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AC KNOWLED GME NTS
We thank Signal Hill Petroleum and NodalSeismic for granting us
permission to use the Long Beach Array data, and we thank
Breitburn Energy and LA Seismic for permission to use the
Rosecrans Array data. We acknowledge J. P. Avouac, R. Bürgmann,
Y. Ma, and W. Frank for helpful discussions. This research was
supported by NSF awards EAR-1214912 and EAR-1520081 and by
the Terrestrial Hazard Observation and Reporting Center at
Caltech. The seismic data are property of Signal Hill Petroleum and
Breitburn Energy. Data are available for noncommercial use
through a license agreement with the data owners that includes
but is not limited to a nondistribution agreement. Please contact
the authors for additional information.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/354/6308/88/suppl/DC1
Materials and Methods
Figs. S1 to S7
References (49, 50)
23 December 2015; accepted 31 August 2016
10.1126/science.aaf1370
SOLAR CELLS
Quantum dot–induced phase
stabilization of a-CsPbI3 perovskite
for high-efficiency photovoltaics
Abhishek Swarnkar,1,2 Ashley R. Marshall,1,3 Erin M. Sanehira,1,4
Boris D. Chernomordik,1 David T. Moore,1 Jeffrey A. Christians,1
Tamoghna Chakrabarti,5 Joseph M. Luther1*
We show nanoscale phase stabilization of CsPbI3 quantum dots (QDs) to low temperatures that
can be used as the active component of efficient optoelectronic devices. CsPbI3 is an
all-inorganic analog to the hybrid organic cation halide perovskites, but the cubic phase of bulk
CsPbI3 (a-CsPbI3)—the variant with desirable band gap—is only stable at high temperatures.
We describe the formation of a-CsPbI3 QD films that are phase-stable for months in ambient air.
The films exhibit long-range electronic transport and were used to fabricate colloidal perovskite
QD photovoltaic cells with an open-circuit voltage of 1.23 volts and efficiency of 10.77%.
These devices also function as light-emitting diodes with low turn-on voltage and
tunable emission.
H
ybrid organic-inorganic halide perovskites,
with the common formulation ABX3 (where
A is an organic cation, B is commonly Pb2+,
and X is a halide), were first applied to
photovoltaics (PVs) as methylammonium
lead triiodide (CH3NH3PbI3) in 2009 (1). Perovskite PV devices processed from solution inks now
convert >22% of incident sunlight into electricity,
which is on par with the best thin-film chalcogenide and silicon devices, but durability of the
semiconductor presents a major technical hurdle
to commercialization. Under environmental stress,
CH3NH3PbI3 dissociates into PbI2 and CH3NH3I,
the latter of which is volatile (2).
Thus, an all-inorganic structure without a volatile organic component is highly desired. The
all-inorganic Pb-halide perovskite with the most
appropriate band gap Eg for PV applications is
cubic (a) CsPbI3 (Eg = 1.73 eV) because geometrical constraints of the perovskite structure require a large +1 A-site cation, and Cs+ is the most
feasible. However, below 320°C, the orthorhombic
(d) phase (Eg = 2.82 eV) is thermodynamically
1
Chemical and Materials Science, National Renewable Energy
Laboratory (NREL), Golden, CO 80401, USA. 2Department of
Chemistry, Indian Institute of Science Education and Research
(IISER), Pune 411008, India. 3Department of Chemistry and
Biochemistry, University of Colorado, Boulder, CO 80309, USA.
4
Department of Electrical Engineering, University of Washington,
Seattle, WA 98195, USA. 5Metallurgical and Materials
Engineering, Colorado School of Mines, Golden, CO 80401, USA.
*Corresponding author. Email: joey.luther@nrel.gov
sciencemag.org SCIENCE
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that generally depends on the asperity size and
stress drop and on the resistance of the matrix.
This effective radius Re controls the range of interaction between asperities. The ratio between Re
and the interasperity distance D determines the
ability of asperities to break together in seismic
events, despite the intervening creep, and thus
influences the statistics of the earthquake catalog.
When Re/D is large, ruptures can involve multiple
asperities. This strong interaction regime potentially leads to a scale-free, power-law earthquake
size distribution (Fig. 3A) and temporal clustering
(Fig. 3C), as observed at shallow depths. When
Re/D is small, asperities tend to break in isolation.
In this weak interaction regime, seismicity is
temporally uncorrelated and, if asperities have
a characteristic size, the earthquake size distribution is scale-bound, as observed in the deep NIF
beneath LB. A systematic decrease of Re/D with
increasing depth may result from several processes,
which are not necessarily independent. One possibility is a rheological control: Re may decrease
with depth due to increasing velocity strengthening of the creeping matrix or decreasing stress
drop within the asperities. Another possibility is
a geometrical (or structural) control: At larger
depths, the range of asperity sizes (and, hence,
of Re) may be narrower or D may be larger (e.g.,
due to lithological variations).
preferred (3). Nevertheless, groups have explored
CsPbX3 compounds as PV materials, but films of
a-CsPbI3 undergo immediate transformation to
the orthorhombic phase when exposed to ambient conditions (4). Attempts to stabilize the cubic
phase through alloying with Br– have been explored
because CsPbIBr2 shows a much reduced d to a
phase transition temperature of 100°C (3). However, the composition change leads to an undesired
increase in the band gap. We show that nanocrystal surfaces can be used to stabilize a-CsPbI3
at room temperature, far below the phase transition
temperature for thin film or bulk materials. We
further show that we can control the electronic
coupling of quantum dots (QDs) to produce airstable, efficient PV cells (initial efficiency above
10%) based on this all-inorganic material.
Many physical properties differ between
nanometer-sized and bulk crystalline materials
of the same chemical compound. One such example is the structural phase in which the constituent
atoms are arranged. For example, the semiconductors CdS and CdSe embody a rock salt structure at high pressure. However, the solid-solid
phase transition point between the rock salt phase
and the hexagonal wurtzite phase can vary greatly
in temperature and pressure as a function of crystal
size (5, 6). Manipulated size-dependent phase
diagrams have been explored in a variety of material systems, with advantageous properties of the
crystals emerging at reduced dimensions in oxides
(such as TiO2), lanthanides (such as NaYF4) (7),
metals (such as Ag) (8), and ferroelectrics (such as
the perovskite BaTiO3) (9).
Synthetic protocols of colloidal halide perovskite QDs have recently been reported (10–17).
CsPbX3 QDs exhibit improved room-temperature
cubic-phase stability and attractive optical properties for a wide range of applications (11, 18–22).
Experiments on size- and shape-dependent optical properties (11, 23–25), surface chemistry (26),
and other photophysics (27) are being explored for
CsPbBr3 QDs. However, previous studies were unable to achieve a-CsPbI3 QDs that were stable
enough for extensive characterization or to be
used in PV cells.
We present an improved synthetic route and
purification approach of CsPbI3 QDs. Once purified, the QDs retain the cubic phase for months
in ambient air and even at cryogenic temperatures.
A method for perovskite QD film assembly is described that allows for efficient dot-to-dot electronic transport while retaining the phase stability
of the individual QDs. The PV cells produced from
this approach have the highest power conversion
efficiency (PCE) and stabilized power output (SPO)
of any all-inorganic perovskite absorber, produce
1.23 V at open circuit (among the best of any
perovskite PV cells), and also function as lightemitting diodes (LEDs), emitting visible red light
with low turn-on voltage.
The tunability of the band gap via size control
due to quantum confinement is shown in Fig. 1.
The series of CsPbI3 QDs, with varied size (band
gap), were synthesized with the addition of Csoleate to a flask containing PbI2 precursor, as first
described by Protesescu et al. (11)—here, using
SCIENCE sciencemag.org
Fig. 1. Characterization of CsPbI3 QDs. (A) Normalized UV-visible absorption spectra and photographs
of CsPbI3 QDs synthesized at (a) 60°C (3.4 nm), (b) 100°C (4.5 nm), (c) 130°C (5 nm), (d) 150°C (6.8 nm), (e)
170°C (8 nm), (f) 180°C (9 nm), and (g) 185°C (12.5 nm).The numbers in parentheses are the average size
from TEM. (B) Normalized photoluminescence spectra and photographs under UV illumination of the QDs
from (A). (C) High-resolution TEM of CsPbI3 QDs synthesized at 180°C. (D) XRD patterns of QDs synthesized at (from bottom to top) 60°, 100°, 170°, 180°, and 185°C, confirming that they crystallize in the cubic
phase of CsPbI3.
Fig. 2. Phase stability of CsPbI3 QDs. (A) Powder XRD patterns and (B) UV-visible absorption spectra,
normalized at 370 nm, of CsPbI3 QDs synthesized at 170°C and stored in ambient conditions for a period of
60 days. (Inset) The slight blue shift that is seen in the excitonic peak with extended storage. (C) Rietveld
refinement fitting of CsPbI3 QD XRD pattern, revealing pure cubic-phase CsPbI3.
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RE S EAR CH | R E P O R T S
100 °C
150 °C
180 °C
550
600
650
700
750
Wavelength (nm)
4000
MeOAc Washed
As-Cast Film
3000
2000
1000
Wavenumber (cm-1)
Fig. 3. CsPbI3 QD films. (A) UV-visible absorption (solid lines) and PL spectra (dashed lines) of CsPbI3
QDs in solution (blue) and as-cast films (black) for QDs synthesized at 100°, 150°, and 180°C. (B) FTIR
spectra showing the IR transmission of a CsPbI3 QD film as cast (black) and after treating with MeOAc (red).
injection temperatures between 60° and 185°C to
control the size (28). This produces QDs solubilized by noncrystalline iodide and oleylammonium
surface ligands (26). Unpurified QDs transform to
the orthorhombic phase within several days (fig.
S1) (28), as in previous reports (29, 30). However,
we developed a process to purify the QDs by using
methyl acetate (MeOAc), an antisolvent that removes excess unreacted precursors without inducing agglomeration. Using this extraction
procedure, we found that the QDs are stable in
the cubic phase for months with ambient storage.
The excitonic peak of CsPbI3 shifted between
585 and 670 nm, corresponding to QD sizes between 3 and 12.5 nm, respectively. The corresponding normalized photoluminescence (PL) spectra
of the samples are shown in Fig. 1B, along with a
photograph of the QDs in hexane. Upon ultraviolet (UV) excitation, emission was in the orange
(600 nm) to red (680 nm) color range, corresponding to a band gap between 2.07 and 1.82 eV
(photographs showing PL from dried QD powders
are shown in fig. S2) (28). The full width at halfmaximum of the PL for the smallest QDs was
83 meV and increased slightly for the larger sizes,
whereas the PL quantum yield varied from 21 to
55% for different sizes (fig. S3) (28).
In contrast to the instability of the cubic phase
of bulk CsPbI3 at room temperature, QDs have
been reported to retain the cubic phase because
of the large contribution of surface energy (Fig.
1D) (11, 31). The softer basic nature of I– as compared with Br– results in weaker acid-base interactions between the halide and the oleylammonium
ligand (a hard acid) in the case of CsPbI3, compared
with that of CsPbBr3 (30, 32). Therefore, the isolation of CsPbI3 QDs is more difficult than that of
CsPbBr3 QDs because of the loss of ligand during
extraction, causing agglomeration and conversion
to the orthorhombic phase. Thus, we found that
MeOAc, which isolates the QDs without full removal of the surface species, is critical to the phasestable devices described below.
The high-resolution transmission electron micrograph (TEM) of the sample synthesized at 180°C
(Fig. 1C) shows an interplanar distance of 0.62 nm,
which is consistent with the (100) plane of cubic
phase CsPbI3 (24, 31, 33). In Fig. 2, A and B, powder
x-ray diffraction (XRD) patterns and UV-visible absorption spectra confirm the absence of diffraction
peaks or the high-energy (~3 eV) sharp absorption
characteristic of orthorhombic phase formation
(31), even after 60 days of storage in ambient conditions. Additionally, the QDs remained in the
cubic phase even after the solution was cooled to
77 K, further demonstrating the expanded temperature stability of the cubic phase.
Rietveld refinement of the XRD patterns (Fig.
2C) (28) allowed us to quantify the contribution
from cubic and orthorhombic phases. No detectable orthorhombic phase was found. Additionally, lattice parameters of three different size CsPbI3
QD samples were estimated (Table 1). The lattice
parameter values showed a size dependence and
were lower than the previously measured experimental value (6.2894 Å at 634 K) of bulk cubic
CsPbI3 (33). Our measurements were performed
Fig. 4. CsPbI3 optoelectronic devices. (A) Schematic (with TEM image of QDs) and (B) SEM cross-section of the CsPbI3 PV cell. (C) Current density–voltage
curves of a device measured in air over the course of 15 days. The black diamond represents the stabilized power output of the device at 0.92 V, as shown in fig. S9.
(D) External quantum efficiency (black, left ordinate) and integrated current density (blue, right ordinate) of the device. (E) EL spectra of CsPbI3 PV cell (CsPbI3 QDs
synthesized at 170°C) under forward bias. (Inset) A photograph of the luminescent device. (F) PL (dashed lines) and EL (solid lines) spectra of completed devices
fabricated by using CsPbI3 QDs synthesized at 170° and 180°C, demonstrating size quantization effects in the completed devices.
94
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Abs./PL (norm.)
Solution
Film
Transmission (a.u.)
R ES E A RC H | R E PO R TS
RE S EAR CH | R E P O R T S
QD size (TEM)
QD size (Rietveld)
a (Å)
Rwp
8 nm
9 ± 1 nm
6.231 ± 0.002
3.42
9
nm
10 ± 1 nm
6.220 ± 0.002
6.50
.....................................................................................................................................................................................................................
.....................................................................................................................................................................................................................
15.5
nm
17 ± 2 nm
6.189 ± 0.002
7.79
.....................................................................................................................................................................................................................
at 297 K, whereas high temperatures are required
to characterize bulk cubic CsPbI3. A similar increase in lattice parameter with decreasing particle
size has been reported in other systems and attributed to electrostatic relaxation with decreasing
crystal size (34).
In order to use these highly phase-stable
a-CsPbI3 QDs in optoelectronic devices, we developed a method to cast electronically conductive
QD films. The QDs were first spin-cast from octane
then dipped in a saturated MeOAc solution of
either Pb(OAc)2 or Pb(NO3)2 (neat MeOAc was
used as a control). This process was repeated
multiple times—typically, three to five—to produce
QD films with thicknesses between 100 and 400 nm.
The optical absorption and PL spectra (Fig. 3A,
for three samples with indicated reaction temperature) show that in each case, the film absorbance and PL was red-shifted ~20 nm from that
of the QDs in solution, whereas the tunable emission properties of the films indicate that quantum
confinement is preserved. Fourier-transform infrared (FTIR) spectra show the removal of organic
ligands from the film with exposure to neat MeOAc
(Fig. 3B), given the near absence of C–H modes
near 3000 cm−1 or below ~2000 cm−1 belonging to
oleylammonium, oleate, or octadecene. We therefore attribute the preserved phase stability of the
QDs in the films to the size of the crystals (given the
quantum confined optical properties) independent
of the surface species. However, we found that prolonged annealing at temperatures >200°C causes
further grain growth and thus induces a phase
transition to the orthorhombic phase (fig. S4 and
table S1) (28). Additional strategies to preserve
the phase in sintered QD films are being explored
(35). We have observed cubic-phase CsPbI3 with
edge length up to 50 nm using the solution-phase
synthesis described here.
We also probed the interaction of Pb2+ salts
with QDs in solution and on films by monitoring
the fluorescence (fig. S5) (28). Titration of a small
amount of Pb(OAc)2 dissolved in MeOAc to the
QD solution showed an enhancement in PL, suggesting improved surface passivation. The surface treatments increase the PL lifetime over that
of neat QD films, which highlights the importance
of surface chemistry in this QD system (fig. S6 and
table S2) (28). Titrations with only MeOAc caused
fast PL quenching. Similarly, dip-coating of the
QD film in a saturated solution of Pb(OAc)2 in
MeOAc resulted in a PL enhancement of ~350%
compared with dip-coating in MeOAc alone.
We fabricated PV cells with CsPbI3 QD films
as the photoactive material. A schematic of the
SCIENCE sciencemag.org
device architecture is shown in Fig. 4A, and a
scanning electron micrograph (SEM) cross-section
image of the reported device with 9 nm QDs is
shown in Fig. 4B. The reverse-scan current densityvoltage (JV) curves showed an open-circuit voltage
(VOC) of 1.23 V, and 10.77% PCE for a 0.10 cm2 cell
made and tested completely in ambient conditions
(relative humidity ~15 to 25%) (Fig. 4C). The hysteresis along with SPO of a device scanned at
various sweep rates is shown in fig. S7 (28). Furthermore, the PCE improved from its initial value
over the course of 60 days storage in dry but ambient conditions (fig. S8) (28). In fig. S9 (28), we
show the SPO of the cell by measuring the current
density while the device is biased at 0.92 V. In Fig.
4D, the spectral response of the PV cell is shown,
indicating a band gap of 1.75 eV for this film. We
compare QD devices to thin-film CsPbX3 perovskite solar cells following literature reports, which
have thus far reported at 9.8% PCE and SPO as
high as 6.5% (4, 31, 36). The QD devices show
improved JV-scan efficiency, operational stability,
and tolerance to higher relative humidity levels
(figs. S10 and S11 and table S3) (28). The VOC is
remarkably higher than that of other QD solar
cells (typically <0.7 V) and among the highest VOC
in all perovskite PV cells for band gap values
below 2 eV (fig. S12, stabilized VOC) (28). We
have not optimized the device architecture or
the QD film-treatment scheme. We found that
dip-coating spin-cast films in neat MeOAc and
MeOAc saturated with Pb(OAc)2 or Pb(NO3)2 all
work reasonably well (JV-scanned PCE > 9%) in
PV devices. Large diffusion lengths and mobility
values have been measured in CsPbBr3 QDs by
means of terahertz spectroscopy (37); however, a
better understanding of the electronic coupling
is critical to maximizing long-range transport in
QD perovskite films.
Given the PL properties of these perovskite QDs,
we explored their use as LEDs. The PV devices
produced bright visible electroluminescence (EL)
when biased above VOC (Fig. 4E, inset). The EL had
a low turn-on voltage near the band gap of the
CsPbI3, with increasing intensity at larger applied
biases (Fig. 4E). These spectra provide direct evidence that quantum confinement is retained in
the complete devices, which is critical to retaining the improved cubic-phase stability, as seen by
the shift in both the EL and PL spectra of devices
with different-size QDs (Fig. 4F). The synthesis of
normally unstable material phases stabilized
through colloidal QD synthesis provides another
mechanism for material design for PVs, LEDs,
and other applications.
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AC KNOWLED GME NTS
We thank J. van de Lagemaat, W. Tumas, H. Choi, M. Beard, and
J. Berry for helpful discussions and B. To for SEM imaging. We
acknowledge support from the Center for Advanced Solar
Photophysics, an Energy Frontier Research Center funded by the
U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences for quantum dot coupling and solar cell
structures. Device durability and structural phase characterization
was performed within the hybrid perovskite solar cell program of
the National Center for Photovoltaics funded by the U.S.
Department of Energy, Office of Energy Efficiency and Renewable
Energy, Solar Energy Technologies Office under contract DE-AC3608GO28308DOE. The original conception and QD synthesis was
performed under the Laboratory Directed Research and
Development program at NREL. A.S. acknowledges the Bhaskara
Advanced Solar Energy fellowship funded by the Department of
Science and Technology, government of India, and Indo-U.S.
Science and Technology Forum (IUSSTF). E.M.S. acknowledges a
NASA Space Technology Research Fellowship. D.T.M. acknowledges
the NREL Director’s Fellowship. All data in the paper and
supplementary materials are available. An application has been made
for a provisional patent (U.S. patent application no. 62/343,251).
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/354/6308/92/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S12
Tables S1 to S3
References (38, 39)
1 June 2016; accepted 7 September 2016
10.1126/science.aag2700
7 OCTOBER 2016 • VOL 354 ISSUE 6308
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REFERENCES AND NOTES
Table 1. Results of the Rietveld refinement. a, lattice parameter; Rwp, weighted-profile R factor.
Quantum dot−induced phase stabilization of α-CsPbI3
perovskite for high-efficiency photovoltaics
Abhishek Swarnkar, Ashley R. Marshall, Erin M. Sanehira, Boris D.
Chernomordik, David T. Moore, Jeffrey A. Christians, Tamoghna
Chakrabarti and Joseph M. Luther (October 6, 2016)
Science 354 (6308), 92-95. [doi: 10.1126/science.aag2700]
Maintaining a stable phase
For solar cell applications, all-inorganic perovskite phases could be more stable than those
containing organic cations. But the band gaps of the former, which determine the electrical conductivity
of these materials, are not well matched to the solar spectrum. The cubic structure of CsPbI 3 is an
exception, but it is stable in bulk only at high temperatures. Swarnkar et al. show that surfactant-coated
α-CsPbI3 quantum dots are stable at ambient conditions and have tunable band gaps in the visible
range. Thin films of these materials can be made by spin coating with an antisolvent technique to
minimize surfactant loss. When used in solar cells, these films have efficiencies exceeding 10%, making
them promising for light harvesting or for LEDs.
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