A Methoxydiphenylamine-Substituted Carbazole Twin Derivative: An Efficient Hole-Transporting Material for Perovskite Solar Cells

Article abstract of DOI:10.1002/anie.201504666

The small-molecule-based hole-transporting material methoxydiphenylamine-substituted carbazole was synthesized and incorporated into a CH₃NH₃PbI₃ perovskite solar cell, which displayed a power conversion efficiency of 16.9

Full text of DOI:10.1002/anie.201504666

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201504666
German Edition: DOI: 10.1002/ange.201504666
Perovskite Solar Cells Very Important Paper
A Methoxydiphenylamine-Substituted Carbazole Twin
Derivative: An Efficient Hole-Transporting Material for
Perovskite Solar Cells**
Paul Gratia, Artiom Magomedov, Tadas Malinauskas, Maryte Daskeviciene,
Antonio Abate, Shahzada Ahmad, Michael Grätzel, Vytautas Getautis,* and
Mohammad Khaja Nazeeruddin*
Angewandte
Chemie
Angew. Chem. Int. Ed. 2015, 54, 11409 –11413
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Abstract: The small-molecule-based hole-transporting mate-
rial methoxydiphenylamine-substituted carbazole was synthe-
sized and incorporated into a CH3NH3PbI3 perovskite solar
cell, which displayed a power conversion efficiency of 16.91%,
the second highest conversion efficiency after that of Spiro-
OMeTAD. The investigated hole-transporting material was
synthesized in two steps from commercially available and
relatively inexpensive starting reagents. Various electro-optical
measurements (UV/Vis, IV, thin-film conductivity, hole mobi-
lity, DSC, TGA, ionization potential) have been carried out to
characterize the new hole-transporting material.
been made to develop alternative molecules with similar
performance. However, most of these molecules fail to show
performance similar to that of Spiro-OMeTAD.[6–12] The only
hole-transporting material (HTM) without the spiro motif,
known to date that demonstrated device efficiencies close to
15% requires custom-made boronic acids as precursors for
the final synthesis.[13]
Here we report a new hole-transporting twin molecule
(V886), based on methoxydiphenylamine-substituted carba-
zole, with performance very similar to that of Spiro-
OMeTAD. Moreover, it does not require an extensive and
expensive synthetic procedure. The high solubility of V886 in
organic solvents (e.g. > 1000 mgmLÀ1 in chlorobenzene)
makes this molecule very appealing for applications with
solution processing. Perovskite solar cells employing V886 as
a HTM show power conversion efficiency up to 16.91%,
which is to the best of our knowledge, one of the highest
reported values for a small-molecule-based HTM. Further-
more, its simple two-step synthesis the ready availability of
the starting materials makes this HTM very appealing for
commercial prospects of perovskite solar cells.
T
he hybrid organic–inorganic methylammonium lead iodide
perovskite has been intensively investigated by Mitzi et al.,
for semiconductor and opto-electronic applications.[1] Due to
the absorption properties and very high molar extinction
coefficient of methylammonium lead iodide perovskite,
Miysaka et al. have used it as a sensitizer in dye-sensitized
solar cells in combination with iodine/iodide as liquid electro-
lyte.[2] The perovskite materials are soluble in most polar and
protonated solvents and the fabricated perovskite-sensitized
solar cells exhibited very low efficiency and instability. To
overcome the disadvantage of the solubility of the perovskite
absorber layer, a solid-state dye-sensitized solar cell was
constructed using Spiro-OMeTAD as a hole conductor[3]
along with the perovskite.[4,5]
The synthesis of V886 (1345.61 gmolÀ1) involves the click
reaction of 1,2-bis(bromomethyl)benzene with 3,6-dibromo-
carbazole, followed by a palladium-catalyzed C–N cross-
coupling reaction with 4,4’-dimethoxydiphenylamine
(Scheme 1). More detailed information on the synthesis can
be found in the Supporting Information (SI).
The performance of V886 was tested in CH3NH3PbI3-
based solar cells using a mesoporous TiO2 photoanode and an
Au cathode following a procedure based on antisolvent
engineering developed by Seok et al. (see SI).[14] The per-
ovskite device with V886 shows a maximum power conver-
sion efficiency (PCE) of 16.91% under AM 1.5 G illumina-
tion, while PCE values exceeding 14% are routinely
observed. The measured fill factor was 0.73, the current
density (Jsc) 21.38 mAcmÀ2, and the open-circuit voltage
1.085 V (Figure 1). The best device from the same batch of
solar cells, prepared following the same device fabrication
procedure but using Spiro-OMeTAD as hole-extracting layer,
displayed a PCE of 18.36% as shown in Figure S1 (SI). The
cross-section scanning electron micrograph of the best V886
device is shown in Figure 2.
However, the synthesis of Spiro-OMeTAD is prohibi-
tively expensive since it includes reaction steps that require
low temperature (À788C), and sensitive (n-butyllithium or
Grignard reagents) and aggressive (Br2) reagents. In addition,
high-purity sublimation-grade Spiro-OMeTAD is required to
obtain high-performance devices. Tremendous efforts have
[*] P. Gratia, Dr. A. Abate, Prof. Dr. M. Grätzel,
Prof. Dr. M. K. Nazeeruddin
Group for Molecular Engineering of Functional Materials
and Laboratory for Photonics and Interfaces
École Polytechnique FØdØrale de Lausanne
1015 Lausanne (Switzerland)
E-mail: mdkhaja.nazeeruddin@epfl.ch
A. Magomedov, Dr. T. Malinauskas, Dr. M. Daskeviciene,
Prof. V. Getautis
Department of Organic Chemistry, Kaunas University of Technology
Radvilenu pl. 19, 50254 Kaunas (Lithuania)
E-mail: vytautas.getautis@ktu.lt
Dr. S. Ahmad
Department Abengoa Research, C/Energía Solar
no1, Campus Palmas Altas, 41014 Sevilla (Spain)
Prof. Dr. M. K. Nazeeruddin
Center of Excellence for Advanced Materials Research (CEAMR)
King Abdulaziz University, Jeddah (Saudi Arabia)
[**] We acknowledge funding from the European Union Seventh
Framework Programme FP7/2007-2013 (grant no. 604032),
ENERGY.2012.10.2.1, and NANOMATCELL, (grant no. 308997. We
thank Dr. V. Gaidelis and Dr. V. Jankauskas for their help with
ionization potential and xerographic time-of-flight measurements,
and Sadig Aghazada for his assistance with CV measurements. A.A.
has received funding from the European Union’s Seventh Frame-
work Programme (grant no. 291771.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201504666.
Scheme 1. Synthetic route to the hole-transporting material V886.
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Figure 3. UV/Vis absorbance spectra of a) a solution of V886 in
chlorobenzene, b) V886 chemically oxidized by addition of 10 mol%
FK209. Upon chemical oxidation, peaks at 628 nm and 814 nm appear.
Figure 1. Current–voltage characteristics of perovskite solar cells using
Spiro-OMeTAD (red) and V886 (black) as hole-transporting materials.
The power conversion efficiency of the V886 perovskite solar cell is
16.91%, the open-circuit voltage Voc =1.085 V, fill factor FF=0.734,
and current density Jsc =21.38 mAcm2. The power conversion efficiency
of the Spiro-OMeTAD perovskite solar cell is 18.36%.
the oxidized V886 does not absorb in the infrared part of the
spectrum, as shown in Figure S3 (SI), making this hole
conductor an interesting option for the semitransparent
devices that are part of a hybrid tandem architecture.[17]
Moreover, solubility tests show the very high solubility of
V886 in some organic solvents like THF and chlorobenzene
(> 1000 mgmLÀ1).
Lateral thin-film conductivity of V886 and Spiro-
OMeTAD layers was measured on a spin-coated thin film
(chlorobenzene) on OFET substrates (Table 1). In the
Table 1: Comparison of key properties of Spiro-OMeTAD and V886.
Compound
Spiro-OMeTAD
V886
Ip [eV][a]
5.00
5.12
5.04
5.27
CV
Ip [eV vs. vacuum]
TOF mobility m0 [cm2 VÀ1 sÀ1 [b]
]
4”10À5
5”10À4
4.7”10À4
2”10À5
6”10À4
4.2”10À5
TOF mobility m [cm2 VÀ1 sÀ1 [c]
]
lateral conductivity
Figure 2. Cross-section scanning electron micrograph of the best
device containing a 100 nm thick layer of V886 as a hole conductor.
(10 mol% FK209) [ScmÀ1
]
[a] Ionization potential was measured from films by the photoemission
in air method. [b] Mobility value at zero field strength. [c] Mobility value
at 6.4”105 VcmÀ1 field strength.
These perovskite solar cells typically exhibit hysteresis in
the current–voltage curve, but according to recent work by
Unger et al., very slow scans lead to hysteresis-free current-
voltage curves.[15] In the same way, the hysteresis of the V886-
containing perovskite solar cell decreases with decreasing
scan rate as shown in Figure S2 (SI). Fast scan rates
(> 20 mVsÀ1) lead to significant hysteresis, while scans
recorded at 20 mVsÀ1 and 10 mVsÀ1 show only little hyste-
resis.
Figure 3 shows the difference in absorbance upon chem-
ical oxidation of V886 by the cobalt(III) complex FK209.[16] In
its pristine form, V886 absorbs only in the UV region below
450 nm, while the chemically oxidized V886 exhibits visible
absorption bands at 628 nm and 814 nm, showing the
formation of the oxidized species. Since the CH3NH3PbI3
perovskite absorption cuts off at 780 nm, the absorption of
the oxidized V886 at 823 nm does not compete. Additionally,
absence of FK209, no Ohmic contact could be observed and
the measured currents were very small compared to the
currents measured upon addition of FK209. In the case of
oxidized V886, Ohmic contacts are formed with gold and the
conductivity upon addition of 10 mol% of FK209 is extracted
by making a linear fit and using Ohmꢀs law. The conductivity
of oxidized V886 was determined to be 4.2 ” 10À5 ScmÀ1,
indicating that the chemical oxidation of V886 actually results
in doping, that is, an increase of carrier concentration and
hence conductivity (Figure 4). It is about one order of
magnitude lower than the conductivity measured in the
doped Spiro-OMeTAD (4.7 ” 10À4 ScmÀ1). However, it is
sufficient for good performance in devices with about 100 nm
thick HTM films.
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Figure 5. Thermogravimetric heating curve of V886 (heating rate
108CminÀ1).
Figure 4. Conductivity plot of a doped V886 and Spiro-OMeTAD thin
film (log scale and linear scale). Ohmic contacts between doped V886/
Spiro-OMeTAD and the gold contacts are formed. The conductivity of
Spiro-OMeTAD is 4.7”10À4 ScmÀ1 and that of V886 4.2”10À5 ScmÀ1
.
The hole mobility of V886 films (Table 1) was measured
by the xerographic time-of-flight method (XTOF). XTOF
measurements indicate that the hole-drift mobility of V886 is
2 ” 10À5 cm2 VÀ1 sÀ1 at weak electric fields and 6.4 ”
10À4 cm2 VÀ1 sÀ1 at a field strength of 6.4 ” 105 VcmÀ1. The
mobility values are of the same order of magnitude as the
ones for Spiro-OMeTAD measured in this work and reported
elsewhere.[18]
When one considers the use of an organic material for
optoelectronic applications it is important to know its solid-
state ionization potential (Ip). The solid-state ionization
potential of undoped V886 was measured by photoelectron
spectroscopy in air (PESA) and compared to the Ip of V886 in
solution, determined by cyclic voltammetry (CV). The differ-
ence between the energies measured in the film (5.04 eV) and
in the solution (5.27 eV) could be attributed to the different
measurement techniques and conditions (solution in CV and
solid film in the photoemission method). The CV measure-
ment also reveals that the HTM undergoes a reversible one-
electron oxidation and the second oxidation peak is at 5.43 eV
(Figure S4).
Figure 6. Differential scanning calorimetry: first and second heating
curves of V886 (heating rate 108CminÀ1).
steps from commercially available materials, and is thus
useful for large-scale production of perovskite solar cells. The
good solubility and film-forming properties allows the
formation of thicker films of high quality. We strongly believe
that V886 can be also an ideal candidate for other optoelec-
tronic applications such as OLEDs and solid-state dye-
sensitized solar cells.
Differential scanning calorimetry (DSC) and thermogra-
vimetric analysis (TG) measurements reveal that V886 films
are amorphous and show good thermal stability; the material
starts to decompose at temperatures around 3908C (Figure 5).
DSC scans demonstrated that V886 is fully amorphous and
shows the glass transition temperature Tg at 1418C (Figure 6).
It is higher than that of Spiro-OMeTAD (1258C),[18] indicat-
ing a more stable amorphous state. Since V886 is fully
amorphous, it is legitimate to link the lateral conductivity to
the vertical conductivity, which is a more realistic parameter
for characterization of a solar cell based on the architecture
described here.
In conclusion, we have demonstrated a small-molecule
hole conductor (V886) that yields very high current density
(> 21 mAcmÀ2) and overall efficiency close to 17% in
CH3CH2NH3PbI3-based solar cells. V886 is less expensive
than the spiro hole conductors: it can be synthesized in two
Keywords: carbyazoles · thin-layer conductivity ·
organic hole transporters · perovskite solar cells ·
power conversion efficiency
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Received: May 26, 2015
Published online: July 16, 2015
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