Facile synthesis of a hole transporting material with a silafluorene core for efficient mesoscopic CH3NH3PbI3 perovskite solar cells

Article abstract of DOI:10.1039/c6ta01776b

A novel electron-rich small-molecule, 4,4′-(5,5-dihexyl-5H-dibenzo[b,d]silole-3,7-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (S101), containing silafluorene as the core with arylamine side groups, has been synthesized via a short efficient route. When S101 was incorporated into a CH₃NH₃PbI₃ perovskite solar cell as a hole transporting material (HTM), a short circuit photocurrent density (J_sc) of 18.9 mA cm^-2, an open circuit voltage (V_oc) of 0.92 V, and a fill factor (FF) of 0.65 contributing to an overall power conversion efficiency (PCE) of ～11% which is comparable to the PCE obtained using the current state-of-the-art HTM 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) (η = 12.3%) were obtained. S101 is thus a promising HTM with the potential to replace the expensive spiro-OMeTAD due to its comparable performance and much simpler and less expensive synthesis route.

Full text of DOI:10.1039/c6ta01776b

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DOI: 10.1039/C6TA01776B.
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Facile Synthesis of Hole Transporting Material with Silafluorene
Core for Efficient Mesoscopic CH NH PbI Perovskite Solar Cells
3
3
3
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
d
b
e
d
Anurag Krishna, Dharani Sabba, Jun Yin, Annalisa Bruno, Liisa J. Antila, Cesare Soci, Subodh
b,c
b,c
Mhaisalkar, * Andrew C. Grimsdale
*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
novel electron-rich small-molecule, 4,4'-(5,5-dihexyl-5H- Snaith et al. [1], the growth in efficiencies has been
dibenzo[b,d]silole-3,7-diyl)bis(N,N-bis(4-methoxyphenyl)aniline unprecedentedly rapid so that a certified PCE of 20.1% has
S101), containing silafluorene as core with arylamine side groups, recently been reported [8-10]. A typical solid state perovskite
has been synthesized via short efficient route. When solar cell comprises a nanocrystalline mesoporous TiO anode,
incorporated into a CH NH PbI perovskite solar cell as a hole a perovskite absorbing layer, a hole transporting layer and a
transporting material (HTM), a short circuit photocurrent density metallic counter electrode. In addition to the quality of the
(
a
2
3
3
3
-
2
(Jsc) of 18.9mAcm , an open circuit voltage (Voc) of 0.92 V, a fill perovskite film, the performance of perovskite solar cells is
factor (FF) of 0.65 contributing to an overall power conversion also strongly affected by the use of charge transporting layers,
efficiency (PCE) of ~11% was obtained which is comparable to the with high efficiencies only being obtained when using an HTM.
PCE obtained using the current state-of-the-art HTM 2,2’,7,7’- In spite of the high price of spiro-OMeTAD, which is due to its
tetrakis(N,N’-di-p-methoxyphenylamine)-
9,9’-spirobifluorene multistep low yielding synthesis and difficult and expensive
(
Spiro-OMeTAD) (η=12.3%). S101 is thus a promising HTM with the purification process, it is the most widely used HTM for
potential to replace the expensive spiro-OMeTAD due to its perovskite based solar cells, as it has produced the highest
comparable performance and much simpler and less expensive device efficiencies to date. However, there has been a
synthesis route.
concerted research effort to design and synthesize alternative
HTMs, which are more economical and show device
performance comparable to spiro-OMeTAD [4,11]. Several
solution processable conjugated polymers including P3HT, and
poly(triarylamine) (PTAA) have been employed as HTMs,
however their device results to date have been markedly
inferior to that of spiro-OMeTAD [12]. A number of small
molecule HTMs based on ethylenedioxythiophene [13],
cruciform oligothiophenes [14], pyrene [15], triptycene [16],
triazines [17], conjugated quinolizino acridine [18], carbazole
In recent years, organic–inorganic metal halide perovskites, in
3 3 3
particular methyammonium lead iodide (CH NH PbI ) have
emerged as very promising light harvesting materials for high
performance and low cost photovoltaic technology [1-3]. The
methyammonium lead halide perovskites have properties such
as excellent ambipolar mobility, large exciton diffusion lengths,
high extinction coefficients, direct bandgap and excellent
absorption throughout the UV-Vis-NIR spectrum which makes
them excellent light harvesting materials [4-7]. Since the
reports of an efficient solid state photovoltaic device using a
perovskite as light absorber by Grätzel, Park et al. [2] and
[
[
19], planar triphenylamine [20] furan [21], silolothiophene
22], methoxydiphenylamine-substituted carbazole [23],
tetrathiafulvalene [24], phenoxazine [25], po-spiro-OMeTAD
26], spiro-thiophene [27], [2,2] paracyclophane [28] and
[
ethene-tetraarylamine [29] cores have also been reported. The
best performing HTMs show similar or superior performance
to spiro-OMeTAD in some aspects but no material has yet
emerged clearly superior in all aspects of device performance,
particularly in terms of cost, device efficiency and stability.
Hence it remains a key challenge to make simple, cost effective
and high performing HTMs.
The present study reports a novel HTM S101 (Fig. 1) based on
an electron rich silafluorene core. Silole and its derivatives
have been widely used in organic electronics because of their
excellent properties including good redox stability in air, and
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high charge carrier mobilities [30-32]. In this work an electron that S101 is also a promising HTM with potentially relatively high
rich silafluorene is used as core, to which are linked hole- hole mobility.
accepting triphenylamine moieties while hexyl chains are Together with its predicted material properties, the short relatively
attached to the silicon atom to produce high solubility in efficient synthesis route of S101 makes it an attractive HTM for any
organic solvents and so improve processability. The S101 potential perovskite solar cell industry. The synthesis route of S101
molecule has planar structure which is desirable for promoting (Scheme 1) is shorter and less expensive as compared to spiro-
intramolecular π-delocalization and intermolecular π-π OMeTAD (see Scheme S1 and Table S4 in ESI for analysis of relative
stacking, which favour both, charge carrier mobility and synthesis costs). The commercially available relatively inexpensive
lifetime, boosting device efficiency [33]. Silafluorene core 3,7-dibromo-5,5-dihexyl-5H-dibenzo[b,d]silole was Suzuki coupled
based materials also has been used in organic light emitting with a triphenylamine boronic acid derivative [35] to give S101 in
diodes [34]. S101 was synthesized from
a relatively 45% yield from an unoptimised reaction. NMR spectroscopy and
inexpensive commercially available Silafluorene derivative in a MALDI-TOF mass spectrometry confirmed the molecular structure
single step with an acceptable yield (45%). In addition to of S101.
extensive investigation of its material properties, we have also
investigated the performance of S101 in mesoscopic
perovskite solar cells in terms of I-V characteristics and charge
carrier dynamics. The devices fabricated using S101
demonstrated efficiency ~11% which is comparable to the
values obtained using the state-of-the-art HTM spiro-OMeTAD.
Scheme 1. Synthetic route for S101.
Fig. 1 shows the chemical structure of the S101 and spiro-OMeTAD
together with the electronic density distribution of their frontier
orbitals calculated by density functional theory (DFT). For S101, the
highest occupied molecular orbital (HOMO) level shows π-bonding
character, spreading over the whole molecule; while the lowest
unoccupied molecular orbital (LUMO) shows π*-antibonding Fig. 1(a) Chemical structure of S101, electronic density distribution of lowest
unoccupied molecular orbital (LUMO) and highest occupied molecular
character being predominately localized on the central silafluorene
orbital (HOMO) for S101; (b) chemical structure of spiro-OMeTAD, electronic
density distribution of LUMO and HOMO for spiro-OMeTAD.
and two benzene units. The calculated HOMO energy of S101 (-4.49
eV) is quite close to that of spiro-OMeTAD (-4.48 eV), which would
promote effective hole transfer from perovskite to S101.
The optoelectronic properties of S101 have been systematically
The hole transfer rate of hole-transporting materials can be
investigated and compared with those of spiro-OMeTAD. In the UV-
described by the Marcus-Hush equation which is dominated by two
Vis absorption spectra (shown in Fig. 2a), absorption peaks of S101
important parameters, namely, (1) reorganization energy and (2)
and spiro-OMeTAD were observed at 381 nm and 384 nm,
electronic coupling, which can be obtained from DFT calculations
respectively. The absorption onset wavelengths of S101 and spiro-
(
(
see the Computational Methods in Supporting Information). Fig. S1
supporting information) demonstrates the predicted crystal
OMeTAD are 434 nm and 420 nm, respectively which correspond to
band gaps of 2.86 eV and 2.95 eV, respectively. The onset of
absorption in S101 is slightly red shifted with respect to spiro-
OMeTAD which can be attributed to the more extended
conjugation in the backbone of S101 due to the presence of two
extra phenyl rings between the nitrogens. From the cyclic
voltammograms of S101 and spiro-OMeTAD, shown in Fig. 2b, the
pair of redox peaks for S101 is observed to be highly reversible,
showing that it has excellent electrochemical stability. The HOMO
energy level was calculated from the CV data using the following
structures of S101 and spiro-OMeTAD with all possible hole-
hopping pathways. The molecular structure of S101, with a planar
silafluorene core, has much smaller intermolecular distances,
resulting in larger electronic coupling between HOMO levels of
neighbouring molecules, which can improve the hole transporting
mobility (see Table S1 and S2 in ESI). S101 shows smaller hole
reorganization energy (0.13 eV) than spiro-OMeTAD (0.15 eV) and
predicted hole mobilities for the simulated charge transport
2
pathways (0.215, 0.141, and 0.049 cm /V·s) which are one order of
+
equation: EHOMO= -5.1-(Eox,HTM vs. Fc/Fc ) (eV), where Eox,HTM vs.
magnitude higher than those of spiro-OMeTAD (0.0058, 0.0046, and
+
Fc/Fc is the onset of oxidation potential of ferrocene, which is used
2
0
.0532 cm /V·s). These values are comparable with our previously
as reference and -5.1eV is the redox potential of ferrocene [36]. The
HOMO levels of S101 and spiro-OMeTAD as calculated from CV are
synthesized high performing furan-based HTM F101 [21], showing
2
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5
.32 eV and 5.15 eV, respectively while the reported HOMO energy
level of CH NH PbI is 5.44 eV [2] which indicates that S101 has
3
3
3
favourable energetics for the hole transfer (Fig. 2c). The energetics
of the system employing S101/spiro-OMeTAD are represented in
Fig. 2d and Table 1 summarizes the optical and electrochemical
properties of the two HTMs. Differential scanning calorimetry (DSC)
showed S101 has a weak glass transition (T
g
) at 45 °C and a melting
= 124 °C, T = 245 °C
point (T ) of 143 °C (see Figure S5 in ESI); cf. T
m
g
m
for spiro-OMeTAD. The cross-sectional scanning electron
microscopy (SEM) images shown in Figure 3d displays the various
3 3 3
interfaces of the S101 HTM based CH NH PbI solar cell indicating
we have achieved formation of a well-defined hybrid structure with
clear interfaces. From the FE-SEM image the thicknesses of the
TiO , perovskite and the HTM layers can be estimated to be ~350,
2
5
50 and 200 nm, respectively. To investigate the charge-carrier
mobility of S101 and spiro-OMeTAD, Field effect transistors were
fabricated. To determine the charge carrier mobility, field effect
transistors (FET) were fabricated using spiro-OMeTAD and S101.
Hole mobilities evaluated from FET transfer characteristics (shown
Fig. 3 Current density vs. voltage curve under AM 1.5G illuminations (100
-2
mWcm ).
-4
2
in Figure S6 in ESI) are 1.2 x 10 cm /Vs (at V = -50 V; V = - 40 V) for
d
g
-5
2
The current–voltage (J–V) characteristics of perovskite solar cells
employing S101 and spiro-OMeTAD as HTMs, together with devices
without HTM are shown in Fig. 3 and summarized in Table 2. The
PCEs of the best performing device with S101 and spiro-OMeTAD as
HTM are 11% and 12.3% respectively. Our study also showed that
the device fabricated without the HTM had a PCE of only around 4.7
spiro-OMeTAD, and 7.2 x 10 cm /Vs (at V = -50 V; V = - 40 V) for
d
g
S101. Contrary to the predictions, Spiro-OMeTAD thus has slightly
higher field-effect mobility than S101, which suggests that spiro-
OMeTAD based devices should show slightly higher fill factors.
%
, which confirmed that a HTM must be an integral component of
the high efficiency perovskite solar cells. The short circuit current
densities (J ) of the cells fabricated using spiro-OMeTAD and S101
sc
2
2
are 19.7 mA/cm and 18.9 mA/cm , respectively. The slightly higher
current using spiro-OMTAD can be attributed to its higher HOMO
level as compared to S101, which gives spiro-OMeTAD a higher
driving force for charge transfer from the perovskite to the HTM.
The devices fabricated from both HTMs exhibit similar fill factors.
The devices with S101 showed a slightly lower open circuit voltage
(
Voc = 0.92V) than devices employing spiro-OMeTAD (Voc = 0.975V).
However, from our CV data, it is observed that HOMO energy level
for S101 is deeper than spiro-OMeTAD which should result in higher
Voc. As the Voc depends on (1) the splitting of the Fermi levels for
photogenerated charges, (2) the recombination, and (3) the
energetics of the devices [15, 16, 21], the slightly lower voltage in
devices fabricated with S101 as HTM might be due to high
recombination which suggests S101 has slightly inferior charge
extraction capability as compared to spiro-OMeTAD.
Fig 2(a) Absorption spectra for spiro-OMeTAD and S101, (b) cyclic
voltammograms for spiro-OMeTAD and S101, c) band diagram for the device
with new HTM S101 and spiro-OMeTAD, and d) cross-sectional SEM image Table 2. Summary of device parameters; open circuit voltage (Voc), current
2
of device fabricated with S101.
density (Jsc), fill factor (FF) and efficiency (η). Area=0.2 cm .
2
a
HTM
J
SC (mA/cm )
V
OC(V)
FF(%)
ηmax (%)
ηavg (%)
Table 1. Optical and electrochemical properties of S101 and spiro-OMeTAD.
S101
18.9
19.7
8.54
0.92
0.975
0.76
65
64.2
72
11
12.3
4.7
10.6
12.0
4.3
a
E
b
λ
max
λ
onset
g
E
HOMO
E
LUMO
Spiro-OMeTAD
W/O HTM
HTM
(nm)
(nm)
(eV)
(eV)
(eV)
S101
381
384
434
420
2.86
2.95
-5.32
-5.15
-2.46
-2.2
[
a] Average PCE is based on six cells in a single batch.
Spiro-OMeTAD
[
a] Optical band gap (E
g
) obtained from the onset value of absorption (λonset). [b]
To further elucidate the low Voc obtained from S101 devices, the
charge-carrier dynamics at the perovskite/ HTM interface and hole
LUMO calculated by LUMO = HOMO + E
g
.
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extraction ability were investigated using steady-state and time- means S101 has potential to replace the more expensive spiro-
resolved photoluminescence (PL) measurements, in which the OMeTAD. This work also lays out a strategy for the design and
efficient quenching of the steady-state PL and the reduction of the development of new cost effective and efficient HTMs for
PL lifetime are indicators of efficient charge extraction at the PSCs.
perovskite/HTM interface. From the PL spectra (Fig. 4a), it is
evident that both HTMs significantly quench perovskite emission Acknowledgments
signal, with HTM spiro-OMeTAD having a slightly better PL
quenching efficiency (ca. 93%) relative to S101 (ca. 89%). Time- This work was funded by National Research Foundation (NRF)
Singapore (CRP Award no.: NRF-CRP4-2008-03 and NRF–CRP14–
014–03 , Singapore Ministry of Education (MOE2013-T2-044) and
the Singapore-Berkeley Research Initiative for Sustainable Energy
SinBeRISE) CREATE Programme. Authors would like to thank Dr.
resolved PL measurements were carried out with excitation at 404
2
nm and monitoring the entire emission spectral range (Fig. 4b). The
pristine perovskite (CH
about 2.7 ns, whereas this was shorter in CH
samples. CH NH PbI /spiro-OMeTAD samples showed a decay time
3
NH
3
PbI
3
) exhibited a radiative life time of
(
3
NH PbI /HTM
3
3
Francesco Maddalena for the helping in fabricating FETs.
3
3
3
of 0.8 ns whereas CH NH PbI /S101 samples showed a longer decay
3
3
3
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Journal of Materials Chemistry A
Page 6 of 6
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DOI: 10.1039/C6TA01776B
Facile Synthesis of Hole Transporting Material with Silafluorene Core for Efficient
3 3 3
Mesoscopic CH NH PbI Perovskite Solar Cells
a
b
d
b
e
d
Anurag Krishna, Dharani Sabba, Jun Yin, Annalisa Bruno, Liisa J. Antila, Cesare Soci, Subodh
b,c
b,c
Mhaisalkar, * Andrew C. Grimsdale *
A power conversion efficiency of 11% has been obtained from a perovskite solar cell using a new
silafluorene-based hole transporting material made by a short efficient synthesis.