Making benzotrithiophene derivatives dopant-free for perovskite solar cells: Step-saving installation of π-spacers by a direct C-H arylation strategy

Article abstract of DOI:10.1039/c9ta09777e

Previously, benzotrithiophene (BTT)-based hole-transporting materials (HTMs) had to be doped with three chemical agents for high-efficiency perovskite solar cells (PSC). This work describes significant progress by which the first dopant-free BTT core-based HTM (YKP03) with EDOT spacers was readily accessed by step-saving direct C-H arylation. PSCs with dopant-free YKP03 showed promising efficiencies of up to 16.15% with potential long-term storage stability.

Full text of DOI:10.1039/c9ta09777e

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Making benzotrithiophene derivatives dopant-free
for perovskite solar cells: Step-saving installation of
p-spacers by a direct C–H arylation strategy†
Cite this: J. Mater. Chem. A, 2019, 7,
4765
2
Received 4th September 2019
Accepted 15th October 2019
a
b
a
a
Yi-Kai Peng,‡ Kun-Mu Lee,‡ Chang-Chieh Ting, Ming-Wei Hsu
a
and Ching-Yuan Liu
*
DOI: 10.1039/c9ta09777e
rsc.li/materials-a
Previously, benzotrithiophene (BTT)-based hole-transporting mate- based HTMs a potential alternative to the commercially avail-
13,14
rials (HTMs) had to be doped with three chemical agents for high- able spiro-OMeTAD.
We noticed that, however, so far
efficiency perovskite solar cells (PSC). This work describes significant accessing BTT core-based HTMs must rely on a multistep
progress by which the first dopant-free BTT core-based HTM (YKP03) approach involving tedious substrate prefunctionalization and
with EDOT spacers was readily accessed by step-saving direct C–H the employment of either costly or somehow unstable organo-
arylation. PSCs with dopant-free YKP03 showed promising efficiencies boron reagents required in the Suzuki coupling reaction.
of up to 16.15% with potential long-term storage stability.
Moreover, in order to achieve high efficiencies, previously re-
ported perovskite solar devices using BTTs as hole-transporters
must be doped with three kinds of chemical agents including 4-
tert-butylpyridine (TBP), lithium bis(triuoromethanesulfonyl)
Incorporation of
a hole-transporting material (HTM) in
perovskite-based solar cells has become a prerequisite for the
enhancement of device performance and stability. Among
imide (Li-TFSI), and an expensive organometallic complex
1
3,4
(
FK209). Therefore, in this work, we wish to demonstrate
various organic hole-transporters, facile synthesis and device
engineering of small-molecule HTMs is of particular interest
since these p-conjugated oligo(hetero)aryls possess several
advantages such as well-dened molecular weight, broad
signicant progress regarding the synthesis and device prop-
erties of BTT-based HTMs (Fig. 1). The major breakthroughs
and improvements are listed as follows: (1) a step-saving new
synthetic route to three BTT core-based HTMs has been devel-
oped utilizing the more challenging but straightforward three-
fold C–H activation/arylation reactions; (2) the rst BTT-
containing dopant-free HTM (YKP03) has been successfully
synthesized by a modied methodology under pre-screened/
2
synthetic diversity, and lower cost. Recently, perovskite solar
cells (PSCs) using high planarity core-based HTMs, such as
15
3,4
5–7
8–10
benzotrithiophenes, tetrathienylbenzenes, and truxenes,
have been shown to exhibit promising power conversion effi-
ciencies (PCE) of up to 18.8%. Among the above small mole-
cules, we had much interest in the preparation, conjugation
extension, and applications of benzotrithiophene (BTT) with
C
3h symmetry, due to its simpler and cost-effective two-step
11
synthesis using inexpensive starting materials and reagents.
In addition to the consideration of synthetic cost, the pla-
narized BTT core usually demonstrated exceptional hole-
3,4,12
transporting characteristics,
both of which make BTT-
a
Department of Chemical and Materials Engineering, National Central University,
Jhongli District, Taoyuan 320, Taiwan, Republic of China. E-mail: cyliu0312@ncu.
edu.tw
b
Department of Chemical and Materials Engineering, Green Technology Research
Center, Chang Gung University, Department of Pediatrics, Chang Gung Memorial
Hospital, Linkou, Taoyuan 333, Taiwan, Republic of China. E-mail: kmlee@mail.
cgu.edu.tw
1
13
†
Electronic supplementary information (ESI) available: NMR spectra ( H and C)
of compounds BTT-3, YKP06 and YKP03. See DOI: 10.1039/c9ta09777e
These authors contributed equally to this work.
Fig. 1 BTT core-based hole-transporting materials: step-saving new
synthesis, facile installation of p-spacers, and dopant-free properties.
‡
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optimized reaction conditions. The 3,4-ethylenedioxythiophene Table 1 Ligand screening for the threefold direct C–H/C–Br coupling
a
reactions
(
EDOT) moiety could be facilely inserted as p-spacers for YKP03
that led to stable PSC devices with a promising efficiency of
6.15%, free of addition of any commonly used chemical
1
doping agents.
First, we performed a synthetic analysis/comparison taking
the high-performance BTT-3 (ref. 3) as an example (Scheme 1).
Prior to the Suzuki coupling reaction, as reported in the litera-
ture, benzotrithiophene (BTT, 1a) was brominated by the NBS
reagent to generate tribromobenzotrithiophene (1aa). In addi-
tion, the end-group (2a) must be converted into its corre-
sponding boronic ester derivative (2aa) through a three-step
approach including Br/Li-exchange, transmetallation and
ligand displacement with pinacol. Ultimately, the required
organoboron reagent was produced. Coupling of 1aa with 2aa
under general Suzuki reaction conditions afforded the desired
BTT-3. Undoubtedly, this is a reliable synthetic strategy to
access most target organic HTMs. However, in order to lower
the cost of a promising HTM, and simultaneously, to avoid the
preparation and treatment of toxic or unstable organometallic
e
Entry
Ligand
Yield (%)
1
2
3
4
5
6
7
8
9
PPh
P(o-tolyl)
P(m-tolyl)
P(p-tolyl)
3
Trace
30
Trace
5
10
21
18
27
14
Trace
Trace
20
Trace
60
3
3
3
Tris(o-anisyl)phosphine
IPr
P(Cy)
3
P(adamantyl)
SPhos
Dppe
Dppp
Xantphos
DPEphos
P(o-tolyl)
P(o-tolyl)
P(o-tolyl)
2
(nBu)
1
1
0
1
reagents such as organotins or organoborons during synthesis, 12
we envisaged that BTT (1a) would directly undergo the threefold 13
C–H bond activation/arylation reactions with 2a under pre-
optimized palladium catalysis conditions, by which BTT-3
would be obtained in a more step-efficient and user-convenient
manner.
b
c
14
15
16
3
3
3
71
55
d
Hence, as shown in Table 1, we decided to perform ligand
screening for the optimization of the threefold C–H/C–Br
coupling reaction of 1a. The rst experiment was conducted
with triphenylphosphine, giving BTT-3 in only trace amount
(
entry 1). We examined other commonly used tri(aryl)phos-
phine ligands such as tritolylphosphines (entries 2–4). Among
three regioisomers (ortho-, meta-, and para-), P(o-tolyl) was
3
found to be relatively promising since the target product was
isolated in 30% yield (entry 2). The more electron-rich tris(o-
a
Unless specied, the threefold C–H/C–Br coupling reactions were
performed with BTT (1a: 0.30 mmol) and the end-group (2a: 1.05
mmol) in the presence of Pd(OAc)
AcOH (60 mol%), and K CO
2
(15 mol%), ligand (30 mol%),
ꢀ
2
3
(1.08 mmol) in DMF (3 mL) at 125
C
b
c
ꢀ
under N
2
for 30 h. DMF (1 mL). DMF (1 mL) at 150 C for 30 h.
d
e
DMF (1 mL) without AcOH. Isolated yields.
anisyl)phosphine led to the formation of BTT-3 in unsatisfac-
tory yield (10%, entry 5). In addition to phosphine ligands, the
carbene-type ligand (IPr) was also tested. However, it did not
further improve the yield (21%, entry 6) compared to the result
of entry 2. Next, two kinds of tri(alkyl)phosphine ligands
including P(Cy)
affording BTT-3 in 18% and 27% yields, respectively (entries 7–
). A low yield was still obtained when the aryl–alkyl hybrid-type
3 2
and P(adamantyl) (nBu) were investigated,
8
phosphine ligand (SPhos) was employed (14%, entry 9). Further,
the coupling reaction was performed with a number of biden-
tate ligands, three of which were shown to be inefficient (trace,
entries 10, 11, and 13), whereas the sterically rigid xantphos
gave the product in 20% yield (entry 12). Therefore, P(o-tolyl)3
was selected for subsequent optimizations including the alter-
ation of the reaction concentration and temperature. We were
pleased to observe a considerable yield improvement when the
Scheme 1 Two approaches to access BTT-3: multistep synthesis vs.
the straightforward threefold C–H/C–Br coupling reactions.
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solvent loading was reduced from 3 mL to 1 mL (60%, entry 14),
and meanwhile the yield can be further enhanced by setting the
ꢀ
reaction temperature to 150 C (71%, entry 15). Finally, we
wondered if the reaction would proceed without the acid addi-
tive. It was found, however, that BTT-3 was isolated with
a diminished yield (55%, entry 16) in the absence of acetic acid.
Based on the optimum reaction conditions obtained in entry
15, we proceeded to synthesize other target HTMs incorporating
p-spacers (YKP06 and YKP03). Prior to the key steps, a series of
building blocks (2b–d: p-spacer + end-group) were efficiently
prepared by direct C–H monoarylation. As shown in Scheme 2,
the reaction of methoxy-substituted bromotriphenylamine 2a
Scheme 3 Synthesis of YKP06 and YKP03 by the optimized threefold
with excess thiophene under typical palladium-catalyzed C–H direct C–H/C–Br coupling reactions.
activation conditions furnished the desired one-side-arylated
thiophene derivative 2b (63%). Bromination of 2b with NBS
gave 2c in good yield (86%). Likewise, 3,4-ethyl- since we were unable to access 2e. We then decided to exchange
enedioxythiophene (EDOT) underwent direct C–H activation/ the role of both reactants: direct C–H arylation of molecule 2d
arylation with 2a, leading to the formation of product 2d in with the tribrominated BTT 1b led to the production of target
moderate yield (66%). It is worth noting that by adjusting the YKP03 in 30% yield (with acetic acid). The isolated yield was
reaction time (6 h), generation of the undesired diarylated improved (61%) as pivalic acid was employed. We assumed that
EDOT byproduct could be suppressed. However, we failed to different acid additives might exhibit dissimilar reactivities
16
obtain 2e aer carrying out NBS bromination of 2d. We found toward a single C–H activation/arylation substrate (2d).
that molecule 2e was prone to decomposition shortly aer
Prior to device fabrication, BTT-3, YKP06 and YKP03 were
isolation (viscous dark liquid was observed).
evaluated by measuring their optical, electrochemical, thermal
In Scheme 3, the threefold C–H arylation was performed by and electrical properties and the results are collected in Table 2.
opt
the coupling of 1a (BTT) with building block 2c under optimum YKP06 and YKP03 possess smaller optical bandgaps (DE
g
¼
opt
reaction parameters (entry 15, Table 1), affording YKP06 (thio- 2.60 eV) than BTT-3 (DE
g
¼ 2.87 eV) due to the inserted p-
phene as the p-spacer) in 44% yield. On the other hand, the spacers (thiophene for YKP06; EDOT for YKP03) that extended
synthetic route to YKP03 (EDOT as the p-spacer) was modied the conjugation lengths. The highest occupied molecular
orbital energy (EHOMO) of each HTM was estimated by per-
forming cyclic voltammetry (CV), showing that all HTMs
exhibited appropriate HOMO energy levels (EHOMO ¼ ꢁ5.16 ꢂ
ꢁ
5.37 eV) located between the EHOMO of the perovskite layer
(
E
HOMO ¼ ꢁ5.43 eV) and gold electrode (EHOMO ¼ ꢁ5.10 eV). The
thermal characteristics of the three molecules were investigated
by thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). Compared to the data for BTT-3 provided in
the literature (T
duction of a thiophene unit as the p-spacer considerably low-
ꢀ
ꢀ
3
d
¼ 470 C; T
g
¼ 159 C), we found that intro-
ꢀ
ered the decomposition temperature of YKP06 (Td ¼ 208 C)
while the glass transition temperature was slightly reduced (T
g
ꢀ
¼
148 C). On the other hand, YKP03 also displayed a relatively
ꢀ
ꢀ
lower T
d
of 411 C, whereas its T
g
was shown to be signicantly
enhanced (213 C), implying that EDOT might be regarded as
a superior p-spacer candidate for benzotrithiophene-based
HTMs. In addition, hole mobility data of HTMs in the space-
charge limited current (SCLC) region were obtained by fabri-
cating each molecule with the device conguration of FTO/
PEDOT:PSS/HTM/Ag. We were pleased to nd that YKP06 and
ꢁ
4
YKP03 demonstrated better hole mobilities (m
h
¼ 4.53 ꢃ 10
;
ꢁ
4
2
ꢁ1 ꢁ1
5
1
.80 ꢃ 10 cm V
s
) than the reported BTT-3 (m
h
¼ 2.66 ꢃ
ꢁ4
2
ꢁ1 ꢁ1
0
cm V
s ), indicating that the installation of p-spacers
enabled an improvement in the conductivity of BTT-based
HTMs. It is worth noting that, under dopant-free conditions,
YKP03 has been shown to exhibit a comparable hole mobility
ꢁ
4
2
ꢁ1 ꢁ1
(m
¼ 5.74 ꢃ 10
cm
V
s ), making it promising for
Scheme 2 Synthesis of building blocks 2b–e (p-spacer + end-group)
using direct C–H monoarylation as the key step.
h
subsequent solar cell applications.
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Table 2 The optical, electrochemical, thermal and electrical properties of BTT-3, YKP06 and YKP03
opta
g
b
c
d
d
ꢀ
ꢀ
2
ꢁ1 ꢁ1
HTMs
DE
[eV]
E
HOMO [eV]
E
LUMO [eV]
T
[ C]
T
g
[ C]
Hole mobility (cm V
s )
ꢁ
ꢁ
ꢁ
4
4
4
BTT-3
YKP06
YKP03
2.87
2.60
2.60
ꢁ5.37
ꢁ5.34
ꢁ5.16
ꢁ2.50
ꢁ2.74
ꢁ2.56
470
208
411
159
148
213
2.66 ꢃ 10
4.53 ꢃ 10
5.80 ꢃ 10
a
opt
DE
CH Cl
g
¼ 1240/lintersect, lintersect refers to the intersection of UV/Vis absorption and photoluminescence spectra (measurements performed in
b
c
2
2
).
T
E
HOMO ¼ ꢁ[E1/2 (vs. Ag/AgClsat'd) + 0.197 (vs. NHE) + 4.500] eV (CV measurements performed in CH
Cl
2 2
).
ELUMO ¼ EHOMO
+
opt
d
DE
g
.
d
was obtained at 5% weight loss of HTMs.
Perovskite solar cells using BTT-3, YKP06 and YKP03 as hole-
Under forward and reverse scanning, as shown in Fig. 2a, the
transport layers were fabricated. The photovoltaic performance devices demonstrated small hysteresis effects and the YKP03-
data are summarized in Table 3. In general, each molecule was based dopant-free PSCs exhibited the best performance. In
ꢀ
dissolved in chlorobenzene and then heated to 90 C for 20 Fig. 2b, we show the spectra of the photon-to-current conversion
minutes in order to obtain a clear solution. Two doping agents, efficiency (IPCE) of perovskite solar cells fabricated with each
4-t-butylpyridine and lithium bis(triuoromethanesulfonyl) HTM. It was found that the devices based on YKP03 (dopant-
imide, were added to the above solution before spin-coating was free conditions) achieved nearly 80% IPCE from the wave-
carried out. Perovskite solar cells based on BTT-3 showed length of 450 nm to 650 nm and exhibited a higher IPCE value
a power conversion efficiency (PCE) of 13.79% which is lower from 670 nm to 770 nm than those using other HTMs. This is
than the literature reported value of 18.20%. We speculated that consistent with their remarkable Jsc value shown in Table 3. As
the enhanced PCE might be attributed to the use of an addi- shown in Fig. 2c, we also measured the steady-state power
3
tional additive (FK209 as the third dopant). Compared to BTT- output of PSC devices using the more efficient YKP03 as the
3, solar cells with YKP06 or YKP03 exhibited superior open- HTM through a continuous PCE (h) vs. time (t) experiment
circuit voltage (Voc ¼ 0.990–1.020 V) and ll factor (FF ¼ (voltage set at 0.82 V). It was found that both curves (YKP03 with
6
2
3
7.20–68.90%) while the short-circuit current density (J
¼
and without dopants) reached Jmax quickly and remained stable
0.86–20.96 mA cm ) was shown to be lower than that of BTT- for over 300 seconds.
, which led to PCEs of 13.92–14.63%. We assumed that the YKP In order to study the hole-transfer process at device inter-
sc
ꢁ
2
series had higher hole mobilities because of their better lm faces, steady-state photoluminescence (PL) was measured based
formation. For the PSCs with YKP06, we assumed that charge on the device structures of glass/perovskite and glass/
recombination might take place at the perovskite/HTM inter- perovskite/HTM (PL spectra provided in Fig. S6†). The
face due to its lower-lying ELUMO (ꢁ2.74 eV, Table 2), thus giving perovskite-only device showed a strong PL band, while the ones
a relatively lower value of JSC. In the case of YKP03 fabricated fabricated with HTMs displayed a signicant reduction of the
under dopant-free conditions, the devices demonstrated PL emission. Compared with other synthesized HTMs, YKP03
improved photovoltaic parameters (V ¼ 1.030 V/J ¼ 23.07 mA exhibited a more effective PL quenching, implying that it may
oc
sc
ꢁ
2
cm /FF ¼ 68%), resulting in a promising PCE of 16.15% which extract holes efficiently from the perovskite layer. Besides, we
is better than that of the doped solar cells with other HTMs also carried out time-resolved PL (TRPL) experiments (Fig. S7†).
synthesized in this work. Moreover, the performance of YKP03- The tted average lifetime for YKP03/perovskite is 7.84–8.50 ns,
based dopant-free PSCs was close to that of the reference cells which is superior than those for BTT-3/perovskite and YKP06/
using the commercially available spiro-OMeTAD as the HTM perovskite (12.99 ns and 11.35 ns). This result also indicated
(PCE ¼ 17.59%).
that YKP03 possesses a better charge extraction capability.
a,b,c
Table 3 The photovoltaic performance of perovskite solar cells using BTT-3, YKP06 or YKP03 as the hole-transport layer
ꢁ
2
HTMs
Voc [V]
Jsc [mA cm
]
FF [%]
PCE [%]
BTT-3
Best
0.960
22.56
63.67
13.79
Average
Best
0.940 ꢄ 0.030
21.82 ꢄ 1.21
20.96
63.91 ꢄ 3.36
13.11 ꢄ 0.69
YKP06
0.990
67.20
13.92
Average
Best
1.000 ꢄ 0.020
1.020
20.44 ꢄ 0.43
65.0 ꢄ 2.57
68.90
13.28 ꢄ 0.64
14.63
YKP03
20.86
Average
Best
Average
Best
0.980 ꢄ 0.040
20.40 ꢄ 1.12
23.07
70.40 ꢄ 3.51
14.07 ꢄ 0.81
YKP03 (dopant-free)
spiro-OMeTAD
1.030
68.00
16.15
1.010 ꢄ 0.030
1.050
22.75 ꢄ 0.40
69.06 ꢄ 1.71
72.10
15.86 ꢄ 0.41
17.59
23.24
Average
1.050 ꢄ 0.020
22.26 ꢄ 0.76
73.64 ꢄ 2.67
17.21 ꢄ 0.26
a
b
The statistical data were obtained based on 6–8 cells. Unless specied, the perovskite solar cells were fabricated with the addition of two doping
agents: 4-t-butylpyridine and lithium bis(triuoromethanesulfonyl)imide. Reverse scans.
c
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Fig. 3 (a) PCE vs. device oxidation time curves and (b) PCE vs. long-
term storage stability of the perovskite solar cells using YKP03 as the
HTM.
solar cells exhibited exceptional stability and the PCEs retained
over 80% of the best value for more than 800 h, whereas the
devices with two doping agents were found to be far less stable
since their PCEs dropped drastically within 500 h. The chemical
dopants (TBP and Li-TFSI) are nitrogen-containing compounds
which may absorb water during the device oxidation process
and long-term storage test. In particular, the Li-TFSI salt is very
Fig. 2 (a) The photocurrent density vs. voltage (J–V) characteristic hygroscopic because of its ionic characteristics. Therefore, we
curves; (b) the incident photon-to-current conversion efficiency speculated that both hygroscopic chemical dopants might
(
IPCE) spectra of perovskite solar cells based on BTT-3, YKP06 or
inuence the lm formation/quality of YKP03-based devices (in
particular the hydrophilic perovskite-based layer might be
partly destroyed), thus leading to an inferior PCE. In contrast,
under dopant-free conditions, YKP03-based PSCs exhibited
better PCEs and potential long-term storage stabilities.
YKP03; (c) the steady-state power output: PCE (h) vs. time (t) curves of
perovskite solar cells based on YKP03.
We further explored the device oxidation time and long-term
stability focusing on perovskite solar cells with the most
promising YKP03 (Fig. 3). With two doping agents, perovskite
solar cells showed the best PCE of 14.63% without the need for
the device oxidation process (Fig. 3a). However, the efficiency
rapidly declined to ꢂ7% when the device was exposed to an
In summary, we provide a step-saving new route to benzo-
trithiophene (BTT) core-based hole-transporting materials
(BTT-3, YKP06 and YKP03) via Pd-catalyzed direct C–H arylation
under optimized reaction conditions. Among three HTMs,
YKP03 incorporating EDOT as the p-spacer demonstrated
promising power conversion efficiencies of up to 16.15% when
employed in perovskite solar devices, without the use of any
chemical doping agents. Moreover, YKP03-based dopant-free
solar cells also displayed impressive long-term stabilities,
making it one of the potential candidates for replacing the
costly and doping-necessary spiro-OMeTAD.
ꢀ
ambient atmosphere for 4 h (25 ꢄ 2 C and 50–70% RH),
presumably owing to the hygroscopic chemical dopants that
caused device instability. In contrast, although the dopant-free
solar cells required at least 3 h of oxidation to achieve
a comparable PCE (ꢂ14%) to that of the doped ones, they
appeared to remain stable and the PCE continued climbing to
the highest value (16.15%) thereaer within 3 h. More impor-
tantly, as shown in Fig. 3b, experiments for long-term storage
stability were also conducted in a glove box under 5% RH. The
result indicated that, under dopant-free conditions, perovskite
Conflicts of interest
There are no conicts to declare.
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Acknowledgements
Financial support provided by the Ministry of Science and
Technology (MOST), Taiwan (MOST 107-2113-M-008-001 and
1
08-2628-E-182-003-MY3), National Central University (NCU),
9 K. Rakstys, A. Abate, M. I. Dar, P. Gao, V. Jankauskas,
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is gratefully acknowledged.
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