A Cost-Effective D-A-D Type Hole-Transport Material Enabling 20% Efficiency Inverted Perovskite Solar Cells?

Article abstract of DOI:10.1002/cjoc.202100022

High-performance, cost-effective hole-transport materials (HTMs) are greatly desired for the commercialization of perovskite solar cells (PVSCs). Herein, two new HTMs, TPA-FO and TPA-PDO, are devised and synthesized, which have a donor-acceptor-donor (D-A-D) type molecule design featuring carbonyl group-functionalized arenes as the acceptor (A) units. The carbonyl group at the central core of HTMs can not only tune frontier molecular orbital (FMO) energy levels and surface wettability, but also can enable efficient surface passivation effects, resulting in reduced recombination loss. When employed as HTMs in inverted PVSCs without using dopant, TPA-FO with one carbonyl group yields a high power conversion efficiency (PCE) of 20.24%, which is among the highest values reported in the inverted PVSCs with dopant-free HTMs. More importantly, the facile one-step synthetic process enables a low cost of 30 USD g^–1 for TPA-FO, much cheaper than the most studied HTMs used for high-efficiency dopant-free PVSCs. These results demonstrate the potential of D-A-D type molecules with carbonyl group-functionalized arene core in developing the low-cost dopant-free HTMs toward highly efficient PVSCs.

Full text of DOI:10.1002/cjoc.202100022

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中
国 化 学
Cite this paper: Chin. J. Chem. 2021, 39, 1545—1552. DOI: 10.1002/cjoc.202100022
A Cost-Effective D-A-D Type Hole-Transport Material Enabling 20%
Efficiency Inverted Perovskite Solar Cells†
Jiachen Huang,a,b Jie Yang,a Huiliang Sun,*,a Kui Feng,a Qiaogan Liao,a,c Bolin Li,a He Yan,*,b and Xugang Guo*,a
a Department of Materials Science and Engineering, Southern University of Science and Technology (SUSTech), No. 1088, Xueyuan Road,
Shenzhen, Guangdong 518055, China
b Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China
c School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China
Keywords
Energy conversion | Donor-acceptor systems | Interfaces | Hole-transport materials | Inverted perovskite solar cells
Main observation and conclusion
High-performance, cost-effective hole-transport materials (HTMs) are greatly desired for the commercialization of perovskite solar
cells (PVSCs). Herein, two new HTMs, TPA-FO and TPA-PDO, are devised and synthesized, which have a donor-acceptor-donor (D-A-D)
type molecule design featuring carbonyl group-functionalized arenes as the acceptor (A) units. The carbonyl group at the central core
of HTMs can not only tune frontier molecular orbital (FMO) energy levels and surface wettability, but also can enable efficient surface
passivation effects, resulting in reduced recombination loss. When employed as HTMs in inverted PVSCs without using dopant,
TPA-FO with one carbonyl group yields a high power conversion efficiency (PCE) of 20.24%, which is among the highest values re-
ported in the inverted PVSCs with dopant-free HTMs. More importantly, the facile one-step synthetic process enables a low cost of 30
USD g–1 for TPA-FO, much cheaper than the most studied HTMs used for high-efficiency dopant-free PVSCs. These results demon-
strate the potential of D-A-D type molecules with carbonyl group-functionalized arene core in developing the low-cost dopant-free
HTMs toward highly efficient PVSCs.
Comprehensive Graphic Content
*E-mail: sunhl@sustech.edu.cn; hyan@ust.hk; guoxg@sustech.edu.cn
View HTML Article
Supporting Information
† Dedicated to Department of Chemistry, SUSTech, on the Occasion of Her
10th Anniversary.
Chin. J. Chem. 2021, 39, 1545－1552
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
1545

Huang et al.
Concise Report
state-of-the-art inverted PVSCs,[9,12] but it is synthesized at a high
cost of 1980 USD g–1 and often needs p-type doping or surface
treatments.[6,13-14]
Background and Originality Content
Organic−inorganic metal halide perovskite solar cells (PVSCs)
are a promising candidate for next-generation photovoltaic tech-
nology.[1] Benefiting from the continuous effort on improving
perovskite composition and film quality,[2] interfacial optimiza-
tion,[3] and device structures,[4] the power conversion efficiencies
(PCEs) of PVSCs have been boosted up to over 25% recently.[1b]
PVSCs can be divided into conventional (n–i–p) and inverted (p–i–
n) structures. Typically, the n–i–p structures are fabricated with
the n-type TiO2 or SnO2 as the electron-transporting materials
(ETMs) and 2,2,7,7‐tetrakis(N,N‐di‐p‐methoxyphenylamine)‐9,9‐
spiro‐bifluorene (Spiro‐OMeTAD) as hole-transport materials
(HTMs).[5] Although most of the high efficiency was achieved by
the n–i–p structures, such devices usually suffer from severe hys-
teresis.[6] Additionally, Li+ dopants for improving the conductivity
of Spiro‐OMeTAD bring adverse effects on device ambient stabil-
ity.[7] By contrast, the p–i–n structures have several distinctive ad-
vantages, including reduced hysteresis and low-temperature pro-
cessing.[6] The p–i–n structures also offer opportunities for inte-
grating PVSCs on silicon solar cells.[4a] However, the device per-
formance of inverted PVSCs lags behind those of conventional
PVSCs.[8] In terms of inverted PVSCs, HTMs play an essential role
in charge collection or blocking at the interface and can also affect
the crystal growth of perovskite.[9] Therefore, considerable efforts
have been devoted to developing dopant-free HTMs for improving
the efficiency of inverted PVSCs.
Small molecular HTMs with well-defined molecular structure
and good reproductivity show great potential for achieving effi-
cient inverted PVSCs.[10d,15] Among various design strategies, do-
nor–acceptor-donor (D-A-D) molecular geometry has recently
received more attention and been investigated in dopant-free
small molecule HTMs (Figure 1).[16] The D-A-D molecule structure
can be ready to modify the frontier molecular orbital energy levels
of HTMs to form a suitable energy alignment with perovskite by
selecting appropriate D and A units, especially in decreasing the
highest occupied molecular orbital (HOMO) energy level.[17]
Moreover, the D-A-D molecular backbone is beneficial to con-
struct dopant-free HTMs due to the strong intramolecular charge
transfer (ICT) and high dipole moment, which could induce
self-doping and built-in potential to promote charge extraction
without adding dopants.[16a,18] On the other hand, it is well desira-
ble to decrease the synthetic cost of the dopant-free HTMs. How-
ever, previous D-A-D molecular HTMs in inverted PVSCs were of-
ten synthesized via multiple steps, preventing their broad applica-
tions.[16d-16f,19]
Scheme 1 The one-step synthetic routes to TPA-FO and TPA-PDO
In this work, two cost-effective D-A-D type dopant-free mo-
lecular HTMs, TPA-FO and TPA-PDO, were designed and synthe-
sized via facile one-step reaction with a decent yield of around
75%. Both HTMs feature carbonyl group-functionalized arene as A
unit and 4-(bis(4-methoxyphenyl)amino)phenyl (TPA)[20] as termi-
nal D unit. Two HTMs show distinct photoelectrical and surface
properties due to the different number of carbonyl groups.
Moreover, the carbonyl groups can increase the dipole moment
and wettability,[21] and passivate the electron-poor defects of the
perovskite as Lewis base groups, resulting in reduced recombina-
tion loss.[22] When used as the dopant-free HTMs in inverted
PVSCs, TPA-FO-based PVSCs exhibit a champion PCE of 20.24%
with an open-circuit voltage (Voc) of 1.09 V, a short-circuit current
density (Jsc) of 22.76 mA·cm−2, and a fill factor (FF) of 81.9%. The
device based on TPA-PDO shows an inferior PCE of 17.02% with a
Voc of 1.03 V, a Jsc of 20.44 mA·cm−2, and an FF of 80.9%. It is
worth mentioning that the synthetic cost of TPA-FO and TPO-PDO
can be reduced to 30 and 35 USD g–1. These results demonstrate
the potential of our proposed D-A-D molecular design principle
with carbonyl group-functionalized arene as A unit for low-cost
and efficient dopant-free HTMs.
Figure 1 The recent development of D-A-D dopant-free small molecular
HTMs utilized in n-i-p and p-i-n perovskite solar cells.
Results and Discussion
Compared to inorganic HTMs, organic HTMs have low-tem-
perature solution processability and tunable optoelectrical and
morphological properties to match with perovskite.[10] For exam-
ple, polymer-based HTMs, poly(3,4-ethylenedioxythiophene):
poly-(styrenesulfonate) (PEDOT:PSS) and poly(triarylamine) (PTAA),
are the most widely used HTMs for inverted PVSCs.[11] However,
the acidity and hygroscopicity of PEDOT:PSS cause deterioration to
device stability. PTAA achieved the highest efficiencies in the
TPA-FO and TPA-PDO in Scheme 1 were synthesized by a
one-step Suzuki coupling reaction, and the corresponding route is
presented in Scheme S1,[23] and the synthetic cost was calculated
in Table S1. TPA-FO and TPA-PDO consist of TPA as D units, and
carbonyl group-functionalized arenes, fluorenone and phenan-
threnedione, as the rigid planar A units, respectively. The ultravi-
olet-visible (UV-vis) absorption spectra of TPA-FO and TPA-PDO in
both chlorobenzene solution and films are presented in Figure 2a.
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Inverted Perovskite Solar Cells
Chin. J. Chem.
In the solution, both exhibit two characteristic absorption peaks
for TPA-FO (383 and 492 nm) and TPA-PDO (387 and 581 nm). The
absorption in the visible light can be attributed to the ICT from
the HOMO on the TPA donor unit to the lowest unoccupied mo-
lecular orbitals (LUMO) on the acceptor unit, while the absorption
in the UV region can be ascribed to π–π* transition.[17b,24] The ab-
sorption peak of TPA-PDO is red-shifted compared to TPA-FO due
to the stronger ICT effect of more electron-deficient phenan-
threnedione core. Compared to their absorption in solution, both
TPA-FO and TPA-PDO films show bathochromic shifts of the ab-
sorption onsets, with the optical band gap (Eg) of 2.06 and 1.65 eV,
respectively. Such phenomena denote increased intermolecular
interactions in the solid state.[17b]
Bꢀssler’s model.[25] Thus, neat TPA-FO film shows sufficient hole
mobility, implying it as a potential dopant-free HTM in inverted
PVSCs.
Figure 2 (a) UV-Vis absorption spectra of TPA-FO and TPA-PDO in chloro-
benzene solution and thin films. (b) Energy levels of TPA-FO and TPA-PDO
derived from the CV curves.
Besides, the optical transmission of HTMs deposited on ITO
was also measured because HTMs are the front layers before the
active layer and sufficient transmittance of HTM is the prerequi-
site for high Jsc of the inverted PVSCs. The results in Figure S5
manifest that the transmittance of both TPA-FO and TPA-PDO in
the visible range is reduced only slightly compared with the pris-
tine ITO substrate due to the absorption of TPA-FO and TPA-PDO,
while TPA-FO is lightly more transparent than TPA-PDO.
Figure 3 AFM height images of the HTM films for (a) TPA-FO and (b)
TPA-PDO; Water contact angle images for the (c) TPA-FO and (d) TPA-PDO
HTMs on ITO substrates; Top-view SEM images of the prepared perovskite
films on (e) TPA-FO and (f) TPA-PDO.
Table 1 Optical and electrochemical properties of TPA-FO and TPA-PDO
HTMs
Name λonset,filma/nm Eg,opt b/eV HOMOc/eV LUMOd/eV μh/(cm2·V–1·s–1)
The topographic morphology of the HTMs films was investi-
gated via atomic force microscopy (AFM). The AFM image of a
TPA-FO film in Figure 3a reveals that TPA-FO film shows a
smoother surface than the TPA-PDO one in Figure 3b, with root‐
mean‐square (RMS) roughness: 1.15ꢁnm versus 2.4ꢁnm. Further-
more, the surface wettability of two HTMs was investigated be-
cause a moderately hydrophobic surface is supposed to facilitate
the growth of larger high-quality perovskite crystals along with
impending water penetration for enhancing ambient device sta-
bility, while sufficient wettability is the prerequisite for the at-
tachment of the polar perovskite precursor solution on sub-
strates.[14a] The contact angle measurements (Figures 3c and 3d)
demonstrate that TPA-FO has a modest contact angle of 40.3° to
water, which is much higher than that of TPA-PDO (16.6°). The de-
crease in contact angle may originate from more carbonyl groups
on TPA-PDO with a higher dipole moment and thus stronger wet-
tability.[21] Consequently, the perovskite grown on TPA-FO shows
larger grain sizes compared to that grown on TPA-PDO, as re-
vealed by the scanning electron microscopy (SEM) images in Fig-
ures 3e and 3f. Besides, a higher intensity of X-ray diffraction (XRD)
pattern was observed from perovskite films grown on TPA-FO,
indicating its better film crystallinity (Figure S9). We have calcu-
lated crystal size using Scherrer’s equation based on XRD analysis
and the corresponding results are listed in Table S2. Average crys-
talline size of perovskites grown on TPA-FO is determined to be
47.11 nm compared to that (45.49 nm) of TPA-PDO. The higher
value for that on TPA-FO may be ascribed to the relatively more
hydrophobic surface for TPA-FO film, which also agrees with the
larger grain sizes based on SEM results (Figures 3e, f).
TPA-FO
TPA-PDO
602
751
2.06
1.65
–5.33
–5.37
–3.27
–3.72
4.19 × 10–5
9.34 × 10–6
a Films were spin-coated using 10 mg·mL–1 solution with a speed of 2000
r/min. b Estimated from the film absorption edge using the equation: Eg
opt
= 1243/λonset (eV). EHOMO = –5.1–(EOX–E1/2 (Fc/Fc+)) (eV). ELUMO = EHOMO
+
c
d
Egopt (eV).
It is well recognized that the HTM/perovskite interfacial band
alignment is decisive in the interfacial hole transport and deter-
mining the final PVSC performances. As shown in Table 1, the
HOMO energy levels of TPA-FO and TPA-PDO were derived from
the onset oxidation potentials by cyclic voltammetry (CV) meas-
urements (Figure S6) and found to be –5.33 and –5.37 eV, respec-
tively. The LUMO energy levels were determined to be –3.27 and
–3.72 eV for TPA-FO and TPA-PDO by ELUMO = EHOMO + Eg. Hence,
increasing from one carbonyl group in TPA-FO to two carbonyl
groups in TPA-PDO downshifts both the HOMO and LUMO energy
levels simultaneously but shows a more significant impact on
LUMO level. Ultraviolet photoelectron spectroscopy measure-
ment (Figure S7) also demonstrated a similar trend. Based on the
space-charge-limited current (SCLC) measurement method, the
hole mobility of neat TPA-FO film is determined to be 4.19 × 10–5
cm2·V–1·s–1 (Table 1 and Figure S8), which is higher than that (9.34
× 10–6 cm2·V –1·s –1) of neat TPA-PDO film and comparable to that of
widely used PTAA (~10–5-10–4 cm2·V –1·s –1).[10d] The relatively low
charge mobility of TPA-PDO can be explained as the sacrifice of
the excessive dipole moment of TPA-PDO, according to the
XPS measurements were conducted to confirm the possible
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Huang et al.
Concise Report
interactions between carbonyl groups as Lewis base groups in
HTMs and undercoordinated Pb2+ in perovskite (Figure 4a). The
signals of the Pb2+ in pristine perovskite exhibit two main single
peaks from the excited electrons of the 4f orbitals accompanied
by two small peaks that can be ascribed to the defects of unsatu-
rated Pb2+.[26] As shown in Figures 4a and 4b, after coating a thin
layer of TPA-FO or TPA-PDO on the top of perovskite, the small
peaks of Pb 4f disappeared, and shifts of Pb 4f peaks and O 1s
peaks to lower binding energy can be detected in both TPA-FO/
perovskite and TPA-PDO/perovskite film, which can be ascribed to
the interactions between carbonyl groups and Pb2+ in both films.
Moreover, Fourier transform infrared (FTIR) spectroscopy (Figure
5) was utilized to further verify the interaction between HTM and
perovskite.[3c] The peaks of 1717 and 1678 cm−1 are appointed to
the symmetric stretching vibrations of C=O bonds (ν(C=O)) of the
pristine TPA-FO and TPA-PDO, respectively. To distinguish differ-
ences between HTM and HTM/perovskite, maximum peak at 1505
cm−1 was normalized as the reference, which is allocated to
stretching vibrations of aromatic C=C bonds and is not sensitive to
Pb2+. The FTIR spectroscopy of both HTM/perovskite blends shows
similar weakened vibration strength for ν(C=O) by 53%, further
validating the interaction between perovskite and the carbonyl
groups of HTMs. However, it should be noted that a slightly
smaller Pb 4f shift was observed in TPA-PDO/perovskite compared
to that of TPA-FO/perovskite films. This phenomenon indicates
that although both HTMs can interact with perovskite, TPA-FO
exhibits stronger interaction with Pb2+ ions than TPA-PDO. The
slightly weaker interaction between TPA-PDO and Pb2+ ions may
be that Pb2+ and 9,10-phenanthrenedione can only form a semi-
quinonate structure, that is to say, only one carbonyl group can
interact with Pb2+ ion without chelation effect of two carbonyl
groups, which is supported by a previous report.[27]
BCP/Ag (Figure 6b) and tested the photovoltaic performance. Fig-
ure 6a illustrates the J-V curves of the best-performing PVSCs in
both reverse and forward scan directions under the AM 1.5G solar
illumination, while the corresponding photovoltaic parameters are
summarized in Table 2.
Figure 6 (a) J–V curves of the best performing TPA-FO- and TPA-PDO-
based PVSCs. (b) Statistical analysis of the device performance (20 devices)
with the device architecture of inverted PVSCs insides. (c) EQE spectra and
integrated Jsc curves. (d) Stabilized power output at the maximum power
point.
The TPA-FO-based device shows a high PCE of 20.24% in the
reverse scan with a Voc of 1.09 V, a Jsc of 22.76 mA·cm−2 and an FF
of 81.85%, and a PCE of 19.85% was achieved in the forward scan.
In comparison, a moderate PCE of 17.02% with a Voc of 1.03 V, a
Jsc of 20.44 mA·cm-2, and an FF of 80.91% was obtained for the
TPA-PDO based devices in the reverse scan, while a PCE of 16.46%
in the forward scan. The hysteresis of the devices can be de-
scribed by the hysteresis index (HI) of the followed equation:[6c]
The HIs of TPA-FO- and TPA-PDO-based devices were calculated to
be 1.9% and 3.2%, respectively.
Figure 4 XPS spectra of the (a) Pb 4f and in the perovskite films without
or with coating TPA-FO or TPA-PDO. The black dash lines indicate the
unsaturated Pb2+ signals. (b) O 1s signal in TPA-FO and TPA-PDO on the
perovskite or the ITO.
Table 2 Photovoltaic performance of the best inverted PVSCs based on
TPA-FO and TPA-PDO as the HTMs (with average performance of 20 de-
vices in paranthesis)
Scan di-
HTM
Voc/eV Jsc/(mA·cm–2)
FF/%
PCE/%
rection
TPA- Forward
FO Reverse
1.09
1.09
22.83
22.76
79.93
81.85
19.85
20.24
(1.08±0.004) (22.30±0.31) (81.91±0.78) (19.80±0.20)
TPA- Forward
PDO Reverse
1.02
1.03
20.34
20.44
79.30
80.91
16.46
17.02
(1.02±0.015) (20.24±0.28) (79.35±1.11) (16.39±0.35)
Additionally, external quantum efficiency (EQE) spectra (Figure
6c) were recorded, and the TPA-FO-based devices show higher
EQE response over the whole absorption spectrum of the perov-
skite active layer when compared with the TPA-PDO‐based devic-
es. The integrated Jscs of TPA-FO (22.10 mA·cm−2) and TPA-PDO
(20.17 mA·cm−2) based devices matched well with the Jscs ob-
tained from J-V measurements.
To further verify the reliability of device performance, we
measured current densities as a function of time at the maximum
power point and the stabilized PCE through 180-second tracing.
Figure 5 FTIR spectra of TPA-FO and TPA-PDO with perovskite together
with pristine TPA-FO and TPA-PDO as the control.
To evaluate the device performance of two D-A-D type small
molecules as dopant-free HTMs in inverted PVSCs, we fabricated
PVSCs with device architecture of ITO/HTM/perovskite/LiF/C60/
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As shown in Figure 6d, the stabilized PCE of 19.60% at 0.94 V for
the TPA-FO-based device and 16.76% at 0.88 V for the TPA-PD-
based devices, respectively, are in agreement with J–V scan re-
sults. Besides, the statistical distribution of the photovoltaic pa-
rameters of TPA-FO-based devices is narrower than that of
TPA-PDO, suggesting better reproducibility of PVSCs on TPA-FO
(Figures 6b and S10).
Figure 8 Devices ambient stability measurement for the unencapsulated
PSVCs based on TPA-FO and TPA-PDO HTMs with a relative humidity of
(40±5)%, 25 oC.
application. We evaluated the device stability of TPA-FO- and
TPA-PDO-based PVSCs in the ambient environment (Figure 8) and
inert N2 atmosphere (Figure S13). Although both unencapsulated
devices degraded in a high-humidity environment due to the air-
and water-sensitive nature of perovskite, it is clear that
TPA-FO-based devices degraded much slower than TPA-DPO
based devices. As shown in Figure S12, the Voc of both remained
stable over the duration time, but the FF of both declined similarly
from ~80% to 55%. Different degrading behaviors occurred in Jsc;
88% of original Jsc was kept for TPA-FO based device in compari-
son to only 55% left for that of TPA-PDO. TPA-FO based PVSCs can
maintain over 55% of the initial PCE after storage for 100 h, while
only 34% for that of TPA-DPO based devices (relative humidity
Figure 7 (a) PL and (b) TRPL spectra of the perovskite films deposited on
ITO, ITO/TPA-FO, and ITO/TPA-PDO.
Table 3 Fitted parameters of photoluminescence decay curves of
TPA-FO/perovskite and TPA-PDO/perovskite films with pristine perovskite
film on ITO as control
Sample
A1/%
τ1/ns A2/% τ2/ns τavga/ns
ITO/perovskite
9.3
25.7
17.4
20.0 90.7 100.9
13.12 74.3 78.9
16.7 80.0 82.0
99.3
75.3
79.2
ITO/TPA-FO/perovskite
ITO/TPA-PDO/perovskite
o
a τavg refers to the average lifetime, which is calculated as follows: τavg
=
45%—55%, 25 C), which can be attributed to the more hydro-
2
(A1τ12 + A2τ2 )/(A1τ1 + A2τ2).
phobic surface and thus better film quality mentioned above, for
protecting films from water permeation. Also, better intrinsic
stability of TPA-FO based devices is achieved with a PCE above 19%
after the storage of 70 d, while relatively low stability of TPA-PDO
based devices is observed with the decline of PCE to only 80% of
its initial value (Figure S13).
To investigate possible reasons for the enhanced Jsc, Voc, and
FF of the devices, we conducted photoluminescence (PL) meas-
urement of the perovskite films deposited on TPA-FO and TPA-
PDO to study the hole extraction and charge recombination in the
perovskite and at HTM/perovskite interfaces. The PL and TRPL
results are plotted in Figure 7. All the PL emission peaks are lo-
cated at 774 nm, and efficient PL quenching occurs in both
TPA-FO/perovskite film and TPA-PDO/perovskite film compared to
the perovskite film on ITO (Figure 7a). To be specific, TPA-FO HTM
can quench PL of perovskite film more efficiently than TPA-PDO
HTM, which may result from either charge transfer between per-
ovskite and HTM or more severe recombination at interfacial
trap/defects in the perovskite/HTMs interfaces.[14b,28] We ob-
tained time-resolved photoluminescence (TRPL) of the above
three perovskite films to further analyze the charge carrier life-
times. As shown in Figure 7b, TRPL spectra for all three samples
are fitted by the bi-exponential decay model, and the corre-
sponding fitting parameters are summarized in Table 3. The fast
lifetime (τ1) refers to charge transfer quenching of PL or defect-
driven recombination rate, while the slower lifetime (τ2) comes
from the non-radiative charge recombination quenching of PL.[29]
TPA-FO/perovskite exhibits the shortest τ1 of 13.1 ns compared to
those τ1 of 16.7 ns and 20.0 ns for TPA-PDO/perovskite and
ITO/perovskite, respectively. In addition, the portion of τ1 moder-
ately increases from 17.4% in TPA-PDO/perovskite to 25.7% in
TPA-FO/perovskite. Besides, the averaged PL lifetime of the
ITO/perovskite was calculated to be 99.3 ns. The average lifetimes
for the perovskite films spun on TPA-PDO decreased to 79.2 ns,
while the further reduced average lifetime is 75.3 ns for the per-
ovskite on TPA-FO. TRPL results suggest that charge recombina-
tion of perovskite on both D-A-D HTMs are suppressed, and hole
transfer is more efficient at the interface between perovskite and
TPA-FO.
Conclusions
In summary, we reported two new dopant-free D-A-D type
small molecular HTMs with carbonyl group-functionalized arenes
as the acceptor units and TPA groups as the donor co-unit, which
can be synthesized via a facile one-step Suzuki coupling reaction,
showing distinct photophysical and electrochemical properties
depending on the number of carbonyl groups. Benefiting from
suitable FMO energy level alignment, moderate wettability, pas-
sivation effect, and good crystallinity of perovskite films on HTM
substrates, the device based on one-carbonyl-group-functional-
ized TPA-FO yielded an optimal PCE of 20.24% with a negligible
hysteresis, which is among the highest values reported for in-
verted PVSCs based on dopant-free HTMs. Our results demon-
strate that the molecule design of D-A-D type HTMs with carbonyl
group-functionalized arene cores can pave the way to develop the
cost-effective dopant-free HTMs toward highly efficient PVSCs.
Experimental
Materials
Anhydrous N,N-dimethylformamide (DMF, 99.8%), anhydrous
dimethyl sulfoxide (DMSO, 99.8%), anhydrous ethanol (99.8%),
anhydrous chlorobenzene (CB, 99.8%), and all other solvents were
purchased from Sigma-Aldrich. Lead (II) iodide (PbI2) and lead (II)
chloride (PbCl2), methylammonium iodide (MAI), formamidinium
iodide (FAI), and C60 were purchased from p-OLED. BCP (99%) was
obtained from TCI. All the materials were used as received. The
The long-term stability of PVSCs is essential for practical
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Huang et al.
Concise Report
synthesis details are shown in Supporting Information.
Am. Chem. Soc. 2009, 131, 6050‒6051; (b) Ma, Y.; Wang, S.; Zheng, L.;
Lu, Z.; Zhang, D.; Bian, Z.; Huang, C.; Xiao, L. Recent research
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963; (c) Ma, C.; Park, N.-G. A realistic methodology for 30% efficient
perovskite solar cells. Chem 2020, 6, 1254‒1264.
Characterization and measurements
1
The H NMR and 13C NMR spectra were tested on the Bruker
Ascend 400 MHz spectrometer. High-resolution mass spectra were
measured by ThermoScientificTM Q-Exactive. UV-Vis optical ab-
sorption spectra were tested on a Shimadzu UV-3600 UV-VIS-NIR
spectrophotometer. Atomic force microscopy (AFM) measure-
ments were tested using a Dimension Icon Scanning Probe Micro-
scope (Asylum Research, MFP-3D-Stand Alone). Cyclic voltamme-
try measurements were conducted under an argon atmosphere
using the CHI760E voltametric workstation with an argon-satu-
rated supporting electrolyte solution of 0.1 mol/L tetra-n-butyl
ammonium hexafluorophosphate (Bu4NPF6) in CH3CN, using a
platinum working electrode, a platinum wire counter electrode,
and a silver wire reference electrode. And the ferrocene/ferroce-
nium redox couple (Fc/Fc+) was used as the external standard for
all the measurements with a scanning rate of 50 mV·s−1.
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Device fabrication
PVSCs were fabricated with an architecture of ITO/HTMs/
perovskite/LiF/C60/BCP/Ag. ITO glass was cleaned by sequentially
washing with detergent, deionized water, acetone, and isopropa-
nol. The substrates were dried by the dry air and treated by UV
ozone for 15 min. (FA0.17MA0.94PbI3.11 0.95(PbCl2)0.05 precursor solu-
)
tion was prepared by mixing (1.17 mol/L) PbI2, (0.13 mol/L) PbCl2,
(0.2 mol/L) FAI and (1.1 mol/L) MAI in DMF/DMSO (V/V = 9/1).
o
The solution was stirred for 2 h at 60 C and then cooled down to
room temperature. TPA-FO or TPA-PDO HTMs were prepared by
dropping the CB or toluene solution (2 mg·mL–1 for the optimized
devices) on the substrates and then spin-coated at a rate of 4500
r/min for 30 s, followed by thermal annealing at 120 oC for 15 min.
Before the perovskite spin-coating process, the precursor solution
was filtered by a 0.45-μm Teflon-filter. Perovskite films formed by
spin coating the precursor solution with a two-step spin-coating
program (500 r/min for 3 s, 5000 r/min for 27 s). In the last 20 s of
the procedure, 125 µL antisolvent (CB) was dropped on the per-
o
o
ovskite films quickly and annealed at 60 C for 1 min plus 100 C
for 10 min. A thin layer of LiF (1 nm) and 30 nm of a layer of C60
was deposited on the perovskite active layer by thermal evapora-
tion at a high vacuum (~3 × 10–5 Pa). Then a thin layer of BCP (0.6
mg/mL in ethanol) was spin-coated as cathode buffer layers at a
rate of 2500 r/min. Finally, ~100 nm Ag back electrode with an
area of 0.037 cm2 was deposited by thermal evaporation at a high
vacuum (~3 × 10–5 Pa).
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.202100022.
Acknowledgement
J.H. and J.Y. contributed equally to this work. H.S. thanks the
National Natural Science Foundation of China (NSFC, No. 21801124).
X.G. is grateful to the Shenzhen Science and Technology Innova-
tion Commission (No. JCYJ20180504165709042). We are grateful
for the assistance of SUSTech Core Research Facilities. PL and TRPL
characterizations were supported by Shenzhen Key Laboratory of
Full Spectral Solar Electricity Generation (FSSEG) from the Shen-
zhen Key Laboratory Project (No. ZDSYS201602261933302).
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Manuscript received: January 8, 2021
Manuscript revised: February 9, 2021
Manuscript accepted: February 18, 2021
Accepted manuscript online: February 20, 2021
Version of record online: XXXX, 2021
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Chin. J. Chem. 2021, 39, 1545－1552