9,9′-bifluorenylidene derivatives as novel hole-transporting materials for potential photovoltaic applications

Article abstract of DOI:10.1016/j.dyepig.2019.108031

Novel 9,9′-bifluorenylidene derivatives were designed to study the effect of alkyl chain length on selected physical properties. The structure of the synthesized compounds was confirmed by using NMR spectroscopy (¹H, ¹³C, H–H COSY, H–C HMQC, H–C HMBC) and elemental analysis. They showed high thermal stability and undergo decomposition in the range of 388–400 °C. As was revealed by DSC investigations, they can be converted from crystalline to amorphous materials with relatively high glass transition temperature. The replacement of the alkyl chains from ethyl to butyl resulted in a significant negative impact on melting and glass transition temperatures. The synthesized derivatives undergo reversible electrochemical oxidation and reduction and showed a very low energy band gap (1.47 and 1.79 eV). They intensively absorb the light up 550 nm and also exhibited a week absorption band in the range of 550–750 nm. Their hole transporting ability was tested in perovskite solar cells. Additionally, the effect of the doping concentration of Li⁺ on photovoltaic device performance for these compounds was investigated. It should be stressed found that 9,9′-bifluorenylidene derivative substituted with ethyl units applied as hole transporting materials in perovskite solar cells demonstrated the highest device efficiency of 7.33% higher than of the spiro-OMeTAD utilized for preparation of the reference cell (4.40%).

Full text of DOI:10.1016/j.dyepig.2019.108031

Journal Pre-proof
9,9′-bifluorenylidene derivatives as novel hole-transporting materials for potential
photovoltaic applications
Slawomir Kula, Agnieszka Pająk, Agata Szlapa-Kula, Angelika Mieszczanin, Paweł
Gnida, Marek Lipiński, Ewa Schab-Balcerzak
PII:
S0143-7208(19)32071-6
DOI:
https://doi.org/10.1016/j.dyepig.2019.108031
DYPI 108031
Reference:
To appear in: Dyes and Pigments
Received Date: 2 September 2019
Revised Date: 5 November 2019
Accepted Date: 6 November 2019
Please cite this article as: Kula S, Pająk A, Szlapa-Kula A, Mieszczanin A, Gnida Paweł, Lipiński M,
Schab-Balcerzak E, 9,9′-bifluorenylidene derivatives as novel hole-transporting materials for potential
photovoltaic applications, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.108031.
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9
,9’-Bifluorenylidene derivatives as novel hole-transporting materials for potential
photovoltaic applications
a*
a,b
a
a
Slawomir Kula , Agnieszka Pająk , Agata Szlapa-Kula , Angelika Mieszczanin ,
c
b
a,c*
Paweł Gnida , Marek Lipiński , Ewa Schab-Balcerzak
a
Institute of Chemistry, Faculty of Mathematics, Physics and Chemistry, University of Silesia, Szkolna 9 St., 40-
07 Katowice, Poland
0
b
Institute of Metallurgy and Materials Science, Polish Academy of Sciences, Reymonta 25 St., 30-059 Krakow,
Poland
Centre of Polymer and Carbon Materials, Polish Academy of Sciences,34 M. Curie-Sklodowska Str., 41-819
Zabrze, Poland
c
Abstract: Novel 9,9’-bifluorenylidene derivatives were designed to study the effect of alkyl
chain length on selected physical properties. The structure of the synthesized compounds was
1
13
confirmed by using NMR spectroscopy ( H, C, H-H COSY, H-C HMQC, H-C HMBC) and
elemental analysis. They showed high thermal stability and undergo decomposition in the
range of 388-400°C. As was revealed by DSC investigations, they can be converted from
crystalline to amorphous materials with relatively high glass transition temperature. The
replacement of the alkyl chains from ethyl to butyl resulted in a significant negative impact on
melting and glass transition temperatures. The synthesized derivatives undergo reversible
electrochemical oxidation and reduction and showed a very low energy band gap (1.47 and
1
.79 eV). They intensively absorb the light up 550 nm and also exhibited a week absorption
band in the range of 550-750 nm. Their hole transporting ability was tested in perovskite solar
+
cells. Additionally, the effect of the doping concentration of Li on photovoltaic device
performance for these compounds was investigated. It should be stressed found that 9,9’-
bifluorenylidene derivative substituted with ethyl units applied as hole transporting materials
in perovskite solar cells demonstrated the highest device efficiency of 7.33 % higher than of
the spiro-OMeTAD utilized for preparation of the reference cell (4.40 %).
Keywords: 9,9’-bifluorenylidene derivatives, Buchwald-Hardwig reaction, electrochemistry,
holes transporting materials, perovskite solar cells (PSC)
1

1
. Introduction
Intensive technological development in the world causes a rapid increase in demand for
electricity. The result is an increase in electricity prices, which affects many aspects of human
life. Nowadays, most of the methods used to generate electricity are based on scarce natural
resources (coal, gas and oil). These methods largely expose the natural environment to
overexploitation and pollution. On the other hand, nuclear energy raises concerns of people
living in the vicinity of the power plant. That is why renewable energy sources are an
extremely interesting alternative. Among them, new generation photovoltaic (PV) cells
deserve special attention. They allow the production of electricity without significant
interference in the natural environment. Examples of such devices are perovskite (PSC) [1-10]
and dye-sensitized solar cells (DSSC) [11-20]. In recent years, these types of PV devices have
achieved high power conversion efficiency (PCE) and PCE of PSC and DSSC has reached
2
4% [2] and 14% [17], respectively. However, in their case, further, development is necessary
for the field of materials creating the active layers of cells. Many examples of inorganic
9,12,16,21-23] and organic [3,6,8-10,12-13,16-17,19] chemical compounds investigated in
[
the aspect of materials used to construct cells have been described in the literature. Special
attention is paid for low molecular weight organic compounds due to many advantages such
as efficient and repeatable synthesis, high purity even in industrial conditions, the well-
defined structure that can be confirmed by many methods and possibility to modify properties
(optical, electrochemical and even solubility) by changing functional groups or replacement
of substituents [3,6,8,10,12-13,17]. Among them, organic molecules that possess hole
transportation ability - p-type organic semiconductors, so called hole transporting materials
(HTMs) receive much attention for applications in PV technology. They play a significant
role in the regeneration of the oxidized state light absorbers and transportation of the holes to
the counter-electrode both in DSSC and PCS [24-25]. Thus, HTMs can be applied as solid-
state electrolyte instead of the liquid electrolyte used in the DSSC which can generate
potential leakage problems limited to this technology. In 1998, first time, p-type
semiconductor,
that
is
2,2’,7,7’-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9’-
spirobifluorene known as Spiro-OMeTAD successfully replaced the conventional liquid
electrolyte as the hole-transporting conductor [26]. In turn, in 2012, Spiro-OMeTAD was
utilized with the perovskite resulted in a great breakthrough in PCE of PSC [27-28]. To date,
Spiro-OMeTAD is the most popular organic HTM used in PSC [6-10,29] and DSSC
technology [24,30]. Unfortunately, it requires a multi-step synthesis [29,31], which
significantly affects its price (about 1g/$450), and thus the cost of photovoltaic devices. Thus,
2

many research groups focus on the development of the new HTMs. As a cheaper alternative
to Spiro-OMeTAD, 2,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-bifluorenylidene
has been presented [32]. PSC cells with such compound used as HTM exhibited high power
conversion efficiency (PCE) 17.8%. Inspired by this result we have undertaken a synthesis
and investigation of two new 9,9’-bifluorenylidene derivatives, such as 2,2’,7,7’-tetrakis(N,N-
di-p-ethoxyphenylamine)-9,9'-bifluorenylidene (M-1) and 2,2’,7,7’-tetrakis(N,N-di-p-
butoxyphenylamine)-9,9'-bifluorenylidene (M-2) toward the replacement of the popular spiro-
OMeTAD. The compounds were designed to meet general HTM requirements such as
appropriate energy level in relation to the light absorber, good stability, solution
processability, smooth thin layer forming capacity and low-cost and environment-friendly
synthesis. The designed compounds comprising a 9,9’-bifluorenylidene core with bis(4-
alkoxyphenyl)amines were obtained in the Buchwald-Hartwig reaction, and their structure
1
13
was confirmed by NMR spectroscopy ( H, C, H-H COSY, H-C HMQC, H-C HMBC) and
elemental analysis. Then, their thermal, electrochemical and absorption properties, from the
point of view of the alkyl chains length effect, were discussed and compared with reported
2
,2’,7,7’-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-bifluorenylidene [32]. Finally, the
prepared novel 9,9’-bifluorenylidene derivatives were tested as HTM in the perovskite solar
cells.
2
. Experimental Section
2
.1. Materials
All starting materials and chemicals were commercially available and were used without
further purification. Materials, measurements and methods are available in Supporting
information. 2,2',7,7'-Tetrabromo-9,9'-bifluorenylidene (3) was prepared according to the
literature [32].
2
.2. Typical procedure for the synthesis of 1-bromo-4-alkoxybenzenes (1-A and 1-B)
A mixture of 4-bromophenol (7,78 g, 45,00 mmol), potassium carbonate (31,10 g, 225,00
mmol) and 360 ml acetone stirred for 10 minutes. Next, bromoalkane (58,50 mmol)
bromoethane or 1-bromobutane) was added and the mixture was heated at reflux for 24h,
(
under argon atmosphere. After the post-reaction mixture was filtered and the solvents were
evaporated from the filtrate using a rotary evaporator. Crude product was purified using
column chromatography.
3

1
-bromo-4-ethoxybenzene (A-1)
After purification by silica gel column chromatography (hexane : ethyl acetate, 10:1), product
1
1
-A was obtained as colorless oil with 86% yield. H NMR (400 MHz, CDCl ) δ 7.36 (d, J =
3
B2
B3
B5
8
.6 Hz, 2H, H ), 6.77 (d, J = 8.6 Hz, 2H, H ), 4.00 (q, J = 6.9 Hz, 2H, H ), 1.40 (t, J = 6.9
B6 13
B4
B2
B3
Hz, 3H, H ). C NMR (100 MHz, CDCl ) δ 158.16 (C ), 132.30 (C ), 116.38 (C ),
3
B1
B5
B6
1
12.71 (C ), 63.77 (C ), 14.83 (C ).
1
-bromo-4-butoxybenzene (A-2)
After purification by silica gel column chromatography (hexane : ethyl acetate, 5:1), product
1
1
8
2
-A was obtained as colorless oil with 92% yield. H NMR (400 MHz, CDCl ) δ 7.37 (d, J =
3
B2
B3
B5
.5 Hz, 2H, H ), 6.78 (d, J = 8.4 Hz, 2H, H ), 3.92 (t, J = 6.4 Hz, 2H, H ), 1.80 – 1.73 (m,
B6
B7
B8 13
H, H ), 1.54 – 1.45 (m, 2H, H ), 0.99 (t, J = 7.4 Hz, 3H, H ). C NMR (100 MHz,
B4
B2
B3
B1
B5
B6
CDCl ) δ 158.40 (C ), 132.29 (C ), 116.42 (C ), 112.66 (C ), 68.05 (C ), 31.35 (C ),
3
B7
B8
1
9.33 (C ), 13.94 (C ).
2
.3. Typical procedure for the synthesis of bis(4-alkoxyphenyl)amines (2-A and 2-B)
A mixture of 1-bromo-4-alkoxybenzene (40,00 mmol) (1-bromo-4-ethoxybenzene (A-1)
or 1-bromo-4-butoxybenzene (A-2)), 4-alkoxyaniline (32,00 mmol) (4-ethoxyaniline or 4-
butoxyaniline), potassium carbonate (14.72 g, 12.80 mmol), L-proline (1.62 g, 12.80 mmol),
o
copper iodide (1.22 g, 6.40 mmol) and 50 ml DMSO was stirred and heated at 100 C for 48h,
under argon atmosphere. After then the post-reaction mixture was quenched with distilled
water and extracted with ethyl acetate. The organic phase was collected and the solvent was
evaporated using a rotary evaporator. Crude product was purified using column
chromatography.
bis(4-ethoxyphenyl)amine (B-1)
After purification by silica gel column chromatography (hexane : ethyl acetate, 3:1), product
1
2
-A was obtained as beige solid with 45% yield. H NMR (400 MHz, DMSO) δ 7.48 (s, 1H,
B0
B2
B3
H ), 6.89 (d, J = 8.9 Hz, 4H, H ), 6.79 (d, J = 8.9 Hz, 4H, H ), 3.93 (q, J = 7.0 Hz, 4H,
B5
B6 13
B4
H ), 1.29 (t, J = 7.0 Hz, 6H, H ). C NMR (100 MHz, DMSO) δ 152.03 (C ), 137.98
B1
B2
B3
B5
B6
(
C ), 118.02 (C ), 115.19 (C ), 63.18 (C ), 14.78 (C ).
bis(4-butoxyphenyl)amine (B-2)
After purification by silica gel column chromatography (hexane : ethyl acetate, 5:1), product
1
2
-B was obtained as pale brown solid with 39% yield. H NMR (400 MHz, DMSO) δ 7.47
B0
B2
B3
(
s, 1H, H ), 6.89 (d, J = 8.9 Hz, 4H, H ), 6.79 (d, J = 8.9 Hz, 4H, H ), 3.87 (t, J = 6.5 Hz,
4

B5
B6
B7
B8
4
H, H ), 1.69 – 1.62 (m, 4H, H ), 1.47 – 1.37 (m, 4H, H ), 0.92 (t, J = 7.4 Hz, 6H, H ).
1
3
B4
B1
B2
B3
C NMR (100 MHz, DMSO) δ 152.25 (C ), 138.00 (C ), 118.04 (C ), 115.24 (C ),
B5
B6
B7
B8
6
7.44 (C ), 30.94 (C ), 18.78 (C ), 13.72 (C ).
2
.4. Typical
alkoxyphenylamine)-9,9'-bifluorenylidene (M-1 and M-2)
A mixture of bis(4-alkoxyphenyl)amine (4,00 mmol) (bis(4-ethoxyphenyl)amine (B-1) or
procedure
for
the
synthesis
of
2,2’,7,7’-tetrakis(N,N-di-p-
bis(4-butoxyphenyl)amine (B-2)), 2,2',7,7'-tetrabromo-9,9'-bifluorenylidene (0,78 mmol,0,50
g), t-BuONa (4.60 mmol, 0,45 g) and 20 ml toluene stirred for 20 minutes, under argon
atmosphere. Next, [Pd dba ] (0.11 mmol, 0,10 g) and Xphos (0.21 mmol, 0,10 g) were added
2
3
and the mixture was heated at reflux for 24h also under argon atmosphere. After the post-
reaction mixture was diluted with THF and flashed through a plug of MgSO . The organic
4
phase was collected and the solvent was evaporated using a rotary evaporator. Crude product
was purified using column chromatography.
2
,2’,7,7’-tetrakis(N,N-di-p-ethoxyphenylamine)-9,9'-bifluorenylidene (M-1)
After purification by silica gel column chromatography (hexane : THF, 5:1), isolated
compound was dissolved in THF and dropped into MeOH, precipitate was collected by
filtration, washed with MeOH and dried. M-1 was obtained as pale brown solid with 59%
1
A3
A6
yield. H NMR (400 MHz, THF) δ 8.02 (s, 4H, H ), 7.27 (d, J = 8.2 Hz, 4H, H ), 6.82 (d,
B2
A5+B3
B5
J = 9.0 Hz, 16H, H ), 6.76 – 6.72 (m, 20H, H
), 3.94 (q, J = 6.9 Hz, 16H, H ), 1.34 (t, J
B6 13
B4
A7
B1
=
7.0 Hz, 24H, H ). C NMR (100 MHz, THF) δ 156.00 (C ), 148.16 (C ), 142.34 (C ),
A4
A2
B2
A1
A3
A6
1
40.02 (C ), 136.25 (C ), 126.62 (C ), 125.54 (C ), 121.73 (C ), 120.25 (C ), 116.08
B3+A5
B5
B6
+
(C
), 64.28 (C ), 15.44 (C ). HRMS (ESI): calcd. for C H N O [M] 1348.6284,
90 84 4 8
found 1348.6289.
,2’,7,7’-tetrakis(N,N-di-p-butoxyphenylamine)-9,9'-bifluorenylidene (M-2)
2
After purification by silica gel column chromatography (toluene), isolated compound was
dissolved in THF and dropped into EtOH, precipitate was collected by filtration, washed with
1
EtOH and dried. M-1 was obtained as brown solid with 53% yield. H NMR (400 MHz,
A3
A6
B2
THF) δ 8.02 (s, 4H, H ), 7.27 (d, J = 8.2 Hz, 4H, H ), 6.82 (d, J = 8.9 Hz, 16H, H ), 6.76
A5+B3
B5
B6
–
1
6.73 (m, 20H, H
), 3.89 (t, J = 6.4 Hz, 16H, H ), 1.77 – 1.68 (m, 16H, H ), 1.54 –
B7
B8 13
.45 (m, 16H, H ), 0.97 (t, J = 7.4 Hz, 24H, H ). C NMR (100 MHz, THF) δ 156.18
B4
A7
B1
A4
A2
B2
A1
(
C ), 148.20 (C ), 142.39 (C ), 140.06 (C ), 136.32 (C ), 126.56 (C ), 125.71 (C ),
A3
A6
B3+A5
B5
B6
B7
1
21.97 (C ), 120.37 (C ), 116.09 (C
), 68.57 (C ), 32.67 (C ), 20.34 (C ), 14.39
B8
+
(
C ). HRMS (ESI): calcd. for C H N O [M] 1572.8788, found 1572.8767.
1
06 116
4
8
5

3. Result and discussion
3
.1.
Synthesis and structural characterization
The synthesis of two new 9,9'-bifluorenylidene derivatives, that is, 2,2’,7,7’-tetrakis(N,N-
di-p-ethoxyphenylamine)-9,9'-bifluorenylidene
and
2,2’,7,7’-tetrakis(N,N-di-p-
butoxyphenylamine)-9,9'-bifluorenylidene denoted as M-1 and M-2 started from preparation
of the necessary substrates (Fig. 1). First, 1-bromo-4-alkoxybenzenes (A-1 and A-2) were
synthesized in the reactions 4-bromophenol with bromoalkanes (bromoethane or 1-
bromobutane) based on the procedure described in the literature [33]. Both of these syntheses
were carried out with the potassium carbonate. Acetone was used as the solvent, which is
much cheaper and easier to evaporate than recommended by literature DMF [34-35] or
DMSO [36-37]. The synthesized 1-bromo-4-alkoxybenzenes (A-1 and A-2) were purified by
column chromatography and used in further reactions without additional purification. The
procedure used proved to be extremely simple and efficient giving the yield equal 86 and 92%
for A-1 and A-2, respectively. Then, bis(4-alkoxyphenyl)amines (B-1 and B-2) were obtained
in reactions 1-bromo-4-alkoxybenzenes (A-1 or A-2) with 4-alkoxyanilines (4-ethoxyaniline
or 4-butoxyaniline) analogous to the procedures described in the literature [33, 38]. Syntheses
were carried out in the presence of potassium carbonate, L-proline and copper iodide. The use
of the mentioned reagents allowed for a reduction of the reaction costs compared to the
catalysts recommended in the literature [39-40]. DMSO was used as the solvent. The
synthesized compounds also were purified by column chromatography. As a result of the
reactions bis(4-alkoxyphenyl)amines (B-1 and B-2) were obtained with yield 45 and 39% for
2
-A and 2-B, respectively.
Finally, 9,9'-bifluorenylidene derivatives (M-1 and M-2) were obtained by Buchwald-
Hardwig reactions bis(4-alkoxyphenyl)amines (B-1 and B-2) with 2,2',7,7'-tetrabromo-9,9'-
bifluorenylidene (3) (Fig. 1). The syntheses were carried out based on the procedure described
in the literature [32]. Xphos, t-BuONa and [Pd dba ] were used as the catalytic system. The
2
3
use of Xpos turned out to be easier and safer than the popular tritert-butylphosphine [41-42].
Toluene was used as a reaction medium. The reaction products were purified by column
chromatography and precipitation from an alcohol solution (M-1 – methanol, M-2 – ethanol).
In summary, synthesized derivatives were obtained with good yield (59 and 53% for M-1 and
M-2).
6

H
2
N
O
Br
Br
O
R
R
R
O
O
NH
Br R, K CO
K CO , CuI, L-proline
2 3
2
3
Acetone, rfx, 24h
DMSO, 100 C, 48h
o
O
R
O
R
R
R
R
R
OH
B-1, B-2
O
O
R
O
O
N
N
N
N
[
Pd (dba) ]
2 3
A-1, A-2
t-BuONa, Xphos
Toluene, rfx, 24h
O
Br
Br
Br
Lawesson's reagent
Toluene, rfx, 24h
Br
Br
M-1
M-2
Br
O
O
R
R
R = C H5 (1) or C H9 (2)
3
2
4
Fig. 1. The synthesis route and chemical structures of all prepared compounds
In order to better understand the structure of the 9,9’-bifluorenylidene derivatives (M-1
1
13
and M-2), they were analyzed by NMR spectroscopy ( H, C, H-H COSY, H-C HMQC and
H-C HMBC). All analyzes for M-1 and M-2 were performed in THF-d8 as solvent. NMR
spectra are shown in supporting information. Signals from atoms in the core of the molecule
are assigned the letter A, while atoms in substituents are assigned the letter B. In the case of
1
H NMR spectra (for both compounds), in aromatic zone we observed: singlet (8.02 ppm)
from A3 protons, doublet (7.27 ppm) from A6 hydrogens, doublet (6.82 ppm) from B2
hydrogens and multiplet (6.76 – 6.72 ppm) formed as a result of overlapping signals from A5
and B3 protons. In the aliphatic zone for M-1 we can notice a quartet (3.94 ppm) from B5
hydrogen and a triplet (1.34 ppm) from B6 protons. In turn, for M-2 we observe a triplet (3.89
ppm) from B5 hydrogen, two multiplets (in the range 1.77 - 1.68 ppm and 1.54 - 1.45 ppm)
from B6 and B7 hydrogen and a triplet (0.97 ppm) from B8 protons respectively. On the other
1
3
hand, on the C NMR spectra, we observe a series of signals in the range from 116.08 -
56.18 ppm. They are come from carbon atoms in the 9,9'-bifluorenylidene core and from
1
carbon atoms in the phenyl rings of substituents. Moreover in the range of 14.39 - 68.57 ppm
we can see signals to carbon atoms in aliphatic chains. Interestingly, the analysis of the H-C
HMQC and H-C HMBC spectra allowed for a detailed assignment of the obtained signals to
carbon atoms in M-1 and M-2 structures (Fig. 2). This made it possible to understand the
structure of synthesized compounds given their high degree of symmetry.
7

Fig. 2. H-C HMQC of M-2
3
.2. Thermal studies
The thermal properties of the synthesized 9,9'-bifluorenylidene derivatives (M-1 and M-2)
were determined using differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA). The obtained results were summarized in Table 1. The representative DSC
traces during two heating scans for M-1 (in DSC measurements) are shown in Fig. 3. Upon
the first heating scan, M-1 exhibited melting endotherm indicating its crystalline nature. In the
case of 9,9'-bifluorenylidene derivative with butyl substituents (M-2) in DSC thermograms
registered during first heating scan the glass transition temperature (T ) before melting point
g
was seen confirms its semicrystalline character (Fig. S2). The second heating scan, when the
isotropic liquid was rapid cooled, revealed the glass transition temperature (T ) and for M-1
g
o
exothermic crystallization (T ) which let to obtain new crystal which melted at 268 C. Thus,
c
they showed behaviour typical for molecular glasses, which can provide uniform, transparent
amorphous thin films contrary to low molecular weight compounds with a strong tendency for
crystallization. It could be observed that modification of the alkyl chains from ethyl (M-1) to
o
o
o
butyl (M-2) causes a significant lowering of T (from 288 C to 95 C) and T (from 132 C to
m
g
o
9
5 C). However, the replacement of ethyl with butyl group let to favour amorphous phase
formation. On the other hand, the length of the alkyl chains only slightly affects the thermal
8

stability of the tested derivatives. The beginning of thermal decomposition determined as the
o
temperature of 5% weight loss (T ) was detected above 400 C with the difference between M-
5
o
1
and M-2 being only 13 C (Fig. S3). As was mentioned similar to M-1 and M-2, 9,9’-
bifluorenylidene derivative with methoxy substituents, that is, 2,2’,7,7’-tetrakis(N,N-di-p-
methoxyphenylamine)-9,9'-bifluorenylidene denoted as KR216 has been reported [32]. Its
thermal properties were added to Table 1 for comparison with M-1 and M-2. However, note
that experiential conditions of DSC and TGA measurements are not the same. It is seen that
the presence of methoxy units (KR216) increases T , T , Tc and T .
m
g
5
In summary, it is worth emphasizing that both compounds exhibited melting point (Tm)
significantly below T . However, taking into account the operating temperature of devices, the
5
presence of ethyl chains in 9,9'-bifluorenylidene derivative (M-1) is beneficial to obtain better
thermal properties, that is higher T and T . Considering the thermal properties of the new
m
g
9
,9'-bifluorenylidene derivatives it can be pointed out that compound with ethyl substituents
(
M-1) showed slightly higher T and T and lower T compare to typical HTM spiro-
5
m
g
OMeTAD (T =388, T =248 and T =167°C [41]).
5
m
g
Table 1. Thermal Properties of M-1 and M-2
DSC TGA
Code
a
o
b
b
c
o
c
o
d o
Tm [ C] T [°C] T [°C] T [ C] T [ C] T
[ C]
g
c
5
10
max
M-1
M-2
288
95
309
132
89
157
212
–
228
401
388
417
428
414
-
445, 691
435, 709
-
[32]
KR216
a
b
registered in I heating run registered in II heating run,
T5 and T10 temperature of 5% and 10% weight loss, respectively.
Temperature of maximum decomposition rate.
c
d
Fig 3. DSC thermograms of M-1 registered in the first and second heating run
9

3
.3.
Redox behavior
It is worth remembering that before organic materials as HTMs can be used in PV devices
they have to exhibit compatible energy levels. Its oxidation potential should be lower than that
of the light harvester, that is, perovskite in PSC or dye sensitizer in DSSC. In view of the
above fact, obtained compounds were tested using cyclic voltammetry (CV) and differential
pulse voltammetry (DPV), which are a useful technique to study redox behavior and to get
information about ionization potential (IP) and electron affinity (EA) being closely related to
HOMO and LUMO energy, respectively. The research was carried out in CH Cl solution in
2
2
0
.1 M Bu NPF as the supporting electrolyte. The electrochemical oxidation and reduction
4 6
onset potentials were used for estimation of the IP and EA energies of the 9,9’-
bifluorenylidene derivatives (assuming that IP of ferrocene equals − 5.1 eV) [43]. The
obtained results are gathered in Table 2, whereas their CV voltammograms are depicted in
Fig. 4.
Table 2. Redox potentials, IP and EA energies and electrochemical band gaps (E )
g
a
b
c
Code
E1red
V]
E2red
[V]
E1ox
[V]
E2ox
[V]
E3ox
[V]
E4ox
[V]
IP(CV)
[eV]
EA(CV)
[eV]
Eg(CV)
[eV]
[
-
1.44 d
-1.76 d
0.03 d
0.2 d
0.39 d
1.03 d
M-1
M-2
-5.13
-5.00
-3.66
-3.21
1.47
1.79
(-1.35)
(-1.65)
(0.03)
(0.18)
(0.35)
(1.01)
-
1.55 d
-1.89 d
-0.10
0.07 d
0.24 d
0.89 d
d
(-1.52)
(-2.86) (-0.18) (0.01)
(0.15)
(0.74)
a
b
c
d
IP = -5.1 - Eox
;
EA = -5.1 – Ered
;
Eg(CV) = Eox (onset) −Ered (onset)
;
determined by DPV measurements;
Both compounds are characterized by two-stage reversible reduction (Fig. 4) contrary to
similar compound KR216, for which no reduction potential was observed in the CV
voltammograms [32]. The first reduction process (-1.44 V for M-1 and -1.55 V for M-2) most
likely corresponds to the reduction on the core 9.9'-bifluorenylidene [44-45]. The obtained
values of the cathode peak potentials allowed determination of EA, which were -3.66 eV and -
3
.21 eV in the case of M-1 and M2, respectively. In the next step, an oxidation process was
performed. It was found that the tested compounds undergo multi-stage oxidation. The first
three peaks were reversible and the last was irreversible (Fig. 4-b). In the case of both
+
2+
compounds (M-1 and M-2) the oxidation is due to the formation of cations first M , than M ,
3
+
4+
next M and finally M similar as was described in the literature for spiro-OMeTAD [46].
The IP values for M-1 and M-2 is -5.13 eV and -5.00 eV, respectively, which classifies them
as good HTM for PSC because are located above the valence band of perovskite CH NH PbI
3
3
3
(-5.44 eV) [47]. Thus, the smooth hole extraction can be expected. The IP values are similar
1
0

to the HOMO energy level of spiro-OMeTAD 5.20 eV or 5.04 eV reported in the literature
41] and [32], respectively. The ionization potential of the reported 9.9'-bifluorenylidene with
[
methoxy groups (KB216 [32]) was 5.09 eV was between IP value of compound containing
ethoxy (M-1) and butoxy (M-2) units. The prepared 9,9’-bifluorenylidene derivatives showed
higher EA level than the conduction band (CB) of perovskite which may prevent charge
recombination [48]. In the case of spiro-OMeTAD reduction process was not observed [41].
What is very important, these derivatives have small energy gaps. Take into account the
discussed processes, the introduction of longer alkyl chains into the compound structure
resulted in a shift towards more negative values of both reduction and oxidation process.
(a)
(b)
Fig. 4. CV spectra of M-1 and M-2 derivatives (reduction (a) and oxidation (b)
processes)
3
.4. UV-vis absorption spectroscopy
The optical properties of obtained 9,9’-bifluorenylidene derivatives (M-1 and M-2) were
studied in solution and the solid-state as a thin film on the glass substrate. Three solvents,
THF, chlorobenzene and dichloromethane were applied for UV-Vis measurements. Moreover,
the thin films of studied compounds without and with a dopant (4-tBP with different amount
of Li-TFSI) were prepared. Such additives were applied as p-type dopants in the next step of
investigations concerns of testing of the prepared 9,9’-bifluorenylidene derivatives as HTM in
PCSs. As chemical doping is a powerful tool to improve the charge carrier transport and the
electrical conductivity of organic semiconductors. The representative UV-Vis absorption
spectra were shown in Fig. 5 and the optical parameters were collected in Table S1.
1
1

a)
b)
1,4
1,2
1,0
0,8
0,6
0,4
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
M-1 film
M-1 8.75 µ
M-1 17.5 µ
M-2 film
M-2 8.75 µ
M-2 17.5 µ
M-2 35
M-2 solution in CB
L
L
L
L
L
µL
M-1 35
µ
M-1 solution in CB
0,2
0,0
300
350
400
450
500
550
600
650
700
750
800
300
350
400
450
500
550
600
650
700
750
800
Wavelength [nm]
Wavelength [nm]
Figure 5. The absorption spectra of (a) M-1 and (b) M-2 in the solid state and in
-6
chlorobenzene solution (c = 5 x 10 mol/L)
The synthesized M-1 and M-2 exhibited three (chlorobenzene) or four (CH Cl , THF)
2
2
strong absorption bands and a weak and broadband around 650 nm. As can be expected a lack
of impact of 9,9’-bifluorenylidene derivatives (M-1 and M-2) chemical structure on UV-vis
spectra were observed. As shown in the Table S1 the solvent polarity has no effects on the
position of absorption bands maximum (λmax) and molar absorption coefficient of investigated
compounds. The presented in literature spiro-OMeTAD showed in dichloromethane, THF and
chlorobenzene three or two absorption bands with λmax around 300, 400 and 625 nm, about
3
00 and 378 nm and at 380 and 530 nm, respectively [32, 49-51]. The molecules M-1 and M-
showed almost the same absorption band in the visible region with λmax at 463 as similar
2
reported KB216 [32]. However, in the case of M-1 and M-2 additional weak absorption band
located at 682 nm was detracted. Compare electronic absorption properties of the 9,9’-
bifluorenylidene derivatives in solution and the solid-state it was found that they exhibited
similar position of λmax, which suggest a lack of strong intermolecular π-π stacking of 9,9’-
bifluorenylidene derivatives in the thin-film state. Moreover, no influence of increasing the
amount of dopant in a film on UV-vis spectra was found.
3
.5. AFM measurements
Keep in mind that the HTMs should form the high quality of smooth thin layer, the
prepared films were characterized by atomic force microscopy (AFM) by determination of its
roughness and thickness. The AFM micrographs of the selected films are depicted in Fig. 6.
The AFM measurements indicate that the film surfaces were quite planar with root-mean-
square (RMS) roughness low values estimated from 1.7 to 9.2 nm (cf. Table S2). It was found
that neat films prepared from both 9,9’-bifluorenylidene derivatives exhibited the same RMS
value. In the case of M-1 addition of Li-TFSI resulted in the soothing of film surface and
1
2

reduction of RMS from 5.8 to 1.7 nm was seen. The correlation between the RMS values and
the amount of Li-TFSI was not observed. The prepared films were characterized by
thicknesses in the range of 123 – 248 nm. The film obtained from M-1 are slightly thicker
compare to M-2 films. In both cases, the thickness increasing together with higher amount of
Li-TFSI.
a
b
c
d
Figure 6. The AFM micrographs of the prepared films (a) a neat M-1, (b) M-1 with 8.75 µL of
Li-TFSI, (c) M-1 with 15.5 µL of Li-TFSI and (d) M-1 with 35 µL of Li-TFSI
3
.6. Photovoltaic investigations
To investigate the effect of a new 9,9’-bifluorenylidene derivatives on the performance of
perovskite cells, the devices with the structure FTO/b-TiO /m-TiO /perovskite/Au and
2
2
FTO/b-TiO /m-TiO /perovskite/HTM/Au were constructed (cf. Fig. 7-a) and detailed
2
2
fabrication process is given in Supplementary Materials). Additionally, except for 9,9’-
bifluorenylidene derivatives as HTM spiro-MeOTAD was applied. FTO was applied as
1
3

cathode (the electron collecting electrode), whereas Au was used as anode (the hole collecting
electrode). Perovskite layer was prepared by two step method. The photovoltaic tests were
done for non-encapsulated devices. The measurements were carried out in Standard Test
Conditions (STC). The representative current–voltage (I–V) characteristics of the studied
devices are given in Fig. 7-b, whereas photovoltaic parameters are collected in Table 3.
(a)
(b)
Fig. 7. (a) Scheme of the prepared perovskite solar cells and (b) the representative
current–voltage (I–V) characteristics of the tested devices with and without additives of Li-
TFSI on the left and on the right side, respectively
In the first step of study, the device without HTM was prepared. As can be seen from
Table 3 such device without HTM yielded the low PV parameters giving low PCE. Next, the
cells with 9,9’-bifluorenylidene derivatives (M-1 and M-2) doped with 4-tBP and Li-TFSI
were fabricated. It should be stressed that different HTMs may need different doping
+
+
concentration of Li . In our investigations, the impact of Li concentration was tested for both
,9’-bifluorenylidene derivatives. Considering this effect on device performance, it was found
9
that the dopant additive in value of 17.5 µL let to obtain off the highest PCE for both tested
new HTMs (cf. Table 3). Thus, for reference cell based on commercially available spiro-
MeOTAD also the 17.5 µL of Li-TFSI was applied. The highest device performance exhibited
1
4

cell with synthesized M-1. Because the device with M-1 showed the highest value of PCE in
previous tests it was decided that only M-1 and spiro-MeOTAD was used for subsequent
studies concern preparation of devices without dopant. As can be expected the Li-TFSI is
essential to render a low series resistances and to facilitate photocurrent generation. The
utilization of dopant resulted in increase of PCE from 2.40 to 4.40, from 4.53 to 7.33 and
from 2.63 to 4.13% for cell with spiro-MeOTAD, M-1 and M-2, respectively. Surprisingly,
the PSC based on spiro-MeOTAD without dopant exhibited a lower value of PCE compare to
device without HTM. The low efficiency resulted from the low value of open circuit voltage
(
V ), which may be a consequence of incomplete covering of perovskite with spiro-
oc
MeOTAD. Considering the PV parameters of the constructed devices given in Table 3, it was
seen that the best PV performance showed device based on 9,9’-bifluorenylidene derivative
substituted with ethyl units (M-1) Similar results, that is, shorter alkyl chains in HTMs
improved perovskite solar cells parameters have been recently reported for spiro-bifluorene
derivatives [41]. The different alkoxy chains effect on oxidation potential (cf. Table 2) and
may lead to different molecular packing in solid-state, which can be reflected in device
efficiency. It can be noticed that device based on M-1 showed higher PV parameters than that
of PSC based on the standard spiro-OMeTAD. One of the reason may be the amount of
+
dopant, which Li concentration was adjusted to investigated 9,9’-bifluorenylidene
derivatives. On the other hand, it seems that the M-1 displayed higher absorption in the
visible region compare to spiro-OMeTAD. For an explanation of the observed phenomenon,
additionally study covering hole mobility measurements, which are out of scope of the
presented work, are necessary.
Table 3. Photovoltaic properties of fabricated perovskite solar cells
V
[
Li-TFSI
J
sc
V
[mV]
oc
FF
[-]
PCE
[%]
Device of structure
2
µL]
[mA/cm ]
FTO/b-TiO /m-TiO /perovskite/Au
-
12.2
631
0.36
2.90
2
2
FTO/b-TiO /m-TiO /perovskite/spiro-MeOTAD/Au
0.00
17.50
13.6
14.2
482
691
0.35
0.43
2.40
4.40
2
2
FTO/b-TiO /m-TiO /perovskite/spiro-MeOTAD/Au
2
2
FTO/b-TiO /m-TiO /perovskite/M-1/Au
0.00
8.75
15.8
15.2
752
713
0.36
0.39
4.53
4.50
2
2
FTO/b-TiO /m-TiO /perovskite/M-1/Au
2
2
FTO/b-TiO /m-TiO /perovskite/M-1/Au
17.50
16.7
823
0.51
7.33
2
2
FTO/b-TiO /m-TiO /perovskite/M-1/Au
35.00
8.75
17.50
35.00
13.3
12.0
12.0
10.8
776
667
769
628
0.59
0.31
0.42
0.48
6.40
2.63
4.13
3.40
2
2
FTO/b-TiO /m-TiO /perovskite/M-2/Au
2
2
FTO/b-TiO /m-TiO /perovskite/M-2/Au
2
2
FTO/b-TiO /m-TiO /perovskite/M-2/Au
2
2
VLi-TFSI - volume of additive of solution Li-TFSI;
J
sc - short-circuit current; Voc - open-circuit voltage; FF - fill factor; PCE - power conversion efficiency;
-
without HTM.
1
5

The cell based on the reported 9,9’-bifluorenylidene derivative with methoxy units (KR216)
exhibited better PV parameters (J =22.3 mAcm-1, V =1023 mV, FF= 77 and PCE=17.8%)
sc
oc
[
32]. Moreover, presented in [32] device with spiro-MeOTAD showed better performance
compared to our PCS. However, it should be stressed that the structure of both cells, that is,
presented herein and reported [32] are significant differences. Thus, a direct comparison of
the effect of utilization of KR216 and M-1 or M-2 as HTM is impossible.
4
. Conclusions
Summarizing, it can be pointed out that:
•
new HTM materials with 9,9’-bifluorenylidene core and different alkoxy chains (ethyl
or butyl) substituted to the para position of diphenylamine were synthesized with
good yield,
•
the presence of butyl units: significantly lowered T , T and to a lesser extent impacts
m
g
on thermal stability, does not effect on UV-vis absorption range and shifted IP and EA
to slightly higher value,
•
•
device with a neat compound bearing ethoxy units gave significant PCE = 4.53 %,
using of doped 9,9’-bifluorenylidene derivative with shorted alkoxy substituent lead to
the highest PCE value of 7.33 % of PSC being higher than that obtained for reference
cell based on spiro-OMeTAD,
•
the obtained results indicate that 9,9’-bifluorenylidene derivative with ethoxy groups
is promising hole transport compounds, which may lead to the development of highly
efficient solar cells. In our further study it will be tested as solid electrolyte in DSSC.
Acknowledgements
The authors thank dr H. Janeczek for DSC measurements and dr M. Siwy for elemental
analysis measurements. This work was supported by National Science Centre of Poland
Grant: No. 2016/23/B/ST8/02045, Grant: No. 2018/31/B/ST8/03294 and University of Silesia
Rector's grant for the best students (Angelika Mieszczanin).
References:
[
1] Jung M, Ji SG, Kim G, Seok SI. Perovskite precursor solution chemistry: from
fundamentals to photovoltaic applications. Chem Soc Rev 2019;48:2011-2038.
2] Aydin E, De Bastiani M, De Wolf S. Defect and Contact Passivation for Perovskite Solar
Cells. Adv Mater 2019;31:1900428-1900448.
3] Ali Said A, Xie J, Zhang Q. Recent Progress in Organic Electron Transport Materials in
[
[
1
6

Inverted Perovskite Solar Cells. Small 2019;15:1900854-1900877.
4] Xie H, Yin X, Guo Y, Liu J, Que W, Wang G. Recent Progress of Flexible Perovskite
Solar Cells. Phys Status Solidi RRL 2019;13:1800566-1800589.
5] Xu WJ, Kopyl S, Kholkin A, Rocha J. Hybrid organic-inorganic perovskites: Polar
properties and applications. Coordin Chem Rev 2019;387:398–414.
6] Yang X, Wang H, Cai B, Yu Z, Sun L. Progress in hole-transporting materials for
perovskite solar cells. J Energy Chem 2018;27:650–672.
7] Hawash Z, Ono LK, Qi Y. Recent Advances in Spiro-MeOTAD Hole Transport Material
[
[
[
[
and Its Applications in Organic–Inorganic Halide Perovskite Solar Cells. Adv Mater
Interfaces 2018;5:1700623-1700645.
[
8] Zhao X, Wang M. Organic hole-transporting materials for efficient perovskite solar cells.
Mater Today Energy 2018;7:208-220.
9] Bakr ZH, Wali Q, Fakharuddin A, Schmidt-Mende L, Brown TM, Jose R. Advances in
[
hole transport materials engineering for stable and efficient perovskite solar cells. Nano
Energy 2017;34:271–305.
[
10] Vivo P, Salunke JK, Priimagi A. Hole-Transporting Materials for Printable Perovskite
Solar Cells. Materials 2017, 10, 1087-1132.
11] Li G, Sheng L, Li T, Hu J, Li P, Wang K. Engineering flexible dye-sensitized solar cells
for portable electronics. Sol Energy 2019;177:80–98.
12] Cole JM, Pepe G, Al Bahri OK, Cooper CB. Cosensitization in Dye-Sensitized Solar
Cells. Chem Rev 2019;119:7279−7327.
13] Błaszczyk A. Strategies to improve the performance of metal-free dye-sensitized solar
Cells. Dyes Pigments 2018;149:707–718.
14] Mohiuddin O, Obaidullah M, Sabah C. Improvement in dye sensitized solar cells from
past to present. Opt Quant Electron 2018;50:377-404.
15] Roslana N, Yaacob ME, Radzi MAM, Hashimoto Y, Jamaludin D, Chen DG. Dye
[
[
[
[
[
Sensitized Solar Cell (DSSC) greenhouse shading: New insights for solar radiation
manipulation. Renew Sust Energ Rev 2018;92:171–186.
[
16] Sharma K, Sharma V, Sharma SS. Dye-Sensitized Solar Cells: Fundamentals and Current
Status. Nanoscale Res Lett 2018;13:381-426.
17] Lee CP, Li CT, Ho KC. Use of organic materials in dye-sensitized solar cells. Mater
Today 2017;20:267-283.
18] Gong J, Sumathy K, Qiao Q, Zhou Z. Review on dye-sensitized solar cells (DSSCs):
Advanced techniques and research trends. Renew Sust Energ Rev 2017;68:234-246.
[
[
1
7

[
19] Kumavat PP, Sonar P, Dalal DS. An overview on basics of organic and dye sensitized
solar cells, their mechanism and recent improvements. Renew Sust Energ Rev 2017;78:1262-
287.
20] Mehmood U, Al-Ahmed A, Al-Sulaiman FA, Malik MI, Shehzad F, Khan AUH. Effect
1
[
of temperature on the photovoltaic performance and stability of solidstate dye-sensitized solar
cells: A review. Renew Sust Energ Rev 2017;79:946-959.
[
21] Urbani M, de la Torre G, Khaja Nazeeruddin MK, Torres T. Phthalocyanines and
porphyrinoid analogues as hole- and electron-transporting materials for perovskite solar cells
Chem Soc Rev 2019;48:2738-2766.
[
22] Gil B, Yun AJ, Lee Y, Kim J, Lee B, Park B. Recent Progress in Inorganic Hole
Transport Materials for Efficient and Stable Perovskite Solar Cells. Electron Mater Lett
019;15:505–524.
23] Elseman AM, Sajid S, Shalan AE, Mohamed SA, Rashad MM. Recent progres
concerning inorganic hole transport layers for efficient perovskite solar cells. Appl Phys A
019;125:476.
24] Hsu CY, Chen YC, Lin RYY, Hob KC, Lin JT. Solid-state dye-sensitized solar cells
2
[
2
[
based on spirofluorene (spiro-OMeTAD) and arylamines as hole transporting materials. Phys
Chem Chem Phys 2012;14:14099–14109.
[
25] Yu Z, Sun L. Recent Progress on Hole‐ Transporting Materials for Emerging
Organometal Halide Perovskite Solar Cells. Adv Energy Mater 2015;5:1500213.
26] Bach U, Lupo D, Comte P, Moser JE, Weissortel F, Salbeck J, Spreitzer H, Grätzel M.
[
Solid-state dye-sensitized mesoporous TiO2 solar cells with high photon-to-electron
conversion efficiencies. Nature 1998;395:583-585.
[
27] Kim HS, Lee CR, Im JH, Lee KB, Moehl T, Marchioro A, Moon SJ, Humphry-Baker R,
Yum JH, Moser JE, Grätzel M, Park NG. Lead Iodide Perovskite Sensitized All-Solid-State
Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci Rep-Uk
2
012;2:591-597.
28] Lee MM, Teuscher J, Miyasaka T, Murakami TN, Snaith HJ. Efficient Hybrid Solar
Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science
012;338:643-647.
29] Gangala S, Misra R. Spiro-linked organic small molecules as holetransport materials for
perovskite solar cells. J Mater Chem A 2018;6:18750-18765.
[
2
[
1
8

[
30] Chevrier M, Hawashin H, Richeter S, Mehdi A, Surin M, Lazzaroni R, Dubois P, Ratier
B, Boucle J, Clement S. Well-designed poly(3-hexylthiophene) as hole transporting material:
A new opportunity for solid-state dye-sensitized solar cells. Synthetic Met 2017;226:157-163.
[
31] Agarwala P, Kabra D. A review on triphenylamine (TPA) based organic hole transport
materials (HTMs) for dye sensitized solar cells (DSSCs) and perovskite solar cells (PSCs):
evolution and molecular engineering. J Mater Chem A 2017;5:1348–1373.
[
32] Rakstys K, Saliba M, Gao P, Gratia P, Kamarauskas E, Paek S, Jankauskas V,
Nazeeruddin MK. Highly Efficient Perovskite Solar Cells Employing an Easily Attainable
Bifluorenylidene-Based Hole-Transporting Material. Angew Chem Int Ed 2016;55:7464-
7
468.
[
33] Shellaiah M, Rajan YC, Lin HC. Synthesis of novel triarylamine-based dendrimers with
4
6
N ,N -dibutyl-1,3,5-triazine-4,6-diamine probe for electron/energy transfers in H-bonded
2
+
donor-acceptor-donor triads and as efficient Cu sensors. J Mater Chem 2012;22:8976–8987.
34] Nagura K, Takemoto Y, Moronaga S, Uchida Y, Shimono S, Shiino A, Tanigaki K,
[
Amano T, Yoshino F, Noda Y, Koizumi S, Komatsu N, Kato T, Yamauchi J, Tamura R.
Preparation of Robust Metal-Free Magnetic Nanoemulsions Encapsulating Low-Molecular-
Weight Nitroxide Radicals and Hydrophobic Drugs Directed Toward MRI-Visible Targeted
Delivery. Chem Eur J 2017;23:15713-15720.
[
35] Yan JJ, Xie HL, Weng L, Yang S, Hai-Liang Zhang HL, Manipulating ordered structure
of ionic liquid crystalline polymers through tuning the alkyl spacer length. Polymer
014;55:6504-6512.
36] Kapf A, Eslahi H, Blanke M, Saccone M, Giese M, Albrecht M. Alkyloxy modified
pyrene fluorophores with tunable photophysical and crystalline properties. New J Chem
019;43:6361-6371.
37] Lee SK, Lee WH, Cho JM, Park SJ, Park JU, Shin WS, Lee JC, Kang IN, Moon SJ.
2
[
2
[
Synthesis and Photovoltaic Properties of Quinoxaline-Based Alternating Copolymers for
High-Efficiency Bulk-Heterojunction Polymer Solar Cells. Macromolecules 2011;44:5994-
6
001.
[
38] Furer SO, Bozic-Weber B, Neuburger M, Constable EC, Housecroft CE. Heteroleptic
copper(I) sensitizers with one versus two hole-transporting units in functionalized 2,9-
dimethyl-1,10-phenanthroline ancillary ligands. RSC Adv 2015;5:69430–69440.
[
39] Shimogawa H, Murata Y, Wakamiya A. NIR-Absorbing Dye Based on BF ‐ Bridged
2
Azafulvene Dimer as a Strong Electron-Accepting Unit. Org Lett 2018;20:5135-5138.
1
9

[
40] Nguyen WH, Bailie CD, Burschka J, Moehl T, Grätzel M, McGehee MD, Sellinger A.
Molecular Engineering of Organic Dyes for Improved Recombination Lifetime in Solid-State
Dye-Sensitized Solar Cells. Chem Mater 2013;25:1519-1525.
[
41] Liu F, Bi S, Wang X, Leng X, Han M, Xue B, Li Q, Zhou H, Li Z. Similar or different:
the same Spiro-core but different alkyl chains with apparently improved device performance
of perovskite solar cells. Sci China Chem 2019;62:739–745.
[
42] Jeon NJ, Lee HG, Kim YC, Seo J, Noh JH, Lee J, Seok SI. o‐ Methoxy Substituents in
Spiro-OMeTAD for Efficient Inorganic−Organic Hybrid Perovskite Solar Cells. J Am Chem
Soc 2014;136:7837-7840.
[
43] Bujak P, Kulszewicz-Bajer I, Zagorska M, Maurel V, Wielgus I, Pron A, Polymers for
electronics and spintronics. Chem Soc Rev 2013;42:8895-8999.
44] Fan Y, Barlow S, Zhang S, Lina B, Marder SR. Comparison of 3D non-fullerene
[
acceptors for organic photovoltaics based on naphthalene diimide and perylene diimide-
substituted 9,9’-bifluorenylidene. RSC Adv 2016;6:70493-70500.
[
45] Eakins GL, Cooper MW, Gerasimchuk NN, Phillips TJ, Breyfogle BE, Stearman CJ.
Structural influences impacting the role of the 9-ylidene bond in the electronic tuning of
structures built upon 9-fluorenylidene scaffolds. Can J Chem 2013;91:1059–1071.
[
46] Nguyen WH, Bailie CD, Unger EL, McGehee MD. Enhancing the Hole-Conductivity of
Spiro-OMeTAD without Oxygen or Lithium Salts by Using Spiro(TFSI) in Perovskite and
2
Dye-Sensitized Solar Cells. J Am Chem Soc 2014;136:10996−11001.
[
47] Ma S, Zhang H, Zhao N, Cheng Y, Wang M, Shena Y, Tu G. Spiro-thiophene derivatives
as hole-transport materials for perovskite solar cells. J Mater Chem A 2015;3:12139–12144.
48] Kang SH, Lu C, Zhou H, Choi S, Kim J, Kim HK. Novel π-extended porphyrin-based
[
hole-transporting materials with triarylamine donor units for high performance perovskite
solar cells. Dyes Pigments 2019;163:734–739.
[
49] Ganesan P, Fu K, Gao P, Schenk K, Scopelliti R, Luo J, Wong LH, Grätzel M,
Nazeeruddin MK. A simple spiro-type hole transporting material for efficient perovskite solar
cells. Energy Environ Sci 2015;8:1986-1991.
[
50] Zhang F, Wang Z, Zhu H, Pellet N, Luo J, Yi C, Liu X, Liu H, Wanga S, Lia X, Xiao Y,
Zakeeruddin SM, Bi D, Grätzel M. Over 20% PCE perovskite solar cells with superior
stability achieved by novel and low-cost hole-transporting materials. Nano Energy
2
017;41:469–475.
51] Howie WH, Harris JE, Jennings JR, Peter LM. Solid-state dye-sensitized solar cells
based on spiro-OMeTAD. Sol Energ Mat Sol C 2007;91:424–426.
[
2
0

Supporting Information
9
,9’-Bifluorenylidene derivatives as novel hole-transporting materials for potential
photovoltaic applications
a*
a,b
a
a
Slawomir Kula , Agnieszka Pająk , Agata Szlapa-Kula , Angelika Mieszczanin ,
c
b
a,c*
Paweł Gnida , Marek Lipiński , Ewa Schab-Balcerzak
a
Institute of Chemistry, Faculty of Mathematics, Physics and Chemistry, University of Silesia, Szkolna 9 St., 40-
07 Katowice, Poland
0
b
Institute of Metallurgy and Materials Science, Polish Academy of Sciences, Reymonta 25 St., 30-059 Krakow,
Poland
c
Centre of Polymer and Carbon Materials, Polish Academy of Sciences,34 M. Curie-Sklodowska Str., 41-819
Zabrze, Poland
*Corresponding author at: Institute of Chemistry, University of Silesia, 9 Szkolna Str., 40-006 Katowice,
Poland. E-mail address: * slawomir.kula@us.edu.pl; phone: (+4832) 359 16 42 (S. Kula).
Table of Contents
1
2
3
4
5
6
7
8
9
1
. Materials...........................................................................................................22
. General methods - measurements.....................................................22
. Thermal characteristics (DSC and TGA).........................................24
. Redox behavior.............................................................................................25
. Photographs....................................................................................................26
. UV-vis spectra...............................................................................................26
. RMS data..........................................................................................................27
1
13
. H NMR and C NMR spectra................................................................27
. HRMS spectra................................................................................................42
0.Literature .........................................................................................................43
2
1

1
. Materials
4
-Bromophenol (98%, TCI), potassium carbonate (anhydrous pure p.a., Chempur),
bromoethane (98%, Sigma-Aldrich), 1-bromobutane (98%, TCI), 4-ethoxyaniline (98%, TCI),
-butoxyaniline (98%, TCI), l-proline (99%, TCI), Sodium tert-butoxide (t-BuONa, 97%,
Sigma-Aldrich), tris(dibenzylideneacetone)dipalladium(0) ([Pd dba ], 99%, Fluorochem),
4
2
3
Xphos (97%, Sigma-Aldrich), Magnesium sulfate (MgSO , pure p.a., Chempur), CDCl (99.8
4
3
atom % D, Sigma-Aldrich), DMSO-d6 (99.9 atom % D, Sigma-Aldrich), THF-d8 (≥99.5
atom % D, Sigma-Aldrich), hexane (95% pure, Chempur), ethyl acetate (pure, Chempur),
acetone (pure, Chempur), dimethyl sulfoxide (DMSO, pure p.a., Chempur), toluene (pure p.a.,
Chempur), tetrahydrofuran (THF, pure p.a., Chempur), methanol (MeOH, pure p.a.,
Chempur), ethanol (EtOH, 96% pure p.a., Chempur or POCH), hydrochloric acid (HCl, 36-
3
8%, Chempur). All reactions were carried out under argon atmosphere. Thin layer
chromatography (TLC) was carried out on silica gel (Merck TLC Silica Gel 60 F ). Silica
2
54
gel from Merck was used for column chromatography. Surfactant (Hellmanex III, Hellma
Analytics), fluorine doped tin oxide coated glass slides (FTOs, 7 Ω/sq, Sigma-Aldrich),
tetraethyl orthotitanate ((C H O) Ti, assay ≥95% Merck), paste Ti-Nanoxide T600/SP
2
5
4
(Solaronix), lead iodide (PbI2, assay 99%, Sigma-Aldrich), anhydrous N,N-
dimethylformamide (DMF, assay 99.8%, Sigma-Aldrich), methylammonium iodide (MAI,
Solaronix), isopropanol (IPA, POCH), chlorobenzene (C H Cl, POCH). 2,2′,7,7′-
6
5
Tetrakis(N,N-di-p-methoxyphenylamine)-9,9-spirobifluorene (spiro-MeOTAD), 4-tert-butyl
pyridine (TBP), lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI), acetonitrile (99.8%,
Sigma-Aldrich).
2
. General methods - measurements
Bruker Avance 400 MHz instrument used to record the NMR spectra in CDCl , DMSO-
3
d and THF-d (as solvent). The HRMS measurements were performed using QTOF Maxis
6
8
Impact Bruker mass spectrometer equipped with an electrospray (ESI) ion source and q-TOF
type mass analyzer. The recorded data were processed using Data Analylis 4.1 software
package. The analyzed samples were dissolved in a mixture of MeCN/CHCl /HCOOH.
3
Differential Scanning Calorimetry (DSC) was performed by TA-DSC 2010 apparatus, under
nitrogen atmosphere with heating/cooling rate of 20 ºC/min and using aluminum sample pans.
Thermogravimetric analysis was performed on Perkin Elmer Thermogravimetric Analyzer
Pyris 1 TGA at a heating rate of 15 °C/min under nitrogen. Eco Chemie Autolab
PGSTAT128n potentiostat was used to the electrochemical measurements. Researches were
2
2

carried out using glassy carbon electrode (diam. 2 mm), platinum coil and silver wire as
working, auxiliary and reference electrode, respectively. Potentials are referenced with respect
to internal standard (ferrocene (Fc)). Differential pulse voltammetry and cyclic voltammetry
experiments were conducted in a standard one-compartment cell, in dichloromethane, under
argon. As the supporting electrolyte was used 0.1M Bu NPF (Aldrich, 99%). The
4
6
-4
3
concentration of compounds was equal 1.0·10 mol/dm . Deaeration of the solution was
achieved by argon bubbling through the solution for about 10 min before measurement. All
electrochemical experiments were carried out under ambient conditions and an argon
atmosphere was maintained over the solution during measurements. UV-Vis spectra were
recorded in dichloromethane, tetrahydrofuran and chlorobenzene solution as the concentration
-6
of 5*10 mol/L using the Evolution 220 Thermo Scientific UV-Vis Spectrophotometer and
UV-Vis-NIR Jasco-V-570 Spectrophotometer. AFM micrographs were obtained using a
TopoMetrix Explorer, working in the contact mode in the air, in the constant force regime.
Fabrication of perovskite solar cells (PSCs)
The solar cells of structures: FTO/b-TiO /m-TiO /perovskite/Au and FTO/b-TiO /m-
2
2
2
TiO /perovskite/compound/Au without and with various amount of Li-TFSI additive were
2
prepared. The fluorine doped tin oxide coated glass slides (FTOs, 2 x 2 cm) were first cleaned
with warm deionised water with Hellmanex III (1 % solution) for 5 minutes in an ultrasonic
bath. Subsequently, the FTOs were rinsing in hot deionised water (~100 °C) twice. Further
step was that FTOs were put into isopropanol in an ultrasonic bath for 5 minutes and again
rinsing in hot deionised water. The next step was prepared a blocking layer b-TiO on FTO
2
(1.14 g tetraethyl orthotitanate in 10 mL EtOH and 0.2 mL HCl) [1]. The fresh solution was
spin coated on the surface of FTO at 2000 r.p.m. for 15 seconds and dried at 200 °C for 10
minutes. Then samples were annealed at 500 °C for 30 minutes in air to calcination. Then
mesoporous m-TiO layer was screen-printed on FTO/b-TiO and was also dried at 200 °C for
2
2
1
0 minutes. Finally samples were annealed at 500 °C for 30 minutes in air.
Perovskite preparation
A two-step method was used to prepare perovskite layer. In the first step was made
solutions of PbI (1.39 g) in anhydrous DMF (3.5 mL) and MAI (0.1 g) in isopropanol (11
2
mL). The solution PbI in DMF was first stirring at room temperature for a few minutes, next
2
at 70 °C for 30 minutes and was spin coated on the hot FTO with TiO layers at 2000 r.p.m.
2
for 30 seconds, next were dried at 70 °C for 3 minutes and 90 °C for 5 minutes. The second
2
3

step of method was dipping in solution of MAI in IPA for 15 minutes and after samples were
dried at 90 °C for 30 minutes. Before dipping every samples were pre-wetting by IPA.
Hole-transporting material (HTM) preparation
M-1, M-2 and spiro-MeOTAD solutions were prepared according to the description
included in the publication [2]. The composition of HTM solution was 72.3 mg of the HTM in
1
mL of chlorobenzene, 28.8 µl of 4-tert-butyl pyridine and 8.75 µl or 17.5 µl or 35 µl of
lithium bis(trifluoromethanesulfonyl)imide (0.002 mol/mL). The prepared solution was spin
coated on the perovskite layer at 4.000 r.p.m. for 30 seconds. Finally, the gold electrode was
-6
deposited by thermal evaporation (~ 10 mbar) on the devices.
Measurements of photovoltaic cells
Current–voltage (I–V) characteristics of the prepared devices were measured using the
PET Photo Emission Tech Inc. Model SS 200AAA class solar simulator in STC (Standard
2
Test Conditions) conditions (temperature of the cell 25 °C, solar irradiance
̶
1000 W/m , air
2
mass 1.5 spectrum
̶
AM1.5). Active area of the tested cells was 0.25 cm .
3
. Thermal characteristics (DSC and TGA)
Fig S1. DSC thermograms of M-1
2
4

Fig S2. DSC thermograms of M-2
1
100
M-1
M-2
M-1
M-2
0
80
-1
-2
60
-3
40
-4
20
-5
0
-
6
2
0
100
200
300
400
500
600
700
800
900
00
300
400
500
600
700
800
o
o
Temperature [ C]
Temperature [ C]
-1
Fig. S3. TGA thermograms of M-1 and M-2 recorded at a heating rate of 15ºC min
4
. Redox behavior
(a)
(b)
Fig. S4. DPV voltamogramms of M-1 and M-2
2
5

5
. Photographs
(a)
(b)
Fig. S5. Photographs of HTM-1 (a) and HTM-2 (b) in solid state under visible light.
6
. UV-vis spectra
Table S1. UV-Vis spectroscopic data of obtained M-1 and M-2 in solution and in the solid
state
5
a
Code
M-1
Solution λmax [nm] (ε·10 )
Film
A neat
With Li-TFSI
8.75 µL) (17.5 µL) (35 µL)
(
b
b
b
CH Cl
chlorobenzene
THF
2
2
289(1.51)
-
288(1.39)
311(1.47)
387(1.31)
463(0.42)
682(0.04)
287(1.60)
313(1.68)
385(1.34)
463(0.50)
698(0.04)
-
-
-
-
3
3
4
6
14(1.63)
87(1.43)
66(0.46)
97(0.04)
317(1.46)
390(1.29)
468(0.44)
691(0.03)
-
311
389
471
315
389
471
314
389
471
316
389
471
M-2
287(1.43)
-
-
-
-
3
3
4
7
15(1.54)
86(1.19)
66(0.43)
03(0.02)
317(1.62)
390(1.29)
467(0.51)
690(0.04)
311
389
471
315
389
472
314
390
471
314
390
472
a
3
-1
-1
b
-6
3
ε – molar absorption coefficient, [dm ·mol ·cm ]. c = 5*10 mol/dm
2
6

7
. RMS data
Table S2. Collected values of RMS and thickness of prepared layers
Structure of layer
RMS [nm]
5.8
Thickness [nm]
M-1
156
195
211
248
123
134
135
151
M-1:TBP:Li-TFSI (8.75 µL)
M-1:TBP:Li-TFSI (17.5 µL)
M-1:TBP:Li-TFSI (35 µL)
M-2
M-2:TBP:Li-TFSI (8.75 µL)
M-2:TBP:Li-TFSI (17.5 µL)
M-2:TBP:Li-TFSI (35 µL)
2.1
2.9
1.7
5.8
3.1
6.0
9.2
1
13
8
. H NMR and C NMR spectra
1
H NMR of 1-bromo-4-ethoxybenzene (A-1)
2
7

H-H COSY of 1-bromo-4-ethoxybenzene (A-1)
1
3
C NMR of 1-bromo-4-ethoxybenzene (A-1)
2
8