Reducing Defects Density and Enhancing Hole Extraction for Efficient Perovskite Solar Cells Enabled by π-Pb2+ Interactions

Article abstract of DOI:10.1002/anie.202102096

Molecular doping is an of significance approach to reduce defects density of perovskite and to improve interfacial charge extraction in perovskite solar cells. Here, we show a new strategy for chemical doping of perovskite via an organic small molecule, w

Full text of DOI:10.1002/anie.202102096

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Accepted Article
Title: Reducing defects density and enhancing hole extraction for
efficient perovskite solar cells enabled by π-Pb2+ interaction
Authors: zhongmin zhou, Li-rong Wen, Yi Rao, Mingzhe Zhu, Ruitao Li,
Jingbo Zhan, Linbao Zhang, Li Wang, Ming Li, and Shuping
Pang
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COMMUNICATION
Reducing defects density and enhancing hole extraction for
2
+
efficient perovskite solar cells enabled by π-Pb interaction
[
a]†
[b] †
[a]
[a]
[a]
[a]
[c]
Li-rong Wen, Yi Rao, Mingzhe Zhu, Ruitao Li, Jingbo Zhan, Linbao Zhang,* Li Wang,*
[
a]
[b]
[a]
Ming Li, Shuping Pang, and Zhongmin Zhou*
[
a]
Prof. L.-r. Wen, M. Zhu, R. Li, J. Zhan, Prof. L. Zhang, Prof. M. Li, Prof. Z. Zhou
Taishan scholar advantage and characteristic discipline team of Eco-chemical process and technology
College of Chemistry and Molecular Engineering
Qingdao University of Science and Technology
Qingdao 266042, P. R. China
E-mail: : zhang_linbao@126.com; zhouzm@qust.edu.cn
[
b]
c]
Y. Rao, Prof. S. Pang
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences,
Qingdao 266101, P. R. China;
Dalian National Laboratory for Clean Energy, Dalian 116023, P.R. China
[
Prof. L. Wang
College of Materials Science and Engineering
Qingdao University of Science and Technology
Qingdao 266042, P. R. China
E-mail: : liwang718@qust.edu.cn
[†] These authors contributed equally to this work.
the development of PSCs.[4] Varieties of strategies have been
exploited, among them the chemical doping is an efficient
approach to reducing nonradiative recombination, improving
carriers transportation and/or tuning fermi level, thus resulting in
excellent device performance.[5] For example, PCBM has been
generally utilized to enhance the photoelectron transport in
perovskite.[6] Wu et al. introduced PCBM through anti-solvent into
Abstract: Molecular doping is an of significance approach to reduce
defects density of perovskite and to improve interfacial charge
extraction in perovskite solar cells. Here, we show a new strategy for
chemical doping of perovskite via an organic small molecule, which
2
+
features a fused tricyclic core, showing strong intermolecular π-Pb
2
+
2+
interactions with under-coordinated Pb in perovskite. This π-Pb
interactions could reduce defects density of the perovskite and
suppress the nonradiative recombination, which was also confirmed
by the density functional theory calculations. In addition, this doping
2
+
via π-Pb interactions could deepen the surface potential and
downshift the work function of the doped perovskite film, facilitating
the hole extraction to hole transport layer. As a result, the doped
device showed high efficiency of 21.41% with ignorable hysteresis.
This strategy of fused tricyclic core-based doping provides a new
perspective for the design of new organic materials to improve the
device performance.
3
FA0.85MA0.15Pb(I0.85Br0.15) to construct graded heterojunction to
improve the photoelectron collection and reduces charge
recombination loss.[6c] Besides, Cu(thiourea)I was utilized to
x
passivate the trap state of perovskite (MAPbI Cl3-x) and increase
the depletion width of bulk heterojunction, which could accelerate
hole transport and reduce charge recombination.[5d] Recently,
Huang group doped perovskite (MAPbI
3
) with F4TCNQ to change
Due to their high light absorption coefficient, direct and tunable
bandgap, long carrier diffusion length, metal halide perovskite
materials have gained significant attention in the last decade.[1]
Recent extensive research into the perovskite solar cells (PSCs)
has led to power conversion efficiency (PCE) from the initial 3.8%
to a certificated 25.5% and thus makes them as competitive
candidates for photovoltaic applications.[2] However, the current
PCE is still far below the limit value of ~ 30% according to the
theoretical Shockley-Queisser limit for single-junction solar cells,
thus there is still large room for improvement.[3] Intrinsic high
defects density of perovskites and mismatched energy-level
alignment at the interfaces within device could lead to robust
charge recombination and inefficient charge extraction, limiting
the work function of MAPbI and increase the carrier (hole)
3
concentration and thus greatly improved device performance.[5e]
Inspired by all above-mentioned demonstrations, we
synthesize a new organic small molecule 1-(4-bromophenyl)-6,7-
diphenylimidazo[5,1,2-cd]indolizine (PDPII, Figure 1), with
limidazo[5,1,2-cd]indolizine as core and benzene substituents as
peripheral unit. The molecule features a fused tricyclic core,
showing strong intermolecular π-Pb2+ interactions with under-
coordinated Pb2+ and thus reducing trap densities in perovskite.
In addition, the chemical doping of 1 deepens the surface
potential and increases the work function of perovskite films,
facilitating the hole extraction from perovskite into hole transport
1
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boundaries and on the surface of perovskite films, which is also
confirmed by the sequence of photoluminescence (PL) maps of
perovskite films taken in situ (Figure 1 e, f). We can see from Fig.
1
f that the blue dots (PDPII) are uniformly distributed in the
perovskite film compared to the Fig. 1e with no any bule dots
existence in the film. Then the conventional devices were
assembled with the configuration of F-doped SnO
2
(
FTO)/compact TiO (c-TiO )/SnO /Perovskite/Spiro-OMeTAD/
2
2
2
Au. The optimal doping concentration of compound 1 for device
performance was optimized as shown in Fig. 2a. Fig. 2b shows
the photocurrent density versus photovoltage (J-V) curves of the
optimal device with and without doping. The optimal device with
doping compound 1 under reverse-scan direction exhibited a
2
short-circuit current density (JSC) of 23.39 mA/cm , an open-circuit
voltage (VOC) of 1.12 V, a fill factor (FF) of 0.80, and the high PCE
Figure 1. (a) The synthetic route to the compound 1. (b) The schematic diagram
of the deposition method of perovskite film and the compound 1 located at the
of 21.41%. This optimal device showed ignorable hysteresis with
grain boundary and on the surface after film formation and strongly interacted
with perovskite component (Pb2+). SEM images of perovskite films without
2
a JSC of 24.19 mA/cm , a VOC of 1.10V, a FF of 0.80, and the PCE
doping (c) and with doping (d). In situ photoluminescence maps (excitation at
of 21.34% under forward-scan direction. However, the device
without doping showed the PCEs of 20.22% and 18.22% under
reverse-scan and forward-scan direction, respectively. The
hysteresis index (HI = (PCEreverse − PCEforward)/ PCEreverse) of the
optimal devices with and without doping were calculated to be
405 nm and emission between 425-475 nm) of perovskite films without doping
(
e) and with doping (f).
layer (HTL) while reducing the charge recombination at this
interface. As a result, the PCE of 21.41% was obtained with less
hysteresis (21.41% and 21.34% under reverse and forward scan
directions) compared with the reference device (20.22% and
0
.3% and 9.9%, respectively. Therefore, the doping of the
compound 1 reduces largely the hysteresis of the device, which
1
8.22% under reverse and forward scan directions).
may originate from the reductive defect density in doped
PDPII can be readily synthesized in two steps from commercial
perovskite, reduced charge recombination and efficient charge
extraction.[9] The steady-state photocurrent output was conducted
compounds. Figure 1a shows the synthetic route of 1 through the
typical reaction of 2-aminopyridines and α-bromo-ketones in
alkaline environment followed by hot electrochemical synthesis,
which provides an environmentally-friendly approach avoiding the
usage of toxic oxidants and costly additives. The compound 1 was
under maximum power point (MPP) bias to confirm the reliability
of the devices. As shown in Figure 2c, the device with doping
exhibits stable photocurrent after 100s continuous illumination
compared to continuous photocurrent decrease of control device,
indicating that the doping of compound 1 can improve both PCE
and light stability. The integration of the external quantum
efficiency (EQE) spectra of the devices with and without doping
are shown in Figure 2d. The device with doping achieves an
x
introduced into the perovskite (FA0.85MA0.15Pb(I0.85Br0.15)3-xCl ) film
via commonly utilized antisolvent dripping approach as shown in
Figure 1b.[7] Figure 1c, d exhibit the scanning electron microscopy
(
SEM) images of perovskite films without and with doping the
compound 1. There is no obvious difference in the grain size
between the perovskite films with and without the doping, which
is consistent with the cross-sectional morphology as shown in
Figure S1. This doping is different from the as-reported results,
which exploited varieties of dopants to modulate the
crystallization of the perovskite films, showing large difference in
film morphology.[8] What’s more, we don’t observe new peaks
appearing as demonstrated by X-ray diffraction (XRD) pattern
spectra in Figure S2, indicating that there are no new phases or
intermediates obtained after doping. Noting that the less intensity
of pattern peaks in doped film may result from the thin layer of
compound 1 on the surface of perovskite film. We can see from
Figure 1d that the compound 1 is distributed at the grain
2
integrated JSC of 22.28 mA/cm while that of control device shows
2
2
2.11 mA/cm , which are consistent with the measured values.
Figure 2 e-h show the performance parameters of devices with
and without doping, indicating the distinct improvement of devices
with doping compound 1.
To confirm the interaction between the compound 1 and
perovskite, we performed X-ray photoelectron spectroscopy
(
XPS) analysis of the perovskite films. The limidazo[5,1,2-
cd]indolizine core has strong  system, which is expected to form
-Pb2+ with the under-coordinated Pb2+ to reduced defects density
of the perovskite.[10] As shown in Figure 3a, our hypothesis was

2
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Angewandte Chemie International Edition
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COMMUNICATION
2
2
1
2
0
8
25
25
20
15
10
5
100
80
60
40
20
0
25
20
For. 0mM
Rev. 0mM
For. 1mM
Rev. 1mM
2
0
0
1
nM
nM
15
15
10
5
0
1
mM @ 0.935 V
mM @ 0.913 V
1
0
5
0
0
0
0
0.2
0.5
1.0
2.0
5.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0
20
40
60
Time (s)
80
100
300
400
500
600
700
800
Concentration (mM)
V
oc
/V
Wavelength (nm)
1
1
1
.15
.10
.05
25
22
21
20
19
18
8
8
7
7
2
0
8
6
2
2
2
2
4
3
2
1
0
1.0
Concentration (mM)
0
1.0
0
1.0
Concentration (mM)
0
1.0
Concentration (mM)
Concentration (mM)
Figure 2. (a) PCEs of devices based on various concentration (mmol/ml) of the compound 1. (b) J–V curves for the best-performing devices based on the perovskite
2
films with and without doping under illumination of AM 1.5G 100 mW/cm . The inset is the corresponding performance parameters. (c) Maximal steady-state
photocurrent output of the champion devices based on the perovskite films with and without doping at the maximum power point. (d) EQE curves of PSCs based
on the perovskite films with and without doping. The reverse scanning performance parameters (e) Voc, (f) Jsc, (g) FF, (h) PCE of 15 devices based on the perovskite
films with and without doping.
verified by the peaks shift (from 141.85 to 141.30 eV for Pb 4f5/2
orbit while from 137.05 to 136.4 eV for Pb 4f7/2 orbit), indicating
the higher electron density environment around Pb2+ cation
created by the compound 1. The quality of the perovskite films on
Figure 3d, the trap-filled voltage (VTFL) can be obtained through a
kink point between the ohmic and the trap-filled regimes.[13] The
defects density (N) is calculated from the Equation (2) below:
2
푒푁퐿
푉TFL
=
(2)
glass
photoluminescence (PL) and time-resolved photoluminescence
TRPL) decay measurements. In Figure3b, the PL intensity of the
was
further
evaluated
by
the
steady-state
ꢁ휀휀0
Where e is the elementary charge, L is the thickness of perovskite
film, ɛ is the relative dielectric constant of the perovskite and ɛ is
(
0
perovskite film with doping compound 1 is significantly enhanced
compared to that of the control film, indicating the high radiative
recombination of the perovskite film with doping, which also
suggests that the nonradiative recombination is restrained in
doped perovskite film. Additionally, the PL peak of the doped film
shows a blueshift of 2 nm compared with that of the control film,
indicating that there is a decrease of defects density resulting
the vacuum permittivity. As shown in Figure 3d, the defect density
was calculated to be 1.66 × 1016 and 3.39 × 1016 cm−3 for
perovskite films with and without doping, which points out that the
doping of compound 1 can effectively reduce the electron defects
density of the perovskite film. Besides, the recombination
dynamic in PSCs is frequently investigated through the
dependency of VOC on light intensity according to the following
equation (3),
possibly from the passivation interaction between the  electrons
of the compound 1 and the Pb2+ in perovskite.[11] Figure 3c shows
푉OC = 푛푘푇/푞ln(퐼)
(3)
where n, κ, T, and q are ideal factor, Boltzmann constant, absolute
temperature and the electron charge. [14] When the ideal factor is
close to 1, the effect of Shockley-Read-Hall recombination can be
neglected. From the slope of the curves in Figure 3e, the fitting
result decreased from 1.67 to 1.40 for doping device, suggesting
that the doping of compound 1 can reduce the defects density and
suppress charge recombination, in well consistent with the XPS,
PL, TRPL and SCLC measurement results. Besides, the transient
photovoltage (TPV) decay of perovskite films with and without
doping PDPII were also measured. As shown in Figure S3 that
the film with doping shows longer charge recombination constant,
indicating better carrier extraction of the doped film.
the TRPL decay curves. The fitted parameters are summarized in
Table S4. The average PL lifetimes (τave) were calculated from the
following equation (1):[12]
2
∑
퐴
ꢀ
푖
푖
휏ave = ∑
(1)
퐴 ꢀ
푖
푖
We obtained the τave values are 67 ns and 44 ns for perovskite
films with and without doping, respectively. As our expected, the
τave is enhanced after doping, showing that the compound 1
reduces the nonradiative recombination of the perovskite film. In
order to quantitatively estimate the defects density in perovskite,
the space charge limited current (SCLC) method with the device
2 2
structure of FTO/c-TiO /mp-TiO /Perovskite/PCBM/Au was
exploited. The J-V curves were obtained in the dark. As shown in
A better insight into the interactions between the compound 1
3
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∆r퐻m = −488.66 kꢉꢊl ꢋꢌlꢍ1 (1 Hartree = 627.509469 kcal mol-1)
and Gibbs free energy with ∆ 퐺 = −48ꢎ.ꢏꢐ kꢉꢊl ꢋꢌlꢍ1. Although
1
1
1
1
0
0
0
0
0
.6
.4
.2
.0
.8
.6
.4
.2
1
w/o
w
0
1
mM
mM
0
1
mM
mM
0
.1
r m
accompanying with the decrease of entropy ( ∆r푆m
=
0
.01
−ꢐ5.63 ꢉꢊl ꢋꢌlꢍ1Kꢍ1), the strong exothermal effects suggested a
thermodynamically spontaneous process at room temperature in
gas phase.
.0
1
0.001
0
44
142
140
138
136
134
700
750
800
850
50
100
Binding Energy (eV)
Wavelength (nm)
Time (ns)
0
1
mM
mM
0
.001
0
1
mM
mM
1.1
1
E-4
E-5
E-6
E-7
0
1
.40kT/q
1
1.0
1.67kT/q
1
1
VTFL
To further gain insight into the mechanism responsible for the
improvement of device performance, kelvin probe force
microscopy (KPFM) was utilized to study the surface potential
change of perovskite films. Figure 4a, b are the topography
images of the perovskite films without and with doping. The
corresponding contact potential difference (CPD) images are
shown in Figure 4c, d, respectively. A deeper surface potential
was clearly observed for the perovskite film with doping, as
0
.9
.01
0.1
1
10
Light intensity (mW cm-2)
100
Voltage (V)
Figure 3. (a) Pb 4f XPS spectra of the control and the doped films. (b) PL
spectra and (c) time-resolved PL decays of the perovskite films with and without
doping. (d) The dark J–V curves of devices based on the perovskite films with
and without doping. (e) Voc of devices as function of illumination intensity. (f)
The ideal model of interaction between the compound 1 and Pb cation in
perovskite.
2
+
and perovskite was illustrated by density functional theory (DFT)
2
calculations, where the perovskite was assumed to be PbI -
reflected from CPD values (
̶ 200 ± 16 mV for doping film and ̶ 95
terminated system (there are detailed calculation parameters in
the Supporting Information). The optimized model of the
±
18 mV for control film) in Figure 4e. The ultraviolet photoelectron
spectroscopy (UPS) analysis was then exploited to verify the
KPFM results. As shown in Figure S4 and S5, the Fermi levels
and valance band edges are obtained as ˗4.61 eV and ˗5.64 eV,
2
+
compound 1 and perovskite (Pb ) is shown in Fig. 3f with shortest
length of 2.45 Å. Partial electron density of C=C bond donated to
Pb2+ cation causes slight distortion of π-plane and the decrease
of the bond order. In computational chemistry, the interaction
energy between A and B is determined by 퐸iꢂt, which is estimated
by the following equation (4):[15]
˗
4.92 eV and ˗5.83 eV for perovskite films without and with doping,
respectively. The doping of compound 1 leads to a downward shift
of Fermi level, which is consistent with the KPFM results. The
deeper CBM of doped perovskite film can reduce the
recombination possibility of electrons from the perovskite with the
퐸iꢂt = 퐸AB − (퐸 + 퐸 ) + ꢃSSE (basis set superposition error)
A
B
holes in HTL, which is beneficial to the enhancement of VOC
.
corr
=
퐸
− (퐸 + 퐸 )
(4)
AB
A
B
Additionally, the Fermi level is closer to valence band edge of the
doping film compared to that of the control film (Figure 4f),
enhancing hole extraction and benefiting for the higher VOC in
doped device. [17]
where 퐸AB is the energy of complex AB, 퐸 + 퐸 is the sum
A
B
corr
energy of monomers, and 퐸AB ꢄ푠 the counterpoise corrected
energy of the complex. The corrected complexation energy was
simulated to be -152.03 kcal/mol. We also analyzed the predicted
thermochemistry data of the above ideal model in gas phase at
0
.0
-
0.1
0.2
2
98.15 K under 1 atm. Assuming the reaction is
-
M + Pbꢁꢅ → M ⋯ Pbꢁꢅ
-0.3
0
1
mM
mM
-
0.4
0.0
0.5
1.0
1.5
2.0
Position (μm)
Where M represents the doping molecule. Since in any certain
temperature, there are
E
vac
CBM
4.05 eV
-
CBM
4.24 eV
e-
×
×
-
Fermi level
4.61 eV
-
spiro-
- 4.92 eV OMeTAD
SnO
2
Fermi level
∆r퐻m = ∑products(ꢆ + 퐻corr) − ∑reactaꢂts(ꢆ + 퐻corr)
ꢇ
ꢇ
perovskite
doped
perovskite
-
5.64 eV
VBM
- 5.83 eV
h
+
VBM
∆r퐺m
=
ꢈ
(ꢆ + 퐺corr) −
ꢈ
(ꢆ + 퐺corr)
ꢇ
ꢇ
products
reactaꢂts
Figure 4. Surface morphology and potential of (a), (b) AFM, and (c), (d) KPFM
images of pristine and doped perovskite films. (e) Potential profiles along the
indicated line. (f) Energy band diagram of the charge transportation layers and
perovskite films with and without doping.
and
∆r퐺m = ∆ 퐻 − 푇∆r푆m
r m
where ∆ 퐻 , ∆ 푆 and ∆ 퐺 are the change of enthalpy, entropy
r
m
r
m
r
m
To check whether Br in compound 1 offers additional benefit in
improving device performance, we synthesized Cl-, I- substituent
PDPIIs (see the details in Supporting Information). Both devices
with doping Cl-PDPII and I-PDPII showed better PCEs than that
of the devices without doping. However, the devices with doping
and Gibbs free energy per mol reaction.[16] ꢆꢇ + 퐻corr and ꢆ +
ꢇ
퐺corr are the sum of electronic and thermal enthalpy, the sum of
electronic and thermal free energy, respectively. It can be easily
figured out that the reaction was favored by both enthalpy with
4
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the compound 1 (Br-PDPII) showed the best results (Figures S6
and Table S5), mainly due to higher VOC and FF values.
Shao, P. Mulligan, J. Qiu, L. Cao, J. Huang, Science 2015, 347, 967–
9
70.
a) A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc.
009, 131, 6050–6051; b) https://www.nrel.gov/pv/cell-efficiency.html,
accessed: September 2020).
[
2]
2
Finally, the stability of the device with doping was investigated.
As shown in Figure S7, the un-encapsulation device with doping
(
[3]
B. Ehrler, E Alarcón-Lladó, S. W. Tabernig, T. Veeken, E. C. Garnett, A.
Polman, ACS Energy Lett. 2020, 5, 3029–3033.
1
exposed to air with a relative humidity (RH) of around 20%
[
4]
a) S. Xiong, J. Song, J. Yang, J. Xu, M. Zhang, R. Ma, D. Li, X. Liu, F.
Liu, C. Duan, M. Fahlman, Q. Bao, Sol. RRL 2020, 4, 1900529; b) X. Zhu,
M. Du, J. Feng, H. Wang, Z. Xu, L. Wang, S. Zuo, C. Wang, Z. Wang, C.
Zhang, X. Ren, S. Priya, D. Yang, S. (Frank) Liu, Angew. Chem. Int. Ed.
retained ~ 94% of the initial PCE after 1300h of storage, whereas
an un-encapsulation device without doping kept ~ 87% of the
original value under the same conditions. The light stability of
encapsulated devices with and without doping was test as shown
in Figure S8. The doped device remained basically unchanged
after 270 h continuous irradiation at MPP condition. However, the
control device decayed to 60% of its original PCE after 240 h. We
2020, DOI: 10.1002/anie.202010987; c) S. Chen, Q. Pan, J. Li, C. Zhao,
X. Guo, Y. Zhao, T. Jiu, Sci. China Mater. 2020, 63, 2465–2476; d) L. K.
Ono, S. (Frank) Liu, Y. Qi, Angew. Chem. Int. Ed. 2020, 59, 6676–6698;
e) W. Qi, X. Zhou, J. Li, J. Cheng, Y. Li, M. J. Ko, Y. Zhao, X. Zhang, Sci.
Bull. 2020, 65, 2022–2032; f) M. Abdi-Jalebi, M. I. Dar, S. P. Senanayak,
A. Sadhanala, Z. Andaji-Garmaroudi, L. M. Pazos-Outón, J. M. Richter,
A. J. Pearson, H. Sirringhaus, M. Grätzel, R. H. Friend, Sci. Adv. 2019,
5, eaav2012; g) E. H. Anaraki, A. Kermanpur, M. T. Mayer, L. Steier, T.
also conducted high humidity (RH 80%) and high temperature
Ahmed, S.-H. Turren-Cruz, J. Seo, J. Luo, S. M. Zakeeruddin, W. R.
Tress, T. Edvinsson, M. Grätzel, A. Hagfeldt, J.-P. Correa-Baena, ACS
Energy Lett. 2018, 3, 773−778.
o
(
150 C) measurement of perovskite films without and with doping.
We took pictures of the films both from front and back sides. As
shown in Figure S9, the control film began to decay at 0.5h under
RH 80%. However, the doped film shows sign of degradation at
around 2h. Figure S10 shows the thermal stability of perovskite
[
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Angewandte Chemie International Edition
10.1002/anie.202102096
COMMUNICATION
Table of Contents
2
2
1
1
5
0
5
0
5
0
For. 0mM
Rev. 0mM
For. 1mM
Rev. 1mM

-Pb2+
0.0
0.2
0.4
0.6
0.8
1.0
1.2
V
oc
/V
Featuring a fused tricyclic core, an organic small molecule was intentionally syntheszied to reduce defects density and improve hole
transportation in perovskite devices via π-Pb2+ interaction, confirmed by multiple characterizations and simulation.
6
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