Mesoscopic CH3NH3PbI3/TiO2 heterojunction solar cells

Article abstract of DOI:10.1021/ja307789s

We report for the first time on a hole conductor-free mesoscopic methylammonium lead iodide (CH₃NH₃PbI₃) perovskite/TiO₂ heterojunction solar cell, produced by deposition of perovskite nanoparticles from a solution of CH₃NH₃I and PbI₂ in γ-butyrolactone on a 400 nm thick film of TiO ₂ (anatase) nanosheets exposing (001) facets. A gold film was evaporated on top of the CH₃NH₃PbI₃ as a back contact. Importantly, the CH₃NH₃PbI₃ nanoparticles assume here simultaneously the roles of both light harvester and hole conductor, rendering superfluous the use of an additional hole transporting material. The simple mesoscopic CH₃NH₃PbI ₃/TiO₂ heterojunction solar cell shows impressive photovoltaic performance, with short-circuit photocurrent J_sc= 16.1 mA/cm², open-circuit photovoltage V_oc = 0.631 V, and a fill factor FF = 0.57, corresponding to a light to electric power conversion efficiency (PCE) of 5.5% under standard AM 1.5 solar light of 1000 W/m ² intensity. At a lower light intensity of 100W/m², a PCE of 7.3% was measured. The advent of such simple solution-processed mesoscopic heterojunction solar cells paves the way to realize low-cost, high-efficiency solar cells.

Full text of DOI:10.1021/ja307789s

Communication
pubs.acs.org/JACS
Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells
Lioz Etgar,*,†,§ Peng Gao,† Zhaosheng Xue,‡ Qin Peng,† Aravind Kumar Chandiran,† Bin Liu,‡
Md. K. Nazeeruddin,† and Michael Gratzel†
̈
†Laboratoire de Photonique et Interfaces, Institut des Sciences et Ingen
Switzerland
́
ierie Chimiques, Ecole Polytechnique Fed
́
er
́
ale de Lausanne,
́
‡Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore
§Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
The direct band gap, large absorption coefficient,24,25 and
high carrier mobility26,27of organo-lead halide perovskites
render them very attractive for use as light harvesters in
mesoscopic heterojunction solar cells. Their electronic proper-
ties can be tailored, allowing for, e.g., layered materials to be
formed by controlling the distance and the electronic coupling
between the inorganic sheets according to the structure of the
organic component employed. The layered perovskites have
high stability in dry air.
ABSTRACT: We report for the first time on a hole
conductor-free mesoscopic methylammonium lead iodide
(CH3NH3PbI3) perovskite/TiO2 heterojunction solar cell,
produced by deposition of perovskite nanoparticles from a
solution of CH3NH3I and PbI2 in γ-butyrolactone on a 400
nm thick film of TiO2 (anatase) nanosheets exposing
(001) facets. A gold film was evaporated on top of the
CH3NH3PbI3 as a back contact. Importantly, the
CH3NH3PbI3 nanoparticles assume here simultaneously
the roles of both light harvester and hole conductor,
rendering superfluous the use of an additional hole
transporting material. The simple mesoscopic
CH3NH3PbI3/TiO2 heterojunction solar cell shows
impressive photovoltaic performance, with short-circuit
photocurrent Jsc= 16.1 mA/cm2, open-circuit photovoltage
Voc = 0.631 V, and a fill factor FF = 0.57, corresponding to
a light to electric power conversion efficiency (PCE) of
5.5% under standard AM 1.5 solar light of 1000 W/m2
intensity. At a lower light intensity of 100W/m2, a PCE of
7.3% was measured. The advent of such simple solution-
processed mesoscopic heterojunction solar cells paves the
way to realize low-cost, high-efficiency solar cells.
These perovskite nanopigments are notably easy to prepare
and to deposit by simple solution processing via spin- or dip-
coating. The perovskite precipates from a solution of PbI2 and
methylammonium iodide, forming CH3NH3PbI3 crystals within
the pores and on the surface of the mesoscopic TiO2 substrate.
The ionic and covalent interaction between the metal cations
and the halogen anions creates inorganic octahedra, while the
cationic alkylammonium head groups provide charge balance to
the structure. Previous reports used CH3NH3PbI3 nanocrystals
as sensitizers in photoelectrochemical cells with liquid electro-
lyte.28−30 However, the performance of these systems rapidly
declined due to dissolution of the perovskite. This problem was
alleviated by replacing the electrolyte with a solid-state organic
hole conductor.31 Very recently, the tin iodide based perovskite
CsSnI3 has been employed as a hole conductor together with
N719 as sensitizer in solid-state dye-sensitized solar cells,
yielding a power conversion efficiency (PCE) of 8.5%.32
Here we report on a CH3NH3PbI3 perovskite/TiO2
heterojunction solar cell using anatase nanosheets with
dominant (001) facets as the electron collector. The pervoskite
acts as an absorber and at the same time as a hole conductor,
rendering superfluous the use of an additional p-type material
for transporting positive charge carriers. This simple meso-
scopic heterojunction solar cell achieved a remarkable photo-
voltaic performance, with short-circuit photocurrent (Jsc) of
16.1 mA cm−2, a fill factor (FF) of 0.57, and open-circuit
voltage (Voc) of 0.631 V, corresponding to a light to electric
PCE of 5.5% under 1 sun intensity. This is the first report on
the successful use of CH3NH3PbI3 simultaneously as an
absorber and hole-transporting material in a heterojunction
solar cell.
ue to their large optical cross section, nanocrystalline
Dpigments are attractive light harvesters in solar
conversion systems. In particular quantum dots (QDs) have
attracted a lot of attention due to their tunable band gap.1−3
A
variety of strategies have been applied to integrate QDs into
solar cells, including QD-polymer hybrid solar cells, QD-
Schottky barrier solar cells, QD-sensitized titanium dioxide
(TiO2) solar cells, and QD hybrid bilayer solar cells.4−12 Recent
investigations have focused on depleted heterojunction devices,
employing a mesoscopic wide band gap semiconductor oxide
such as TiO2 or ZnO as a thin spacer layer between the QDs
and the conducting transparent oxide current collector.13−21
Efficiencies of 5−6% were observed with these simple
structures. In addition, a tandem QDs solar cell with the
same structure has been demonstrated.18 Multiple exciton
generation (MEG) was also witnessed in a similar QDs based
solar cell structure.22,23 While such heterojunction QD solar
cells show promising photovoltaic performance, they still face
problems such as low stability, low open-circuit voltage, and fast
carrier recombination which prevent them from achieving
higher efficiencies.
The synthesis of CH3NH3PbI3 and deposition on the
mesoporous TiO2 film was carried out by spin-coating of a
40 wt% precursor solution of CH3NH3I and PbI2 in γ-
butyrolactone. Upon drying at room temperature, the film
Received: August 6, 2012
Published: October 8, 2012
© 2012 American Chemical Society
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coated onto the TiO2 darkened in color, indicating the
formation of CH3NH3PbI3 in the solid state, confirmed by X-
ray diffraction (XRD) spectroscopy (see Figure 2C).
Figure 1 presents a scheme of the device structure and its
energy level diagram. The conduction and valence bands of the
respectively. The change in contrast in Figure 2A above the
compact TiO2 layer indicates penetration of the CH3NH3PbI3
nanocrystals into the pores within the anatase nanosheet film.
Figure 2C shows XRD patterns of the CH3NH3PbI3 on TiO2
and on microscope glass. The peaks of CH3NH3PbI3 on the
microscope slide match closely with those of CH3NH3PbI3 on
TiO2.
Figure 3A exhibits J−V characteristics of the mesoscopic
CH3NH3PbI3/TiO2 heterojunction photovoltaic cell under 1
Figure 1. (A) Scheme of the device structure. (B) Energy level
diagram of the CH3NH3PbI3/TiO2 heterojunction solar cell.
CH3NH3PbI3 permit electron injection into the TiO2 and hole
transport to the gold back contact, respectively. The bottom
section of the device acts as an electron collector and is
composed of a 100 nm thick hole-blocking compact TiO2 film
deposited onto the FTO transparent glass front contact,
followed by deposition of a ca. 500 nm thick layer of the TiO2
nanosheets. The light is absorbed by the CH3NH3PbI3
nanoparticles, which were deposited onto the TiO2 by the
spin-coating technique. A gold contact was evaporated on top
of the CH3NH3PbI3 thin film.
Figure 2D shows a typical XRD pattern of TiO2 nanosheets
synthesized at 180 °C. All the diffraction peaks are indexed to
the anatase phase of TiO2 (JCPDS No. 21-1272), indicating
that the obtained product is pure anatase TiO2. Figure 2A,B
shows high-resolution scaning electron microscopy (HR-SEM)
images of the cross section of the solar cell and the TiO2 film,
Figure 3. (A) J−V characteristic of the lead iodide perovskite/TiO2
mesoscopic heterojunction solar cell. (B) IPCE spectrum of the
device. Inset: absorption spectra of the perovskite layer on the TiO2.
sun illumination. Under standard reporting conditions, i.e., AM
1.5 solar light at 1000 W/m2, the device produced Voc = 0.631
V and Jsc = 16.1 mA cm−2, with FF = 57%, corresponding to a
PCE of 5.5% (Table 1). The PCE increases at 100 W/m2
intensity to 7.28% with Jsc = 2.14 mA cm−2, FF = 62%, and Voc
= 0.565 V.
Table 1. Photovoltaic Device Parameters of the
CH3NH3PbI3/TiO2 Solar Cell
sun intensity
Jsc (mA/cm2)
Voc (mV)
FF
PCE (%)
10
2.1
565.8
631.6
0.62
0.57
7.28
5.5
100
16.1
The incident photon to current conversion efficiency (IPCE)
specifies the ratio of extracted electrons to incident photons at a
given wavelength. The IPCE spectrum (Figure 3B) is plotted as
a function of wavelength of the impinging light. The solid-state
CH3NH3PbI3/TiO2 heterojunction solar cell shows an
excellent photocurrent response from 400 to 800 nm, with
the IPCE reaching a maximum of 90% in the wavelength range
Figure 2. (A) High-resolution scanning electron microscopy (HR-
SEM) picture of the cross section of the CH3NH3PbI3/TiO2
heterojunction solar cell. The dark areas can be attributed to the
CH3NH3PbI3 which penetrates into the mesoporous TiO2 film. (B)
HR-SEM of the cross section of the TiO2 nanosheets film alone. (C)
XRD pattern of the CH3NH3PbI3 deposited on a TiO2 nanosheet. (D)
XRD pattern of an individual TiO2 nanosheet.
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of 400−540 nm and decreasing at longer wavelengths until 800
nm. Integration of the IPCE spectrum over the AM1.5 solar
emission yields a photocurrent density of 16.2 mA/cm2, in
reasonable agreement with the measured values. The inset of
Figure 3B shows the absorption spectrum of the perovskite on
the TiO2 film.
Solar Cell Fabrication. Thin, dense TiO2 layers of ∼100
nm thickness were deposited onto a SnO2:F conducting glass
substrate (15 Ω/cm, Pilkington) by the spray pyrolysis
method.34 The deposition temperature of the TiO2 compact
layer was 450 °C. TiO2 nanosheet films of ∼0.5 μm thickness
were spin-coated onto this substrate, using the TiO2 nanosheets
with 001 dominant facets. The TiO2 layer was annealed at 500
°C for 30 min in air. The substrate was immersed in 40 mM
TiCl4 aqueous solutions for 30 min at 70 °C and washed with
distilled water and ethanol, followed by annealing at 500 °C for
30 min in air.
The synthesis of CH3NH3PbI3 on the TiO2 surface was
carried out by dropping a 40 wt% precursor solution of
equimolar CH3NH3I and PbI2 in γ-butyrolactone onto the
TiO2 film. Film formation was induced by spin-coating (2000
rpm, 30 s) under glovebox conditions.
The film coated on the TiO2 changed color upon drying at
room temperature, indicating the formation of CH3NH3PbI3 in
the solid state. The CH3NH3PbI3 film was annealed under
argon for 15 min at 100 °C.
Finally the counter electrode was deposited by thermal
evaporation of gold under a pressure of 5 × 10−5 Torr. The
active area was 0.12 cm2. After the preparation, the cells were
allowed to be exposed to air.
Photovoltaic Characterization. Photovoltaic measure-
ments employed an AM 1.5 solar simulator equipped with a
450 W xenon lamp (model 81172, Oriel). Its power output was
adjusted to match AM 1.5 global sunlight (100 mW/cm2) by
using a reference Si photodiode equipped with an IR-cutoff
filter (KG-3, Schott) in order to reduce the mismatch between
the simulated light and AM 1.5 (in the region of 350−750 nm)
to less than 2%, with measurements verified at two PV
calibration laboratories [ISE (Germany) and NREL (USA)]. I−
V curves were obtained by applying an external bias to the cell
and measuring the generated photocurrent with a Keithley
model 2400 digital source meter. The voltage step and delay
time of photocurrent were 10 mV and 40 ms, respectively. A
similar data acquisition system was used to determine the
monochromatic incident photon to electric current conversion
efficiency. Under full computer control, light from a 300 W
xenon lamp (ILC Technology, USA) was focused through a
Gemini-180 double monochromator (Jobin Yvon Ltd., UK)
onto the photovoltaic cell to be tested. The monochromator
was incremented through the visible spectrum to generate the
IPCE(λ), defined by IPCE(λ) = 12400(Jsc/λφ), where λ is the
wavelength, Jsc is short-circuit photocurrent density (mA cm−2),
and φ is the incident radiative flux (mW cm−2). Photovoltaic
performance was measured by using a metal mask with an
aperture area of 0.12 cm2. The measurments were preformend
under bias light. The cross section of the device was measured
by using a Zeiss Jemini FEG-SEM instrument at 5 kV with
magnification of 250K×.
It is important to note the that same cells made from
standardly used TiO2 nanoparticles (i.e., 101 exposed facets and
20 nm particles) achieved a PCE of only 2.8% at 1 sun
intensity. This PCE is lower than that achieved using 001 TiO2
nanosheets. As was discussed in our previous work,20 the
improved photovoltaic performance of the nanosheets
compared to nanoparticulate films may be attributed to the
higher concentration of ionic charge of the exposed (001) faces
compared to the (101) facets, thus strengthening the
attachment of the QDs to the TiO2 surface and screening the
electrons injected in the conduction band of the oxide.
In summary, the present work establishes for the first time
that CH3NH3PbI3 nanocrystals can act both as an efficient light
harvester and as a hole transporter in solar cells comprising a
meoscocopic CH3NH3PbI3/TiO2 heterojuntion. The role of
the TiO2 nanoplatelets is to accept electrons and transport
them to the front collector. This eliminates the need to employ
an additional organic or inorganic hole conductor, whose
infiltration in the mesoporous TiO2 has often posed difficulties
in the past. There remains room for further very substantial
improvement of the PCE, in particular by augmentation of the
FF and Voc through control of the layer’s crystallinity. This will
be the focus of our continuing investigations. The fact that the
perovskite is stable in dry ambient air and can be deposited by
low-cost solution processing opens up new avenues for future
development of high-efficiency, low-cost photovoltaic cells.
CH3NH3I Synthesis. CH3NH3I was synthesized by reacting
30 mL of methylamine (40% in methanol, TCI) and 32.3 mL
of hydroiodic acid (57 wt% in water, Aldrich) in a 250 mL
round-bottom flask at 0 °C for 2 h with stirring. The precipitate
was recovered by putting the solution on a rotary evaporator
and carefully removing the solvents at 50 °C. The yellowish raw
product methylammonium iodide (CH3NH3I) was washed
with diethyl ether by stirring the solution for 30 min, a step
which was repeated three times, and then finally recrystallized
from a mixed solvent of diethyl ether and ethanol. After
filtration, the solid was collected and dried at 60 °C in a vacuum
oven for 24 h.
Synthesis and Purification of TiO2 Nanosheets. Syn-
thesis of the nanosheets followed the typical experimental
procedure.33 A Ti(OBu)4 (10 mL, 98%) and hydrofluoric acid
(0.8 mL, 47%) solution was mixed in a 150 mL dried Teflon
autoclave which was kept at 180 °C for 24 h, yielding well-
defined rectangular sheet-like structures with a side length of 30
nm and a thickness of 7 nm. After reaction the dispersion was
cooled to room temperature, and the white powder was
separated by high-speed centrifugation and washed with
ethanol, followed by several distilled water washings.
Caution: Hydrofluoric acid is extremely corrosive and a contact
poison; it should be handled with extreme care! Hydrofluoric acid
solution is stored in Teflon containers for use.
The crystallographic properties of the perovskite structures
deposited on FTO-TiO2 and a microscopic slide were
investigated using XRD with a Bruker D8 Discover apparatus.
The instrument was set in “locked coupled” mode, and the
acquisition was done in θ−2θ mode for every 0.1° increment
over the Bragg angle range of 10−60°.
AUTHOR INFORMATION
■
Corresponding Author
lioz.etgar@mail.huji.ac.il
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Carole Graetzel for fruitful discussions. This
work was partially supported by EU FP7 project ENERGY-
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