, resulting in the
longest observed dN−I and correspondingly the lowest ES1 of
2.27 eV. Interestingly, when crystallized as a thin film, the same
pyrene-O-propyl-NH3I molecule that creates (pyrene-O-propyl-
NH3)2PbI4 can form a different perovskite phase with a higher
ES1 of 2.53 eV (Figure S7). This high ES1 phase has a larger
inorganic-to-inorganic layer spacing than does (pyrene-O-
propyl-NH3)2PbI4, suggesting that the alkyl linker in this
phase adopts a more extended conformation. This conforma-
tion may decrease the dN−I and thus raise ES1 as compared to
(pyrene-O-propyl-NH3)2PbI4, which contains an intramolecular
hydrogen bond.
As noted previously, the naphthalene-O-propyl-NH3I mole-
cule can form both (naphthalene-O-propyl-NH3)2PbI4 and
(naphthalene-O-propyl-NH3)2PbI4·(C4H6O2)0.5. These perov-
skites demonstrate a structural trend similar to that of the two
layered perovskites formed by the pyrene-O-propyl-NH3I
molecule. The propyl linker in (naphthalene-O-propyl-
NH3)2PbI4 adopts an extended conformation, and the material
exhibits a high ES1 of 2.57 eV (Figure 2D). The second layered
perovskite, (naphthalene-O-propyl-NH3)2PbI4·(C4H6O2)0.5, has
a lower ES1 of 2.35 eV, and in two of the four molecules within
the asymmetric unit of the crystal structure the propyl linker is
disordered between two conformations (Figure 2C). In one, it
adopts an extended conformation, but in the other it exhibits
the same type of intramolecular hydrogen bond observed in
(pyrene-O-propyl-NH3)2PbI4. In both (naphthalene-O-propyl-
NH3)2PbI4·(C4H6O2)0.5 and (pyrene-O-propyl-NH3)2PbI4, the
propyl linker can form a six-membered intramolecular ring
when the ammonium cation hydrogen bonds to the aromatic
ether. When this intramolecular hydrogen bonding occurs, it
reduces the optical bandgap by increasing dN−I within the
structure. These results demonstrate the structural role that the
organic cation can play in dictating the optical properties of the
resultant perovskite and suggest new strategies to tune the
optoelectronic properties by influencing dN−I through intra-
molecular bonding and steric control.
Optical absorption spectroscopy was used to determine the
energy of the S1 exciton peak (ES1) in thin film samples of each
layered perovskite (Table 1 and Figure S2). Photoluminescence
experiments were performed on thin film and single-crystal
samples. The expected emission from the S1 exciton was
observed only in the naphthalene containing layered perov-
skites (Figure S3). In all of the pyrene and perylene containing
layered perovskites, both single-crystal and thin film samples
showed no fluorescence signal above the autofluorescence
background of the glass. This likely indicates enhanced
nonradiative recombination processes in these materials that
results in little or no observable fluorescence. These n = 1
layered perovskites have characteristically large exciton binding
energy in the range of 250−500 meV that results from the
dielectric confinement of the 2D-inorganic layer.26 The large
binding energy is apparent in the absorption spectra with a
pronounced lowest energy electronic transition, which is
assigned to the S1 exciton followed by a broader absorption
onset typically assigned to the bandgap absorption (Figure S4).
The precise determination of the bandgap of these materials
can be difficult to ascertain from the absorption spectra at room
temperature. Thus, we used the ES1 transition as an indication
of bandgap energy for these materials because this absorption
has been shown to produce photocurrent and is thus important
to photovoltaic performance.27 A wide range of ES1 values
(2.27−2.57 eV) have been observed for the layered perovskites
investigated here. Increasing ES21−values are correlated with
increasing distortion of the PbI4 octahedra as compared to
the ideal, lowest energy arrangement in which Pb−I−Pb bond
angles along the central plane of the inorganic layer are 180°
(Figure S5).28,29 The role that the organic cation plays in
dictating octahedral distortion and thus the optical properties of
layered perovskites has been previously linked to the position of
the positively charged ammonium cation with respect to the
negatively charged inorganic lattice.28 Our materials generally
follow this previously defined model, except for (naphthalene-
O-propyl-NH3)2PbI4, which has the highest ES1 despite having
moderate penetration of the ammonium cation into the
inorganic lattice (Figure S6). Recently, low octahedral
distortion has been observed in an n = 1 layered perovskite
containing tertiary ammonium groups.21 In this structure,
substitution on the ammonium group may increase the distance
between the cationic nitrogen and the negatively charged
inorganic lattice. In our materials, we have observed that ES1
values increase linearly with decreasing average cationic
nitrogen to peripheral-iodide distance (dN−I) (Figure 2A). As
dN−I decreases, the electrostatic interaction between the
cationic nitrogen and negatively charged inorganic lattice
increases, resulting in octahedral distortion and a higher ES1.
Out-of-Plane Conductivity. To investigate how the
electronic character of the organic cation affects out-of-plane
conductivity, we measured the relative out-of-plane conductiv-
ity along the layered axis of single crystals for all of the layered
perovskites reported in this work (except (naphthalene-O-
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propyl-NH3)2PbI4·(C4H6O2)0.5 due to the small sizes of
crystals) as well as three aliphatic control crystals ((butyl-
NH3)2PbI4, (hexyl-NH3)2PbI4, and (octyl-NH3)2PbI4). These
measurements were carried out using the devices described in
Figure 3A, which used single crystals to eliminate the effects of
grain boundaries on conductivity. These measurements were
performed on multiple devices per crystal as well as multiple
crystals of the same layered perovskite (Table S2).
Representative current−voltage traces used for determining
the out-of-plane conductivity of these crystals are provided in
Figure S8. These traces are linear at low bias, which suggests
ohmic contact between electrodes and the various crystals. The
small series resistance introduced by the electrodes should be
systematic and inherently incorporated into all of these
measurements. Short circuits in this device geometry (direct
contact between FTO and gold) generate many orders of
magnitude higher current density than measurements through
the crystals. Thus, we expect the contact resistance to be small
as compared to that of the perovskite. The conductivity results
show that the median conductivity spans close to 4 orders of
magnitude, and the results are summarized in Figure 3B (under
illumination) and Figure S9 in the dark (see Table S3 for
numerical data). Generally, layered perovskites containing
aliphatic cations were found to have the lowest conductivity,
followed closely by the naphthalene-containing layered perov-
skites. Layered perovskites with pyrene and perylene cations
were found to have substantially higher conductivity than those
containing aliphatic moieties. This trend in conductivity mirrors
the better alignment of energy levels between perylene and
pyrene and the inorganic lattice (Figure 1C). This result
indicates that the electronic character of the aromatic moieties
within the organic galleries can be utilized to enhance out-of-
plane conductivity in these materials.
In the design of these molecules, we included a variable
linker length to study the effect of spacing between aromatic
moieties and inorganic layers on out-of-plane conductivity.
When comparing the three aliphatic controls and (naphthalene-
O-propyl-NH3)2PbI4, we found that their conductivities are on
the same order of magnitude with a slight trend toward lower
out-of-plane conductivity with increasing inorganic-to-inorganic
layer spacings (Figure S10). Interestingly, a different trend was
observed for layered perovskites containing pyrene. In this
subset of layered perovskites, (pyrene-O-ethyl-NH3)2PbI4 and
(pyrene-O-butyl-NH3)2PbI4 have effectively the same high
conductivity of 4 × 10−5 S/m, while the conductivity of
(pyrene-O-propyl-NH3)2PbI4 is 2 × 10−6 S/m, which is
considerably lower. This indicates that there is an additional
structural aspect that is more significant than just the spacing
between the inorganic layer and the aromatic core. We assume
that n = 1 lead halide perovskites with the same aromatic
moiety have similar carrier concentrations. Therefore, the large
differences in conductivity should be due to mobility
differences caused by the structural conformations of the
aromatic cations that are not seen in the aliphatic controls.
Examination of crystal structures for the three pyrene-
containing layered perovskites indicates that in the higher
conductivity (pyrene-O-ethyl-NH3)2PbI4 and (pyrene-O-butyl-
NH3)2PbI4, the organic cations exhibit edge-to-face type π-
stacking interactions when viewed down the layered axis across
the van der Waals gap (Figure 4A and B). However, for the
lower conductivity (pyrene-O-propyl-NH3)2PbI4, an edge-to-
edge arrangement was observed in the crystal structure (Figure
4C).
Figure 3. (a) Schematic representation of the layered perovskite
indicating the direction of conductivity measurements (left), and
schematic of the device used to measure the out-of-plane conductivity
and a colorized SEM of a single crystal drop-cast on a silicon wafer
cleaved to reveal the cross section (right) (note the crystal is flat over
its width (0.5 mm) and makes good contact with the substrate). (b)
Conductivity of nine different n = 1 layered perovskites with either an
aliphatic cation or a cation containing naphthalene, pyrene, or perylene
(higher conductivity is observed for the pyrene and perylene samples,
which possess better energy level alignment with the inorganic lattice).
FTO refers to fluorine-doped tin oxide.
Similar to the pyrene-containing layered perovskites, there is
a significant difference in out-of-plane conductivity between the
two naphthalene-containing layered perovskites. The lowest
conductivity sample of all of the layered perovskites measured
was (naphthalene-O-propyl-NH3)2PbI4, which has a conduc-
tivity of 1 × 10−8 S/m, while (naphthalene-O-ethyl-NH3)2PbI4
has a significantly higher conductivity of 3 × 10−7 S/m. In the
higher conductivity (naphthalene-O-ethyl-NH3)2PbI4, we ob-
serve edge-to-face type interactions (Figure 4d), whereas in the
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Figure 4. Crystal structures of n = 1 layered perovskites ranked from left to right by highest out-of-plane conductivity to lowest where the structures
in panels a and b have similar conductivity values: (a) (pyrene-O-ethyl-NH3)2PbI4, (b) (pyrene-O-butyl-NH3)2PbI4, (c) (pyrene-O-propyl-
NH3)2PbI4, (d) (naphthalene-O-ethyl-NH3)2PbI4, and (e) (naphthalene-O-propyl-NH3)2PbI4. The stacking of aromatic cores is highlighted with
dashed lines within the organic galleries, and the square insets show a view down the layered axis from the perspective of an eclipsed aromatic core
(viewing direction is denoted by the black arrow in the crystal structures).
lower conductivity (naphthalene-O-propyl-NH3)2PbI4, the
structure reveals a staggered edge-to-edge stacking. This trend
between the type of packing across the van der Waals gap and
out-of-plane conductivity is the same as in the pyrene-
containing layered perovskites. Last, (perylene-O-ethyl-
NH3)2PbI4 shows edge-to-edge type interactions (see Figure
S11), although it has the highest recorded out-of-plane
conductivity of all of the layered perovskites studied here. We
attribute this to a better energy level alignment with the
inorganic lattice as compared to materials containing the
pyrene, naphthalene, or aliphatic cations.
The results described above suggest that, in addition to the
electronic character of the aromatic moiety, their structural
arrangement within the organic galleries plays a role in the
observed out-of-plane conductivity. Specifically, edge-to-face
interactions across the van der Waals gap yield enhanced
conductivity as compared to edge-to-edge interactions. This is
likely due to the comparatively better orbital overlap in the out-
of-plane direction of the aromatic moieties, which is known to
facilitate more efficient charge transport in organic single
crystals.30−32 This observation implies that future efforts to
improve conductivity need to focus on how one controls the
spatial arrangements of electronically active cations within the
perovskite crystal. Finding strategies to further enhance orbital
overlap possibly through face-to-face interactions may further
improve out-of-plane conductivity.
should exhibit enhanced photovoltaic device performance.
Photovoltaic performance is of course also affected by optical
absorption, crystallinity, and morphology of the active layer.
Thus, we chose to focus on (pyrene-O-propyl-NH3)2PbI4 for
photovoltaic devices because it has the lowest energy S1 exciton
and therefore its absorption better matches the solar spectrum.
Although this layered perovskite did not have the highest
conductivity, it offered the best balance of properties for
photovoltaic performance. As noted previously, thin films of
(pyrene-O-propyl-NH3)2PbI4 can crystallize into two different
phases, and thus we selected annealing conditions (140 °C for
1 min) that resulted in predominant formation of (pyrene-O-
propyl-NH3)2PbI4 rather than the higher ES1 phase.
Our best reproducible device architecture (Figure 5A) using
(pyrene-O-propyl-NH3)2PbI4 achieved a power conversion
efficiency (PCE) of 1.14
0.11% with a champion device
performance of 1.38% (Figure 5B). These devices had a Voc of
1.02 0.03 V, Jsc of 2.43 0.14 mA/cm2, and fill factor of 0.46
0.02% across 27 devices (Figure S12). To our knowledge,
this is the highest reported PCE for an n = 1 layered perovskite
solar cell. This result is noteworthy given that grazing-incidence
wide-angle X-ray scattering (GIWAXS) of our thin films shows
a clear preferential orientation of the inorganic layers parallel to
the substrate (Figure 5C), while the previously reported
highest-efficiency n = 1 device had inorganic layers oriented
perpendicular to the substrate.12 Furthermore, our device is an
order of magnitude higher efficiency than any other reported n
= 1 layered perovskite with the inorganic layers oriented
parallel to the substrate. We attribute this enhancement in
performance at least partially to enhancements in out-of-plane
Device Fabrication and Photovoltaic Performance.
Given that poor out-of-plane conductivity is the chief limitation
to the application of n = 1 layered perovskites in photovoltaic
devices, the higher-conductivity layered perovskites in this work
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Figure 5. (a) Energy levels of the layers in the (pyrene-O-propyl-NH3)2PbI4 layered perovskite solar cell. (b) I−V curve for the champion device
with 1.38% efficiency (only forward sweep is shown for clarity, but the average includes forward and reverse curves where little hysteresis was
observed (Figure S13)). (c) Grazing-incidence wide-angle X-ray scattering of (pyrene-O-propyl-NH3)2PbI4 crystallized on a PEDOT:PSS-coated
substrate. (d) AFM height and phase images for (pyrene-O-propyl-NH3)2PbI4 crystallized on PEDOT:PSS. (e) Photograph showing the difference
in appearance of three substrates dipped in water: the 3D perovskite with the formula (CH3NH3)PbI3 and the n = 4 layered perovskite with the
formula (butyl-NH3)2(CH3NH3)3Pb4I13 turn yellow instantly revealing chemical degradation, whereas the n = 1 layered perovskite (pyrene-O-
propyl-NH3)2PbI4 retains its original color.
conductivity in the crystalline material. Preliminary devices with
(pyrene-O-propyl-NH3)2PbI4 showed that the PCE was
significantly lower in low-crystallinity samples as compared to
high-crystallinity samples, indicating that the photovoltaic
performance is dependent on the crystalline phase rather
than grain boundaries or defect states (Figure S14). External
quantum efficiency results are shown in Figure S15. These
results show no clear contribution of the pyrene absorption to
the efficiency of the overall photovoltaic device. This is
expected given the low absorption of pyrene within the visible
spectrum and indicates that photovoltaic performance observed
is due to the layered perovskite. With respect to charge
transport layers, a low work function PEDOT:PSS formulation
was used for better energy level alignment with the active layer,
and no electron transport layer was added because the pyrene-
O-propyl-NH3I molecule forms a native electron-transporting
capping layer on the top surface of the active layer. A more
detailed discussion of this capping layer, each device layer,
device optimization, and UPS measurements is included in
Figure S16 and ST 2.
a charge transport layer to achieve photovoltaic function
because butylammonium iodide does not form an electron-
transporting capping layer on the surface of the active layer,
unlike (pyrene-O-propyl-NH3)2PbI4. Thus, to compare the two
layered perovskites, we added a thin layer of 1-propoxypyrene
on both (pyrene-O-propyl-NH3)2PbI4 and (butyl-NH3)2PbI4
active layers to mimic the native electron transport layer on the
(pyrene-O-propyl-NH3)2PbI4 active layer. These control
devices demonstrate that 1-propoxypyrene functions as an
electron transport for layered perovskite active layers, and that
in this device architecture (pyrene-O-propyl-NH3)2PbI4 per-
forms significantly better than (butyl-NH3)2PbI4 as a photo-
voltaic active layer.
As previously discussed, GIWAXS was utilized to compare
the orientation of the crystalline domains in these thin films. In
both (pyrene-O-propyl-NH3)2PbI4 and (butyl-NH3)2PbI4
(Figure S17), the films were found to be crystalline and
oriented with the 2D layers parallel to the substrate. We
analyzed the orientation of the thin films using Herman’s
orientation factor S to define orientational ordering.33 An S
value of 1 indicates perfect parallel orientation of domains, S =
0 indicates isotropic domains, and S = −0.5 indicates perfect
perpendicular orientation of domains. By analyzing data along
the azimuthal angle χ from 0° to 90° within the q range of the
The PCE of our devices using (pyrene-O-propyl-NH3)2PbI4
is at least 1 order of magnitude higher than control devices with
(butyl-NH3)2PbI4 as the active layer using the same device
architecture (Table S4). The (butyl-NH3)2PbI4 device required
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measured both with the microscope lamp turned to full power
(MLC-150C illuminator, 150 W EKE halogen lamp) and in the
absence of any light. The conductivity was calculated using the linear
region of the I−V curve (voltage sweep from −5 to 5 V) and
parametrized based on individual device thicknesses. The thickness of
each device was determined by optical profilometry (Zygo 3D Optical
Profiler and Bruker Contour GT-K). The averages of multiple devices
per crystal and across several crystals were taken to determine the out-
of-plane conductivity of a given structure (Table S2).
Crystal Structure Determination. Single-crystal X-ray data were
collected at room temperature on a Kappa Apex 3 diffractometer. For
control crystals with previously reported structures, partial data sets
were collected for unit cell determination (between 60 and 90 frames)
to validate against the previously reported structures. For all other
crystals, a complete data set was collected for structure solution and
refinement. In all cases, the face-indexing tool in Apex 3 was utilized to
verify the direction of the layered axis for electrical measurements
(Figure S21). Using Olex 2, crystal structures were solved using the
ShelXS structure solution program using direct methods and refined
using ShelXL refinement package using least-squares minimizations.
The inorganic lattice was refined anisotropically without restraints.
The organic cations were refined anisotropically with the exception of
hydrogen atoms, which were placed in idealized positions. Additional
restraints to the bond lengths and thermal ellipsoids for the organic
cations were applied where necessary. Structural information for seven
layered perovskites in this work is provided in Table 1 and Table S1.
CCDC nos. 1840802−1840808 contain the supplementary crystallo-
graphic data for this Article.
Substrate Preparation. Glass and patterned ITO substrates
(Thin Film Devices, 20 Ω/□) were cleaned by sequential sonication
in hexane, soapy water, milli-Q water, and a 1:1:1 mixture of
isopropanol, acetone, and methanol. After drying, clean substrates
were treated for 20 min in a UV-ozone cleaner (Bioforce
Nanosciences). Poly(3,4-ethylenedioxythiophene) polystyrenesulfo-
nate in toluene (PEDOT:complex) (Ossila) was diluted to one-half
of its original concentration with toluene and then filtered through a
0.2 μm PTFE filter before spin-coating at 5000 rpm with a 5000 rpm/s
ramp rate for 30 s. PEDOT:complex (Ossila) films were then annealed
at 120 °C for 10 min in a nitrogen-filled glovebox. Poly(3,4-
ethylenedioxythiophene) polystyrenesulfonate in water (PEDOT:PSS)
(Heraeus) was filtered through a 0.45 μm nylon filter before spin-
coating at 5000 rpm with a 1000 rpm/s ramp rate for 60 s.
PEDOT:PSS (Heraeus) films were then annealed at 150 °C for 20
min in a nitrogen-filled glovebox.
Thin Film Preparation. Thin film samples other than those
intended for device active layers were prepared by spin-casting 2:1
molar ratios of organic ammonium iodide salt:lead(II) iodide (TCI
Chemical) dissolved in 20−30 wt % in 50 vol % DMF/50 vol %
DMSO onto freshly cleaned glass slides at 2000 rpm. With respect to
solubility, we found that a combination of DMF and DMSO as a
solvent mixture was very helpful in dissolving the poorly soluble bulky
organic cations in our layered perovskite materials. From this solvent
mixture, we spin-coated and annealed crystalline thin films of each of
the layered perovskite materials. Each of the layered perovskites
crystallized easily in thin films except for that containing the perylene
derivative. These substrates were then annealed on a hot plate at 110
°C for 30 min. The resulting thin films were characterized by powder
X-ray diffraction (PXRD) (Scintag XDS2000 using Cu Kα radiation at
40 kV and 1−20 mA current), ultraviolet−visible absorption
spectroscopy (PerkinElmer LAMBDA 1050), steady-state photo-
luminescence (Horiba Nanolog Fluorimeter), and grazing incidence
wide-angle X-ray scattering (GIWAXS) (Beamline 8-ID-E Advanced
Photon Source, Argonne National Laboratory). GIWAXS was
performed with a beam energy of 10.92 keV and with a 0.14°
incidence angle. The unit cell determined from single-crystal X-ray
diffraction was compared by overlaying a predicted pattern34 onto the
GIWAXS patterns to verify the formation of the same phase in thin
films and single crystals (Figure S1 and ST 1).
(600) peak of (pyrene-O-propyl-NH3)2PbI4, we found that S =
0.95, and within the q range of the (002) peak of (butyl-
NH3)2PbI4 we found that S = 0.97. This indicates strong
alignment of the layered axis parallel to the substrate for both
thin films (see ST 3 for derivation details). When analyzed by
atomic force microscopy, thin films of (pyrene-O-propyl-
NH3)2PbI4 exhibit significant roughness and small grain sizes
(Figure 5D). This is potentially caused by the low diffusivity of
the bulky organic cation during the annealing process. The
small grain size observed with this technique may explain some
of the azimuthal peak broadening observed in GIWAXS.
Importantly, the active layer in these devices is chemically
stable. Although the complete devices suffer from limited
stability in atmospheric conditions due to reactions between the
active layer and the cathode material, the active layer itself is
exceptionally stable; it can withstand immersion in water for
several minutes without any decomposition to lead(II) iodide
(Figure S18). This property is unique when compared to the
prototypical 3D perovskite (CH3NH3)PbI3, the n = 4 layered
perovskite (butyl-NH3)2(CH3NH3)3Pb4I13, and surprisingly the
(butyl-NH3)2PbI4 (Figures 5E, S19, and S20). This result
demonstrates that the added hydrophobic bulk of large
aromatic cations may act to enhance stability while maintaining
higher conductivity as compared to aliphatic cations of similar
size. This may result in additional stability and conductivity in
higher-n layered perovskites.
CONCLUSIONS
■
The out-of-plane conductivity in chemically stable n = 1 layered
perovskites has been increased by several orders of magnitude
using aromatic cations in the organic layers to better match
their energy levels with those of inorganic layers. When used in
photovoltaic devices, these layered perovskites yielded the
highest reported power conversion efficiency (1.38%) for an n
= 1 layered perovskite. We showed that intramolecular
hydrogen bonding in the organic cations and supramolecular
π-stacking interactions between them can result in narrower
bandgaps and greater out-of-plane conductivity. Our results
suggest there is great potential for functional improvement in
chemically stable layered perovskites focusing on the design of
the molecular and supramolecular structure of the organic
layers.
EXPERIMENTAL SECTION
■
Single-Crystal Preparation for Structure Evaluation and
Conductivity Measurements. All single-crystal samples were
prepared using vapor diffusion of dichloromethane into γ-butyr-
olactone (GBL) solutions containing a 2:1 molar ratio of organic
ammonium iodide salt:lead(II) iodide. The concentration of GBL
solution was critical to obtain high-quality samples for both single-
crystal X-ray diffraction and for electrical measurements (Table S5 and
ST 4). In all cases, plate-like crystals, which ranged in color from
yellow to red, appeared in ca. 24 h. These plate-like crystals form due
to preferential crystal growth along the 2D inorganic galleries. To
measure conductivity along the layered axis of these plate-like crystals,
they were separated from their mother liquor and washed (details in
ST 4) before being drop-cast on a freshly cleaned unpatterned FTO
substrates (Solaronix, 15 Ω/□). Small square gold pads of 113 μm ×
113 μm × 50 nm were then thermally evaporated on top of the crystals
through a TEM shadow mask (Electron Microscopy Sciences T200-
Cu), and conductivity was measured on the single-crystal samples in a
two-electrode geometry (Signatone probe station, Agilent 4155C
semiconductor parameter analyzer). The conductivity was then
measured through the thickness of the crystals and normalized by
pad area and crystal thickness. Out-of-plane conductivity was
Device Fabrication. We designed the layers of our photovoltaic
devices as shown in Figure 5A. The devices in this study were
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fabricated in a nitrogen-filled glovebox. Pyrene-O-propyl-NH3I and
lead iodide were dissolved at a 2:1 molar ratio in a 25 wt % solvent
mixture of 50 vol % DMF/50 vol % DMSO and heated for 30 min at
65 °C. For optimized (pyrene-O-propyl-NH3)2PbI4 devices, the active
layer was spin-cast onto PEDOT:PSS-coated, patterned ITO
substrates for 40 s at 4000 rpm with a ramp time of 7 s. The active
layer was then annealed for 1 min at 140 °C. 50 nm silver contacts
were thermally evaporated onto the substrates at a pressure of <10−6
mbar using a shadow mask to define 4 mm2 devices. More details
about the device fabrication have been included in ST 2. Finally,
devices were tested in air on a Newport solar simulator with a Keithley
2400 sourcemeter with a voltage ramp rate of 0.2 V/s.
Resource (NSF ECCS-1542205); the State of Illinois and
International Institute for Nanotechnology (IIN). This work
made use of the EPIC, Keck-II, and SPID facilities of
Northwestern University’s NUANCE Center, which has
received support from the Soft and Hybrid Nanotechnology
Experimental (SHyNE) Resource (NSF ECCS-1542205); the
MRSEC program (DMR-1720139) at the Materials Research
Center; the International Institute for Nanotechnology (IIN);
the Keck Foundation; and the State of Illinois, through the IIN.
This work made use of the J. B. Cohen X-ray Diffraction
Facility supported by the MRSEC program of the National
Science Foundation (DMR-1720139) at the Materials Research
Center of Northwestern University [LCP1]. This work made
use of the Northwestern University Micro/Nano Fabrication
Facility (NUFAB), which is partially supported by Soft and
Hybrid Nanotechnology Experimental (SHyNE) Resource
(NSF ECCS-1542205), the Materials Research Science and
Engineering Center (NSF DMR-1720139), the State of Illinois,
and Northwestern University. This research used resources of
the Advanced Photon Source, a U.S. Department of Energy
(DOE) Office of Science User Facility operated for the DOE
office of Science by Argonne National Laboratory under
Contract No. DE-AC02-06CH11357.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.8b03659.
■
S
X-ray crystallographic data for (naphthalene-O-ethyl-
NH3)2PbI4 (CIF)
X-ray crystallographic data for (naphthalene-O-propyl-
NH3)2PbI4 (CIF)
X-ray crystallographic data for (naphthalene-O-propyl-
NH3)2PbI4·(C4H6O2)0.5 (CIF)
X-ray crystallographic data for (pyrene-O-ethyl-
NH3)2PbI4 (CIF)
X-ray crystallographic data for (pyrene-O-propyl-
NH3)2PbI4 (CIF)
X-ray crystallographic data for (pyrene-O-butyl-
NH3)2PbI4 (CIF)
X-ray crystallographic data for (perylene-O-ethyl-
NH3)2PbI4 (CIF)
Synthetic details, characterization, experimental details,
and additional figures as described in the text (PDF)
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AUTHOR INFORMATION
Corresponding Author
*s-stupp@northwestern.edu
ORCID
James V. Passarelli: 0000-0001-7918-9392
Daniel J. Fairfield: 0000-0001-5880-2055
Mark P. Hendricks: 0000-0003-1295-9879
Hiroaki Sai: 0000-0002-4268-2148
Samuel I. Stupp: 0000-0002-5491-7442
■
Notes
The authors declare no competing financial interest.
∇J.V.P. and D.J.F. contributed equally.
ACKNOWLEDGMENTS
■
This work was primarily supported by the U.S. Department of
Energy, Office of Science, Basic Energy Sciences, under award
no. DE- FG02-00ER45810. Work on single-crystal conductivity
measurements was supported by the Air Force Research
Laboratory under agreement number FA8650-15-2-5518.
N.A.S. was supported by the Department of Defense (DoD),
Air Force Office of Scientific Research, through the National
Defense Science and Engineering Graduate (NDSEG) Fellow-
ship, 32 CFR 168a. N.A.S. and J.V.P. also acknowledge support
from Northwestern University through a Ryan Fellowship. We
would like to thank Michelle Chen for photoluminescence
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