Inverted planar perovskite solar cells with dopant free hole transporting material: Lewis base-assisted passivation and reduced charge recombination

Article abstract of DOI:10.1039/c7ta02440a

Three novel triarylamine derivatives (TPACs: TPAC0M, TPAC2M and TPAC3M), sharing the same conjugation scaffold but possessing different numbers of methoxy units, were designed for high performance hole transport materials (HTMs) of p-i-n planar perovskite solar cells (PSCs). Cyclic voltammetry and absorption results showed that their energy levels would be beneficial to work as HTMs of PSCs, and time-resolved photoluminescence and transient photo-voltage studies proved that TPAC-based PSCs had better charge extraction and more suppressed non-radiative recombination properties than PEDOT:PSS-based PSCs, leading to superior power conversion efficiency (PCE). We confirmed that the methoxy units introduced to the triarylamine derivatives did not cause noticeable changes in their optical properties, such as absorption and bandgap, electrical conductance, measured by conductive atomic force microscopy, and mobility, but the charge extraction and recombination behaviors of TPAC-based PSCs could be improved by increasing the number of methoxy units in the arylamine moiety. X-ray photoelectron spectroscopy revealed that methoxy units could act as a Lewis base passivating the defect sites at the interface between HTM and perovskite, consequently resulting in an increase of their PCE to 17.54% without any dopants.

Full text of DOI:10.1039/c7ta02440a

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Jeon, I. K. Lee, J. Zhang, H. Jeong, J. Park, J. Bang, T. K. Ahn, H. Shin, B. G. Kim and H. J. Park, J. Mater.
Chem. A, 2017, DOI: 10.1039/C7TA02440A.
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Page 1 of 22
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DOI: 10.1039/C7TA02440A
Inverse Type Planar Heterojunction Perovskite Solar Cells with
Dopant Free Hole Transporting Material: Lewis Base-Assisted
Passivation and Charge Recombination
Sang Jin Park,1,† Seolhee Jeon,2,† In Kyu Lee,2,† Jing Zhang,1 Huiseong Jeong,1,3 Ji-Yong
Park,1,3 Jiwon Bang,4 Tae Kyu Ahn,5 Hee-Won Shin,5 Bong-Gi Kim2,* and Hui Joon Park1,6,*
1Department of Energy Systems Research, Ajou University, Suwon 16499, Republic of Korea
2Department of Organic and Nano System Engineering, Konkuk University, Seoul 05029,
Republic of Korea
3Department of Physics, Ajou University, Suwon 16499, Republic of Korea
4Nano convergence Materials Center, Korea Institute of Ceramic Engineering & Technology,
Jinju 52851, Republic of Korea
5Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea
6Department of Electrical and Computer Engineering, Ajou University, Suwon 16499,
Republic of Korea
*Corresponding Authors: huijoon@ajou.ac.kr, bgkim2015@konkuk.ac.kr
†S.J.P., S.J. and I.K.L. contributed equally to this work.
Keywords: triarylamine, dopant free hole transporting material, perovskite solar cell, charge
recombination, organic electronics
1

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Abstract: Three novel triarylamine derivatives (TPACs: TPAC0M, TPAC2M and TPAC3M),
DOI: 10.1039/C7TA02440A
sharing the same conjugation scaffold but possessing the different number of methoxy unit,
were designed for high performance hole transport materials (HTMs) of perovskite solar cells
(PSCs), especially p-i-n inverse-type planar heterojunction PSCs, and synthesized through a
simple suzuki-type coupling reaction. From cyclic voltammetry and absorption results, it was
shown that their energy levels would be beneficial to work as a HTM of PSCs. In addition,
time-resolved photoluminescence measurement and transient photo-voltage study proved that
TPACs-based PSCs had better charge extraction and more suppressed non-radiative
recombination properties than PEDOT:PSS-based PSCs, leading to superior power
conversion efficiency (PCE). Furthermore, we confirmed that the applied methoxy units to the
triarylamine derivatives did not cause noticeable changes on their optical properties, such as
absorption and bandgap, electrical conductance, measured by conductive atomic force
microscopy, and mobility, but the charge extraction and recombination behaviors of TPACs-
based PSCs could be improved by increasing the numbers of methoxy unit in arylamine
moiety, expected to act as Lewis base that could passivate the defect sites at the interface
between HTM and perovskite, analyzed by X-ray photoelectron spectroscopy depth
measurement, consequently resulting in an increase of their PCE to 17.54 % without any
dopants.
2

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DOI: 10.1039/C7TA02440A
Introduction
Organic semiconductors have been intensively investigated, as a potential candidate, to
replace inorganic semiconductor or conductor, because of their unlimited modification ability
and intrinsically flexible characteristic. Especially, exhaustive explorations on the charge
transfer mechanism in organic semiconductors have enlarged their possible application areas,
such as organic light emitting diode (OLED),1 photovoltaic (PV) device2 and field-effect
transistor (FET).3 Recently, organic-inorganic hybrid perovskite solar cells (PSCs), which
utilized a perovskite material as photoactive layer, have drawn great attention because of
relatively higher power conversion efficiency (PCE), compared to the PV devices adopting an
organic semiconductor as active layer.4 Perovskites are efficient light harvesting materials
with bipolar transport property and long carrier diffusion length, but it has been shown that
organic semiconductor-based hole transport materials (HTMs), which extract holes from the
perovskite layer and transport them to an electrode, could play a crucial role for high
efficiency PSCs.5 In order to achieve higher light-to-electrical energy conversion efficiency in
PSCs, the organic HTMs should have an appropriate energy level alignment with perovskites
and a good charge carrier mobility to minimize both electrical potential loss and charge
recombination. For this purpose, organic semiconductors having strong crystallization
behaviour6 or versatile feasibility of chemical doping7 were dominantly considered as
effective HTMs in PSC, because of their superior electrical mobility in comparison with
amorphous organic semiconductors.
In OLEDs, triarylamine derivatives are predominantly considered as HTMs, because
the amine atom is relatively easy to lose one electron and the resulting radical cation, potential
hazard for long-term stability of electronic devices, can be stabilized by resonance effect of
adjacent aryl substituents.8 However, the inherently low electrical mobility of the triarylamine
derivative, due to amorphous-like characteristics, has restricted their broad application to
3

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9
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PSCs, unless they were chemically doped to improve the electrical charge mobilitDyO.I:S10i.n10c3e9/Cth7TeA02440A
chemical doping would cause potential issues, such as high cost, complicate fabrication and
dopant-induced device degradation,10 devising highly efficient dopant-free HTM is one of
inevitable demands to make further advancement of PSCs.
Meanwhile, most recent PSCs have either n-i-p-type or p-i-n-type configurations.
Even though the highest efficiencies over 20% PCE have been reported from n-i-p-type
configuration PVs, built on thick mesoporous (mp) metal-oxide scaffold, their complicated
preparation procedures and needs for high temperature annealing hampered the extension of
PSC technology to low-cost flexible devices that can be fabricated by a high-speed roll-to-roll
process.4 Moreover, without the mp metal-oxide layer, n-i-p-type PSCs often have severe
hysteretic behavior. Therefore, the necessity of high performance HTM, which can realize p-i-
n type planar heterojunction (HJ) PSCs by simple one-step solution-process at low
temperature as well as those with long-term stability is continuously increasing.
In this contribution, three triarylamine derivatives were newly synthesized and their
optical and electrical properties were evaluated as HTMs of p-i-n inverse-type planar HJ PSCs.
When they were adopted as HTMs in this configuration, overall power conversion efficiency
(PCE) reached over 17% without any chemical doping treatment. When the optical and
electrical properties of the synthesized triarylamine derivatives were explored, it was shown
that the Lewis-base effect of the methoxy group at the triarylamine derivative backbone,
analyzed by X-ray photoelectron spectroscopy (XPS) depth measurement, played an
important role to increase charge extraction property and to reduce charge recombination,
ultimately resulting in better device performances including the enhancement of short circuit
current (Jsc) and open circuit voltage (Voc). In addition, the structure dependent charge
extraction and recombination tendencies were systematically correlated by both time-resolved
photoluminescence (TRPL) and transient photo-voltage (TPV) studies.
4

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DOI: 10.1039/C7TA02440A
Results and discussion
As illustrated in Fig. 1 and Scheme S1, we designed and synthesized three novel triarylamine
derivatives (TPACs: TPAC0M, TPAC2M and TPAC3M) sharing a same conjugated
backbone to which the different numbers of methoxy unit attached. All triarylamine
derivatives were obtained from one- or a two-step simple chemical reaction, and their
1
structures were fully characterized by means of H-NMR and mass spectroscopy. When the
absorption characteristics of the obtained TPACs were compared, they showed nearly same
absorption spectra both in solution and film state (Fig. S1), reflecting an amorphous-like
physical nature which is commonly shown in triarylamine derivatives.11 Introduction of
electron donating methoxy groups to the triarylamine (TPAC2M) or carbazole (TPAC3M)
moieties caused absorption red-shift slightly (15 nm) or little change, respectively. However,
the introduced methoxy groups noticeably changed the molecular energy levels in the TPACs.
The highest occupied molecular orbital (HOMO) energy levels of TPACs were directly
measured by means of cyclic voltammetry (CV) and the lowest unoccupied molecular orbital
(LUMO) energy levels were estimated with their absorption onset. As shown in Fig. S2 and
Fig. 1b, TPAC2M containing two methoxy units at the diphenylamine moiety exhibited
relatively shallow HOMO level, rather than TPAC0M having no methoxy unit. The additional
introduction of the electron donating methoxy group to the phenyl carbazole moiety did not
significantly change molecular energy levels, including optical band-gap (TPAC3M). Based
on the obtained energy levels, we could speculate that TPACs have enough potential to
function as efficient HTMs in PSC, because their shallow HOMO levels, compared to that (-
5.4 eV) of perovskite, would be beneficial to extract hole from perovskite layer and the
shallow LUMO levels would effectively block electron transfer from perovskite layer to HTM,
respectively.12
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DOI: 10.1039/C7TA02440A
H3CO
a
N
N
N
N
H3CO
OCH3
TPAC0M
TPAC2M
TPAC3M
c
ab
b
-1.71 eV
TPAC3M
-4.96 eV
-1.73 eV
-1.83 eV
-3.9 eV
Al
-4.3 eV
PCBM61
-6.0 eV
ZnO
TPAC2M
-4.3 eV
Al
PCBM / ZnO
TPAC0M
-5.18 eV
-4.4 eV
CH3NH3PbI3
TPAC
ITO
ITO
-4.7 eV
-4.98 eV
500 nm
-5.4 eV
Hole transporting layer
nm
5
nm
5
nm
5
nm
5
d1
d2
d3
d4
PEDOT:PSS
TPAC0M
TPAC3M
TPAC2M
(RMS: 1.72)
(RMS: 0.29)
(RMS: 0.26)
(RMS: 0.30)
2.5
0
2.5
2.5
0
2.5
0
0
‐2.5
‐5
‐2.5
‐5
‐2.5
‐2.5
‐5
2 μm
2 μm
2 μm
‐5
2 μm
Fig. 1. a) Detailed chemical structures of obtained novel triarylamine derivatives for dopant
free hole transporting materials. b) Energy level of each layer of perovskite solar cell. c)
Cross-sectional SEM of TPACs-based perovskite solar cell (TPAC3M-based device as a
representative). AFM height images of d1) PEDOT:PSS, d2) TPAC0M, d3) TPAC2M, and
d4) TPAC3M on ITO electrode. RMS values of PEDOT:PSS, TPAC0M, TPAC2M, and
TPAC3M on ITO are 1.72, 0.29, 0.30, and 0.26 nm, respectively.
6

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To investigate the performances of TPACs as dopant free HTMs, we fabricated p-i-n
DOI: 10.1039/C7TA02440A
inverse type planar HJ PSCs as following configuration: ITO / HTM / CH3NH3PbI3 / [6,6]-
phenyl-C61-butyric acid methyl ester (PCBM) / ZnO / Al. Additionally, PEDOT:PSS, one of
the most widely utilized organic conductors, was also applied as a HTM to relatively compare
the charge extraction and transfer efficiency in PSCs. Atomic force microscopy (AFM)
topological images of HTMs in Fig. 1d confirmed that the height variation of the surface of
TPACs on ITO was within a nanometer, of which the root mean square (RMS) roughness was
around 0.3 nm, much lower than that of PEDOT:PSS (1.7 nm). Therefore, TPACs could more
efficiently planarize the rough ITO surface than PEDOT:PSS, and it is expected that their
uniformity would not induce any morphological difference in the following perovskite
photoactive layer. The optimized film formation condition of TPACs was 20 mg/ml at 1000
rpm (Table S1), giving the thickness in the range of 40 nm – 50 nm (Fig. 1c). In Fig. 2a and
Table 1, the performances of PSCs, which utilized newly obtained TPACs and PEDOT:PSS
as HTMs, are summarized. All TPACs-based PSCs exhibited better device performances than
the PEDOT:PSS-based PSCs, and in particular, PSCs with TPAC3M showed the best
performance over 17% of PCE without any chemical doping treatment. TPACs-based PSCs
also showed better stability than PEDOT:PSS-based PSCs (Fig. S3). All J-V characteristics of
our PSCs showed negligible hysteresis depending on scan directions, indicating stable
operation of each cell (Fig. S4).
Table 1. Summary of device performance of perovskite solar cell adopting different hole
transporting materials
HTM Materials
PEDOT:PSS
TPAC0M
Voc (V)
Jsc (mA/cm2)
18.23 (18.57)
19.80 (20.73)
21.68 (22.58)
22.11 (22.79)
FF
PCE (%)
0.86 (0.87)
0.92 (0.91)
0.98 (0.99)
0.99 (1.00)
0.73 (0.78)
0.72 (0.74)
0.72 (0.71)
0.75 (0.78)
11.44 (12.60)
12.92 (13.92)
15.20 (15.77)
16.58 (17.54)
TPAC2M
TPAC3M
Numbers are average values of forward and reverse scan data, measured from over 30 devices for each
condition (values in parentheses are from the best performing devices).
7

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DOI: 10.1039/C7TA02440A
b
a
103
25
20
15
10
5
102
101
100
10-1
10-2
PEDOT:PSS
TPAC0M
TPAC2M
TPAC0M
TPAC2M
TPAC3M
0
TPAC3M
-5
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (V)
Voltage (V)
PEDOT:PSS
TPAC0M
c
1.0
1.0
c1)
c2)
0.5
0.0
0.5
0.0
c5)
-0.5
-1.0
-0.5
-1.0
-0.10
-0.05
0.00
0.05
0.10
-0.10
-0.05
0.00
0.05
0.10
V(V)
V(V)
TPAC3M
TPAC2M
1.0
0.5
1.0
0.5
c3)
c4)
0.0
0.0
-0.5
-1.0
-0.5
-1.0
-0.10
-0.05
0.00
0.05
0.10
-0.10
-0.05
0.00
0.05
0.10
V(V)
V(V)
Fig. 2. a) J-V curves of PEDOT:PSS- and TPACs(TPAC0M, TPAC2M, and TPAC3M)-based
perovskite solar cells. All data were measured at AM 1.5 G (100 mW·cm−2 intensity). b)
Measured logJ-V plots under dark conditions for hole-only devices. The calculated hole
mobility values of TPAC0M, TPAC2M, and TPAC3M from the SCLC method are 1.210-5,
1.010-5, 1.110-5 cm2V-1s-1, respectively. c) I-V curves of c1) PEDOT:PSS, c2) TPAC0M,
c3) TPAC2M and c4) TPAC3M measured by c-AFM. The mean values of resistances
(standard deviations), measured from PEDOT:PSS, TPAC0M, TPAC2M, and TPAC3M by c-
AFM, are 2.1 (0.9), 1.1 (0.6), 1.1 (0.6), and 1.5 (0.8) kΩ, respectively. c5) Schematic diagram
of c-AFM measurement.
The enhanced Jsc of TPACs-based PSCs could be originated from the better charge
transfer efficiency in HTM or/and the effective charge extraction from the perovskite layer.
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Since both PEDOT:PSS and TPACs have negligible optical density on the highDpOhI:o10to.10n3-9f/lCu7TxA02440A
region, the most amounts of photocurrent could be generated from the applied perovskite
photo-absorber after absorption of same amount solar light. In order to evaluate the charge
transfer efficiency in HTM, we measured electrical conductivity of HTM in vertical direction
by conductive atomic force microscopy (c-AFM), as shown in Fig. 2c. Interestingly,
resistances measured from all HTMs showed almost similar level (1~2 kΩ) and their variation
was within standard deviation, suggesting that charge transport behavior may be not be a
crucial factor to explain the improved photocurrent generation of TPACs. This was further
confirmed by mobility values in vertical direction measured by hole-only devices, which
utilized high work-function molybdenum oxide (MoO3) instead of ZnO to block the injection
of electrons from the Al cathode, using the space-charge-limited current (SCLCs) method at
low voltage: J = 9εoεrμV2/8L3, where ε0εr is the permittivity of the component, μ is the carrier
mobility, and L is the thickness.13 As shown in Fig. 2b, the measured hole mobilities from
PSCs with TPACs were similar (TPAC0M = 1.210-5 cm2V-1s-1, TPAC2M = 1.010-5
cm2V-1s-1, and TPAC3M = 1.110-5 cm2V-1s-1).
To examine the charge extraction ability of doping-free TPACs from perovskite
photoactive layer as HTM, we prepared Quartz(Q)/doping-free TPACs (TPAC0M, TPAC2M
or TPAC3M)/CH3NH3PbI3 samples and measured their PL lifetimes by time-correlated single
photon counting (TCSPC), maintaining Q/PEDOT:PSS/CH3NH3PbI3 as a reference to TPACs.
The PL decay curves of each sample were convoluted using bi-exponential functions and
were plotted in Fig. 3a. The Q/PEDOT:PSS/CH3NH3PbI3 showed an averaged PL lifetime
(ave) of 7.34 ns; a fast decay component, 1, of about 2.1 ns and slow decay component, 2, of
12.1 ns. Those PL lifetimes were shortened with applying a HTM to TPACs-based samples.
Especially, TPAC3M with three electron-donating methoxy units showed the shortest PL
lifetime of ave=3.68 ns, compared to those (ave=4.27 ns, ave=4.98 ns) of TPAC2M with two
9

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methoxy units and TPAC0M without functional groups, respectively, indicating DthOaI:t10T.10P3A9/CC7T3A02440A
has about a twofold increase in charge extraction rate compared to that of PEDOT:PSS, which
is advantageous to get better photocurrent generation. The detailed information about bi-
exponential fitting result is in Table S2. This photogeneration trend is well-matched with Jsc
values of TPACs- and PEDOT:PSS-based PSCs; TPAC3M: 22.79 mA/cm2 > TPAC2M:
22.58 mA/cm2 > TPAC0M: 20.73 mA/cm2 > PEDOT:PSS: 18.57 mA/cm2. The EQE signals
of PSCs with TPAC3M and PEDOT:PSS are in Fig. S5. From the PL decays and Jsc result,
we could also conclude that the electron-donating methoxy units attached to triarylamine
(TPAC2M) or carbazole (TPAC3M) moieties could play a crucial role to improve the device
performances, which will be discussed in detail.
a
b
1000
100
10
PEDOT:PSS
TPAC0M
PEDOT:PSS
TPAC0M
TPAC2M
1000
TPAC2M
TPAC3M
TPAC3M
1
100
0.1
0
10
20
30
40
50
60
0.6
0.7
0.8
0.9
1.0
Time (ns)
Voc
e
d
d
c
c
e
O 1s (TPAC3M-only, at 0 min sputtering)
O 1s (Perovskite on TPAC3M, at 1.8 min sputtering
100
80
60
40
20
0
Pb 4f
O 1s
528
532
536
540
0.0
0.4
0.8
1.2
1.6
2.0
528
532
536
540
Binding Energy (eV)
Sputtering time (min)
Binding Energy (eV)
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Fig. 3. a) PL decay curves of perovskite/HTM layers on quartz at peak emissionDwOIa: 1v0e.10le39n/gC7tThA02440A
(770 nm), excited with 670 nm from quartz side. Black, red, blue and green colors represent
PEDOT:PSS, TPAC0M, TPAC2M and TPAC3M. b) Recombination lifetime vs. Voc plots of
complete cells having PEDOT:PSS (black), TPAC0M (red), TPAC2M (blue) and TPAC3M
(green) as a HTM, which measured by transient photovoltage (TPV) experiment. XPS spectra
showing the O 1s region of (c) TPAC3M-only sample at 0 min sputtering and (d) perovskite-
casted TPAC3M sample at 1.8 min sputtering. e) XPS depth profiles of perovskite-casted
TPAC3M sample.
In addition to Jsc, we explored the recombination behavior of photo-carriers in PSC
devices using TPV measurement to clarify the Voc variation among devices. As we have
shown in Fig. 2a and Table 1, all TPACs-based PSCs showed superior Voc to PEDOT:PSS-
based PSCs, even though HOMO levels of TPAC-series are comparable (TPAC0M = 5.18
eV) to or even lower (TPAC2M = 4.98 eV, TPAC3M = 4.96 eV) than that of PEDOT:PSS
(known to be about 5.2 eV). Moreover, TPAC3M having the lowest HOMO level due to the
electron donating methoxy units in its conjugated backbone, produced higher Voc than that of
TPAC0M without any methoxy unit in the same conjugated backbone. The Voc values of the
devices are 1.00 V > 0.99 V > 0.91 V > 0.87 V for TPAC3M, TPAC2M, TPAC0M,
PEDOT:PSS, respectively. Therefore, we speculated that the recombination behavior of
photo-carrier in those PSC structures, which directly influenced their Voc, would be different
between TPACs and PEDOT:PSS and that it could be further manipulated by changing the
number of electron donating methoxy units, attached to their conjugated backbone. To
investigate the charge recombination behavior in actual device structures, we used the TPV
measurement.14 Fig. 3b shows the averaged charge recombination lifetimes (τn) from three
types of TPACs- and PEDOT:PSS-based PSCs, measured with an increment in Voc. PSCs
with doping-free TPACs as HTMs revealed much longer τn than that of PEDOT:PSS as a
HTM in the entire range of Voc. Furthermore, the more methoxy units TPAC conjugated
backbone has, the longer τn TPAC-based PSCs have. The longer τn would be beneficial to
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reduce recombination event and to increase the possibility of charge extraction in actual
DOI: 10.1039/C7TA02440A
device structures. Consequently, the longer τn provided better performances of devices
including Voc. The change of recombination behavior in device structure in an accord with the
utilized HTMs is well-matched with their Voc variation.
The aforementioned changes in kinetics of excitons and free carriers depending on
kinds of HTMs suggest that trap density at the interface between HTM and perovskite could
be reduced by introducing methoxy units to TPACs. It has been reported that Lewis bases
such as thiophene, pyridine and methoxy group could passivate defect sites such as under-
coordinated Pb atoms on the crystal surface and at the grain boundaries, often generated
during the annealing process of perovskites, resulting in decreasing non-radiative
recombination in perovskite films.15 In order to clarify if methoxy group at TPAC2M and
TPAC3M worked as Lewis base at the interface between TPACs HTM and perovskite, XPS
depth analysis was performed to directly investigate the interfacial chemical states. Fig. 3c
and d show XPS spectra showing the O 1s regions of the TPAC3M-only sample and
perovskite-casted TPAC3M sample. As for the signal of perovskite-casted TPAC3M sample,
the perovskite/TPAC3M interface reference point (around 1.4 ~ 1.6 min of sputtering time),
in which the high Pb 4f fraction of perovskite layer significantly decreased (Fig. 3e), was
firstly clarified, and the O 1s signal at 1.8 min of sputtering time was utilized for the
comparison with that of bare TPAC3M sample. As shown in XPS spectra, higher binding
energy O 1s feature (around 538 eV), indicative of a direct interaction of oxygen sites as
Lewis base with possible defect sites such as halide vacancy of perovskite,16 was clearly
observed. Although TPACs contain amine showing stronger Lewis basicity than oxygen of
methoxy unit, the amine atom of the triarylamine moiety is concealed by the peripheral three
phenyl units. Thus, methoxy units attaching to the end of molecule could have higher chance
to react with the defect site of perovskite layer. This methoxy group effect was obvious when
charge extraction and recombination behaviors of PSCs with three types of TPACs having
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different number of methoxy unit were compared, as we showed earlier. As the number of
DOI: 10.1039/C7TA02440A
introduced methoxy unit increases from TPAC0M to TPAC3M, charge extraction could be
enhanced and recombination be effectively suppressed, which provides higher Jsc and Voc to
PSCs.
Conclusions
In summary, we designed and synthesized three novel triarylamine derivatives, which can
control carrier extraction and recombination behaviors in PSCs as a high performance HTM
by changing the number of electron donating methoxy unit, even without any dopants. As
increasing the numbers of methoxy unit in arylamine moiety, expected to act as Lewis base
that could passivate the defect sites at the interface between HTM and perovskite, we
confirmed that charge extration from perovskite to HTM was improved and recombination in
PSC device was suppressed, increasing their Jsc and Voc. Consequently, the PCEs of p-i-n
inverse type PSCs adopting TPACs as dopant-free HTMs gradually improved from TPAC0M
(13.92%) having no methoxy unit to TPAC3M (17.54%) containing three methoxy groups in
arylamine moiety.
Experimental
Synthesis
All starting materials were purchased from commercial suppliers (Aldrich and TCI). Detailed
synthetic procedures are summarized in supporting information and synthesized compounds
1
were fully characterized with H-NMR, 13C-NMR, mass and elemental analysis. (Fig. S6-
S14). In addition, the thermal transition and degradation behaviors of final compounds
(TPAC0M, TPAC2M and TPAC3M) were characterized with differential scanning
calorimetry and thermal gravimetric analysis (Fig. S15), respectively.
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DOI: 10.1039/C7TA02440A
TPAC0M
Under inert condition, 9-phenylcarbazole-3-boronoic Acid (2g, 6.97mmol), 4-
bromotriphenylamine (2.052g, 6.33mmol) and Pd(0) (0.374g, 0.32mmol) were placed into
round bottom flask. After adding tetrahydrofuran and 2M of K2CO3 solution (48mL/16mL,
3:1 ratio), the mixture was vigorously stirred at 60℃ for 8 hours. Then, the mixture was
cooled down to room temperature and quenched by adding deionized water. After extraction
with methylene chloride, the organic layer was collected and dryed with MgSO4. The residual
solid was removed by filteration and the solution was evaporated under vacuo. Final
compound, TPAC0M, was collected through column chromatography (MC/n-Hexane: 1:3)
and consequent recrystallization under MC/n-Hexane mixture, in yield 75%. In order to
improve purity, the obtained product was sublimed and used for device fabrication. 1H-NMR
(400MHz, Acetone) δ 8.65 (s, 1H), 8.45 (d, 1H), 7.89-7.82 (m, 5H), 7.80 (d, 2H), 7.69 (t, 1H),
7.63-7.52 (m, 3H), 7.50-7.41 (m, 5H), 7.28 (d, 2H), 7.24 (d, 4H), 7.19 (t, 2H); 13C NMR
(CDCl3, 150 MHz, ppm) 147.87, 146.56, 141.35, 140.18, 137.73, 136.32, 133.04, 129.93,
129.29, 127.99, 127.50, 127.09, 126.12, 125.19, 124.47, 124.24, 123.91, 123.53, 122.75,
120.37, 120.06, 118.36, 110.03, 109.94; m/z LC/MS 486.62 and anal. calcd. for C36H26N2 : C,
88.86; H, 5.39; N, 5.76; found: C, 88.74; H, 5.40; N, 5.86.
TPAC2M
Final product was similarly obtained with the synthetic procedure TPAC0M. 9-
Phenylcarbazole-3-boronoic Acid (2.052g, 7.15mmol), 4-Bromo-4’,4”-dimethoxytriphenyl-
1
amine (2.5g, 6.51mmol) and Pd(0) (0.385g, 0.33mmol) were used to obtain TPAC2M. H-
NMR (400MHz, Acetone) δ 8.59 (s, 1H), 8.43 (d, 1H), 7.89-7.66 (m, 8H), 7.55 (t, 3H), 7.43 (t,
1H), 7.21 (d, 4H), 7.11 (d, 2H), 7.06 (d, 4H), 3.93 (s, 6H); 13C NMR (CDCl3, 150 MHz, ppm)
141.32, 140.03, 137.77, 129.92, 127.46, 127.07, 126.41, 126.07, 125.11, 123.89, 123.56,
120.37, 120.01, 118.15, 114.73, 109.98, 109.90, 55.54; m/z LC/MS 546.67 and anal. calcd.
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for C38H30N2O2: C, 83.49; H, 5.53; N, 5.12; O, 5.85; found: C, 83.50; H, 5.51;DNOI,: 150..110339;/CO7T,A02440A
5.86.
TPAC3M
Final product was similarly obtained with the synthetic procedure TPAC0M. 9-(4-
methoxyphenyl)-carbazole-3-boronoic ester (2.00g, 5.01mmol), 4-Bromo-4’,4”-dimethoxy-
triphenylamine (1.748g, 4.50mmol) and Pd(0) (0.268g, 0.23mmol) were used to obtain
1
TPAC3M. H-NMR (400MHz, Acetone) δ 8.58 (s, 1H), 8.41 (d, 1H), 7.80 (d, 1H), 7.74 (d,
2H), 7.67 (d, 2H), 7.58-7.33 (m, 6H), 7.21 (d, 4H), 7.11 (d, 2H), 7.05 (d, 4H), 4.01 (s, 3H),
3.92 (s, 6H); 13C NMR (CDCl3, 150 MHz, ppm) 158.89, 155.75, 147.50, 141.81, 141.22,
140.52, 134.39, 133.05, 130.36, 128.51, 127.75, 126.40, 126.01, 125.07, 123.66, 123.32,
121.43, 120.34, 119.77, 118.13, 115.12, 114.74, 109.92, 109.85, 55.63, 55.54; m/z LC/MS
576.70 and anal. calcd. for C39H32N2O3: C, 81.23; H, 5.59; N, 4.86; O, 8.32; found: C, 81.25;
H, 5.56; N, 4.88; O, 8.31.
Cyclic voltammetry measurement
Ag/AgNO3 served as the pseudo reference electrode, ferrocene was used for calibration, and
the supporting electrolyte was tetrabutylammonium hexafluorophosphate (TBAHFP) in
acetonitrile (0.1 M). Oxidation and reduction potential of each TPAC series were directly
measured under the scan rate of 100 mV/s, after preparation of solution in dichlromethan (1.0
mM).
Device fabrication
Acetone, isopropyl alcohol (IPA), and deionized (DI) water were sequentially used to clean
the patterned ITO-coated glass substrates via ultrasonication. The ITO substrates were treated
with oxygen plasma for 5 min before use to improve the wettability of following solution. For
PEDOT:PSS HTM, Clevios PVP AI 4083 was deposited on the ITO substrate by spin-casting
at 3000 rpm for 30 s and annealed using a hot plate at 150 °C for 20 min. For TPACs-based
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perovskite solar cells (PSCs), TPAC0M, TPAC2M or TPAC3M layer was formed on the ITO
DOI: 10.1039/C7TA02440A
substrate by spin-casting their solution (1:1 chlorobenzene and acetonitrile mixed solvent, 20
mg/ml concentration) at 1000 rpm for 30 s instead of PEDOT:PSS. After forming a hole
transport layer (HTL) on the ITO substrate, the samples were transferred to a N2 box.
CH3NH3PbI3 perovskite solution was prepared by dissolving PbI2 (Sigma Aldrich) and
CH3NH3I (1:1 molar ratio), synthesized following the literature,4 in the solvent mixture of γ-
butyrolactone (GBL) and dimethylsulfoxide (DMSO) (7:3 v/v) for a total concentration of 1.1
M in a N2 atmosphere. The solution was stirred at 70 °C for at least 12 h before being used.
Then, spin-casting the solution along with toluene drop-casting achieved a uniform perovskite
layer.4a PCBM (20 mg/ml in chlorobenzene) was then spin-cast on top of the perovskite layer
and ZnO layer was added by spin-casting ZnO NP solution (Sigma Aldrich) on PCBM as an
electron transport layer (ETL). Those perovskite photoabsorber, PCBM and ZnO layers were
prepared in a N2 box. Finally, the samples were transferred into a thermal evaporator and Al
(80 nm) was thermally deposited at a base pressure of 4 × 10−6 torr. Over 30 devices were
prepared for each process condition. As for hole-only devices for hole mobility calculation, 30
nm MoO3 was thermally deposited on TPACs without shadow mask instead of ZnO, and Al
deposition through the shadow mask completed device fabrication with the following
configuration: transparent substrate/ITO/PEDOT:PSS/TPACs/MoO3/Al.
Device characterization
A solar simulator (PEC-L01, Peccell Technologies, Inc.) with an AM 1.5G filter was used to
provide 100 mWcm-2 of illumination on the PV cells, with the intensity calibrated using a Si
photodiode, equipped with an infrared cut-off filter (KG5) to reduce spectral mismatch. J-V
characteristics were obtained using an Ivium technology Ivium compactstat by scanning the
J-V curves at a 0.05 Vs-1 scan rate. UV–visible absorption spectra were recorded with a Jasco
V760 UV-Vis NIR spectrophotometer in the 400-850-nm wavelength range at room
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temperature. SEM images were obtained by Hitachi S-4800. TRPL curves were recorded
DOI: 10.1039/C7TA02440A
using a commercial TCSPC system (FluoTime 200, PicoQuant). Samples were excited by
picosecond diode laser of 670 nm (LDH-P-C-670, PicoQuant) with a repetition rate of
(4MHz). The emitted PL was collected by a fast photomultiplier tube (PMT) detector (PMA
182, PicoQuant) with a magic angle (54.7°) arrangement. The incident angle of excitation
pulse was set to be about 30° with respect to the sample. The resulting instrumental response
function was about 160 ps in full-width-half-maximum. And the PL decays were measured at
the emission peak (770±5nm) for perovskite. In addition, a cut-off filter (FF01-692nm,
Semrock) was applied to block the remaining scattering and the excitation intensity was
controlled using a neutral density filter set in order to avoid unwanted nonlinear effects. TPV
decay measurement was performed using a nanosecond OPO laser system (INDI-40-10,
Spectra-Physics) as a small perturbation light source and a Xe lamp (LS-150-XE ABET) as a
bias light source. The devices were directly connected to a digital oscilloscope (DSO-X
3054A, Agilent) and the input impedance was set to be 1 MΩ for an open circuit condition.
The bias light intensity was controlled by neutral density filters for various Voc and the
generated voltage transient (ΔV) by an attenuated laser pulse of 550 nm did not exceed 20 mV.
All the films, utilized to measure absorption, TRPL, and TPV, were the same as those for
solar cell devices. To investigate conductivity of HTL, local I-V measurements were carried
out using conventional AFM (XE-100, Park Systems). Platinum-coated AFM tip has < 40 nm
tip radius for better electrical contact, and force constant is 2.7 N/m to avoid destruction of
HTLs. XPS depth profiling was applied to investigate the buried interface between TPACs
and perovskite under the conditions of an Ar ion gun power of 2 kV and 1 mA on a raster area
of 22 mm, yielding an equivalent sputtering rate of 10 nm·min-1 of SiO2. All the binding
energies of the XPS data were calibrated with reference to the C–C bond in C 1s (284.5 eV).
17

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DOI: 10.1039/C7TA02440A
Acknowledgments
This research was supported by Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future
Planning (NRF-2015R1C1A1A02036659) and the Ministry of Education (NRF-
2014R1A1A2056403). This research was also supported by the Industrial Fundamental
Technology Development Program (10063268) funded by the Ministry of Trade, Industry &
Energy (MOTIE) of Korea and by the National Research Council of Science and Technology
(NST) through Degree and Research Center (DRC) Program(2015).
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DOI: 10.1039/C7TA02440A
Table of Contents entry
Triarylamine derivatives containing electron donating methoxy units provide outstanding
device performance when they are adopted as dopant-free hole transporting layer in inverse
type planar heterojunction perovskite solar cell (PSC). The methoxy units introduced to a
conjugated scaffold critically contribute to the reduced charge recombination during PSC
operation, which can be verified by time-correlated single-photon counting measurement and
transient photo-voltage study.
1