Simple synthesis and molecular engineering of low-cost and star-shaped carbazole-based hole transporting materials for highly efficient perovskite solar cells

Article abstract of DOI:10.1039/c7ta04762b

Perovskite solar cells (PrSCs) have emerged as a very promising technology in the field of photovoltaics by demonstrating power conversion efficiencies (PCEs) soaring from 3.9% to above 22% within the past eight years. To date, perovskite solar cells mainly depend on spiro-OMeTAD to perform a key role as a hole transporting material (HTM). However, the complicated multi-step synthetic procedures and high-cost purification process for spiro-OMeTAD limited its potential for commercial application. Herein, three new carbazole-based HTMs with a starburst structure, coded as SGT-405(3,6), SGT-410(3,6) and SGT-411(3,6) via tuning the substitution position from the (2,7) to the (3,6) position of the carbazole moiety, have been successfully synthesized via three-step synthesis from commercially available reagents and investigated for highly efficient perovskite solar cells. By adopting this strategy, among them, molecularly engineered carbazole derivative SGT-405(3,6) exhibits significantly increased T_g (192.7 °C), improved film forming ability, reduced hole reorganization energy and enhanced hole mobility compared to its parent molecule SGT-405(2,7) and spiro-OMeTAD. Owing to the promising properties of SGT-405(3,6), meso-porous type PrSCs employing SGT-405(3,6) showed a remarkable PCE of 18.87%, which is better than that of the photovoltaic device employing spiro-OMeTAD (17.71%). To the best of our knowledge, the achieved PCE (18.87%) is the highest value reported for devices with the structure of FTO/compact TiO₂/meso-porous TiO₂/CH₃NH₃PbI_3-xCl_x/HTM/Au employing small-molecular HTMs. Meanwhile, owing to the simple synthesis of SGT-405(3,6), compared with SGT-405(2,7) previously developed by our group, synthesis cost was much lowered, resulting in low cost compared to the spiro-OMeTAD and SGT-405(3,6), by approximately three times. Furthermore, the long-term device stability of PrSCs was enhanced for SGT-405(3,6) to some extent compared to those of other HTMs studied here due to the good uniform capping layer of SGT-405(3,6) on top of the perovskite layer and the prevention of moisture penetration into the perovskite layer. Therefore, SGT-405(3,6) is a promising low-cost and efficient non-spiro type HTM with potential to replace expensive spiro-OMeTAD for PrSCs.

Full text of DOI:10.1039/c7ta04762b

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
rsc.li/materials-a

Page 1 of 28
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DOI: 10.1039/C7TA04762B
Simple Synthesis and Molecular Engineering of Lowꢀcost and Starꢀshaped
Carbazoleꢀbased Hole Transporting Materials for Highly Efficient
Perovskite Solar Cells
a+
a+
b
a,*
Chunyuan Lu , In Taek Choi , Jeongho Kim and Hwan Kyu Kim
a
Global GETꢀFuture Lab. and Department of Advanced Materials Chemistry, Korea
University, Sejong 300ꢀ79, Korea
b
Department of Chemistry, Inha University, 100 Inhaꢀro, Incheon 402ꢀ751, Korea
*To whom correspondence should be addressed: Email: hkk777@korea.ac.kr
+
These authors equally contributed to this work.
Electronic supplementary information (ESI) available. See DOI:

Journal of Materials Chemistry A
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Abstract
Perovkite solar cells (PrSCs) have emerged as a very promising technology in the field of
photovoltaics by demonstrating power conversion efficiencies (PCEs) soaring from 3.9% to
above 22% within the past eight years. To data, perovskite solar cells mainly depend on
spiroꢀOMeTAD to perform a key role as a hole transporting material (HTM). However, the
complicated multiꢀstep synthetic procedures and highꢀcost purification process for
spiroꢀOMeTAD limited its potential for commercial application. Herein, three new
carbazoleꢀbased HTMs with starburst structure, coded as SGTꢀ405(3,6), SGTꢀ410(3,6) and
SGTꢀ411(3,6) via tuning the substitution position from (2,7) to (3,6) position of carbazole
moiety, have been successfully synthesizedvia threeꢀstep synthesis from commercial
available reagentsand investigated for highly efficient perovskite solar cells. By adopting this
strategy, among them,molecularly engineered carbazole derivativeSGTꢀ405(3,6) exhibits
significantly increased T
g
(192.7℃), improved film forming ability, reduced hole
reorganization energy and enhanced hole mobility compared to its parent molecule
SGTꢀ405(2,7) and spiroꢀOMeTAD. Owing to the promising properties ofSGTꢀ405(3,6),
mesoꢀporous type PrSCs employing SGTꢀ405(3,6) showed a remarkable PCE of 18.87%,
which is better than the photovoltaic device employing spiroꢀOMeTAD (17.71%).To the best
of our knowledge, the achieved PCE (18.87%) is the highest value reported for devices with
2 2 3 3 x
the structure of FTO/compactTiO /mesoꢀporousTiO /CH NH PbI3ꢀxCl /HTM/Au employing
smallꢀmolecular HTMs. Meanwhile, owing to the simple synthesis of SGTꢀ405(3,6),
compared with SGTꢀ405(2,7) previously developed by our group, synthesis cost was much
lowered, resulting in low cost compared to the spiroꢀOMeTAD and SGTꢀ405(3,6), by
approximately three times. Furthermore, the longꢀterm device stability of PrSCs was
enhanced for SGTꢀ405(3,6) to some extent compared to other HTMs studied here due to the
good uniform capping layer of SGTꢀ405(3,6) on top of perovskite layer and the prevention of
the moisture penetration into the perovskite layer. Therefore, SGTꢀ405(3,6) is a promising
lowꢀcost and effective nonꢀspiro type HTM with potential to replace expensive
spiroꢀOMeTAD for PrSCs.
Key words: holeꢀtransporting materials, carbazole, simple synthesis, hole mobility, film
forming ability, perovskite solar cells

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DOI: 10.1039/C7TA04762B
Introduction
Perovskites have been widely employed as efficient light absorbing layer for photovoltaic
1
, 2
cellsdue to their broad spectral absorption from visible to nearꢀinfrared, high charge carrier
3
4, 5
6
mobilities, long charge diffusion lengths, small exciton binding energy (ca. 50 meV),
7
solutionꢀprocessing abilities. Very recently, the certified power conversion efficiency (PCE)
8
of perovskite solar cells has skyrocketed to over 22%. Due to the rapid development of
perovskite solar cells (PrSCs), it was elected as one of the top ten emerging technologies of
9
2016.
10
Holeꢀtransporting material (HTM) is a necessary component for efficient PrSCs, since
HTMs play a critical role in efficiently extracting holes from perovskite to HTM layer and
simultaneously transmitting to counter electrode in order to decrease in charge recombination
and loss of Voc. So far, spiroꢀOMeTAD is still the most commonly used hole transporting
material for efficient PrSCs. Unfortunately, complicated synthetic scheme and highꢀcost
purification procedures (sublimation) for spiroꢀOMeTAD result in efficient but costly
materials, because the synthesis of spiroꢀOMeTAD involved five reaction steps that require
low temperature (ꢀ78℃), and sensitive (nꢀbutylꢀlithium or Grignard reagents) and aggressive
11
(Br
2
) reagents. In addition, the purities in HTMs can act as trap site for holes, thus
highꢀpurity sublimationꢀgrade spiroꢀOMeTAD isrequired to obtain highꢀperformance
12
devices. The complicated synthesis and highꢀcost purification procedures (sublimation) of
spiroꢀOMeTAD hindered its potential for commercialization of efficient PrSCs. Therefore it
is necessary to develop more economical and efficient alternative nonꢀspiro type HTMs to
spiroꢀOMeTAD.
Carbazole derivatives have attracted much attention due to their interesting photochemical
13, 14
properties.
Increased attention on carbazoleꢀbased derivatives could be explained by the
features of carbazole derivatives's: low cost of the 9Hꢀcarbazole starting material, good
chemical and environmental stability, versatility of the carbazole reactive sites that can react
with a wide variety of functional groups to fine tuning of its electronic and optical
15ꢀ17
properties.
In our previous work, we reported the synthesis and characterization of
carbazoleꢀbased HTMs with twoꢀ or threeꢀarm chemical structures, which were linked
through phenyleneꢀderived central core units. We also investigated their application in
1
8, 19
CH
3 3 3
NH PbX ꢀbased perovskiteꢀsensitized solar cells.
In this work, in order to further decrease the synthesis cost and improve the hole transporting
18
capability, we designed and synthesized three derivatives of (SGTꢀ405(2,7), SGTꢀ410(2,7)

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1
9
and SGTꢀ411(2,7) ) via tuning the substitution position from the 2,7 to the 3,6 position of
carbazole, which are coded as SGTꢀ405(3,6), SGTꢀ410(3,6)and SGTꢀ411(3,6). It is worth to
mention that the substitution position tuning from ( 2,7) to (3,6) position of carbazole moiety
not only simplified synthesis routes (Figure 2 and Figure 3), but also improved its hole
transporting capability, which can be illustrated by its improved smooth film forming ability
and hole mobility compared to SGTꢀ405(2,7).With the device structure of FTO/compact
TiO /mesoꢀporous TiO /CH NH PbI Cl /HTM/Au, the SGTꢀ405(3,6) showed a PCE of
2
2
3
3
3ꢀx
x
18.87%, which is better than the photovoltaic devices based on spiroꢀOMeTAD (17.71%). To
the best of our knowledge, the achieved PCE (18.87% ) is the highest value reported for the
devices with structure of FTO/compact TiO /mesoꢀporous TiO /CH NH PbI Cl /HTM/Au
2
2
3
3
3ꢀx
x
employing smallꢀmolecular HTMs. Therefore, the simple synthesis and good performance of
SGTꢀ405(3,6)make it a very promising candidate for industrial production in the near future.
Results and discussion
The carbazole moiety consists of pyrrole ring fused to two benzene rings. The nitrogen in the
pyrrole ring makes carbazole an electronꢀdonating unit (pꢀtype). Besides, the carbazole unit
itself possesses good thermal stability, and further it can be functionalized with a variety of
substituent to tune its thermal, photoꢀphysical and electrochemical properties for
1
6,17,20
21
OLED
for organic semicondutors
high electronꢀdonating ability and chemical stability. In our previous work, carbazole
and solar cells. Therefore, carbazole derivatives have attracted much attention
2
2ꢀ26
due to their good thermal stability, lowꢀcost starting material,
18
derivatives based on the (2,7) substitution position of carbazole unit, SGTꢀ405(2,7) and
1
9
SGTꢀ411(2,7) exhibited good photovoltaic performance when employed as HTMs for
MAPbI ꢀbased perovskite solar cells. However, the preparation of 2,7ꢀdibromocarbazole is a
3
challenging task, which involves the nitration of 4,4'ꢀdibromo biphenyl by nitric acid and
27ꢀ29
cadogan reductive cyclization, as shown in Figure 2.
Herein, we try to tune the
substitution position from the 2,7 to 3,6 position of carbazole moiety, as shown in Figure 2.
Compared to 2,7ꢀdibromocarbazole,3,6ꢀdibromocarbazole is commercially available and
costꢀeffective material. In addition, because the electron donating ability of the 3,6ꢀcarbazole
3
0,
is higher due to the fact that the N atom is at the para position in 3,6ꢀcarbazole derivatives,
3
1
we can expect that the newly designed 3,6ꢀcarbazole derivatives may have better hole
transporting property. Therefore, we designed and synthesized new three carbazole
derivatives, which are code as SGTꢀ405(3,6), SGTꢀ410(3,6) and SGTꢀ411(3,6) (see Figure
1
). The synthetic scheme for new designed molecules are shown in Figure 3.

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Figure 1. Molecuar structures of SGTꢀ405(3,6), SGTꢀ410(3,6) ,SGTꢀ411(3,6) and
SGTꢀ405(2,7).
Figure 2. Synthetic diagram of substitution position tuning of carbazole moiety for lowꢀcost
carbazoleꢀbased small molecule HTMs for perovskite solar cells.

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Figure 3. The synthetic routes for SGTꢀ405(3,6), SGTꢀ410(3,6) and SGTꢀ411(3,6).
Figure 4. Normalized UVꢀVis absorption and photoluminescence spectra of SGTꢀ405(3,6),
SGTꢀ410(3,6), SGTꢀ411(3,6) and reference HTMs (spiroꢀOMeTAD and SGTꢀ405(2,7) )
measured in DCM solution. (Note: The UVꢀVis absorption spectra of SGTꢀ405(3,6) coincide
with that of SGT410(3,6) ).

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The normalized UVꢀvis absorption spectra of three new developed HTMs and reference
HTMs (spiroꢀOMeTAD and SGTꢀ405(2,7)) measured in dichloromethane are shown in
Figure 4. The maximum absorption peaks of SGTꢀ405(3,6), SGTꢀ410(3,6), SGTꢀ411(3,6)
and reference molecules of spiroꢀOMeTAD and SGTꢀ405(2,7) are summarized in Table 1.
All new HTMs showed strong absorption bands at 270ꢀ310 nm, which are attributed to the
πꢀπ* transitions of carbazole and SGT units, and weak absorption bands at 330ꢀ400 nm,
32
corresponding to the nꢀπ* transitions of carbazole and SGT units. It is worth stating that no
absorption peak of SGTꢀ405(3,6), SGTꢀ410(3,6) and SGTꢀ411(3,6) observed in visible
region compared to spiroꢀOMeTAD is truly beneficial in terms of subduing the incident
18, 33
photon loss absorbed by the HTM layer itself.
Compared to the λmax of spiroꢀOMeTAD
(385 nm), the λmax of SGTꢀ405(3,6), SGTꢀ410(3,6) and SGTꢀ411(3,6) are blue shifted to 306
nm, indicating that much weakened conjugation in SGTꢀ405(3,6), SGTꢀ410(3,6) and
SGTꢀ411(3,6) than that of spiroꢀOMeTAD. The only chemical structure difference between
SGTꢀ405(2,7) and SGTꢀ405(3,6) was shown in Figure 1. Even though the substitution
position tuning from the (2,7) to the (3,6) of carbazole moiety leads to approximately 36 nm
red shift of λmax, compared to SGTꢀ405(2,7), SGTꢀ405(3,6) exhibits a much decreased
absorption peak in the wavelength region of 350ꢀ425 nm. From the above absorption profiles,
it could be confirmed that synthesized new HTMs can act as HTMs without interfering with
18, 34
the incident light absorbing process by perovskite layer.
The maximum emission peaks of
SGTꢀ405(3,6), SGTꢀ410(3,6) and SGTꢀ411(3,6) are located at 466 nm, 466 nm and 530 nm,
respectively. From the intersection of absorption and emission spectra, their bandꢀgap can be
estimated and summarized in Table 1.
The HOMO energy level was derived from the first oxidation potential, whereas the LUMO
energy level was determined by subtracting the bandꢀgap energy from the HOMO energy
level. In order to determine the first oxidation potential (E ) of new developed HTMs, cyclic
ox
voltammetry (CV) experiments were conducted in DCM solution. The diagram of CV curves
are shown in Figure 5 and the corresponding energy levels are collected in Table 1. The
HOMO energy level of spiroꢀOMeTAD measured is ꢀ5.25V, which is similar to the value of
35
previous reported result. The HOMO energy levels of new synthesized HTMs (ꢀ5.26 eV for
SGTꢀ405(3,6) and SGTꢀ410(3,6), ꢀ5.24 eV for SGTꢀ411(3,6) ) are very close to that of
spiroꢀOMeTAD (ꢀ5.25 eV) and well aligned with the valence band of perovskite (ꢀ5.43 eV).
The energy level alignment of the related materials used in this study are illustrated in Figure
6
. The suitable HOMO energy levels of new developed HTMs allow holes to be extracted
from perovskite to HTMs layer. In addition, LUMO energy levels of all new designed HTMs
are much higher than the conduction band of perovskite, indicating that new designed HTMs
could act as electron block layer and thus reduce the charge recombination processes.

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Figure 5. Cyclic voltammograms of SGTꢀ405(3,6), SGTꢀ410(3,6), SGTꢀ411(3,6) and
spiroꢀOMeTAD and the FC/FC+ redox couple in THF with 0.1 M
6
tetraꢀnꢀbutylammoniumhexafluorophosphate (TBAPF ) as the supporting electrolyte with a
ꢀ1
scan rate of 50 mV S .

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Table 1. Photophysical and electrochemical properties of new HTMs, SGTꢀ405(2,7) and
spiroꢀOMeTAD.
a)
b)
c)
d)
f)
λabs (nm)
ε
λem
Eox
Egap
HOMO
(eV)
LUMO
(eV)
Hole Mobility
HTMs
ꢀ
1
ꢀ1
2
ꢀ1 ꢀ1
M cm
(nm) (V vs NHE) (eV)
(cm ꢁV ꢁs )
(
)
3
3
3
3
2
06
60
06
60
85
118070
35420
ꢀ
4
SGTꢀ405(3,6)
SGTꢀ410(3,6)
466
464
0.76
0.76
2.92
2.92
ꢀ5.26
ꢀ5.26
ꢀ2.34
ꢀ2.34
1.8×10
117710
35310
ꢀ
5
5.27×10
8.74×10
135630
135630
40690
ꢀ
5
SGTꢀ411(3,6)
SGTꢀ405(2,7)
spiroꢀOMeTAD
306
530
424
425
0.74
2.91
3.03
3.05
ꢀ5.24
ꢀ5.25
ꢀ5.25
ꢀ2.33
ꢀ2.22
ꢀ2.20
3
2
60
70
123480
64210
86440
46800
58800
60000
e)
ꢀ4
358
0.75
1.05×10
3
3
90
07
ꢀ
5
366
85
0.75
8.32×10
3
6
a) Cyclic voltammograms measurements were carried out in THF solution with 0.1M TBAPF as supporting
electrolyte, ferrocene/ferrocenium(E(Fc/Fc+) = 630 mV vs. NHE) as reference.
b)Calculated from the intersection of the normalized absorption and emission spectrameasured in DCM
solvent.(Figure 4).
c)HOMO=ꢀ(Eox+4.5 eV).
d)LUMO=HOMO+Egap
.
18
e)Eox of SGTꢀ405(2,7)are collected from our published paper.
f)Pure HTM without doping.

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Figure 6. a) The energy level alignment of the related materials used in this study, b)
Configuration of solar cell device used in this study, c) Crossꢀsection of FEꢀSEM image of
representative perovskite solar cells.
The thermal properties of various HTMs were studied by thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC).The corresponding data are summarized in Table
2
. And Figure 7 shows that all of the new designed small molecules have high decomposition
temperature (Tdec) located in the range of 372~400℃, with a weight loss of 5%, which is
similar to that of spiroꢀOMeTAD (417℃), indicating newly developed HTMs possess good
thermal stability. It is already reported that HTMs possessing higher glass transition
temperature can reduce the tendency to orientated aggregation (crystallization) upon
3
6,37
heating.
morphology stability of amorphous film upon long time device storage or heating. Among
them, SGTꢀ405(3,6) exhibited very high glass transition (T ) of 192.7℃) (Figure 8).
Comparison of T of SGTꢀ405(3,6) with that of SGTꢀ405(2,7) shows that substitution
position tuning from (2,7) to (3,6) position of carbazole moiety leads to the increase of T by
g
The glass transition temperature (T ) of HTMs is an important indicator for the
g
g
g

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44.1 ℃. In addition, the glass transition temperature of SGTꢀ405(3,6) is much higher than
that of spiroꢀOMeTAD (125.8℃). However, there are no glass transition peaks detected for
both SGTꢀ410(3,6) and SGTꢀ411(3,6). Additionally, crystallization process is detected at
233℃ for SGTꢀ405(3,6) during first heating scan, whereas no crystallization was observed
during the cooling and second and third heating step. In applying small molecular materials
for electronic and optoelectronic devices, the morphology stability upon heating is often
required, since phase transitions could take place during device operation (e.g., solar cells
sitting continuously in direct sunlight can get quite hot, especially in hot climatic
conditions).Therefore, we studied the morphology stability of various HTM films spinꢀcoated
onto glass substrate under different heating condition (w/o heating or heating at 130 ℃ for
1hour)by polarized optical microscopy in atmosphere. When annealing treatment (130℃ , 1
hour) was applied on various HTM spinꢀcoated films, enlarged crystallization areas were
observed in the case of SGTꢀ405(2,7), SGTꢀ410(3,6), SGTꢀ411(3,6) and spiroꢀOMeTAD
films, while almost no change of surface morphology of SGTꢀ405(3,6) was observed,
indicating that SGTꢀ405(3,6) exhibits the best morphology stability upon heating (stable
g
amorphous ) due to its very high T of 193℃ . (Figure S1).
Figure 7. Thermogravimetric analysis data of SGTꢀ405(3,6), SGTꢀ410(3,6), SGTꢀ411(3,6)
ꢀ1
and reference HTMs (spiroꢀOMeTAD and SGTꢀ405(2,7) ) with scan rate of 10℃ min under
atomosphere .
N
2

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Figure 8. Differential scanning calorimetry traces of SGTꢀ405(3,6), SGTꢀ410(3,6),
SGTꢀ411(3,6) and other HTMs (spiroꢀOMeTAD and SGTꢀ405(2,7)) with scan rate of 10℃
ꢀ1
2
min under N atomosphere.

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Table 2. Thermal properties of new developed HTMs and spiroꢀOMeTAD.
a)
a)
a)
b)
HTMs
Tg (℃)
Tm (℃)
Tcr (℃)
Tdec (℃)
SGTꢀ405(3,6)
SGTꢀ410(3,6)
SGTꢀ411(3,6)
SGTꢀ405(2,7)
spiroꢀOMeTAD
192.7℃
332.8℃
233℃
396℃
372℃
372℃
396℃
417℃
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
148.6℃
125.8℃
328.8℃
247.4℃
212.2
ꢀ
a) Glass transition (collected from the second circle of DSC curve), melting
ꢀ1
2
andcrystallization temperatures observed from DSC(10℃min under N atmosphere)
ꢀ1
b) Degradationtemperature observed from TGA measurements (10℃min under N
2
atmosphere)
The uniformity of HTM capping layer spinꢀcoated onto perovskite film was dramatically
influenced by the solubility and aggregation behavior (related to the molecular structure) of
3
8
HTMs. It has been widely demonstrated that smooth film forming ability of HTM is very
important for the creation of good interfacial contact of perovskite/HTM/Au, which could
39, 40
significantly affect the hole extraction and the charge recombination in device.
Therefore
surface morphology of various spinꢀcoated HTMs film were investigated by FEꢀSEM. As
shown in Figure 9,spinꢀcoated SGTꢀ405(3,6) film exhibits very smooth and uniform capping
layer onto perovskite layer, which could be ascribed to its highly twisted nonꢀplanar
41
molecular structure. As for spinꢀcoated spiroꢀOMeTAD film, there are some nanoꢀsized
needleꢀshaped islands appeared as shown in Figure 9f, indicating that spiroꢀOMeTAD tends
4
2, 43
to oriented aggregation or crystallization due to its intrinsic feature,
which is in
agreement with the previous reported result that some very small crystals were observed in
41
the spinꢀcoated film of spiroꢀOMeTAD. The chemically structural difference of three new
synthesized HTMs was shown in Figure 1. Long alkyl chains in SGTꢀ410(3,6) and
SGTꢀ411(3,6) molecules were introduced to increase their solubility. However, a surprising
phenomenon was observed, when compared to uniform SGTꢀ405(3,6) film: Some
sphereꢀshaped islands were observed in the spinꢀcoated films of SGTꢀ410(3,6) and
SGTꢀ411(3,6), which could be ascribed to the aggregation induced by the additional van der
38
Waals interactions stemming from the long alkyl chains.
In order to further study the top morphology of each HTM film spinꢀcoated on top of
perovskite layer, atomic forced microscopy was used to measure surface roughness of various
HTM capping layers. As shown in Figure 10, the perovskiteꢀonly film without HTMs shows
the largest surface roughness (RMS) of 15.989 nm (see Figure 10a), After the deposition of
HTMs on top of perovskite films, the roughness was significantly decreased. The value of
surface roughness decreases in the order of SGTꢀ411(3,6), spiroꢀOMeTAD, SGTꢀ410(3,6),

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SGTꢀ405(2,7) and SGTꢀ405(3,6). The 3DꢀAFM image of top morphology of SGTꢀ405(3,6)
see Figure 10b) indicates that SGTꢀ405(3,6) leads to the formation of uniform and full
(
coverage capping layer on top perovskite film, which is agreed with the result from FEꢀSEM
pictures, as shown in Figure 9b. It should be addressed that the surface morphology and
roughness should have a strong influence on the photovoltaic performances. The relatively
high fill factor of 78.65 of SGTꢀ405(3,6) is partially contributed to its good film forming
ability. The relatively low fill factor of SGTꢀ410(3,6) and SGTꢀ411(3,6) is partially ascribed
to their roughness surface morphology.
x
Figure 9. a) SEMꢀ top view of the MAPbCl3ꢀxI film. bꢀf) The top view of various HTM
capping layers on the perovskite films; b) SGT405(3,6); c) SGT410(3,6); d) SGT411(3,6); e)
SGT405(2,7); f) spiroꢀOMeTAD.

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Figure 10. AFM images of top morphology of various films. a) MAPbCl3ꢀx
I
x
film; bꢀf) the
top morphology of various HTM capping layers on the perovskite films: b) SGT405(3,6); c)
SGT410(3,6); d) SGT411(3,6); e) SGT405(2,7); f) spiroꢀOMeTAD.
Hole mobility is another significant parameter to consider in the design of new HTMs for
perovskite solar cells. Herein, the hole mobility of each new HTM synthesized was measured
44, 45
by using spaceꢀchargeꢀlimited current (SCLC) method.
by space charge and is given as:
The current density Jsc is limited
9
ꢆꢇ
ꢈꢉ
ꢀ
ꢁ ꢂꢃ ꢃ
ꢄ ꢅ
8

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−
12
where J is the current density, µ is the hole mobility, ε
0
vacuum permittivity (8.85 × 10
F
−1
m
), ε is the dielectric constant of the material (normally taken to approach 3 for organic
r
4
6
semiconductors) , V is the applied bias, and d is the film thickness. The hole only device
structure used in this work: FTO/PEDOT:PSS/HTM/Au. Fitting the JꢀV curves (Figure11)
for each HTM applying to the equation evaluates the hole mobilities to be listed in Table 1.
ꢀ5
2
ꢀ1 ꢀ1
The value of hole mobility (8.32×10 cm ꢁV ꢁs ) of spiroꢀOMeTAD obtained here is very
ꢀ5
2
ꢀ1 ꢀ1
47
close to the value of 8.67×10 cm ꢁV ꢁs reported in a previous literature. As shown in
ꢀ5
2
ꢀ1 ꢀ1
Table 1, SGTꢀ410(3,6) exhibits the lowest hole mobility (5.27×10 cm ꢁV ꢁs ).
ꢀ4
2
ꢀ1 ꢀ1
SGTꢀ405(3,6) exhibits the highest hole mobility (1.8×10 cm ꢁV ꢁs ), followed by
ꢀ4
2
ꢀ1 ꢀ1
ꢀ5
SGTꢀ405(2,7) (1.05×10 cm ꢁV ꢁs ). SGTꢀ411(3,6) shows comparable value of 8.74×10
2
ꢀ1 ꢀ1
cm ꢁV ꢁs to that of spiroꢀOMeTAD.
Figure 11. JꢀV characteristic of thin films in holeꢀonly devices.
Space charge limited current (SCLC) measurements indicated that SGTꢀ405(3,6) exhibited
.8 times higher hole mobility than that of SGTꢀ405(2,7), but these two molecules show quite
1
similar chemical structure to each other. To elucidate this from theoretical approach, density
functional calculations were performed by using the B3LYP/6ꢀ31G(d,p) level with Gaussian
09 program. And spiroꢀOMeTAD used as a reference compound was calculated together
using the same basis set and calculating method. The optimized geometry structures, detailed
energy levels of frontier molecular orbitals and hole reorganization energy (λ) are
summarized in Figure 12. The reorganization energy (λ) represents the relaxation of the
molecule when one electron is withdraw or donated. Generally, based on the anatomy of a
Marcusꢀtype electronꢀtransfer mechanism between the molecules, the small reorganization
45
energy implies the fast transfer rate for electrons or holes to some extent. As shown in

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Figure 12, the hole reorganization energy of spiroꢀOMeTAD is 153.3 meV which is close to
45
the value reported in previous literature. The λ values of SGTꢀ405(3,6) and SGTꢀ405(2,7)
molecules are 95 meV and 117.3 meV, respectively, indicating that SGT405ꢀ(3,6) exhibits a
very small hole reorganization energy, much smaller than that of SGTꢀ405(2,7) and
spiroꢀOMeTAD, which agrees with the trend of hole mobility values measured by SCLC
method. Furthermore, the λ value of 95 meV of SGTꢀ405(3,6)is indeed comparable to the
hole reorganization energies computed for the best organic pꢀtype semiconducting materials
48
(
e.g., heteroacenes or oligoacenes). These findings demonstrated that the substitution
position tuning from (2,7) to (3,6) position of carbazole moiety not only simplified synthesis
route, but also decreased its hole reorganization energy.
Figure 12. Results of DFT calculation (optimized molecular geometry, electron distributions
ofHOMO and LUMO energy levels and hole reorganization energy (λ)) of SGT405ꢀ(3,6),
SGTꢀ405(2,7) and spiroꢀOMeTAD).

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Figure 13. Timeꢀresolved PL spectra of glass/perovskite and glass/perovskite/various HTMs
measured at an excitation wavelength (393nm), and emission wavelength (750nm).
The interfacial hole transfer process between new HTM and perovskite can be investigated
3
2
by TRꢀPL (timeꢀresolved photoluminescence) measurements. Figure 13 shows TRꢀPL
spectra acquired from various perovskite films spinꢀcoated with different HTMs. Compared
with the neat perovskite films, the PL intensity is quenched much more significantly for
spiroꢀOMeTAD and SGTꢀ405(3,6), followed by SGTꢀ405(2,7), SGTꢀ411(3,6) and then
SGTꢀ410(3,6), indicating that SGTꢀ405(3,6) exhibits better hole extraction ability compared
to that of SGTꢀ405(2,7), SGTꢀ411(3,6) and SGTꢀ410(3,6).
From the characterization of newly synthesized HTMs by UVꢀVis absorption and
photoluminescence spectroscopy, cyclic voltammetry, thermogravimetric analysis,
differential scanning calorimetry,scanning electron microscope, atomicforce microscopy
andspaceꢀchargeꢀlimited current (SCLC) measurement, we can expect that the
newlydevelopedHTMs have the potential to be applied as hole transporting materials for
PrSCs.The high hole mobility of SGTꢀ405(3,6) could potentially lead to high Jsc value for
perovskite solar cells. Moreover, the high hole mobility and excellent film forming ability of
34
SGTꢀ405(3,6)could potentially lead to higher fill factor of the operating devices.
To evaluate the ability of synthesized compounds acting as HTM for PrSCs, the mesoꢀporous
type configuration of FTO/compactꢀTiO /mesoporous TiO /CH NH / HTM/Au in
Figure 6b was used in this work. All devices were fabricated using one step deposition
2
2
3
3 x
PbCl3ꢀxI
49
method with toluene dropping processes and the detailed procedure is described in
supporting information section. Different component layer of representative complete device
can be seen clearly from the crossꢀsection scanning electron microscopy (SEM) image in
1
8
Figure 6c. For comparison, spiroꢀOMeTAD and SGTꢀ405(2,7) were employed as reference
HTMs in this study.

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The JꢀV curves of perovskite solar cells with different HTMs were shown in Figure 14a. And
their detailed parameters of photovoltaic performances were summarized in Table 3.
2
spiroꢀOMeTADꢀbased device gives a PCE of 17.71% with Jsc of 22.01mA/cm , FF of 76.58,
V
oc of 1.051 V. SGTꢀ405(3,6)ꢀbased PrSC device exhibits the highest PCE up to 18.87%,
2
mainly coming from the enhanced Jsc of 22.93 mA/cm and remarkable high FF of 78.65,
2
while SGTꢀ405(2,7)ꢀbased device exhibits a PCE of 18.0% with Jsc of 22.32 mA/cm , FF of
7
6.32, Voc of 1.057 V. The FF is related to series resistance (R
order to obtain a high FF, the solar cells should have a low R
words, the HTM layer should effectively block the electron and transport the hole from
to the Au electrode. The fabrication conditions used for all devices are the same,
s
) and shunt resistance (Rsh). In
50
s
and a high Rsh
.
In other
MAPbCl3ꢀxI
3
except for HTM. Therefore, the enhanced hole mobility and better film forming ability of
SGTꢀ405(3,6) compared to other HTMs used in this work, rendered SGTꢀ405(3,6) based
2
PrSC with enhanced Jsc of 22.928 mA/cm , remarkable high FF of 78.65 and the lowest R
s
of
30.9ꢂ. The trend of fill factors of PrSCs device employing various HTMs followed a trend
s
identical to the hole mobility of various HTMs, ascribed to a reduced R (series resistance). A
s
straightforward method of estimating R for a solar cell is to calculate the slope of the J−V
51
curve near Voc. As shown in Table 3, SGTꢀ410(3,6) and SGTꢀ411(3,6) based PrSCs showed
relatively high series resistance of 44.4 ꢂ and 45.2 ꢂ, respectively. Figure 14b shows the
monochromatic incident photon conversion efficiency (IPCE) spectra of perovskite solar cells
employing different HTMs, and the integrated photocurrent densities are in good agreement
with the Jsc measured from JꢀV measurements.
Figure 14. a) Current density (J)–voltage (V) curves of the bestꢀperforming solar cells with
SGTꢀ405(3,6), SGTꢀ410(3,6), SGTꢀ411(3,6), SGTꢀ405(2,7) and spiroꢀOMeTAD (measured
ꢀ2
under 100 mWꢁcm AM1.5G solar illumination); b) corresponding IPCE curves.

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Table 3. Summary of photovoltaic parameters for the bestꢀperforming devices.
2
HTMs
J
sc (mA/cm )
Voc (V)
FF (%) η (%) Average η (%)
s
R (ꢂ))
SGTꢀ405(3,6)
SGTꢀ410(3,6)
SGTꢀ411(3,6)
SGTꢀ405(2,7)
22.928
20.574
21.190
22.321
1.0466
1.0461
1.0348
1.0567
1.0507
78.65
75.25
75.22
76.32
76.58
18.87 18.31±0.44
16.20 15.54±0.41
16.50 15.68±0.54
18.00 17.18±0.82
17.71 16.93±0.48
30.9
44.4
45.2
32.6
40.2
spiroꢀOMeTAD 22.009
Figure 15. Device statistics of (a) J , (b) V , (c) fill factor and (d) PCE of the perovskite
sc
oc
solar cells employing various HTMs.

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Figure 16. Current (J)–voltage (V) curves of the bestꢀperforming dopantꢀfree solar cells
employing various HTMs, SGT405ꢀ(3,6), SGTꢀ410(3,6), SGTꢀ411(3,6) and SGTꢀ405(2,7)
and spiroꢀOMeTAD.
Table 4. Summary of photovoltaic parameters for the bestꢀperforming dopantꢀfree device
using different developed HTMs.
2
HTMs
J
sc (mA/cm )
V
oc (V)
FF (%) η (%)
s
R (ꢂ))
SGTꢀ405(3,6)
SGTꢀ410(3,6)
SGTꢀ411(3,6)
SGTꢀ405(2,7)
21.625
15.837
14.246
20.699
0.9504
0.936
0.9581
0.9415
0.99
41.27
36.47
23.64
40.27
38.16
8.48
5.41
3.23
7.85
8.13
189
306
865
253
202
spiroꢀOMeTAD 21.519
To investigate the effect of HTMs on the cells stability, aging tests of the perovskite solar
cells without encapsulation were performed under the same dark storage conditions (in
atmosphere, at room temperature and the humidity in the range of 15% ꢀ 20% ). As shown in
Figure 17, SGTꢀ405(3,6) exhibited the best device stability among five types of HTMs based
PrSCs, while SGTꢀ411(3,6) based devices renderedthe poorestdevice stability. The device
stability test results revealed thatthe improved stability of SGTꢀ405(3,6)ꢀbased cell compared
to other HTMꢀbased devices could be attributed to the uniform HTM capping layer on the top
4
0, 43
of perovskite layer, preventing the moisture penetration into the perovskite layer.

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Figure 17. The longꢀterm device stability tests for PrSC devices with various HTMs (in
atmosphere, at room temperature and the humidity in the range of 15% ꢀ 20%). The inserted
picture is the corresponding graph of normalized power conversion efficiency as function of
time.
Cost evaluation
To compare the synthesis cost of SGTꢀ405(3,6) with spiroꢀOMeTAD and SGTꢀ405(2,7), the
lab synthesis cost of SGTꢀ405(3,6) and SGTꢀ405(2,7) were estimated by using the model
52
proposed by Osedach et al. More detail information about the synthesis cost is available in
supporting information section.
Table 5. Comparison of the estimated material costs.
[
a]
HTMs
Material cost ((€/g
29.65
)
Commercial price((€/g)
SGTꢀ405(3,6)
SGTꢀ405(2,7)
ꢀ
[b]
88.08
ꢀ
[c]
spiroꢀOMeTAD
96.09
186ꢀ580
[
[
a] Material cost estimated for the synthesis of 1 g of product.
18
b] Synthetic scheme taken from our published paper.

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53
[
c] Taken from the reference.
Conclusions
In this study, three new carbazoleꢀbased starburst small molecule HTMs, which are coded as
SGTꢀ405(3,6), SGTꢀ410(3,6) and SGTꢀ411(3,6), were designed and synthesized on the basis
1
8, 19
of our previous work
via tuning the substitution position from (2,7) to (3,6) position of
carbazole moiety. This makes them easily synthesized via threeꢀstep from commercially
available starting materials. They were characterized by UVꢀVis absorption and
photoluminescence spectroscopy, cyclic voltammetry, scanning electron microscope and
atomic force microscopy and space charge limited current (SCLC) measurement. Among
them, SGTꢀ405(3,6) exhibits high glass transition temperature (192.7℃), good solubility,
excellent film forming ability and high hole mobility compared to spiroꢀOMeTAD.
Meanwhile SGTꢀ405(3,6) shows very small hole reorganization energy (95 meV). These
analytic results demonstrate that the substitution position tuning from (2,7) to (3,6) position
of carbazole moiety not only simplified synthesis route, but also improved its hole
transporting performance, which can be illustrated by its improved smooth film forming
ability, reduced hole reorganization energy (λ) and increased hole mobility compared to
SGTꢀ405(2,7). Owing to the promising characteristics of SGTꢀ405(3,6), with the device
2 2 3 3 x
structure of FTO/compact TiO /mesoꢀporous TiO /CH NH PbI3ꢀxCl /HTM/Au, the device
employing SGTꢀ405(3,6) as HTM affords an remarkable conversion efficiency of 18.87%,
which is better that of spiroꢀOMeTAD based devcie (17.71%) under the same fabrication
conditions. To the best of our knowledge, the achieved PCE (18.87%) is the highest
conversion efficiency among the device with structure of FTO/compact TiO
2
/mesoꢀporous
TiO /CH NH /HTM/Au employing smallꢀmolecular HTMs. In addition, the
2
3
3 x
PbI3ꢀxCl
longꢀterm device stability of PrSCs was enhanced for SGTꢀ405(3,6) compared to other
HTMs studied here due to the good uniform capping layer of SGTꢀ405(3,6) on top of
perovskite layer and the prevention of the moisture penetration into the perovskite layer. Our
work demonstrate SGTꢀ405(3,6) is an promising alternative nonꢀspiro type small molecular
HTM with lowꢀcost and excellent performance for existing cost ineffective and
syntheticallyꢀchallenging spiroꢀOMeTAD in perovskite solar cells.

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Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded
by the Korean government (MSIP) through the Midꢀcareer Researcher Program
(2014R1A2A1A10051630) as well as Climate Change Program (2015M1A2A2056543)and
the Functional Districts of the Science Belt Support Program, Ministry of Science, ICT and
Future Planning (2015K000287).
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Journal of Materials Chemistry A
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DOI: 10.1039/C7TA04762B
Graphic abstract
Simple Synthesis and Molecular Engineering of Low-cost and Star-shaped
Carbazole-based Hole Transporting Materials for Highly Efficient Perovskite
Solar Cells
a
a
b
a,*
Chunyuan Lu , In Taek Choi , Jeongho Kim and Hwan Kyu Kim
a
Global GET-Future Lab. and Department of Advanced Materials Chemistry, Korea University,
Sejong 300-79, Korea
b
Department of Chemistry, Inha University, 100 Inha-ro, Incheon 402-751, Korea