Lawrence Berkeley National Laboratory
Recent Work
Title
Thermochromic halide perovskite solar cells.
Permalink
https://escholarship.org/uc/item/06v5r17g
Journal
Nature materials, 17(3)
ISSN
1476-1122
Authors
Lin, Jia
Lai, Minliang
Dou, Letian
et al.
Publication Date
2018-03-01
DOI
10.1038/s41563-017-0006-0
Peer reviewed
eScholarship.org Powered by the California Digital Library
University of California
Articles
https://doi.org/10.1038/s41563-017-0006-0
1
Department of Chemistry, University of California, Berkeley, California, USA.
2
Materials Sciences Division, Lawrence Berkeley National Laboratory,
Berkeley, California, USA.
3
Department of Physics, Shanghai University of Electric Power, Shanghai, China.
4
Davidson School of Chemical Engineering,
Purdue University, West Lafayette, Indiana, USA.
5
Berzelii Center EXSELENT on Porous Materials, Department of Materials and Environmental Chemistry,
Stockholm University, Stockholm, Sweden.
6
Lawrence Livermore National Laboratory, Livermore, California, USA.
7
Kavli Energy NanoScience Institute,
Berkeley, California, USA.
8
Department of Materials Science and Engineering, University of California, Berkeley, California, USA. Jia Lin, Minliang Lai and
Letian Dou contributed equally to this work. *e-mail: p_yang@berkeley.edu
S
mart windows with reversible and persistent colour changes to
modulate visible light transmittance have been reported using
electrochromic, thermochromic, and liquid crystal materi
-
als
1–3
. The transparency of such windows is controlled simply by
absorbing or reflecting sunlight without actually converting the
solar energy into a useful form. To date, semi-transparent photo
-
voltaics have been demonstrated for power-generating windows
that provide shading, lighting, and power output
4,5
, but without any
colour-changing characteristics. Photovoltaic windows with switch
-
able transparencies—smart photovoltaic windows—which can har-
vest and manage the incoming solar energy have been developed
only by combining semi-transparent solar cells with additional
chromic components to form multi-junction tandem devices
6–8
. It
is highly desirable to develop a stable and photoactive material pos
-
sessing two intrinsic states that have large colour contrast, one with
high transparency to ensure the greatest brightness, and the other
with strong light absorption to produce sufficient electrical energy,
where the two states can be reversibly switched back and forth in
response to the external environment. Materials with structural
phase transitions have been found inherently linked to substantially
different optical, electronic, and/or thermal properties due to dis
-
tinct atomic arrangements of each specific crystal structure
9,10
. In
particular, non-volatile and fully-reversible phase transitions sug
-
gest the possibility of applications in smart photovoltaic windows.
The concept has not been realized because most of the semiconduc
-
tors cannot be switched between a transparent phase and a non-
transparent phase reversibly, without deteriorating their electronic
properties.
Recently, halide perovskites of an ABX
3
structure [A = CH
3
NH
3
+
(MA), HC(NH
2
)
2
+
(FA), Cs
+
; B = Pb
2+
, Sn
2+
; X = I
—
, Br
—
, Cl
—
] have
emerged as intriguing photovoltaic materials and become a rap
-
idly evolving field
11,12
. The prototypical organic− inorganic hybrid
perovskite methylammonium lead iodide (MAPbI
3
) was reported
to undergo multiple temperature-dependent phase transitions, with
a change from the tetragonal to the cubic phase at ~60 °C, within
the solar cell operating temperature range
13,14
. However, the struc-
tural properties vary only little between the two phases, with slight
tilting of the three-dimensional metal-halide octahedral network.
Consequently, both the optoelectronic properties and solar cell per
-
formances do not alter significantly upon phase transition. Another
promising halide perovskite material is the purely inorganic ver
-
sion, caesium lead iodide/bromide (CsPbI
3−x
Br
x
, 0 ≤ x ≤ 3). These
inorganic perovskites have achieved significantly enhanced thermal
and environmental stability, and are considered to be potentially on
a par with the organic− inorganic hybrid species in terms of intrin
-
sic solar cell performance ability
15,16
. Substantial structural changes
occur in these inorganic perovskites upon phase transitions, often
between a room-temperature non-perovskite phase (low-T phase)
and a high-temperature perovskite phase (high-T phase)
17,18
.
These two phases feature distinct optoelectronic properties such
as the bandgap, photoluminescence (PL) quantum efficiency, and
charge carrier mobility and lifetime
14,15
. Here we examine the struc-
tural phase transition behaviours in such inorganic mixed halide
perovskite CsPbI
3−x
Br
x
thin films. The large structural changes
induced by phase transitions lead to films with two switchable char
-
acteristic states with distinct visible transparencies and photovoltaic
device efficiencies, making them promising candidates for smart
photovoltaic windows.
Figure1a schematically illustrates the crystal structure change
between the low-T and high-T phases of CsPbI
3−x
Br
x
. The low-T to
high-T phase transition occurs upon reaching the transition tem
-
perature by thermal heating (in inert or ambient condition) with
the high-T phase being kinetically trapped and metastable in an
inert environment when cooled to room temperature. Critical to
Thermochromic halide perovskite solar cells
Jia Lin
1,2,3
, Minliang Lai
1
, Letian Dou
1,2,4
, Christopher S. Kley
1
, Hong Chen
1
, Fei Peng
5
,
Junliang Sun
5
, Dylan Lu
1,2
, Steven A. Hawks
1,2,6
, Chenlu Xie
1
, Fan Cui
1
, A. Paul Alivisatos
1,2,7,8
,
David T. Limmer
1,2,7
and Peidong Yang
1,2,7,8
*
Smart photovoltaic windows represent a promising green technology featuring tunable transparency and electrical power gen-
eration under external stimuli to control the light transmission and manage the solar energy. Here, we demonstrate a ther-
mochromic solar cell for smart photovoltaic window applications utilizing the structural phase transitions in inorganic halide
perovskite caesium lead iodide/bromide. The solar cells undergo thermally-driven, moisture-mediated reversible transitions
between a transparent non-perovskite phase (81.7% visible transparency) with low power output and a deeply coloured
perovskite phase (35.4% visible transparency) with high power output. The inorganic perovskites exhibit tunable colours and
transparencies, a peak device efficiency above 7%, and a phase transition temperature as low as 105 °C. We demonstrate excel-
lent device stability over repeated phase transition cycles without colour fade or performance degradation. The photovoltaic
windows showing both photoactivity and thermochromic features represent key stepping-stones for integration with buildings,
automobiles, information displays, and potentially many other technologies.
© 2018 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
NATURE MATERIALS | VOL 17 | MARCH 2018 | 261–267 | www.nature.com/naturematerials
261
Articles
Nature Materials
achieving full phase reversibility is the back-conversion of the high-
T to low-T phase. It is known that the metastable high-T phase of
CsPbI
3
can be transitioned to the low-T phase by reheating to about
200 °C in an inert atmosphere
19
. When Br is added, the high-T phase
is more stable, and difficult to revert to the low-T phase by mild
heating. Instead, we find that, in general, moisture exposure effec
-
tively triggers this phase transition at room temperature. Methanol
and ethanol vapours can also trigger the phase transition, but not
as efficiently as moisture (Supplementary Fig. 1a). Fig.1b shows
the visually distinct images of the non-coloured low-T and orange-
red-coloured high-T phase CsPbIBr
2
films, suitable for win-
dow applications. These transition processes could be potentially
leveraged during normal device operation where sunlight is used to
heat the film and drive off moisture (or assisted by extra heating),
inducing a phase transition to the photovoltaically active high-T
phase, while moisture ingress results in a subsequent phase transi
-
tion back to the transparent low-T phase.
We use CsPbIBr
2
as a model system to characterize in detail the
low-T and high-T phases. The powder X-ray diffraction (XRD)
pattern of CsPbIBr
2
in the low-T phase matches well with the sim-
ulated pure iodide (CsPbI
3
) and bromide (CsPbBr
3
) low-T ortho-
rhombic phases, with the corresponding peak positions sitting
between them (Fig.1c). The crystal structural details of CsPbIBr
2
are derived from rotation electron diffraction (RED) combined
with fitting of the XRD pattern, identifying the low-T phase
with space group Pmnb and a = 4.797 Å, b = 9.982 Å, c = 17.184 Å
10 20 30 40 50
(220)
(211)
(210)
(111)
(200)
(110)
(100)
High-T
c
1 cm
Heating
Moisture
Low-T High-T
a b
e
f
g h
d
10 20 30 40 50
(032)
(015)
(023)
(021)
(002)
(012)
2θ (°) 2θ (°)
2
θ
(°)
Low-T
CsPbBr
3
CsPbI
3
400 500 600 700
800
CsPbIBr
2
Low-T
High-T
Wavelength (nm)
CsPbIBr
2
20 40 60 80
0
10
20
30
40
Transition time (h)
x = 2
x = 1.5
Low-T
High-T
x = 1
020406080 100
0.00
0.25
0.50
High-T
Low-T
Absorbance (a.u.)
Absorbance, PL (a.u.)
Number of cyclesRelative humidity (%)
91215182
12
4
Low-T
High-T
Fresh
After cycles
High-TLow-T
Intensity (a.u.)
Intensity (a.u.)
Intensity (a.u.)
Fig. 1 | Phase transitions of inorganic halide perovskites. a, Schematics of the low-T to high-T phase transition by heating and the high-T to low-T
transition by exposure to moisture. Caesium and halide atoms are shown in green and red, respectively. The low-T phase is represented by one-
dimensional chains of edge-sharing lead-halide octahedra, whereas in the high-T phase the octahedra share corners. b, Photograph of the low-T phase
(non-coloured) and high-T phase (orange-red-coloured) thin films. c,d, Powder X-ray diffraction (XRD) patterns of the CsPbIBr
2
(red) low-T (c) and high-T
films (d) deposited on a glass substrate compared with the simulated CsPbI
3
(blue) and CsPbBr
3
(black) low-T orthorhombic and high-T cubic phases.
e, Absorption (dashed lines) and PL (solid lines) spectra of the low-T (black) and high-T (red) CsPbIBr
2
films. f, Variation of the high-T to low-T phase
transition time of CsPbI
3−x
Br
x
films measured at different humidity conditions with RH = 20− 80%. Error bars indicate the standard deviation. The inset
shows the corresponding photographs of the high-T and low-T phase thin films (from left to right: x = 1, 1.5, and 2). The high-T phase films display different
colours from dark brown to orange-red, while the low-T phase ones are fully transparent. g, The stable and reversible switching of the absorption (550 nm)
of the three CsPbIBr
2
thin films over 100 phase transition cycles. h, XRD patterns of the low-T (black) and high-T (red) phase films before and after 100
cycles of phase transitions, showing full reversibility of the crystal structures.
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262
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(Supplementary Fig.2). A similar peak shift has been observed
for the CsPbIBr
2
high-T phase as compared to the CsPbI
3
and
CsPbBr
3
cubic phases (Fig.1d). The unit cell of the high-T phase
is determined to be a = 5.926 Å with the space group Pm-3m
(Supplementary Fig.3 and Table1). The large changes in the crys
-
tallographic structures indicate the first-order nature of the phase
transition. The phenomenon of phase transition is different from
the other observed transition behaviours between different colour
states in halide perovskites accompanied by chemical reactions and
new compound formation
20
. During the high-T to low-T transi-
tion process, the characteristic high-T XRD peak gradually dimin-
ishes while the low-T peak appears upon continuous exposure to
moisture, and finally the high-T peak completely vanishes, indi
-
cating full conversion to the low-T phase (Supplementary Fig.1b).
Accordingly, the film shows a macroscopically homogeneous and
gradual colour change across the entire region (Supplementary
Fig.1c and Supplementary Videos1, 2) that tracks well with the
XRD data. Fig.1e quantitatively shows the optical absorption and
PL spectra of the high-T and low-T phase thin films with light
absorption above the band edge at about 2.1 and 2.9 eV, respec
-
tively. In the PL emission spectra, the high-T phase shows a nar-
row band edge emission, whereas the emission becomes much
broader and weaker for the low-T phase.
The high-T to low-T phase transition rate is found to be strongly
dependent on the composition (x = 1, 1.5 and 2) and relative humid
-
ity (RH = 20–80%, Fig.1f). The inset in Fig.1f shows the photographs
of both the high-T and low-T phase films. The reversibility of the
structural transitions between the low-T and high-T phases of the
inorganic perovskite CsPbIBr
2
is monitored by the absorption spec-
tra, which show no apparent changes after more than 100 repeated
cycles (see Fig. 1g). Furthermore, no shift of the characteristic
peaks or emergence of any impurity peaks (such as CsI, CsBr, PbI
2
or PbBr
2
) is observed in the XRD patterns of either phase after
repeated phase transition cycles (Fig.1h). It is worth noting that, as
compared to the freshly prepared high-T phase film, the (110)/(100)
peak intensity ratio initially increases and subsequently remains
stable during cycling, corresponding to a phase-transition-induced
variation of the CsPbIBr
2
thin-film morphology, but without a sig-
nificant change of the top surface roughness and grain dimensions
(Supplementary Fig.4). This observation can be explained by the
rearrangement of crystal orientations in the thin film to reach the
lowest surface energy.
The moisture adsorbed on the inorganic halide perovskite film
surface can effectively catalyse the high-T to low-T phase transi
-
tion at room temperature by introducing vacancies into the crys-
tal lattice and lowering the free-energy barrier to nucleation
21
.
Shown in Fig.2a, free-energy calculations of a molecular model
of CsPbI
3
show that there is a significant enhancement of halide
vacancies when a thin water film is in contact with the perovskite
interface
22
, as pictured in Fig.2b. This is a consequence of the
large solvation enthalpy of halide ions and their accompany
-
ing low vacancy formation energy
23
. The characteristic time for
a vacancy to be created at the interface is computed to be 1 ms,
with an equilibrium concentration that depends on the relative
humidity as in Fig.2c and is up to five orders of magnitude larger
than the expected defect concentration in the bulk of the mate
-
rial. Additional free-energy calculations shown in Fig.2d report
that there is a significant reduction of the surface tension between
the low-T and high-T structures in the presence of halide vacan
-
cies. This reduction results from mitigating the ionic bonding
constraints within the interfacial region of the lead-halide octa
-
hedra, which share corners in the high-T phase but share edges
ab
e
z /Å
48012
Δ F(z)(eV I
–
)
0
0.25
0.5
0.75
10
–2
10
–3
10
–4
10
–5
0.0 0.20.4 0.60.8 1.0
c
p/p
o
ρ
v
Δ G(Q)(eV nm
–2
)
0
0.5
1.0
1.5
2.0
Q
0.2 0.4 0.6 0.8
d
0 1.0
10
6
10
5
10
4
10
3
10
2
10
1
0.20.4 0.6 0.
81.0
f
CsPbIBr
2
CsPbI
2
Br
ρ
v
×10
–3
τ/s
ρ
v
= 0.0
ρ
v
= 0.01
Fig. 2 | Mechanism of the moisture-triggered phase transition in inorganic perovskites. a, Reversible work, ∆ F(z), to transfer an I
—
atom from the solid
perovskite to the thin adsorbed water layer, where z is the direction perpendicular to the interface. b, Snapshot from a molecular dynamics simulation
near the top of the barrier in a showing the lead halide sub-lattice in grey and orange, the Cs
+
ions in green, the oxygens of the water in red and their
hydrogens in white. c, Vacancy concentration, ρ
v
, per unit cell as a function of relative humidity, p/p
o
. d, Free energy to transform the high-T phase into the
low-T phase, ∆ G(Q), with or without an I
—
vacancy, where Q is an order parameter that interpolates between the two crystal phases. e, Snapshot from a
molecular dynamics simulation of the interface between the low-T and high-T phases with the same colouring as in b. f, Average nucleation time, τ, as a
function of vacancy concentration for CsPbIBr
2
and CsPbI
2
Br.
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Articles
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in the low-T phase (see Fig.2e). This reduction of the free-energy
barrier to nucleation at the sites of halide vacancies results in a
transition rate that changes exponentially in the presence of a
vacancy, as quantified in Fig.2f for CsPbIBr
2
and CsPbI
2
Br. The
moisture triggered phase transition was further characterized by
ambient pressure X-ray photoemission spectroscopy (AP-XPS).
When exposing to water vapour, the adsorbed oxygen amount
on surface increases but does not change with the probing depth
(Supplementary Fig.5), indicating that water is adsorbed only on
the surface without penetrating interior of the lattice. The results
reveal that the phase transition is fundamentally different from
the hydration/dehydration process observed in hybrid perovskite
MAPbI
3
24–26
, which suffers from decomposition after water mol-
ecule intercalation.
a
ZnO
CsPbI
3-x
Br
x
NiO
x
FTO/glass
Al/ITO
Al/ITO
ZnO
FTO
NiO
x
200 nm
CsPbI
3-x
Br
x
b
cd
e
f
gh
–0.2 0.0 0.2 0.4 0.6 0.8 1.0
–10
–5
0
5
Low-T
High-T
Semi-transparent
Voltage (V)
Current density (mA cm
–2
)
400 500 600 700 800
0
20
40
60
80
100
Low-T
High-T
Wavelength (nm)
Low-T High-T
0
100
200
300
400
0
2
4
6
8
PCE (%)
0
Composition x
123
CsPbI
3–x
Br
x
350 400 450 500 550 600
0
20
40
60
80
0
2
4
6
8
Wavelength (nm)
EQE (%)
Low-T
High-T
Integrated Jsc (mA cm
–2
)
–10
–5
0
5
Voltage (V)
Current density (mA cm
–2
)
Transition temperature (°C)
Transmittance (%)
–0.2 0.0 0.2 0.4 0.6 0.8 1.0
Low-T
High-T
Sputtered ITO
Fig. 3 | Characterization of phase transition solar cell devices. a, Schematic drawing of the solar cell architecture of glass/FTO/NiO
x
/CsPbI
3−x
Br
x
/ZnO/Al
or ITO. b, Cross-sectional scanning electron microscopy (SEM) image showing the homogenous and pinhole-free absorber active layer and the
high-quality ZnO layer (without top electrode). c, Photocurrent density-voltage (J–V) characteristics of both the high-T and low-T phase solar cells
(Al electrode) using CsPbIBr
2
as the active component (reverse scan at 20 mV s
−1
), proving that the solar cell performance is greatly affected by the phase
transition process. d, The external quantum efficiency (EQE) spectra and integrated J
SC
. The integrated EQE matches the measured J
SC
data to within
4%. e, The low-T to high-T phase transition temperature and solar cell performance (reverse scan at 20 mV s
−1
) as a function of composition. Error bars
indicate the standard deviation. A very high bromide concentration (x > 2.5) results in the stabilization of the high-T phase at room temperature, which can
hardly be fully reverted back to the low-T phase. f, The image of the semi-transparent CsPbIBr
2
solar cell device in the (left) transparent mode and (right)
orange-red-coloured photovoltaic mode with the sputtered ITO as the top transparent electrode. g, J–V curves of the semi-transparent device. h, The
transmittance spectra of the semi-transparent device in the low-T (black curve) and high-T (red curve) phases. The transmittance of the sputtered ITO
layer (thickness ~120 nm) is also given for reference.
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