A sequential condensation route as a versatile platform for low cost and efficient hole transport materials in perovskite solar cells

Article abstract of DOI:10.1039/c9ta05121j

In an effort to diminish the cost of perovskite solar cells (PSCs) with regard to hole transport materials (HTMs), we employed an easily attainable condensation route to synthesize a cheap and efficient HTM. Using a newly engineered small organic molecule

Full text of DOI:10.1039/c9ta05121j

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Sequential Condensation Route as a Versatile Platform for Low Cost and
Efficient Hole Transport Material in Perovskite Solar Cell
Babak Pashaei,a Hashem Shahroosvanda*, Mohsen Ameri,b Ezeddin Mohajeranib, Mohammad Khaja Nazeeruddinc*
aGroup for Molecular Engineering of Advanced Functional Materials (GMA), Chemistry Department, University of Zanjan,
Zanjan, Iran
b Photonics of Organic Materials and Polymers (POMP lab), LASER & Plasma research institute, Shahid Beheshti University, Iran
c
Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences and Engineering, Ecole polytechnique fédérale
de Lausanne, CH-1951 Sion, Switzerland
Abstract: in an effort to diminish the cost of perovskite
solar cells (PSCs) with regards to the hole transport
materials (HTMs), we employed an easily attainable
condensation route to synthesis a cheap and efficient
HTM. Using newly engineered small organic molecule,
N,N'-(naphthalene-1,5-diyl)bis(1-(2,3-
26 The total estimated material cost for 1 gram of spiro-OMeTAD
is about 92 $. Moreover, total estimated chemical waste for the
synthesis of 1 gram of spiro-OMeTAD is 3.6 (kg/g). Finally, the
commercial price for 1 gram of spiro-OMeTAD reach to 170-425
$.26-27 Besides the high price of spiro-OMeTAD, stability in PSC,
is the main limitation for commercialization.
diphenylquinoxalin-6-yl)-1-phenylmethanimine), coaded
as BEDN, the power conversion efficiency (PCE) reached
to 17.85%, comparable to the state-of-the-art HTM spiro-
OMeTAD (19.50%). The BEDN’s estimated cost is 1.38
($/g), which is considerably cheaper than spiro-OMeTAD,
92 ($/g). The low cost and high efficiency are promising
perspective in commercialization of perovskite solar cells.
The main strategy to synthesize small molecule HTMs is
employing palladium-catalyzed C−N cross-coupling using an
expensive catalyst such as Pd/BINAP.28-30 Therefore, the
actual promising way to reduce the cost of reaction pathway is
replacing of Pd catalyst with a cheap and efficient catalyst or
completely removing it. Interestingly, a large number of
condensation methods do not need to use any catalyst or hard
condition.31-32 Newly synthesized HTM based on the
condensation procedure is less prone to undergo undesirable
side reactions which seen in the corresponding catalyzed–base
reactions due to diminished yields or catalyst deactivation,
leading to achieving high product yield (> 80%). Therefore, the
specific property of newly synthesized HTM based on
condensation methods is the simplified synthesis process compared
to other HTM, which are known to date.
Despite the development of several newly synthesized small
organic HTMs based on C-N cross coupling for PSCs application,20
the finding of new strategies to facilitate the synthesize pathways
needs more efforts. Herein, we introduce an exceedingly cheap
PSC device using new HTM based on sequential condensation of
phenanthrene ligand. Only two sequent ketone-amine
condensations without adding any catalyst or additive at mild
condition were carried out to obtain BEDN with high purity and
product yield.
Introduction
The rapid development of organic/inorganic metal-halides
perovskite solar cells (PSCs) has amazed the solar cells research
community.1-2 Only after seven years from the beginning, more
than ten thousand researchers are currently being involved in PSCs
progress, worldwide.3 The rapid growth of efficiency, reaching
from about 3.8%4 to phenomenal value of 24.2%,5 in just a few
years from the discovery of the first perovskite nanocrystals as a
light absorber in DSSCs in 2009,4 has turned this field to a “gold
rush “6. PSCs have the potential to operate efficiently for
widespread deployment, but several obstacles still exist. Firstly,
achieving a reliable judgment about the long-term stability under
extensive light soaking at a temperature of 60 °C is seemingly a
challenging step.7-12 In recent years, a number of strategies and
protocols have been reported to address this issue in the near
future.13 Secondly, the environmental issues with the materials at
the heart of PSCs, namely lead metal, cannot be neglected.14 Hence,
fundamental understanding of the mechanism of degradation and
composition of employed materials could be useful to reach to
novel formulations and materials with the reduced cost and higher
Results and discussion
The synthetic routes of BEDN are shown in Scheme 1. The total
yield of BEDN is 60%, which was prepared by a straightforward
two-step condensation synthesis. BEDN was purified by
recrystallization and characterization (1H NMR, see the ESI†
Figures S1andS2).
efficiencies.15-16 17
. As of the essential requirement to fabricate an
efficient device is to minimize the charge recombination losses and
to control the trapping phenomena at the perovskite/hole transport
materials (HTMs) interface.18-19 Accordingly, the discovery of new
HTMs to employ in optoelectronic devices is still an open issue,
and an enormous improvement on the efficiency is achieved by
identifying novel HTMs including polymers, small organic
molecules, and inorganic materials.20 In this paradigm, small
organic molecules have remarkable advantages such as the facility
in the synthesis process with high yield and purity, which is of great
interest for industrial production.21 The most well-known organic-
HTM in PSCs is spiro-OMeTAD, which has recorded a high power
conversion efficiency (PCE) in the early days of PSC research.22
However, the big problem to the commercialization of spiro-
OMeTAD is its costly synthesis process with costly sublimation
steps for purification. In particular, six difficult steps of synthesis
and purification process is needed to obtain pure spiro-OMeTAD
along with highly cost Pd catalyst in inert atmosphere condition.23-
Ethanol
O
O
N
N
O
O
H2N
H2N
CH3COOH
90 o
C
DPDA
BED
N
N
N
CH3COOH
127 o
O
NH2
BEDN
N
N
C
N
N
N
NH2
0.5 mol
1 mol
BED
Scheme 1. Molecular structure and synthetic routes for BEDN.

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To get more insight about the activity of newly investigated
HTM, we compared the differences and similarities in all
photophysical, photochemical and photoelectrochemical properties
of BEDN with a reference spiro–OMeTAD. The UV-Vis spectra of
newly synthesized HTM exhibit similar absorption peaks as spiro–
OMeTAD as summarized in Table 1.
This result indicates that the new HTM presents a good
hydrophobicity and can be used as suitable HTM in PSCs. This
means that new HTM prevents water penetration to the perovskite
surface below the HTM, and thus does not destroy the perovskite,
which is highly sensitive to moisture. To address the film forming
ability and distribution of new HTM on the surface, FE-SEM of
glass/BEDN and glass/TiO2/BEDN was compared to spiro-
OMeTAD which shown that there is not any pin-hole or defect on
the coated layers (ESI-Figures S3-S5).
Figure 1. (a) UV-Vis and photoluminescence spectra of BEDN and spiro-
OMeTAD in CHCl3 (Inset: the zoomed image of the visible region of
absorption spectra of BEDN and spiro-OMeTAD). (b) Cyclic
voltammograms of BEDN and spiro-OMeTAD. (c) Differential scanning
calorimetry of BEDN and spiro-OMeTAD (Mentioned value in the image
is related to glass transition temperature (Tg)). (d) The contact angle of
BEDN.
Figure 2. Device architecture based on FTO/compactTiO2/meso-
TiO2/(FAPbI3)0.85(MAPbBr3)0.15/BEDN/Au. (b) The relative energy levels
in the PSCs based on BEDN and spiro-OMeTAD. (b) Cross-sectional SEM
image of PSCs devices based on BEDN.
The inset of Figure 1a depicts the absorption band of BEDN and
spiro-OMeTAD at the visible region, which shows that the ε of
BEDN is smaller than corresponding spiro–OMeTAD.33 The
photoluminescence (PL) of BEDN and spiro–OMeTAD recorded
in chloroform shows a strong band at 427 and a shoulder at 575 nm,
while spiro-OMeTAD exhibited only one emission band at 421 nm,
which is in agreement with earlier references.34 The
electrochemical behaviors of BEDN and spiro–OMeTAD have
been investigated by using cyclic voltammetry (CV) and BEDN
exhibited quasi-reversible oxidation/reduction processes in the
positive potential range (Figure 1b). The first oxidation half–wave
of BEDN was observed at 0.58 V vs Ag/AgCl which is 0.02 V
lower than spiro-OMeTAD, confirming that BEDN more easily
oxidized to BEDN+ than spiro–OMeTAD to spiro–OMeTAD+.35
UV-Vis, PL and CV spectra were employed to estimate the
HOMO, LUMO and band gap of newly investigated HTM, which
is summarized in Table 1. Differential scanning calorimetry (DSC)
measurements indicated that BEDN has higher 136 oC glass
The newly investigated small-molecule BEDN was tested as
HTMs in solution-processed PSC devices. A stack of thin layers
onto fluorine-doped tin oxide (FTO) coated glass was prepared that
serves as the backbone of device. The device was made as standard
configuration including TiO2-blocking layer/TiO2-mesoporous
layer as electron-transporting layer (ETM)/perovskite absorber
((FAPbI3)0.85(MAPbBr3)0.15) and the newly engineered molecule as
HTM.36 The solubility of BEDN in chlorobenzene (CB) is very
good which supports the uniformity of deposited HTM layer. To
enhance the conductivity of cells, a small amount of dopants was
used in both cases, BEDN and spiro-OMeTAD. For a purposeful
comparison, solar cells with spiro-OMeTAD based HTM and
without any HTM were also fabricated.
o
transition temperature (Tg ) than spiro-OMeTAD (Tg = 125 C;
Figure 1c),14 which confirms a higher thermal stability of BEDN
with respect to spiro. DSC also proved that BEDN starts
crystallization at 164 oC and spiro-OMeTAD at 158 oC, which are
both far above temperatures for conventional device operation. To
examine the hydrophobicity properties of BEDN, the water contact
angle measurement was carried out (Figure 1d). The water contact
angle for BEDN coated films was 83.2, as is shown in Figure 1d.
2

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Table 1. Spectroscopic and electrochemical data for BEDN and spiro-OMeTAD.
d)
c)
e)
f)
a)
b)
휆푎푏푠
휆푒푚
EOX
Tg
휂quenching
Hole Mobilityg)
Conductivityg)
(S cm-1)
Eo-o
EHOMO
(eV)
ELUMO
(eV)
HTM
(V)
0.58
0.6
(oC)
(cm2 V-1 s-1)
9.08 × 10-5
4.10 × 10-5
(eV)
3.14
3.04
(nm)
(nm)
427
421
BEDN
301, 357 (sh), 532 (sh)
304, 388, 525 (sh)
-5.18
-5.21
-2.02
-2.17
136
125
0.94
0.89
6.2 × 10-5
3.4 × 10-5
spiro-OMeTAD
a,b)InCHCl3 solutions (1×10-5 M).c)Eo-o was calculated from the intersection of absorption and emission spectra. e)From CV measurements,
E1/2 = 1/2(Epa+Epc); 0.1 M acetonitrile/tetrabutylammonium perchlorate (TBAP) versus Ag/AgCl at scan rate of 80 mV/s. e)EHOMO = -(E1/2
(vs. Fc/Fc+) + 4.8). f)ELUMO = EHOMO+ Eo–o g)Pure HTM without doping.
.
The schematic illustration and the cross-section scanning electron
microscopy (SEM) images (the different layers can be seen from
displayed in Figure 2b. The current density over the applied voltage
figure of PSC devices is shown in Figure 3a and the obtained data
are summarized in Table 2. Obviously, all photovoltaic
performances of PSC devices based on molecularly engineered
HTM are significantly improved compared to the PSC without
HTM, confirming the effective role of BEDN in device. On the
other hand, compared with spiro-OMeTAD, which exhibited a JSC
of 22.16 mA cm-2, VOC of 1.10 V and consequently PCE of 19.50%
which are close to the BEDN-based device with JSC of 20.64 mA
cm-2 and VOC of 1.03 V and PCE of 17.85%.
investigated by recording forward and reverse scanning of current
against applied voltage (Figure 3(a)). The results shown that the
forward scanning mode is slightly lower than reverse ones,
indicating the hysteresis phenomenon, which is about less than 1%
in both samples.
Figure 3b shows the incident photon-to-current efficiency (IPCE)
of the PSCs based on BEDN and spiro-OMeTAD as a function of
wavelength, indicating the good agreement between the JSC
obtained from the IPCE and J-V measurement. Obviously, the
IPCE from BEDN HTM is similar to spiro-OMeTAD, reaching
above to 85% conversion. However, as shown in the IPCE spectra,
there is a loss in photon conversion in the range of 550−750 nm for
the BEDN, which is not the case for spiro-OMeTAD. We attribute
this loss to the existence of traps and recombination sites at the
perovskite/HTL interface, compared to spiro-OMeTAD.37
Table 2. Summarized photovoltaic parameters derived from
current−voltage curves for the best samples.
VOC
[V]
1.03
1.03
1.10
1.10
JSC
[mA cm-2]
20.60
20.64
22.14
FF
[%]
79
84
76
PCE
[%]
16.76
17.85
18.51
19.50
HTMs
RS-BEDN
FS-BEDN
RS-spiro-OMeTAD
FS-spiro-OMeTAD
22.16
80
These results are particularly interesting since the PCE of PSC
based on BEDN was almost doubled compared to PSC without
HTM. PSCs for 20 samples of BEDN and spiro-OMeTAD in the
same condition were also fabricated (ESI.S2). The general trend in
PCE for both BEDN and spiro-OMeTAD was the same with the
maximum values ones.
Figure 6. Statistical distribution of perovskite solar cells performance with
BEDN and spiro-OMeTAD.
After observing the J-V and IPCE spectra, we hypothesized that
perhaps the reason for the difference in the photovoltaic
performance was due to inefficient hole transport.19 To test this
behavior, the dynamics of hole transfer from perovskite to HTM
was investigated by employing steady state PL of the
TiO2/perovskite and TiO2/perovskite/HTM. As shown in Figure 4a,
samples have shown a PL peak at 760 nm originated from the
perovskite (FAPbI3)0.85(MAPbBr3)0.15. When a BEDN is cast onto
the perovskite layer, the perovskite PL quantum yield of
perovskite/BEDN film was significantly decreased in respect to the
bare perovskite film.
(a)
(b)
Figure 3. J-V (a) and IPCE (b) curves of PSCs based on BEDN and spiro-
OMeTAD HTMs. Forward and reverse scanning of J-V is shown in Figure
3(a) by doted and solid lines, respectively.
In order to investigate reproducibility of PSCs based on BEDN, at
least 15 individual devices were fabricated using BEDN and spiro-
OMeTAD. Statistical distribution of the device performances is
shown in Figure 6. The average of performance of PSCs of the
devices with BEDN and spiro-OMeTAD are 16.05% and 17.60%,
respectively. The general trend in PCE for both BEDN and spiro-
OMeTAD was the same with the maximum values ones. The
hysteresis parameter of both BEDN and spiro-OMeTAD was
3

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(a)
(b)
Figure 4. (a) The PL of the substrate FTO/TiO2/HTMs and
FTO/TiO2/perovskite. (b) Time resolved PL spectra measured at a
wavelength near band gap that yields maximum PL signal upon
exciting pristine and BEDN and spiro-OMeTAD coated perovskite
samples at 405 nm.
Compared with the spiro-OMeTAD based cell, which acquires a
PL quenching efficiency (ηquench = 0.89), the BEDN-based cell
records a substantially higher ηquench = 0.94 (by 11%). The origin
of the quenching of PL spectrum of perovskite/BEDN can be
Figure 5. HOMO–LUMO molecular distribution, energy levels
and calculated oxidation potentials of the spiro-OMeTAD and
BEDN. ΔGox is the Gibbs free energy of oxidation.
attributed to charge carrier extraction across the interface between
25
BEDN and (FAPbI3)0.85(MAPbBr3)0.15
.
For
a
deeper
understanding of the charge transfer dynamics in the newly
investigated HTM, the time resolved photoluminescence
spectroscopy (TRPLs) was also performed. A bi-exponential decay
function was employed to fit the lifetime of the perovskite/HTM
because the TRPS shown a non-exponential decay. The value of τ2
of (FAPbI3)0.85(MAPbBr3)0.15, BEDN/(FAPbI3)0.85(MAPbBr3)0.15
and spiro-OMeTAD/(FAPbI3)0.85(MAPbBr3)0.15 is 16.20, 10.15 and
9.95 ns, respectively. From the results of steady-state PL and
TRPS, the difference of charge separation ability in BEDN and
spiro-OMeTAD is quite small.36 The electronic structures of the
spiro-OMeTAD and BEDN HTMs calculated by density functional
theory (DFT) are shown in Figure 5. As seen in the Figure, the
HOMO and LUMO of BEDN are separately localized on the
naphthalene core and the DPDA moieties, respectively. The BEDN
HOMO is stabilized slightly compared to spiro-OMeTAD, which
can be seen in their oxidation potential. The higher calculated
oxidation potential for BEDN (0.1 eV) relative to spiro-OMeTAD
(0.03 eV) indicates that the higher energy is required to oxidize the
molecule. This higher potential in BEDN is probably due to the
lack of repulsive interactions between the two molecular halves in
spiro-OMeTAD.33 Although the PCE of the BEDN HTM is not
higher than that of spiro-OMeTAD, the comparable efficiency still
supports that the BEDN is a promising HTM for PSC.
Charge transport properties is another important parameter to
consider in the design of new HTMs in PSCs. The hole mobility
values of the pristine HTMs were measured by a space-charge
limited currents (SCLCs) method according to literature12,38-41
(ESI†. Figure. S6,) and the data is listed in Table 1. The mobility
of spiro-OMeTAD is similar to the data reported in the literature,
indicating the reliability of the SCLC technic.42-43 The extracted
hole mobility for pristine BEDN is 9.08 × 10-5 cm2 V-1 S-1, which
is almost five times higher than that of spiro-OMeTAD (4.10 × 10-5
cm2 V-1 S-1). The high hole mobility and PCE of this new
engineered molecule make it good candidate to work in PSCs. The
conductivity of pristine BEDN is also slightly higher than that of
pristine spiro-OMeTAD (Table 1). The conductivity of BEDN after
addition additive was 1.2 × 10-4 S cm-1. The PSC based on dopant-
free BEDN presented the better performance in comparison with
dopant-free spiro-OMeTAD (ESI, Figure S8, Table S2). However,
in spite of showing high conductivity and hole mobility of BEDN
without additives, spiro-OMeTAD showed a slightly better
performance after the addition of LiTFSI and t-BP additives.
Furthermore, the durability of photovoltaic performances of
devices based on BEDN and spiro-OMeTAD was further probed
under standard conditions (Figure 7). The performances of devices
based on two HTMs decreased very slowly in 500 h. After this time,
BEDN and spiro-OMeTAD based devices retained 93% and 90%
of initial PCEs.
4

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tetrabutylammonium perchlorate (TBAP) in chloroform. The
current density–voltage (J–V) curves were measured using a solar
simulator (Newport, Oriel Class A, 91195A) with a source meter
(Keithley 2420) under 100 mA cm-2 illumination (AM 1.5G) and a
calibrated Si-reference cell certificated by NREL. The J–V curves
of all devices were measured by masking the active area with a
metal mask of area 0.096 cm2.
Synthesis
of
N,N'-(naphthalene-1,5-diyl)bis(1-(2,3-
diphenylquinoxalin-6-yl)-1-phenylmethanimine) (BEDN)
The synthesis of newly investigated HTM is based on simple
condensation between carbonyl and amine moieties without any
future column chromatography. First, BED precursor was prepared
from the condensation of benzil (420 mg, 2 mmol) with DPDA
(424mg, 2 mmol) in the mixture of 50 ml ethanol and 2 ml of acetic
acid. The mixture was refluxed for 4 h and then the yellow
precipitated solid was filtered and washed with cold ethanol to
remove the unreacted ligands. In following, the pure BED
precursor was achieved through the crystallization of product in
ethanol (700 mg, 90 %). Analysis data: 1H NMR (250 MHz,
CDCl3), 8.53 (s, 1H), 8.28 (s, 2H), 7. 7.89-7.92 (d, 2H), 7.26-7.64
(m, 13H). 13C NMR (250 MHz, CDCl3), 195 (C=O), 155, 154,
138, 132.81, 132.45, 130, 129.88, 129.86, 129.76, 129.16, 128.51,
128.37.
Figure 7. Changes with time of the photovoltaic performance parameters
of the PSC devices with BEDN and Spiro-OMeTAD.
Table 3 shown that the price of newly engineered HTM is much
lower than spiro-OMeTAD.23-26 (Please see supplementary, S2 for
more details).
Finally, condensation between synthesized BED (386 mg, 1 mmol)
and diamine naphthalene (78 mg, 0.5 mmol) in acetic acid for 12 h
produced yellow precipitated BEDN. After washing and re-
crystallization in acetic acid, pure BEDN was achieved (285 mg,
60 %). Analysis data: 1H NMR (250 MHz, CDCl3), 8.30 (d, 1H),
8.26 (s, 1H), 8.17 (d, 1H), 7.81-7.84 (d, 3H), 7.71 (t, 2H), 7.58-7.61
(m, 2H), 7.43-7.46 (m, 4H), 7.34-7.36 (m, 4H). 13C NMR (250
MHz, CDCl3), 195.43, 155.28, 154.73, 142.56, 139.89, 138.78,
138.38, 137.07, 133.56, 131.77, 130.19, 130.10, 129.88, 129.65,
129.52, 129.17, 128.55. CHN: Anal. calcd. For C64H42N6 (%): C,
85.882; H, 4.734; N, 9.391 Found (%): C, 85.889; H, 4.739; N,
9.398 ESI-MS: m/z 894.34, [M-H]+.
Table 3. Survey of estimated chemical synthesis cost and waste streams
for different HTMs.
HTM
BEDN
2
6
1.38
92
0.2
0.25 (0.0)
3.6 (1.0)
-
spiro-OMeTAD
39.46
170-425
Fabrication of Cell
Conclusion
Perovskite solar cells were fabricated on fluorine-doped tin oxide
(FTO) coated glass substrates. Part of the glass substrate coated
with FTO was etched with Zn powder and HCl 2 M ethanol
solution. Then, the substrates were washed carefully with distilled
water, detergent, acetone, ethanol, isopropanol and then treated
with an ultraviolet/O3 cleaner for 15 min. On these substrates, a
solution of HCl and titanium diisopropoxide bis(acetylacetonate)
in anhydrous ethanol was coated with spin-coating method at 2000
r.p.m. for 30 s. Then, the substrates were heated at 500 ℃ for 30
minutes and cool down to room temperature. Mesoporous TiO2
layer diluted in ethanol was deposited by spin-coating at 2000
r.p.m. for 10 s to achieve 300-400 nm thick layer. After that, the
substrates were sintered again at 500 ºC for 30 minutes. The PbI2
solution was coated on mesoporous TiO2 layer for 5 s at 6500 r.p.m.
and dried at 70 ℃. The mixed perovskite precursor solution was
prepared by dissolving PbI2 (1.15 M), FAI (1.10 M), PbBr2 (0.2 M),
and MABr (0.2 M) in a anhydrous solvent DMF:DMSO=4:1
(volume ratio). The perovskite solution was spin-coated in a two-
step procedure at 1,000 and 6,000 r.p.m. for 10 and 30 s
respectively. Chlorobenzene (110 μl) was dropped on the spinning
substrate at the 20 s in the second step and then films were annealed
at 100 °C for 90 min in the glove box. Following this step, the
BEDN and spiro-OMeTAD were deposited by spin-coating at 4000
r.p.m. for 20 s. The HTM solutions were prepared by dissolving the
HTM in chlorobenzene at a concentration of 78 mM, with the
In summary, to achieve a low cost HTM for using in efficient PSCs,
we successfully replaced the palladium-catalyzed C-N coupling
pathway with a simple sequential condensation route. Surprisingly,
this newly designed pathway included only two-step without any
purification, hard condition and catalyst. In particular, the PCE of
PSC based on molecularly engineered HTM is comparable with
benchmark spiro-OMeTAD, resulting in 17.85 and 19.50%,
respectively.
Experimental
All starting materials were purchased from Aldrich Chemical and
Merck Companies and used without further purification. 1H and 13
C
NMR spectra were recorded on Bruker Advance 250 MHz
spectrometers with CDCl3 and D6-DMSO as solvent. Chemical
shifts were calibrated against TMS as an internal standard. UV/Vis
and emission spectra were measured using an Ultrospec 3100 pro
spectrophotometer and AvaSpec-125 spectrophotometer in CHCl3
solution. Eo-o obtained using interception of absorption and
emission spectra. The Electrochemical studies were accomplished
by using SAMA500 potentiostat electrochemical analyzer with
conventional three electrode cell, a Pt disk as the working
electrode, a Pt wire as the counter electrode. Ag/AgCl was used as
the reference electrode and the supporting electrolyte was 0.1 M
5

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addition of 18 µL LiTFSI (from a stock solution in acetonitrile with
concentration of 1.0 M), 29 µL of tert-butyl pyridine (from a stock
solution in chlorobenzene with concentration of 1.0 M). Finally, a
80 nm Au electrode was deposited by thermal evaporation under
high vacuum.33,44
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and A. Hagfeldt, Energy Environ. Sci., 2016, 9, 1989-1997.
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and M. Grätzel, Adv. Funct. Mater., 2014, 24, 3250-3258.
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Chem. A, 2014, 2, 17115-17121.
Computational method details
The ground-state geometries were fully optimized using density
functional theory (DFT) with the B3LYP hybrid functional at the
basis set level of 6-31G*, and the frontier molecular orbitals were
drawn using an isovalue of 0.03 a.u. All calculations were
performed using Gaussian 09 package in the PowerLeader
workstation. The molecular orbitals were visualized using Gauss
View 5.0.8.
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M. Herz and H. J. Snaith, Energy Environ. Sci., 2014, 7, 982-988.
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J. Phys. Chem. Lett., 2015, 6, 1543-1547.
PL quenching efficiency
PL quenching efficiency was calculated using the following
formula:
푃퐿푏푎푟푒 ― 푃퐿푞푢푒푛푐ℎ
휂푞푢푒푛푐ℎ
=
푃퐿푏푎푟푒
Where PL bare and PL quench are integrated PL intensities of
perovskite on sapphire substrates without and with the quenching
HTM layer.
15. Z. Hawash, L. K. Ono, S. R. Raga, M. V. Lee and Y. Qi, Chem.
Mater., 2015, 27, 562-569.
Synthesis cost estimation of 1 gram BEDN
16. L. K. Ono, S. R. Raga, M. Remeika, A. J. Winchester, A. Gabe
and Y. Qi, J. Mater. Chem. A, 2015, 3, 15451-15456.
17. T. A. Berhe, W.-N. Su, C.-H. Chen, C.-J. Pan, J.-H. Cheng, H.-
M. Chen, M.-C. Tsai, L.-Y. Chen, A. A. Dubale and B.-J. Hwang,
Energy Environ. Sci., 2016, 9, 323-356.
We estimated the synthesis cost of 1 gram BEDN according to the
cost models of Pablo et al.24 and Osedach et al.23 The price of the
materials used has been obtained from Merck, Sigma Aldrich,
DeJong companies. We compared the price of 1 gram of this new
HTM with the price of 1 gram of spiro-OMeTAD, which is reported
in the literature (Please see the ESI, Table S1).33,45
18. M. Grätzel, Nat. Mater., 2014, 13, 838.
19. Y. H. Lee, J. Luo, R. Humphry‐Baker, P. Gao, M. Grätzel and
M. K. Nazeeruddin, Adv. Funct. Mater., 2015, 25, 3925-3933.
20. C. Rodríguez-Seco, L. Cabau, A. Vidal-Ferran and E.
Palomares, Acc. Chem. Res., 2018, 51, 869-880.
Conductivity Measurements
21. A. Magomedov, S. Paek, P. Gratia, E. Kasparavicius, M.
Daskeviciene, E. Kamarauskas, A. Gruodis, V. Jankauskas, K.
Kantminiene and K. T. Cho, Adv. Funct. Mater., 2018, 28,
1704351.
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and S. I. Seok, J. Am. Chem. Soc., 2013, 135, 19087-19090.
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Sci., 2013, 6, 711-718.
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Chem. A, 2015, 3, 12159-12162.
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Paek, V. Jankauskas and M. K. Nazeeruddin, Angew. Chem., 2016,
128, 7590-7594.
Glass substrates without conductive layer were cleaned carefully
with detergents, deionized water, acetone and ethanol, respectively.
Then to remove remaining organic residues, the substrates were
sintered at 500 ºC. A thin layer of nanoporous TiO2 was coated on
the glass substrates by spin-coating with a diluted TiO2 paste (PST-
20T) with ethanol (1:3, mass ratio). After that, TiO2 film was
sintered in the oven at 500 ° C. Then a solution of HTM in
chlorobenzene (concentrations similar to photovoltaic devices) was
deposited by spin-coating on TiO2 layer. Finally, an Au layer was
deposited on top of the HTM layer by thermal evaporation.
Mobility Measurements
26. A. J. Huckaba, P. Sanghyun, G. Grancini, E. Bastola, C. K.
Taek, L. Younghui, K. P. Bhandari, C. Ballif, R. J. Ellingson and
M. K. Nazeeruddin, ChemistrySelect, 2016, 1, 5316-5319.
27. A. T. Murray, J. M. Frost, C. H. Hendon, C. D. Molloy, D. R.
Carbery and A. Walsh, Chem. Commun., 2015, 51, 8935-8938.
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1267.
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1157.
30. P. Ruiz-Castillo and S. L. Buchwald, Chem. Rev., 2016, 116,
12564-12649.
The hole mobility of HTMs has been investigated according to
literature.39-40 The devices of BEDN and spiro-OMeTAD were
fabricated with the structure ITO/PEDOT:PSS/HTM/Au. The hole
mobility values were calculated using the Mott–Gurney law.41
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