Non-Planar and Flexible Hole-Transporting Materials from Bis-Xanthene and Bis-Thioxanthene Units for Perovskite Solar Cells

Article abstract of DOI:10.1002/hlca.201900056

Two new hole-transporting materials (HTMs), BX-OMeTAD and BTX-OMeTAD, based on xanthene and thioxanthene units, respectively, and bearing p-methoxydiphenylamine peripheral groups, are presented for their use in perovskite solar cells (PSCs). The novelty of the newly designed molecules relies on the use of a single carbon-carbon bond ‘C?C’ as a linker between the two functionalized heterocycles, which increases the flexibility of the molecule compared with the more rigid structure of the widely used HTM spiro-OMeTAD. The new HTMs display a limited absorbance in the visible region, due to the lack of conjugation between the two molecular halves, and the chemical design used has a remarkably impact on the thermal properties when compared to spiro-OMeTAD. BX-OMeTAD and BTX-OMeTAD have been tested in ([(FAPbI₃)_0.87(MAPbBr₃)_0.13]_0.92[CsPbI₃]_0.08)-based PSC devices exhibiting power conversion efficiencies of 14.19 and 16.55 %, respectively. The efficiencies reached, although lower than those measured for spiro-OMeTAD (19.63 %), are good enough to consider the chemical strategy used as an interesting via to design HTMs for PSCs.

Full text of DOI:10.1002/hlca.201900056

HELVETICA
chimica acta
Accepted Article
Title: Non-Planar and Flexible Hole Transporting Materials from Bis-
Xanthene and Bis-Thioxanthene units for Perovskite Solar Cells
Authors: Javier Urieta-Mora, Inés García-Benito, Iwan Zimmermann,
Juan Aragó, Pedro D. García-Fernández, Giulia Grancini,
Agustín Molina-Ontoria, Enrique Ortí, Nazario Martin, and
Mohammad Khaja Nazeeruddin
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the VoR from the journal website shown below when it is published
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To be cited as: Helv. Chim. Acta 10.1002/hlca.201900056
Link to VoR: http://dx.doi.org/10.1002/hlca.201900056
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10.1002/hlca.201900056
Helvetica Chimica Acta
Non-Planar and Flexible Hole Transporting Materials from Bis-
Xanthene and Bis-Thioxanthene units for Perovskite Solar Cells
Javier Urieta-Mora,‡,a,b Inés García-Benito,‡,c Iwan Zimmermann,c Juan Aragó,d Pedro D. García-
Fernández,b Giulia Grancini,c Agustín Molina-Ontoria,b* Enrique Ortí,d* Nazario Martín,a,b*
Mohammad Khaja Nazeeruddinc*
a
Departamento Química Orgánica, Facultad C. C. Químicas, Universidad Complutense de Madrid, Av. Complutense s/n, 28040
Madrid (Spain). Homepage: http://www.ucm.es/info/fullerene/. E-mail: nazmar@ucm.es
b
IMDEA-Nanociencia, C/ Faraday 9, Ciudad Universitaria de Cantoblanco, 28049 Madrid, Spain. E-mail:
Agustin.molina@imdea.org
c Group for Molecular Engineering of Functional Materials and Laboratory for Photonics and Interfaces EPFL VALAIS, CH-1951
Sion, Switzerland. E-mail: mdkhaja.nazeeruddin@epfl.ch
d
Instituto de Ciencia Molecular, Universidad de Valencia, Catedrático José Beltrán 2, 46980 Paterna, Spain. Homepage:
http://www.molmattc.com/portal/. E-mail: enrique.orti@uv.es
‡ These authors have contributed equally to the work.
A special collection on the honor of François Diederich
Abstract
Two new hole-transporting materials (HTMs), BX-OMeTAD and BTX-OMeTAD,
based on xanthene and thioxanthene units, respectively, and bearing p-
methoxydiphenylamine peripheral groups are presented for their use in perovskite solar
cells (PSCs). The novelty of the newly designed molecules relies on the use of a single
carbon−carbon bond “C−C” as a linker between the two functionalized heterocycles,
which increases the flexibility of the molecule compared with the more rigid structure of
the widely used HTM spiro-OMeTAD. The new HTMs display a limited absorbance in
the visible region, due to the lack of conjugation between the two halves, and the chemical
design used has a remarkable impact on the thermal properties when compared to spiro-
OMeTAD. BX-OMeTAD and BTX-OMeTAD have been tested in
([(FAPbI3)0.87(MAPbBr3)0.13 0.92[CsPbI3]0.08)-based PSC devices exhibiting power
]
conversion efficiencies of 14.19 % and 16.55 %, respectively. The efficiencies reached,
although lower than those measured for spiro-OMeTAD (19.63%), are good enough to
consider the chemical strategy used as an interesting via to design HTMs for PSCs.
Introduction
Photovoltaic technologies have raised as one of the most promising alternatives for
harvesting energy in the search of a society based on non-fossil fuels. In this context,
organic–inorganic hybrid perovskite solar cells (PSCs), in which the perovskite has the
general formula ABX3 (A = cation, such as methylammonium (MA+), formamidinium
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(FA+) or cesium (Cs+); B = Pb2+; X = halide), have stepped into the spotlight of
photovoltaic devices since the groundbreaking discovery of Miyasaka and co-workers.[1]
In just a few years, efficiencies of PSCs have been boosted from the initial 3.8 % to 23.7
% certified[2] due to the outstanding properties of the perovskite,[3] such as long charge-
carriers diffusion lengths,[4-5] panchromatic light absorption with tunable band gap,[6-7]
high charge-carrier mobilities[8-9] and solution-processed fabrication. All these attractive
properties can be easily tuned by precisely engineering of the cations and/or halides in
the composition of the perovskite.[10-12] Taking advantage of its singular ambipolar
behavior, organic–inorganic perovskites can be sandwiched between n-type and/or p-type
semiconducting layers in planar or mesoporous architectures.[13] Generally, the best
performing devices incorporate small organic molecules[14-15] as hole-transporting
materials (HTMs) for an efficient extraction of the photogenerated holes, improving their
transport to the counter-electrode and acting as an energetic barrier to block the electron
migration to the cathode. Inorganic salts[16-18] or classical polymeric structures[19-21] can
also be used as HTMs although they present certain drawbacks such as batch-to-batch
reproducibility, difficult purification, and broad molar mass distribution. In this sense,
small organic molecules offer potential advantages such as well-defined molecular
weight, easy purification, and less batch-to-batch variations.
An extensive assortment of organic molecules have been reported with the ultimate goal
of outperforming the commonly used benchmark HTM 2,2’,7,7’-tetrakis (N,N-di-p-
methoxyphenylamine)-9,9’-spirobifluorene, namely spiro-OMeTAD. Unfortunately, the
preparation of the spirobifluorene central scaffold present in spiro-OMeTAD requires a
complicated synthetic protocol, which significantly limits its practical compatibility for
low-cost and large-scale photovoltaic applications.[22] Small organic molecules have
attracted special attention as HTMs because of their simple deposition processes (from
solution or vapor) and their molecular structure tunability. The design and synthesis of
HTMs have provided a wide variety of molecular structures helping to fine-tune their
properties; in particular, the optical and electrochemical properties. Despite this
versatility, a clear relationship between the molecular structure of the HTM and the
photovoltaic efficiency when used in PSCs has not been yet established. For this reason,
a wide variety of strategies have been described for the preparation of HTMs “a la carte”.
Among these strategies, the combination of large p-conjugated structures and electron-
rich fused heterocycles has resulted to be one of the most effective, favoring the
intermolecular p–p stacking and, therefore, improving the hole-carrier mobility.[23-30]
Conversely, spiro-like structures have also been extensively studied as organic scaffolds
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for mimicking the outstanding properties of spiro-OMeTAD, reaching values of up to
22.6 %,[15] due to the balance of their electronic and morphological properties and
solution processing.[31-33]
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
O
S
H3CO
H3CO
N
N
N
N
OCH3
OCH3
H3CO
H3CO
N
N
N
N
OCH3 H3CO
OCH3 H3CO
N
N
N
N
OCH3
OCH3
O
S
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
spiro-OMeTAD
BX-OMeTAD
BTX-OMeTAD
Figure 1. Chemical structures of BX-OMeTAD, BTX-OMeTAD and spiro-OMeTAD.
Following the aforementioned well-established strategies, herein we present the
photovoltaic performance of two new HTMs, in which the twisted spiranic carbon atom
between the two halves is replaced by a covalent ‘‘C–C’’ bond. BX-OMeTAD and BTX-
OMeTAD consist of two xanthene or thioxanthene units connected through sp3-
hybridized carbon atoms, rendering a more flexible conformation than those of spiro-like
structures (Figure 1). In addition to the detailed optical, electrochemical and thermal
properties of the new HTMs, rationalized with the help of density functional theory (DFT)
calculations, mesoporous PSCs were prepared in combination with the state-of-the-art
perovskite [(FAPbI3)0.87(MAPbBr3)0.13 0.92[CsPbI3]0.08. Power conversion efficiencies of
]
up to 16.55% are derived from mesoscopic PSCs using the new HTMs, which, although
lower than those obtained for the benchmark spiro-OMeTAD, are good enough for
practical applications.
O
O
Br
Br
Br
Br
b)
a)
X
X
X
X = O, S
1a, X = O
1b, X = S
2a, X = O
2b, X = S
OMe
OMe
c)
OMe
OMe
X
X
MeO
MeO
N
N
N
N
OMe
OMe
d)
N
N
MeO
X
OMe
3a, X = O
3b, X = S
OMe
OMe
BX-OMeTAD, X = O
BTX-OMeTAD, X = S
Scheme 1. Synthesis of BX-OMeTAD and BTX-OMeTAD. Reagents and conditions: a) Br2, AcOH,
.
120 °C, 1a: 69%, 1b: 74%; b) BH3 SMe2, THF, 66 °C, 2a: 75%, 2b: 92%; c) bis(4-
methoxyphenyl)amine, Pd2(dba)3, XPhos, NatBuO, toluene, 100 °C; d) n-BuLi, Fe(acac)3, THF, –78
°C to r.t, BX-OMeTAD: 52%, BTX-OMeTAD: 52%;
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Results and discussion
The preparation of the symmetric BX-OMeTAD and BTX-OMeTAD HTMs was carried
out by stepwise synthesis following well-established protocols under an inert atmosphere
(Scheme 1). The synthetic methodology along with the experimental details are given in
the Supplementary Information. Starting from xanthone or thioxanthone, a bromination
reaction provided the corresponding dibrominated intermediate in excellent yields. The
required xanthene and thioxanthene units were readily prepared upon reduction of the
.
carbonyl groups in the presence of borane dimethyl sulfide complex (BH3 SMe2),
followed by the Buchwald−Hartwig reaction with the p-methoxydiphenylamine. Finally,
homocoupling of the two halves was achieved by treating 3a and 3b with n-BuLi in the
presence of Fe(acac)3 in moderate yields. The structures of BX-OMeTAD and BTX-
OMeTAD and the corresponding intermediates were fully characterized by means of
1
standard spectroscopic techniques such as H and 13C nuclear magnetic resonance
(NMR), Fourier transform infrared (FTIR), and high-resolution MALDI-TOF (HR-MS)
techniques (see the Supporting Information for further details). As representative
spectroscopic features, both HTMs showed the corresponding aliphatic and aromatic
signals. The chemical structures of both 3a and 3b were ascertained by HR-MS.
The structural and electronic properties of the BX-OMeTAD and BTX-OMeTAD HTMs
were initially studied by performing density functional theory (DFT) calculations at the
B3LYP/6-31G** level in the presence of CH2Cl2 as solvent. The BX and BTX cores, the
pendant diphenylamine DPA moiety, and the reference spiro-OMeTAD compound were
also computed for comparison purposes (see the Supporting Information for full
computational details). Figure 2 displays the minimum-energy B3LYP/6-31G**-
optimized structures calculated for the BX and BTX cores. Similar structures are
computed for the BX-OMeTAD and BTX-OMeTAD HTMs (Figure S1), and Figure S2
shows the most representative bond lengths computed for both systems. The molecular
structure is determined by the xanthene and thioxanthene units, which are folded along
the middle axis of the central heterocyclic ring and point in different directions (up and
down) within the same molecule to minimize the steric interactions (Figure 2). The
deviation from planarity of the xanthene and thioxanthene moieties is characterized by
the folding angle α that defines the tilting of the lateral benzene rings, which preserve
their planarity, with respect to the average molecular plane. Angle α is calculated to have
a smaller value for the xanthene unit in the BX core (34º) than for the thioxanthene moiety
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10.1002/hlca.201900056
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in the BTX core (44º). The incorporation of the four DPA arms to the central cores
increases the folding of the xanthene and thioxanthene units and, compared to the pristine
cores, slightly higher values are computed for angle α (38º and 47º for BX-OMeTAD and
BTX-OMeTAD, respectively, Figure S1).
Figure 2. Top (left) and side (right) views of the B3LYP/6-31G**-optimized geometries calculated
in CH2Cl2 solution for the BX (a) and BTX (b) cores. The value of angle α characterizing the folding
of the xanthene and thioxanthene units is quoted.
The benzene rings condensed to the central heterocyclic ring both in the xanthene and in
thioxanthene units preserve their characteristic delocalized structural pattern with similar
carbon–carbon (CC) lengths in the 1.394−1.401 Å range (Figure S2). For both the BX
and BTX cores, the C(sp3)−C(sp3) bond linking the xanthene and thioxanthene halves is
calculated to have a long length of ~1.600 Å. The main difference in terms of bond lengths
between the BX and BTX cores is the lengthening of all bonds forming the central
heterocyclic ring as a consequence of the substitution of oxygen in BX by sulfur in BTX,
the largest difference being found for the carbon–sulfur bonds. For BX-OMeTAD and
BTX-OMeTAD, the insertion of the four DPA units hardly causes any significant change
in the structural shape and in the bond distances of the spiro-like cores. Regarding the
latter, a lengthening of the CC bonds to which the DPA groups are anchored is obtained
(Figure S2).
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Figure 3. Energy diagram displaying the frontier molecular orbitals computed at the B3LYP/6-31G**
level in CH2Cl2 for the DPA unit, the BX and BTX cores, the BX-OMeTAD and BTX-OMeTAD
HTMs and the spiro-OMeTAD reference compound.
Figure 3 displays the frontier molecular orbitals calculated for the DPA unit, the BX and
BTX cores, the BX-OMeTAD and BTX-OMeTAD HTMs and the spiro-OMeTAD
reference compound. The highest-occupied molecular orbital (HOMO) of the BX core,
computed at ‒5.84 eV, is noticeably lower in energy than the HOMO of DPA (‒4.72 eV)
indicating a weaker electron-donor character. The substitution of oxygen by sulfur in
BTX causes a slight but non-negligible destabilization of the HOMO level (‒5.69 eV).
For the four-armed BX-OMeTAD and BTX-OMeTAD HTMs, the insertion of the DPA
moieties results in a significant destabilization of the HOMO level, which is now
computed at ‒4.60 and ‒4.65 eV, respectively, thus approaching the HOMO level
calculated for spiro-OMeTAD (‒4.44 eV). The HOMO of both BX-OMeTAD and BTX-
OMeTAD spreads primarily over the central covalent carbon–carbon linkage cores with
contributions from the DPA moieties (mostly from the nitrogen atoms). It should be noted
that an important charge transfer occurs from the central cores, which hold a total net
charge of 0.81e and 0.73e for BX-OMeTAD and BTX-OMeTAD, respectively, to the
peripheral DPA groups. The electronic structure of the BX-OMeTAD and BTX-
OMeTAD HTMs is therefore predicted to be significantly polarized.
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Figure 4. a) Differential pulse voltammetry recorded in CH2Cl2 containing 0.1
M
tetrabutylammonium hexafluorophosphate (TBAPF6) at a scan rate of 100 mV s‒1. b) UV-vis
absorption spectra (solid line) and fluorescence emission spectra (dashed line) of spiro-OMeTAD,
BX-OMeTAD and BTX-OMeTAD in CH2Cl2.c) Thermogravimetric analysis curves recorded under
nitrogen at 10 °C min−1 of heating rate. d) Differential scanning calorimetry curves under nitrogen at
a heating rate of 20 °C min−1 (second cycle).
The electrochemical properties of the novel HTMs and spiro-OMeTAD were analyzed
by differential pulse voltammetry (DPV) (Figure 4a) in TBAPF6/CH2Cl2 at a scan rate of
100 mV s–1. The recorded oxidation potentials and the derived HOMO energies are
gathered in Table 1. BX-OMeTAD exhibits three oxidation waves centered at 0.83, 1.25
and 1.34 V versus normal hydrogen electrode (NHE). The change of the oxygen atom in
the central core by sulfur, resulted in a slightly increase of the donor ability (0.79, 1.21
and 1.30 V versus NHE). Spiro-OMeTAD also shows three oxidation waves centered at
0.70, 0.82 and 1.03 V, in which the first oxidation potential is cathodically shifted by 0.09
and 0.13 V in comparison with that of BTX-OMeTAD and BX-OMeTAD, respectively.
The estimated HOMO energies inferred from the first oxidation potential for BX-
OMeTAD, BTX-OMeTAD and spiro-OMeTAD are –5.27, –5.23 and –5.14 eV,
respectively. Therefore, the novel HTMs feature an excellent band alignment with the
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valence band edge of the triple cation [(FAPbI3)0.87(MAPbBr3)0.13 0.92
]
[CsPbI3]0.08
perovskite (ca. –5.7 eV).
Table 1. Electrochemical and optical properties of BX-OMeTAD, BTX-OMeTAD and spiro-OMeTAD.
HTM
Eox1/2 [V][a] EHOMO [eV][b]
λmax, abs [nm][c] λmax, em [nm][c] E0-0 [eV][d] ELUMO [eV][e]
BX-OMeTAD
BTX-OMeTAD
spiro-OMeTAD
0.83
0.79
0.70
‒5.27
‒5.23
‒5.14
309
310
386
414
403
419
3.24
3.32
3.05
‒2.03
‒1.91
‒2.11
[a] Determined from DPV measurements vs. normal hydrogen electrode (NHE). [b] EHOMO is estimated in eV by EHOMO = ‒4.44 eV ‒
Eox1/2. [c] λmax of absorption and emission measured in CH2Cl2 solutions. [d] E0-0 was determined at the intersection of normalized
absorption and emission spectra. [e] ELUMO was estimated by ELUMO = EHOMO + E0-0
.
To gain insight into the oxidation processes, the oxidized species (up to the tetracation)
of BX-OMeTAD and BTX-OMeTAD were calculated at the B3LYP/6-31G** level in
CH2Cl2. Table S2 gathers the charges accumulated by the constituting fragments (central
core and DPA units) as oxidation occurs. For BX-OMeTAD•+, the charge for the radical
cation species are drawn from the core (ca. 0.36e) and the DPA units (0.16e from each
unit). A slightly larger charge removal from the core (ca. 0.40e) is calculated for BTX-
OMeTAD•+. Upon further oxidation (dication, trication and tetracations), the charge is
gradually extracted from both the core and the four DPA units. For the tetracations, the
excess of positive charge is still more concentrated in the core (ca. +1.70e) than in the
DPA units (ca. +0.57e, Table S2). This indicates that the xanthene and thioxanthene cores
are directly involved in the oxidation processes.
To evaluate the capability of the BX-OMeTAD and BTX-OMeTAD systems as HTMs,
hole reorganization energies (λ) were estimated at the B3LYP/6-31G** level in gas phase
(see the Supporting Information for full computational details). The λ values calculated
for BX-OMeTAD and BTX-OMeTAD are 0.120 and 0.124 eV, which are notably
smaller than the values obtained for its constituting fragments (0,192 and 0.198 eV for
the BX and BTX cores, respectively, and 0.274 eV for the DPA units). The decrease in λ
results from to the fact that charge extraction involves both the core and the peripheral
DPA moieties. BX-OMeTAD and BTX-OMeTAD therefore exhibit low reorganization
energies similar to that computed for spiro-OMeTAD (0.139 eV).[25] The small λ values
together with the adequate energy level alignment with the valence band edge of the
perovskite indicate the potential of this family as HTMs for PSC devices.
The optical properties of BX-OMeTAD and BTX-OMeTAD were investigated in
solution as depicted in Figure 4b and compared with spiro-OMeTAD. Maximum
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absorption and emission wavelengths are listed in Table 1. The derived optical bandgaps
(E0–0) were estimated from the intersection of the absorption and emission bands. BX-
OMeTAD and BTX-OMeTAD show similar absorption patterns in dichloromethane
with an intense absorption band below 350 nm, which is broader for the latter. An asset
of these kind of systems as HTMs for PSCs is their lack of conjugation, which keeps the
visible range of the spectrum transparent, in contrast to the spiro-OMeTAD. The lack of
absorption in the visible region determines that the new HTMs exhibit relatively high
energies for the lowest energy 0–0 transition (E0–0), which ensures a good electron-
blocking capability. The emission spectra show broad bands with maximum emission
peaks centred at 414 and 403 nm for BX-OMeTAD and BTX-OMeTAD, respectively.
The optical absorption and fluorescence of spiro-OMeTAD are shifted towards higher
wavelengths compared to BX-OMeTAD and BTX-OMeTAD owing to the enhanced p-
conjugation of the spirocyclic central scaffold in spiro-OMeTAD.
The absorption spectra of BX-OMeTAD and BTX-OMeTAD was theoretically
simulated from the B3LYP/6-31G** time-dependent DFT (TDDFT) calculation of the
lowest-energy 80 singlet excited electronic states (Sn) in the presence of CH2Cl2 (Figure
S3) The theoretical spectra correctly reproduce the shape of the experimental spectra with
an intense band approximately centred at 310 and 314 nm for BX-OMeTAD and BTX-
OMeTAD, respectively, in perfect agreement with the experimental bands peaking at 309
and 310 nm, respectively. The simulated absorption bands come from a plethora of
electronic transitions close in energy and a simple assignment is not easy. Nevertheless,
a dominant electronic transition is predicted for both BX-OMeTAD (S0 → S10) and BTX-
OMeTAD (S0 → S7) (see Figure S3 and Table S1). These electronic transitions are of
π→π* nature, and imply excitations with a partial charge transfer from the central BX
and BTX moieties to the peripheral DPA units (Table S1 and Figure S4).
To elucidate the thermal stability of the pristine HTMs, thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) were performed (Figure 4c and 4d)
and compared with spiro-OMeTAD. Detailed thermal features are listed in Table 2. From
TGA, both HTMs possess relatively high decomposition temperatures (Tdec) of 334 and
360 °C (at a weigh loss of 1 %) for BX-OMeTAD and BTX-OMeTAD, respectively,
but significantly lower than that measured for spiro-OMeTAD (449 °C). The difference
in Tdec is due to the relatively more rigid structure of spiro-OMeTAD.[34] The
morphological study of the new compounds was conducted using DSC analysis. As show
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in Figure 4d, BX-OMeTAD shows a similar morphological behavior to that of spiro-
OMeTAD. During the first heating scan, both molecules exhibit a discernible glass
transition (Tg) and melting temperature (Tm) along with an additional crystallization
process (Tc) which is not observed upon cooling. After consecutive heating/cooling cycles
only rather small changes in the Tg are detected (Figures S21). On the other hand, BTX-
OMeTAD, displays a three-step thermal behavior with a crystallization process (Tc = 267
°C) and a sharp endothermic peak corresponding to a melting temperature of 313 °C. No
crystallization process was observed upon cooling and only a glass transition (Tg = 119
°C) was detected during the second heating scan.
Table 2. Thermal properties of BX-OMeTAD and BTX-OMeTAD in comparison with spiro-OMeTAD.
HTM
Tdec (ºC)[a]
Tm (ºC)[b]
Tc (ºC)[b]
Tg (ºC)[c]
BX-OMeTAD
BTX-OMeTAD
spiro-OMeTAD
334
280
190
132
-
360
313
267
449
247
174
131
[a] Decomposition temperature determined from TGA (1 % weight loss). [b] Melting temperature (Tm) and
[c]
crystallization temperature (Tc) determined from the first cycle of the DSC.
Glass transition
temperature (Tg) determined from the first cycle. All experiments were performed under N2 atmosphere.
The performance of the new HTMs, BX-OMeTAD and BTX-OMeTAD, was evaluated
in perovskite-based solar cells (PSCs) using the state-of-the-art triple cation perovskite
[(FAPbI3)0.87(MAPbBr3)0.13 0.92[CsPbI3]0.08 (Table 3). The relative position of the frontier
]
energy levels of the HTM materials with respect to those of the perovskite and the TiO2
and Au electrodes is illustrated in Figure 5a. Devices were fabricated from a stack of thin
films deposited onto a fluorine-doped tin oxide (FTO) conductive glass that serves as
support (see the Supporting Information for details). The layers forming the PSC device
were deposited as follows. First, as anode, a 30 nm blocking layer is applied by spray
pyrolysis onto the FTO followed by 250 nm mesoporous titania. Second, the perovskite
layer was spin-coated in a single-step by spinning the perovskite precursor solution at
2000 rpm for 12 s and at 5000 rpm for 30 s. After 15 s of the second step, 100 mL of
chlorobenzene was poured onto a rotating substrate to obtain a smooth perovskite layer
after annealing for 1 h at 100 °C. The composition of the perovskite layer is
[(FAPbI3)0.87(MAPbBr3)0.13 0.92[CsPbI3]0.08. Third, the HTM layer was deposited by spin-
]
coating a 20 mM solution of the BX-OMeTAD and BTX-OMeTAD derivatives in
chlorobenzene using tert-butylpyridine, Li-TFSI and FK-209 as dopants. The thickness
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of the HTM layer was optimized to be around 100 nm for both molecules. Finally, the
gold electrodes were thermally evaporated using a shadow mask. The individual forming-
layers of the PSC architecture can be seen from the cross-sectional scanning electron
microscopy (SEM) images as shown in Figure 5b and 5c. As a general trend, the
presented chemical structures seem to favor the formation of homogeneous thin films
with a good morphology. The current density-voltage (J−V) curves of the best-
performing conventional PSCs (reverse bias) using BX-OMeTAD and BTX-OMeTAD
as HTMs, under 1 sun AM 1.5G (100 mW cm–2) simulated sunlight, are shown in Figure
5d and their related photovoltaic parameters are collected in Table 3. Both HTMs were
compared with the reference spiro-OMeTAD. BTX-OMeTAD-based PSCs exhibit the
highest power conversion efficiency (PCE) up to 16.55 %, mainly arising from an
enhanced fill factor (FF) of 69.8% and a short-circuit current (Jsc) of 23.35 mA cm−2. On
the other hand, devices incorporating BX-OMeTAD yield a significantly lower PCE of
14.19 % with a Jsc of 22.93 mA cm−2, a FF of 61.3% and a Voc of 1.01 V with pronounced
hysteresis (see Figure S7 and S8).
Figure 5. a) Schematic energy levels diagram for the different layers of PSC including the new HTMs
and spiro-OMeTAD. b) Cross-section scanning electron microscopy (SEM) image of a PSC device
using BX-OMeTAD as HTM. c) Cross-section scanning electron microscopy (SEM) image of a PSC
device using BTX-OMeTAD as HTM. d) Current density−voltage (J−V) curves of BX-OMeTAD,
BTX-OMeTAD
and
spiro-OMeTAD
in
combination
with
[(FAPbI3)0.87(MAPbBr3)0.13 0.92
]
[CsPbI3]0.08-based devices. e) External quantum efficiency (EQE)
plots of BX-OMeTAD, BTX-OMeTAD and spiro-OMeTAD with corresponding integrated current.
f) Conductivity of BX-OMeTAD, BTX-OMeTAD and spiro-OMeTAD measured on substrates with
interdigitated gold electrodes with a channel length of 2.5 µm.
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10.1002/hlca.201900056
Helvetica Chimica Acta
The performance of PSCs incorporating spiro-OMeTAD as HTM exhibited a PCE up to
19.63 %. The photovoltaic behavior of the doped spiro-OMeTAD-based PSCs were in
good accordance with state-of-the-art results reported in the literature.[12]
The electrical conductivity of the HTMs thin films was measured in a lateral
configuration between 2.5 mm spaced gold contacts. Thin films of the new HTMs were
deposited onto substrates having interdigitated gold electrodes by using the same
procedure as for best device preparation, adding 6 mol% and 20 mol% of FK209 as a
dopant in BX-OMeTAD and BTX-OMeTAD, respectively. As it can be seen in Figure
5f, BX-OMeTAD and BTX-OMeTAD exhibit low conductivity values of 2.2 x 10-8 and
1.0 x 10-7 S cm-1, respectively, which remarkable lower than that of spiro-OMeTAD. The
low conductivity arising from these HTMs is most likely the reason of the low FF
obtained for their devices, resulting in high series resistance.
The short circuit currents experimentally found by J−V measurements match well with
the ones calculated from the integrated external quantum efficiency (EQE) measurements
shown in Figure 5e (± 5% mismatch).
Table 3. Device performance of PSC devices incorporating BX-OMeTAD and BTX-OMeTAD as HTMs
and the [(FAPbI3)0.87(MAPbBr3)0.13 0.92
]
[CsPbI3]0.08 perovskite. The figures of merit are compared with
those obtained for the reference spiro-OMeTAD.
HTM
Voc (mV) Jsc (mA cm‒2) FF (%) PCE (%)
BX-OMeTAD
BTX-OMeTAD
spiro-OMeTAD
1010
1015
1061
22.93
23.35
23.57
61.3
69.8
78.5
14.19
16.55
19.63
Conclusions
In summary, we have successfully designed and synthetized two new flexible, spiro-
OMeTAD analogues, as hole-transporting materials (HTMs) for perovskite solar cells
(PSCs). The new HTMs (BX-OMeTAD and BTX-OMeTAD) incorporate a covalently
linked bis-xanthene(BX) or bis-thioxanthene (BTX) core decorated with four peripheral
p-methoxydiphenylamine (PDA) units. The synthetic route toward their preparation
requires an easy and attainable four-step synthesis. Both HTMs exhibit a negligible
absorbance in the visible, owing to their lack of conjugation, and therefore useful for
tandem solar cells. They exhibit slightly deeper HOMO levels (~0.1 eV) compared to
spiro-OMeTAD, and can be used for an efficient hole-charrier extraction from the
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10.1002/hlca.201900056
Helvetica Chimica Acta
perovskite. The flexibility of their central core has a relevant impact on the thermal
properties when compared to the more rigid structure of spiro-OMeTAD. The novel
derivatives possess high thermal stability (up to 330 °C), good solubility and high LUMO
energy to block electrons and suitable HOMO level to extract holes from perovskite. The
new
compounds
were
employed
in
state-of-the-art
[(FAPbI3)0.87(MAPbBr3)0.13 0.92
]
[CsPbI3]0.08 perovskite solar cells and reached relatively
moderate performances, lower than those obtained for spiro-OMeTAD (19.63 %). Light-
to-energy conversion of up to 16.55 % and 14.19 % were obtained for BTX-OMeTAD
and BX-OMeTAD, respectively, with noticeable hysteresis in line with the poor
conductivity values. This work clearly reveals the importance of the molecular design in
HTMs for addressing the efficiency of the PSC. Interestingly, the presence of a more
flexible single C−C bond connecting the two halves of the HTM core has a strong impact
on the geometrical and electronic properties and, thereby, on the efficiency of the
fabricated PSC devices.
Conflicts of interest
There are no conflicts to declare.
Acknowledgments
We thank the Swiss National Funds for Scientific Research contract number
200020L_172929/1, Project “Tailored Design and in-depth understanding of perovskite
solar materials using in-house developed 3D/4D nanoscale ion-beam analysis”. Borun
New Material Technology generously supplied the high-quality spiro-OMeTAD. The
authors also thank the European Research Council (ERC-320441-Chirallcarbon), the
Spanish Ministry of Economy and Competitiveness MINECO (projects CTQ2017-
83531-R, CTQ2015-71154-P, CTQ2016-81911-REDT and Centro de Excelencia Severo
Ochoa SEV-2016-0686, CAM (FOTOCARBON project S2013/MIT-2841), the
Generalitat Valenciana (PROMETEO/2016/135 and SEJI/2018/035) and European Feder
funds (CTQ2015-71154-P). J.A. is grateful to MINECO for a “Ramon-y-Cajal”
fellowship (RyC-2017-23500). I.G.B and G.G. acknowledge the Swiss National Science
Foundation (SNSF) funding through the Ambizione Energy project Hyper (Grand
number PZENP2_173641).
Notes and references
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10.1002/hlca.201900056
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