Benzotrithiophene-based hole-transporting materials for 18.2 % perovskite solar cells

Article abstract of DOI:10.1002/anie.201511877

New star-shaped benzotrithiophene (BTT)-based hole-transporting materials (HTM) BTT-1, BTT-2 and BTT-3 have been obtained through a facile synthetic route by crosslinking triarylamine-based donor groups with a benzotrithiophene (BTT) core. The BTT HTMs we

Full text of DOI:10.1002/anie.201511877

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International Edition: DOI: 10.1002/anie.201511877
German Edition: DOI: 10.1002/ange.201511877
Solar Cells
Benzotrithiophene-Based Hole-Transporting Materials for 18.2%
Perovskite Solar Cells
Agustín Molina-Ontoria+, Iwan Zimmermann+, InØs Garcia-Benito, Paul Gratia,
Cristina Roldµn-Carmona, Sadig Aghazada, Michael Graetzel,
Mohammad Khaja Nazeeruddin,* and Nazario Martín*
Abstract: New star-shaped benzotrithiophene (BTT)-based
hole-transporting materials (HTM) BTT-1, BTT-2 and BTT-3
have been obtained through a facile synthetic route by
crosslinking triarylamine-based donor groups with a benzotri-
thiophene (BTT) core. The BTT HTMs were tested on
solution-processed lead trihalide perovskite-based solar cells.
Power conversion efficiencies in the range of 16% to 18.2%
were achieved under AM 1.5 sun with the three derivatives.
These values are comparable to those obtained with todayꢀs
most commonly used HTM spiro-OMeTAD, which point them
out as promising candidates to be used as readily available and
cost-effective alternatives in perovskite solar cells (PSCs).
anionic substitution. As an example, the formamidinium (FA)
based perovskite FAPbI3 shows excellent light harvesting
properties due to its lower band gap energy (Eg),[11] whereas
by using the methylammonium mixed halide perovskite
MAPbIxBr3Àx higher Eg and open circuit voltage (Voc) can
be obtained.[12] Improved energy conversion efficiencies are
also observed when combining the two perovskite materials
in the compositional modification (FAPbI3)1Àx(MAPbBr3)x
that was recently presented by Seok et al.,[13] reporting
power conversion efficiencies (PCEs) above 20%. The
ambipolar behavior of the perovskite allows its combination
either n–i–p or p–i–n configurations with electron transport-
ing (ETMs) and/or with hole-transporting (HTMs) materials.
These interface layers play an important role for transporting
and blocking the charges. Awide number of HTMs have been
synthesized and investigated in combination with perovskite
absorber ranging from classical semiconducting polymers to
small molecules.
Focusing on the latter, different central cores have been
used in the state-of-the-art photovoltaic devices, such as 9,9’-
spirobifluorene,[14] spiro-liked derivatives,[15] thiophene deriv-
atives,[16] triphenylamine,[17] bridged-triphenylamines,[18,19]
pyrene,[20] 3,4-ethylenedioxythiophene,[21] linear p-conju-
gated,[22–24] triptycene,[25] tetraphenylethene,[26] silolothio-
phene or triazines,[27] all of them namely decorated with
diarylamines, triarylamines and/or carbazole derivatives.
With this approach, the best power conversion efficiency
obtained to date, up to 20%,[13] has been achieved by using
the polymer poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine]
(PTAA). According to their limited absorption in the visible
region, these materials can act as hole-transporters without
interfering the spectral response of the photoactive material.
Nevertheless, there are also fewer examples of HTMs
absorbing in the visible and near-infrared region, that is, by
incorporating pentacene,[28] S,N-heteropentacene,[29] benzodi-
thiophene derivatives,[30,31] phenoxazine[32] and thiolated
nanographene,[33] and which exhibit PCEs over 15%. The
low band-gap HTM layer enhances light absorption in the
device, and exhibits beneficial effects. Actually, it has been
previously reported that photons absorbed by low band-gap
HTM results in a noticeable improvement of photocurrent.[34]
In our case, the BTT series absorbs below 420 nm, where the
perovskite strongly absorbs and, therefore, it has no influence
on the solar cell efficiency.
S
ince its first use as light absorber in a sensitized solar cell by
Miyasaka and co-workers,[1] organic–inorganic methylammo-
nium (MA) lead halide MAPbX3 (X = I, Br) perovskites have
experienced a scientific research blast for photovoltaic
applications.[2–7] Organometal trihalide perovskites exhibit
exceptional intrinsic properties such as light absorption from
visible to near-infrared range, high extinction coefficient, long
electron–hole diffusion lengths, a direct band gap as well as
high charge carrier mobilities, among others.[8–10] Further-
more, the perovskite material is relatively versatile and its
electronic properties can be widely tuned by cationic or
[*] Dr. A. Molina-Ontoria,[+] I. Garcia-Benito, Prof. Dr. N. Martín
IMDEA-Nanociencia, C/ Faraday 9
Ciudad Universitaria de Cantoblanco, 28049 Madrid (Spain)
E-mail: nazmar@ucm.es
Dr. I. Zimmermann,[+] P. Gratia, Dr. C. Roldµn-Carmona, S. Aghazada,
Prof. Dr. M. K. Nazeeruddin
Group for Molecular Engineering of Functional Materials
Institute of Chemical Sciences and Engineering, EPFL VALAIS
1951 Sion (Switzerland)
E-mail: mdkhaja.nazeeruddin@epfl.ch
Prof. Dr. M. Graetzel
Laboratory of Photonics and Interfaces, Institute of Chemical
Sciences and Engineering, EPFL VALAIS
1015 Lausanne (Switzerland)
Prof. Dr. N. Martín
Departamento Química Orgµnica, Facultad C. C. Químicas
Universidad Complutense de Madrid
Av. Complutense s/n, 28040 Madrid (Spain)
Homepage: http://www.ucm.es/info/fullerene/
[+] These authors contributed equally to this work.
Supporting information for this article (detailed synthesis procedure,
device preparation, additional SEM images, hysteresis curves, device
statistics, detailed characterization including XRD, CV, DSC, TG,
NMR, MS) can be found under:
Hitherto, the most studied HTM in PSC has been the
2,2,7,7-tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobi-
fluorene (Spiro-OMeTAD). Perovskite solar cells employing
this semiconductor have achieved PCEs values over 19% by
http://dx.doi.org/10.1002/anie.201511877.
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combining different strategies such as the interface engineer-
synthetic step, bromination of BTT by employing NBS leads
ing[35] between the charge transporting and extracting layers
as well as by improving the formation of the perovskite crystal
via the generation of a Lewis base adduct.[36] Unfortunately,
the preparation of the spirobifluorene central core present in
this material requires a complicated synthetic protocol which
makes it a relatively expensive material. Besides, sublimation-
grade of the Spiro-OMeTAD is an essential requirement in
order to achieve high PCEs devices.[23]
to the formation of the desired tribromo-derivative. Finally, p-
methoxydiphenylamine or p-methoxydiphenylamine-substi-
tuted carbazole were introduced by Buchwald–Hartwig
amination reaction to obtain tri-substituted BTT derivatives
BTT-1 and BTT-2. On the other hand, p-methoxytriphenyl-
amine units were covalently linked to the BTT core by
a Suzuki cross-coupling reaction, affording BTT-3. The new
compounds were obtained as stable solids in a good overall
yield (see Supporting Information).
In this work, we report a new family of ease to prepare
HTMs based on
a
benzo[1,2-b:3,4-b’:5,6-b’’]trithiophene
As previously mentioned, the new HTM molecules BTT-
1, BTT-2 and BTT-3 together with spiro-OMeTAD as
a reference were tested on perovskite-based solar cells. The
energy levels of the HTM materials are shown in Figure 2. As
(BTT) as a central core endowed with p-methoxydiphenyl-
amine (BTT-1), p-methoxydiphenylamine-substituted carba-
zole (BTT-2) and p-methoxytriphenylamine (BTT-3). These
compounds exhibit remarkable PCEs values when used in
organometal trihalide perovskites solar cells (Figure 1). BTT
Figure 2. Left: Cross sectional images of devices fabricated with BTT-1,
BTT-2 and BTT-3 on MAPbI3 perovskite. The thickness of the hole-
transporting layer is indicated. Right: Energy diagram of the different
components in a perovskite solar cell.
can be observed in the Figure, the HOMO levels of the BTT-
1 and BTT-2 molecules are very similar to the one for spiro-
OMeTAD, whereas the BTT-3 exhibits slightly lower HOMO
level, which closely matches the energy level of the valence
band edge of the MAPbI3 perovskite. The cell architecture
used in this study is shown among the high resolution
scanning electron microscopy (SEM) cross-section images
displayed in Figure 2. At the anode, a compact and meso-
porous layer of TiO2 was deposited on top of a fluorine-doped
tin oxide (FTO) coated glass and used as the electron
collector, while the cathode consists of a gold electrode
thermally evaporated onto the HTM layer. The photoactive
material MAPbI3 is sandwiched in between these layers, as
well as the modified perovskite (FAPbI3)0.85(MAPbBr3)0.15
recently introduced by Seok,[13] which was also tested for
comparison. Both perovskite layers were fabricated by using
a single step spin coating procedure similar to the protocol
previously described in the literature.[37] As a summary, the
perovskite precursor solution was spin-coated at 4000 rpm for
30 s. 10 s prior to the end of the spinning program, 120 mL of
chlorobenzene were poured onto the spinning substrate to
obtain a homogeneous dense perovskite film after annealing
Figure 1. Chemical structures of BTT-1, BTT-2 and BTT-3.
core exhibits a C3h symmetry with three thiophene rings fused
to a benzene central ring. The planarized star-shaped
structure of the BTT is foreseen to promote an effective p–
p intermolecular interaction, which could eventually lead to
efficient hole-transporting properties.
The synthetic pathways and the experimental details for
the preparation of the new family of HTMs are given in the
Supporting Information (SI). The BTT central core was
synthesized by a stepwise approach to obtain the tribromo-
BTT, which involves the iodination of the 1,3,5-trichloroben-
zene, Sonogashira cross-coupling reaction and ring closing
reaction with sodium sulfide in NMP. In a subsequent
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for 1 h at 1008C. Compared to the standard perovskite
devices, the composite material has a thicker capping layer of
around 400 nm, three times larger than the value observed
when using the standard perovskite. The hole-transporting
materials presented in this study were then spin-coated on top
of the perovskite layer at 4000 rpm for 15 s. The detailed
procedure for the device fabrication is given in the SI.
The photovoltaic performance of the devices under AM
1.5 G conditions are shown in Figure 3 and the detailed results
be found in the SI. The results show excellent performances
for all the benzotrithiophene derivatives, being BTT-3 slightly
better than its analogous BTT-1 and BTT-2. Power conversion
efficiencies (PCE) above 18.2% were observed on standard
MAPbI3 perovskite solar cells, with almost no hysteresis when
tested under the described conditions. The average PCE of
17.7 Æ 0.4% indicates a very good reproducibility for this
molecule. Up to date such a high PCE value could be only
achieved when using either spiro-OMeTAD or PTAA as
hole-transporting materials. Comparing the results obtained
for the three derivatives, the main difference when using BTT-
3 is the high short circuit current density (Jsc = 21.9 mAcmÀ2).
In fact these values are very close to those obtained with the
spiro-OMeTAD reference. Devices with BTT-2, on the other
hand, show a remarkably high open circuit voltage of almost
1.1 V, resulting in PCEs up to 17% (average: 15.6 Æ 1.1%).
BTT-1 shows the lower performance, which might be also
related to its low solubility in chlorobenzene compared to
BTT-2 and BTT-3, making it more difficult to obtain uniform
films and reproducible results, which can be seen from the
relatively high standard deviation on the average PCE value
of 13.9 Æ 1.5%. Nevertheless, PCE values of up to 16% are
still possible to be obtained by using this molecule. Compar-
ing the devices prepared with the composite perovskite
(FAPbI3)0.85(MAPbBr3)0.15 and BTT-HTM molecules, the
resulting performances are very similar. As previously
observed, BTT-1 is the least effective HTM, leading to
lower fill factor and Jsc. On the contrary, compounds BTT-2
and BTT-3 behave similarly and excellent PCE values of up to
17.5% are obtained. As a general trend, the presented
chemical structures seem to favor the formation of homoge-
neous thin films with a good morphology and conductivity,
which lead to excellent fill factor values (FF > 70%) com-
parable to those obtained for spiro-OMeTAD. These exper-
imental findings suggest a high carrier mobility in these new
materials.
The electrical conductivity of the BTT-based thin films
was measured in a lateral configuration between 2.5 mm
spaced gold contacts. Thin films of the BTT molecules were
deposited onto substrates having interdigitated gold electro-
des by using the same procedure as for device preparation,
adding 3 mol% of FK209 as a dopant in all of them. Figure 4
shows the comparison of the conductivity determined for the
three derivatives. The values were extracted using a linear fit
and Ohmꢀs law. As can be observed in the inset of Figure 4,
BTT-1 exhibits the lowest conductivity (6.0 ” 10À6 ScmÀ1),
followed by BTT-2 (1.3 ” 10À5 ScmÀ1). The conductivity of
BTT-3 is the highest, reaching 2.79 ” 10À5 ScmÀ1. Although
this value is still about one order of magnitude lower than that
of Spiro-OMeTAD[23] it seems to be sufficient for obtaining
a high performance when a thin HTM layer is used.
Figure 3. I–V curves showing the performance of perovskite solar cells
prepared by using BTT-1, BTT-2 and BTT-3 as HTM: standard MAPbI3
perovskite (top) and compositional modified perovskite (FAPbI3)0.85
-
(MAPBr3)0.15 (bottom). The J–V curves obtained for the reference can
be found in the SI.
The trend in conductivity clearly supports the observation
of the device performance obtained for the three different
BTT molecules. The thickness of the individual BTT layers
was adjusted to obtain the best efficiencies. According to the
results, the lower conductivity of BTT-1 requires a very thin
layer in order to have a good performance. Values for the
shunt resistance of the three derivatives, estimated from the
slope close to short circuit current, are very similar for the
are summarized in the table below. Current–voltage (J–V)
curves were recorded by applying a forward bias with a scan
rate of 10 mVsÀ1 in order to minimize the hysteresis effects
stemming from the perovskite material.[38] Hysteresis scans
showing the performance under forward and reverse bias can
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ENERGY.2012.10.2.1; NANOMATCELL, grant agreement
no. 308997, and H2020-ICT-2014-1, grant agreement no.
643791. N.M. thanks the European Research Council ERC-
320441 (Chirallcarbon), MINECO of Spain (CTQ2014-
52045-R), the CAM (FOTOCARBON project S2013/MIT-
2841), and the Alexander von Humboldt Foundation.
Keywords: benzotrithiophenes · perovskites ·
power conversion efficiency · solar cells · triarylamines
How to cite: Angew. Chem. Int. Ed. 2016, 55, 6270–6274
Angew. Chem. 2016, 128, 6378–6382
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Education, Research and Innovation (SERI), CEPF special
energy funds and the European Union Seventh Framework
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604032
of
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