Ladder-type heteroacene-based dopant-free hole-transporting materials for efficient and stable CsPbI2Br perovskite solar cells

Article abstract of DOI:10.1016/j.dyepig.2021.109368

Dopant-free hole-transporting materials (HTMs) play a vital role in improving the power conversion efficiencies (PCEs) and stability for all-inorganic perovskite solar cells (PVSCs). In this work, two donor-acceptor-donor (D-A-D) type HTMs (L2 and L2-T) were designed and synthesized by using a ladder-type heteroacene core. Compared to L2-T with thiophene spacers, L2 presents suitable energy levels, smooth surface morphology, and improved hole mobility. Consequently, CsPbI₂Br PVSC based on L2 delivers a decent PCE of 12.41%, which outperforms the counterpart based on L2-T (11.07%). Whereas, the control device based on dopant-free 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) reveals a relatively low PCE of 9.95%. In addition, both the L2- and L2-T-based devices can maintain over 85% of their original PCEs after 1000 h storage demonstrating their excellent shelf stability. This work highlights the good potential of the ladder-type heteroacene building block for developing dopant-free HTMs toward high-performance CsPbI₂Br PVSCs.

Full text of DOI:10.1016/j.dyepig.2021.109368

Dyes and Pigments 191 (2021) 109368
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Ladder-type heteroacene-based dopant-free hole-transporting materials for
efficient and stable CsPbI Br perovskite solar cells
2
a
,
b
a
a,
b
a,*
Hao Liu , Qisheng Tu , Di Wang , Qingdong Zheng
a
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao West Road, Fuzhou,
Fujian, 350002, China
b
School of Physical Science and Technology, ShanghaiTech University, 100 Haike Road, Shanghai, 201210, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Dopant-free hole-transporting materials (HTMs) play a vital role in improving the power conversion efficiencies
Perovskite solar cells
Hole-transporting materials
Dopant-free
(
PCEs) and stability for all-inorganic perovskite solar cells (PVSCs). In this work, two donor-acceptor-donor (D-A-
D) type HTMs (L2 and L2-T) were designed and synthesized by using a ladder-type heteroacene core. Compared
to L2-T with thiophene spacers, L2 presents suitable energy levels, smooth surface morphology, and improved
Ladder-type heteroacene
Stability
2
hole mobility. Consequently, CsPbI Br PVSC based on L2 delivers a decent PCE of 12.41%, which outperforms
′
′
the counterpart based on L2-T (11.07%). Whereas, the control device based on dopant-free 2,2 ,7,7 -tetrakis(N,N-
′
di-p-methoxyphenylamine)-9,9 -spirobifluorene (Spiro-OMeTAD) reveals a relatively low PCE of 9.95%. In
addition, both the L2- and L2-T-based devices can maintain over 85% of their original PCEs after 1000 h storage
demonstrating their excellent shelf stability. This work highlights the good potential of the ladder-type heter-
2
oacene building block for developing dopant-free HTMs toward high-performance CsPbI Br PVSCs.
1
. Introduction
molecular
π
- stacking, resulting in relatively low hole mobility [14,15].
π
Thus, the Spiro-OMeTAD always needs to be chemically doped with
dopants such as lithium bis(trifluoromethanesulfonyl)imide (LiTFSI),
Organic-inorganic hybrid perovskite solar cells (PVSCs) have
received tremendous attention due to their rapidly increased power
conversion efficiencies (PCEs) from 3.8% in 2009 to 25.5% currently [1,
tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III)
tris(bis(tri-
fluoromethylsulfonyl)imide)) (FK209), and 4-tert-butylpyridine (t-BP)
to enhance the electrical properties. Nevertheless, most dopants possess
ionic diffusion/migration issues and are of the hydrophilic feature,
which would aggravate device degradation [16]. Therefore, it is needed
to develop alternative dopant-free HTMs for efficient and stable
all-inorganic PVSCs.
2
]. However, the organic-inorganic hybrid perovskites may exhibit poor
stability against the light and heat because of the volatilizable organic
+
A-site cations such as methylammonium (MA ) and formamidinium
+
(
FA ) [3,4]. Therefore, replacing the organic cations with inorganic
cations to construct all-inorganic perovskites is an emerging strategy in
order to improve the stability [5,6]. Hitherto, CsPbI and CsPbI Br are
3
2
Donor-acceptor-donor (D-A-D) design strategy has been successfully
used in high-performance dopant-free HTMs because the intramolecular
charge transfer (ICT) feature of the resulting materials could induce self-
doping to facilitate charge extraction [17,18]. Meanwhile, the band
gaps, energy levels, and charge transport properties can be easily
adjusted by introducing different donor and acceptor units [19]. To
date, a number of efficient D-A-D type HTMs for organic-inorganic
hybrid PVSCs have been reported [19–22]. For example, Guo et al.
designed and synthesized a novel D-A-D type HTM (MPA-BTTI) with
well-matched energy levels, moderate hole mobility, and superior film
morphology, leading to an outstanding PCE of 21.17% for
organic-inorganic hybrid PVSCs [20]. Compared with the enormous
two widely used perovskites for all-inorganic PVSCs, and the best PCEs
over 16% have been achieved with improved thermal- and
photo-stability [7–10].
On the other hand, high-performance HTMs are crucial to achieving
efficient and stable PVSCs [11,12]. The HTMs not only play an essential
role in promoting hole transport and reducing charge recombination,
but also prevent the device from being corroded by moisture, oxygen,
′
′
and electrode [13]. It is well known that 2,2 ,7,7 -tetrakis(N,N-di-p--
′
methoxyphenylamine)-9,9 -spirobifluorene (Spiro-OMeTAD) is the
dominated HTM for high-performance PVSCs. However, the orthogonal
molecular conformation of Spiro-OMeTAD is not beneficial to form close
*
Corresponding author.
E-mail address: qingdongzheng@fjirsm.ac.cn (Q. Zheng).
https://doi.org/10.1016/j.dyepig.2021.109368
Received 19 January 2021; Received in revised form 29 March 2021; Accepted 30 March 2021
Available online 6 April 2021
0
143-7208/© 2021 Elsevier Ltd. All rights reserved.

H. Liu et al.
Dyes and Pigments 191 (2021) 109368
efforts devoted in the D-A-D type HTMs for organic-inorganic hybrid
PVSCs, much less attention has been paid on the development of
dopant-free HTMs for all-inorganic PVSCs in spite of their advantages in
stability.
12H). 13C NMR (CDCl
3
, 101 MHz, ppm): 161.65, 156.44, 149.33,
144.70, 140.26, 139.93, 129.60, 127.15, 126.58, 126.50, 125.07,
123.33, 119.79, 116.18, 114.93, 111.54, 55.60, 49.52, 37.11, 31.96,
31.79, 30.08, 29.80, 29.67, 29.39, 26.79, 22.74, 14.20. High-resolution
Ladder-type heteroacenes are promising building blocks for efficient
D-A-D type HTMs because their energy levels could be easily tuned
through the employment of different ladder-type heteroacenes [20,
mass spectroscopy (HRMS) (MALDI) m/z: calcd for C88
1378.7554; found: 1378.7519. Elemental Analysis (EA) (%) calcd for
: C, 76.59; H, 7.74: N, 4.06; found: C, 76.83; H, 7.61; N,
106 4 6 2
H N O S :
88 106 4 6 2
C H N O S
2
3–25]. At the same time, the coplanar configuration can effectively
3.82.
strengthen the packing in film state, leading to enhanced charge
π-π
transport [26,27]. Moreover, the ladder-type heteroacene-based HTMs
may exhibit good thermal and chemical stability, which is essential for
stable PVSCs [28]. As an electron-deficient ladder-type heteroacene,
2.2.2. Synthesis of L2-T
TPTI-Br (0.100 g, 0.107 mmol), compound 5a (0.236 g, 0.349 mmol)
3 4
and Pd(PPh ) (0.015 g, 0.013 mmol) in dry toluene (25 mL) was
′
′
thieno[2 ,3 :5,6]pyrido[3,4-g]thieno[3,2-c]-isoquinoline-5,11(4H,
0H)-dione (TPTI) has been successfully used in organic solar cells with
degassed with nitrogen for 30 min and then reacted at reflux for 24 h.
After cooling to room temperature, the solution was extracted with
1
intriguing photovoltaic performance [29,30]. The nitrogen (N) atoms
located at the two electron-withdrawing lactam groups in TPTI can
provide sites for introducing alkyl chains which is important to modify
the solubility and aggregation. Meanwhile, sulfur (S) atoms and car-
bonyls in TPTI can act as Lewis base to passivate surface defects of
perovskite [31–33]. Therefore, TPTI may be an excellent acceptor core
for dopant-free HTMs. However, to the best of our knowledge, TPTI has
never been used to construct HTMs for PVSCs. Based on these consid-
erations, two novel D-A-D type dopant-free HTMs (L2 and L2-T) were
designed and synthesized by using TPTI as acceptor unit. 4-Methox-
y-N-(4-methoxyphenyl)-N-phenylaniline (TPA) was chosen as a donor
unit, which is closely related to the hole injection/transportation
2 2 4
CH Cl and dried over anhydrous MgSO . The organic phase was
concentrated by a rotary evaporator to get a brown solid. Then, the
crude product was purified by column chromatography with petroleum
ether/CH
2 2
Cl (1/1, v/v) as eluent to obtain L2-T (0.135 g, 82.3% yield)
1
as a red solid. H NMR (CDCl
3
, 400 MHz, ppm) 8.67 (s, 2H), 7.40 (d, J =
8.5 Hz, 4H), 7.21 (m, 2H), 7.08 (m, 10H), 6.99 (s, 2H), 6.92 (d, J = 8.5
Hz, 4H), 6.85 (m, 8H), 4.19 (s, 4H), 3.82 (s, 12H), 2.01 (s, 2H),
1
3
1.51–1.07 (m, 48H), 0.92–0.72 (m, 12H). C NMR (CDCl
3
, 101 MHz,
ppm) 161.24, 156.21, 148.64, 144.93, 140.49, 139.41, 137.57, 134.28,
129.20, 126.91, 126.46, 126.33, 125.67, 125.52, 123.26, 122.35,
120.17, 116.10, 114.85, 112.61, 55.58, 49.30, 37.18, 32.01, 31.85,
31.79, 30.15, 29.85, 29.77, 29.45, 26.86, 22.77, 14.23, 14.20. HRMS
[
34–36]. Both of the newly synthesized HTMs exhibited well-matched
(MALDI) m/z: calcd for C96 , 1542.7308; found: 1542.7374.
H
110
N
4
O
6
S
4
energy levels, smooth film morphologies, and high hole mobilities.
The dopant-free L2-based device achieved an impressive PCE of 12.41%,
with an open-circuit voltage (VOC) of 1.21 V, a short-circuit current
EA (%) calcd for C96H N O S : C, 74.67; H, 7.18: N, 3.63; found: C,
110 4 6 4
74.92; H, 7.14; N, 3.40.
ꢀ 2
density (JSC) of 13.73 mA cm , and a fill factor (FF) of 75.02%, which is
superior to the device based on L2-T (11.07%). In contrast, the
dopant-free Spiro-OMeTAD-based device showed an inferior PCE of
2.3. Device fabrication
The indium tin oxide (ITO) glass substrates were cleaned by using
detergent, distilled water, acetone, and isopropanol in sequence. Before
the deposition of electron transport layers, the substrates were cleaned
by ultraviolet ozone for 15 min. Then the SnO2 nanoparticles (5%,
diluted by water) were spin-coated on the substrates at 3000 r.p.m. for
9
8
.95%. Furthermore, L2- and L2-T-based devices could maintain over
5% of their initial PCEs after 1000 h demonstrating their excellent
long-term stability.
◦
◦
2
. Experimental section
20 s, and then annealed in ambient air at 120 C for 10 min and 150 C
for 20 min. After cooling down to room temperature, the substrates were
subjected to ultraviolet ozone treatment for 5 min and then transferred
2
.1. Materials
2
into a glove box. For the CsPbI Br perovskite film deposition, the
Unless otherwise noted, all chemicals were purchased from Aldrich
perovskite solution was prepared by dissolving 276.6 mg of PbI , 220.2
2
Inc., Adamas-beta Ltd., Suna Tech Inc., and Energy Chemical without
further purification. Lead iodide (PbI ) and lead bromide (PbBr ) were
mg of PbBr , and 311.8 mg of CsI in 1 mL of DMSO. Then, the solution
2
2
2
was spin-coated onto the SnO layer at 500 r.p.m. for 5 s and 3000 r.p.m.
2
◦
purchased from TCI. Cesium iodide (CsI) was purchased from Xi’an
Polymer Technology Crop. Spiro-OMeTAD was purchased from Shenz-
for 30 s, followed by thermal annealing via a two-step process at 42 C
◦
for 30 s and 160 C for 10 min, respectively. The HTMs (L2, L2-T, and
ꢀ 1
hen Feiming Science and Technology Co., Ltd. The SnO
2
colloid pre-
Spiro-OMeTAD with a concentration of 20, 15, and 20 mg mL ,
cursor was purchased from Alfa Aesar (Tin (IV) oxide, 15% in H
colloidal dispersion).
2
O
respectively.) were dissolved in chlorobenzene (CB) without additives.
The hole-transporting layer was deposited on top of the perovskite layer
at 3000 r.p.m. for 30 s. The resulting L2 and L2-T films were thermally
◦
2
2
.2. Synthesis of HTMs
.2.1. Synthesis of L2
annealed at 80 C for 10 min to improve the device performance. Finally,
1
3
0 nm MoO and 100 nm Ag cathode were deposited on the HTM layer.
′
2
,8-Dibromo-4,10-bis(2-hexyldecyl)-4,10-dihydrothieno[2 ,3’:5,6]
2.4. Characterization of materials and devices
pyrido[3,4-g]thieno[3,2-c]isoquinoline-5,11-dione (TPTI-Br, 0.100 g,
0
.107 mmol), compound 3a (0.142 g, 0.329 mmol), and Pd(PPh
3
)
4
1H NMR and 13C NMR spectra were measured on a Bruker AVANCE-
(
0.016 g, 0.014 mmol) were dissolved in toluene (25 mL) and K CO
2
3
3
400 spectrometer in CDCl solutions with tetramethylsilane (TMS) as
aqueous solution (2 M, 6 mL). After being degassed with nitrogen for 30
reference. HRMS measurements were carried out using Thermo Scien-
tific Q Exactive HF Orbitrap-FTMS. Elemental analysis (EA) was per-
formed by Elementar vario MACRO cube. Thermogravimetric analysis
◦
min, the mixture was stirred and reacted at 100 C for 24 h. Upon
cooling down to room temperature, the solution was extracted with
◦
CH
product was purified by column chromatography with petroleum ether/
CH Cl (1/1, v/v) as eluent to obtain L2 (0.120 g, 81.6% yield) as a
yellow solid. H NMR (CDCl
.5 Hz, 4H), 7.10 (m, 10H), 6.94 (d, J = 8.5 Hz, 4H), 6.86 (m, 8H), 4.22
s, 4H), 3.80 (s, 12H), 2.01 (s, 2H) 1.51–1.04 (m, 48H), 0.94–0.67 (m,
2
Cl
2
and dried over anhydrous MgSO
4
. After concentration, the crude
(TGA) was measured on Mettler TGA/SDTA851 at a heating rate of 20 C
ꢀ 1
min . Differential scanning calorimetry (DSC) curves were recorded on
◦
ꢀ 1
2
2
Mettler DSC 822 at a heating rate of 10 C min . Ultraviolet–visible
(UV–vis) spectra were collected on a Lambda 365 UV–vis spectropho-
tometer. Steady-state photoluminescence (PL) was measured by a fluo-
rescence spectrometer (Edinburgh Instruments, FLS980). Ultraviolet
1
3
, 400 MHz, ppm): 8.77 (s, 2H), 7.44 (d, J =
8
(
2

H. Liu et al.
Dyes and Pigments 191 (2021) 109368
photoelectron spectra (UPS) for the thin films were carried out by using
X-ray Photoelectron Spectroscopy (Thermo Fisher, ESCALAB 250Xi).
Atomic Force Microscopy (AFM) measurements were conducted by
using Bruker Dimension ICON in the tapping mode. The J-V character-
istics of the devices were obtained using a Keithley 2400 Source Meter
and L2-T exhibit distinctly melting peaks and crystallization peaks,
demonstrating high crystallinities for both of them in solid-state.
3
.3. Photophysical and electrochemical properties
ꢀ
2
under simulated AM 1.5G illumination (100 mW cm ) with an Oriel Sol
A simulator (Newport). The light intensity had been accurately cali-
The optical absorption of L2 and L2-T were measured in dilute
3
ꢀ 5
chloroform solution (1.0 × 10 M) and thin-film states, and the cor-
responding data are listed in Table 1. In solution, as shown in Fig. 1a,
they both exhibit strong absorption bands in the visible region
brated with a National Renewable Energy Laboratory (NREL)-certified
silicon reference cell. The external quantum efficiency (EQE) was
measured with an EQE measurement system (Newport). The details of
cyclic voltammetry (CV) and hole mobility measurements were pro-
vided in the Supporting Information.
(
400–550 nm), which mainly originates from the ICT transitions be-
tween donor unit and acceptor unit. The absorption peak (λmax) of L2 is
located at 450 nm. Nevertheless, L2-T shows a bathochromically shifted
absorption with a λmax of 480 nm, which could be attributed to the
enhanced electron-donating effect induced by the thiophene spacers.
The result can be further confirmed by the electrostatic surface potential
3
. Results and discussion
3
.1. Synthesis
(
ESP) analysis, which was calculated by density functional theory (DFT)
at B3LYP/6-311G** level. As shown in Fig. 1c and d, L2-T exhibits an
extended negative electrostatic potential region in comparison with L2,
suggesting an increased ICT in L2-T. In thin-film state, L2 and L2-T
exhibit red-shifted absorption bands with distinct shoulder absorption
peaks, indicating strong intermolecular interaction in films, which is in
favor of efficient charge transfer. Meanwhile, the optical band gaps
The synthetic routes of L2 and L2-T are shown in Scheme 1, and the
details are afforded in the experimental section. Compounds 3a, 5a and
the brominated monomer TPTI-Br were prepared according to the re-
3 4
ported procedures [29,37,38]. Using Pd(PPh ) as catalyst, L2 was then
synthesized in 81.6% yield through the Suzuki coupling reaction be-
tween TPTI-Br and compound 3a. With the same palladium catalyst, the
Stille coupling reaction between TPTI-Br and compound 5a produced
L2-T in 82.3% yield. The chemical structures of L2 and L2-T were
(Egaps) of L2 and L2-T were calculated to be 2.25 and 2.10 eV, respec-
tively, according to the onsets of absorption wavelength (λonset) in films.
To further understand their stokes shifts, PL spectra of L2 and L2-T were
measured. As presented in Fig. S3, both of them reveal larger stokes
1
13
characterized by H NMR, C NMR, HRMS, and EA, respectively. Both
HTMs exhibit good solubility in common organic solvents such as
dichloromethane, chlorobenzene, and toluene, implying their excellent
solution processability.
Table 1
Photophysical and charge transport properties of L2 and L2-T.
a
b
c
d
2
ꢀ 1 ꢀ 1
HTM
λ
abs (nm)
E
gap (eV)
EHOMO (eV)
E
LUMO (eV)
μ
(cm
V
s
)
3
.2. Thermal stability
1.51 × 10ꢀ
1.05 × 10ꢀ
4
L2
450
480
2.25
2.10
ꢀ 4.95
ꢀ 4.77
ꢀ 2.70
ꢀ 2.67
4
L2-T
To investigate the thermal properties of L2 and L2-T, TGA and DSC
a
Absorption spectra were measured in dilute chloroform solution.
measurements were conducted. From the TGA curves (Fig. S1), the
b
c
Optical band gaps were calculated by the equation: Egap = 1240/λonset (eV).
HOMO in film was estimated by using the UPS measurement.
LUMO in film was calculated according to the equation of EHOMO = EHOMO
decomposition temperatures (T
d
s), defined as 5% mass loss, were esti-
E
◦
mated to be 420.5 and 446.6 C for L2 and L2-T, respectively, suggesting
their excellent thermal stability, which is essential for the operational
stability of PVSCs in high temperature. As displayed in Fig. S2, both L2
d
E
+
E
gap
.
Scheme 1. Synthetic routes for L2 and L2-T: (i) 4-iodoanisole, CuI, KOH, 1,10-phenanthroline, toluene, reflux. (ii) bis(pinacolato)diboron, KOAc, Pd(dppf)Cl
2
, DMF,
◦
◦
Cl, THF, ꢀ 78 ◦C. (v) Br
8
1
0
C. (iii) 2-bromothiophene, Pd(PPh
3
)
4
, K
2
CO
3
, H
2
O, toluene, 100 C. (iv) n-BuLi, SnBu
3
2 3 3 4 2 3 2
, CHCl , RT. (vi) Pd(PPh ) , K CO , toluene, H O,
◦
00 C. (vii) Pd(PPh
3
)
4
, toluene, reflux.
3

H. Liu et al.
Dyes and Pigments 191 (2021) 109368
Fig. 1. The UV–vis absorption spectra of HTMs (a) in dilute chloroform solution and (b) in film. The electrostatic potential (ESP) of L2 (c) and L2-T (d).
2
Fig. 2. (a) CV voltammograms of L2 and L2-T. (b) Energy level diagram of CsPbI Br, L2, and L2-T. (c) UPS spectra of L2 and L2-T films. (d) The J-V curves of hole-
only devices based on L2, L2-T, and Spiro-OMeTAD, respectively.
4

H. Liu et al.
Dyes and Pigments 191 (2021) 109368
shifts of around 120 nm compared with that of Spiro-OMeTAD (39 nm)
perovskite film. Particularly, the L2 film exhibits a smooth and uniform
morphology with the smallest RMS value, which is expected to form
better ohmic contact between perovskite film and electrode thereby
reducing undesirable current leakage in the resulting PVSCs.
[
39], suggesting that they have significant structural deformation be-
tween ground state and excited state and a good molecular flexibility of
the excited state, which is favorable for a better pore-filling of the HTMs
with an improved hole extraction capability [40–42].
To study the charge transport properties of L2 and L2-T, the hole
To further evaluate the electrochemical properties of L2 and L2-T, CV
measurement was performed, and the CV curves are shown in Fig. 2a.
The highest occupied molecular orbital (HOMO) energy levels were
calculated according to the formula of EHOMO = -(ϕox + 4.82) (eV) with
mobilities (
ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS)/HTM/
MoO /Ag through space-charge-limited current (SCLC) method, and the
J-V curves are shown in Fig. 2d. The of dopant-free L2 film was
, slightly larger than that of L2-T
), which can be partly ascribed to the
μ
) were measured with the architecture of ITO/poly(3,4-
3
μ
+
ꢀ 4
2
ꢀ 1 ꢀ 1
s
Fc/Fc as an internal standard, where ϕox is the onset oxidation po-
evaluated to be 1.51 × 10 cm V
ꢀ 4
2
ꢀ 1 ꢀ 1
tential. For L2, the HOMO energy level was estimated to be ꢀ 5.12 eV.
Whereas, L2-T displays an up-shifted HOMO energy level of ꢀ 5.02 eV
due to the electron-donating thiophene spacers. Moreover, the UPS
measurement was further employed to determine the energy levels in
films. The cut-off regions and the onset regions for L2 and L2-T are
film (1.05 × 10 cm
V s
smooth and uniform morphology of the L2 film. Under the same device
fabrication conditions, the Spiro-OMeTAD film affords significantly
ꢀ
5
2
ꢀ 1 ꢀ 1
reduced
μ
of 1.40 × 10 cm V
s . The enhanced charge transport
property for L2 and L2-T is beneficial for efficient charge transfer and
low charge recombination.
F
shown in Fig. 2c. The energetic gaps between Fermi levels (E s) and
HOMO energy levels were estimated to be 0.38 and 0.31 eV for L2 and
L2-T, respectively. Meanwhile, the corresponding work functions were
evaluated to be 4.57 and 4.46 eV. Thus, the HOMO energy levels were
determined to be ꢀ 4.95 and ꢀ 4.77 eV for L2 and L2-T in the film state,
respectively, which are in agreement with the results from the CV
measurements. In the two HTMs, L2 exhibits a deeper-lying HOMO
energy level, which could contribute to enhanced hole transfer and
reduced energy loss. On the other hand, the lowest unoccupied molec-
ular orbital (LUMO) energy levels of L2 and L2-T in film-states were
calculated to be ꢀ 2.70 and ꢀ 2.67 eV, respectively, according to the
equation of ELUMO = EHOMO + Egap. Compared with the conduction band
3.5. Photovoltaic performance
To investigate the photovoltaic performances of L2 and L2-T, typical
n-i-p devices were prepared with the architecture of ITO/SnO2/
CsPbI Br/HTM/MoO /Ag (Fig. 4a). The J-V curves of PVSCs based on
2
3
three HTMs are shown in Fig. 4b, and the corresponding parameters are
tabulated in Table 2. The best-performing device based on L2 exhibits an
ꢀ 2
impressive PCE of 12.41% with a VOC of 1.21 V, a JSC of 13.73 mA cm
,
and a FF of 75.02%. By contrast, the L2-T-based counterpart shows a
relatively low PCE of 11.07% with a V of 1.16 V, a JSC of 13.56 mA
OC
ꢀ 2
of CsPbI
2
Br perovskite (ꢀ 4.26 eV) [43], the LUMO energy levels of L2
cm , and a FF of 70.63%. It can be found that L2-based device presents
increased VOC and FF values than those of the device using L2-T as HTM.
The reason for the higher VOC in L2-based device is probably related
with the deeper-lying HOMO level of L2. On the other hand, L2-based
and L2-T are high enough to block the electrons transfer from the con-
duction band of perovskite to the back-metal electrode.
2
device delivers a shunt resistance (Rsh) of 8.19 kΩ cm and a series
3
.4. Film morphologies and charge transport properties
2
resistance (R
s
) of 7.82 Ω cm (Table 2). Whereas, the device based on L2-
2
T shows an inferior ohmic contact with a reduced Rsh of 3.63 kΩ cm and
The tapping mode AFM was conducted to probe the morphologies of
2
an increased R
s
of 9.22 Ω cm , which is partly responsible for its
of L2-T-based device could be
the small-molecule films when coated on the surface of perovskite. As
shown in Fig. 3, the root-mean-square (RMS) roughnesses of L2, L2-T
and Spiro-OMeTAD films were determined to be 2.5, 3.9, and 4.2 nm,
respectively, significantly lower than the value of 21.2 nm for pristine
decreased FF. The lower Rsh and higher R
s
attributed to the rougher morphology and lower hole mobility of L2-T,
which would increase charge recombination and leakage current in
the surface and restrain the hole transfer between perovskite layer and
HTM [44–46]. For the comparison purpose, the devices with dopant-free
Spiro-OMeTAD were fabricated under the same conditions, and the
corresponding PCE decreased to 9.95% with a VOC of 1.16 V, a JSC of
ꢀ 2
1
3.77 mA cm and a FF of 62.51%. The EQE spectra for the devices
were measured to verify the accuracy of the JSC values obtained from the
J-V curves. As shown in Fig. 4c, both L2- and L2-T-based devices show
strong photon-response in the wavelength range from 350 nm to 650
nm. The integrated JSC values for L2-, L2-T-, and Spiro-OMeTAD-based
ꢀ 2
devices were estimated to be 13.62, 13.37, and 13.71 mA cm
,
respectively, which agree with the results from the J-V curves within 2%
mismatches. Additionally, the stabilized power output efficiencies of the
devices based on L2, L2-T, and Spiro-OMeTAD were recorded. As shown
in Fig. S5, under the maximum power-point conditions and 400 s
continuous illumination, the stabilized PCEs of L2-, L2-T-, and
Spiro-OMeTAD-based devices were 11.82%, 10.53%, and 9.34%,
respectively, which agree with the results from the J-V scan. It can be
found that both L2- and L2-T-based devices could maintain over 95% of
their initial values after 400 s of operation. Whereas, the device based on
Spiro-OMeTAD exhibited relatively poor stability, whose PCE decreased
by more than 5%, indicating the better operational stability of the L2-
and L2-T-based PVSCs.
3
.6. Hydrophobic properties and long-term stability of devices
The contact angle tests were performed to analyze the surface
Fig. 3. AFM height images of (a) the pristine perovskite and the perovskites
wettability of HTMs. As shown in Fig. S4, L2 and L2-T reveal strong
◦
coated with (b) L2, (c) L2-T, and (d) Spiro-OMeTAD.
hydrophobic properties with contact angles of 92.2 and 92.5 ,
5

H. Liu et al.
Dyes and Pigments 191 (2021) 109368
Fig. 4. (a) Schematic diagram of the device structure with n-i-p configuration. (b) J-V curves of the optimized devices based on L2, L2-T, and Spiro-OMeTAD. (c)
Corresponding EQE spectra, and (d) long-term stability of the encapsulated devices in air.
Table 2
Photovoltaic parameters of the devices based on L2, L2-T, and Spiro-OMeTAD.
a
ꢀ 2
2
2
HTM
L2
V
OC (V)
J
SC (mA cm
)
FF (%)
PCE (%)
R
s
(Ω cm )
R
sh (kΩ cm )
1.21
1.19 ± 0.01)
1.16
1.14 ± 0.01)
1.16
1.14 ± 0.02)
13.73
75.02
12.41
7.82
9.22
17.54
8.19
3.63
7.56
(
(13.39 ± 0.37)
13.56
(73.39 ± 2.35)
70.63
(11.73 ± 0.42)
11.07
L2-T
(
(13.34 ± 0.50)
13.77
(67.68 ± 2.42)
62.51
(10.32 ± 0.47)
9.95
Spiro-OMeTAD
(
(13.38 ± 0.69)
(59.46 ± 3.13)
(9.05 ± 0.47)
a
In the parentheses are average values based on more than 20 devices.
respectively, which is beneficial for improving the stability of the cor-
responding devices. However, Spiro-OMeTAD is of relatively weak hy-
hole mobilities than Spiro-OMeTAD. The CsPbI
2
Br PVSC based on L2
showed the highest PCE of 12.41% due to its well-matched energy
levels, smooth film morphology, and increased hole mobility. Whereas,
the device based on dopant-free Spiro-OMeTAD displayed a relatively
low PCE of 9.95%. Moreover, both dopant-free L2- and L2-T-based de-
vices could maintain over 85% of their initial PCEs after 1000 h storage
suggesting their excellent shelf-storage stability. These results indicate
that the ladder-type TPTI is a promising building block to construct D-A-
◦
drophobic nature with a reduced contact angle of 76.5 . The long-term
stability of PVSCs with encapsulation was measured in air at room
temperature. As shown in Fig. 4d, the device based on L2 maintained
over 85% of its initial PCE after 1000 h. Meanwhile, L2-T-based device
displayed relatively better stability than L2, retaining over 90% of its
original PCE at the same storage conditions. In contrast, the device based
on dopant-free Spiro-OMeTAD revealed relatively inferior stability by
maintaining 73% of its original PCE under the same conditions.
2
D type HTMs for efficient and stable CsPbI Br PVSCs.
CRediT authorship contribution statement
4
. Conclusion
Hao Liu: synthesized the hole-transporting materials, fabricated and
tested the solar cell devices, wrote the manuscript. Qisheng Tu: fabri-
cated and tested the solar cell devices, wrote the manuscript. Di Wang:
fabricated and tested the solar cell devices, All authors discussed the
results and commented on the final manuscript. Qingdong Zheng:
designed the experiments, wrote the manuscript, directed this project.
In summary, two novel D-A-D type dopant-free HTMs (L2 and L2-T)
were designed and synthesized using ladder-type TPTI as the acceptor
unit and TPA groups as donor units. The planar molecular structure can
significantly strength the intermolecular
π
-
π
stacking, leading to an
excellent charge transport property. Finally, L2 and L2-T showed higher
6

H. Liu et al.
Dyes and Pigments 191 (2021) 109368
[19] Yu XY, Li Z, Sun XL, Zhong C, Zhu ZL, Li ZA, et al. Dopant-free
Declaration of competing interest
dicyanofluoranthene-based hole transporting material with low cost enables
efficient flexible perovskite solar cells. Nano Energy 2021;82:105701.
20] Wang Y, Chen W, Wang L, Tu B, Chen T, Liu B, Yang K, Koh CW, Zhang XH, Sun H,
Chen GC, Feng XY, Woo HY, Djurisic AB, He ZB, Guo XG. Dopant-free small-
molecule hole-transporting material for inverted perovskite solar cells with
efficiency exceeding 21%. Adv Mater 2019;31(35):1902781.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[
[
21] Guo HX, Zhang H, Shen C, Zhang DW, Liu SJ, Wu YZ, Zhu WH. A coplanar
Acknowledgements
π
-extended quinoxaline based hole-transporting material enabling over 21%
efficiency for dopant-free perovskite solar cells. Angew Chem Int Ed 2020;60(5):
674–9.
2
We thank financial support from the National Natural Science
Foundation of China (No. U1605241), the Natural Science Foundation of
Fujian Province for Distinguished Young Scholars (No. 2019J06023),
the Key Research Program of Frontier Sciences, CAS (No. QYZDB-SSW-
SLH032).
[
[
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22] Tian Y, Tao L, Chen C, Lu HF, Li HP, Yang XC, Cheng M. Facile synthesized fluorine
substituted benzothiadiazole based dopant-free hole transport material for high
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Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
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