Hydrogen-Bonded Dopant-Free Hole
Transport Material Enables Efficient
and Stable Inverted Perovskite Solar
Cells
Rui Li1†, Chongwen Li2†, Maning Liu3, Paola Vivo3, Meng Zheng1, Zhicheng Dai1, Jingbo Zhan1, Benlin He2, Haiyan Li2,
Wenjun Yang1, Zhongmin Zhou1* & Haichang Zhang1*
1Key Laboratory for Rubber-Plastics of Ministry of Education/Shandong Province, School of Polymer Science and
Technology, Qingdao University of Science and Technology, Qingdao 266042, 2Institute of Materials Science and
Engineering, Ocean University of China, Qingdao 266100, 3Hybrid Solar Cells, Faculty of Engineering and Natural
Sciences, Tampere University, FI-33014 Tampere
*Corresponding authors: zhouzm@qust.edu.cn; haichangzhang@qust.edu.cn; †R. Li and C. Li contributed equally to
this work.
Cite this: CCS Chem. 2021, 3, 3309–3319
Although many dopant-free hole transport materials
(HTMs) for perovskite solar cells (PSCs) have been
investigated in the literature, novel and useful molec-
ular designs for high-performance HTMs are still need-
ed. In this work, a hydrogen-bonding association
system (NH⋯CO) between amide and carbonyl is
introduced into the pure HTM layer. Our study demon-
strates that the hydrogen-bonding association can
not only significantly increase the HTM’s hole trans-
port mobility and functionalize the surface passiv-
ation to the perovskite layer, but also form Pb–N
coordination bonds at the interface to promote the
hole extraction while hindering the interfacial charge
recombination. As a result, the PSCs based ondopant-
free hydrogen-bonded HTMs can achieve a champion
power conversion efficiency (PCE) of 21.62%, which is
around 32% higher than the pristine PSC without the
hydrogen-bonding association. Furthermore, the
dopant-free hydrogen-bonded HTMs based device
shows remarkable long-term light stability, retaining
87% of its original value after 500 h continuous
illumination, measured at the maximum power point.
This work not only provides a potential HTM with
hydrogen-bonding association in PSCs, but also
demonstrates that introducing hydrogen bonding in-
to the materials is a useful and simple strategy for
developing high-performance dopant-free HTMs.
Keywords: hydrogen bonding, dopant-free hole
transport material, inverted perovskite solar cell, high
efficiency, stability
RESEARCH ARTICLE
Received: Sept. 15, 2021 | Accepted: Oct. 27, 2021
DOI: 10.31635/ccschem.021.202101483
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3309
Introduction
The power conversion efficiency (PCE) of perovskite
solar cells (PSCs), the leading third-generation low-cost
photovoltaic technology, has increased remarkably
from 3.8% in 2009 to 25.5% in 2020, which is compara-
ble to that of commercialized crystalline silicon.1 After
light absorption in the PSCs, the generated electrons
and holes need to be transported through the perov-
skite layer and collected at the adjacent charge selective
interfaces. A recent report2 highlights the fact that the
performance of PSCs is dominated by swift hole trans-
port (hole injection rate ∼1 ns) rather than relatively slow
electron transfer (electron injection rate ∼11 ns). This
suggests that hole transport materials (HTMs) play a
key role in the impressive progress of PSCs. Currently,
state-of-the-art PSCswith conventional n–i–p structures
utilize 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenlamine)-
9,9-Spirobifluorene (Spiro-OMeTAD) and poly-triaryla-
mine (PTAA) as standard HTMs. However, Spiro-OMeTAD
and PTAA are not only tremendously expensive but suffer
from low mobility (<1 × 10−5 cm2 V−1 s−1) and limited
conductivity (<3 × 10−7 S cm−1).3 Thesematerials thus need
dopants such as bis(trifluoromethane)sulfonimide lithium
salt (Li-TFSI) to increase their hole mobility and conduc-
tivity that may lead to device degradation due to the (1)
sophisticated oxidation process associated with unde-
sired ion migration and (2) chemical interaction with the
down-lying perovskite layer.4 Hence, to overcome the
drawbacks of doped HTMs, during the last several years
a wide range of novel low-cost dopant-free HTMs have
been reported as alternatives to the state-of-the-art
HTMs.5–11
A good dopant-free HTM is expected to simultaneously
display high hole transport mobility and well-matched
frontier molecular orbitals (FMOs) with the perovskite
valence band edge. Furthermore, to reduce the energy
losses at the interface, the HTM needs to passivate the
unavoidable defects present in the perovskite layer. In
HTMs, holes need to be efficiently transferred within and
between individual molecules (intra-/intermolecular con-
jugation hole transport). To enhance the intramolecular
conjugation hole transport, donor–acceptor (D–A) type,
D–D type, strong planarity, and large π-conjugation mo-
lecular designs have mainly been used for dopant-free
HTM.12–16 Charge-carrier mobility between the adjacent
molecules significantly increases if the materials exhibit
self-assembling properties that can be exploited to gen-
erate ordered structures.17,18 However, well-organized
nanostructures are difficult to obtain with classic solution
processing methods due to the disorder packing. Intro-
ducing a hydrogen-bonding functionality in the materials,
resulting in hydrogen-bonded material superstructures,
can be a suitable method to fulfil the above-mentioned
critical requirements. Molecules with fused hydrogen-
bonding have been reported to exhibit high charge-carrier
mobility and conductivity in organic field-effect transis-
tors (OFETs), as demonstrated by our previous work and
by other groups as well.19–23 Materials with high hole mo-
bility in OFETs might be suitable as dopant-free HTMs for
PSCs. To the best of our knowledge, there are only two
articles that claim the application of hydrogen-bonding
functionalized HTMs in PSCs.24,25 Kaneko et al.16 proposed
small molecular dopant-free HTMs to fabricate n–i–p type
PSCs with PCE of 14.5%, but the authors did not focus on
the inverted p–i–n structure. In addition, they synthesized
the molecules containing multilarge size alkyl chains to
enhance the solubility of the small molecules. In most
cases, large alkyl-chains result in poor packing and low
hole mobility. Later, Más-Montoya et al.25 synthesized hy-
drogen-bonded small molecules with no alkyl chain for p–
i–n PSCs with PCE of 15.9%, which remained stable for
more than 1200 swith a variation of less than 1%. However,
due to the poor solubility of the molecules, the device
fabrication procedure relied on the thermal evaporation
technique, which increases the complexity and cost of the
device fabrication. Thus, making dopant-free HTMs by the
vapor method cannot be easily and widely used by most
research groups (Supporting Information Table S1).
Herein, we report dopant-free D–A–D HTMs with tert-
butoxylcarbonyl (t-Boc)-substituted diketopyrrolopyrrole
(DPP) as acceptors, capped at both ends with electron-
rich units of triphenylamine (TPA), named TPADPP-Boc.
DPP is the most widely used chromophore in OFETs,
which enables extremely high hole mobility,19,26 and TPA
is a popular unit to build high-performance HTMs.27–31 In
the designed molecule, the t-Boc units could be easily
decomposed into carbon dioxide and isobutylene gas
upon the thermal annealing process.32 Meanwhile, the
N–H units emerge, and the hydrogen-bonded 3,6-di(5-
N,N-bis(4-methoxyphenyl)aniline-thiophene-2-yl)pyrrolo
[3,4-c]pyrrole-2,5(1H,4H)-dicarboxylate (TPADPP) was
formed between the N–H units and the C=O units from
the neighboring DPP core (Figure 1). The application of
solution-processable hydrogen-bonded dopant-free
HTMs is studied in this work. The results showed that
hydrogen-bonding association increases the holemobility
of HTMs from 1.11 × 10−4 to 3.09 × 10−4 cm2 V−1 s−1, resulting
in a significantly enhanced PCE of PSCs from 16.36% to
21.62% (32% enhancement), which indicates that hydro-
gen-bonded materials are promising candidates as dop-
ant-free HTMs for efficient PSCs.
Experimental Methods
Synthesis of TPADPP-Boc
TPA-Bo (0.188 g, 0.42 mmol), DPP-Boc-Br (0.131 g,
0.2 mmol), and potassium carbonate aqueous solution
(2 M/L 5 mL) were added into freshly distilled toluene
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(15 mL) (Scheme 1). The mixture was degassed with nitro-
gen, followed by the addition of tetrakis(triphenylpho-
sphine)palladium (0.0115 g, 0.01 mmol). The mixture
reacted for 36 h at 100 °C under N2 protection. After
cooling the solution to room temperature, it was extracted
with dichloromethane and deionized water twice and
dried over anhydrous MgSO4. The crude product was
purified by column chromatography (silica gel, dichloro-
methane) to afford compound TPADPP-Boc as a blue
solid (0.195 g, yield: 88%). 1H NMR (500 MHz, d1-CHCl3,
δ) (ppm): 8.31–8.32 (d, 2H), 7.42–7.44 (d, 2H), 7.24–7.25 (d,
2H), 7.08–7.10 (d, 2H), 6.85–6.90 (q, 4H), 3.81 (s, 12H), 1.63
(s, 18H). 13C NMR (125 MHz, d1-CHCl3, δ) (ppm): 159.31,
156.44, 151.67, 149.56, 149.14, 140.02, 136.71, 135.74,
127.16, 126.97, 124.63, 122.72, 119.50, 114.85, 109.62, 85.71,
55.51, 55.45, 27.75. Microanalysis found C, 69.45%; H,
5.27%; N, 5.05%; S, 5.79% (C, 69.42%; H, 5.28%; N,
5.06%; S, 5.79%).
Synthesis of TPADPP
TPADPP-Boc (0.055 g, 0.05 mmol) was thermally
annealed on a hot plate at 185 °C for 10 min (Scheme 2).
The dark blue product of TPADPPwas obtained (0.045 g,
Figure 1 | (a) Chemical structures of materials, illustration of decarboxylation of PADPP-Boc and hydrogen-bonding
formation. (b) TGA spectrum of TPADPP-Boc from room temperature to 185 °C (heating rate 50 °C/min). Once the
temperature reached 110 °C, thematerial was kept at this condition for 800 s. (c) FT-IR spectra of TPADPP-Boc at 185 °C
for 150 s. (d) Cyclic voltammograms of the materials as thin films deposited on ITO. Solution: 0.1 M TBAPF6/Acetonitrile.
Potentials calculated versus ferrocene. Scan rate: 100 mV s−1; T = 20 °C.
N
N
O
O
Boc
Boc
DPP-Boc-Br
S
S
Br
Br
N
O
O
B
O
O
+
N
N
O
O
S
S
NN
O
OO
O
O
O
O
O
Pd(PPh3)4, K2CO3/H2O
Toluene
TPA-Bo
TPADPP-Boc
Scheme 1 | Synthetic route of TPADPP-Boc. The synthesis route and NMR spectrum of the starting product of TPA-
DPP are shown in Supporting Information Figures S9–S13 and Schemes S1–S4.
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yield: 100%). Due to the poor solubility of the TPADPP, the
NMR spectra were not measured. Microanalysis found C,
71.49%;H, 4.69%; N, 6.18%; S, 7.05% (C, 71.50%;H, 4.67%; N,
6.18%; S, 7.07%).
Perovskite (FA0.8Cs0.2PbI2.96Br0.04) solution
The 1.35 M FA0.8Cs0.2PbI2.96Br0.04 perovskite precursor
solution was obtained by dissolving 137.6 mg of forma-
midinium iodide, 52 mg of cesium iodide, 447.2 mg of
lead iodide, 11 mg of lead bromide, and 5.2 mg of lead
thiocyanate in a mixed solvent of N,N-dimethylforma-
mide and dimethyl sulfoxide with a ratio of 3:1. The
precursor solution was stirred at 60 °C for 3 h before
use.
Device fabrication
The indium-doped tin oxide (ITO) glass substrates with
an optical transmission of >80% in the visible range and
sheet resistance of 8−10 Ω−2 were purchased from Tech-
no Print Co., Ltd. (Chiba, Japan). The patterned ITO
substrate was first cleaned using a surfactant, then
washed with sequential sonication in deionized water,
ethanol, and acetone for 10 min, respectively. Finally, it
was subjected to UV/ozone treatment for 30min before
utilization. The HTM layers were fabricated by spin-
coating them at 3000 rpm for 30 s, then annealing at
100 and 180 °C for 10 and 30 min, respectively. The
perovskite films were prepared by dripping 100 μL of
the perovskite precursor solution on substrates fol-
lowed by spin-coating at 500 rpm for 2 s and 4000 rpm
for 50 s. 750 μL of diethyl ether was dripped at the 25th
second of the second step. Then the films were trans-
ferred to a preheated hot plate at 65 °C for 2 min and
then to a 100 °C hot plate for 15 min. After the formation
of the perovskite film, the C60 electron transporting
layer (30 nm) and 2,9-Dimethyl-4,7-diphenyl-1,10-phe-
nanthroline (BCP) hole-blocking layer (10 nm) were
evaporated successively. A 100 nm thick of Ag cathode
was thermally evaporated under a reduced pressure of
2 × 10−5 Torr to achieve a complete device via a metal
shadow mask of 0.094 cm2.
Results and Discussion
Design and synthesis of the TPA-DPP
Figure 1 shows the molecular structure of TPADPP-Boc.
The synthetic route to the target TPADPP-Boc was
straightforward via the Suzuki coupling reaction be-
tween dibromominated DPP and 4-broate ester-4-N,N-
bis(4-methoxyphenyl)aniline (TPA-Bo) (see Supporting
Information). Compared to PTAA or Spiro-OMeTAD
HTMs, the cost of the TPADPP-Boc is significantly lower
($12.69/g) (Supporting Information Tables S2–S8). The
two t-Boc units of the TPADPP-Boc enable good solubil-
ity in most common organic solvents, rendering their
solutions processable. Upon thermal annealing, the t-Boc
units were decomposed, while the TPADPP-Boc was
converted into TPADPP. TPADPP contains two lactam
units that can form intermolecular hydrogen-bonding
pairs (NH⋯OC, Figure 1a).23 To validate the formation of
the TPADPP with fused hydrogen bonding after the
decarboxylation of the t-Boc groups, thermogravimetric
analysis (TGA), elemental analysis, and Fourier transform
infrared (FT-IR) experiments were conducted. As shown
in Figure 1b, the TPADPP-Boc decomposed under 185 °C
heating for 10 min with a weight loss of around 18.19%,
which matched well with the weight percentage of the
t-Boc units in TPADPP-Boc (18.08%). In addition, the
elemental analysis of the annealed TPADPP-Boc
matched well with the TPADPP element composition.
These observations indicate that the t-Boc units could
be easily decomposed through the thermal annealing
process and the TPADPP-Boc converted into TPADPP.
To further confirm the formation of the hydrogen bond-
ing between the neighboring TPADPP molecules in the
thin film, the FT-IR spectra were measured under 185 °C
for varied periods. As can be seen from Figure 1c, during
the thermal annealing at 185 °C for 150 s, the absorption
peak at 1753 cm−1 (C=O stretching of t-Bocunits) gradually
decreased and finally disappeared, which can be ascribed
to the decarboxylation. Meanwhile, the N-Boc units chan-
ged into N–H groups. Once the N–H group emerged, the
hydrogen-bonding association was formed between the
N–H units and the C=O units from the neighboring
N
N
O
O
S
S
NN
O
OO
O
O
O
O
O
185 oC / 10 min
H
N
N
H
O
O
S
S
NN
O
O
O
O
TPADPP-Boc TPADPP
Scheme 2 | Synthetic route of TPADPP.
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TPADPP. This led to the following observations: (1) A
broad absorption peak between 2700 and 3255 cm−1
emerged, typically the peak located at 3128 cm−1, which
is ascribed to the hydrogen-bonded NH stretching vibra-
tion. (2) The absorption peak of the carbonyl group
from the DPP core located at 1673 cm−1 was shifted to
1643 cm−1, indicating that the isolated C=O groups were
bonded with NH units (C=O⋯H–N). (3) The amide I signal
shifted to lower wavenumbers (C=O stretching, from 1673
to 1643 cm−1), while the amide II signal shifted to higher
wavenumbers (N–H bending, from 1563 to 1596 cm−1),
which consequently confirmed the formation of a second-
ary amine.33–36 The FT-IR results agreed well with those
reported for published hydrogen-bonded systems.
The UV–vis absorption spectra of both molecules in
the thin-film state are shown in Supporting Information
Figure S1. Both materials exhibited two absorption peaks
between 590 and 665 nm. After the decarboxylation, the
peaks were significantly red-shifted by 25 nm, which could
be ascribed to the fact that the hydrogen-bonding associ-
ation in the TPADPP thin film resulted in strong aggrega-
tion.17 From the absorption onset, the optical bandgaps of
1.68 eV for TPADPP-Boc and 1.52 eV for TPADPP were
estimated (Supporting Information Table S9). The electro-
chemical properties of the materials were studied using
cyclic voltammetry (CV) and ultraviolet photoelectron
spectroscopy (UPS). From the onsets of anodic oxidation
and cathodic reduction, the highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) energy levels of TPADPP-Boc were calculated as
−5.25 and −3.72 eV, while the TPADPP showed slightly
higher HOMO energy levels (−5.21 eV) and lower LUMO
energy levels (−3.70 eV, Figure 1d). The high HOMOenergy
levels of the two materials can be ascribed to the strong
electron donor character of bis(4-methoxyphenyl)amine.
The HOMO energy levels calculated from CV curves
matched well with the results obtained by UPS (−5.25 and
−5.20 eV for TPADPP-Boc and TPADPP, respectively,
Supporting Information Figure S2). The FMO energy levels
of the twomolecules were higher than the valence band of
the perovskite, indicating that the two materials are po-
tentially suitable as HTMs for only transferring the holes
while blocking the electron transfer from the perovskite
layer to HTM layers.
Device properties
To build an efficient PSC device, the morphology, quality,
and contact angle of the HTMs thin films should be
carefully considered. The surface morphology of the
HTMs was characterized by atomic force microscopy
(AFM). The TPADPP-Boc film was annealed at 110 °C for
10 min to remove the residual chlorobenzene. Under this
condition, the TPADPP-Boc exhibited thermally stable
properties (Supporting Information Figure S3). To get rid
of the two Boc groups and obtaining the TPADPP-Boc
film, further thermal treatment under 185 °C for 10 min
was used. As shown in Figures 2a and 2b, both film
surfaces were fully covered by the HTMs. However, the
TPADPP film showed lower roughness and was more
uniform than the TPADPP-Boc film. Generally, the perov-
skite materials are difficult to deposit on organic HTM
layers, due to their low wettability. To check the wetta-
bility of both films, the contact angles of both films were
measured. Figures 2c and 2d show that the TPADPP film
exhibits a much lower contact angle than TPADPP-Boc
(TPADPP-Boc: 92°; TPADPP-61°), indicating the ease of
perovskite film deposition. This might be ascribed to the
fact that, after removing the Boc units, the NH functional
groups, which are more hydrophilic, emerged.
High hole transportmobility plays a key role in designing
high-performance dopant-free HTMs. Normally, HTMs
would not need additional doping process if they exhibit
hole mobilities up to 10−3 ∼ 10−4 cm2 V−1 s−1.37 In this work,
the charge transport properties of both films with and
without Boc groups were evaluated by utilizing the
space-charge-limited-current (SCLC) method.38 The dark
I–V characteristics of the hole-only devices with configu-
ration ‘ITO/poly(3,4-ethylenedioxythiophene) (PEDOT):
poly(styrenesulfonate) (PSS)/hole transport layer (HTL)/
Ag’ were evaluated (Figure 2e). The hole mobilities were
extracted by the Mott–Gurney Law (1):
μ=
8JDL
3
9ɛ0ɛrV
2
ð1Þ
where ɛ0 is the vacuum permittivity, ɛr is the relative
dielectric constant (assuming ɛr = 3 for organic materials),
L is the thickness ofHTM, JD is the dark current density, and
V is the applied voltage. The determined thickness of both
films under optimal concentration was 20 nm. As a result,
the hole mobilities of the films with and without Boc
groups were calculated as 1.11 × 10−4 and 3.09 × 10−4 cm2
V−1 s−1, respectively, compared to 6.94 × 10−4 cm2 V−1 s−1 of
classic PTAA (Supporting Information Figure S4). This
indicates that both pristine films can efficiently perform
as hole transport materials in PSCs. Compared to
TPADPP-Boc, the hole transport mobility of TPADPP was
enhanced by almost three times, which can be ascribed to
the hydrogen-bonding formation.
Scanning electron microscopy (SEM) was used to char-
acterize the surface morphology of the perovskite films
atop the HTMs with and without Boc groups. As shown in
Figures 3a and 3b, both perovskite films are uniform and
fully covered to prevent direct contact with the cathode
and anode and in turn eliminating current leakage.39 Be-
sides, there is no discernible thickness change of perov-
skite films, as shown in Supporting Information Figure S5.
Figure 3c shows the UV–vis absorption spectra of the
perovskite samples. The absorption of both the perovskite
films is almost the same at 780 nm, which can be attrib-
uted to the similar size and thickness of the perovskite
grains. To investigate the influence of the HTMs with and
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Figure 3 | Top-view SEM images of perovskite films based on (a) TPADPP-Boc and (b) TPADPP. (c) UV–vis absorption
spectra. (d) XRD pattern spectra. (e) Steady-state PL spectra. (f) TRPL decay transient spectra.
Figure 2 | AFM images of (a) TPADPP-Boc and (b) TPADPP thin films on ITO. Contact angles of (c) TPADPP-Boc and
(d) TPADPP films with respect to water droplets. (e) The SCLC measurements of hole-only devices with configuration
“ITO/PEDOT:PSS/HTM/Ag”.
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without Boc groups on the crystallinity of perovskite films,
the X-ray diffraction (XRD) analysis was performed. As
shown in Figure 3d, the 2θ angle of all diffraction peaks of
the perovskite films, which are independent on the under-
layer, is consistent with the previous reports. Both perov-
skite films showed strong diffraction peaks at 14.11° with
full width at half maxima (FWHM) of 0.149° for TPADPP
and 0.167° for TPADPP, respectively, which is assigned to
the (100) planes. The higher peak of perovskite deposited
on the TPADPP film at 14.11° demonstrates its better
crystallinity and more effective charge transport along
the z-axis compared to the perovskite deposited on the
TPADPP-Boc film. This might be ascribed to the fact that,
after removing the Boc units, the NH groups emerged and
passivated the perovskite surface.
After the light absorption, the generated holes need to
be transported through the perovskite layer and collected
at the adjacent HTM layer. This step plays a key role in
achieving high-performance PSCs. To investigate the in-
terfacial hole extraction process at the interface between
the perovskite and HTM, steady-state and time-resolved
photoluminescence (TRPL) measurements were con-
ducted. Figure 3e shows a clearly enhanced PL quenching
when depositing a perovskite film on the TPADPP layer
compared to the case of perovskite atop the TPADPP-Boc
layer. The calculated quenching efficiencies, that is, hole
extraction efficiencies, are 58.4% and 91.0% for TPADPP-
Boc and TPADPP, respectively. This suggests that the NH
groups of TPADPP can form Pb–N coordination-bonds
through the formation of Lewis adducts between under-
coordinated Pb atoms and N atoms at the perovskite/
HTM interface, which can significantly promote interfacial
hole extraction.40 The hole extraction dynamics were
also monitored by comparing the TRPL decays in
Figure 3f. A clear decay acceleration was observed for
glass/perovskite/TPADPP compared to glass/perovskite/
TPADPP-Boc, indicating that the holes at the perovskite/
HTM interface can be swiftly extractedwith the help of the
Pb–N bond, which is highly consistent with PL quenching
data. We then attempted to quantitatively analyze the PL
decay data by using a reported kinetic model, that is, a
one-dimensional charge diffusion model (see the analysis
method in Supporting Information).41,42 A rate equation
(see Supporting Information Equation S1) was used to fit
the PL decay data, including first-order recombination (k1)
via carrier traps, second-order nongeminate-free charge
carrier (electron and hole) recombination (k2), and inter-
facial hole extraction process (kHT). The resulting k1, k2,
and kHT for two HTMs are summarized in Supporting
Information Table S9. TPADPP exhibited reduced k1
(8.5 × 106 s−1) and k2 (1.3 × 10−10 s−1 cm3) compared to those
(k1 = 2.3 10
7 s−1 and k2 = 4.1 × 10−10 s−1 cm3) of TPADPP-Boc,
suggesting that the interfacial Pb–N bond not only pro-
motes hole extraction but also hinders single-carrier trap-
ping recombination by filling the surface traps of the
perovskite. Moreover, the hole extraction rate constant
(kHT = 5.4 × 108 s−1) of TPADPP was nearly one order of
magnitude higher than that (6.1 × 107 s−1) of TPADPP-Boc,
which is also evident by the faster effective lifetime
(τ1/e = 11.5 ns) of TPADPP-based film compared to that
(τ1/e = 24.7 ns) of TPADPP-Boc case (see Supporting
Information Table S9). To investigate the PSC perfor-
mance based on the designed HTMs, the p–i–n configura-
tion of ITO/HTM/Perovskite/C60/BCP/Ag (Figure 4a)
PSCs with an active area of 0.094 cm2 was realized.
Figure 4b shows the energy-level alignment of each layer
of the device. The optimal thickness of the HTM, evaluated
by comparing the device performance with varying con-
centrations of TPADPP solutions (Supporting Information
Figure S6), was ∼20 nm. Figure 4c shows the current
density–voltage (J–V) curves (measuring under AM 1.5G
illumination at 100mWcm−2) of the champion devicewith
negligible hysteresis. TPADPP-based PSC exhibits an
open-circuit voltage (VOC) of 1.115 V, a short-circuit current
density (JSC) of 23.27 mA cm
−2, a fill factor (FF) of 0.833,
and a PCE of 21.62% under the reverse-scan direction. The
parameters of this champion device under the forward-
scan direction include a VOC of 1.118 V, a JSC of 23.24 mA
cm−2, an FF of 0.818, and a PCE of 21.26%. This indicates
the TPADPP is a promising dopant-free HTM. For the
comparison of the hydrogen-bonding association of HTM
on the performance of the PSC, the device with TPADPP-
Boc film as HTM was fabricated, and the corresponding
J–V curves are also shown in Figure 4c. Table 1 sum-
marizes the photovoltaic parameters of the devices with
TPADPP and TPADPP-Boc as HTMs, respectively. After
removing the Boc units from the HTM structure, the VOC
and JSC of the corresponding PSCs were significantly
enhanced, which resulted in a PCE boost from 16.36 up
to 21.62% (32% enhancement). Such a significant perfor-
mance improvement can be ascribed to the following: (1)
The hydrogen-bonding association for theHTMs improves
the hole transport mobility within the HTM layer. (2) The
NH units passivate the perovskite surface, leading to a
perovskite layerwith goodquality and crystallinity. (3) The
NH groups form Pb-N coordination bonds between the
top-coordinated Pb atoms at the interface, which can
significantly improve the hole extraction rate.28 In this
work, the p–i–n PSCs, employing a hydrogen-bonded
dopant-free HTM, achieved a champion PCE of 21.62%,
which is comparable with the recent highest PCE
records.43
The integration of the external quantum efficiency
(EQE) spectra of the champion devices based on the
hydrogen-bonded TPADPP, TPADPP-Boc, and PTAA
HTMs, yielded the photocurrent densities of 22.1, 21.3, and
22.7 mA/cm2, respectively, which are consistent with the
measured JSC (Figure 4d). To check the device reliability,
the PSCs based on TPADPP, TPADPP-Boc, andPTAAwere
tested with the photocurrent output under a fixed bias. As
presented in Figure 4e, there are stable photocurrent
densities and efficiency outputs for the devices with
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TPADPP and PTAA as HTMs compared to those based on
TPADPP-Boc over a period of 300 s under a continuous
AM 1.5G illumination. To further quantitatively estimate the
defects density in a perovskite, the SCLC method with the
device structure ‘ITO/HTM/Perovskite/Spiro-OMeTAD/
Au’ was exploited. The J–V curves were obtained in the
dark. As shown in Supporting Information Figure S7, the
trap-filled voltage (VTFL) was obtained through a kink
point between the ohmic and the trap-filled regimes. The
defects density (N) was calculated based on the following
eq 244:
VTFL =
eNL2
2ɛɛ0
ð2Þ
where e is the elementary charge, L is the thickness of
perovskite film, ɛ is the relative dielectric constant of the
perovskite, and ɛ0 is the vacuum permittivity. The defect
density was calculated as 1.22 × 1016, 5.88 × 1015, and
4.27 × 1015 cm−3 for the devices based on the TPADPP-
Boc, TPADPP, and PTAA, respectively, which revealed that
the perovskite film deposited on TPADPP film showed
much lower defect density than the perovskite deposited
on TPADPP-Boc film. This further indicates that the NH
units of the TPADPP core can passivate the perovskite
surface.
Long-term device stability is one of the most important
challenges for the emerging PSC technology. In this work,
the stability of the PSC based on hydrogen-bonded
TPADPPmaterialswasmeasuredatmaximumpowerpoint
(MPP) under continuous illumination at 100 mW cm−2
(Figure 4f). In addition, we investigated the illumination-
time dependence on the decay of two key photovoltaic
Table 1 | Photovoltaic Parameters (Average and Maximum Values) of 15 Separated Devices Based
on the HTMs with and without Boc, Respectively
HTM VOC/V JSC/mA cm
−2 FF PCE
With Boc Ave. 0.98 ± 0.04 24.02 ± 0.78 0.67 ± 0.02 15.74 ± 0.53
Max. 1.02 22.22 0.72 16.36
Without Boc Ave. 1.11 ± 0.01 23.80 ± 0.29 0.79 ± 0.02 20.89 ± 0.65
Max. 1.12 23.27 0.83 21.62
Figure 4 | (a) Illustration of the device structure. (b) Energy-level diagram for the device. (c) J–V curves of PSCs with
TPADPP-Boc, TPADPP, and PTAA HTMs under illumination of AM 1.5G 100 mW cm2. (d) EQE curves of PSCs with
TPADPP-Boc, TPADPP, and PTAA HTMs. (e) Maximum steady-state photocurrent output of the champion device at
the MPP. (f) Long-term stability tests of devices based on TPADPP under light soaking at MPP (1 sun, with 420 nm UV
light cutoff filter; 35 °C; bias potential 0 V; using glass lid/UV epoxy encapsulation) and under storage conditions.
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parameters (Jsc andVoc). The devicemaintained 87%of its
initial PCE value after 500 h. There is no significant change
inVocduring the illumination time (Supporting Information
Figure S8). However, we could observe a gradual drop in
Jsc, whichmay be caused by the degradation of the perov-
skite layer due to ion migration.
Conclusion
Two D–A–D typed materials, namely TPADPP-Boc and
TPADPP, were designed, synthesized, and applied in the
PSCs as dopant-free HTMs. TPADPP was obtained via the
thermal annealing of TPADPO-Boc. Our studies showed
that, after the decarboxylation, the N-Boc units were con-
verted to NH groups, which then formed hydrogen-bond-
ing (NH⋯CO) with the carbonyl of the neighbouring DPP
core. The hydrogen-bonding association significantly en-
hancedhole transportmobility—upto3.09× 10−4 cm2V−1 s−1
for TPADPP—which is three times higher than that of
TPADPP-Boc. Furthermore, the NH units that emerged
couldnot onlypassivate the surface of the perovskite layer
for a more crystalline perovskite film but also form Pb–N
coordination bonds at the interface between the HTM and
the perovskite layer, which significantly improve the hole
extraction. As a result, the dopant-free hydrogen-bonded
TPADPP based PSCs achieved a champion PCE of 21.62%,
whichbearscomparisonwiththatofclassicaldopedPTAA-
baseddevices(PCEaround22%).However,thepristinePSC
based on TPADPP-Boc showed a PCE of only 16.36%.
Furthermore, the dopant-free TPADPP-based device
showed remarkably high long-term photostability, retain-
ing 87% of its original value after 500 h of continuous
illuminationmeasuredatMPP. Introducinghydrogenbond-
inginthemoleculardesignconcept,suchasamineunits, isa
simple and useful strategy to develop high-performance
solution-processed dopant-free HTMs. This study is
expected to provide a guideline for further development
ofdopant-freeHTMsforhighlyefficientandstable inverted
PSCs.
Supporting Information
Supporting Information is available and includes experi-
mental procedures, Figures S1–S13, and Tables S1–S10.
Conflict of Interest
The authors declare no conflict of interest.
Funding Information
This researchwasmade possible as a result of a generous
grant from the Natural Science Foundation of China
(grant no. 21805151), the Natural Science Foundation of
Shandong Province, China (grant no. ZR2018MB024),
and the Young Taishan Scholars (grant nos. 201909120
and 201909121). M.L. acknowledges the Finnish Cultural
Foundation (no. 00210670) for funding. P.V. thanks the
Jane and Aatos Erkko foundation (project ASPIRE) for
financial support. This work is part of the Academy of
Finland Flagship Programme, Photonics Research and
Innovation (PREIN), Decision No. 320165.
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