LETTERS
PUBLISHED ONLINE: 22 DECEMBER 2013 | DOI: 10.1038/NPHOTON.2013.341
Perovskite solar cells employing organic
charge-transport layers
Olga Malinkiewicz1, Aswani Yella2, Yong Hui Lee2, Guillermo Mı́nguez Espallargas1, Michael Graetzel2,
Mohammad K. Nazeeruddin2 * and Henk J. Bolink1 *
Thin-film photovoltaics play an important role in the quest for
clean renewable energy. Recently, methylammonium lead
halide perovskites were identified as promising absorbers for
solar cells1. In the three years since, the performance of perovskite-based solar cells has improved rapidly to reach efficiencies
as high as 15%1–10. To date, all high-efficiency perovskite solar
cells reported make use of a (mesoscopic) metal oxide, such
as Al2O3 , TiO2 or ZrO2 , which requires a high-temperature sintering process. Here, we show that methylammonium lead
iodide perovskite layers, when sandwiched between two thin
organic charge-transporting layers, also lead to solar cells with
high power-conversion efficiencies (12%). To ensure a high
purity, the perovskite layers were prepared by sublimation in a
high-vacuum chamber. This simple planar device structure and
the room-temperature deposition processes are suitable for
many conducting substrates, including plastic and textiles.
The hybrid organic–inorganic methylammonium lead halide
perovskites, pioneered by Mitzi, are recognized for their excellent
semiconducting properties11,12. Following the pioneering work of
Miyasaka and co-workers1, the ease with which these organic–inorganic hybrid perovskite materials can be prepared and processed
from solution makes them attractive for photovoltaic applications13.
In their studies, the methylammonium lead iodide perovskite was
used as a sensitizer in liquid dye-sensitized solar cells and an efficiency of 3.8% was obtained. The low efficiency results from the
solubility of the perovskite layer in the iodine/iodide redox electrolyte system. High efficiencies have been realized by replacing the
liquid electrolyte with a solid hole-transporting material such as
2,2′ ,7,7′ -tetrakis(N,N-di-p-methoxyphenylamine)-9,9′ -spirobifluorene (spiro-MeOTAD), as reported by the groups of Snaith and
Graetzel2–9. Interestingly, Lee et al.6 showed that a mixed-halide
perovskite on a mesoporous Al2O3 photoanode acts both as a
light absorber and an electron conductor. Etgar et al.8 reported
that the pure methylammonium lead iodide perovskite also acts as
an efficient hole conductor. Recently, Burschka et al.4 published a
high power-conversion efficiency of 15% for a solar cell in which
the perovskite was deposited by a novel sequential deposition
technique on a mesoporous TiO2 film. Ball et al.5 showed that a
thick layer (350 nm) of the mixed-halide methylammonium lead
perovskite (CH3NH3PbI32xClx) leads to efficient solar cells (12%).
They argue that the perovskite functions as the absorber and as
the electron and hole transporter. Additionally, it was suggested
that the excitons dissociate predominantly in the bulk and not
at a donor–acceptor interface. The operating mechanism of
planar perovskite solar cells, however, is not clear. For example,
it was shown that the photoluminescence of a mixed-halide
methylammonium lead perovskite was reduced strongly when
either a hole- or an electron-transporting layer was deposited on
top of it14. A similar effect, but less strong, was observed for thin
films of the pure iodide methylammonium lead perovskite14.
Hence, exciton dissociation may occur both in the bulk and at the
perovskite interface. Currently, the dominant process is not
established and may well depend on the exact nature of the
perovskite film. Up to now, the highest efficiency was reported by
Liu et al., who used a thin metal oxide layer and a sublimed
perovskite layer that leads to solar cells with an efficiency of
15.4%9. High material purity is essential for efficient chargecarrier transport and to prevent exciton quenching, as established
in organic light-emitting diodes and organic photovoltaic (OPV)
devices15,16. Therefore, in these devices vacuum sublimation is
employed frequently to achieve high-purity layers. Solutionprocessed perovskites are generated in situ and therefore may
contain imperfections and impurities. Purification of the perovskite
layer is not possible as it decomposes on dissolving.
An inverted layout, similar to that employed in small molecular
weight and polymeric solar cells, has also been described17,18. In
these bilayer planar heterojunction devices, the perovskite layer
was used as the electron donor in combination with an electron
acceptor, in analogy to a frequently used OPV layout. The holes
are extracted via the transparent conductor poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS). The device
performances, however, were significantly inferior to those of
devices that employ (mesoporous) metal oxides.
Here, we present an inverted thin-film solar cell based on a sublimed methylammonium lead iodide perovskite layer sandwiched
between two very thin electron- and hole-blocking layers that
consist of organic molecules. The organic materials were deposited
using solution-based processes, whereas the CH3NH3PbI3 perovskite and the metal contact were deposited using thermal evaporation under vacuum. These simple devices, free of metal oxide
and prepared using room-temperature processing, reach efficiencies
as high as 12% at 100 mW cm22, which demonstrate their potential
for a wide variety of photovoltaic applications.
The CH3NH3PbI3 perovskite thin film was prepared by coevaporation of the two starting compounds, CH3NH3I and PbI2.
Details of the experimental conditions are given in the Methods
section. These films were characterized using grazing incidence
X-ray diffraction (GIXRD). Initially, excess PbI2 was present
in the film, yet on optimizing the evaporation process, highly
oriented pure CH3NH3PbI3 films were obtained (see Fig 1a and
Supplementary Fig. 1). The films adopt the typical tetragonal structure for CH3NH3MX3 hybrid perovskites (M ¼ Pb, Sn; X ¼ Cl, Br,
I) formed by a three-dimensional anionic framework of PbI6 octahedra with the methylammonium cations in the interstitial space (see
Fig. 1b)19. The roughness of the CH3NH3PbI3 film was evaluated
using atomic force microscopy (AFM) and an image of a typical
1
Instituto de Ciencia Molecular, Universidad de Valencia, C/Catedrático J. Beltrán 2, 46980 Paterna (Valencia), Spain, 2 Laboratory of Photonics and
Interfaces, Swiss Federal Institute of Technology (EPFL), Station 6, Lausanne, CH 1015, Switzerland. * e-mail: mdkhaja.nazeeruddin@epfl.ch; henk.bolink@uv.es
128
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DOI: 10.1038/NPHOTON.2013.341
a 80,000
b
Experimental
ITO
Simulated
70,000
60,000
50,000
40,000
30,000
20,000
10,000
10
10
15
20
25
30
35
40
45
50
2θ (°)
c
d
25
Z (nm)
20
15
10
5
840 nm
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
X (µm)
Figure 1 | Characteristics of the sublimed CH3NH3PbI3 perovskite layer. a, GIXRD pattern with Cu Ka1 radiation (l ¼ 1.54056 Å) of the sublimed thin film
(blue), ITO substrate (red) and calculated for CH3NH3PbI3 with preferred orientation along the (100) and (001) directions. b, Simulation of the crystal
structure of CH3NH3PbI3 perovskite. Pb atoms are placed at the centre of the grey octahedrons, lavender spheres represent iodine atoms and green spheres
represent the methylammonium cations. c, Photograph of a 20-nm-thick film. d, AFM image and profile of a 60-nm-thick film.
scan is given in Fig. 1d, which demonstrates a smooth film with a
root mean square roughness of 5 nm. Figure 1c is a photograph of
a 20-nm-thick film.
As the CH3NH3PbI3 layer is prepared via vacuum sublimation, it
can be implemented easily in different device architectures. To
demonstrate that the CH3NH3PbI3 is capable of performing most
of the roles required to obtain an efficient solar cell and to minimize
the use of costly organic semiconductors, a simple device structure
was chosen. In this structure, which is typical for both organic
photovoltaic and light-emitting devices, a semitransparent organic
conductor was used as the positive charge-collecting contact.
The structure of the device is shown in Fig. 2a, and consists
of a 70 nm PEDOT:PSS layer and a thin layer (,10 nm) of
poly(N,N ′ -bis(4-butylphenyl)-N,N′ -bis(phenyl)benzidine) (polyTPD)
(Fig. 2c) as the electron-blocking layer. On top of this, the
CH3NH3PbI3 was evaporated thermally to a thickness of 285 nm
followed by a thin layer (,10 nm) of (6,6)-phenyl C61-butyric
acid methyl ester (PCBM) as the hole-blocking layer (Fig. 2d)20.
The device was completed by the evaporation of an Au top
electrode (100 nm). Both the polyTPD and the PCBM layers were
deposited using a meniscus-coating process to ensure high-quality
films21. The thickness of the layers was established using
absorbance measurements.
The relevant energy levels of the materials used to prepare the
solar cell are depicted in Fig. 2b. The valence band (VB) and
conduction band (CB) of the CH3NH3PbI3 perovskite are 25.4
and 23.9 eV, versus vacuum, respectively7. On illuminating the
device, excitons are generated in the CH3NH3PbI3 perovskite layer.
It was reported that excitons in CH3NH3PbI3 perovskites are of
the Wannier–Mott type, which implies that they may dissociate in
the bulk of the perovskite layer5,12. Owing to the use of indium–tin
oxide (ITO)/PEDOT:PSS as the hole-collecting contact and Au as
the electron-collecting electrode, the built-in voltage of this device
is small. Hence, to direct the flow of electrons and holes, thin
hole-blocking and electron-blocking layers are incorporated adjacent
to the perovskite layer. PolyTPD and PCBM were selected for this
role as their highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) levels, respectively,
match well with the VB and CB of the perovskite, which allows for
a good transport of holes towards the polyTPD and of electrons to
the PCBM layer (Fig. 2b). As the LUMO of polyTPD is significantly
closer to vacuum compared with that of the perovskite CB, polyTPD
efficiently blocks the flow of electrons. The opposite process, the
blocking of holes, occurs at the perovskite–PCBM interface
because of the lower HOMO of PCBM compared with that of the
perovskite VB. As mentioned above, exciton dissociation may also
occur at the perovskite–polyTPD and perovskite–PCBM interfaces14.
Figure 3a shows the current–voltage (J–V) characteristics of a
typical small-area (0.09 cm2) perovskite solar cell measured in the
dark and under light intensities of 100, 50 and 10 mW cm22. The
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a
b
CH3NH3Pbl3
(285 nm)
PolyTPD
(10 nm)
PEDOT:PSS
(70 nm)
ITO
−3.9
−3.9
CH3NH3Pbl3
PCBM
(10 nm)
PolyTPD
PCBM
−2.4
PEDOT:PSS
(100 nm)
Vacuum
Energy
Au
DOI: 10.1038/NPHOTON.2013.341
−4.8
−5.3
ITO
−5.4
Au
−5.4
−5.4
−5.9
c
d
N
N
OCH3
n
O
Figure 2 | Schematics of the device. a, Stacked layer structure. b, Schematic of the relative energy levels of each layer. c, Chemical structure of the
polyarylamine (polyTPD). d, Chemical structure of PCBM.
a 20
b 100
94.6% sun
49.5% sun
9.3% sun
Dark
18
3
80
14
IPCE (%)
12
10
8
60
2
40
Absorbance
Current density (mA cm−2)
16
6
1
4
20
2
0
0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Voltage (V)
0
400
500
600
700
800
Wavelength (nm)
Figure 3 | Typical J–V and IPCE characteristics of the perovskite solar cell. a, Photocurrent density versus voltage at 100, 50 and 10 mW cm22 and in the
dark. b, IPCE spectrum and absorbance of a 285-nm-thick perovskite layer.
reproducibility from device to device was excellent, with less than
10% deviation. The short-circuit current density (JSC), opencircuit voltage (VOC) and fill factor (FF), respectively, are
16.12 mA cm22, 1.05 V and 0.67, which lead to a power-conversion
efficiency of 12.04% measured at 100 mW cm22. At 50 and
10 mW cm22 the device exhibited very similar efficiencies, 12.04%
and 12.00%, respectively. The high open-circuit potential indicates
that there are negligible surface and sub-bandgap states in the
perovskite film. The device performance under 100 mW cm22 is
remarkable in view of the very thin perovskite film of 285 nm. A
device with cells of larger area (0.98 cm2) was also prepared and
evaluated at 100 mW cm22 (Supplementary Fig. 4). The JSC and
VOC are very similar to the data obtained for the small-area cell
(14.76 mA cm22 and 1.05 V, respectively). The FF, however, is significantly lower (0.52), which is the main reason for the lower
power-conversion efficiency (8.27%) for these larger cells.
130
The incident photon-to-current conversion efficiency (IPCE)
spectra exhibit 74% (Fig. 3b) where the generation of photocurrent
started at 790 nm, in agreement with the bandgap of the
CH3NH3PbI3. It is interesting that the IPCE spectra show a very
steep onset, contrary to the IPCE spectra reported for TiO2 and
Al2O3 mesoscopic-based perovskite cells. A very similar IPCE spectrum as observed for our solar cells was reported for a solar cell
based on a perovskite layer sandwiched between a C60-modified
TiO2 layer and a layer of poly(3-hexylthiophene)22. As such, the
steep onset may be related to the specific interaction of the perovskite with the fullerene molecules. The IPCE spectrum is almost
flat, except for a dip at 630 nm that could result from the oxidized
polyTPD acting as a filter or from reduced absorption of the perovskite. Integrating the overlap of the IPCE spectrum with the AM1.5G
solar photon flux yields a current density of 16.40 mA cm22, which
is in excellent agreement with the measured photocurrent density of
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DOI: 10.1038/NPHOTON.2013.341
16.12 mA cm22 at the standard solar AM1.5G intensity of
100 mW cm22, which confirms that the mismatch between the
simulated sunlight and the AM1.5G standard is negligible. The
absorbance of the CH3NH3PbI3 film is similar to those reported
previously for this material and increases with increasing layer
thickness (Fig. 3b). The absorption extends over the complete
visible spectrum up to 800 nm, with a local maximum around
500 nm (the scale of the graph in Fig. 3b is optical density).
Conclusion
An efficient solid-state, thin-film solar cell was obtained by sandwiching a sublimated CH3NH3PbI3 perovskite layer between two
thin organic charge-transporting layers that function as hole and
electron blockers, respectively, and contacting it via an indium tin
oxide (ITO)/PEDOT:PSS as the hole-extraction and an Au electronextraction contact. The simple device architecture, which is an ntype oxide and scaffold free, coupled with easy room-temperature
fabrication using economically favourable materials, high efficiency
and high reproducibility rivals strongly the established thin-film
photovoltaic technologies. The device power-conversion efficiency
of 12% at 100 mW cm22 is remarkable in view of the very thin
(285 nm) perovskite film. The high Jsc of 16.12 mA cm22 and the
open-circuit potential of 1.05 V reveal that very few electrons and
holes recombine, which demonstrates the effectiveness of the
hole- and electron-blocking layers. We believe that this new class
of perovskite solar cell using organic semiconductors to collect the
charges will find widespread applications to rival photovoltaic
solar cells based on thin films.
Note added in proof: During the revision of this paper, a highly relevant article to this topic was published: P. Docampo, J.M. Ball,
M. Darwich, G.E. Eperon & H.J. Snaith, Efficient organometal
trihalide perovskite planar-heterojunction solar cells on flexible
polymer substrates. Nature Commun. 4, 2761 (2013).
Methods
Materials. Photolithographically patterned ITO-covered glass substrates were
purchased from NaranjoSubstrates. Aqueous dispersions of PEDOT:PSS (Clevios P
VP Al 4083) were obtained from Heraeus Holding and used as received. Poly-TPD
was purchased from ADS Dyesource. PbI2 was purchased from Aldrich and used as
is. CH3NH3I was prepared similarly to a previously published method8; in brief,
CH3NH3I was synthesized by reacting 21.6 ml methylamine (40 wt% in water;
Aldrich) and 30 ml hydroiodic acid (57 wt% in water; Aldrich) in a 250 ml roundbottomed flask at 0 8C for 2 h with stirring. The white precipitate was recovered by
evaporation at 50 8C for 1 h. The product, CH3NH3I, was dissolved in ethanol,
filtered, recrystallized from diethyl ether and dried at 60 8C in a vacuum oven for 24 h.
Device preparation. Devices were prepared on cleaned ITO substrates by spin
coating a thin layer of PEDOT:PSS from the commercial aqueous dispersion (1,200
revolutions per minute for 30 s (Chemmat, 3 cm diameter), which resulted in a
thickness of 70 nm). On top of this layer a thin film of polyTPD was deposited from
a chlorobenzene solution (10 mg ml21) using a meniscus coater and a coating speed
of 2.5 mm s21. Then the substrates were transferred to a vacuum chamber integrated
into an inert glovebox (,0.1 ppm O2 and ,0.1 ppm H2O; MBraun) and evacuated
to a pressure of 1 × 1026 mbar. Two ceramic crucibles were filled with CH3NH3I
(freshly prepared) and PbI2 , which were heated to 70 8C and 250 8C, respectively.
The film thickness was controlled by the PbI2 evaporation at a rate of evaporation of
0.5 Å s21 (see Supplementary Information for more details concerning the
perovskite evaporation process). The PCBM layer was deposited using a
chlorobenzene solution (10 mg ml21) in ambient conditions with a meniscus coater
and a coating speed of 10 mm s21. The device was completed by the thermal
evaporation of the top metal electrode under a base pressure of 2 × 1026 mbar to a
thickness of 100 nm. The solar cells (active area of 0.09 and 0.98 cm2) were then
encapsulated with a glass cover using an ultraviolet-curable epoxy sealant (Ossila E131
Encapsulation Epoxy), with an ultraviolet exposure of 5 min.
Characterization. GIXRD data were collected at room temperature in the 2u range
5–508 on an Empyrean PANalytical powder diffractometer using Cu Ka1 radiation.
Typically, four repeated measurements were collected and merged into a single
diffractogram. Pawley refinements23 were performed using the TOPAS computer
program24 and revealed an excellent fit to a one-phase model with a tetragonal cell
(a ¼ 8.80(2), c ¼ 12.57(2) Å) and space group I4/mcm. Additional peaks that
correspond to the ITO substrate were also observed. The observed and
calculated diffraction patterns for the refined crystal structures are shown in
Supplementary Fig. 3.
Simulation of the crystal structure of CH3NH3PbI3 perovskite was performed
using, as a starting model, the isostructural Sn analogue (CCDC ref. code:
ZZZBWS02), and modifying the cell parameters to those obtained in the
Pawley refinement25.
Absorption spectra were taken using a fibre-optics based Aventes Avaspec2048
spectrometer.
Current–voltage characteristics were recorded by applying an external potential
bias to the cell and recording the generated photocurrent with a digital source meter
(Keithley Model 2400). The light source was a 450 W xenon lamp (Oriel) equipped
with a Schott K113 Tempax sunlight filter (Praezisions Glas & Optik) to match the
emission spectrum of the lamp to the AM1.5G standard. A black mask was used in
the photovoltaic studies to limit the active area of the device. Before each
measurement, the exact light intensity was determined using a calibrated Si reference
diode equipped with an infrared cut-off filter (KG-3, Schott). IPCE measurements
were determined using a 300 W xenon light source (ILC Technology). A Gemini180 double monochromator (Jobin Yvon) was used to select and increment the
wavelength of the radiation that impinged on the cells. The monochromatic incident
light was passed through a chopper run at 1 Hz frequency, and the on/off ratio was
measured by an operational amplifier. IPCE spectra were recorded as functions of
wavelength under a constant white-light bias of approximately 5 mW cm22 supplied
by an array of diodes that emitted white light. The excitation beam from a 300 W
xenon lamp (ILC Technology) was focused through a Gemini-180 double
monochromator (Jobin Yvon) and chopped at approximately 2 Hz. The signal was
recorded using a Model SR830 DSP Lock-In Amplifier (Stanford Research Systems).
Received 16 September 2013; accepted 14 November 2013;
published online 22 December 2013
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Acknowledgements
We thank A. Soriano-Portillo, J. Ferrando, A. K. Chandiran and E. Ortı́ for their assistance
with the sample preparation and characterization, and careful proofreading of the
manuscript. This work was supported by the European Community’s Seventh Framework
Programme (ORION, Grant 229036 and NANOMATCELL, Grant 308997), the Spanish
Ministry of Economy and Competitiveness (MAT2011-24594), the Generalitat Valenciana
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DOI: 10.1038/NPHOTON.2013.341
(Prometeo/2012/053) and the Global Research Laboratory Program, Korea (GLOBASOL,
Grant 309194).
Author contributions
O.M. designed, prepared and characterized the devices, A.Y. and Y.H.L. characterized the
devices, G.M.E. supervised the X-ray characterization and interpretation, M.G. supervised
the work and wrote the manuscript, M.K.N. initiated the research, provided key materials
and wrote the manuscript, H.J.B initiated the research, designed the devices, supervised the
work and wrote the manuscript.
Additional information
Supplementary information is available in the online version of the paper. Reprints and
permissions information is available online at www.nature.com/reprints. Correspondence and
requests for materials should be addressed to M.K.N. and H.J.B.
Competing financial interests
The authors declare no competing financial interests.
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