Molecular engineering of face-on oriented dopant-free hole transporting material for perovskite solar cells with 19% PCE

Article abstract of DOI:10.1039/c7ta01718a

Through judicious molecular engineering, novel dopant-free star-shaped D-π-A type hole transporting materials coded KR355, KR321, and KR353 were systematically designed, synthesized and characterized. KR321 has been revealed to form a particular face-on organization on perovskite films favoring vertical charge carrier transport and for the first time, we show that this particular molecular stacking feature resulted in a power conversion efficiency over 19% in combination with mixed-perovskite (FAPbI₃)_0.85(MAPbBr₃)_0.15. The obtained 19% efficiency using a pristine hole transporting layer without any chemical additives or doping is the highest, establishing that the molecular engineering of a planar donor core, π-spacer and periphery acceptor leads to high mobility, and the design provides useful insight into the synthesis of next-generation HTMs for perovskite solar cells and optoelectronic applications.

Full text of DOI:10.1039/c7ta01718a

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K. Nazeeruddin, J. Mater. Chem. A, 2017, DOI: 10.1039/C7TA01718A.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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DOI: 10.1039/C7TA01718A
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Molecular Engineering of Face-on Oriented Dopant-free Hole
Transporting Material for 19% Perovskite Solar Cells
Kasparas Rakstys,‡a Sanghyun Paek,‡a Peng Gao,a Paul Gratia,a Tomasz Marszalek,b Giulia Grancini,a
Kyung Taek Cho,a Kristijonas Genevicius,c Vygintas Jankauskas,c Wojciech Pisula,b and Mohammad
Khaja Nazeeruddin*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Through judicious molecular engineering, novel dopant-free star- only very few candidates reached PCE values close to or
shaped D-π-A type hole transporting materials coded KR355, exceeding 20%.10-15 However, although PSCs have achieved high
KR321, and KR353 were systematically designed, synthesized and PCE values, the stability remains an issue due to dopant-
characterized. The KR321 has been revealed to form a particular induced degradation of PSC.16-18
face-on organization on perovskite film favoring the vertical charge Traditionally, the hole transporting layer (HTL) for PSCs is
carrier transport and for the first time, we show this particular heavily doped with bis(trifluoromethane)sulfonimide lithium
molecular stacking feature resulted in
efficiency over 19% in combination with mixed-perovskite yl)-4-tert-butylpyridine)cobalt(III)
(FAPbI3)0.85(MAPbBr3)0.15 The obtained 19% efficiency using (FK209). TBP is commonly used as HTL morphology controller,
a
power conversion salt (LiTFSI), 4-tert-butylpyridine (TBP), and tris(2-(1H-pyrazol-1-
tri[hexafluorophosphate]
.
pristine hole transporting layer without any chemical additives or while LiTFSI and FK209 provide the necessary electrical
doping is the highest, establishing that the molecular engineering conductivity.19-20 However, the use of additives is problematic,
of a planar donor core and periphery acceptor leads to high since hygroscopic nature of lithium salt makes the HTL highly
mobility, and the design provides useful insight towards the hydrophilic and Co(III) dopant tends to chemical degradation,
synthesis of next-generation HTM’s for perovskite solar cells and negatively influencing the stability of the entire device.21-22
optoelectronic applications.
Therefore, a promising solution for stabilizing PSCs is the
appropriate choice of dopant-free HTMs. However, the PCE of
pristine HTL based devices are consistently lying around 10%,
with only very few examples over 15%.23-29 Therefore,
development of dopant-free HTMs with both enhanced
moisture resistance and charge transporting properties is
desired to probe their structure-performance correlations
towards the realization of stable and high-efficiency PSCs.
In this work, we have systematically engineered three novel
dopant-free star-shaped donor – π-bridge – acceptor (D–π–A)
type HTMs. Such molecules feature a planar triazatruxene
central core (D), inducing π-stacking for vertical hole
conduction, thiophene-based multiple conjugated arms (π) and
malononitrile (A). Due to strong intermolecular interaction, the
molecules have a great potential to show high charge carrier
mobility, minimizing Ohmic losses of the contact. Moreover,
they combine the advantages of both small molecules, i.e. well-
defined structures and polymers like good thermal,
electrochemical and photochemical stability, together with high
solubility, proper wetting on the perovskite.30 All the three
molecularly engineered HTMs have been applied in PSCs and for
the first time, we show that a highly ordered characteristic face-
on organization could favor vertical charge carrier transport in
Hybrid lead halide perovskite-based solar cells (PSCs) have
attracted significant attention in the photovoltaics due to
inexpensive precursors, simple fabrication methods, and
remarkably high power conversion efficiency (PCE), which
already exceeds commercialized polycrystalline silicon solar
cells.1-8 Typical PSC configuration composes of an electron
transporting material TiO2 (ETM), infiltrated with the perovskite
absorbing material and coated with a hole transporting material
(HTM), which plays an important role facilitating the extraction
and transportation of holes from perovskite to the
corresponding contact and is essential to reach high light-to-
current conversion efficiency.9 To date, a great number of new
promising molecular organic HTMs have been reported, but
a.Group for Molecular Engineering of Functional Materials, Institute of Chemical
Sciences and Engineering, École Polytechnique Fédérale de Lausanne, CH-1951
Sion, Switzerland.
b.Organisch-Chemisches Institut, Ruprecht-Karls-Universitt, Im Neuenheimer Feld
270, 69120 Heidelberg, Germany.
c. Department of Solid State Electronics, Vilnius University, Sauletekio 3, Vilnius
10222, Lithuania.
‡ These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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the perovskite solar cell and a PCE over 19% with improved
stability was achieved using KR321
View Article Online
DOI: 10.1039/C7TA01718A
.
The general synthesis scheme for the preparation of 2,2',2''-
(((5,10,15-trihexyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-
c]carbazole-3,8,13-triyl)tris(4-hexylthiophene-5,2-
diyl))tris(methanylylidene))trimalononitrile KR355
,
2,2',2''-
(((5,10,15-trihexyl-10,15-dihydro-5H-diindolo[3,2-a:3',2'-
c]carbazole-3,8,13-triyl)tris(3,3''-dihexyl-[2,2':5',2''-
terthiophene]-5'',5-diyl))tris(methanylylidene))trimalononitrile
KR321
and
2,2',2''-(((5,10,15-trihexyl-10,15-dihydro-5H-
diindolo[3,2-a:3',2'-c]carbazole-3,8,13-triyl)tris(4,4-dihexyl-4H-
cyclopenta[2,1-b:3,4-b']dithiophene-6,2-
diyl))tris(methanylylidene))trimalononitrile KR353 is shown in
Figure 1.
Figure 2. UV-Vis absorption (solid line) and photoluminescence
(dashed line) spectra normalized at the peak value (left); and
cyclic volammograms of triazatruxene-based HTMs (right).
The triazatruxene donor and malononitrile acceptor groups
were preserved throughout the series, while the π-bridge was
modulated with 3-hexylthiophene, 3,3''-dihexyl-2,2':5',2''-
The optimized geometries for the new HTMs, along with the
HOMO and LUMO pictograms are presented in Figure S7. In all
three molecules, the HOMO orbitals are mostly developed in
the central triazatruxene core along with some thiophene
substituents. After three thiophene rings, the space extension
of the total HOMO is almost negligible. The extension of the
LUMO orbitals mainly in the arms and localized around the 2-
methylenemalononitrile. All calculated energy level values are
in close agreement with those obtained experimentally.
The normalized UV-Vis absorption and photoluminescence (PL)
spectra of new compounds in DCM solutions are shown in
Figure 2. Typically, for the dipolar D-π-A type molecules, sharp
charge-transfer absorption bands were found in the visible
region, with the peak maxima centered at 487, 515 and 569 nm
terthiophene
and
4,4-dihexyl-4H-cyclopenta[2,1-b:3,4-
b']dithiophene, respectively. The synthesis begins with the
construction of functional conjugated arms on triazatruxene
core by Suzuki cross-coupling reaction of desired building
blocks. After formylation with Vilsmeier complex, the aldehyde-
terminated derivatives were converted to final low-bandgap
chromophoric HTMs with malononitrile through the
Knoevenagel condensation reaction, successfully forming
electron-accepting moieties. All detailed synthetic procedures
are described in the supporting information.
for KR355, KR321, and KR353, respectively. Expectedly, KR355
and KR321 have an extra π–π* transition induced absorption
bands in UV region. Photoluminescence spectra show that all
molecules have very large Stokes shifts of around 200 nm
suggesting significant changes in the geometrical configuration
of the molecules upon excitation. The optical bandgap (Eg) is
estimated from the intersection of the corresponding
normalized absorbance and photoluminescence spectra. It is
known that reducing the bandgap of a semiconductor can
enhance the intrinsic electrical conductivity by increasing the
carrier concentration.31 Eg values of 2.13, 2.05 and 1.96 eV were
determined for KR355
HOMO energies of KR355
,
KR321, and KR353, respectively. The
KR321 and KR353 were measured to
,
lie at -5.55, -5.24 and -5.34 eV, respectively, by the cyclic
voltammetry (CV). Solid-state ionization potential (IP) was
measured by the electron photoemission in the air on the thin
films (Figure S12), and measured IP values are fully in agreement
with the HOMO levels, -5.52, -5.18 and -5.38 eV, respectively.
The oxidation potential variation can be attributed to the
changes in the length of the π-bridge moiety. These values are
in
alignment
with
photoactive
perovskite
layer
(FAPbI3)0.85(MAPbBr3)0.15 having a valence band at -5.65 eV and
should favor efficient photogenerated charge transfer at the
interface. Also, the determined LUMO values below -4 eV
should effectively block electron transfer from perovskite to the
HTMs. All optical and electrochemical properties are
summarized in Table 1.
Figure 1. Synthesis route for star-shaped D-π-A HTMs. (a)
Pd(PPh3)4, 2 M aq. K2CO3, THF, 80 °C; (b) POCl3/DMF, DCE, 0 °C–
reflux; (c) CH2(CN)2, Et3N, DCM, RT.
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Table 1. Optical and electrochemical properties of synthesized to patterns with only one isotropic intercolumnar peak (d-
View Article Online
DOI: 10.1039/C7TA01718A
HTMs.
spacing of the first peak is 2.27 nm for KR353 and 2.02 nm for
KR355, Figure S10). The decrease in order and lack of vertical
surface alignment of KR353 and KR355 could lead to the decline
of their device performance.
λem
(nm)a
688
EHOMO
(eV)b
-5.55
-5.24
-5.34
Eg
ELUMO
(eV)d
-3.42
-3.19
-3.38
μ (cm2
V−1 s−1)e
5.0 × 10-7
2.6 × 10-4
1.1 × 10-5
ID
λabs (nm)a
(eV)c
2.13
2.05
1.96
Charge transport properties (Figure S13, Table 1) of the novel
derivatives were studied using space-charge limited current
(SCLC) regime by fabricating hole-only devices on
ITO/PEDOT:PSS/HTM/Au architecture. The calculated vertical
hole mobility value of KR321 is 2.6·10-4 cm2 V−1 s−1, which is one
order of magnitude higher than that of KR353 (μ = 1.1·10-5 cm2
V−1 s−1) and three orders of magnitude higher than KR355 (μ =
5.0·10-7 cm2 V−1 s−1), respectively, indicating improved charge
hopping properties through face-on oriented columnar stacks.
The gain insight into the interface processes and in particular on
the hole transfer at the perovskite/HTM interface, we monitor
the steady-state photoluminescence (PL). Figure S8 shows the
comparison of the PL spectra between the pristine perovskite
and the perovskite interfaced with spiro-OMeTAD as well as
with the series of new molecules presented. The perovskite
samples have a comparable thickness of 300 nm. This enables
us to retain a constant density of absorbed photons for all the
samples investigated. The CW PL spectra have been registered
upon excitation at 650 nm; that enabled to selectively excite the
perovskite and not the HTM. The perovskite/HTMs shows a
reduction of the PL signal with respect to the pristine perovskite
film. This suggests that interfacial hole transfer happens and
quenches the PL signal. In particular, the KR321 shows a similar
quenching as observed for spiro-OMeTAD.
KR355
KR321
KR353
365, 487
401, 515
569
710
721
aMeasured in DCM solution. bMeasured in DCM/tetra-n-
butylammonium hexafluorophosphate (0.1 M) 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 (NHE) by addition of +0.624 V and -
c
4.44 eV to the vacuum, respectively. Estimated from the intersection
d
of the normalized absorbance and emission spectra. Calculated from
ELUMO = EHOMO + Eg. eMeasured by space-charge limited current (SCLC)
regime and fitted using Mott-Gurney law.
It is known that the charge within active layers of a thin film
solar cell will preferentially transfer through vertical direction.
At the same time, C3h symmetrical molecules have the
advantage to form face-on stacking and columnar geometry on
the surface, which will favor vertical charge transport along the
π-π stacking direction. To prove the concept, the
supramolecular organization of the new HTMs on the silica
substrate surface was determined by Grazing-Incidence Wide-
Angle X-ray Scattering (GIWAXS). The corresponding patterns in
Figures 3 and S10 indicate significant differences in self-
assembly between the compounds. The highest order of the
series and a distinct surface arrangement has been found only
for KR321. Interestingly, KR321 shows an ideally characteristic
face-on organization with columnar stacks standing up the
surface as illustrated in the inset of Figure 3a. For this molecular
arrangement, an intensive π-stacking reflection appears out-of-
plane of the pattern and is related to a d-spacing of 0.38 nm.
Further, we found in-plane scattering intensities correspond to
the rectangular lattice of the intercolumnar organization of
KR321 (lattice parameters of a = 6.04 nm and c = 1.46 nm). The
narrow azimuthal intensity distribution of these reflections
suggests a pronounced out-of-plane surface alignment of the
columnar stacks (Figure 3b). The observed face-on orientation
is expected to favor the vertical charge carrier transport and
improve the solar cell efficiency in comparison to KR353 and
KR355. (Figure S10) An identical enhancement in solar cell
performance was observed for face-on arranged donor-
acceptor polymers.32 However, the alignment mechanisms for
organic semiconductors during solution deposition is still under
discussion. One hypothesis is that low aggregation and the
existence of monomeric species in the solution results in a face-
on organization, while aggregates are arranged edge-on in the
film.33,34 The molecular structure plays a fundamental role in the
solution aggregation and hence for the surface ordering.
Normally a face-on orientation was achieved for various small
molecular weight disc-shaped liquid crystalline molecules by
cooling from their isotropic melt between two surfaces.35-37 In
To demonstrate the function of the novel compounds as
dopant-free HTMs, we prepared PSCs with mixed perovskite
+
absorber (FAPbI3)0.85(MAPbBr3)0.15 (MA: CH3NH3 , FA:
+
NH=CHNH3 ). The solar cells preparation is fully described in
supporting information. Figure S11 displays the cross-section
image of the PSC containing KR321, analyzed by field-emission
scanning electron microscope. The device is made by 700 nm
perovskite atop 200 nm thick mesoporous TiO2 layer, which was
deposited on FTO glass coated with 50 nm compact TiO2. The
device is completed by 70 nm thick HTM and 80 nm gold as back
contact.
Figure 3. a) GIWAXS pattern of KR321 film coated from
tetrachloroethane on silica wafer (schematic illustration of the
molecular surface arrangement) and b) azimuthal integration of
the π-stacking and intercolumnar reflections (star indicates
scattering at the beam stop).
contrast to KR321, KR353 and KR355 are poorly ordered leading
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devices were maintained at the maximum powerVpieowiAnrtticdleuOrnilninge
DOI: 10.1039/C7TA01718A
aging and the current-voltage curve was recorded automatically
every 2 h. The efficiency of devices initially decreased in early
time decay. Similar dynamics have been recently observed
demonstrating that a rapid degradation mechanism is activated
by metal electrode migration through HTMs and contacting the
perovskite layer.38,39 The general trend showed significantly
improved durability of the device prepared with dopant-free
KR321, maintaining 60% of its initial PCE after 650 h, while the
PCE of devices with doped spiro-OMeTAD dropped by 80%
under identical conditions.
Figure 4. Current-voltage curves of novel dopant-free HTMs and
doped spiro-OMeTAD as reference (left), IPCE spectra of the
devices (right).
The current density-voltage (J-V) characteristics under the
AM1.5G irradiation at 100 mW cm-2 for the champion PSCs
using dopant-free HTMs are shown in Figure 4, and the
corresponding device output parameters are summarized in
Table 2. PCE histograms are shown in Figure S15 in the
supporting information. The device with dopant-free KR321 as
HTM, which was measured to have ideal columnar stacks
standing up the surface showed an excellent PCE of 19%. The
device exhibited an open-circuit voltage (VOC) of 1.13 V, a short-
circuit current density (JSC) of 21.7 mAcm−2, and a fill factor (FF)
of 0.78, indicating the identical photovoltaic performance as
with heavily doped spiro-OMeTAD reference. In contrast,
devices using dopant-free KR353 and KR355 only yield very low
PCEs of 14.87% and 8.8%, respectively, attributed to the
significantly decreased Jsc and FF, most likely due to the lower
lying LUMO level leading to poor electron blocking and greater
charge recombination.
Figure 5. Normalized maximum power point tracking of
perovskite solar cells prepared in a single experiment, using
KR321 and spiro-OMeTAD as HTMs. The measurement was
performed under UV-filtered simulated sunlight in an argon
atmosphere without any encapsulation for 650 h.
Table 2. Photovoltaic performance of the devices based on
Conclusions
KR321
, KR353, KR355 and spiro-OMeTAD under AM1.5G
illumination (100 mW cm-2).
To conclude, we have successfully synthesized three
symmetrical dopant-free hole transporting materials based on
D-π-A type architecture. For the first time, we show that the
face-on formed columnar stacks of HTM molecules is benefiting
charge transfer within a perovskite solar cell. The optimized
structure of KR321 showed highly ordered characteristic face-
on organization leading to increased vertical charge carrier
transport and a power conversion efficiency over 19% with
improved stability. This result is on par with the heavily doped
spiro-OMeTAD reference, clearly showing the importance of
proper molecular engineering and the great perspective of
dopant-free HTMs in perovskite solar cells and outperforms
most of the other dopant-free HTMs reported to date.
ID
Jsc (mA/cm2)
Voc (V)
1.05
FF
PCE (%)
8.88
KR355
16.01
21.70
19.31
22.25
0.53
0.78
0.69
0.76
KR321
1.13
19.03
14.87
19.01
KR353
1.11
spiro-OMeTAD
1.12
This is also fully in agreement with the result of 2D-WAX
measurement, proving that the advantage of ordered face-on
stacking on the charge transport and hence the device
performance. To show the impact of the hysteresis on device
performance, we reported the J-V traces collected by scanning
the applied voltage at 0.01 Vs-1 from forward bias (FB) to short
circuit (SC) and the other way around (Figure S14). The incident
photon-to-electron conversion efficiency (IPCE) spectra with
integrated JSC values is shown in Figure 4. The integrated
photocurrents calculated from the overlap integral of the IPCE
Acknowledgements
The authors acknowledge financial support from SNSF NRP 70
project; number: 407040_154056, CTI 15864.2 PFNM-NM,
Solaronix, Aubonne, Switzerland, the European Commission
H2020-ICT-2014-1, SOLEDLIGHT project, grant agreement N°:
643791 and the Swiss State Secretariat for Education, Research
and Innovation (SERI) and NPRP award [NPRP 6-175-2-070]
from the Qatar National Research Fund. We thank Manual
Tschumi for designing the stability measurement system.
spectra are 21.2, 18.4, 16.6, and 22.4 mAcm-2 for KR321
, KR353,
KR355 and spiro-OMeTAD, respectively, and are consistent with
those obtained from the experimental J-V measurements.
In Figure 5, the maximum power point tracking (MPO) using
pristine KR321 and doped spiro-OMeTAD layers for hole
transport in corresponding perovskite devices is shown. During
the measurement, unsealed devices were kept in argon
ambience under a constant illumination of 100 mW cm-2. The
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28 F. Zhang, X. Zhao, C. Yi, D. Bi, X. Bi, P. Wei, X. Liu, S. Wang, X.
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