Highly Efficient Perovskite Solar Cells Employing an Easily Attainable Bifluorenylidene-Based Hole-Transporting Material

Article abstract of DOI:10.1002/anie.201602545

The 4,4′-dimethoxydiphenylamine-substituted 9,9′-bifluorenylidene (KR216) hole transporting material has been synthesized using a straightforward two-step procedure from commercially available and inexpensive starting reagents, mimicking the synthetically

Full text of DOI:10.1002/anie.201602545

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201602545
German Edition: DOI: 10.1002/ange.201602545
Solar Cells
Highly Efficient Perovskite Solar Cells Employing an Easily Attainable
Bifluorenylidene-Based Hole-Transporting Material
Kasparas Rakstys, Michael Saliba, Peng Gao, Paul Gratia, Egidijus Kamarauskas,
Sanghyun Paek, Vygintas Jankauskas, and Mohammad Khaja Nazeeruddin*
Abstract: The 4,4’-dimethoxydiphenylamine-substituted 9,9’-
bifluorenylidene (KR216) hole transporting material has been
synthesized using a straightforward two-step procedure from
commercially available and inexpensive starting reagents,
mimicking the synthetically challenging 9,9’-spirobifluorene
moiety of the well-studied spiro-OMeTAD. A power conver-
sion efficiency of 17.8% has been reached employing a novel
HTM in a perovskite solar cells.
OMeTAD. In addition, the novel HTM does not require an
expensive synthetic procedure and was prepared employing
a straightforward two-step strategy. 9-Fluorenylidene moiety
is known to facilitate charge carrier migration conveying
electrons from donor to acceptor throughout the structure
and have never been tested as potential HTMs for PSC.[14]
The general synthesis procedure for the preparation of
2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9’-bifluor-
enylidene KR216 is shown in Scheme 1. Commercially
P
erovskite-based solar cells (PSCs) have rapidly become the
hottest topic in photovoltaics due to their unique optical and
electrical properties, since the first report in 2009 by Miyasaka
et al.[1] The best performing device configuration of PSC is
composed of an electron-transporting material (ETM), typ-
ically a mesoporous layer of TiO2, which is infiltrated with
perovskite absorber material, coated with a hole-transporting
material (HTM) and gold as a back contact.[2–5]
Numerous organic small molecule-based HTMs have
been explored in PSCs, reaching efficiency of 16–18%.[6–11]
To date, 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-
9,9’-spirobifluorene (spiro-OMeTAD) and 2’,7’-bis(bis(4-
methoxyphenyl) amino) spiro[cyclopenta[2,1-b:3,4-b’]dithio-
phene-4,9’-fluorene] (FDT) HTMs reached the highest
reported values over 20%.[12,13] However, the multi-step
synthesis of spiro-type derivatives is prohibitively expensive
and challenging, since it requires low temperature and harsh
acidic and basic conditions. Moreover, high-purity sublima-
tion-grade spiro-OMeTAD is required to obtain high-perfor-
mance devices.
Scheme 1. Straightforward synthetic route for the KR216 HTM. a) Law-
esson’s reagent, toluene, 1108C. b) 4,4’-Dimethoxydiphenylamine, t-
BuONa, Pd2dba3, XPhos, toluene, 1108C.
available and inexpensive precursor 2,7-dibromo-9H-fluo-
ren-9-one have been reacted with 2,4-bis(4-methoxyphenyl)-
1,3,2,4-dithiadiphosphetane-2,4-dithione, well known as Law-
=
essonꢀs reagent, to form 9,9’-ylidene (C C) double bond.
Here we present a molecularly engineered HTM 4,4’-
dimethoxydiphenylamine-substituted 9,9’-bifluorenylidene
KR216 with performance on par with that of spiro-
With this we demonstrate the synthesis of 2,2’,7,7’-tetra-
bromo-9,9’-bifluorenylidene 1 by single-step reaction, accel-
erating the synthetic routes proposed by Wudl and Luh, using
dimerization of the substituted 9-bromofluorene in the
presence of DBU and desulfurdimerization of dithioketals
mediated by W(CO)6, respectively.[15,16] Furthermore, we note
that no column chromatography was required in advance for
the next step. Then, the symmetrical olefin have been
equipped with 4,4’-dimethoxydiphenylamine units using the
[*] K. Rakstys, Dr. M. Saliba, Dr. P. Gao, P. Gratia, Dr. S. Paek,
Prof. Dr. M. K. Nazeeruddin
Group for Molecular Engineering of Functional Materials
Institute of Chemical Sciences and Engineering
École Polytechnique FØdØrale de Lausanne
1015 Lausanne (Switzerland)
À
palladium-catalyzed Buchwald–Hartwig C N cross-coupling
E-mail: mdkhaja.nazzeruddin@epfl.ch
reaction, to release final HTM KR216 having 9,9’-bifluor-
enylidene central core, which mimics 9,9’-spirobifluorene
moiety of well-studied spiro-OMeTAD.
E. Kamarauskas, Dr. V. Jankauskas
Department of Solid State Electronics
Vilnius University
It is well known, that both organic impurities and metal
residues may present in final raw organic semiconducting
materials, and they are known to act as charge carrier traps or
photoquenchers significantly affecting their original proper-
ties, consequently reducing the photovoltaic perfor-
mance.[17,18] Therefore, we have synthesized spiro-OMeTAD
in our lab under the same final cross-coupling step and
Sauletekio 9, Vilnius, 10222 (Lithuania)
Prof. Dr. M. K. Nazeeruddin
Center of Excellence for Advanced Materials Research
King Abdulaziz University
Jeddah (Saudi Arabia)
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201602545.
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purified using both flash chromatography and precipitation
lattice stacked in a highly slipped fashion (Figure 1c,d)
procedures as for KR216, having the same purity grade in
order to achieve a more accurate comparison. All synthetic
procedures and cost estimation are reported in the Supporting
Information.
The chemical structure of KR216 is confirmed by X-ray
diffraction analysis as displayed in Figure 1a. Similar to the
through p···p, C···C and C···O short contacts as well as CH/
p (CH/O) hydrogen bonds. Eight fluorenes are found to have
three different types of orientation. As indicated in Figure 1c
the parallel fluorenes with same orientation is marked with
the same color. When scrutinizing the crystal structure of
KR216, an abundance of non-covalent intermolecular inter-
actions are found with shortest p···p distance of 3.378 Š,
shortest C···C contacts of 3.254 Š and shortest C···O contacts
of 3.090 Š. Similar to that of spiro-OMeTAD, due to the
highly sterically hindered geometry, no close face to face
overlap is observed. However, the small p···p distance[20] and
short C···O contacts are expected to facilitate the facile
establishment of highly efficient charge transport channel.
The thermal behavior of HTMs was determined by
thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) measurements (Table 1). From TGA, it
Table 1: Thermal properties of KR216 and spiro-OMeTAD.
ID
Tm [8C][a]
Tg [8C][a]
Tcr [8C][a]
Tdec [8C][b]
KR216
spiro-OMeTAD
309
234
157
126
228
–
398
417
Figure 1. a) ORTEP drawings of KR216 determined by X-ray crystallog-
raphy indicating four different kinds of dihedral angles between the
two central fluorenes and four diphenyl amine units (hydrogen atoms
omitted for clarity). b) Perspective views of KR216 molecule showing
one typical dihedral angle between the two fluorene units (42.48).
c) View down reciprocal cell axis b of the stacking molecules showing
three different types of orientation of the eight fluorene units (same
orientation with same color). d) Noncovalent intermolecular short
contacts of KR216 in a one-unit cell. Dashed blue lines illustrate p···p
short contacts (3.378 Š, and 3.381 Š) and C···C short contacts
(3.254 Š, 3.313 Š, 3.336 Š, 3.340 Š, 3.350 Š, and 3.370 Š,). Dashed red
lines illustrate C···O short contacts (3.090 Š, 3.105 Š, 3.185 Š and
3.180 Š).
[a] Melting, glass transition, and crystallization temperatures observed
from DSC (108CminÀ1, Ar atmosphere). [b] Degradation temperature
observed from TGA (5% weight loss at 108CminÀ1, N2 atmosphere).
was found that KR216 has a similar to spiro-OMeTAD
(4178C) and relatively high decomposition temperature (Tdec
)
of 3988C, with a weight loss of 5%, indicating good thermal
stability, required for photovoltaic devices.
The thermal transitions of KR216 were studied by DSC
and compared with spiro-OMeTAD (Figure 2). During the
first heating scan, both glass transition (Tg) (1578C) and
melting of the crystals (3098C) are observed, indicating that
the material could exist in both crystalline and amorphous
states. Additionally, crystallization process is detected at
2288C, whereas no crystallization was observed during the
cooling and second heating steps, only the glass transition at
1578C.
Similarly to KR216, spiro-OMeTAD can also exist in both
amorphous and crystalline states.[21] During the first heating,
no crystallization and only melting was detected at 2348C.
The glass transition of spiro-OMeTAD was observed only
during the second heating cycle at 1268C and is lower than
that of KR216, indicating that KR216 has more stabilized
amorphous state.
The normalized UV/Vis absorption spectra of KR216 and
spiro-OMeTAD in THF are shown in Figure 3. Both exhibit
almost identical absorption band in UV region with the
absorption maximum at 390 nm, while the KR216 also shows
a broad optical absorption in the visible region centered at
466 nm, revealing enhanced conjugation through 9-ylidene
double bond with better p-electron delocalization. Optical
band gaps (Eg) determined from the onset of absorption were
2.41 eV for KR216 and 3.00 eV for spiro-OMeTAD, respec-
tively.
ubiquitous spiro-OMeTAD, four diphenyl amine units are
attached to the two central fluorene skeletons with four
different torsion angles with the minimum of 15.28 and the
maximum of 838. This is an indication of different degree of
distortion among the four bulky propeller shaped diphenyl
amine units. This can be understood by Figure 1b, in which
the crystal structure of KR216 is found to be distinct from that
of spiro-OMeTAD, when the sp3 carbon atom in the latter is
replaced by a double bond between two sp2 hybridized carbon
atoms.[19] It is known that the sp2 hybridized p bond cannot
rotate freely and tend to form either cis- or trans-conforma-
tion. So the competition between the planarization and
repulsive steric hindrance lead to a pseudo spiro conforma-
tion and diversified torsion angles. Therefore, the dihedral
angles between the two double-bond connected fluorenes are
measured to be either 42.48 (43.78) or 35.48 (36.58). These
values are much smaller than that of spiro-OMeTAD (89.948)
due to the reason mentioned above (Figure S7-a). This effect
is also reflected in the length of double bond between two
fluorenes, the value of which is also varied according to the
degree of distortion (Figure S7-b).
KR216 was found to crystallize in the triclinic space group
P1. In one unit cell, four independent molecules the crystal
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Table 2: Optical and electrochemical properties comparison.
ID
labs
Eg
EHOMO vs.
HOMO vs.
LUMO
[eV][d]
[nm][a] [eV][b] NHE [V][c]
vacuum [eV][c]
KR216
390,
466
390
2.41 0.65
3.00 0.60
À5.09
À5.04
À2.68
À2.04
spiro-
OMeTAD
[a] Absorption was measured in THF solution. [b] Determined from the
UV/Vis absorption onset. [c] Measured in DCM/tetra-n-butylammonium
hexafluorophosphate (0.1m) solution, using glassy carbon working
electrode, Pt reference electrode, and Pt counter electrode with Fc/Fc+ as
an internal standard. Potentials were converted to the normal hydrogen
electrode by addition of +0.624 V and À4.44 eV to the vacuum,
respectively. [d] Calculated from LUMO=HOMO+Eg.
KR216. The solid-state ionization potential (IP) was measured
by the electron photoemission in air on the thin films (PESA,
Figure S9). KR216 and spiro-OMeTAD have similar IP
values, 5.01 and 5.10, respectively. The small 0.07 eV differ-
ence was noticed to ionize different grade spiro-OMeTAD
samples. Lateral thin-film conductivity of HTM layers was
measured on OFET substrates (Figure S10). The conductivity
of oxidized KR216 was determined to be 3.40 ” 10À5 ScmÀ1,
which is slightly higher of that of lab-grade spiro-OMeTAD
(s = 3.04 ” 10À5 ScmÀ1). More than two times higher conduc-
tivity was obtained for commercial spiro-OMeTAD (s =
6.89 ” 10À5 ScmÀ1), showing a huge negative influence of
remained impurities. Xerographic time-of-flight (XTOF)
measurements were used to characterize charge-transporting
properties of the HTMs (Figures S11, S12). Gaussian-type
hole transport with well-defined transit time was observed in
KR216. The room-temperature zero-field hole-drift mobility
of KR216 was measured to be 4.6 ” 10À5 cm2 VÀ1 sÀ1 and
indicates similar charge hopping properties as for spiro-
OMeTAD (m0 = 1 ” 10À4 cm2 VÀ1 sÀ1). Similarly to conductiv-
ity results, high-purity commercial spiro-OMeTAD has higher
hole-drift mobility, reaching 1.3 ” 10À4 cm2 VÀ1 sÀ1. All elec-
trical properties are summarized in Table 3.
Figure 2. DSC first and second heating curves of KR216 and spiro-
OMeTAD, scan rate 108CminÀ1, Ar atmosphere (top). Thermogravi-
metric analysis (TGA) data of HTMs, heating rate of 108CminÀ1, N2
atmosphere (down).
Table 3: Comparison of electrical properties.
ID
IP
m0
m
s [ScmÀ1 [d]
]
[eV][a] [cm2 VÀ1 sÀ1 [b]
]
[cm2 VÀ1 sÀ1 [c]
]
Figure 3. UV/Vis absorption spectra normalized at the peak value
(left) and cyclic voltammograms (right) of KR216 and spiro-OMeTAD.
KR216
5.01 4.6”10À15
5.10 1”10À4
7.0”10À4
1”10À3
3.40”10À5
3.04”10À5
spiro-OMeTAD
synthesized
spiro-OMeTAD
commercial
To compare the energy levels of the new HTM with spiro-
OMeTAD we performed cyclic voltammetry (CV) measure-
ments. The data derived from the ground-state oxidation
potential (EHOMO) estimated from the cyclic voltammogram
shown in Figure 3 are summarized in Table 2. No reduction
potential was observed in the CV measurement. The HOMO
value of KR216 was estimated to be À5.09 eV versus the
vacuum, which is slightly destabilized compared with that of
spiro-OMeTAD (À5.04 eV). However, the 5 meV difference
is small and hardly affects the hole transfer from the
perovskite (À5.65 eV)[22] to the KR216.
5.03 1.3”10À4
2.1”10À3
6.89”10À5
[a] Ionization potential was measured by the photoemission in air
method from films. [b] Hole mobility value at zero field strength. [c] Hole
mobility value at 3.6”105 VcmÀ1 field strength. [d] Conductivity was
measured using 70 mm HTMs solution in chlorobenzene, doped with
3 mol% FK209.
To demonstrate the ability of KR216 act as HTM, we
prepared PSCs with perovskite as the absorber and compared
with the cells fabricated using the both grades of spiro-
OMeTAD under similar conditions. The HTM layers were
deposited onto the mixed perovskite from a chlorobenzene
solution containing 330 mol% tert-butylpyridine (TBP),
To elucidate the influence of impurities for energy and
electron transfer, electrical properties were determined using
both grades of spiro-OMeTAD and comparing with the
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50 mol% tris(bis(trifluoromethylsulfonyl)imide) (Li-TFSI)
scan, respectively. In comparison, it is slightly higher than that
and 3 mol% FK209 (CoIII oxidant) as additives. Detailed
device fabrication procedure is outlined in SI. The current
density–voltage (J–V) curves of the best devices under
100 mWcmÀ2 AM1.5G solar illumination with reverse and
forward scans are shown in Figure 4, and the extracted
corresponding parameters are summarized in Table 4. Our
of device prepared with our lab-grade spiro-OMeTAD,
17.4% for forward and 16.3% in backward scan, respectively.
This result clearly shows that KR216 is an excellent HTM for
PSC.
Improved photovoltaic performance reaching PCE of
18.4% was observed of the device built on high-purity
commercial spiro-OMeTAD under identical conditions.
Despite, higher VOC, JSC, and FF yielding higher overall
efficiency, there are also less difference obtained between the
forward and reverse scans, confirming that high-purity of
spiro-OMeTAD is required to obtain high-performance
devices. Furthermore, we note that the optimized perovskite
contributed towards the high efficiencies in this work.
The incident photon-to-current efficiency (IPCE)
(Figure 4) of the perovskite devices as a function of wave-
length shows that the device with KR216 as the HTM exhibits
IPCE above 85% from 400 nm covering all the visible region
to 700 nm. Photocurrents obtained from the IPCE data are in
close agreement with those of current–voltage measurements.
In conclusion, we report a novel easily attainable 9,9’-
bifluorenylidene-based hole-transporting material named
KR216, obtained by a straightforward two-step synthetic
route from commercially available and inexpensive starting
materials. The estimated price of KR216 is around 50 times
lower than that of commercial spiro-OMeTAD. A remarkable
power conversion efficiency of 17.8% was obtained for
perovskite solar cells using KR216, which is on par with the
power conversion efficiency of the high-purity commercial
spiro-OMeTAD. The presented result clearly demonstrates
that the 9-fluorenylidene moiety is very promising for future
molecular engineering of HTMs and let us believe that
KR216 has a potential for commercial application in high-
performance PSC.
Acknowledgements
Figure 4. J–V curves of best performing devices prepared with KR216
and spiro-OMeTAD as the reference. Devices were masked with
a black metal aperture of 0.16 cm2 to define the active area. The curves
were recorded scanning at 0.01 VsÀ1 (top) and IPCE spectra as
a function of the wavelength of monochromatic light (down).
The research leading to these results received funding from
the Qatar National Research Fund (grant number NPRP 6-
1752-070; a member of The Qatar Foundation). M.S.
acknowledges support from the co-funded Marie Skłodowska
Curie fellowship, H2020 grant agreement number 665667. We
thank the Swiss State Secretariat for Education, Research and
Innovation (SERI), CEPF Special Energy Fund for EPFL-
Sion, and Prof. Michael Grätzel for allowing to use LPI
facilities.
Table 4: Solar-cell performance parameters, extracted from J–V curves.
ID
Scan
JSC [mAcmÀ2
]
VOC [mV] FF
PCE [%]
direction
FB to SC 22.3
SC to FB 22.4
spiro-OMeTAD FB to SC 20.6
synthesized SC to FB 20.7
spiro-OMeTAD FB to SC 21.4
commercial SC to FB 21.7
1023
1004
1090
1080
1107
1073
0.77 17.8
0.74 16.8
0.75 17.4
0.72 16.3
0.76 18.4
0.77 18.4
KR216
Keywords: bifluorenylidene · electrochemistry ·
hole-transporting materials · photovoltaics · solar cells
How to cite: Angew. Chem. Int. Ed. 2016, 55, 7464–7468
Angew. Chem. 2016, 128, 7590–7594
measurements were taken at a slow scan rate of 10 mVsÀ1,
resembling quasi steady-state conditions.[23,24] The device with
KR216 as HTM possesses an open-circuit voltage (VOC) of
1023 mV, a short circuit current density (Jsc) of 22.3 mAcmÀ2,
and a fill factor (FF) of 0.77, yielding PCE of 17.8% for the
forward and 1004 mV, 22.4 mAcmÀ2, 0.74, 16.8% in backward
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Received: March 12, 2016
Revised: April 1, 2016
Published online: May 9, 2016
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